Deluxe Reverb, too quiet -- bad OT? suggestions?
Bluesbreaker
quiet.
I decided to check the output to get an approximate wattage -- using a
DMM to monitor the voltage across the speakers while playing the amp
dimed. The speaker registers about 3.5VAC avg and 4.0+VAC max, into
an 8ohm speaker.
P = V^2 / R
P = (4)^2 / 8
P = 2W
I've taken some voltage measurements and everything looks pretty
good. Perhaps a bit on the high side. Using the AB763 schematic, my
voltages are:
OT center tap: 446V
Connection "B" (after choke): 445V
Connection "C" (after 1st 10k): 351V
Connection "D" (after 2nd 10k): 384V
6V6 #1 plate: 443V
6V6 #2 plate: 442V
Does this look like a bad OT? Anything else I should check?
jh
I don't see a reason at the moment to misstrust the OT.
It seems, that current flow through the power tubes is very low.
I'd check the screen grid Rs at first and measure the current through
the power tubes.
I bet you have mixed something up with the readings of "C" and "D". A
higher reading at "D" than "C" is IMHO very unlikely, unless something
is *generating* energy in the preamp.
regards
Jochen
WB
> Am 07.02.2010 18:51, schrieb Bluesbreaker:
>> The Deluxe Reverb I'm working on sounds pretty good, but is WAY too
>> quiet.
>>
>> I decided to check the output to get an approximate wattage -- using a
>> DMM to monitor the voltage across the speakers while playing the amp
>> dimed. The speaker registers about 3.5VAC avg and 4.0+VAC max, into
>> an 8ohm speaker.
>>
Dimmed ? WTF is DIMMED ?
Turn it all the way up. You won't reach the 20W until it is cranked.
>> P = V^2 / R
>> P = (4)^2 / 8
>> P = 2W
>> 6V6 #1 plate: 443V
>> 6V6 #2 plate: 442V
>>
>> Does this look like a bad OT? Anything else I should check?
>
Those DC voltages look ok .. normal .
*carefully* play the amp with the meter set on AC in 100vAC
range and you should see a huge voltage swings on the plates.
That huge AC gets inverted in Low AC/High current to drive the
speakers ( An OT is a step down xformer ) .
A pair of 6v6 in a 70 SF are not going to be deafen loud.
Around 5 the amp will sound sweat, then after 7 gritty.
I like a clear boost (like a tube screamer set on clean )
in the front to really push the preamp, unlike the modern
Blues Deluxe that have a over drive *and* a volume.
Bluesbreaker
Here they are (corrected):
OT center tap: 446V
Connection "B" (after choke): 445V
Connection "C" (after 1st 10k): 351V
Connection "D" (after 2nd 10k): 284V
For screen grid R's, are you talking about the 6V6's or all the preamp
tubes also? Am I just checking that they are 1500 ohm (+/- 10%)?
Thanks,
Andy
Bluesbreaker
I wrote "dimed" which is an expression for "volume at 10".
WB
>> Dimmed ? WTF is DIMMED ?
>
> I wrote "dimed" which is an expression for "volume at 10".
Sorry about 'dat. First thing to do is check for
loose connection by tapping on the tubes and such, and test
another speaker ;-} .
Bluesbreaker
I checked the bias -- it was a bit hot, so after adjusting it the
6V6's are running 460V / 16.3mA.
I'll try running the amp and wiggling wires with a wooden chopstick,
to see if I can find any bad connections. Failing that, what could
the problem be?
Lord Valve
Got a scope?
LV
Bluesbreaker
WB
That is the whole art of amp repair !
1. That is always the 1st step is looking for loose parts. Fender
sockets get worn && since they hang upside down it is not uncommon to
have loose socket pins from tubes being swapped.
2. The deluxe is a 2 channel amp .. so if the
lost of volume occurs on both, you can more or
less rule the pre-amp stages out.
3.Pull the reverb and tremolo tubes .. you don't need those working.
now:
3a. check the heaters ! .. tubes need 6vac and be glowing to work !
4. You can trace the AC signal through the stages with a DVM set on AC,
but a scope works wonders.
