Another Hot Rod Deluxe Oscillation distortion

[Magiamusical]

This Fender Hot Rod Deluxe exhibits on both channels an oscillating noise when a chord is played. I checked R78 & 79 and they measure ok, tested the sound using the effect loop and the preamp sound good the power section has the problem. Resoldered the tube socket just in case and measure all the resistors in the power section. Running out of ideas after checking most of the suggestions found in different forums. What I have found ss that the bias voltage is around 8mv when it should be 60mv. I’m including the voltages measure in the power section. Any ideas on the next step. (wish I had an oscilloscope to see the oscillation)

[drewl]

Flip down the main board and check for any loose solder connections.
check your negative bias voltage and see if it varies with the pot if it has one.

Check the ribbon cables between boards and the plus and minus 16 volt supplies, their dropping resistors and diodes.
Check the 470 ohm screen resistors to pin four of the output tubes, that voltage should be close to voltage on pin three.

[Alex R]

it's often filter caps on these.

[Mick Bailey]

Hi and welcome to the forum.

First thing I'd investigate is why the idle current is so low. That 8mV translates to 8mA, or 4mA per tube - and that figure includes screen current. Chances are you're getting lots of crossover distortion. If you could scope it you'd see a rather large notch on each half of the waveform on the zero axis.

This needs sorting out before anything else. Usually the bias trimmer won't have the range to set the bias voltage high enough to reduce the tube dissipation to that extent.

Make sure your meter is working correctly and you're grounding the probe on the chassis. If I get a reading like this I always question my technique or equipment. Make sure your DMM doesn't need batteries - a low battery can give a false reading.
Your B+ should be around 430v and you show 352V. There's also a discrepancy in that you show CP19 and CP20 as having different readings to the plate voltages, so something may be wrong with your measurements - if you look at the schematic these should be the same.

[Magiamusical]

Hi and welcome to the forum.

First thing I'd investigate is why the idle current is so low. That 8mV translates to 8mA, or 4mA per tube - and that figure includes screen current. Chances are you're getting lots of crossover distortion. If you could scope it you'd see a rather large notch on each half of the waveform on the zero axis.

This needs sorting out before anything else. Usually the bias trimmer won't have the range to set the bias voltage high enough to reduce the tube dissipation to that extent.

Make sure your meter is working correctly and you're grounding the probe on the chassis. If I get a reading like this I always question my technique or equipment. Make sure your DMM doesn't need batteries - a low battery can give a false reading.
Your B+ should be around 430v and you show 352V. There's also a discrepancy in that you show CP19 and CP20 as having different readings to the plate voltages, so something may be wrong with your measurements - if you look at the schematic these should be the same.

CP19 and CP20 measure the same (my mistake when I took the notes), I just took the measurement again and tried another Digital Meter I have and the voltages are the same also re-checked the bias point and it has 8mv, if I move the bias pot it goes from 6 - 12mv. I'm trying to see if someone in the area has a scope that I can borrow. Agree that its sound like crossover distortion or that the phase converter is not working making one tube not amplifying one of the phases, have check that area but can't find the cause. The problem has to be from the reverb to the final stage because if I connect the pre-amp to the pa it works fine. Regarding the high voltages I also found it strange that they are low (and the ac input is 120v) but I dont think that would be enough to cause the problem.

[g1]

Regarding the high voltages I also found it strange that they are low (and the ac input is 120v) but I dont think that would be enough to cause the problem.

Your B+, plate, and screen voltages for the power tubes are down almost 100V, so it certainly will lower the idle current substantially.
Find out why the point marked B+ in the power supply is so low and you will probably solve your problem.
What do you measure for AC volts at B+ ?

[Magiamusical]

Your B+, plate, and screen voltages for the power tubes are down almost 100V, so it certainly will lower the idle current substantially.
Find out why the point marked B+ in the power supply is so low and you will probably solve your problem.
What do you measure for AC volts at B+ ?

What I meant was 120ac at the wall not at B+
Will check the power supply circuit, to see whats going on with the low voltage.

THANKS Mick

[g1]

What do you measure for AC volts at B+ ?

What I meant was 120ac at the wall not at B+

Nope , I meant do an AC voltage measurement on the B+ to check for ripple.

[Mick Bailey]

I'm trying to see if someone in the area has a scope that I can borrow.

This is one job where a scope wouldn't be an advantage. It's just a case of measuring AC and DC voltages and a DMM is fine.

I wouldn't at this stage get diverted by the fact that the preamp works, or in scoping signals. The basis for correct operation of the amp is a correct set of voltages. If these are wrong then you won't be able to say for sure if anything else is. Once these are correct and the tubes are pulling the correct current then move on to any remaining problems.

[stokes]

Like AlexR has already said,filter caps.I've had about a dozen of these amps on my bench with oscillations.They use Illinois caps for the filters and I guess they got a bad run.Hot Rod Deluxe,Deville,Blues Jrs,some only a year old,always the filters.I even had 2 amps that had no problems,they were friends of mine.I talked them into changing the filters and they were absolutely astounded at the improvement in tone/response.Use good quality F&T or Sprague Atoms,you wont be dissappointed.