66 Ampeg J12-D

[Justin Thomas]

Hi everyone,

Decided to revisit this one so I can unload it. Issues are low volume, very dark sounding, distortion (but kot unpleasant), cathode voltages in preamp & PI are high, and PI plates are low. Power supply amd power amp check out fine, all resistors checked okay, out of circuit if needed. Replaced most of the coupling caps (none in trem yet). All lytics replaced. Tubes are a good pair of 7591As and preamp tubes are 6C10, not 6BK11.

Schemo here:
http://tubeamplifierparts.com/schema..._schematic.gif

Voltages for preamp tubes, separated by section:

V1
Pin 4: 1.35V
Pin 9: -.6 to +.6V
Pin 10: 75 to 125V

Pin 6: 3.2V
Pin 7: -.35V
Pin 5: .35V

Pin 3: .36V
Pin 11: -.35V
Pin 2: 0V


V2
Pin 4: 3.44V
Pin 9: .3V
Pin 10: 163V

Pin 6: 2.2V
Pin 7: 0V
Pin 5: 150V

Pin 3: 2.0V
Pin 11: .03V
Pin 2: 126V

Also, power tubes G1 showing .03V, but all others spot on within 10%. Right now my main concern is V2. Any suggestions? Thanks!

Justin

[g1]

I take it you tried swapping V1 & V2 and there was not much change in the readings?
How can V2 pin 4 be high and pin 10 also be high? If there is more current than normal, the plate voltage should be lower than normal.

[Justin Thomas]

Thanks g-one,

Yeah, I've swapped the tubes around. The V2 pin 10 being about 15V high seems okay, accounting for modern wall voltages, it's well within tolerance. Why pin 4 is over by 100% is what I don't get either. That, and the PI is way off, too - high cathodes, low plates by ~50V. Seems like everything with that one particular tube is wacky. I'll try reflowing the socket sometime soon and checking the associated grounds, as it's about the only thing from RG's page I haven't tried. I should mention this amp sat unused in a climate controlled store for about 15 years... Are there other parts besides the electrolytics that go horribly wrong?

I'll mention that when I got it the preamp tubes were a 6K11 & 6AV11... putting the 6C10s in DID make the trem work, but no change in volume or tone. Do the 6C10s need a slight circuit tweak? I haven't read of anyone doing one anywhere...

Thanks again,

Justin

[g1]

What are your voltages at points A,B, and C of the power supply?

[Enzo]

Crap, I blanked the post I was writing.

Did you try the amp with the trem tube pulled? A hanging trem can sap your tone.

You have used 6C10, which is a thre triode version of 12AX7, all three triodes amp factor 100. But the 6K11 is the first triode like a 12AU7, amp factor 17, while the remaining ones are 100. That is your input stage triode. SO you have stuck a 12AU7 where a 12AX7 belongs, essentially. I would imagine that would have some effect.

[g1]

Enzo, 6BK11, not 6K11. I believe the BK has all triodes 100 gain and 6C10 is used as replacement.

[Justin Thomas]

Enzo, g-one,

Thanks for the replies. Yes, the 6C10s are in it now, basically 3x12AX7. I pulled the trem tube, didn't take the measurements cuz it's late & I'm tired, but dimed it's quiet enough to normally converse over. Woolly, fuzzy, not much high end with a Tele bridge p'up & tone on 10.

B+ is 385; A, B, C are 385, 374, 294 respectively. The wall usually runs around 125V here, so being 20 or so volts high I consider normal. Seems to me that the PI cathodes are about 30% high and the plates are about 30% low.

This circuit has some strange things (to me) going on...

Justin

[Enzo]

In post #3 he said 6K11.

OK, so trace the signal. Make up a signal tracer from another amp and see what it sounds like stage by stage.

INject a signal at the grid of the PI, pin 7. Or at the volumje control if that is more convenient. How's that sound? Clear or still with symptom? That will at least tell you preamp tube stage or power amp.

Replaced MOST coupler caps? OK which ones were not replaced? Other than trem, as you mentioned.

[Justin Thomas]

Hey guys,

Thanks for the direction. I'll be away from the amp for the weekend but will dig back in Monday hopefully. Found a thread about making a signal tracer from here (thanks Enzo!) so I'll be doing that too. I'll pull the trem tube & remeasure the preamp, and get more specific about the caps, too. And for a signal generator, any reason a guitar/delay pedal se tto infinite won't work for this purpose?

Thanks, and later,

Justin

[Enzo]

I have a number of nice signal generators, but 99% of the time I just run a cord from my bench stereo receiver tuned to a music station. A CD player or Casseette deck would work. I used to have a small Yamaha keyboard - the kind with half size keys - and it had a line out jack. I;d trigger the demo song, and use that as a test signal. ON stage if we needed a steady signal, we'd set something heavy on the organ to push down a key.

[g1]

It looks like the voltage shown on the schem. for V2 pin4 is wrong anyway, so no problem there. (If there were 1.7V there, that would be half a mA of current through the tube, which would mean 60V drop across the plate resistor. That would put point C at 210V which is nonsense, the PI has around 200V at it's plates).
So your V2 voltages seem to be in the ballpark. However, V1 pins 2 and 6 are both shown to be grounded, you listed 3.2V at V1 pin6 ?

[Justin Thomas]

Checking back in...

Enzo, I found an old thread where you described how to make a signal tracer. You said add a high-value resistor from the cap leg to ground. 220K okay, 470K, doesn't matter much? And okay to connect between the cap & sleeve, since that will be connected to grounded chassis anyway? Just makes for a compact assembly... Haven't had a chance to use it yet.

G-one, that 3.2V on V1 pin 6 should say 0.32V - transcription error on my part.

I pulled the trem tube, disconnected the plate supply to the socket @ point B at the board, lifted the wire on pin 5 @ the board. Trying do eliminate anything that might be associated with the trem circuit. No changes on V2. I checked the 2 500pF caps on the PI, both .05 mF caps on V1 pin 10, no change. There's only the 2 .05mF caps coming off pin 9, the tone caps, and the 3 near the power supply (death cap (three-wire cord IS installed), .1 & .22 in heaters), and the vol/tone caps left.

Is it safe to say there's a partial "short" somewhere in the PI? I did some math and both halves are pulling. 5mA more than they should according to the schemo.

Also, one end of the death cap is grounded, the other end is attached to the fuse/power switch junction. Okay to yank that cap entirely and take that wire off the fuse? Cord is otherwise done right...

I'm learnjng, slowly...

Thanks,

Justin

[g1]

The difference in the PI currents is only .5mA, not 5, so no big issue. Nothing looks terribly out of whack as far as your DC voltages go, not enough to create a huge volume loss it would seem.
Scope or signal tracer should show where your big volume drop is occurring.
As far as the cap at the fuse holder, go ahead and remove it.