Different AC ripple on either side of preamp coupling cap

[Blair]

I have a preamp I'm working on currently. I measure 1.2mV AC on the output of the supply which is feeding the plates of a 6CG7 SRPP circuit. I have ~6.1V on my cathodes & 300V on my plates.

On the output cathode side of my 6CG7 tubes, I measure about the same 1.2mV AC ripple. On the other side of the coupling cap, I'm getting 4X that or 3-5mV. Any ideas?

I've posted something like this before, but turned out it was in the power supply.

Thanks for any ideas!

Blair

[kheper]

The cap may be picking up AC hum or RF interference. You may want to switch to DC on the fils, shorten the leads on the cap or whatever.

[DeathRex]

Gosh it's kinda hard to measure 1.2mv. Try an Aikido, the purpose of the second stage is to get rid of the 1.2mv.
Post a schematic.

[Geek]

What's your psu schema look like

[Blair]

Hi guys,

It's a standard 6SN7 SRPP schematic with 1.5K cathode resistors. I added 1.8K stoppers on all grids. 1uF coupling caps on the output and 475K to ground after that. Sorry, I'm on a phone. My computer locked up:(

Anyway, the PS is a 6BW4 into 22uF -- 20H choke -- 68uf -- 5.6K -- 60uf

I moved things around and floated my filaments and now, I get a (muted input) .5mV settled on each channel into a 30K active load (cheapie receiver).

I have never been able to make a super quiet, tube rectified preamp.

One thing for sure. The NOS power transformer definitely puts out mechanical hum. That could be some of it.

[Geek]

Hmmm, have you lifted the heaters or DC supplied them?

Better to cure the cause than mask the problem. Aikido is really a "fudge" than a fix, IMO

Cheers!

[Blair]

Hi Gregg,

Yes, the heaters for the 8CG7s are sitting at 100V reference and the plates of the top tubes are at 309V.

Once my B+ settles, with my meter set on AC mV, it is at .5mV AC ripple, so there appears to be no real loss from the PS to the outputs now. You think adding another small cap section of 3-5uF may help right by the plates?

Thanks!

Blair

[Geek]

Heya,

500uV of ripple on the B+ line is pretty good

So, the tubes are picking up ripple from somewheres.

- The lifted heaters, are they bypassed for AC with a 2.2-10uF cap to ground? That one bit me on a repair job! It hummed like crazy, but all voltages were good. Turned out to be a dry heater lift bypass cap

- Hummy tube (large h-k leakage). Westinghouse and Admiral brands are insane for that Try a different brand if available.

- Wiring layout.

- Ground loop.

Cheers!

[DeathRex]

I was just looking into this yesterday. Once you elevate the filaments, what size cap do you need on the DC going to the filaments? Most I've seen is 10uf.

[Blair]

I usually use 1.2uf ploys there because they are cheap at surplus.

I moved things around and got between .3 and .5mV on the outputs.



Not sure if that is good enough or not. He has 80s model KG4 speakers. 94db. Mine are 91db. I can hear the power transformer over the audible hum in the speakers. That's the worst part about old stock. You never know what it's going to do under load:(