SS driven OTs?

[Paul1453]

It took me quite a while to get a limited understanding of our tube driven OTs.

So, I'm just wondering what the major differences are with a SS driven OT if there are any.

There are lots of unloved SS PA amps on Ebay.

They seem to have similar output settings compared to tube PAs by just looking at them.

Are the SS PA OTs with the multiple load selection options completely different than the tube driven OTs?

[eleventeen]

You'll probably find SS amps bearing an OT in the distinct minority. That was and is certainly one of the great attractions of SS output sections...that the impedance and ampacity of a complementary or series pair of output transistors is directly compatible with driving a speaker. Meaning, no need for a heavy and costly OT. If you *did* find such a rare animal, I'd imagine the OT would not be anywhere near what tubes want to see. Transistors can handle considerable currents at 35-50-70 volts. Tubes cannot wiggle the power necessary to drive a speaker when a pair of 6L6, say, is maxed out at 100 mils. A 2N3055 can handle 10 amps!

[Paul1453]

Thanks 11!

I just noticed a lot of SS PA amps that seem to have the same type of output settings as the tube PAs.

I've just been skipping all SS junk gear, but these got me wondering.

I knew some SS amps omit an OT, but didn't know if they all did.

Then I thought it might be a difference in load, if a SS amp did use an OT.

If you look at a 30,50,100 Watt tube PA, they often want $100+ for a rusted piece of junk with no tubes in unknown condition sold as-is.

You can find a similar SS amp in working condition sometimes for $50 including shipping.

[eleventeen]

The number of parts adaptable from the SS world to the tube world is vanishingly small. As in "none". Knobs, rubber feet, that kind of thing. You'll find the pots are usually in the 1K-2500-5K ohm range, maybe a 10K; Annoyingly, you *may* find power transformers that look promising based upon their sheer size. Of course, you'll find no heater winding on such a tranny. But even worse, I have a fair number of PTs (from SS organs) with several 100 volt and 80 volt windings. So the initial thought is that maybe those windings could be placed in series and get rectified using a bridge. Nope, they (the ones I have) are "nested"; they share a CT, they are wound like the primary of an ultra linear output tranny. You can't place any of the windings in series. Useless. That is, unless you're looking for an 8-pound bias transformer that needs to supply 1-2 mils on a bad day. Bleh!

[PRR]

Tube amps "require" a transformer to get from the few-K impedance of tubes down to the few-Ohm impedance of speakers.

Crystals (transistors) conduct current far better, can drive speakers direct, OTs are expensive, so most domestic transistor gear skips the OT.

The big exception is PA. If the speaker lines are short they may be run at 8, 16, 32 ohms. But the lines may be hundreds of feet. I have a 500 foot line in REAL BIG (0.2" conductor) wire, thousands of bucks, about 0.8 Ohms round trip. If I ran that to a 4 Ohm speaker I'd lose 16% of my power. Using more affordable wire I could lose half my power.

Material costs tend to suggest a Line Impedance near 100 Ohms. That keeps copper not-huge and also acceptable insulation costs. For larger powers this leads to High Voltage and More Rules. Therefore amps 15W to 70W will tend to have a "70V" winding, making line impedance 333 to 70 Ohms. In other cases, nominal 25V is the limit, making 15W-100W line impedance 41 to 6 Ohms. Both taps are usually provided, and only a transformer can churn the same power into two very different impedances.

A SS amp can be designed for any impedance. High impedance suggests high voltages. But 40V transistors are far cheaper than 40oV transistors; and current gain must be reduced to get the hi-volt rating. Even with a transformer to give the final impedance, SS designers will usually design in the few-Ohm range, because the transistors are readily available at good price.

So the basic PA amp could be a transformerless 8-Ohm job stolen from the Hi-Fi division, plus a transformer to get the higher impedances used in long-line PA work.

All non-tiny audio amplifiers work push-pull. In tubes, since we don't have P-type tubes, and usually need a transformer anyway, we use the OT to combine the push and the pull. This is possible with transistors, and you may find one. But since 1970 there has been much work in transformerless push-pull transistor amps for the Hi-Fi world. These reached high performance and elegant designs. So the PA OT is not tasked with combining the push and the pull, and has no CT/split winding for that.

[Paul1453]

So if I understood your meaning PRR.

A pre 1970 SS PA might possibly have a tube compatible OT.
Or it could be a complete bust.
Much more research on the specific make and model is required to know for sure.

I just noticed that when I search for the cheapest PA amps SS designs dominate the listings.
Then I noticed that some of the older SS PAs have all the same impedance load choices as the tube PAs do.
That made me wonder if the OTs were in any way similar and maybe usable in a tube amp?

I'm always on the lookout for unrecognized parts value in cheap gear.   

[jjasilli]

A pre 1970 SS PA might possibly have a tube compatible OT.
The fact that some SS PA amps have an OT that "puts out" 6 ohms does not mean that it's useful for tubes.  The SS OT primary must match the low output impedance of transistors.  Tubes have high output impedance and must have a primary OT winding to match it.


