# Running tubes on low voltages by design

Discussion in '"Boat Anchor" & Classic Equipment' started by W5INC, Jun 30, 2019.

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A good friend who now lives with his family in Lafayette In and is a prof. at Purdue is a keyboardist but mainly collects Vintage Tube organs. We rebuilt many of his organ's Amp circuits, typical of the era.

So here is a schematic modified for and updated for the Allen:

Pheel

#### Attached Files:

• ###### Allen Organ PS.pdf
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112.3 KB
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T

TNX Mr. Pheel for the updated version of the Allen amp schematic. My schematic doesn't show any plate voltage for the 12AY7, but since the OD3s are in series @ 300V and then going across a 27K resistor for the 250V reading, it then goes thru a 100K resistor to feed the plate. With my bad ciphering skills the plate of the 12AY7 should be fed with about 50V it looks like. The cathode is being run at 1.4 volts, which after reading your previous post I can see why it is being done that way. Your friend must like these amps, so I guess that is a good thing. TNX agn Mr. Pheel.

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What I am doing is working with MatLab simulations and relying on the voltage measurements from the full schematic to be accurate at the various cathode and plate points.

With the tube characteristics inserted, the results of the simulation for the 12AY7 stage from the full schematic was as follows:

Vs = 250V (Supply voltage)
Vp = 160V (Plate Voltage pins 1 and 6)
RL = 100000 (calculated plate load resistance for Vs and Vp but matches schematic)
VKBIAS = 1.4V (Given)
Rk = 1.5k (Given)
Ik and Ip = 0.9 mA (from VKBIAS and Rk values given above)
AV12AY7 = 15.7143 or approx. 16.

So the dc plate voltage on each 12AY7 plate should be 160V assuming 0.9 mA of current through each tube and a 250V dc supply at each 100k plate resistor common feed point.

A 100 mV p-p AC sine wave voltage on the input should result in approx. 1.6V p-p at the plate.

The Vdrop across the 27k calculates out to be approx. 50 volts@1.8 mA. 300V - 50V = 250Vs.

Pheel

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Here are the Predicted Stage gains:

Pheel

#### Attached Files:

• ###### Allen Organ Amp Gain Stages.pdf
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Thank You Mr. Pheel for all of the great information. Very much appreciated!

New meters arrived today, so I have to go buy a new hole saw to make sure I get a nice clean cut for the meters. The metal is rather thick maybe 12 ga so I want no problems cutting it. I will be looking into the 6SN7 voltage problems this weekend and see if that can be cleared up. Seeing Light from an end of the tunnel is always a good thing.

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How big is that hole?
I'd consider a punch. Much easier!
Beg, borrow, or steal if you have to.

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A knockout punch, might do it. But the pilot hole will be pretty hefty for that size guy. The meter body is 1 1/4" in diameter, so the hole needs to be a tad larger. Easy to do with a round file for final fitting. There is enough room to mount the meters directly under the KT88s to keep the wiring very simple.

Last edited: Jul 12, 2019 at 4:49 AM
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I like Greenlee punches. PM sent.

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Nice meters.

Here is another schematic (suggested) for the output stage and I'll bow out for now.

Pheel

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182.9 KB
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Tnx Mr. Pheel for that most recent update. It will come in mighty handy to say the least.

Somebody in the past put their own circuit designs into these amps and it isn't looking good as resistor values are all over the board when compared to the schematic. There aren't many parts in this amp, so whatever pieces aren't in the original schematics or Mr. Phell's updates will be jettisoned. Since the amp is big getting to pieces won't be much of a problem either. I'm learning to navigate Mouser's website pretty good.