crazy buzzing distortion

[surefirespider]

Hi, i'm at a loss here. this is like the third amp i've built lately and all three of them have this crazy buzzing when the tube starts overdriving. I'm using 6sl7gt preamp and 6l6 power tube from a 5y3 rect.tube. I've tried lowering the cathode resistors. changed the layout thinking maybe that would've helped, everything has been changed out and still won't go away. the only thing that makes it go away is keeping it at low volumes before any overdrive. I ran a distortion peddle into it and it sounded better but only because it covered it up. Here's a schmatic, if someone would like to help
. this is a completly new build.

[loudthud]

I never really liked the sound of un-bypassed cathodes. I think it sounds better to fully bypass the cathodes and make up for the additional gain by attenuating between stages.

[surefirespider]

Originally Posted by loudthud

I never really liked the sound of un-bypassed cathodes. I think it sounds better to fully bypass the cathodes and make up for the additional gain by attenuating between stages.

So your saying put bypass caps across the cathode resistors and then what a voltage divider between the stages?

[Chuck H]

Well the amp looks straight foreward enough. +1 on the cathode bypass caps but I don't think a lack of them should cause "crazy buzzing distortion". It would help a lot if you had access to a scope. Then you could actually watch whats happening as the amp begins to overdrive and trace the any wave form abnormalities to their source.

Are you playing all the amps through the same speaker cabinet or type of speaker?

With simple amps (especially with simple amps) it's important to get everything right for good overdrive. There's just so few things happening with most single ended amps that if even one thing isn't ideal it can sound bad. I have never built with 6sl7 tubes so I can't say if your load and bias R's are ideal.

Are all your amps using the same OT?

Example: If the OT is a little "hashy" sounding and your using a bright speaker, odds are the amp will have bad sounding distortion. This as it applies to the simple, single ended amps.

Chuck

[surefirespider]

each of these has been played through different speakers, at first I thought that it was speakers tried different cabs i had. just bought a couple of new speakers tried them and still the same, I don't think I could have eight bad speakers. I built this last one hoping it would be resolved but not. Could it be the board that i'm using? it's old bakelite board I cut and mounted everything on.it's the only thing that wasn't replaced when i completely redid the whole build.

[surefirespider]

picture of the inside of the chassis. I installed the bypass caps on the cathodes, it increased the gain of course. but also made this thing hum five time what it was. and if i take the cap off the poweramp cathode it will decrease the hum. this is why i'm thinking the board is conductive.

[Chuck H]

It's easy enough to find out if your board is conductive. Just set your DMM to low DCV, jam the red probe into the board near a high voltage eyelet and touch the chassis with the black probe. If you get a voltage reading your board is conductive.

Looking at the photo I think your problem is a parasitic oscillation due to your layout. Your OT is right under your preamp and your actual speaker output is very near your first amplification stage. Basically you have the amps output and input both on the same end of the amp and that's a design no no. Radiant fields within the amp have the wrong parts of the circuit sharing signal. There is no good fix short of a complete teardown and rebuild with an entirely different layout If there's room you could try moving the OT and speaker output to the other end of the chassis. Even if you have to scootch the PT over a bit. Know that with the PT and OT right next to each other the laminates on either transformer should be arranged at 90* of each other.

Chuck

[surefirespider]

thanks Chuck, it's no problem changing things around. it might take me a little while to do it, but if it helps then it'll be worth the time. This sight is great!

[MWJB]

Your volume control pulls down the value of the 6L6 grid load resistor (220k at pin 5).

You could try a coupling cap between the vol pot & the 220K, but honestly, I'd be thinking about putting the volume & tone control between the halves of the 6SL7. At the moment you have a lot of series resistance between the 6SL7 halves with the pre pot down, or little attenuation between the halves with the pre pot up.

[surefirespider]

Ok i checked my board and i'm getting a reading from 1.7mv-3mv depending where i check it on the board. And I have tried the volume and tone between the stages of the preamp tube and it was the same. i took the master volume and tone control out of the circuit and didn't make much differance.

[MWJB]

Remove/bypass the 100ohm resistor after the rectifier (looks like you have 150?), I'd also be thinking about bumping up the B+ voltage, do you have a GZ34/ss rectifier you can try?

Increase the voltage rating on the 6L6 cathode bypasscap to 100v.

I don't like the way your wires are routed round the 6SL7, try disconnecting it and rewiring so that the input wire is at the input jack side of the tube & pre volume connects to the 6L6 side of the tube.

Rewire the plate wires, these can be long & should sweep around the tube base, low to the chassis floor. Keep the grid wires away from the plate & heater wires.

You still want something like a coupling cap to stop the volume pulling down the 6L6 grid load.

What value are your plate resistors at the 6SL7, schem says 270K, but the resistor bands don't look right (should be red/violet/yellow). 110v on the preamp plate is very low, the amp will distort from the off.

How is the hum affected by pulling the preamp tube?

Your grounds seem to converge under the tone pot - ground the power amp grounds to a PT bolt, ground the preamp cathodes & filter cap to the input jack ground lug - you could well have a ground loop.

How are your heaters referenced to ground.

[surefirespider]

Ok i'll try this, I have the board out and moved the ot down the chassis. i'll post what my findings are when it's completed. the pt has a center tap for the 6.3 windings. my bad, yes the resistors are 470k i was just changing things out to figure this thing out.

[surefirespider]

Ok here goes I moved the ot away from the preamp side towards the pt more. turned the socket around, installed a blocking cap between the vol. pot and the 220k. the problem isn't any better, it sounds crackly, fizzy,buzzing noises on low notes.

[surefirespider]

well I hate to say it but sometimes we all think we know it and really we all make mistakes. I just walked out my front door for a quick smoke, and there's a package from ups, oh my a pair of old jenson c10pf i bought for ten bucks on ebay. I put one of them in this amp and OMG, no crazy buzzing crackling pour sounding amp anymore. it's like night and day. I guess this proves to me that the cheep new speakers arn't quight as good as I thought. there's still alot to learn about building amps so i'll still be around asking questions. Thanks so much for all your help, it opened my eyes to different ways of looking for problems and different opinions of what to do .