New build New question

studiodunn · Post by **studiodunn** » Mon Dec 09, 2024 8:59 pm

*New pics Below*

[original post]
I Just finished this Express build and everything was great from the first firing....well, almost everything.

Plate to cathode is 401v, with -33v @ bias. When i try to bias el34s to around 40ma with the MV opened up, the amp gets unhappy. rolling the MV back stops the problem.

This is a pretty straight copy of Kelly, with a few tweaks from other variants but nothing I would consider out of spec.

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https://youtube.com/shorts/7Sx81KkdQl8?feature=share

YouTube · Post by **xtian** » Mon Dec 09, 2024 10:06 pm

Disconnect NFB loop from OT tap. How does this affect the issue?

studiodunn · Post by **studiodunn** » Mon Dec 09, 2024 10:37 pm

xtian wrote: ↑Mon Dec 09, 2024 10:06 pm Disconnect NFB loop from OT tap. How does this affect the issue?

That makes the issue come on faster, so worst

YouTube · Post by **xtian** » Mon Dec 09, 2024 10:49 pm

So you've got some major instability, I guess, and since you've got a non-standard layout, lead dress might be the issue. Oh yeah, I see one major no-no: your green OT primary (output tube socket pin 3) is touching the phase inverter grid wire (pin 7, white wire).

studiodunn · Post by **studiodunn** » Mon Dec 09, 2024 11:36 pm

xtian wrote: ↑Mon Dec 09, 2024 10:49 pm So you've got some major instability, I guess, and since you've got a non-standard layout, lead dress might be the issue. Oh yeah, I see one major no-no: your green OT primary (output tube socket pin 3) is touching the phase inverter grid wire (pin 7, white wire).

Thanks xtian!

studiodunn · Post by **studiodunn** » Tue Dec 10, 2024 6:10 am

This is a layout mistake on my part. The original layout did not have a MV and the grid stoppers were on the board. I put the ppimv in and forgot to add new resistors off board. Dumb mistake on my part.

studiodunn · Post by **studiodunn** » Tue Jan 07, 2025 5:39 pm

Got the amp/cabinet finished and she screams. I put the HV windings on a switch and love running a pair of 6k6's in it. They give up a very vintage growl that I love in this circuit.
I added a 6 position switch (shift) that chooses the TS coupling cap 10p through 1000p. This does a really cool job of shifting the mid frequency and letting the amp be cleaner and fatter.

Cheers and Happy New Years to Y'all.

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Littlewyan · Post by **Littlewyan** » Tue Jan 07, 2025 8:52 pm

Looks really cool! Nice work! Love the cabinet.

statorvane · Post by **statorvane** » Wed Jan 08, 2025 9:13 am

Looks really cool! Nice work! Love the cabinet.

+1

studiodunn · Post by **studiodunn** » Thu Jan 16, 2025 6:52 pm

Quick question for the masses - has anyone experienced there (in this case Express) amp just sounding better with an impedance mismatch?

I have been playing the amp allot through several cabinets and plugged into the cabinet I had mainly been playing the amp through, and it just sounded better. It's a pure 64 ported 1x12 loaded with a 12K-85.
The amp sounded so much better, i looked to see what might have changed and sure enough, the amp was set to 8ohm and the cab is 16. Is this just a combination of small variations in the circuit and parts that would make the mismatch work better?

The Amp Garage

New build New question

New build New question

Re: New build bias / PPIMV weirdness

Re: New build bias / PPIMV weirdness

Re: New build bias / PPIMV weirdness

Re: New build bias / PPIMV weirdness

Re: New build bias / PPIMV weirdness

Re: New build bias / PPIMV weirdness

Re: New build fixed and finished

Re: New build fixed and finished

Re: New build fixed and finished - NEW Question