ST-70 rebuild concept?

[Blair]

Hi Guys,

I have two big power transformers from Baldwin organs, and 4 A-470 output transformers.

Originally, I was going to tie the primary and secondaries of each pair of A-470s together and run PPP EL-34, but I keep reading nasty things about paralleling this way. Valid? I don't know.

Anyway, I think what I am planning on doing is putting a single driver stage per amp into two separate PP EL34 amps. This will allow me to parallel the secondaries to make these mono, or I can have two identical amps driven by the exact same signal for Bi-Amping.

What would be a good driver stage for this project?

The only thing I cannot figure out is the feedback. How can this be accomplished using two output transformers, but one front end?

Thanks!

Blair

[EWBrown]

I've never attempted to venture into this territory, but might it be better to run each A470 with its own pair of EL34s, then just connect the two OPTs' secondaries in parallel

THis is akin to the orignal ST70's mono/stereo switch selection, but with a bit of a new twist:

The sets of EL34s could be driven by a single ST70 type (Use Shannon's ST70 driver, better than the original 7199 design) of driver circuit, just use individual interstage coupling caps and bias adjustment (or cathode resistor) for each power tube. NFB loop would still connect from OPT "high" 8 or 16 ohm, back to the VA stage's cathode, as in the original PP EL34 design.

Just a SWAG on my part...

/ed B

[Blair]

Right,

That's my thoughts also regarding the mono operation with the secondaries strapped for mono.

I was thinking about a Poseidon front end or similar with individual coupling caps.

I'll try to draw something up soon.

Thanks!

Blair

[kheper]

or I can have two identical amps driven by the exact same signal for Bi-Amping.

Blair

The bi-amping scenario is more appealing than the paralleling.

Depending on how much voltage and current the Baldwins can supply, I was thinking KT88s for the woofers and EL34s (or maybe 5881s) in triode mode for the mid-ranges and tweeters.

2 KT88s @ ~375V should do ~40W audio output, and 2 EL34s in triode mode @ ~375V should do ~13W.

[Blair]

I think my B+ is going to be closer to 450V.

I thought about that also. Using 6550 or KT88 in triode for bass duty. Will the A-470 handle the current for 6550s or KT88s?

Thanks!

Blair

[kheper]

Will the A-470 handle the current for 6550s or KT88s?

Blair

From the new, triode A470 specs, "This type of transformer can be used with many tube types, such as 6550, KT88, KT90". "Max DC per side (suggested) 80 ma". Might need to under-bias them. I think A470s can do better than 80ma per side, though.

[rmyauck]

Why not if you can use each ST-70 for each speaker so you will have if your speakers allow one channel driving the lows and the other the Hi's. Bob Latino suggested it on his form as better than one stereo amp driving the lows and the other the highs because you get better separation between the channels.

You could also experiment with triode wiring one of the channels if you like that better also.

If you can do the above you would get rid of all the neg. parallel issues!

Just my 2 cents!

Regards!


Randy

[Blair]

Yes, that's the concept. Bi-Amp "monoblocks".

It is just a unique build. To hold me over until I can build something with a little more power.

Thanks!

Blair

[kheper]

Well, if the amps need to use identical output tubes, my idea is out. But, even if your mid-ranges and tweeters can handle ~35W, the result would sound unbalanced and unpleasant. Triode mode (for the mids and tweeters) + inline attenuators?

http://www.rothwellaudioproducts.co.uk/ ... ators.html

[Blair]

I plan to bi-amp by breaking my crossover at the woofer and midrange tweeter filters.

I'm not sure if it will sound too unbalanced.

[Blair]

How's this for a layout?

[battradio]

Hi Blair ,

I just rebuilt this baldwin for a friend and the B+ is closer to 415 volts than 450 , with 5u4GB in it .Solid state 440 would be about what you would get .

Mark

[Blair]

Hi Mark,

That's about right. I have 429V B+ with a single pair of el34s right now with SS rectification and a 25V Zener on the center tap to drop the B+.

These are also "bucked" with the 5V winding to drop it a little also.

Blair

[Blair]

Nice build BTW!

[Blair]

Or, does this layout work better? I've never used tube rectifiers in a build. This layout would allow for a few GU4s



Any benefit to tube rectification?

I kind of like the long "front to back" layout better.

Blair