My hoffman Twin Reverb - new problem

[proaudioguy]

I found another JBL in my garage so I installed it in the amp and put the amp back together (without fixing the buzzy distortion thing) so I could just enjoy it at normal levels for a while and its all jacked up.


I used another amp to check the power tubes, they are fine.
I used another amp to probe the signal.  Its good all the way to the power tube grids but with quite a bit of noise after the PI, I assume from the less filtered AC.


The plate of the Reverb tube is wiggling like an oscillator.  Pulling that tube out makes no difference to this problem.


When turning the amp on from standby instead of the pop of the speakers, its more of a buzzing sound that comes up quick like an old TV set or microwave when first turned on, then it settles.  IN order to get audible signal to the speakers, I have to crank the amp and its real noisy.


I probed the plates of the power tubes but it was pure rizz.  The OT measures .8 on the secondary and 38-39 from one side of the primary to the CT and 40-42 on the other side of the primary to the CT.


Clearly I broke something but all the tubes seem to be fine.


VOLTAGES on ALL the plates ARE LOWER THAN NORMAL.  About 50 volts lower on the preamp tubes.


Voltages on all the plate pins with all tubes removed are around 413.


BIas voltage is MORE NEGATIVE than usual.


Any clues?

[proaudioguy]

The voltage out of the PT is around 320VAC per leg to ground (CT is grounded).
Voltage out of the rectifier which sees 50µF of capacitance with the amp in standby is just over 300VDC.  I have Diodes on order.  Worth a try.   Replacing the 3A 1000V IN5408s with 1A 1000V IN4007s.  I feel like the diodes are fine and I’m dropping the voltage somewhere else, but I can’t imagine where.

[Latole]

   I feel like the diodes are fine and I’m dropping the voltage somewhere else, but I can’t imagine where.

Read ohms boths side you'll know how are diodes good or bad ;


https://www.electronicshub.org/test-a-diode/

[sluckey]

The voltage out of the PT is around 320VAC per leg to ground (CT is grounded).
Voltage out of the rectifier which sees 50µF of capacitance with the amp in standby is just over 300VDC.

Are you sure the 50µF is always connected to the rectifier, IOW, before the standby switch? Hoffman's schematic shows all filter caps after the switch.

Measuring only 300VDC at the rectifier would be normal ***IF*** there is no filter cap connected.

[proaudioguy]

Sluckey, I came to update you all, but YOU are a genius sir.  I would like to sit next to you at your bench and just watch cause you always seem to nail the issues at hand, but I just wish I could bounce this stuff off you directly so I didn’t have to wait 2 weeks to get your wisdom!


4 of the 6 diodes were BAD.  I replaced them and the voltage did not go up so I went under the chassis and pulled off the cap cover and lifted a leg off the first 100µF and its OPEN.  The second one is good.  I just ordered 2 100/350 F&Ts and just for fun I ordered 3 more 20/500s because I may not need them today but when I do, I don’t want to wait another week.


I’m hoping between the diodes and the caps, that I can get straight on the reverb driver freaking out.
I’m quite sure that cap wasn’t open last week.  I’m not sure what did it but its a TAD I bought 8-12 years ago and of course the amp is barely used.


As long as I had to order, I ordered all the electrolytics for the DRII which needed a cap job when I got it 8 years ago and its been sitting here waiting for it ever since.  What do you guys use for the 4µF caps?  I ordered 2x8µF for each position and I guess I’ll run them in series which is kind of lame.  They are sprague atom at least.


On the twin reverb AB763, the 2x70s, which in my case are 2x 100s in series are before the standby switch.  On the DR schematic the 2 16µF in paralllel are before the standby switch as well.  My Concert II also has those first 2 caps before the standby switch.

[sluckey]

I’m quite sure that cap wasn’t open last week.  I’m not sure what did it but its a TAD I bought 8-12 years ago and of course the amp is barely used.

An unfortunate characteristic of electrolytic caps is that they can go bad even if just sitting in a parts bin. They can slowly dry out if not sealed properly and a dried out cap usually loses it's capacitance, eventually to the point of being "open".

Carbon comp resistors will also go bad just sitting in a parts bin. They always drift to a higher value. A CC resistor's value is related to how tightly packed the carbon particles are. Tighter packed means lower resistance. Over time the particles will become less packed and the value increases.