5f6a bassman - feedback oscillation? Help!

[j-stylez]

I am in the process of rebuilding my 5f6a bassman with some extras. My build was working fine, but now after having reassembled it, I am running into some problems:
There is low frequency oscillation when raising the MASTER above a certain level. It's like a tremolo sound, whoop whoop whoop, very deep. Once started it will not stop unless I turn off the amp and power it back on. It doesnt sound like puttering motorboating noise, more like a very deep wou wou wou wou wou

Volume and Tone controls, presence, have no effect on the oscillations. Same goes for the reverb circuit that I have encorporated (Spring reverb driven with a solid state mosfet circuit). I am not using the other half of v1 yet, I built only one channel. Will use the other for tremolo once i sorted this out. I have added a PPIMV with a dual gang pot. This was already there when i first built it, so I am thinking this should also not be the problem.
I have solid state rectification with a voltage doubler circuit.

I have tried switching OT transformer leads, this also did not solve the problem. I have tried disconnecting the NFB, the problem still persists.

The problem persists in both fixed bias and cathode biased mode ( I added a switch to have both ).

I just hope its not the positioning of components (Tubes / Transformers), I am working with a small enclosure, and there is no room to change anything. I am not having any hum problems, so I am thinking this shouldnt be the problem.

I am wondering whether it could be a grounding problem. I am using a star ground scheme, where there are 2 star points. Preamp, PI, Pots, etc all go to a ground point to the chassis at the input jack.
Power amp, wall outlet ground lug, center tap, power tube cathodes, bias circuit are grounded to a point at the other end of the chassis.
Speaker Jack is grounded to the chassis.

I have shielded the grid wires for the power tubes, and put 3k3 grid stoppers on the tube socket pins.

Voltages are:

V1: 1: 232 2: 3: 1,92 4: 5: 6: 7: 8: 9:
V2: 1: 208 2: 3: 1,32 4: 5: 6: 362 7: 208 8: 210 9:
V3: 1: 282 2: 30 3: 50 4: 5: 6: 290 7: 32 8: 50 9:

V4: 1: 50 2: 3: 491 4: 493 5: -51 6: 495 7: 8: 9:
V5: 1: 50 2: 3: 491 4: 493 5: -51 6: 495 7: 8: 9:

see pictures below for the layout.

If anyone can help me out - it would be greatly appreciated!

[loudthud]

Where are the rest of the filter capacitors? Does the amp have a choke?

[j-stylez]

Yes, using a choke.
I thought the Reservoir caps from the doubler would be enough to replace the Main filters. Do you think it could be a lack of filtering?

[loudthud]

It looks like you are using 100uF caps in the circuit where the caps are in series, one end of the transformer winding goes to the node between the caps, the other end of the winding connects to two diodes. That should be enough for the main reservior cap, but you need more caps downstream for the screens, PI and preamp.

[j-stylez]

Ohhh...I see your point! I am missing the two 20uF caps - one right after the choke and before the 4k7 dropping resistor, and the other one after the 4k7 dropping resistor

Is this likely to be the cause of such a problem? I think it is, the next filter cap after those that are missing is the 8uF for the preamp tubes! And the preamp isnt making any problems.
I'll have to get some appropriate caps for the high voltage first, until then I can't test it.

[loudthud]

Yes, missing caps can cause the kind of putt-putt-putt oscillation you describe. The caps need a 500V rating if you don't have and use a Standby switch. Before the tubes warm up, the full Voltage flows down the string of resistors with almost no drop. Finding an 8uF 500V may be difficult. You can use 10uF to 22uF.

[j-stylez]

thanks for the advice - I used caps in series to achieve the rating so I can try it out right now - I found 2x33uF 450V and 2x22uF 500V, but to be safe i put them in series (my B+ is pretty high).

Regarding the standby switch - I did some research about that, and thought that its not really necessary. I know I'm not using a tube rectifier, so my voltage isnt coming up slowly - but then again, I read that the dreaded "cathode stripping" does not happen in guitar amplifiers, because the voltages are not high enough for it to be problematic.

Edit: Well, it wasnt the caps. I installed them but the problem persists - no change. (Its also not leaking caps, i checked the coupling caps to the grids)

Any more ideas are welcome

[j-stylez]

The AMP is silent with the Phase Inverter tube pulled. It must be here where the Problem lies then?
Its not the pi tube, i replaced that and it still oscillates then. But without the pi tube, no oscillation.

[überfuzz]

Could you show us your power supply schematic. Are the DC feed nodes separated?

[j-stylez]

This is my power section:



My mosfet based reverb circuit also feeds from the point where V1 is connected to B+.

PI has its own node, V1 and V2 share a node with the reverb circuit.

I have isolated the problem further. When I disconnect the reverb tank by unplugging the 2 jacks from the amp chassis, the oscillation stops!
Does that point to a grounding problem in that area?
With the reverb tank connected and mix and dwell controls on zero (no reverb in the circuit), the problem persists- with the tank disconnected, all is well...

I have my reverb inserted between the PI and the tone control - my problem stops when i pull the PI tube. So i am pretty sure it must be with the reverb.
Ill check next if it is phyiscal feedback from the reverb tank. What is the best place to ground the reverb jacks to, and also the reverb transformer?

The reverb jacks are isolated from the chassis and grounded at the star ground close to the input. The reverb OT is also grounded at that point.

[j-stylez]

Wow...it was only physical feedback from the vibrations of the amp to the reverb tank...or maybe of the transformer to the reverb tank or something like that! I positioned the tank a bit away from the amp, and the problem stopped!

[überfuzz]

Okay, then you sorta nailed it. Question then is what makes the reverb-circuit unstable..? Well, you have to install the tank in the amp eventually don't you? :-)

Have you tried to feed DC to the reverb circuit from a isolated node? It looks as if you feed V1 and V2 from the same node. Just to rule it out you can try to add a dropping resistor + capacitor between V1 and V2.