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I'm still working on this amp, it's all pretty much working except for the phono preamp. It's modeled after the Marantz 7, using 6N2Ps and a 6N3P. I had a bit too much gain, and too much hum, so I put in a bit of local feedback on the second amplifier. With that feedback in, I have about 3mVPP of hum out of the preamp. I am happy with that. Without the feedback and the phono hooked up, I'm getting 20mVPP of hum out of the preamp. The hum about drops in half with the phono disconnected from the amp, AC connected to the phono does not matter, and the ground wire is connected. The phono is a Techniques SL-BD22.
The preamp does work, and Eddie Howard sounds fine, but it hums between songs.
So no phono connected and feedback = 3mVPP
No phono connected and no feedback = 20mVPP
Phono connected and feedback = 20mVPP
Phono connected and no feedback = 30MvPP

Could be a few things... bad grounding, bum interconnects, oscillating tube, leaky heater-cathode, ground drain wire not making connection at the tonearm.

First, I'd rule out the table/interconnects as the cause.

The phono works fine on my Sansui SS amp. The filaments are DC, and when I remove filament power, the waveform doesn't change, well for about 6 seconds, then it goes away.

Probably bad filtering +U. If You have big gain, You can make filter CLCLC /with two chocks/.

The filtering is C-6CJ3-L-C-regulator-C. I can't measure any ripple with my oscope at 2mv.

The phono input has a direct connection to the star ground, and the phono amp has it's own direct connection to the star ground. I did try connecting them together with another ground, creating a ground loop, and got more hum.

Should I just have one ground from the star to the preamp then to the phono input?

Scope the PSU rails after the regulator?

I'll recheck when I get home.

Reg is against deviations of +U, but I doubt that it decrease ripples.....Don't forget, You have very big gain, and every very small noise will be amplified....

azazello wrote:Reg is against deviations of +U, but I doubt that it decrease ripples.....Don't forget, You have very big gain, and every very small noise will be amplified....

That's the tough part.

Fig 1: The upper wave is channel without feedback, lower with feedback no phono connected @ 10mv/div

Fig 2: Same with phono connected

Fig 3: B+ at 2mv/div

Fig 4: Oscope probe connected to ground, at the same place that it's grounded.

I whipped out the Eiko ST70 and it has 70mvpp hum on the phono preamp out. But there is no difference between having the phono connected or not.

I got the hum down to 3mVPP by unwinding the two channels and ground. I tried using shielded wire, but it made it worse. Now I have a black, white and red wire loose, but placement is key.

Check the ground in the wall.

I had to fix a popping sound from the relays, and I found I only had 4.5 volts to the phono filaments. I got a new resistor in 6.2 volts on the filaments, and the hum didn't change. I replaced the ground wire with a proper one, and hum is at 6mVPP, which I think is fine. I get 6mVPP hum and 2VPP music.
This one is very nearly done, I haven't seen the top of it in 3 months.
But now I have to find out why the Eico has so much hum.

I removed a bunch of alligator clips and soldered in the correct components. Then I removed the 2 200ma 2H chokes and put in a 400ma 0.6H choke. My AC ripple after the chokes going to the output tubes, went from 60mv to 117mv. At 4 volts of ripple I couldn't tell there was any more hum, so 117mv is fine. Going to the 6CG7 drivers, I have 7mv ripple.
My phono amp hum went from 6 to 3mv, by removing that too close choke.
I played Isolated System and Supremacy, and the amp got really hot, being upside down. It puts out a little over 300 watts of heat.