Express clone noise on high notes

[Tubetwang]

markr14850

Beautifull work on these pictures...

[markr14850]

Hey Folks,

Thanks for all the suggestions. I really appreciate your help!

I've tried all the following:
- 33k grid stopper on v1 pin 2
- 33k grid stopper on v1 pin 7
- coax from volume to v1 pin 2
- relocated NFB wire
- move treble wire away from others
- routed the wire to v1 pin 6 clockwise around the socket

None of these had any effect on the noise.

I was wrong about the 1M grid ref resistor not being there. It was there, hiding under the big plastic input jack.

All coax is tied down only at one end - the end away from the tubes.

The only thing that seems to change the amount of the noise is if I mismatch the output impedance. With an 8 ohm speaker, if I select 4 ohms it gets worse. If I select 16 ohms, it gets better. Does this imply that the problem should be no earlier in the chain than the PI?

Thanks,
--Mark

[mlp-mx6]

What happens if you disconnect the NFB wire?

[markr14850]

> What happens if you disconnect the NFB wire?

No change in the noise.

[Fischerman]

I'm having some difficulty with the description of the noise...it's very difficult to describe a noise. But re-reading the op and seeing that nothing else helped...one thing you might try (just to see if it even addresses the problem) would be to put a grid stopper on the grid of V2 (the third stage)...a big one...like 100k. IIRC, I had something similar to what you describe during some tweaking but that was a Komet or Concorde pre...that third stage was just getting hit too hard.

[markr14850]

I tried a grid stopper on V2. No change.

Someone locally suggested that it could be "ghost notes" caused by insufficient/faulty power supply filtering. (Does that make sense? I only notice the noise on very high notes.)

I'm going to replace all the PS caps and see if that does anything.

Thanks for all the ideas everyone!

--Mark

[billyz]

I noticed your Heater leads to the power tubes runs right under the output jacks. Might try moving them to the inside of the chassis.

[keithrick]

I noticed your Heater leads to the power tubes runs right under the output jacks. Might try moving them to the inside of the chassis.

Also, I do not see if you have a center tap or fake center tap for your heater leads. If not, you need to add the 100 ohm resistors to ground from the first powertube as in the Franchesca pics.

You could also lift those diodes on the power tubes off of the heater leads

Good luck!

[WhopperPlate]

Thread revival! Did the replacement ps caps do the trick?

Thanks

Charlie

[markr14850]

Nope. Never did figure that one out. Just moved on.

[Richie]

jurgen,
I'm just guessing because I can't really see it but if you look at the ground buss right behind where the grid wires connect to the board, there is what appears to be the ground connection for that shield.

I used shielded for the input wire, the wire from the volume to the second stage, and for my power tube grid wires (I used the 2-conductor+shield just like this amp did) and my amp isn't microphonic and it doesn't appear to be oscillating (nothing audible and the amp puts out full power)...and it's very quiet regarding noise/hum. I also used a 22k grid stopper on the input wire (although it wasn't microphonic/oscillating without it...there was just so much gain and brightness I felt it was needed). But it's never sounded that great so...whatever.

What OT did the amp use?