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Old 10th June 2008, 03:37 PM   #1
tikiroo is offline tikiroo  Australia
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Default Potential project for some inherited transformers

Hi, this is my first post on DiyAudio, although I've been lurking awhile.

I have a set of power and output transformers that I obtained from my grandfather some time ago and I want to put them to use rather than them gathering dust in the shed! They were originally used in a pair of Mullard 5-10's that he constructed in the early 1960's. I'm after a few suggestions for potential projects.

Transformers are:

2 x Partridge P4013 power transformers
300-0-300 @135mA, 2 x 6.3v ct@2A, 1 x 0-5-6.3 @2A

2 x Partridge P3667 output transformers
8k primary (with 6k taps), ??? secondary - there are two windings but the label has rubbed off

1 x Partridge P4014 output transformer (C-core)
8k primary with 43%UL taps, 4 secondary windings (16 ohm with all in series)

If I had two of the P4014 I would go for the Baby Huey (EL84) that seems to be quite popular, but I'm out of luck. I don't really want to use EL84's in pentode mode with the P3667 transformers, so what to do?

Would a triode connected EL34 PP be possible using the 8k P3667's? I should be able to get ~400V out of the transformer with solid state rectification (current might be marginal?), and the P3667's are rated for 15W.

Any suggestions are welcome This would be my first valve amp project, but I have built a few solid state amps.

Steve
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Old 10th June 2008, 04:58 PM   #2
kevinkr is offline kevinkr  United States
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I'm not sure about this, but I think the 6K primary tap could be used as a UL connection on the P3667 transformer. I was not able to find specific information on your transformers with a quick google, but this should work.

I don't think this transformer is suitable for use with EL34 because the plate currents for good performance with an EL34 are significantly higher than with the EL84, and 8K does not represent an optimum load for EL34 in triode connection either.

Triode connected EL84 will give you 7 - 8 watts output depending on operating point chosen, and IMO 7 watts of triode connected EL84 power sounds very good. (I have an amp based on Scott iron that is in this power range.)
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Old 11th June 2008, 01:25 AM   #3
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Steve,

The 6 KOhm taps could be used for UL connections, but the amount of local NFB applied to the screen grids will be different than that seen with the "typical" 40% UL tap. As I mull things over, it seems this UL connection will be very triode like. I expect a good deal less than 2X pure triode delivers.

"El Cheapo" circuitry requires approx. 360 V. of B+. You should be able to obtain that from the 300-0-300 winding using 5R4 rectification and CLC filtration. The pair of 6.3 V./2 A. filament windings will provide plenty of heater current for a 12AT7 splitter driver, an 'AL5 B- rectifier and a pair of 6V6 family O/P tubes.
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Old 11th June 2008, 09:05 AM   #4
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I can understand why you would want to put Partridge transformers to good use, they should be excellent.

I would agree with Kevin's view that the current needed to run EL34s could be too much for those OP trannies. I expect you can get excellent performance from EL84s in UL PP.
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Old 11th June 2008, 02:17 PM   #5
tikiroo is offline tikiroo  Australia
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Thanks for your responses guys, all very helpful. I had thought about using the 6k taps for UL mode but taps are effectively 85% (measured using ratio of winding resistances) rather than the standard 20% or 43%, too far away for me to be confident of using them in this manner. Triode connected EL84's would be less "risky" but only 5W - maybe parallel PP using the 6k taps might work (normal recommended load for a single triode connected pair being 10k)?

The PP EL34 triode mode I was thinking of was the "alternative" one mentioned in the Mullard data sheets and in "Mullard circuits for audio amplifiers" (I have the second edition from 1960), where a 8-12k load is used with a 235 ohm shared cathode resistor (no bypass). Maximum power output is reduced a little from that achieved with the more usual 5k load, about 13W, but distortion is also reduced to <1%. I thought I might get away with this mode of operation given the output transformers seem fairly rugged (they weigh 3.2 pounds each).

I suppose if all else fails I could consider pentode operation, using some of the more modern (compared to the 5-10) circuits floating around.
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Old 11th June 2008, 03:07 PM   #6
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by tikiroo
Thanks for your responses guys, all very helpful. I had thought about using the 6k taps for UL mode but taps are effectively 85% (measured using ratio of winding resistances) rather than the standard 20% or 43%, too far away for me to be confident of using them in this manner. Triode connected EL84's would be less "risky" but only 5W - maybe parallel PP using the 6k taps might work (normal recommended load for a single triode connected pair being 10k)?

The PP EL34 triode mode I was thinking of was the "alternative" one mentioned in the Mullard data sheets and in "Mullard circuits for audio amplifiers" (I have the second edition from 1960), where a 8-12k load is used with a 235 ohm shared cathode resistor (no bypass). Maximum power output is reduced a little from that achieved with the more usual 5k load, about 13W, but distortion is also reduced to <1%. I thought I might get away with this mode of operation given the output transformers seem fairly rugged (they weigh 3.2 pounds each).

