Sope question

[bsco]

Thanks for that long reply.....much appreciated......It is a master Volume which looks like stock....I'll check out your suggestions.......and as "Arnold" would say......"I'LL Be Back".........Thanks for the help.....

[g1]

When you talk about 400hz superimposed on an "AC power supply" waveform, to me it sounds like big ripple. Are the main filter caps ok?

[bsco]

When you talk about 400hz superimposed on an "AC power supply" waveform, to me it sounds like big ripple. Are the main filter caps ok?

I measured the DC and AC voltage at the plates of the output tubes to see if there was any change when the controls were cranked.....they varied the dc levels slightly....like 20 volts or so.....no AC measured on the plates.....plate voltage to the other pre-amp tubes look ok as well....It is getting late here......Will sleep on it and have another go at it tomorrow.......Thanks for the reply.......
P.S. I noticed that this unit does not have a "Line Out"...but the schematic shows that it has one.....It looks as if the FBR is coming off the line out jack (820 ohms/1 Watt), which is tapped off the speaker out jack...please correct me if I am wrong.....I am going to have a look at a regular twin Reverb with no Master Volume......Thanks for all the input so far......Just to let everybody here know... this unit had been left it in storage for many years.....the speakers are not original.....they are not Jensen's......

[g1]

Does it have a pull switch on the master? If so, here's a schematic with no line out jack.

[bsco]

Does it have a pull switch on the master? If so, here's a schematic with no line out jack.

Yes...it has a pull switch...Thanks for the schematic....
Cheers....

[bsco]

I checked this out after.....I pulled the reverb tubes and the tremelo tube....I left in the first two pre-amp tubes and the PI.....I measured the plate voltage to V1 and V2 while watching the output on the scope....V1 measures approx 230V on both plates...V2 is closer to 250V....tone controls set at midway....Master Volume on 10.....reverb and tremolo controls at min....channel two tone controls at midway.....channel two volume at zero..

When I crank up the volume for the clean channel once I hit say 4 or 5, the plate voltage drops by 25volts roughly...the higher the input volume the lower the voltage....and the worse the oscillation.....I swapped the two channel preamp tubes with another two good tubes......same thing.....I check the new power supply caps that he had purchased and had installed himself and they tested good for capacity and esr....esr was 2.6 on my "Blue ESR Meter"....the FBR is 820 ohms.....

.so.....to me , and please correct me if I am wrong, this feedback problem that I am having is power supply related somehow?? I also had to replace the supply diodes as one was broken and I replaced all six to keep everything balanced there....using 3A ones....oh..and there is a new set of output tubes in there as well and one screen resistor was open and that was replaced.....and I also installed 4 cathode resistors on the output tubes as well.....1 ohm, 1 watt, 1 %.....suggestions..........comments........feel free to fire away.........
Cheers

[bsco]

I had a chance this morning to check out the cathode bypass caps......one cap in particular was only 8uf instead of 25uf but the esr was excellent.....the other bypass caps and excellent capacitance but a higher ESR...I replaced them to see.......same thing...there is no change.....

[Tom Phillips]

Did you check the continuity of each of the three 20uF filter cap negative leads to chassis ground?
Check from the cap lead all the way to chassis. Not just to an eyelett or adjacent wire.

[Jazz P Bass]

I
When I crank up the volume for the clean channel once I hit say 4 or 5, the plate voltage drops by 25volts roughly...the higher the input volume the lower the voltage....and the worse the oscillation.....

Is that plate voltage drop with a signal injected.

If so, that is to be expected.

What you do not want (regardless of cap tests) is the signal riding on the power supply node ABOVE the plate resistor.
If the signal rides on that node, then the cap for that node is not doing it's job.

[Chuck H]

It's not unusual for the "other" electrolytic caps in an amp to get skipped when someone does a "cap job". Strangely, some suggested procedures skip those caps while recommending the replacement of certain film caps in an amp!?! Anyhoo... What about the bias supply caps. I don't know how close the amp is to full power when the oscillation occurs, only that you said it happens with the master volume up. The power tube grid circumstances can shift as the tube reaches full power and the waveform begins to compress. I might suspect the bias supply caps. Some ugly stuff can happen there if those caps are bad. I'll guess you've already seen that the idle current measures correct and that the power tubes are reasonably good and somewhat balanced.?.

[bsco]

Did you check the continuity of each of the three 20uF filter cap negative leads to chassis ground?
Check from the cap lead all the way to chassis. Not just to an eyelett or adjacent wire.

Checked that out and everything is good......no resistance there.....

[bsco]

Is that plate voltage drop with a signal injected.

If so, that is to be expected.

What you do not want (regardless of cap tests) is the signal riding on the power supply node ABOVE the plate resistor.
If the signal rides on that node, then the cap for that node is not doing it's job.

Yes...the plate voltage will drop with the level of the signal....

[bsco]

It's not unusual for the "other" electrolytic caps in an amp to get skipped when someone does a "cap job". Strangely, some suggested procedures skip those caps while recommending the replacement of certain film caps in an amp!?! Anyhoo... What about the bias supply caps. I don't know how close the amp is to full power when the oscillation occurs, only that you said it happens with the master volume up. The power tube grid circumstances can shift as the tube reaches full power and the waveform begins to compress. I might suspect the bias supply caps. Some ugly stuff can happen there if those caps are bad. I'll guess you've already seen that the idle current measures correct and that the power tubes are reasonably good and somewhat balanced.?.

The amp was running with the master volume up full and the channel volume at around 6 with a 200mV signal......I disconnected the bias caps and they check good for capacity and ESR....the output tubes are a new matched set......I did notice that one tube measures a lower bias current than the other three.....I am going to check that out to see if that follows the tube or stays with the socket....8ma in the difference......the other three tubes measure roughly 30mA....or I should say 30mV across the one ohm cathode resistor.....I'll do some more checking and post it........

[bsco]

I connected a .01uf/600V cap on the end of my scope probe and measured the signal on the plates.....the sine wave does bounce ever so slightly but there is no signs of modulated waves or weird distorted complex waveforms.......When I connect the scope to the dummy load, then it happens.....I have the probe set to X10 and the scope to AC coupling....I am going to pull the speaker jack out of the amp and see if there is any corrosion where the body of the jack is mated to the chassis.....might as well check the extension jack as well......Ill let you know how I make out.......

[g1]

I think there have been a few mentions of the feedback resistor, but I'm not sure if you tried disconnecting the feedback loop?

P.S. I think it would be negligent if I didn't mention that the proper spelling is soap.