HUM! Need some help...

[MHProd]

Thanks guys!

Yes it was a LarMar type. I reinstalled the grid leaks and connected their junction to the bias supply. On Pin 5 of the power tubes, I have -48V.

B+ 435V on the plates. KT66 are biased at 37ma. No red plating. Bias circuit works fine.

The VVR scaled the whole amp, so no parallel B+.

I looked under the board and all is good. Checked all the grounds for continuity. All good.

If I add a tube in the PI, that's when the issue start. Any measurements I should take on the PI without the tube ?

[matt h]

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[MHProd]

No, I doubt it comes from that as it was working fine with the VVR/PPIMV.

Trying to figure out what's wrong with the PI. Any validity to measurements without the tube?

[Littlewyan]

Ok lets take a step back. You're issues are hum and fuses blowing. Now how do you hear hum if the fuses blow with all the valves in? Do you hear it with just the power tubes installed?

[Stevem]

Just because the output tubes do not red plate does not mean that one or both of them are not shorted, if you have enough spare fuses pull one tube out at a time and see if the fuse holds, also with even just one good output the amp will have output and play and may have hum!
I would also just for kicks check the level of ac voltage ripple on the bias voltage.

[MHProd]

Ok lets take a step back. You're issues are hum and fuses blowing. Now how do you hear hum if the fuses blow with all the valves in? Do you hear it with just the power tubes installed?

The hum is only present if there's a tube in V3. With the recto and power tubes in, I can turn the amp on and all is well. That's when I measured the bias and B+ I posted above. If I put a tube in V3, I can hear a continuous low pitch sound. That is with the amp on a variac brought up to about 85V. If I bring it up to 120V, the fuses blow.

The low pitch sound comes in as soon as the power tubes start to warm up, so not immediately.

Would measurements on V3 with no tube reveal anything?

[Littlewyan]

Something must be shorted. Have you checked all connections to V3? I cant see all of the wiring in the photo but what I have seen looks ok. Also I'm guessing the hum is still there when V1 and V2 have been pulled?

[matt h]

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[MHProd]

Yes, I still get the noise when V1 and V2 are pulled with just V3 in. If i remove V3 then the noise is gone.

It is strange as I haven't touch anything upstream or within the PI.

Can we say that since the power tubes bias correctly, the B+ is OK and I have no noise, the OT CT and choke connections are OK?

I'm confused because all I did was to relocate the OT CT and one side of the choke which were originally connected to the VVR along with the bias supply...

[Littlewyan]

Where are they relocated to? Have you checked it against the schematic? I find the best way is to go through the amp whilst following the schematic. Then you'll know everything is hooked up ok.

[MHProd]

As per original schematic. To one side of the mains 32uF.

Should I take measurements without the power tubes but with the preamp tubes in?

[Littlewyan]

Have you accidently shorted the choke out maybe? That would explain it.

[MHProd]

So, without the power tubes I get the following:

B+ 470V measured at the mains 32uF where the OT CT and one side of the choke are:

V1:
1. 229
3. 1.8
6. 233
8. 1.8

V2:
1. 189
6. 340
7. 189
8. 191

V3:
1. 259
3. 44
6. 253
8. 44

Looks within specs I believe.

So,I guess something is definitely wrong in the communication between the power amp and the PI.

Not sure about the choke since I only moved one side of it and it went to the right spot. Checked the soldering which looked fine. Unsoldered and resoldered just to be sure.

[Littlewyan]

Measure the resistance between the two ends of the choke. Can you also measure the voltage on the otherside of the choke?

[Stevem]

At this point I would unhook both ends of the transformers B+ feeding the rectos and with all the tubes out and a 1/2 amp fast blow fuse in place fire it up and see if the fuse holds to at least rule out the PT!