I suppose if all else fails I could consider pentode operation, using some of the more modern (compared to the 5-10) circuits floating around.

There's no risk at all, running at the 15% tap will simply give you something just a little closer to triode mode operation than the oft recommended 20%. (EL84 only, don't do this with the alternative circuit you mentioned above.)

You might get away with the alternative configuration you describe as the combined plate current will be in the vicinity of 70mA which is in the same territory as is typical for a conservatively run pair of EL84.
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Old 11th June 2008, 07:51 PM   #7
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Hi,
If you went for the Baby Huey with pentode mode outputs you can expect the output impedance to be adequately low. The Plate to Plate feedback does an excellent job of lowering output impedance to levels comparable to a cathode follower (hard to believe but true). Don't know exactly what output impdance the Baby Huey claims, but I have used this approach in another amp and it works fine.

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Old 12th June 2008, 12:02 AM   #8
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Steve,

The "Baby Huey" idea is excellent. Gingertube, who did the design work, is a fellow "Aussie". Comparative geographic closeness can't hurt.

Another possibility is a triode wired "El Cheapo Grande". Triode wired 7591s will work on those 6 Ohm taps. Run the "idle" current for a PP pair as "hot" as possible, consistent with O/P trafo limits. You will have to SS rectify the B+ in ECGs, using your power trafos. Also, combination bias is indicated on the PP pair to safely allow 330 KOhm grid leak resistors. That value keeps 12AT7 splitter driver gain up and works well with 100 nF. coupling caps. from the splitter.
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Old 12th June 2008, 12:51 PM   #9
tikiroo is offline tikiroo  Australia
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Quote:
Originally posted by Eli Duttman
Steve,

The "Baby Huey" idea is excellent. Gingertube, who did the design work, is a fellow "Aussie". Comparative geographic closeness can't hurt.

Another possibility is a triode wired "El Cheapo Grande". Triode wired 7591s will work on those 6 Ohm taps. Run the "idle" current for a PP pair as "hot" as possible, consistent with O/P trafo limits. You will have to SS rectify the B+ in ECGs, using your power trafos. Also, combination bias is indicated on the PP pair to safely allow 330 KOhm grid leak resistors. That value keeps 12AT7 splitter driver gain up and works well with 100 nF. coupling caps. from the splitter.
Eli, you've sold me on the "El Cheapo Grande", I get maximum use out of the power transformers and the sonic benefits of PP triodes. The 7591's look nicer than EL84's as well . I should be able to run the idle current at 50-60 mA. Just a couple of quick questions before I read the 58 pages of the El Cheapo thread:

-What do mean by "ECGs" when talking about solid state rectification?
- By combination bias do you mean a single cathode resistor, or something else?

Thanks for your help, also the others that have made suggestions.

Steve
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Old 13th June 2008, 01:09 AM   #10
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Quote:
What do mean by "ECGs" when talking about solid state rectification?
ECG = "El Cheapo Grande"

Quote:
By combination bias do you mean a single cathode resistor, or something else?
Combination bias is a method that uses both a RC network in the cathode circuitry and a negative supply in the grid circuitry. As the name suggests, it's a combination of "fixed" and cathode bias.

Combination bias has several virtues. The cathode RC network stabilizes the operating point, which allows the use of grid leak resistor values greater than that usually seen, when a negative bias supply is employed. Obtaining the bulk of the bias from a negative supply avoids "wasting" B+ rail Volts. Use a well matched pair of "finals" and a single, shared, RC cathode bias network. I recommend 100 Ohms and 470 μF. in the bias network. A single trim pot. serves to set the "idle" current in the PP pair.

-------------------------------------------------------------------------------------------------------

A B+ rail voltage of approx. 390 is to be expected from a 300-0-300 V. rectifier winding in combination with SS rectification. The 'T7 should have 200-220 V. on its anode and IB should be 3 mA. Losses in a "check valve" decoupling network are under 2 V. A total of 62 KOhms will load the 'T7 anodes. Obtain that total from a 50 KOhm inductive wirewound resistor and a 12 KOhm Carbon film part.

The power trafo has 3 filament windings. Phase up and connect in parallel the "indentical" pair. This parallel pair will energize the heaters of the 3 signal tubes and the heater of an indirectly heated vacuum rectifier that slows B- rise down. The 3rd filament winding will be voltage multiplied along the lines of the 2nd schematic shown here. A total of 6 multiplier stages will be used. Schottky diodes will be in the 1st 5 stages and a vacuum rectifier in the 6th stage. You will tap the multiplier after 3 stages to get the instant on C- bias supply. All 6 stages form the B- supply needed by the splitter/driver circuitry.

What sort of signal source(s) are planned for use with the ECG monoblocks?

Has consideration been given to volume control, channel balance, and source selection?

Do you have something like a 6X4, 6X5, or 7Y4 in your parts pile?

BTW, 7591 screen grids exhibit some toughness. So, use a 180 Ohm Carbon composition resistor to tie each of the 2X g2 connections to the plate.
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