There are two methods : Start at the power stage and go backwards,
or establish what is working early on and move forward. YMMV .
I do both .
Run a CD or MP3 player plugged to an input channel .. measure
the AC at the tip of the 1/4 jack.
5. A typical guitar signal will measure 100mv AC on Pin 2 of v1 ..
input, then amplified by 100 (12ax7 has MU of 100 ), so you
see expect to ~1V AC on input to the next stage, pin 2 of v3 .
5a. Use the one hand behind the back in the pocket trick when
you are probing if you are newbie. 400vdc really won't kill you,
but it hurts like hell. The pops and crackles can spook you when
you probe.
If you don't reply to this email .. I made a mistake about telling
you 400vDC won't kill you ;-} .
6. I vaguely recall the PI will be around 4 to 5 AC input to
power stage. You should expect to see a pretty large ( 40+ vAC)
AC on the plates.
When you localize the AC signal disappearing you found your problem.
Likely a open cap, or resistor.
I don't have tricks to see if a OT is bad.
Lord Valve
Did I mention that you were a fucking retard?
Read your post for details.
Lord Valve
Expert (please obsess)
jh
the 1500 ohms are the so called "grid stoppers" a the the control grids.
Those ain't what i was talking about. I talked about the 470 ohms which
come from "B". The lead to the "screen grids" of te 6V6es. The name is
not my invention. Those are the only tubes in your amp, whioch have
screen grids. The others are triodes.
BTW 16mA is still cold for a Deluxe. But then it should sound
"suboptimal" but the power would be there.
Check the voltages a the cathodes of the PI and the other voltages at
the preamp tubes.
Check whether both channels are there the same way, or if one has power
and the other not.
somewhere in the circuit you are loosing the signal.
regards
Jochen
Lord Valve
> Yes, I do have a scope and a signal generator.
Load the amp's output with an 8-ohm resistor, 40 watts or better.
Monitor the signal across the resistor with
your scope. Also connect youyr DVM
across the load resistor, set for ACV.
It needs to be a true RMS type for 1KHz;
if yours isn't, make your test signal 200
Hz.
Connect the output of your sig generator
to the amp's input. Set it for 1 KHz, 20-30mV
output. Turn the reverb and trem depth
controls to zero. Dime the bass and treble
controls. Turn the volume control up until
you see a soft clip. If you can't make a
soft clip, increase the output from the
generator.
If you see a good-looking wave at the
output, read the voltage from your DVM,
square it, and divide by the load resistor's
value. (8, in this case.) That will give you
the RMS output of the amplifier in watts.
You should be seeing 18-22 watts for
a DR. If you have this much output across
the resistor, suspect the speaker. If not,
monitor the amp's AC current draw (at
the variac) when it's putting out as much
power as it is capable of. A DR will draw
about an amp at full power. If it's drawing
an amp and it's only putting out 2 or 3
watts of funky-looking sine wave, you
probably have a shorted secondary in
the output transformer. The short may
well be *outside* the transformer, so
carefully inspect the wires inside the
amp which go to the output jacks, and
inspect the wires which go down to the
speaker. Pull the speaker plug out of
the output jack and read across it with
your DVM set to low ohms; you should
see something in the area of 6 ohms or
so. If you see one ohm or a fraction
of an ohm, you have a short in the
speaker wiring; this usually occurs
in the jack. Pop the plastic cap off
it and eyeball it; the wire sometimes
frays where it passes through the
plug, shorting the hot lead to ground.
Telll me what you find out doing the
above tests, and I'll give you more
tests to do if you haven't nailed the
problem.
LV
fendertechnicalsupport
<bluesb...@gmail.com>wrote:
If all the volts are +- 10%, the bias is +-5%, current drain around
1/2 - 3/4 amp at max output suspect the speaker. Sub to verify.
Doesn't an AB763 use 6L6? Volts won't be the same.
WB
>
> Telll me what you find out doing the
> above tests, and I'll give you more
> tests to do if you haven't nailed the
> problem.
>
>
GOper Loser whining has slowed I see.
Too Long in The Wasteland
>
> Doesn't an AB763 use 6L6? Volts won't be the same.
