new buuld, what's wrong?
Phil_S
problem?
This amp: http://home.comcast.net/~psymonds/Maxie-tone.htm
No shorts, it passes the dim bulb limiter test. With tubes in sockets, HT
sends 580vac to the rectifiers. Full wave diode rectified B+ at the first
filter cap, standby open, is over 500V, which made me really nervous because
I used 450V caps. I figure 580 * .7 = 409. How's it going 100v above that?
OK, closed the standby, which interrupts the B+ rail between the first
filter cap and the choke. Closing the standby (on position) completes the B+
rail to the choke and the second filter cap. B+ drops to 122vdc (tubes in
sockets). The choke is 556 ohms, unknown inductance (H), but it came from a
cathode biased amp running a pair of 6L6. Aside from this amp being fixed
bias, the use of the choke is same as the donor amp. I don't know if the
donor amp was working.
The B+ rail seems OK. Voltage is dropping from 120 to around 110 as I go
from node to node, suggesting to me that part is built OK. If we had *real*
voltage, I'd know for sure.
One other problem is the bleed resistor. What did I do wrong? One end to the
rectified output (where the diodes meet and feed the first filter cap) and
the other end to ground. They were bleeding down the live B+ and there was
no meaningful voltage with them in circuit. This is lifted from the circuit
right now and is a comparatively minor problem.
Is the choke just sucking all the voltage out of the amp? Should I just
replace it with a resistor? What value might be appropriate here, especially
in light of the outrageously high voltage coming from the rectifier when
standby is open.
Well...thanks. Pictures might be available tonight.
Feel free to return to the tea party. Sorry for the interruption.
Phil
WB
> You guys feel like taking a break to give me a hand trouble shooting the
> problem?
>
> This amp: http://home.comcast.net/~psymonds/Maxie-tone.htm
>
> Is the choke just sucking all the voltage out of the amp? Should I just
> replace it with a resistor? What value might be appropriate here,
> especially in light of the outrageously high voltage coming from the
> rectifier when standby is open.
>
The schematic doesn't show a choke ... 550o is a lot .. They
generally run 100 ohm, 40H range , and some amps don't use them.
I'added chokes to get rid of extreme ripple on B1+, .. hell even just
B+ .
With a unknown choke .. unknown current rating ... I would :
short the choke ( you can simply use a jumper and short the bitch ) ...
> Feel free to return to the tea party. Sorry for the interruption.
>
Free the tea leafs ! They have been tormented enough . Return them to
India where they below .. No more refuge for tea baggers !
Phil_S
"WB" <nos...@no.com> wrote in message
news:4b451b77$0$5342$bbae...@news.suddenlink.net...
> Phil_S wrote:
>> You guys feel like taking a break to give me a hand trouble shooting the
>> problem?
>>
>> This amp: http://home.comcast.net/~psymonds/Maxie-tone.htm
>
>>
>> Is the choke just sucking all the voltage out of the amp? Should I just
>> replace it with a resistor? What value might be appropriate here,
>> especially in light of the outrageously high voltage coming from the
>> rectifier when standby is open.
>>
> The schematic doesn't show a choke ... 550o is a lot .. They generally
> run 100 ohm, 40H range , and some amps don't use them.
> I'added chokes to get rid of extreme ripple on B1+, .. hell even just B+
> .
>
>
> With a unknown choke .. unknown current rating ... I would :
>
> short the choke ( you can simply use a jumper and short the bitch ) ...
>
>
LOL! I've already got it removed. You don't see a choke on the schematic?
L1? In the B+ rail, next to the standby switch, S2?
Anyhow, there should be something between the first filter cap to the OT CT
and the second filter cap that feeds the screens. I think I'm going with 1K
5W. If that's no good, I've got 225 5W, one of those brown ones.
>
>> Feel free to return to the tea party. Sorry for the interruption.
>>
>
> Free the tea leafs ! They have been tormented enough . Return them to
> India where they below .. No more refuge for tea baggers !
Ummm...yuh.
