Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Zelbrek on January 13, 2020, 11:22:46 am
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Hi all:
I scored a couple of OPT's from a Rowe jukebox of fairly recent vintage (it played CD's, not vinyl records). I know the original circuit used beefy transistors to deliver 250W into two 8-Ohm speakers, and the OPT's are some heavy metal (heh-heh).... If I could determine the output impedance of the original circuit, would that allow to me to figure out whether the OPT's would "match up" with a potentially powerful tube-based amp circuit?
I have schematics for the Rowe unit, but am too dumb to decipher them to calculate what that output impedance is/was. Is there a general rule of thumb, or shortcut, that I could use for a quick go/no-go assessment, or do I need to get out my rusty slide rule and actually think? Put another way, does anybody know of tube-based circuits that successfully employed these bulky monsters?
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I have schematics for the Rowe unit
if you could post the output section with tranny that would be great, typically S.S. parts isn't very tube friendly
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Usually OT for SS circuit has a low primary impedance if a "standard" 4-8-16 ohm speaker is connected to the secondary
or, if you prefer, they have a low turn ratio between primary and secondary windings
Franco
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Here's some articles with data. I haven't read them carefully. It seems that the output transistors need some manner of coupling the the speakers. OTP's are one, seldom used, solution. However because of the impedenances involved, I doubt that the SS OTP would suit output tubes - unless enough tubes were used to reduce output impedance to SS levels. IOW an OTP-less tube amp could use this OTP!
https://www.passdiy.com/project/articles/arch-nemesis (https://www.passdiy.com/project/articles/arch-nemesis)
https://www.passdiy.com/project/articles/arch-nemesis (https://www.passdiy.com/project/articles/arch-nemesis)
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Read this...
https://www.radioremembered.org/outimp.htm
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To shooter, kagliostro, jjailli, sluckey and others who may respond:
Many thanks. I had already made some voltage in/voltage out measurements using a crappy Chinese auto-former and equally crappy Chinese VOM. My results seemed so bizarre that I had to stop and question my entire methodology, and thus I posted my original query. I can already see, based on your detailed, thoughtful (and timely!) responses, that these OPT's won't be very useful in an amp that uses EL84's, 6V6's, or any other commonly-used output tubes.
I'm so worried about giving newbies ideas which might lead to trauma or death that I'm going to refrain from giving any specific information (schematics, voltage readings, etc.), and end this topic by stating it as a general rule that the SS jukebox transformers in question should be left alone, because you can easily buy a suitable new or used OPT for your tube-amp project, which has its specs clearly labeled, for less than the price of a good DMM, Variac or emergency-room visit... "Move along folks; nothing to see here."
But: I have recently started to figure out that the output tubes we guitar players know and love (6V6's, EL34's, KT88's etc.) represent just a tiny fraction of the gazillion different vacuum-tube devices that exist, and that an OPT that would be totally inappropriate for these tubes just might end up being useful to experimenters who know how to decipher a tube data sheet. I really am a serious Dumpster-diver who hates to see good stuff get crammed into landfills, and if I can find an inexpensive, readily-available vacuum tube that could be used with these hefty OPT's, I will post some (hopefully) thoughtful commentary.
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get crammed into landfills
I take mine to the local scrap yard, pennies a pound in your pocket and they recycle it for you :icon_biggrin:
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There are tubes like UL84
https://www.thetubestore.com/45b5-ul84 (https://www.thetubestore.com/45b5-ul84)
http://www.r-type.org/exhib/aai0038.htm (http://www.r-type.org/exhib/aai0038.htm)
http://www.r-type.org/pdfs/ul84.pdf (http://www.r-type.org/pdfs/ul84.pdf)
that are fine with low impedance load (UL84 class A SE with B+ 170V is fine with 2K load)
may be that putting in parallel some of this tubes you can use that OT (may be)
you must measure the turns ratio of the transformer to know if you can use it
K
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I see you got this sorted, but for lurkers:
...OPT's from a Rowe jukebox ... beefy transistors to deliver 250W into two 8-Ohm speakers.... would "match up" with a potentially powerful tube-based amp circuit? ... ... ...
A transistor can pull 60V 6A, about 10 Ohms.
A tube can pull 400V 0.2A, about 2,000 Ohms.
(Solid crystal conducts a LOT better than empty vacuum.)
I got a gearbox from a 140MPH motorcycle. I got a 10MPH tractor needs a gearbox. Will it work? Not a good fit.
You only got the OTs? The complete amplifier would have some use.
Are you sure of the "8 Ohms"? Some jukes supported multiple speakers all over a space with local volume control and low-cost wiring; 25V or 70V distribution. It is "possible" to work few-Watt tubes at 70V signal level. But you would need a stupid number of small tubes to justify a 250W OT.
Some combination of windings *might* transform 2 to 4 to 8 to 16, to get a better match to an odd-Ohms speaker. But almost nobody owns the 250 Watt tube amp which might need that. And using a re-matching transformer on an arbitrary transistor amp risks burn-up from the near-zero winding impedance for subsonics and DC error.
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You only got the OTs? The complete amplifier would have some use.
A year or so ago, I was riding my bicycle to the liquor grocery store and saw an abandoned jukebox in a vacant lot. After making my purchases, I sped home to grab some tools. Over the course of a few evenings, I stripped everything that I thought might be useful or valuable, and then basically did nothing. After reading your query, I just now looked through the stuff I kept, and AFAIK there still remain the workings of a powerful SS amplifier and a couple of low-end 8-Ohm 12" speakers (the magnets don't look very bulky IMHO).
If I were in the market for a home stereo set-up, I might be able to use the parts, but since all I want is a guitar amp I'm calling this a waste of time. All I got were cockroach egg casings, moldy mouse turds and busted knuckles. Hanta virus anyone? And yet: The speakers might(?) be useful in my dream 5W-10W homebrew amp... As everybody knows, them Celestions don't come cheap, and for good reason.
Getting back to my original post, let me say that I did read through the various articles and datasheets you all were kind enough to provide. There is a lot of good information for the experimenters out there, but I ain't one. And if anybody wants a clear explanation of exactly why an OPT from a SS amp wouldn't work with a tube amp, look no further than the thoughtful and incisive posts above.