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Shell 2020P revisited

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Plimpington2

Super Member
Folks,

Time once again to be burdened by my Shell 2020P “Manhasset” integrated amplifier on my bench off and on for 3 years now. I have gotten the line stage working great. The phono stage, on the other hand, has the most horrific hum imaginable. Almost ever effort to quiet it down has failed. I rebuilt the whole thing. I even ran a grounding bus to one point for the whole amp. No joy.

Each channel uses 1/2 of a 12AX7 and 1/2 of a 12AU7. RIAA is passive and of the feedback variety. For whatever reason, only the front tube is DC heated. This off the common cathode of the output tubes. Runs around 9V. I could adjust this a bit by changing out the cathode resister, but I hardly think that’s the issue.

The front tube IS the problem, I feel. It begins to hum when my finger gets close to it. Humming reduces, but still bad, when I touch the chassis. I am going to rebuild it again, and change the switching scheme so that the phono stage sits entirely in front of the source selector (right now, it kinda sits “between” it. It’s an odd layout). The phono input will go directly to the first grid of the phono stage (with suitable load resister to ground, of course).

AND, I’m going to rectify the heaters of both tubes. The question is, is there a realistic limit to how much current I can draw through the common cathode of the OP tubes? What is the practical consequence of pulling, say, 600ma through the cathodes rather than just 300ma? AND, just WHERE does this current come from anyway? OR, am I better off throwing a bridge and capacitor across the back end of the heater chain and hope for the best?
 
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If its humming when you get close to it, that's not the heater making 60 hz.

When they powered the filament with DC parallel to the cathode resistor of the output stage, the current comes from the B+ supply.

I'd start by removing the ground which you added.
Then check that the ac to the heaters is referenced to ground
 
the output tubes are only going to flow what they flow. There is no asking more of it. If its an EL84 design, 4 of those is going to idle around 150ma which is just right for one 12a*7 heater, or some number of them in parallel. The problem is the bias voltage also happens to be around a dozen volts, which means either one tube, or you do what Fisher and others did to run multiples and tickle some voltage back to the grid so the grid-cathode voltage is where it needs to be, but the cathode-ground voltage is enough to run your tube heaters. The problem there is that making the cathode voltage higher effectively makes the plate voltage lower which reduces output power.

you can apply the zener diode fix that @dcgillespie came up with for the Fisher stuff that uses this arrangement and probably eliminate the resistor that is in parallel with the tube heater. That ought to bump the heater voltage some and drop idle current on the output stage a bit as well.

if you could post the schematic again that would help
 
trainbuftony said:
If its humming when you get close to it, that's not the heater making 60 hz.

When they powered the filament with DC parallel to the cathode resistor of the output stage, the current comes from the B+ supply.

I'd start by removing the ground which you added.
Then check that the ac to the heaters is referenced to ground
Click to expand...

Well, now the whole phono stage is out. So, back to square one. I have a clean slate at the moment.

The AC to the second stage of the phono stage (12AU7) and to the line stage (a 12AX7) is the distal end of a long AC chain, grounded through a hum pot. So it has a ground reference.

Yes, very sensitive to my hand being close to it. Which is some sort of grounding issue. What frequency is that likely to be?
 
gadget73 said:
the output tubes are only going to flow what they flow. There is no asking more of it. If its an EL84 design, 4 of those is going to idle around 150ma which is just right for one 12a*7 heater, or some number of them in parallel. The problem is the bias voltage also happens to be around a dozen volts, which means either one tube, or you do what Fisher and others did to run multiples and tickle some voltage back to the grid so the grid-cathode voltage is where it needs to be, but the cathode-ground voltage is enough to run your tube heaters. The problem there is that making the cathode voltage higher effectively makes the plate voltage lower which reduces output power.

you can apply the zener diode fix that @dcgillespie came up with for the Fisher stuff that uses this arrangement and probably eliminate the resistor that is in parallel with the tube heater. That ought to bump the heater voltage some and drop idle current on the output stage a bit as well.

if you could post the schematic again that would help
Click to expand...

I’ve thought about that. But I’m ok with the heater voltage all else being equal. It’s the hum that’s killing me. I can’t see hanging another 12A*7 off the cathodes without having to bend over backwards with lots of other changes.

I think what I’m leaning towards is putting a bridge and a big cap off the end of the AC heater chain (after the pre-amp heater), and run both the phono heaters off that. And then pay very close attention to grounding.
 
Have you tried swapping the 12AX7 / 12AU7 tubes with other one's? It sounds like a microphonic tube maybe. :idea:

Since you have shown no pics, do any of these tubes need shields? :dunno:
 
dumping R67 for a 12v zener diode would get you more heater voltage on the 12ax7 and it ought to drop the current in the output stage at the same time.
 
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