Stephen Cowell
"Phil_S" <psymond...@comcast.net> wrote
> OK, closed the standby, which interrupts the B+ rail between the first
> filter cap and the choke. Closing the standby (on position) completes the
> B+ rail to the choke and the second filter cap. B+ drops to 122vdc (tubes
> in sockets). The choke is 556 ohms, unknown inductance (H), but it came
> from a cathode biased amp running a pair of 6L6. Aside from this amp being
> fixed bias, the use of the choke is same as the donor amp. I don't know if
> the donor amp was working.
>
> The B+ rail seems OK. Voltage is dropping from 120 to around 110 as I go
> from node to node, suggesting to me that part is built OK. If we had
> *real* voltage, I'd know for sure.
>
> One other problem is the bleed resistor. What did I do wrong? One end to
> the rectified output (where the diodes meet and feed the first filter cap)
> and the other end to ground. They were bleeding down the live B+ and there
> was no meaningful voltage with them in circuit. This is lifted from the
> circuit right now and is a comparatively minor problem.
Something wrong with the PT... how much voltage drops across the
choke? When you put the bleeder back in, does most of the voltage
drop *before* the choke, or at the bleeder? I'm guessing that
your PT is not giving enough current; put the bleeder back in (take the
tubes out) and find out who's the pig sucking down the B+.
BTW, the bleeder needs to be located where it can pull down the
caps no matter what position the standby is in... as is, you can still
get a major shock.
__
Steve
.
jh
hi Phil,
as Steve already wrote; look which component sucks down B+. BTW these
Variations are typically normal in an AC30. Without tubes in there, B+
rises substantially.
Perhaps it's not a "real" problem. How hot does the PT get and how does
it behave under (playing) load not just in idle; Class A or AB?
BTW I can't see the real schematic only the small thumb.
And - what the heck is this garden hose tubing?
;-)
regards
Jochen
WB
>> Free the tea leafs ! They have been tormented enough . Return them
>
> Ummm...yuh.
Would you like Wingnuts with your tea ?
Phil_S
bleeders. They are presently lifted. Even after lifting them, the circuit
was still wrong. I pulled the choke and put in a 1K 5W. This fixed the B+
rail. B+ nodes starting at the first filter cap: 371-364-342-332-322. All
seems well here now. In terms of expectations, if the HT secondary is ~580,
expected B+ would be 580*.7=406, and taking off 5% for loading brings it
down to ~385; so 371 is in the ball park.
There is other relatively good news on other voltages, except for V1, a
6SL7. B+ at 322v goes across a 100K plate load R that is shared by both
plates. Plate voltage drops to 130v. I'd much prefer to push this closer
to 200v. One cathode r/c is 1K/24u and the other is 1K/0.86Uu (not 0.68;
found a bargain bin regular film cap for 25 cents). Cathode voltage is ~1V
on both sides. What to do here?
Bad news on the 6L6's pulled from that Sears organ(?) chassis. They are
very pretty looking Sylvania's and I so much wanted them to be good. Alas,
the voltage drop from plate to plate (across the OT primary) is in the mV
range, over a primary R of 118 ohms. 350mV/118z = ~3mA. This is true
through the range of bias voltage from -24 to -36. I've concluded they are
toast. Since they are so pretty looking, I think I'm going to figure out
how to turn them into a paperweight and see if it will fetch $100 on
fleebay. Sell it to a rich dentist?
More...see reply to Jochen.
Thanks.
Phil
Phil_S
news:hi4315$ut9$1...@news.eternal-september.org...
>
> hi Phil,
>
> as Steve already wrote; look which component sucks down B+. BTW these
> Variations are typically normal in an AC30. Without tubes in there, B+
> rises substantially.
> Perhaps it's not a "real" problem. How hot does the PT get and how does it
> behave under (playing) load not just in idle; Class A or AB?
>
I've concluded the choke is a pig. I've removed it. See above reply to
Steve.
> BTW I can't see the real schematic only the small thumb.
>
I tested it with IE and Firefox. Have you tried either clicking on it
(maybe double click) or right clicking and selecting "view image"? It comes
into the browser inconsistently for reasons I don't understand. Clicking on
it should bring it into view. Please give me some additional feedback on
this. I can modify the resolution on it. Thanks.
>
> And - what the heck is this garden hose tubing?
