Marhsall JCM 800 mod 2203, Bias Drift, amp way too bright, DI jack resistors burned
Thorny
in for new tubes. I have a nice set of E34Ls for it from LV and was
thinking of Teslas for the preamp. I opened the amp and it has likely
had the power tranny replaced at one time, and probably caps replaced
(appears that way). The power tranny is a Dagnal T7-8. The
electrolytic caps all test ok with my ESR meter. The resistors on the
sockets all test ok.
Problem 1) the only visable problem I see is the DI jack has two
damaged resistors on it of unknown value. I did not see them on the
schematic. They need to be replaced if someone knows the values. I
have figured that should not be a problem unless he uses the DI. But
I will fix if I can get the correct values.
Problem 2) damn this amp is shrill and bright. I can set the treble
on 3 or less and it is still shrill. I removed the tesla ECC83 and I
put in a 12AX7EH and it rolled the highs off a little, but it still it
way too bright. It has bass, but owner would like more. The owner
wanted it to be less bright I am sure.
Problem 3) The bias drifts - WAY TOO MUCH. I am used to things
heating up, but this just gradually keeps on going until it is too
much. The voltage is about 430V on the sockets. I tried to bias
around 20w SD. I have checked all the caps with my ESR and they test
fine.
What do you all suggest checking for these problems that I haven't
addressed so far?
Thanks in advance for your help.
Thorny
Phil Allison
"Thorny" <jthor...@sigecom.net> wrote in message
news:ugg4evgcuean4k046...@4ax.com...
> Got a JCM 800 - the horizontal jack model, not the vertical. It came
> in for new tubes. I have a nice set of E34Ls for it from LV and was
> thinking of Teslas for the preamp. I opened the amp and it has likely
> had the power tranny replaced at one time, and probably caps replaced
> (appears that way). The power tranny is a Dagnal T7-8. The
> electrolytic caps all test ok with my ESR meter. The resistors on the
> sockets all test ok.
>
> Problem 1) the only visable problem I see is the DI jack has two
> damaged resistors on it of unknown value. I did not see them on the
> schematic. They need to be replaced if someone knows the values. I
> have figured that should not be a problem unless he uses the DI. But
> I will fix if I can get the correct values.
** There is a 2k7 from the 8 ohm tap in series with a 100 ohm to
ground. From the join of these two there is a 560 ohm to the tip of the
socket. Gives a 600 ohm source at -30 dB compared to the speaker out.
>
> Problem 3) The bias drifts - WAY TOO MUCH. I am used to things
> heating up, but this just gradually keeps on going until it is too
> much. The voltage is about 430V on the sockets. I tried to bias
> around 20w SD. I have checked all the caps with my ESR and they test
> fine.
** Bias drift ( as measured on pin 5 ) has several likely causes:
1. The 10 uF, 160 volt bias supply filter caps are leaky and much more
so when hot - test with hot air or just replace those old blue ones on
sight.
2. The EL34 tubes have got a leakage problem - often the plastic of
the base is leaky when hot and conducts between the screen and grid pins.
4 and 5 )
3. The octal sockets are contaminated and leaking as above when hot -
replacement is the only cure for this - use ceramics ones, they never go
leaky.
............... Phil
RoccaforteAmps
>so when hot - test with hot air or just replace those >old blue ones on
>sight.
> 2. The EL34 tubes have got a leakage problem - >often the plastic of
>the base is leaky when hot and conducts between the >screen and grid pins.
>4 and 5 ) >>
Tube bases are not the cause for bias drift,
leakage/gas is.
<<
> 3. The octal sockets are contaminated and leaking >as above when hot -
>replacement is the only cure for this - use ceramics >ones, they never go
>leaky.
>>
There is no benifit for using ceramic tube sockets in guitar amplifiers.
Ceramic tube sockets are really for transmitting circuits, high frequency loss.
We dont have that problem here.
Also, I've replaced more arched ceramic sockets
than any other kind in Marshalls.
Keep working on Marshalls Phil.
Thorny
far:
1) Tried another set of tubes. Still "drifts"
2) Rebuilt the bias circuit - putting in 22mfd/160v caps, and new
diodes in bias and rectification circuit. Still seems to be drifting
up.
My news ready was not updating for a little while, so I did all this
before reading your posts. I cleaned up a bunch of solder joints and
resoldered a suspect ground wire. I will check the sockets - have not
yet.
I noticed a little bit of scratchiness on my guitar when I used the
volume control. It is a new pot in the guitar. Would DC leakage from
the amp cause these problems I am having? How would you go about
finding the source?
