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Old 31st December 2009, 10:24 PM   #1
martyh is offline martyh  United States
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Default Ballantyne output Iron

I have some Ballantyne T-25 amplifiers that I salvaged a few years back. They are kind of a funky two amps in one deal with an se 6bq5 monitor amp and a PP 7027 amp on the same chassis. Anyway, I am wondering if anyone knows about the quality of the iron in these amplifiers. The outputs for the 7027s have a pretty large core for a low power amp and are marked

46570
23423

Any experience with these or info on who made them?

Thanks,
Marty
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Old 1st January 2010, 12:38 AM   #2
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Marty,

You already know enough to make something good out of the trafos used with 7027s. The 7027 is a member of the 6L6 clan. Use SED (St. Petersburg) 6L6GC "finals" and Mullard style circuitry. Unlike Williamson style circuitry, Mullard style works quite well with so/so "iron". If the trafos are "super/duper", so much the better.
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Old 1st January 2010, 12:55 AM   #3
martyh is offline martyh  United States
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Thanks Eli,

I got a bunch of NOS 7027s on the same dumpster dive but I was thinking along the lines of class A PP 6b4g. Iím guessing that you are thinking of phase shift and feedback when you recommend a mullard rather than a Williamson but I donít want to use any around the transformer so hopefully they will work out.
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Old 1st January 2010, 02:10 AM   #4
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Marty,

Genuine NOS 7027s are money.

"Typical" 6L6 "iron" is 6.6 KOhms end to end, while 2A3/6B4G trafos are 5 KOhms end to end. Distortion and power O/P will go down, if the "iron" is mated to power triode "finals".

It's long odds the O/P trafos you salvaged were intended for use inside a GNFB loop. Most "vintage iron" was engineered that way. Sonic shortcomings, particularly at the frequency extremes, may plague you, should GNFB be omitted.

Since triodes are on your mind, an interesting possibility is triode wired 6L6GCs operating in (sic) Class "A2". Mullard style topology still gets used, but you buffer the LTP with MOSFET source followers DC coupled to the O/P tubes' control grids. Tubelab's Power Drive shows how it's done.
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Old 1st January 2010, 12:22 PM   #5
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There is no way to know by looking at the outside of a transformer how it will perform. No way around it, you have to measure it.

The secondary inductance is some indication of the lowest frequency it will handle. Technically you could measure all of the transformer's inductance, resistance and interwinding capacitance and get a picture that way. But in practice the best way is to run signal through it and see what it does.

The HF response usually will show a "bump" in the response, that is usually the max freq that the xfmr is going to work. If the bump is very large, that is not a good thing either.

Obviously, some xfmrs were wound to be more or less "PA" or speech only, and some were wound to be hi-fi...

edit: oh yeah, you know the amps that they came out of, you might be able to search and find the original specs... that would tell you a lot.



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