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60 Watts A Channel 6528A Tube Amp

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I started this project a few months ago. Wound the transformers and the choke, made the black metal frame so the heavy things won't bend the chassis. I haven't seen much about building amps with 6528A tubes. I have had good luck with them for higher power amps. Maybe someone has comments about this?
 

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My apologies to Kevinkr, I accidentally screwed up his response.

In any case, it's important to note the B+ restrictions (400V), the mandatory warmup before applying B+ (at least 30 seconds), thermal management (there's 30W in heaters alone! See Morgan Jones's solution in his Scrapbook Challenge amp), and getting the load correct.

Kevin said all this better than I did.:D
 
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My apologies to Kevinkr, I accidentally screwed up his response.

In any case, it's important to note the B+ restrictions (400V), the mandatory warmup before applying B+ (at least 30 seconds), thermal management (there's 30W in heaters alone! See Morgan Jones's solution in his Scrapbook Challenge amp), and getting the load correct.

Kevin said all this better than I did.:D


My response seems to have evaporated, it was indeed #2.. :eek: :D

One thing I do remember mentioning is that the plate curves look rather similar to the 6AS7G which has a rather wide swing in transconductance across its plate curves, meaning it is not very linear, should be OK in PP, but may not be the greatest choice for an SE amplifier. (look at how the distance between curves varies across the range)

I believe both VTV and Glass Audio (or one of its compendiums) featured amplifiers based on 6528s.

Here's the link I originally posted: www.radiolamp.su/pdf/115/6/6528.pdf
 
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Rectifier filter board. I think almost all series pass tubes have similar curves, 6080, 6AS7, 6528A, and 6336A and B, which I built a 120 watts a channel amp with. I have never made any amps with delayed B+ except the one I used TV damper diodes 6CT3. All of the amps I made are still going with no problems for 8 years. SE I would use delay.
 

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I just finished this amp using the 6528 tubes, 2 per channel push pull. It didn't work. Single tube works fine, but just paralleling the 2 triodes caused it to oscillate at a very high frequency. Finally I crossed connected the triodes in each tube and that works fine. Anybody know why this happened?

Where oh where have your grid-stoppers gone, oh where oh where can they be? :D
Seriously there is no reason I can think of why the connection approach taken shouldn't be completely agnostic, and I am assuming as drawn that you didn't use grid stopper resistors.. These are a must with relatively high transconductance types as they will otherwise oscillate and usually at inconveniently high frequencies where it isn't always obvious.

SY's comments about inadvertent neutralization in the 6528SE thread are probably relevant. Anyway installing 1K stopper resistors right at the grids should kill it.
 
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.
 
It is clear that these transformers always are heavy and expensive because they are difficult to make. We tried bifilar but that didn't work either. I wish some guru told me how to wind something like this. Interleaving also didnt do the trick.
Next try might be a 4 division bobbin, as few layers as possible but 'heaping up" the wire to minimize capacitance, B+ in the middle, bias at the outside so cold ends of the windings are close to the core and hot ends are close together. We'll see.
This is meant for an ECC99 driving 6528.
Steven
 
The reason for the high freq. oscillation is two fold a .6 pf plate to plate capacitance and stated min grid stop is 500 ohm pre grid . By swapping section in the tubes the coupling is inverted braking the loop gain that made it oscillate . the transformer load may well provide the grid stop load need. Sorry if I came lat to the party . Where are you finding 6528 at a reasonable price ? Given the coated graphite plates if keep under 30 watts will last a very very long time.
 
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