Not much of a Tech support huh ? You must be in India ?
Bluesbreaker
the same results.
> Doesn't an AB763 use 6L6? Volts won't be the same.
You'll find a number of Fender amps that are labeled AB763. For
example, the AB763 Super Reverb is different than the AB763 Deluxe
Reverb. Most of the circuit is very similar, but there are very
observable differences. For example, the AB763 Super has treble,
middle, bass while the AB763 Deluxe has only treble and bass. Also,
the Super has a bright switch, 6L6 output tubes, different PT and OT,
and larger filter caps. Probably other differences as well.
Bluesbreaker
Thanks for taking the time to suggest some things for me to try.
Super Bowl kept me busy this evening, so I'll try to make some more
progress tomorrow.
On a side note, I'm gonna need to spend a little time with my scope.
I noticed it was repeatably showing 4x higher AC values than my DMMs.
I double-checked I wasn't using a funky probe (never heard of a 4x
probe anyway). I'm sure I was doing something bone-headed -- Argh.
My good news of the day: This afternoon, I got my buddy's Super
Reverb working nicely. I really does sound like a dream. He was
planning on selling it, but after hearing it (in its restored glory)
he's decided not to part with it. Smart move. Maybe the best
sounding dry, un-enhanced tone I've ever heard. I'm so loving his
amp. I'm thinking of doing 1 last thing to it -- adding two diodes to
reduce the PIV as seen by the rectifier. Its such a cheap/easy mod
that I doubt there's a downside, but I figured I'd throw it out there.
Thanks again!
Bluesbreaker
results.
I followed LV's advice on performing a few tests:
I connected a 30.4mV @ 200Hz signal to the input (I used the normal
channel).
Output connected to an 8ohm (200W) dummy load.
Scope connected to output load.
I dialed up the volume until I noticed a prominent notch beginning to
form on the output signal.
I measured the output voltage. 2.47V
2.47V @ 8ohm ~= 0.76W
(I laughed a bit after seeing this -- I've managed to acquire the very
rare "Fender Pee-Shooter")
Next I checked the current draw.
0.81A at the setting used to measure output voltage.
0.93A at full tilt.
I carefully check the output jacks, wiring, and speaker cable.
Everything looks good.
Thanks again for all the help!
Andy
Lord Valve
Those symptoms are a dead giveaway for a shorted OPT secondary.
But - just makin' sure - you *are* using
the "SPEAKER" jack and not the "EXT
SPKR" one, right? Because if you're
using the wrong one, the output will
have a half-assed dead short across
it caused by the (probably corroded)
grounded tip shunt contact in the SPEAKER
jack. You'd be surprised how often
people do this...and since the tip shunt
is usually dirty or corroded, its resistance
will be something north of nil point zippity-shit,
and the speaker (or the test load) will get
enough signal to make a small amount of
sound.
No offense, but I've seen old Fenders
brought in many times for "low output"
that had the plug in the wrong hole.
Bluesbreaker
both jacks last week -- 1 was corroded beyond repair. I've triple-
checked the jack wiring using a Fender Super on the bench right next
to it as a reference. But I'll go look at it again now.
Bluesbreaker
close enough in specs to safely substitute in and test. It was pulled
from a Zenith phonograph running a pair of EL84's. I used the OT
briefly in my 18W amp and it sounded fine. I'm thinking that the
primary is probably 8K, whereas I believe the Deluxe uses a 6.5K
primary. Is this just a stupid idea?
I guess I should invest in a variac if I'm gonna screw around with
mystery transformers, eh?
Bluesbreaker
would be a safe substitute.
The schematic reads:
12watt OT
9k5 primary
8ohm secondary
Bluesbreaker
circuit and temporarily rig the little Zenith OT in. The amp sounded
much louder, clearer, and better -- and it no longer exhibited the
strange ghost notes I was hearing before. I figured the ghost notes
were from a bad cap somewhere and that I would be troubleshooting that
next. Turns out it was the OT also.
I just ordered a replacement OT -- now breathing a sigh of relief that
the mystery is solved.
Thanks a zillion for all the great help, advice, and tips.
Regards,
Andy