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Why no tiny output transformers?

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Hammond 125 FSE has band 10 - 50000 with the same amplitude form in my SE 2A3 RCA and SE 300B JJ whit excellent sound. Small 125s start from about 40-50 Hz. I tried DSE with 6V6 Silvania. 125s are very suitable for small amps with good results. /150 - 15000 in instruction mean to go to expensive Hammonds :) /
 
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I have built a 6ak6 + PC92 stereo amplifier with the Hammond 125ASE, because I had space constraints. They cost more than bigger transformers and I am not happy about the sound. The low frequency attenuation is noticeable. Still searching for a SE transformer about the same size of the 125ASE but with better specifications.
 
... Thanks for letting us know that they might be better than advertised.

My 125ESE seem to be. From one of my posts in the Tubelab sub forum:

"The conditions were plate set for 10K, secondary 8 ohm, the load was 8 ohm resistive, current through the OPT is 50 mils at 415 volts.

I set 1 Khz as the 0 dB point at 1 watt output. 100 Hz was -1db, 70 Hz was - 2 dB, and 55 Hz was - 3dB. The high frequency response was flat to 6 kHz, where it began a very gentle roll off out past 30 kHz. 10 kHz was down about -0.5 dB, 15 kHz was about -0.8 db, 20 kHz was -1.5 dB and 30 kHz was - 2 dB."

I measured the inductance and it was higher than specified, but off the top of my head, don't recall what it was. They also perform pretty well on the SMPTE IMD test. IMD gets better with higher current through them.

My test equipment is pretty mediocre, but likely better than nothing. I don't know anything about the other OPTS in the 125 line.

edit: they were hooked up to 801A's.

Win W5JAG
 

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The world used to be full of 1 Watt tube amps. Kitchen radios and phonographs.

These all used a cut-down of 25L6 working around 105V and 2K load, almost always with 4 Ohm speaker. These parts used to be trash. They are now hard to find; you probably have to buy an ugly radio to get one. "Fidelity" was in line with the original low-low $9.95 price and flashy cabinet (the actual radio from loop to speaker may have cost under $2).

At a different class, transformer 250V radios for communications: it was common to load a small power pentode at 250V-300V and 15K-25K for about a Watt. The market did not demand extended bass. While that market vanished, a guy named Leo bought a bunch for reverb drivers in the big guitar amps. Fender numbered these parts as #125A20B & #022921. The "3.5W" is optimistic: it will do that above 150Hz but to get there you need well over 300V supply (Fender did run these with 12AT7 at 400V). With 150V and a 6AU6 you can get near 1/3rd Watt.
 
Without knowing what output tube you will use, what quiescent current, and what configuration, it is very hard to pick an output transformer.

Take an EL84, and a single ended transformer with 5K Ohm primary, 50mA DC rating, and 20H primary. It will give completely different low frequency results for an EL84 that is triode wired (rp ~ 1500 Ohms), Ultra Linear wired (rp ~ 3000 Ohms or more), or Pentode wired (rp ~ 38,000 Ohms). 20H has 5,026 Ohms of inductive reactance at 40Hz, the low note of an electric bass guitar. No, I do not build guitar amps, but that is just a real world example of a fairly low note. The inductive reactance will be in parallel with the reflected impedance of the loudspeaker (reflected impedance of 5k Ohms, wherever the loudspeaker impedance matches the output tap of the transformer, this typically only happens at a few frequencies, as it rises and falls above and below its "rated" loudspeaker impedance).

Without negative feed back, the Pentode mode will have significant bass roll off, and a damping factor of about 0.13, yes thats right, less than unity. Also, since the Pentode is essentially a current source (vs the 5k Ohm primary), the leakage reactance of the transformer can cause the high frequencies to be rising in amplitude just as if you had turned up a Treble knob on a preamp. Of course we will have more power than Triode wired, but there will also be more harmonic and intermodulation distortions. You might not be satisfied without negative feedback to lessen these effects. This has the highest gain without feedback, but with feedback some gain goes away.

The Ultra Linear will have a slight roll off of the lower bass frequencies, and a damping factor of about 1.66 (at least it is greater than unity). The harmonic and intermodulation distortions will be less than the Pentode wired mode, and the power will be slightly less. Again, you can use negative feedback to lessen these effects. This has medium gain, but with negative feedback, some of that gain goes away.

The Triode wired will have the best low frequency response, and the lowest distortion. The tradeoff is lower power. The upside is that you do not have to use negative feedback. The gain is preserved (no negative feedback) This can also be the simplest circuit.

All of these modes can be used successfully if implemented properly.

One of your constraints is size. A creative push pull design might not take up any more room than an SE design; you can have a much smaller output transformer for the same frequency range and distortion, allowing for one more (small) output tube per channel.

Since this for low power, the amp may not have to have very much gain. So go with Triode wired. For Stereo single ended, one 12AU7, and either 2 EL84s, or 2 6S4s might do the trick. For Stereo push pull, one 12AY7, and either 4 EL84s, or 4 6S4s might do the trick. (for this configuration, you use a current source in the output stages to do the phase splitting, which lets you get away with only one triode to drive the output stage).
 
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I did not see a maximum DC rating for the 8W SE from Antique. And 230 Ohm DCR will give ~ 1dB loss if the transformer is used as a 2500 Ohm primary when the output is loaded with 1/2 the output tap rating.

All of these small cores will either be DC limited, or low frequency bandwidth limited (versus rp), or low frequency power bandwidth limited (actually all of these). But that is true for large SE transformers, they will have these limits too, just more power, or lower frequencies, etc.

I have to go back to one of my initial questions: What will the amplifier configuration, current, and topology be.

Start a design by picking an output transformer, and make the rest of the amp work well with it. Or start by picking an output tube, current, and mode, and pick an output transformer to work with it.

We need to know one or the other, or we can not say, or even guess, how well the amp might perform.
 
.......Start a design by picking an output transformer, and make the rest of the amp work well with it.
Or start by picking an output tube, current, and mode, and pick an output transformer to work with it.....

So, I made a deal on the internet, and the seller bundled two SE transformers that he had laying around. The physical size is very close to the Hammond 125bse and Edcor XSE10 series transformeres. Transformer ratio was checked using 1 KHz test tone and a scope. Voltage ratio is 42,5:1. I guess this leaves me with a transformer primary impedance between 7 and11K (assuming 4-6 ohm speakers). There are no taps in the primary nor on secondary site. Hence UL is not an option and speaker choice will be limited with respect to impedance. Most of my speakers are 6 ohm. No idea of what max current the transformer can handle. For what it's worth the only markings on the trafos are 598-B and 598-A/KV (different mounting bracket-both measure the same)

Any recommendation on what tube may be suitable?

Lars
 
42.5:1 turns ratio gives you 10.8K:6R impedance ratio. That work suit a 211 or 845 as fas as impedance goes, but those tubes will probably flow too much current for that transformer, however if you hook up a 3-4 ohm load it would work well with 6V6 types... Including 6P1P, 6P6S, 6AQ5, 6005 etc.
 
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