Thanks again.
Thorny
On 08 Jun 2003 16:05:40 GMT, roccafo...@aol.com (RoccaforteAmps)
wrote:
Phil Allison
"RoccaforteAmps" <roccafo...@aol.com> wrote in message
news:20030608120540...@mb-m10.aol.com...
> <<philA:
> > 1. The 10 uF, 160 volt bias supply filter caps are >leaky and much
more
> >so when hot - test with hot air or just replace those >old blue ones on
> >sight.
> > 2. The EL34 tubes have got a leakage problem - >often the plastic of
> >the base is leaky when hot and conducts between the >screen and grid
pins.
> >4 and 5 ) >>
>
>
> Tube bases are not the cause for bias drift,
> leakage/gas is.
** How the fuck would you know ??????
> > 3. The octal sockets are contaminated and leaking >as above when
ot -
> >replacement is the only cure for this - use ceramics >ones, they never go
> >leaky.
> >>
>
> There is no benifit for using ceramic tube sockets in guitar amplifiers.
** Even more stupid than his last comment.
> Ceramic tube sockets are really for transmitting circuits, high frequency
loss.
> We dont have that problem here.
** There are other worthwhile benefits this cowboy is unaware of.
> Also, I've replaced more arched ceramic sockets
> than any other kind in Marshalls.
** I have never seen an "arched" socket. Ceramic is far harder to
carbonise than phenolic or plastic so it is rare to see one damaged by
arcing.
> Keep working on Marshalls Phil.
** Keep pulling it harder Doug - it will come right off soon.
............. Phil
RoccaforteAmps
Phil Allison
> Phil, go suck on a can of WD40.
** You really are a pathetic, excuse for a bullshit artist Doug.
Building amps does not prove you know a damn thing about them - those
dumb pommy bitches who built all those crappy Marshalls sure didn't.
................ Phil
Bill Kahle
> Thanks Guys for the great information. I have tried the following so
> far:
>
> 1) Tried another set of tubes. Still "drifts"
>
> 2) Rebuilt the bias circuit - putting in 22mfd/160v caps, and new
> diodes in bias and rectification circuit. Still seems to be drifting
> up.
>
> My news ready was not updating for a little while, so I did all this
> before reading your posts. I cleaned up a bunch of solder joints and
> resoldered a suspect ground wire. I will check the sockets - have not
> yet.
>
> I noticed a little bit of scratchiness on my guitar when I used the
> volume control. It is a new pot in the guitar. Would DC leakage from
> the amp cause these problems I am having? How would you go about
> finding the source?
>
> Thanks again.
>
> Thorny
Hi Thorn,
You probably already checked
it, but I didn't see anything on your
posts; what about the (usually .022)
pi couplers? Seems trivial, but there's a
slim chance that the drift is from dc leakage
there. I see that you've r/o ps & bias filters,
and tubes. Sockets would be a possibility, but
only remote at the -lower- plate voltages you've
posted. What about the ground integrity (output socket solder
joints- pin 1, 8 & the lug + the mechanical connection
to the chassis? Take a quick look at the bias tap wire.
(white?) Make sure it isn't pinched between the board
and the brass standoff and is providing a high resistance ground.
(Been there, seen that! ;-)
When the drift occurs, did you happen to also note
what the voltages (plate & screen & bias) are doing?
If all else fails while trying things around the output,
try removing all tubes except for the '34s. (There
may be an issue in another part of the preamp ckts.
causing unstable loading of the power supply which
could translate to fluctuations in the output tube
idle current. Why? After exhausting possibilities for the
'obvious', isolation is your friend.)
bk
RoccaforteAmps
Oh, ok Mr.WD40.
What a wanker.
Phil Allison
** Doug - you are a damn liar.
.............. Phil
Thorny
>
>On 7-Jun-2003, Thorny <jthor...@sigecom.net> wrote:
>
>> Problem 2) damn this amp is shrill and bright. I can set the treble
>> on 3 or less and it is still shrill. I removed the tesla ECC83 and I
>> put in a 12AX7EH and it rolled the highs off a little, but it still it
>> way too bright. It has bass, but owner would like more. The owner
>> wanted it to be less bright I am sure.
>
>Common problem these days. I think it's the way they design amps now. On
>my JCM800 4211, I turn the treble OFF and work the highs from the Mid
>control instead. From there, I blend in the right amount of Presence to
>compensate.