>
The blue hose is 18ga unshielded twisted pair rated for 600V that came from
some mainframe computer application. It was a bargain eBay purchase. I
have about 10 lengths of 10 ft each. If you want one, send me your mailing
address via email and I'll send you one...I know you are in Germany, but it
won't cost much to put in in an envelope. You, too, can have the blue hose
look!
I had stripped the wire out of the cover and rolled it up on a spool, but it
seemed just perfect as is for the heaters.
> ;-)
>
> regards
>
> Jochen
Meanwhile, I'm going to see if I can get the amp going with a pair of 6V6.
They ought to be good with plate voltage in the high 300's and bias voltage
seems to be in the relevant range.
Phil
Phil_S
"WB" <sp...@there.com> wrote in message
news:4b45d763$0$5326$bbae...@news.suddenlink.net...
Regular nuts would be fine with me, thank you very much.
Stephen Cowell
"Phil_S" <psymond...@comcast.net> wrote
> Meanwhile, I'm going to see if I can get the amp going with a pair of 6V6.
> They ought to be good with plate voltage in the high 300's and bias
> voltage seems to be in the relevant range.
Don't chuck the Sylvanias until you've truly determined
that the OT is good, no wiring errors, etc.
Nice lookin' build, btw.
__
Steve
.
Phil_S
"jh" <j...@org.de> wrote in message
news:hi4315$ut9$1...@news.eternal-september.org...
> BTW I can't see the real schematic only the small thumb.
>
You shamed me into posting a larger update ;-}
Phil_S
"Stephen Cowell" <stephenleeN...@gmail.com> wrote in message
news:hi4rv5$a6t$1...@news.eternal-september.org...
Yeah, I'm not too quick to toss anything. I was half serious out making a
paper weight.
The OT ohms out very reasonably. I ran low voltage on it to confirm turns
ratios and to be sure which 2ndary wire is which, as the label was
incomplete. I have every reason to believe the OT is good. Of course,
we'll see.
Thanks for your comment on how it looks. I am not totally satisfied, but
it's clear that I'm improving. This was another shoehorn build. Fitting in
the power supply caps was a bitch. I built it outside the chassis and
dropped it in. I did not like drilling so late in construction, but
couldn't figure a way around that. At first, I was going to hang it on the
3" vertical side, but it fit on the floor and that solved a few other lead
dress challenges. It was one of those things, damned if you do, damned if
you don't. If I was doing it again, I'd use a 17x8 instead of 17x6. The 3"
height is nice. I began to think in 3 dimensions. I left 1" under the
board, allowing the OT secondaries plenty of room to find their way to the
jacks. One unexpected advantage of the 6" wide was very, very short runs
from the board to the sockets. At the power tubes, if you can see it, grid
stoppers and screen stoppers use no wire at all.
Those yellow generic boards are not bad at all to work with once you get
used to them. Steve at Apex Jr. sells them for $3.95 and Weber sells them
for $5.95. 6 of 1, half dozen of the other, if you are ordering, the
overall parts order plus shipping is the determining factor for all-in
cost.
I ordered parts from Weber. The 2A and 0.6A circuit breakers were less than
buying a fuse and holder, so I gave them a try. Fits in a smaller hole and
lower profile. I think I would use them again.
Stephen Cowell
"Phil_S" <psymond...@comcast.net> wrote
> I ordered parts from Weber. The 2A and 0.6A circuit breakers were less
> than buying a fuse and holder, so I gave them a try. Fits in a smaller
> hole and lower profile. I think I would use them again.
Is that small CB rated for HV? That's hard to do!
__
Steve
.
Phil_S
"Stephen Cowell" <stephenleeN...@gmail.com> wrote in message
news:hi505o$pve$1...@news.eternal-september.org...
I don't know why it wouldn't be OK. AFAIK he sells them for what I'm doing.
One is for line voltage, 120VAC. The other is for the B+ CT. Not supposed
to have anything of significance there -- isn't that the "-0-" part of the
280-0-280? They are labeled for 125VAC, 50VDC. Try them. You'll like
them. Cheap, too, $1.50. You'll pay more for a fuse and holder.
Phil_S
just yet.
Anyway, the good news is, with the NFB pot all the up it squeals like a pig!
Down around 7 it stops. Lead dress? Just too much R? BTW, do I need to
connect the unused outer lug and wiper? I didn't.
