6V6 1987 build sounds like a dying fuzz (sound attached)...

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MHProd
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Joined: Mon Mar 17, 2008 8:37 am

6V6 1987 build sounds like a dying fuzz (sound attached)...

Post by MHProd » Mon Mar 30, 2020 3:49 pm

Finished a new build which is essentially a 1987 circuit with 6V6s. Only change is PI 820/22K.

The (fixed) bias is fine and stable. No volume issue. B+ is around 395V on the 6V6s plates and screens.

The noise is only there when I play. Otherwise, the amp is very quiet. No hum or anything.

When I turn up the volume, the sound breaks up very early and gets into some nasty distortion... think fuzz where the transistor is biased way too cold. The note decays quickly in a bit of a choppy way...

Nodes: Pi 290V, V2 247V, V1 230V.

I think the voltages are fine:
Plates: V1 P1 140V - P6 160V / V2 P1 151V - P6 247V - V3 P1 199V - P6 188V
Also V2 P8 151V, V3 P2/7 28V - P3/8 53V

Heater voltage is good on all tubes.

So far I'm 99% sure:
-Not a component value error (triple checked everything, including measurements)
-Not something wired incorrectly. (traced everything)
-Not a continuity issue (every single connections, including grounds)
-Not DC leaking coupling cap.
-Not the tubes
-Not an external factor (guitar, speaker, cables).

I inspected all joints and redid a few which looked a bit suspicious.

I wiggled the all the tubes while playing, issue still present.
Chopsticked for a while. Nothing.

The only "clue" i have so far is when the presence pot is set between 8 and full a lot of the noise/decay goes away. I checked the PI/NFB (moved wires around, resoldered everything, changed coupling caps...) but it did not change anything.

Unfortunately, I do not have a scope.

Thanks for your suggestions!

danman
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Re: 6V6 1987 build sounds like a dying fuzz (sound attached)...

Post by danman » Mon Mar 30, 2020 6:00 pm

Disconnect the nfb wire from the impedance selector and see if the issue is resolved. The amp will break up a bit sooner without nfb but it shouldn't sound nasty. If this fixes the issue, you may be experiencing positive feedback and the OT primary wires will need to be swapped at the 6v6 sockets pin 3. Positive feedback normally causes a squeal at first startup but it can also cause other weird noises to occur. If this quick test does not help, you can hook the nfb back up and keep looking.

MHProd
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Joined: Mon Mar 17, 2008 8:37 am

Re: 6V6 1987 build sounds like a dying fuzz (sound attached)...

Post by MHProd » Tue Mar 31, 2020 2:45 pm

Thanks Danman.

I forgot to mention I already tried that. Disconnecting the NFB made it a bit better but not much. I swapped the OT primaries anyway and got the feedback...

I'll also mention that the issue is present while plugged in all 4 inputs, even if I strum the strings very lightly and with the volume on 2 (or 6, doesn't make a difference).

I recorded the noise and tried to post an mp3 of it but I get HTTP Error. I messaged Admin but haven't heard back yet.

It's a very specific type of fuzzy sound which has a "screechy" quality to it, almost like an octavia.

Never encountered this before.

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neikeel
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Location: Suffolk, England

Re: 6V6 1987 build sounds like a dying fuzz (sound attached)...

Post by neikeel » Fri Apr 10, 2020 4:45 am

I presume all the jack sockets are new and in good condition?
What sort of silver mica caps are you using?
Whose OT is it?

From what you describe it sounds like one stage overloading another with signal?

Can you post some clear pictures whilst you are waiting for the sound clips to load?
Neil

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