>
>I tried the "unbright" mods that are found on the Internet, but I didn't
>really care for what the did to the sound and put it back. True, the amp
>mellowed out, but it also seemed to lose some of the character. Better to
>adjust the EQ knobs. "Off" ain't neccesarily a bad thing if you don't need
>it.
>
>~kp
I tried some of the unbright mods too, and they helped some. I wound
up putting one of them back in. I can now actually turn the treble on
some before sterilization occurs. Also, C9 had what appeared to have
been a mod, and I put it back to stock and it sounded better to me
too. It is much better, all done in gradual steps. NOW to find the
source of the bias creep.
Thanks.
Thorny
Phil Allison
** You have never said how you know the bias is creeping up.
Is the grid voltage dropping?
Are you monitoring the plate current or AC draw ?
............... Phil
RoccaforteAmps
>>
About what, the WD40 thing?
Its in the archives phil, ROTFLMAO
Phil Allison
> <<philA: ** Doug - you are a damn liar.
>
> About what, the WD40 thing?
** About everything.
You lie to cover up your technical ignornance.
You lie so you can bury the truth in bullshit.
You lie to aggrandise yourself.
................ Phil
Bill Kahle
"Phil Allison" <phila...@optusnet.com.au> wrote in message
news:3ee52be7$0$13748$afc3...@news.optusnet.com.au...
Thought you'd like to know.
bk
Bill Kahle
other thing:
are you able to
'run' the amp up
on the dummies
with a signal applied?
If so, is this when the
runaway condition starts
to rear its f'ugly head? How
does the signal *look*? Normal?
I hate to say it, but Satan could be
living under the bells of the output
transformer. Hmmmmm. I wonder
what story those scorched D.I. passives
could tell if they could talk?
Just a thought.
bk
Thorny
wrote:
*******
I wound up replacing those too - still creepin.
I see that you've r/o ps & bias filters,
>and tubes. Sockets would be a possibility, but
>only remote at the -lower- plate voltages you've
>posted. What about the ground integrity (output socket solder
>joints- pin 1, 8 & the lug + the mechanical connection
>to the chassis? Take a quick look at the bias tap wire.
>(white?) Make sure it isn't pinched between the board
>and the brass standoff and is providing a high resistance ground.
>(Been there, seen that! ;-)
not pinched, but looks like it may have been hot at one
time...connection seems good.
This amp has been monkeyed with, and notices the trannies were loose -
I tightened them down. The power tranny has either been changed, or
removed and put back - appears to be a replacement.
>When the drift occurs, did you happen to also note
>what the voltages (plate & screen & bias) are doing?
seems to be going down - but I am out of time at the moment for
tonight - will resume tomorrow. I need to run it about an hour to be
sure.
>If all else fails while trying things around the output,
>try removing all tubes except for the '34s. (There
>may be an issue in another part of the preamp ckts.
>causing unstable loading of the power supply which
>could translate to fluctuations in the output tube
>idle current.
Tried that tonight - didn't seem to make a difference.
Why? After exhausting possibilities for the
>'obvious', isolation is your friend.)
>
I must have overlooked something - I am going to go back and put in
another set of tubes if it drifts this next time. I may put it away
for a day since work is getting backed up behind this one. I will
report back there is something to report back. It sure sounds good
now while it is working correctly - so that is a plus. Tonight is
already done with, tomorrow is mostly booked, and Wednesday night I
have a class - where does the time go?
>bk
>
Thanks all for your help - I appreciate it!
Thorny
Phil Allison
"Bill Kahle" <cone....@worldnet.att.net> wrote in message
news:oqbFa.117141$cO3.8...@bgtnsc04-news.ops.worldnet.att.net...
> >
> AggrandiZe.
>
> Thought you'd like to know.
** Not spelt like that in my dictionary.
Substitution of "z" for "s" is an American thang like "stabilizer"
for stabiliser.
.............. Phil
Lord Valve
Bill Kahle <cone....@worldnet.att.net> wrote in message
news:oqbFa.117141$cO3.8...@bgtnsc04-news.ops.worldnet.att.net...
>
"Aggrandise" is perfectly OK in OZ, where they speak the
Queen's English (and kiss the Queen's Ass, which is another
story entirely) but "ignornance" is certainly a puzzler. ;-)
Lord Valve
Grammarian/Lexicographer
Phil Allison
"Lord Valve" <LORD_...@prodigy.net> wrote in message
news:f%eFa.2702$Sc5...@newssvr31.news.prodigy.com...
> Lord Valve
> Grammarian/Lexicographer
>
** Related to Conan the Grammarian ?
........... Phil
David Gravereaux
Oh no! You guys have fallen into the spelling wars. Stop now, please.
