Please help me in getting rid of the buzz.

[keithgreenhalgh]

In your schmetic the 91k resistor looks like your feedback resistor. this would nornally connect to the 8ohm tap of the output transformer but it goes nowhere in the schmetic. Why?

[Miles Prower]

I'd suggest replacing the diodes in the 845 plate supply. If you were pulling enough excess current to red plate an 845, you might have poofed a diode in that FWB, or at least inflicted some serious damage, and it's just an accident waiting to happen. It would be helpful if you could o'scope and see what that noise is doing.

That would certainly account for the excess noise.

[rdk845]

Originally posted by keithgreenhalgh
In your schmetic the 91k resistor looks like your feedback resistor. this would nornally connect to the 8ohm tap of the output transformer but it goes nowhere in the schmetic. Why?

91K is connected to 8ohm tap. The line somehow got deleted on the picture. Sorry.

Originally posted by Miles Prower
I'd suggest replacing the diodes in the 845 plate supply. ..... It would be helpful if you could o'scope and see what that noise is doing.

That would certainly account for the excess noise.

That's a good point. I replaced both bridge rectifiers for 10V 845 heater since one of them had a diode gone bad. I checked all other diodes using multimeter in diode mod, they seemed fine, some resistance on the forward direction, infinity to the other.
Oscilloscope is a good idea, but I'm not sure where I can borrow one..

[keithgreenhalgh]

OK Heres what we know. When you first built it there was no hum so we can rule out a wiring error. All cap have been bridged so we can rule out the power supply hum.Wait a minute. If the hum is on both channels,the only thing common to both channels is the power supply. On the diagram of the output stage,there is a 100ufd 400volt cap.did you check this one too.But if it was bad there would only be hum in one channel. It has to be a problem in the power supply! Its the only thing common to both channels.One last thing you could try is pulling the 12ax7 anf the 6l6 to see if the hum goes away.

[rdk845]

First of all, this amp is a commercial amp, so I didn't build it.

And yes, the buzz is coming from both channels, but they sound a little bit different in their pitch.

And since I test bypassed all power section caps, it is more likely a transformer or somewhere in the main section. Left side interstage transformer seems to be slight vibrating. Any way to check this transformer?

As far as main sections goes I grounded grid of 6l6g or shorted input wires ( which is connected to 12AX),to the ground and the buzz was always there. Any other way to check which stage could be at fault?

It's interesting you ask about that 100 micro /400V decoupling cap in main section, since the actual amp board differs from schematic here. Instead of 100 micro /400V for each channel, I see 1 100micro / 16 V capacitor connected to ground via 100ohm resistor, for each channel and lone 100micro /400V capacitor. I don’t know how this 100/400 cap is connected to the rest of main section, since board here is situated in a way that is difficult to detach and flip. All other part of amp seems to be same as the schematic, except in the power section for 845 heater (10V) , there are 2 8200 filtering capacitors instead of 1 per channel.

Any thought would be appreciated...

[SHiFTY]

Are the filaments for the driver tubes referenced to ground somewhere? What does the resistance measure from ground to the filament winding?

[keithgreenhalgh]

Left side interstage transformer seems to be slight vibrating. Any way to check this transformer?

If the schmetic is correct,it would be easy to check the transformer since you would not have to disconnect anything. We do not know the primary and secondary dc resistance but it should be the same on both sides.Also measure to gnd on both pri and sec and from pri to sec(this should measure open).Even if one transformer was bad,i don't see how this would cause hum on both channels. My money is on the power supply. One last option since it is a production amplifier and you have a schmetic is to take it to a repair shop and have them look at it.Since these people repair stuff of a living they could probably find the tble fast orthough it would cost more money.

[ray_moth]

The buzz you describe seems to me more like 60Hz than 120Hz. If that's the case, then either Miles is right about your bridge rectifier being wrecked or it's coming from the power tranny.

It could be picked up through poor grounding or possibly through induction between the transformers. The interstage tranny is particularly vunlnerable because it's followed by an amplifying stage but even the OP tranny could be affected. The orientation of the tranny cores is imortant to avoid inductive pickup. Although it's a commercial amp, that doesn't guarantee that transformer placement is ideal. You could try shielding with some pieces of iron or steel, if you have some handy, to see if it makes a difference.

I think with this type of buzz that making changes to the PS (e.g. bypassing the caps) could be a waste of time.

[keithgreenhalgh]

Originally posted by ray_moth
The buzz you describe seems to me more like 60Hz than 120Hz. If that's the case, then either Miles is right about your bridge rectifier being wrecked or it's coming from the power tranny.

It could be picked up through poor grounding or possibly through induction between the transformers. The interstage tranny is particularly vunlnerable because it's followed by an amplifying stage but even the OP tranny could be affected. The orientation of the tranny cores is imortant to avoid inductive pickup. Although it's a commercial amp, that doesn't guarantee that transformer placement is ideal. You could try shielding with some pieces of iron or steel, if you have some handy, to see if it makes a difference.

I think with this type of buzz that making changes to the PS (e.g. bypassing the caps) could be a waste of time.

Interesting idea but he said it was ok when he got it so it couldn't be the orentation of the transformers because they havn't changed location. My money is still on the power supply tble of some sort.

[rdk845]

Thanks for the inputs. Here are some measurements recommended.

Originally posted by SHiFTY
What does the resistance measure from ground to the filament winding?

They are 0.3 ohm to ground for both of 12AX7 and 6l6g heaters. Onne of the 6.3V secondary seems to be connected to ground.

Originally posted by keithgreenhalgh
Left side interstage transformer ...... We do not know the primary and secondary dc resistance but it should be the same on both sides.Also measure to gnd on both pri and sec and from pri to sec(this should measure open).

Actually the primary seems to be about 300ish and secondary is about 450ish ohm for both channels. Primary to Ground is mega ohms (I guess the 100/400v caps are charged ) and Secondary to ground is 20K and slowly increasing¡¦.

Originally posted by ray_moth
The buzz you describe seems to me more like 60Hz than 120Hz. If that's the case, then either Miles is right about your bridge rectifier being wrecked or it's coming from the power tranny.

Yes. This does seem to be 60HZ. But there was no hum as new, so I'm thinking it is not transformer orientation. The diode seems to be O.K. but I could change that. Any way to check the power tranny?

Originally posted by keithgreenhalgh
My money is still on the power supply tble of some sort.

Can you give me some direction on how to test the power transformer? The cable coming out of power tranny seem to be O.K. on eye, and resistances from gnd to solde points of these cables on pc board seem to be all megaohms.

Thanks again.