Asymmetrical P-P OPT Primary

[rdf]

The output primaries of the the Rowe R-4359a jukebox amp bought for iron (a simple common cathode/cathodyne/p-p pentode circuit) measure ~55 ohms DCR from B+ to one side and ~74 ohms on the other. The effective operating impedances haven't been measured. It was designed for 7868 tubes and rewired a couple days ago for 7591s.

Is the primary DC imbalance a result of 'economical' winding technique or was it common in jukeboxes to maintain a little 2nd HD sweetener? Manufacturers played with the secondary windings, including returning the speakers to an out-of-phase tap of the opposite channel to make up for the narrow driver spacing of the typical box. A distortion analysis though shows pretty good 2nd harmonic cancellation.

[JohnAtwood]

What matters the most in a transformer is the number of turns - this gives the voltage and impedance ratio. If one half of the primary is on the inside, nearest the core, and the other half is on the outside, the outside windings will have a longer length, hence higher resistance, even though they have the same number of turns. It is pretty common for push-pull transformers to have different resistances for each primary half. Some forms of interleaving the windings tend to equalize the resistance, but it is hard to get it exact.

Sometimes when I build an amp, I "build out" the low resistance half by adding an external resistor chosen to make the two halves have the same resistance. Then I can check for current balance by putting a voltmeter from plate to plate. If both the current and resistance is balanced, the DC voltage will be zero.

[rdf]

Thx John. A non-interleaved winding technique seemed the most likely cause but the large difference in resistance had me wondering. As did the measurements, nasty little piece of work this circuit. The manufacturer does in fact exactly what you describe, insert a small value resistor in the plate circuit of one side. Thx again!

[tube-lover]

hi RDF & JohnAtwood,

Many sorry to expose too much informations.

P1~B1 & P2~B2 was Asymmetrical P-P OPT Primary.

But it must PP in same two type of tube such as 6L6,KT66 PP etc.

the reason of this was making for Rich 2nd & 3rd harmonic.

Snce SE OPT was rich in 2nd & 3rd harmonic. If PP use Asymmetrical P-P OPT Primary can increase some of 2nd & 3rd harmonic too. BTW, It need more technical skillful to do this.

thz

thomas

[Geek]

Hi,

Asymmetrical P-P OPT Primary.

Was it was made for guitar?

[rdf]

Originally posted by Geek
Was it was made for guitar?

No, the other device that creates an unholy noise: jukebox.

[tubelab.com]

I have a good supply of P-P OPT's that were made for guitar amps by Schumaker. The DC resistance of one half of the primary is 52 ohms and the other side is 60 ohms. I dissassembled one of these to find out why this is so. There is no interleaving. One primary half is wound first, closest to the core, then the secondary (common, 4, 8 and 16 ohms) is wound next, followed by the other half of the primary. As John pointed out the outer winding has the same number of turns as the inner winding, but the winding path is longer, using more wire, and having a higher DC resistance.

On paper these would suck for HiFi, but I have used them in dozens of HiFi amps including my 300Beast (P-P no feedback at all) with good results. They were rated for 80 watts in the guitar amp world, but are good for about 30 watts in HiFi. Several years ago I bought 200 of these for $16 each, a deal! They still show up on Ebay from time to time.

I have seen similar transformers where the outer winding was wound with a slightly thicker wire (I am always taking things apart). The DC resistance was more balanced, but the transformers didn't sound as good.

[Bandersnatch]

Hey-Hey!!!,
DCR differences between halves of the primary are no reason to exclude interleaving. It just shows that the sections are on average different distances( radially ) from the start of the bobbin.

First example that comes to mind are all the Dynaco outputs...
cheers,
Douglas

[Dave Cigna]

Yeah. Virtually every PP OT that I've measured had different DCR on the two halves, even some that are considered to be high quality. I strongly doubt that any of them were deliberately made asymmetrical. Only the very, very best of the best big money designs went to the trouble of balancing the DCR.

Most of these transformers were loading pentodes which have plate resistances in the tens of thousands of ohms. Ten or twenty ohms in the OT is irrelevant and completely swamped my variations between tubes anyway.

-- Dave

[tube-lover]

hi,

looks the atmosphere was not good. but I would like to say, IF OPT not had interleaving the capacitance will very large. Bad for HF. Another reason was most PP amp OPT design was P1--SG1--B1 & P2--SG2---B2 was use seperate Bobbin, same volumn to winding........why will had different DCR!!! pls considerate!!

there contain little design secret of the OPT. sorry to say I cannot talk more.

thx

thomas