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Old 16th March 2006, 02:30 AM   #1
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Default Two Stereo Tube Amps Into Monoblocks

I have two non-working UL or PP stereo tube amps - EL37-based Knight KB85 models. They both need to be gone over and I'm trying to decide if I should try to keep them identical with an eye towards using them as monoblocks.

What's the scoop on that? People must have done it with Dyna ST70, etc. How well does a stereo amp sound strapped for mono? COnvert to triode and then strap?

Also these will never be stock-looking because Knight used such crappy thin boxes for their other wise great products, so I also wouldn't mind changing the layout. Any ideas on other circuits? I suppose one day EL37s will be gone . . .

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Old 16th March 2006, 12:50 PM   #2
SHiFTY is offline SHiFTY  New Zealand
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If they are both non-working I would suggest you build one good one out of the two, and keep one for spares.

The problem with bridging amps is that you have just far too much stuff in the signal path for it to ever sound good... Unless they have that option to bridge the outputs then it is also quite hard because you have to have one channel out of phase with the other, then take the outputs from the two positive terminals.

BTW EL37 is quite rare, I would keep that in mind. They should create a good amount of power without bridging.

schematics here (scroll down):
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Old 16th March 2006, 06:50 PM   #3
kevinkr is offline kevinkr  United States
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Actually strapping identical stereo tube amps to mono is usually done by connecting the inputs and outputs in parallel. (See old dyna stereo amplifiers as an example) For an 8 ohm load you would select the 16 ohm taps in parallel.

BTL is used mainly in solid state amplifiers. (Bridge tied load which is differential output)

See here for a discussion of this with a little analysis: Take a look at what "Bear" has to say as this is exactly correct.

Incidentally Shifty is right about the sound quality issues, if the two channels are not absolutely identical they tend to fight each and this results in degraded linearity. I have never heard a strapped amplifier sound quite as good as just one channel by itself.

EL37 is basically equivalent to the 6L6GC imho.. It is rather rare now, and if you have a good spare quartet you can get enough money on eBay to cover most of the cost for new chassis and the many new parts required to build a pair of monoblocks.
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Old 22nd March 2006, 08:39 PM   #4
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Thanks for your replies. It's not a very good indictment of tube amps if you have to worry that the two channels together will be too distored, is it?

I have some other EL37 amps that actually work - 2 fisher 100 monos and 2 fisher 80 az monos, plus a great little fisher 500 integrated, so I'll be able swap the 10 EL37s that I do have around for at least 5-10 more years.

I want to do something with the Knights - using one as a parts unit seems cruel. There are all the parts available still - you may just have to burn your own PCB or go to point-to-point wiring.

But I have a Knight stereo EL34-based integrated that is a monster with DC preamp, dual GZ34 recifiers, nice trannies, etc. (but, again really weak sheet metal. It sounds fantasic. Blows me dynas out of the water kb-85s must sound great. (But not as sweet for vocals as my EL84 fisher intigrated.) This leads be to believe the KB-85 is a even sweeter sounding machine.

Okay I'm rambling, sorry. SO how about using amps for surround-sound? Okay, I just answered my question. I really only play vinyl, so that's out. I guess I should trade away the kb-85 projects and go listen to what I brung. Would trade these babies for SME, etc. vintage tonarm.

Sorry to waste your time. Broken delete key

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