Single Ended Hum

[Helmholtz]

To simulate the loading effect by the 33k NFB resistor with MV fully up I used "TSC in the Web" (great tool, allows to overlay plots).
With tone controls in middle position I see a drop of grid signal by around 12dB corresponding to a factor of 4.
Also less bass and significantly increased treble.

The lowered grid signal might not allow the EL84 to achieve its full output of maybe 5W.

The NFB itself will have little effect because power stage gain is only around 1.5.

[leadfootdriver]

Changed a couple things.. Added 2.5K dropping resistors and lower the Power Tubes Cap to 22uf..
Drew up a power string. VDC is with Tubes in..

[Chuck H]

What is the voltage on top of your power tube cathode resistor?

[leadfootdriver]

Trying to figure out how I came up with 150R for the Cathode 10+ years ago..
Measured CV for you but noticed that it changed as I cranked the Master Volume..
Set to 0 I got 7.4dc. Cranked, I got 13.3dc. Is this a byproduct of my custom NFB? LOL

I'm fully prepared to scrap this design.. I'll think I figured out a way to fake what I'm trying to achieve with only 3 Gain Stages, but am ready to try again by adding a 3rd 12ax7 and maybe a single EL34 this time.. I think I can fit it all in there..

[Chuck H]

Measured CV for you but noticed that it changed as I cranked the Master Volume..
Set to 0 I got 7.4dc. Cranked, I got 13.3dc. Is this a byproduct of my custom NFB?

Hmmm...? That's odd. About the only thing "I" can think of that might cause that relative to your FB circuit would be that it is eliminated with the master at zero. Perhaps this is stopping an ultrasonic oscillation causing lower current draw with that setting.

I would expect the lower 7.4V figure for your circuit values and 13.3V seems high for 264Vp and a 150R cathode.

EDIT: If you have DCV on the el84 input grid that might explain the above behavior at settings. Please measure for DC at the el84 input grid.

[Helmholtz]

You can't simply use an EL34 with your transformers.

Still hoping for an answer to my question about lifting the 33k NFB resistor.

[leadfootdriver]

Yeah, I realize I'd need new Transformers.. I'm kinda flip flopping on that idea because I have a different platform I can use.. JCA50. Never really liked the way it sounds.. I've tweaked a few things and it's better, but the overall DNA is not my style.. I would gut this thing and use the transformer for maybe a Fryette Deliverance.



Lifting the 33K NFB makes it super bright as per my bypassing, and it gets louder; probably the volume it's supposed to be. Reconnecting it is almost like a comb filter, canceling out a lot of the brightness. It's fine for this little 8" Speaker. It's fine for practicing. It's a little buzz box and only has so much potential. I think it's better than it was stock! I'll just leave it as a snapshot in time.
Grid volts is 0 with MV at 0 and 11.73vcd with MV cranked.

[Helmholtz]

Grid volts is 0 with MV at 0 and 11.73vcd with MV cranked.

Is this positive DCV wrt ground?

If yes it looks like a leaky tone cap.

[leadfootdriver]

Not impossible because I burned everything! LOL
Thanks.

[Chuck H]

It's fine for practicing. It's a little buzz box and only has so much potential...

I'll just leave it as a snapshot in time.

Well you shouldn't be using it at all regardless if it's for practice or just a novelty because...

Grid volts is 0 with MV at 0 and 11.73vcd with MV cranked.

This indicates that the master volume at zero is grounding the voltage that SHOULD NOT be present at the el84 grid. Considering the voltage divisions happening I'd have to think you have either a fully shorted coupling capacitor or you have a mis wire connecting the HV rail where it shouldn't be. This is not only a concern about damage to the amp but a potentially dangerous situation for you. You seem to be waffling and unconcerned about whether to fix this amp or not. Which raises the question of why you posted about it here.?. But this is a safety issue. So if you ARE NOT going to work on the amp then resolve to never turn it on again and just gut it for parts whenever you get to another project. If you ARE going to work on the amp then we should get to the business of fixing it. Starting with tracing the DC voltage on your signal chain.

Please don't show me another amp and discuss potential projects anymore. You started a thread for THIS amp. THIS amp is still broken and potentially dangerous. Are you going to fix it or not?

[leadfootdriver]

The Bass Tone cap was definitely leaky.. I burned the crap out of it. Replaced it with my last one .. No more DC on the Grid.. Good Teching!

[Tom Phillips]

... If you ARE going to work on the amp then we should get to the business of fixing it. Starting with tracing the DC voltage on your signal chain...

The Bass Tone cap was definitely leaky...

This is also a reminder that it is futile to mod / tweak an amp that is not properly operational.

[leadfootdriver]

RIP 2003-2023

I started playing with Fixed Bias and it just couldn't take it anymore.. I thought I smelled something before it gave up the ghost..
The Bracket says Thermal Fuse built in.. Well, it must be buried in the Windings, rendering this thing a Paper Weight.
No worries.. I just wanted to see if I could get it running to have on the shelf in the future because you never know.

New PT and Choke Coil on the way along with more Tube Sockets so I can add another 12ax7.

[Helmholtz]

If you're still intending to use an EL34, consider that this would require a different OT too.

[leadfootdriver]

Thought about it, but it's a little 8" Combo. The Transformers would be to big to fit on the Chassis and in that little Cabinet..
Maybe if it was a 10 or 12 inch..