EDIT:  A transformer does not have an output impedance per se, though we speak that way for convenience.  The tranny simply has two windings.  It's converting the impedance of whateer source is feeding it.  E.g., if a tranny has an output impednace of 6 ohms, that'e because the secondary winding is matched to the primary winding which is matched to the source impedance.  (Matched does not mean identical -- a tie may match a suit but is not identical to a suit.) So, if you change the source impedance the tranny will no longer have 6 ohms at its output.

[Paul1453]

I'm figuring there is a reason these SS PAs are so much cheaper.

And that even if one has an OT it may be useless for our guitar amp applications.

I know how to test an unknown OT to determine its usefulness.

Those SS PAs with a 4,8,16 ohm and 25V and 70V selection options,
caught my eye and made me ask this question.

I might have to just take a shot on a 50W SS PA with this type of transformer to really learn my lesson.
If I can get one for less than $20 delivered.   

[trobbins]

SS PA amps can be very cheap/throwaway.  A TOA A1061A 60W SS PA has 0-15.5-50-71-100V OT secondaries (ie. 0-4-42-83-165 ohm), and a 2.8Ω PP primary impedance.  Reversing this OT, a 240Ω PP primary (using the 0-50V-100V secondary windings) could drive a 4Ω speaker (using the 2.8Ω PP primary windings).

Going up in PA power rating, eg to a TOA A512A 120W unfortunately lowers the effective PP impedance you can utilise in a reversed application.  So if anything, aim for a lower rated PA amp to give a better impedance outcome.

I have those two PA amps, and had a good look at how to 'valve' them based on what was commonly available in OZ.

What can you do with a 240Ω PP primary ?  Well you could aim to use up those cheap TV tubes - in OZ we often have 6CM5's overflowing the spares bucket.  A quad 6CM5 PP could achieve up to 20-30W output using that OT.  The impedance ratio can also be moved up even further - eg. a 480Ω PP primary can be the design input.

Those PA amps ran mains as well as alternate 12VDC supply, so the PT typically has a 14VAC beefy secondary (9A rating in the TOA A1061A), which can then be used for the large heater current requirement, and the original PA can then be basically re-used as it has the PT, OT, front panel controls etc etc.  But a B+ and screen supply is needed, and of course some effort 

[jjasilli]

What (Where?) is Oz?


EDIT:  slang for Australia???

[Willabe]

EDIT:  slang for Australia???

Yep. 

[PRR]

Generally, if the amp does not have a 300V-400V B+, it aint good for tubes.

Even if the 70V winding had a CT (most don't), it only wants 35V*1.4= 50V peak each side, say 60V-90V of B+, and the power available with practical tubes at 90V is quite small. (As mentioned in other thread.)

Yeah, Tim can find the marginal cases. Outside the US 100V and even 200V speaker lines have been around a while. Some of the fat (large heater-cathode) TV sweep tubes can pull huge current at low voltages.

Of course if you stack enough huge tubes and suffer much heat you can pull respectable power directly in 16 Ohms no OT. Though I think the Futterman wannabes have scoured that market bare.

[trobbins]

Aiming to 'tube' a ss PA amp would not be a classic guitar or hifi amp blueprint, and would only be for those hardened recyclers who like giving old equipment a new life and purpose, or like the idea of a rocky horror / Frankenstein style amp, or are keen on putting TV horizontal deflection tubes to good use.

It would be interesting to hear from the purveyors of 2nd hand junk/opportunity on how many old ss PA amps have the full complement of 50V, 70V and 100V line level outputs - as that is when the OT becomes 'practical' to use.


The ss PA amps I often see on eBay are from the 1980-2000 era from Japan and allow a 12V car battery powering capability, so the ss output stage is low voltage/high current PP, and so drives a low impedance OT primary (eg. 2.8 ohm in the 60W PA amp I have).


As indicated, the whole PA amp enclosure can be reused, as many of the major parts would remain in use. 


Given the low PP impedance available from the OT (when working backwards), then the TV horizontal deflection beasts like 6CD6G, 6DQ6A, 6CM5/EL36 have a small but strong following for utilising in guitar amps, and enjoy low impedance PP loading.

[Ugly Distortion]

I had a memory of the early ss McIntosh hifi amps using OTs (for residential not PA 70V work). I googled and found this if anyone is interested:

http://audiokarma.org/forums/index.php?threads/autotransformers-why-does-mcintosh-use-them.164153/

[HotBluePlates]

I had a memory of the early ss McIntosh hifi amps using OTs (for residential not PA 70V work). ...

Note that an "autotransformer" is not the same thing as a "transformer".

A transformer has a primary and a secondary (and maybe other windings) insulated from each other but on the same core. A transformer can transfer energy from one winding to the other while keeping them isolated at d.c. (because they're not directly connected).

An autotransformer is a single winding on a core, but with multiple taps. When you apply an a.c. voltage to one of the taps, voltages are available at the other taps. These voltages are higher at the taps with more turns than the one where you applied the voltage, and lower at taps with fewer turns. Since this is all one continuous coil, there is no d.c. isolation.

A Variac is a variable autotransformer. It is one big coil on a core, and wall voltage is applied at a tap on the coil. Output voltage is obtained from a brush which can move along the coil, including to turns beyond the tap where wall voltage is applied (to allow output voltage greater than wall voltage). This works because "volts per turn" is constant along the coil.