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Old 28th July 2012, 03:19 PM   #21
7N7 is offline 7N7  United Kingdom
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About ten years ago I built an experimental amplifier using a single 6528 in push-pull. It worked quite nicely. It was all discussed on this thread here. Note it originally was planned for a quartet of 6CK4s; this was no good so it was converted to 6528A.

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Old 29th July 2012, 04:33 PM   #22
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Not that heavy. Most I have made are about the size of a 10 watt output one. The one in this amp is a lot smaller because less voltage drive needed. Maybe 4 watt size? Wire size is #38. I made the 4 division bobbin ones also for my 40 watt and 120 watt a channel ones. These were the 10 watt size for VV52 and 6336A. Any tube with gain of around 20 works fine. It has to be scramble wound on the last 1/3 of winding on hot end to work right. If you don't you can use a lower gain triode to drive it harder. The cap across the windings, plate to grid has to be there if not bifilar. See my amp photo's. All of them I have made have worked fine. But I have been winding transformers for over 40 years.
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Seems strange I never ran into oscillating outputs before since I have been messing with the stuff since the 1960s. None of the other amps ever did it. I was going to use 6CK4 also, but never could get enough of them to get both a match on specs and the tube shape. But they are only good for around 12 watts a pair. 6EW7 was what I finally used. I haven't bought any 6528 tubes for years. I have some extras for sale. I got them all off of eBay. Seems those days are long gone. The amp puts out a lot of heat, but none of the plates have any red at all.
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Old 29th July 2012, 07:03 PM   #23
7N7 is offline 7N7  United Kingdom
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Quote:
Originally Posted by fooeywuffle View Post
[...]
Seems strange I never ran into oscillating outputs before since I have been messing with the stuff since the 1960s. None of the other amps ever did it. I was going to use 6CK4 also, but never could get enough of them to get both a match on specs and the tube shape. But they are only good for around 12 watts a pair. 6EW7 was what I finally used. I haven't bought any 6528 tubes for years. I have some extras for sale. I got them all off of eBay. Seems those days are long gone. The amp puts out a lot of heat, but none of the plates have any red at all.
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I recall that I managed to match up a set of 6CK4s, but they are poor things really; I tested them for distortion and they didn't perform terribly well.
6EW7 on the other hand is an excellent valves; I spent a lot of time testing these and in differential pairs they were excellent.

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Old 22nd December 2012, 04:43 PM   #24
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I had a lot of 6CK4s. I matched them up, but never got 2 that looked alike, so gave up on them. The larger bottle 6EW7s worked great.
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Old 1st October 2013, 02:51 AM   #25
byang is offline byang  United States
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My back was ruined many years ago. I can still move it but hard for me now. The driver transformers have around 3000 turns on each of the 4 windings. They are wound with the first 2000 turns just spindled on, the last 1000 turns are scramble wound. It is split bobbin. Start of winding is B+ so finish is in the middle. Second coil wound over first is wound in the opposite direction and finish is bias supply. So there are 2 sets of coils. The 2 coil sets are wound in opposite directions. The .1 Uf cap is across the plate and grid connections. No cap and the thing resonates secondary coil at around 1 Kc. No way to eliminate this cap unless the thing is wound bifilar. Or interleaved which is a real pain. I tried it and results weren't that good. Also made it larger to fit in all of the insulation. Bifilar takes triple or quad insulated magnet wire which seems impossible to get. If this is confusing, I can draw up diagram of it. Seems expensive commercially made ones also use the added capacitor.
I have built a few 6528/6336 amps
push pull 6336 amps are ok. While 6528 sounds bad in pushpull mode. I ordered 2 section output transformers to built a parallel single ended 6528 amp and suprisely, it is my best sounding amp! Keys for success:transform drive(600 to 10k ),no driver stage. Use ccs at cathodes to auto bias the tubes.
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Old 3rd October 2013, 03:48 PM   #26
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I noticed no real difference between the 6336 and 6528 sound, but the 6336 sure is harder to drive needing high voltage.
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