another hum problem..please help

[thomasdj]

Hi,

I am building a 3ch 100w igh gain amp and I'm almost there..

All three channels has great sound and all the tone stacks works well.

Channel 1 is working perfectly..no hum at all!
Channel 2/3 got a LF hum..my guess is 100/120 hz..

The hum increase a bit when i turn the bass past 0.5 and change shape with the rest of the EQ..the hum also rise with the master volume..If I ground the grid of V2b (see schematic) the hum disappers and the amp is totally quiet.. If I ground the grid of V2a the hum increase and becomes "more square" sounding.. so my guess was that it could be the 1uf cathode cap at V2a which is switched in by RY1'a..I've tried to replace the 1uf cap and changing it's value, but the hum still remain the same...it's the same issue with both channels, 2/3..

I don't think it's the input grid at v2a since it's shared with channel 1..wich is quiet

The hum is not affected by the gain pots at all..

Any suggestions how to solve this issue?

btw..All my voltage are close to the shown in the schematic..My PT has center tapped fillament (grounded at starground 220uf cap)..the heaters are tight twisted and runs underneath the pots and along the chassis' sides.

Thanks

Best regards

Thomas

[thomasdj]

ok..I've tried to elevate the heaters but didn't help at all.

A scope on the speaker out gives me a sawtooth curve..see attached file..


Could this be a leaky 0.022 cap between RY1'b and the 475K grid resistor at
V2b??

As i wrote befor..if I ground V2b grid (pin7), the hum totally disappear! If I ground V2a grid the hum is still there..

Thanks

Thomas

[thomasdj]

Could this be a leaky 0.022 cap between RY1'b and the 475K grid resistor at
V2b??

Tried to replace the 0.022uf cap..didn't helped at all..

help please

Thomas

[tubeswell]

If it happens when you switch channels, it could just be bad grounding. Try different grounding (e.g.; split ground with all preamp grounds, pre-amp filter cap ground, and vol, tone etc going to one of the input jack grounds, and all the other 'high current' grounds going to one of the pT bolts at the other end of the chassis).

[thomasdj]

If it happens when you switch channels, it could just be bad grounding. Try different grounding (e.g.; split ground with all preamp grounds, pre-amp filter cap ground, and vol, tone etc going to one of the input jack grounds, and all the other 'high current' grounds going to one of the pT bolts at the other end of the chassis).

Hi tubeswell!

Thank you for your reply

I went through some seriously troubleshooting with the amp and the hum is almost gone. It turned out to be a bad 0.022 coupling cap between V2a and V2b..

Now the only hum I've got is a hiss sound which i think is coming from the guitar..since it's only present when the guitar volume is turned up. The guitar (ibanez) is not shielded and think this could be the problem. However, when the guitar is not plugged in I see no signs indicating hum on any of the 3 channels. Not even when scoping at the output

Thanks

Thomas

[tubeswell]

Glad to hear you found your hum problem.

Hiss is quite normal but you can put metal film resistors in for all the plate resistors (if you haven't done so already). That should reduce hiss a bit

[thomasdj]

Yes..I'll try that..I'm not 100% sure where the hiss is comming from yet but could be the plate resistors..

Thanks

[Steve Conner]

If the hiss only appears when the guitar volume is turned up, it sounds like it's basically the self-noise of the first tube, or Johnson noise in the resistance of the volume pot or whatever.

Either way, it means that you've probably got the noise level of the amp as good as it's going to be, it may be time to put the lid on and enjoy The one thing I would try is swapping the first tube to see if it's an abnormally noisy one.

[thomasdj]

Hi steve!

You're right! I went to the rehersal today and tried the amp out on the 4x12 cabs..! The amp amp was very silent!

I got one little issue to solve..it got a very low headroom, - almost none. It's even loud or LOUD!!!

Thanks

Thomas