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Opinions on SET transformers for 307a amp

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Planning to build an SET 307a tube amp. Looking for opinions for best output transformers -- some candidates I am considering. Please provide feedback and recommendations on reasonable, high quality candidates:

Hashimoto HC-507U
Lundahl LL1623

Have not used any of these but wish I had the chance to compare the nice ones.

What's your operating point, and what kind of OPT are you looking for - 5K? I'm assuming triode operation since you are saying SET. On the face of it it seems like the kind of OPT you'd use for a single 45 or 4P1L tube.

I use a 4P1L output tube which I'm happy with, and I use a Lundahl LL1682 which is more like 8K into 8 ohms. That's only because I had a pair - otherwise I would have used LL1663, which is 5K fixed output. Cheaper than the much larger LL1623, which has extra windings you may not need. I'd rate these smaller Lundahl OPTs as very good OPTs. To go better, look at Monolith. Ward emailed me the following recommendation for a 5K OPT:

"With an internal resistance of 1K2, the S25 would be the ideal candidate. I would also not regap it to 35 mA since there is plenty of inductance. In this way, you could use it in the future with another tube at e.g. 80 mA. Summit is sonically superior to Etude due to better bandwidth, better saturation specs, better dielectric etc . It also sounds better to my ears."

Hashimoto is obviously a good product, but I'd be more likely to go with Monolith if I wanted to spend more than the Lundahl LL1663 costs.

Look at the 300b output transformer thread - good stuff in it. And look at this thread - Tubelab tried out the 307A at 80mA with a 5K OPT, so the Monolith choice seems appropriate. You could also swap in a 300b if you wanted.

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This is worth to try: TTG-EL84SE - Tube output UL transformer [5kOhm] EL84 / 6V6 SE - Shop Toroidy.pl

I have tested some PP-transformer of Toroidy and those have convinced me about the quality.

That's very cheap. I was under the impression that toroidal OPTs were better for PP than SET because gapping toroids was more of a design problem. Is there truth in that? Menno Van Der Veen says he spent 3 years designing a SE toroid. Plitron do a 5K one to his specs

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...Menno Van Der Veen says he spent 3 years designing a SE toroid. Plitron do a 5K one to his specs...

Technical innovations usually spread to other manufacturers if there are not protective patents.
It could be possible that Toroidy have been able to speed up their product development based on what Plitron have been doing first.
(just my personal assumption)
The reasonable price is due to competitivity of Polish labour cost copared to US and Canada.
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The 307A operating current is a low 43mA, implying that a high current wide gapped opt will not be required.

Is there a downside to having too high current/wide gap for 307a amp? Is the 5K primary the best value in this case? I can order custom from Silk, Thailand. I don't know much about this so your explanation would be helpful. I'd appreciate it.
I also am planning a 307a / VT225 SE amplifier. I already have LL1663 transformers which I used before with EL84 / 6V6 tubes and am very happy with. They are gapped for 50 mA and according to the 307a spec sheet will be just about perfect. iirc the 307a runs about 43 mA at 300Vdc on the plate another 5 or so for G2 at same voltage and the recommended primary impedance at this op is 4500 ohms, the LL1663 is I think a 4800 ohm primary.
The list you have has well regarded names, by reputation I would say Hashimoto but I have never compared them so take my opinion with a grain of salt.

I don't know how much building you do but you could always start with something cheap and then afterwards decide if you think your build is worth spending on an expensive OPT. The one's Artosalo recommended are well priced and have published specifications and he seems to have had good experience with the manufacturer. You could also try Hammond 125ESE's which have multiple primary impedances which would be good for experimenting with.

Good Luck.
...You could also try Hammond 125ESE's which have multiple primary impedances which would be good for experimenting with.

Hammond 125 ESE has too low primary inductance and therefore bad bass end linearity. It is not designed for Hifi-amplifiers.
Below a quote from the manufacturer:

Designed for general purpose or replacement use (not Hi-Fi), in single ended, tube output circuits.
Frequency response: 100 Hz. - 15 Khz at full rated power (+/- 1db max. - ref. 1 Khz).
I tried 307a of course (which is a pentode). You make a good point about the UL screen grid tap. I used one of my XE-20s transformers which I believe has 40-42% taps. I have no idea if this works for 307a though.

I fully agree that certain sweep tubes are excellent performers too. (still) much cheaper than 307a as well...
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Use a tx with a primary impedance specified in the the design. I'm sorry if you thought I was implying that this be changed to suit the fairly low current used in your adopted circuit.

In theory a transformer will perform best when its gap is sized appropriately for the circuit tube current. If it is too large, then a reduction in sensitivity/dynamics can occur. If it is too lsmall, then saturation can occur, leading to distortion and a curtailed low frequency response. It's a bit of a balancing act that those who know more about ouput transformers than I may be able to explain better.

If a transformer is saturating due to overload (which usually occurs at low frequencies), there are three possible remedies: (a) increase the gap; (b) increase the core dimensions; (c) use improved core material. If you increase the gap you lose inductance and sensitivity; if you increase the core size or improve its material (more costly solutions) you do not: hence b and c are the solutions usually adopted by better manufacturers.

However by increasing core dimensions a tx's intrinsic winding capacitance increases, which can lead to a loss of high frequency extension. Good manufacturers are able to balance these factors to give the best balanced performance.

In summary, I would say:
1. avoid transformers that appear to have a small core size in relation to their stated power handling capacity.
2. avoid purchasing a tx that is rated for currents well over spec (unless you are planning on using them in circuits with a greater anode current at a later date).
3. avoid purchasing a tx rated for currents under spec.
4. To my knowledge Silk are a highly reputable manufacterer adept at balancing the design parameters I have discussed to achieve good performing high value for money products.
5. The prices of the Toroidy transformers appear to be remrkably low. However, I have no idea what kind of performance they offer. If their performance is in the same ball park as the Silk, they would represent remarkable value for money.
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