--
David Gravereaux <davy...@pobox.com>
[species: human; planet: earth,milkyway(western spiral arm),alpha sector]
RoccaforteAmps
>>
Related to WD40?
Rich Koerner
week, but this thread looks like it need some help.
So, what the heck.....
Thorny wrote:
>
> Got a JCM 800 - the horizontal jack model, not the vertical. It came
> in for new tubes. I have a nice set of E34Ls for it from LV and was
> thinking of Teslas for the preamp. I opened the amp and it has likely
> had the power tranny replaced at one time, and probably caps replaced
> (appears that way). The power tranny is a Dagnal T7-8. The
> electrolytic caps all test ok with my ESR meter. The resistors on the
> sockets all test ok.
So, we have good filters, screen and grid resistors are on value.
OK,....
> Problem 1) the only visable problem I see is the DI jack has two
> damaged resistors on it of unknown value. I did not see them on the
> schematic. They need to be replaced if someone knows the values. I
> have figured that should not be a problem unless he uses the DI. But
> I will fix if I can get the correct values.
Looks like someone plugged the speaker wire in that jack. It didn't
take long for them to open to a no load condition on the output tranny.
Which, now presents the possibility of fly back hitting the amp.
You will have to look for the signs of fly back.
> Problem 2) damn this amp is shrill and bright. I can set the treble
> on 3 or less and it is still shrill. I removed the tesla ECC83 and I
> put in a 12AX7EH and it rolled the highs off a little, but it still it
> way too bright. It has bass, but owner would like more. The owner
> wanted it to be less bright I am sure.
Oh, it's not your amp. OK.
> Problem 3) The bias drifts - WAY TOO MUCH.
I'm going to assume you refer to Bias VOLTAGE, not the tube's Bias
Current!!!!!!!
Two very different things. That, could mean two very different things
are going on here.
However,...... we'll move along.
> I am used to things
> heating up, but this just gradually keeps on going until it is too
> much.
OK, hold up a second. We have to make sure we are on the same page
here.
Bias voltage drift, is one thing. Tube bias current drift, with an
unchanging bias voltage applied, is another thing, and a sign of a
polished turd for a tube.
Here is what I'd do. With the output tubes PULLED, I 'd monitor the
following all at the same time:
Line voltage
Line current
Bias voltage on the control grid feed ckt
B+ on the primary center tap
Screen supply voltage feed
Note all the voltages over a one hour period.
Record the voltages, and note any change during the one hour time
period.
Any recorded change is now to be analyzed for the drift problem.
If there is no change, and all voltages are constant, continue on below.
Now, plug in those output tubes, and monitor and record those same
readings again.
If they start changing, note the time lapse, and let them go only if the
line current isn't going up too fast.
Have faith in that FAST BLOW fuse YOU ARE going to have in that amp.
Note, which readings are changing, and which are not.
Kill the lights, or shade the tubes. See if one or more are starting to
get red in the fold of the anodes.
This will tell you what is going on.
For example:
If one tube goes red, stay with that tube and its related ckt.
If two on the same bias branch gets red, go to the parts common to that
bias branch.
If all start to show color, go to the bias supply itself.
This, along with your readings, should point to what is going on.
> The voltage is about 430V on the sockets. I tried to bias
> around 20w SD.
I'm still not sure from your choice of words here, that you actually
refer to voltage, and not current.
It's a common mistake using the kathode resistor method.
Which, you are making a VOLTAGE measurement, and converting it to
CURRENT in your head.
This voltage across the kathode resistor, should it change or drift, in
no way indicates with 100% certainty, that the voltage on the control
grid had also changed. That voltage across the kathode resistor can
change for another reason, while the bias voltage on the control grid
remains constant.
For example, the changing of spacing inside of the tube will product
this problem. Give the tube a nice rap with a small rubber coated
screwdriver handle. If that kathode resistor voltage changes, throw
that tube in the trash.
My eyes are closing, so I'm going to hit the sack.
Maybe, I'll get some more thoughts on this tomorrow, when I see what I
rambled on about.
> I have checked all the caps with my ESR and they test
> fine.
>
> What do you all suggest checking for these problems that I haven't
> addressed so far?
>
> Thanks in advance for your help.
>
> Thorny
Regards,
Rich Koerner,
Time Electronics.
http://www.timeelect.com
Specialists in Live Sound FOH Engineering,
Music & Studio Production,
Vintage Instruments, and Tube Amplifiers