60 Watts A Channel 6528A Tube Amp

[fooeywuffle]

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?

[SY]

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.

[bigwill]

That's the longest transformer I've ever seen

[fooeywuffle]

I have made a lot of those long ones so it would fit with the others at the same height. Check my website.

[kevinkr]

Originally Posted by SY

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.

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

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

[fooeywuffle]

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.

[fooeywuffle]

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?

[SY]

See my post in the 6528 se thread.

[kevinkr]

Originally Posted by fooeywuffle

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?
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.

[fooeywuffle]

I think they were eaten by snakes! The cross thing would tend to neutralize it capacitance wise. The first amp I made using one tube per side didn't do it. Neither did the big one using paralleled 6336.
T.E.C. TRIODE AMPS
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