Solid state transformers for tube amp use?

[MadMax]

I have a handful of old, solid-state power amps to strip for parts and was wondering about the transformers. For one thing, some of these have large power transformers, and I keep hearing that size equates to power handling capacity. In the one I have open now (a Bogen CHS-100A), the schematic shows that mains AC is stepped down to 55V AC. With no tubes to drive, why would these things be so robust? And the real question here is, can these things be used to power a small tube amp?
Also, most of these old PAs have multi-tap output transformers. Can they be used in tube amp applications or is there a  difference in input impedance that makes it impractical?
I just hate to waste such heavy iron, so any input is appreciated.

MadMax

[sluckey]

why would these things be so robust?

It's big because it's a 100 watt amp. That 55V transformer has to provide 2 amps of current just to make 100 watts. It's probably rated for about 4 amps.

can these things be used to power a small tube amp?

No. Same goes for the OT.

[MadMax]

Okay, I was sort of expecting the answer to to the power transformers to be no. I haven't seen any tube schematics for something that can run from that low voltage. My hope was that there might be a way to cash in some of that amperage for voltage (yeah, I know that sounds a lot like a good transformer, but I was hoping there was something solid-state that could make it happen).
Anyhow, I really hoped the OT might be adaptable, but then I don't have a good concept of the impedances of solid-state vs. tube circuits.
Hey, that's the beauty of being only half educated, there's a lot of hopefulness in my world!

MadMax

[shooter]

last time I scaped mine the scrapper paid something like 45 cents a pound, I had a couple hundred lbs worth

[MadMax]

Hey, ignoring the center tap for a second, why can't I wire this one backwards (input will be output and vice versa). Since it is stepping down 2:1, it will double voltage 1:2 the other way. If I only use it for 500 mA, it should work for ~250 DC won't it?

[sluckey]

120VAC applied to the secondary would give about 262VAC on the primary. Run that 262VAC thru a FWB and filter cap and you'll have about 370VDC. These are all unloaded voltages.

[Willabe]

I wouldn't put the wall's 120vac on a 55vac secondary.

The secondary wasn't made for that acv.

[MadMax]

Willabe, is this what you mean:

The original secondary was meant to push 2A @ 55VAC (110W). Since R=V/I, then R=55/2, so R=27.5 ohms.
Plug the secondary to 120VAC and since I=V/R, then I=120/27.5, so I=4.36A. Then 4.36A x 120v = 523Watts (more than 4 times its original power).

I haven't taken an actual resistance reading on either winding, but if the above is true, I see what you mean.

[Willabe]

Willabe, is this what you mean:

No, I mean that the transformer maker made that PT to receive a certain ACV to it's primary, so just because the current/wattage #'s seem ok, there's still the ACV fed to the flipped/flopped/reversed primary/secondary PT to consider.

 

[MadMax]

We have got to be saying the same thing. If you are really NOT saying its a power handling issue, then please be specific so I can learn something from this.
A winding is just a piece of wire, its length and guage determine its resistance, but ultimately its protective coating determines how much heat it will tolerate before shorting.

So, if we are saying the power problem I described in my last post IS the issue, can it be solved with a series resistor on the primary side to reduce current flow and bring it back down to 110 +/- Watts?

[Willabe]

If you are really NOT saying its a power handling issue, then please be specific so I can learn something from this.

I thought I was clear;

No, I mean that the transformer maker made that PT to receive a certain ACV to it's primary, so just because the current/wattage #'s seem ok, there's still the ACV fed to the flipped/flopped/reversed primary/secondary PT to consider.

A winding is just a piece of wire,

No it's not, it also is wound around an iron core.

There's more to a PT than power handling. What about the wire voltage rating and the iron uesd? There's much more involved in the design of a PT, transformer engineers have a long list of things to check off when designing a transformer. 

There's core saturation to be considered too. And they do that when they design a PT for a specific primary acv/current and secondary acv/current   

Let's take a more drastic situation, 120acv/6.3acv PT.

The PT company did not build that PT to have 120acv applied to the 6.3acv secondary. So even if the PT can handle the power can it handle the ACV you want to apply it? 

[shooter]

I tried what you're talking about with a EU printer 220vac.  I had a old PT rated 55v 5A on the sec, 120vac primary.  2:1, I figured I had 2.5A to play with and the printer only needed .5A.  It worked, sorta, the printer functioned but the tranny hummed pretty bad, got hot to the touch, and the EMF from the hum knocked out my little mp3 radio.  I don't know the math/science why, just the practical results.  Even tried another tranny, same. 