I've got pops, hums or squeals from V2 and the finals. I've got zip signal
coming from the V1 stage (yes, heaters have voltage). I corrected a wiring
error. The bottom side of c6/c7 was run to the wrong place, but I don't see
how that would have done anything. C4 was the wrong value, changed it from
.02 to .002 and now it's right. I'm not seeing anything else. I'll spend a
bit more time verifying the first stage before I turn it on again.
I've been thinking about V1. The 100K plate load resistor drops B+ 322 to
VA 130 or 192V drop. That's 2mA. Is that right? Not unreasonable for a
6SL7. What can I do to cut the drop in half? Parallel another 100K (easier
than changing to 47K; can tack in for a test). I'm reasoning here (danger!)
that ordinarily you have a 100K for each plate from one node, meaning the
effective R is half because they are parallel. Only using one R means I
should cut it in half?
Phil_S
"Phil_S" <psymond...@comcast.net> wrote in message
news:hi5ema$jfe$1...@news.eternal-september.org...
I caught and fixed a couple of wiring errors, and a couple of missed grounds
(oops!). I realized the concertina will show uneven voltages and now it is
good at about 207 plate and 87 cathode.
Now, I've got a scratch when I probe the plate on V1. That is as close to
the input jacks as I can get to generate noise on the speaker.
Unfortunately, the plates are wired together, so I can't tell if this is one
side, the other side or both.
I have no signal from the input jacks to V1. There is roughly 100v on the
plates and 1V on the cathodes (not shared). I double checked the jack
wiring and it all looks OK to me. I even checked that funky switch that
jumpers the inputs and it is operating correctly, as I can see 33K on each
side of it when it's open and when closed it is 16K,
I changed the tube, too. I'm running out of possibilities here. Any ideas?
A jumper from the input jack to where? Grid of V2? I'm dying to see if I
can get anything out of this amp beside pop, scratch, and squeal!
Stephen Cowell
"Phil_S" <psymond...@comcast.net> wrote
> "Stephen Cowell" <stephenleeN...@gmail.com> wrote
>> "Phil_S" <psymond...@comcast.net> wrote
>>> I ordered parts from Weber. The 2A and 0.6A circuit breakers were less
>>> than buying a fuse and holder, so I gave them a try. Fits in a smaller
>>> hole and lower profile. I think I would use them again.
>>
>> Is that small CB rated for HV? That's hard to do!
>
> doing. One is for line voltage, 120VAC. The other is for the B+ CT. Not
> supposed to have anything of significance there -- isn't that the "-0-"
> part of the 280-0-280? They are labeled for 125VAC, 50VDC. Try them.
> You'll like them. Cheap, too, $1.50. You'll pay more for a fuse and
> holder.
... and it would be worth it, too. 50VDC? And how much
DC are you running through it? Guess what happens when it
tries to open? It will just arc! This will cause self-destruction
of the contacts, at the very least.
Ratings are there for a reason, Phil... pay attention to them.
__
Steve
.
Stephen Cowell
"Phil_S" <psymond...@comcast.net> wrote
> I have no signal from the input jacks to V1. There is roughly 100v on the
> plates and 1V on the cathodes (not shared). I double checked the jack
> wiring and it all looks OK to me. I even checked that funky switch that
> jumpers the inputs and it is operating correctly, as I can see 33K on each
> side of it when it's open and when closed it is 16K,
Whenever you parallel triode sections, you parallel
the plate resistors too. Either run two 100K's, or
put in a 47K, or...
Pull one of the plates loose, so you're only working with
one triode. If this doesn't work, then put a pot in for
the cathode R and start cranking above 1K until you see the
plate V start to float up (your plate is saturated, I'm guessing,
is why you're not seeing signal).
If your tube was cut off (like if you forgot the 1M grid
return resistor) your plate V would be high, not low.
Get that plate V back up by biasing the triode, using
the K resistor.
And who said you didn't have to bias pre tubes?!
__
Steve
.
Stephen Cowell
>
> "Phil_S" <psymond...@comcast.net> wrote
>> "Stephen Cowell" <stephenleeN...@gmail.com> wrote
>>> "Phil_S" <psymond...@comcast.net> wrote
>>>> I ordered parts from Weber. The 2A and 0.6A circuit breakers were less
>>>> than buying a fuse and holder, so I gave them a try. Fits in a smaller
>>>> hole and lower profile. I think I would use them again.
