First Stage Hiss

[sepulchre]

I built another 50 watt amp, sort of a Marshall 2204 with a Bunch of mods. The first one I built is amazing - guess I lucked out. It sounds really nice but what's really special to me is that with no guitar plugged in you can dime everything and it makes NO noise. Ok, a tiny bit of air sound, like someone left a mic on in an empty room. But it's essentially silent. I sold it and the guy brags to me about how it shows up others' amps for its quiet idle.

As I was saying, I built another. This one has a little hum when the master (PPIMV) and pre-PI volume are dimed but not enough to worry about. But when the "Drive" is turned up, between the first and second stages, it starts to hiss. At full volume it's practically roaring. I have swapped out each of the components, one at a time, to isolate the origin... to no avail. I tried various tubes - there are subtle differences but a hurricane still blows gale force.

Now, when turned down somewhat the amp can be used and sounds nice as long as you don't want to drive the second stage much. But that's how you get a great sound from this style of amp.

So, any suggestions?

Stages 1+2 schematic.jpg

[xtian]

Input grid resistor: try going down to 10K, and use metal film. Bigger (physically) resistor is better to avoid noise, so use 1 watt. Same for the two 100K plate resistors.

[sepulchre]

I use 1w throughout, but no metal film. I give it a try.

Thanks!

[Malcolm Irving]

Did you remember to wire up the input jack so that it shorts out the 1M when nothing is plugged in?

[R.G.]

Have you put a scope on it?

Have you swapped out truly all of the parts? including sockets, wire and decoupling caps? If it's truly every single component, then it's some combination. You can get storms of hiss from RF (including self oscillation) being detected by a stage and then amplified.

Do both the previous amp and the current one act different when running side by side, powered from the same wall duplex?

[sepulchre]

xtian - The 100k anode resistors are indeed metal film, Vishay Dale to be specific. I'm waiting for a 10k to try on the input grid. I did try another, I think it's metal oxide. If there's any difference it's only that the hiss is smoother but I can't guarantee that - maybe just hoping.

Malcolm - Yes, the input jack grounds itself with nothing plugged in. To make sure the jack is working I also tried a plug that is grounded. No different.

R.G. - Yes, I put a scope on it. There's a tiny very high freq in the first stage output but it's there even when the Drive is turned down and the hiss is silenced. I expected to see an irregular, jagged trace for the hiss but didn't. I have considered self oscillation but all the leads anywhere near the input end are shielded and the wiring dress is proper (I'm a bit OCD on this point).

No, I have not changed out the tube socket, wire or decoupling caps. I will try that tonight, if have a spare socket. I get my wire from Apex Jr. and have never had a problem, but for diligence sake I will change it. The coupling caps are Sozo in both amps but in #2 the second stage is coupled to the cathode follower network with a Sprague orange drop #715. Both couplers are .022uF.

I can't run the first and second amps side by side; I sold the first. But I tested them both on my bench with the same duplex. Also, #2 does the same thing when I use it in the room where I play; different duplex & breaker. #1 had no problems in either place.

Here's a photo. It's messy from all the component changing I've been doing. I'll clean it up when it's fixed (I have faith).

Stage 1+2.jpg

[sepulchre]

Here are a couple of pics of #1. One pic is earlier on in the build, the other is the final as noted in the pics.

Stages 1+2 a.jpg

Stages 1+2 c.jpg

[R.G.]

R.G. - Yes, I put a scope on it. There's a tiny very high freq in the first stage output but it's there even when the Drive is turned down and the hiss is silenced. I expected to see an irregular, jagged trace for the hiss but didn't. I have considered self oscillation but all the leads anywhere near the input end are shielded and the wiring dress is proper (I'm a bit OCD on this point).

You did know where I was going with that - RF oscillation. The tiny high frequency oscillation does seem to lend some credence to it.

It's probably worth some tinkering to eliminate that. What's the purpose of Csnub, and what happens when it is disconnected? Or just connected to ground, not to the cathode?

Along the same lines, what happens when you tag solder in a 0.1uF cap across the input 1M resistor?

The cap from plate to cathode and cathode to ground reminded me a bit of some oscillator circuits I've seen, and the cathode can look inductive at RF in some circuits, so maybe the first stage is singing a little. That would fit with the drive control cutting it.

[Roe]

In order to reduce hiss some recommend replacing the typical 1m pot with 500k or 250k

[sepulchre]

R.G. - I routed the Csnub to ground and added a 0.1uF across the 1m input ground. There's still some hiss but Way less. It's not as quiet as #1 but I think I just lucked out somehow with that one. I'm going to see if I can borrow it back for some more in depth study; try to figure out what my secret was.

Roe - I intend to replace the 1m Drive pot with a 500k. I do think the pot that's in there now has an issue; it makes some noise when turning it up or down.

Thanks for your help guys. It still has some hum I want to eliminate and who knows? Maybe that will get rid of all the hiss too. (not holding my breath)

[martin manning]

... I intend to replace the 1m Drive pot with a 500k. I do think the pot that's in there now has an issue; it makes some noise when turning it up or down.

Any DC on that pot? I haven't been following closely, but the caps in front of it could be leaking and/or noisy.

[R.G.]

Good point. A leaking coupling cap would drop in hiss. I was just discounting that because OP said he'd replaced the parts.

[sepulchre]

It's been awhile. Been busy.

Anyway, Martin - no DC on that pot. I replaced it to no effect.

RG - I'm pretty sure I did replace the coupling cap, but I'll make sure.

This is so frustrating because the first one I built is dead silent with nothing plugged in to it, even with all the knobs dimed. Can't get back to it just yet; customers awaiting repairs. But when I do I'll likely be back here.

Thanks guys

[Ken Moon]

I've always used 5751 tubes in the first stage of my higher-gain amps, and I bought a bunch of JAN 5751's on eBay, and select for lowest hiss.

Works wonders on amps I've built with the TW preamp.

Of course, I try to do everything "right" in the design and layout, but first-stage hiss is a hard bugger to kill if the next stage has high gain.

[Roe]

...

Roe - I intend to replace the 1m Drive pot with a 500k. I do think the pot that's in there now has an issue; it makes some noise when turning it up or down.

with a 500k pot the 470k & 470pf treble boost should be changed to 220k and 1nf to keep the same mid boost