Is it ok to combine output transformers?

[krzanik]

I have two sets of single ended output transformers with high primary impedances.

One set has a primary impedanced of 13K into 8 Ohms and the other set has a primary impedance of 17.9k into 8 Ohms. These were taken from old reel to reels in which they were driving 4 ohms speakers.

I saw a web site were a guy combined two high impedance output transformers (in series) to get a lower primary. I tried this, combining one of the 13K with one of the 17.9K in series and got a primary of 3362 ohms. Is this ok to do? I should mention that the 13K ohm output transformer is about half the size of the 17.9K ohm output transformer and I wonder if connecting them like this (one big with one small) will put a lot of strain on the smaller transformer? If its ok to do this it would save me from having to buy new output transformer (which would be really great).

The web site I saw this at was: http://www.atatan.com/~s-ito/amp/yakinori.html
If you scroll down there is a diagram of the schematic. Thanks.

[gingertube]

The reason your are light on replies is because this needs some analysis to answer your question and you havent supplied all the data needed.

Note that in the article the transformer primaries are connected in parallel and the secondaries are connected in series. Is this how you have wired the two trannies?

The two secondaries will contribute different voltages (not equal) to the speaker. What load gets reflected to the two primaries will be different as well. What Speaker load are you connecting to the series connected secondaries?

If you answer the above, I or someone else may be able to answer the question.

Cheers,
Ian

[krzanik]

Thanks for the reply. Yes, I have connected the two transformers exactly as you indicated, Primaries in parallel and secondaries in series. My speakers are 8 Ohms.

Hopefully this will help. Thanks.

[planet10]

With those 2 i'd look at using the big one as a parafeed for the little one, Use an EL84 with low inpedance speakers (or perhaps an EL95 if you need to use higher impedance speakers). If they were the same you'd have more options.

dave

[gingertube]

The theory was'nt obvious to me so plugged in some values to play with.

Assume: Vprimary = 200V, rp of output tube = 10KOhms
Looking at the secondaries:

The 17K9 has a turns ratio of 47.3 will generate 4.228V (200/47.3) with an internal source resitance of 4.48 Ohms (10K/47.3^2)

The 13K has a turns ratio of 40.3 will generate 4.963V (200/40.3)with an internal source resistance of 6.154 Ohms (10K/40.3^2)

Total secondary Volts = 9.191
Total Secondary Resistance = 10.63 + 8 Ohm load
With an 8 Ohm load Signal current will be 0.493 mA.

You would get identical results to this if the 17K9 tranny secondary was loaded with 4.1 Ohms and the 13K tranny secondary was loaded with 3.9 Ohms.

Reflecting these to the primary:
On the 17K9 primary side you get 9.143K
On the 13K primary side you get 6.338K

Parallel combination of these is 3.743K

The 13K primary tranny will cetainly have 1.44 times the signal current thru' it and therefore will dissipate 2 times the power.
i.e. 2/3 of the total power will be in the 13K primary transformer.

I may well have screwed up some calcs here - done on the fly. BUT
Should be good enough for guidance. Shows that using 2 transformers like this is best done with identical trannies.

Cheers,
Ian

[krzanik]

Thanks to Planet 10 and gingertube for your replies. It seemed like too simple a solution to my problem...and it was. I'm trying to build with what I have. I'll start looking into parafeed and check out the EL95 tube. I'm learning a lot lately. Thanks.

[planet10]

Originally posted by krzanik
I'll start looking into parafeed

Bottlehead (and Bottlehead forum) is where you should start.

Here is the map for one of the varitions of a friend's parafeed EL84 amp to give you a start

dave

[krzanik]

Thanks Planet 10!