>>>
>>> Is that small CB rated for HV? That's hard to do!
>>
>> I don't know why it wouldn't be OK. AFAIK he sells them for what I'm
>> doing. One is for line voltage, 120VAC. The other is for the B+ CT. Not
>> supposed to have anything of significance there -- isn't that the "-0-"
>> part of the 280-0-280?
I just read this again... you are under the misaprehension
that there is 0amps flowing through your 0.6a circuit
breaker.
1.) Why would it be there in the first place?
2.) When you do a full wave using the CT, it
becomes the ground reference, and the outside
legs both supply the positive side. Examine the
schem until you understand this... it's 'portant.
__
Steve
.
Phil_S
"Stephen Cowell" <stephenleeN...@gmail.com> wrote in message
news:hi62uc$6bd$1...@news.eternal-september.org...
Steve,
I don't understand. The 2A is for the line voltage. AC only, 120V. That's
ok, right? It's only on the PT primary. No DC there, ever, right?
The B+ center tap, is that the problem? What do I need there? I must not
understand what's there. The CT is on the AC side of the circuit. I looked
again and wrote it down so I could get it right here. It says: "91 Series,
0.6A, 125/250VAC, 50VDC. If any real current is on that CB I want it to
blow. Why would it arc? Too much DC voltage? I don't get it. There's no
DC at the CT, is there?
I placed these CB's in such a way that I can drill them out and replace them
with regular fuse holders. I'm ready to do what's safe. Please explain
this a bit more.
Thanks.
Phil
Phil
Phil_S
"Stephen Cowell" <stephenleeN...@gmail.com> wrote in message
news:hi63e9$7ms$1...@news.eternal-september.org...
I put a jumper over the tone stack and a cable into the jack. By thumbing
it, I could clearly hear a very low signal. I just came from that exercise.
I'm thinking two things. One is that I've got to rebias V1 like you said
and I'll start by tacking in another 100K or swapping it for a 47K. Not
sure what I'll do about the cathode, but am guessing it needs to increase
from 1K. Looking at RC-30, there is a set of tables in the back for
Resistance Coupled Amplifiers. The list for 6SL7 appears to suggest Rk up
to the 10K range, but I imagine that would be rather noisy and would require
a big jump in Ra, too. One thing at a time....
The other thought concerns the tone stack. Read here about the tone stack:
http://www.freewebs.com/balijukka/roybeanstack.htm
I think this t/s is swallowing the signal. If I cant hit it hard enough at
the front end, it won't come out the back end. The designer says (thread
over at AX84, direct exchange between us) the back end needs to be 330K.
I've done this with a 500K volume pot and a 1M grid ref in parallel. He
thought this would be fine.
Considering I didn't get much oomph when the t/s was bypassed, I'm inclined
to deal first with V1. At least I know it is working all the way to the
first .02 coupling cap.
I also lifted the NFB loop. For reasons I don't understand, the problem was
worse, not better. I have multiple problems and have to take them one at a
time.
It's OK. This is recreation. This amp *will* run and relatively soon.
Thanks for looking in.
Phil
Phil_S
"Stephen Cowell" <stephenleeN...@gmail.com> wrote in message
news:hi65u9$eli$1...@news.eternal-september.org...
I'm gonna take your word on this one and drill out the hole for the fuse
holder. I am having trouble wrapping my mind around the DC aspect of this.
The CT is ground ref even with a tube rectifier. Same thing?
Are you telling me the ~100 or ~200mA or whatever from the tube
plates+screens is flowing on the CT? Don't the diodes block backwards flow?
I'm trying, really I am. I believe you but this is rather obtuse.
Many amps simply omit the CT fuse and go straight to ground. Some consider
it unnecessary? I wanted an extra margin of safety in the event of a melt
down or whatever. You know, penny of prevention, pound of cure (Oh, the
Britspeak!).
I think you are saying the one for the line voltage is OK as is?
jh
Hi Phil,
there was a difference:
i opened it on the notebook with wide screen and no mouse with a wheel
to zoom in
i opened it on the "real PC" where i have standard screen resolution and
th pooss. to zoom.
