Anyone care to help me finish my build ?

[antolc37]

Hello all, I'm having trouble finishing a custom build I've put together on my spare time, it's basically a Marshall 1959 preamp (bright channel only) in front of a ECC99 in push/pull configuration, all fits in a 1U 19" rack I had lying around.

The amp is working "ok", quite loud and crunchy as expected, the only thing that is bothering me is a loud squeal or hiss (oscillation?) when the volume is pushed beyond 8/10 and I can't seem to be able to pinpoint the source of this (it seems to be around V2a~ when poking with a stick the area after the first coupling cap seems sensitive up to the V2 grid)

So far I've tried switching the 33k grid resistor on V1 with a 68k, I've also redone the shielded cable from the input jack, I've tried grounding V1b too (because it is not used at the moment) without much success.

Amp is split in two boards, the "power" board with diode rectifier bridge and capacitors/self/resistors network for providing high voltages, and the "sound" board with coupling capacitors, tone stack, cathode polarization etc.. each board has a ground bus and the ground buses are connected together to the chassis in a "star" manner. Main plug has its own wire screwed to the chassis also.

Power transformer is toroïdal 30VA, 220v primary (I'm in Europe), 0.085A@200V sec1, 2A@6,3V sec2. I can provide all the voltages if need be of course.

Output transformer is hammond 125C wired for 12,5kohm primary under 8ohms speaker.

As it is my first build, I'm not completely satisfied with the cable management and how some parts of the design ended up (namely heaters), I'd be happy to read your advice on that matter, I don't know if its bad enough to create that hi pitch hiss however, or maybe the choices I made in some component values are bad, anyway I hope you can help me finish this project, I can provide any information one would need for "debugging".

On the picture you'll see that both OT and self are not tied to the chassis yet, OT is supposed to be on the far right corner but I wanted to test the amp before screwing it on.

thanks all

[Lectroid]

Schematic?

[antolc37]

sure here it is, the exercise was interesting I had to trace it back from the actual circuit which showed me two odd things :
- Tone stack is weird
- 22k grid resistor for V4 is too big I don't rember why I put so much

[sluckey]

That tone stack is drawn weirdly but if you rotate the bass and mid pots and line them up under the treble pot you will see a very familiar TMB stack.

Those 22K grid stoppers should not be a problem.

[antolc37]

oh ok that's good to know, I had hoped this was the origin of the hiss though and not the lead dress or something wrong with the layout

[AlNewman]

Maybe you need to adjust the voltage divider after the PI?
Maybe you need to adjust the ratio of the NFB circuit?

You might just be hitting the power tubes too hard.

What happens if you lift the NFB?  Maybe it's as simple as reversing the OT primaries.

[antolc37]

The presence potentiometer reduces the hiss indeed so maybe I could add some more NFB (I've already tried disconnecting it completely), I will try adding a stereo potentiometer after the PI if it doesn't do the trick. You're probably right, I remember that when the power tube was biased cooler (I had only 630R at my disposal) I didn't have the squeal.

[elams1894]

Try moving the Output Transformer away from the ECC99. You could be getting a heap of induced positive feedback from the OT as it looks very close indeed.

[Rontone]

Try moving the Output Transformer away from the ECC99.

Possibly angling it 90 deg at the same time, those brown and blue OT primary wires could be a lot more direct to the power tube then

The NFB wire also has to travel past the OT, the Power Tube and then right next to the heaters, it may be more direct going over the top past V3 instead of the power tube? Or along the chassis between the alloy tube mounts?

[mountainhick]

Maybe it's as simple as reversing the OT primaries.

First thing I'd check. Odds are 50%.