Which output transformer?

[Tapsnap]

I have a push-pull amp schematic that calls for an output transformer with 4K ohm primary and an 8 ohm output. I can't find an output transformer anywhere with those specs. So, my question is: Is it better to get an 8K to 16 ohm transformer or would I be better off with a 5K to 8 ohm transformer?

[terminalgs]

that should be a common Marshall JTM100 ratio.


 you can email hoffman to see if his heyboer marshall OTs are 4K.  also lots of bassmans were 4K.


see  018343  at this link:



https://el34world.com/Transformers/fentrans.htm

[Tapsnap]

Those are both way too big. This is a 10 watt amp. The fender OT you recommended is 50 watt and the Marshall is 100 watt. Do you know of something smaller, perhaps 15 watt?

[sluckey]

Let's see your schematic.

[Tapsnap]

See attachment

[sluckey]

The 6AQ5 is basically a low voltage 6V6. I would want an OT with an 8K or 10K primary in that circuit. Something suitable for a Fender Princeton Reverb.

[PRR]

Hammond 125B would be appropriate.
http://www.hammondmfg.com/125a.htm

[Tapsnap]

Sluckey, can you explain your thinking?

[Tapsnap]

I actually have this OT but I'm not sure if it is capable of handling the output. https://www.tubesandmore.com/products/transformer-output-8-w

[PRR]

The output of that amp is 4 maybe 5 Watts.

In any case, the transformer won't be harmed, just distort bass more.

Just use it.

[pdf64]

Without thinking about it too much, my guess is that the 4K pp load is used as the HT is rather low at 220V. 4k allows more signal current to flow and so get a good power output.

[PRR]

I think this particular amp "design" is a "throw-together".

In "rational" engineering, you pay the big bucks for two 6V6/6AQ5, you supply and load them to get all they got: over 250V and over 6Kct to get 8W or 10W.

On the other hand, there are uses for "small" amps. And sometimes it makes fine sense to use a "popular" tube type instead of scrounging for a smaller "just right" tube. Also to use any odd transformer found under the bench or at the surplus shop.

Also note that the drawing says it is transcribed from old written notes. Which may not have been too exactly documented.

Which is an argument for the "universal" PP OTs. You can _try_ 4K or 8K or maybe 12K. Measured output will vary some; but if you Need Big Output you would not be fooling with peewee plans. The onset and rise of overload, "flavor", will differ when you load "too low" versus "too high". (The amp is not strained in any case: it runs hot at idle and less-hot with loaded signal.)

So yeah, don't think about it too much. Build, fiddle around, play.

[PRR]

> 4k allows more signal current to flow

Limited by Vg2. The 6V6/6AQ5 is not a fat bottle. Won't flow huge current even if you try to suck it. As a pencil-guess, it probably should not be loaded smaller than 2K per plate or it just won't swing most of the plate voltage. That leads to 8Kct for push-pull. It scales somewhat as Vg2 rises, so the 6.6Kct loading of the high-volt Fenders may be appropriate. Conversely I'd be thinking 10K for low-volt amplifiers.

And FWIW: the plan says 4K but my rough math from the idle/roar current numbers suggests more like 6K. I may be wrong, the present drawing may be wrong, or the original transformers may have been marked wrong. Not inclined to worry about it.

[Tapsnap]

PRR, so your saying any OT between 4K and 10K  with an 8ohm output would be OK in this amp? I was lead to believe that the matching of the primary and secondary was more critical than that. So to answer my original question, "Is it better to get an 8K to 16 ohm transformer or would I be better off with a 5K to 8 ohm transformer?" The answer is yes?

[brewdude]

I was under the impression that 10k was the typical primary impedance for a pair of 6AQ5’s in push pull.

I built a 6AQ5/6005 push pull amp with about 240v anode to cathode using an Edcor GXPP10-10k/8R.  I used a VVR to scale the entire amp. 


It’s very fun to play and sounds great.

[brewdude]

PRR, so your saying any OT between 4K and 10K  with an 8ohm output would be OK in this amp? I was lead to believe that the matching of the primary and secondary was more critical than that. So to answer my original question, "Is it better to get an 8K to 16 ohm transformer or would I be better off with a 5K to 8 ohm transformer?" The answer is yes?

What speaker load do you intend to use?

[Tapsnap]

brewdude,  I haven't yet picked one out, but I suppose a single 12", 8ohm 12 to 15watt speaker would work.

[Paul1453]

A cheap 10W line matching OT with a 10K primary to an 8 ohm speaker load works well for PP 6AQ5s at +250Vdc or less!

You can usually pick up an OT like that for around $15 including shipping on Ebay.   

[tubeswell]

See attachment

This is somewhat like a low power supro t-bolty design (for the PI and output stage) but with low-power tubes and a goosed-up front end.


Should sound plenty raunchy

[Tapsnap]

Thanks everyone for your help.

[92Volts]

brewdude,  I haven't yet picked one out, but I suppose a single 12", 8ohm 12 to 15watt speaker would work.

Because negative feedback isn't used and you haven't bought the speaker yet, don't go out of your way to use an 8ohm speaker & OPT if 4 or 16 ohms would make OPT selection easier. Disregarding what else has been said in this thread (higher than 4k might be better), if you couldn't find 4k:8ohms but could find 4k:4ohms, you could just buy the second option plus a 4 ohm speaker.

Continuing to disregard suggestions that (much) higher impedance is better... 5k is so close to 4k I'd consider them equivalent. If you did want 4k, I'd feel more comfortable with that than running a 16 ohm OPT with an 8 ohm speaker, especially if you were pushing the limits of either OPT. In this case, it doesn't matter much.