It worked from the beginning, but i couldn't zoom on the notebook.
The "new version" is better and also works there.
regards
Jochen
jh
Phil,
I guess it's time to use the scope....
see how far the signal gets and where it dissapears
rgards
Jochen
Stephen Cowell
>> DC are you running through it? Guess what happens when it
>> tries to open? It will just arc! This will cause self-destruction
>> of the contacts, at the very least.
>>
>> Ratings are there for a reason, Phil... pay attention to them.
>>
> I don't understand. The 2A is for the line voltage. AC only, 120V.
> That's ok, right? It's only on the PT primary. No DC there, ever, right?
Yes, no problem. On either side of the CB is 120VAC, 0VDC.
The ratings are met.
> The B+ center tap, is that the problem? What do I need there? I must not
> understand what's there. The CT is on the AC side of the circuit.
Nope... when you've got diodes, voltage can only
squirt one way. You've got two half-wave rectifiers
working, one on each side of the CT of the PT.
Lift the CT and nothing happens... no voltage output.
That's why they put the HV fuse there, otherwise
they'd have to either fuse both legs or fuse after
the first filter, leaving it unprotected.
> I looked again and wrote it down so I could get it right here. It says:
> "91 Series, 0.6A, 125/250VAC, 50VDC. If any real current is on that CB I
> want it to blow. Why would it arc? Too much DC voltage? I don't get it.
> There's no DC at the CT, is there?
If one side of the diodes fails shorted, then you'll
have a whole bunch of DC on that breaker. Also,
every time a half-wave pulse comes in, you're looking
at B+ across that circuit breaker. You can't expect
it to open only when the AC cycle is crossing zero...
and those shrimpy contacts won't open far enough
to interrupt 400V. It'll burn for the remainder of
the 16mS of the cycle... and just might light back up
when the next cycle comes along.
Perhaps you've never struck an arc while working
on a hot piece of HV equipment... let me tell you,
it's a good excuse to skitter your skivvies. I struck
one once... a good 1/4" arc, had to power down
the amp to get it to stop. HV DC is strange stuff.
>
> I placed these CB's in such a way that I can drill them out and replace
> them with regular fuse holders. I'm ready to do what's safe. Please
> explain this a bit more.
Circuit breakers are not as good as fuses, IMO.
There is *no* circuit breaker made today that
costs less than $200 that I'd trust with 400VDC
across it... it'd have to be mercury-wetted kinda
thing. 120VAC line voltage a different matter...
and yet, still not as good as a fuse. Notice
who uses circuit breakers... nobody! My Echoplex
has one... it runs a motor and solid-state 30VDC.
Crappy PA heads have thermal shutdowns...
just what you want when the gig's getting good.
Line fuses are slo-blow... circuit breaker OK.
HT fuses are normally quick-blow... how fast
is the 0.6A CB? It's already told you that it's
rated to interrupt 120VAC or 50VDC... and
your application definitely exceeds the first,
and possibly the second, rating.
At least replace the HV CB with a fuse... I'd
feel much better about it. Sorry for the rant.
__
Steve
.
Phil_S
> At least replace the HV CB with a fuse... I'd
> feel much better about it. Sorry for the rant.
> __
> Steve
Hardly a rant, just a good explanation and much appreciated. I did not
understand this. Questions asked and answered, and thank you. I'll drill
it out and replace it ASAP.
OK, thought I'd try... I know, if it looks too good to be true, it almost
always is.
Phil_S
>
> Phil,
>
> I guess it's time to use the scope....
>
>
> see how far the signal gets and where it dissapears
>
>
> rgards
>
> Jochen
LOL, I'll have to figure out how it's done. Maybe I'll dodge the bullet by
rebiasing V1, but don't actually believe I'll get that lucky.
jh
Phil,
just go for it
signal generator to 1kHz 50mV PP
Scope to 10mV/DIV for a 1:10 probe
Hook it up to the Amp - amp shut off
Attach probe to input jack - directly at the contact
Get a stable signal on the scope
Turn the amp on and pass from component to component.