[sluckey]

I've never seen any iron from a SS amp that could be used in a tube amp. Sure, you can jury rig some hodge-podge arrangement that may work, but is it reliable? Not in my opinion.  And why??? Just because you can reverse primary and secondary of a transformer in your mind does not mean it's a good thing to do. I'd use those transformers as they were meant to be used or tie a rope on them and use them for a boat anchor.

[PRR]

If you put "too much" voltage on a transformer winding, you get too much magnetic flux in the iron, and it doesn't do its job. Typically "hummed pretty bad, got hot to the touch", and ultimately burns-up. (Also bad for your electric bill.)

55VAC makes 77VDC. A Doubler gives 154VDC. Run 50L6 for a couple Watts? A Quadrupler makes 300VDC. This is really plenty for 6V6. But who wants a hundred Watts of 6V6 (a dozen bottles)? If it was 2A at 77VDC, it approaches 0.5A at 300V DC. An Octupler approaches 600V @ 0.25A, which is good for 100 Watts of 6550/KT88, but WHAT a clot of capacitors!! The large number does not leverage into bulk-buy, because you need several different V+uFd sizes.

What was wrong with the amp? A  CHS-100A is an excellent amplifier and probably more reliable than most tube amps.

[MadMax]

I sure appreciate those who've weighed in on this. I knew if a transformer from a 100W solid-state could power a 100W tube amp everyone would be doing it. I was hopeful that I could power a 10W tube amp with it though. But I am heeding your advice and giving that thought up. Now where's that microwave oven transformer I had...

There may not be much wrong with the Bogen in question. When I run a guitar signal to the various input options, most are very low volume output. The bridged AUX input gives useable output, but nowhere near 100W. It may just be one of the OP AMPS, so maybe I'll look a little closer at it before using it as an anchor!

[eleventeen]

Two other things:


You'll not have heater voltage anywhere, so whatever you think you are saving....


Most solid state device transformers have multiple secondary windings. I have a few here with (I think) 162 volts and 70 volts. I don't know what it came from, but judging by its general mass, say 3.5" cubed, it can definitely supply some current. So, first thing I think is "put the secondaries in series and follow it with a doubler."


But what you'll find is that in most cases, the secondaries are what I call "nested". They may or may not have center taps, but they are configured like the primary on a ultra linear (output) transformer. There is one winding producing the highest AC volts and at first glance, another winding. But the "second winding" is simply a tap on either arm of the main secondary. That means that the two windings cannot be placed in series because it would short out some portion of the secondary. Bleh. You can of course test continuity simply between the two windings. So the thing is not very useful, like it or not. 

[MadMax]

Good points Eleventeen.
I already have a Bogen C-60 that I converted to guitar amp (I even hard-wired a Boss OD-1 into it with the controls on the side of the custom wooden case I put it in). It was a fun project but I find the sound kind of sterile. I'm sure that these are optimized for hi-fi reproduction, but I'm not sure (yet) how to make them more guitar friendly. In this age of i-pods and CDs, I have no need of an outdated SS amp for hi-fi use.

As to the output transformers... I have to assume that the secondary taps are exactly the same as any other 4, 8, or 16 ohm output transformer. Therefore, if Sluckey is right when he says they're no good for tube OTs, the primary impedance must not be what a tube amp needs. So, before I de-solder this one to take actual readings, is there anyone who has actually measured SS output transformers for tube use, OR (like Silvergun) just put one in to see what happens?

[eleventeen]

I feel fairly safe stating that there's no bloody way a SS output transformer would have anything near the proper impedances for tube use. It's fairly rare to even find a SS output tranny in an amp. This tells you....what? That transistor output stages are already very low impedance. That such a transformer is therefore likely to be very very low impedance, nothing like the 5K - 6.6K 8K we'd like to see for tube use.


That said, maybe the thing would "work". I can't imagine it would work "well". Frankly, I cannot recall the last time I encountered a SS amp that used an OT.

[PRR]

> a SS amp that used an OT.

Fairly common in fixed-install amps which drive very long speaker runs. 70.7V is not a "natural" voltage for a transistor amp, yet gives best economy for runs of several hundred feet. 25V is also popular, and the amp-maker does not want a total re-design and separate model for each voltage. There are also uses for audio amps which deliver 115V, for vary-speed AC motors.

And then the odd ones. Mac's early transistor amp was scaled to like 3 Ohms, used a transformer to give the several impedances popular in hi-fi (also lab work, shaker tables). When transistors were very expensive you found push-pull and even SE transistors with transformers. The classic pocket radio is 0.1 Watts push-pull into a 1K:8 transformer. Many early TVs had a 100V transistor working more or less off wall voltage with 8K:8 OT.