It's easy. That's the reason why you've got a scope. Much easier than
changing components by scrapshot.
regards
Jochen
Phil_S
news:hi7eq7$j21$03$1...@news.t-online.com...
> Phil,
>
> just go for it
>
> signal generator to 1kHz 50mV PP
> Scope to 10mV/DIV for a 1:10 probe
> Hook it up to the Amp - amp shut off
> Attach probe to input jack - directly at the contact
> Get a stable signal on the scope
> Turn the amp on and pass from component to component.
>
> It's easy. That's the reason why you've got a scope. Much easier than
> changing components by scrapshot.
>
> regards
>
> Jochen
OK, I hear you! I cleaned up and cleared a place on the bench for the scope
this morning. What a PITA, as I am not the neatest person. Thank you for
forcing me to clean up ;-}.
I don't have a sig gen. The scope provides a test signal, labeled 1K 1V PP.
I metered it and it really does put out 1KHz (1.024!!) and voltage is
.42VAC. Is this too much voltage for the amp input?
BTW, I built a cable with a 1/4" jack when I got the scope, so, basically,
I'm good to go.
As an alternative, I can dig out my portable CD player (who uses these any
more?) and burn a test CD. I'll have to rig up a cable of some sort.
Hopefully one of my kids has a pair of used up earbuds. What sort of mV
output would I expect from that?
The other alternative is an old laptop (P4 CPU) that I can put in service as
a sig gen, but same issue with a cable needed. What sort of voltage can I
expect to find at the output jack on the sound card?
RichL
> I don't have a sig gen. The scope provides a test signal, labeled 1K
> 1V PP. I metered it and it really does put out 1KHz (1.024!!) and
> voltage is .42VAC. Is this too much voltage for the amp input?
That may saturate the first stage a bit. But just build a simple
voltage divider. Got a spare pot lying around? Anywhere between 100
and 500 K ought to do the trick. Knock it down to the 10-100 mV range.
Phil_S
Oh, never mind about all those questions for using the scope. I used the
signal generated by the scope and it works out fine.
I see some funny shaped waves in some places. Not sure what to make of it.
I would post pictures, but think it isn't necessary.
Here's what I found. At C8 going to the outer lug of the volume pot (R14)
I've got a decent looking sine wave. At the wiper, I got something I don't
fully understand. The probe seems to be very picky about where it makes
contact to show me a wave and this may be a factor. I thought, for a while,
I had wave on the wiper of the volume pot. Later, I did not or it was flat.
It is flat or non-existent a the other end of the wire from the wiper to the
grid of the AX7 half of the 12DW7. For the most part, there is no or not
much signal at the grid. One time, with the scope probe, I generated a
decent pop on the tube pin, and that was encouraging. I was not able to
duplicate it.
It seems to me the amp is eating the signal at the junction of the Volume
pot wiper, R15 (1M grid ref to ground), and the grid pin of the tube.
Limited options here...change the pot, lift the grid ref? I don't know
what. I only have one 12DW7. I tried a 12AU7 and it didn't make a
difference. I could try a 12AT7, 12AY7, or 12AV7. I am not comfortable
with a 12AX7 as the concertina PI on the AU7 half is biased with 10K
resistors.
Here's one other thing. I can hear the 1KHz signal at the speaker.
However, I am guessing there is a great deal of oomph to the signal. It is
.42V at the source, into the input jack, and .22V after the 68K grid stopper
for V1. This is measured with the amp off and the test signal going into
the input jack.
I still can't hear a guitar signal.
Thoughts? Suggestions?
Thanks.
Phil
Phil_S
"RichL" <rple...@yahoo.com> wrote in message
news:GfadndM75o5vUNXW...@supernews.com...
Thanks Rich. I ran the signal into the jack with a modified guitar cable.
That sends it over the 68K and seems to do the trick.
Phil_S
"RichL" <rple...@yahoo.com> wrote in message
news:GfadndM75o5vUNXW...@supernews.com...
Thanks Rich. I ran the signal into the jack with a modified guitar cable.
jh
> stopper for V1. This is measured with the amp off and the test signal
> going into the input jack.
>
> I still can't hear a guitar signal.
>
> Thoughts? Suggestions?
>
> Thanks.
> Phil
Hi phil,
great. As I already said or always say, I'm very proud of you. The
progress you made over the years - respect.
My suggestion for now would be to make some continuity checks with the
amp shut off. If the pot is working correctly, you should see low ohms
between C8 and the grid when it's turned on 10. Mesaure the DC
resistance to ground there you should be in the ballpark of 300k ( the
500k and the 1M in parallel. Turn the pot and look what's happening.
If this all looks reasonable, pull the DW7 and scope it again. Do a lot
of measurements, just to gain some experience with the scope and the
trigger.
IMHO you can also put a 12AX7 in there, just to see whether the AX part
of the DW7 is flaking out. It's not a nice treatment for an AX, but it
should survive. Take one you don't like, e.g. a microphonic one.
regards
Jochen
Phil_S
> Am 09.01.2010 19:48, schrieb Phil_S:
>
><snip embarrassing compliment. Those kind words are appreciated.>
>
>
I've got the amp working now! It turns out the common plate resistor in V1
is an unworkable arrangement, along with the switch to jumper the inputs.
These have been clipped.
The switch doesn't work because it doesn't lift ground on the second pair of
input jacks. I don't know why the common plate load R eats the signal.
The task now is to figure out how to get the second triode working. I left
a space for another plate load resistor (not so dumb sometimes). The
problem is how to mix the inputs without a whole pile of parts.
Can I use a cap from each plate to R10?
http://home.comcast.net/~psymonds/Maxie-tone.htm
It seems to me that it may be enough to rebias V1 to allow more plate
voltage on the common plates. I'm guessing here because I don't really
understand the common plate operation. It seems to me, as plate resistance
declines, we use a lower value plate load resistor. Put in a 47K? I've
been toying with this idea for a while. Maybe double Rk from 1K to 2K?
Phil
Greasehorse
Your substitution box is your friend...glad to see ya still at it
Phil...I am starting to mess with things again...might finish the
November project I put aside...all of my parts are moved around
...guess I'll have to order more stuff...
Phil_S
"Greasehorse" <greas...@hotmail.com> wrote in message
news:uvaik5pu35hdp8h48...@4ax.com...
Yeah, another gizmo to build....thanks. You are right, of course. Real amp
builders make the calculations and know ahead of time, though.
I've got Merlin Blencowe's book. He discusses the common plate load
resistor Just found it in there. His example is for a 12AX7, but it should
work just as well for a 6SL7, which is pretty close. I have a distant
feeling I'm going to end up with 68K as the plate load resistor.
Greasehorse
What results (tones?) are you hoping to gain with C6/C7 setup. and why
not silver also?..
Greasehorse
<psymond...@comcast.net> wrote:
BTW, how easy is this book to read and follow the logic in your
opinion?
Phil_S
"Greasehorse" <greas...@hotmail.com> wrote in message
news:94bmk5h79i25aik3q...@4ax.com...
> What results (tones?) are you hoping to gain with C6/C7 setup. and why
> not silver also?..
I think I ended up with one silver and one film. Not using three?
Unnecessarily complicated to switch. Just looking for some variation in
color. It turns out not to be much and I may just eliminate it.
Phil_S
Actually, it is reasonably well written and not too difficult to follow.
There is so much packed into relatively few pages that sometimes I forget to
look, like with the common anode resistor. (He's a Brit.) It's technical.
He doesn't spoon feed, but it doesn't require an engineering degree either.
IIRC, it's about $40. Lulu.com is the cheapest price. Delivery was
slow...maybe 3 weeks.
Greasehorse
I was wondering if it was worth it. The tone chasers seem to prefer
the silver and that is why I asked about it..not to imply using
three...but you could I guess...I might get a copy of that book off of
Amazon US..a couple of dollars more but will likely get here in a week
or so...
Phil_S
"Greasehorse" <greas...@hotmail.com> wrote in message
news:o4fnk5lia1652ulq4...@4ax.com...
I just checked my parts order. One is silver mica and the other is
polypropylene film. I really can't hear the difference. Maybe I'm a bid
deaf?
Get the book. It's a great centralized compilation of electronics for
guitar preamps as opposed to hi-fi, so it's very practical. Wisely, he
didn't waste time with the PA section of the amp. There isn't much to that
part.