Cheap SE output transformers

[Geek]

Originally posted by Bas Horneman

Geek ..got a schematic for us/me?

Thanks.

Sorry, missed this one

I scrapped the 6EW7 as too insensitive and went back to 6FD7 (they sub for each other, but the 6FD7 U1 has higher gain)

PS circuit for it and writeup.

[Bas Horneman]

Thanks Geek!!

[Geek]

You're welcome!

There's still R&D going on, if you want to check that thread every so often

[G33/33]

Originally posted by Bas Horneman

Geek ..got a schematic for us/me?

Thanks.

found this link ?, but 2 watt only

http://www.audiohobbyist.com/images/6ew7/6EW7sch.JPG


rgds,
Gede

[Geek]

They will do only 2 to 2.5W without NFB.

[tubelab.com]

OK, despite the football game on TV, I got the transformers tested. I have lots of data, too much to post here, so it will be posted on my web site in a few days. The short story follows:

I took my TubelabSE, which is currently running Shuguang 300B's through custom 5K ohm Electra-Print transformers, and measured the vital statistics. I used this amp because it works very well and has excellent 5K transformers. The other transformers were connected into the amp and compared to the Electra-Prints. This amp is running a small power transformer so that 45's, 2A3's and 300B's can be used. The B+ voltage is about 300 to 310 volts with 300B's, depending on the bias current. This (and the 5K load) limits the power to 5 or 6 watts.

Three different low cost transformers were tested. Both electrical and listening tests were performed. The bias was adjusted to the point where the third harmonic was minimized. This was 50 mA on the Electra-Print transformers. The 3 other transformers were tested at the minimum 3H point, and at 50 mA.

Electra-Print:
These are the reference transformer, I have found nothing better for this amp. Best sound is achieved at 50 mA with 300B's. Distortion measures 2.3% at 4.5 watts (mostly 2nd harmonic), frequency response is 14Hz to 28.7KHz, and visible clipping appears just over 5 watts. Specs measured at 1watt: Frequency response 3.6Hz to 29.1KHz, distortion 0.91%. This transformer is only rated for 10 watts, but it is 5 times the size of the others. No visible saturation effects were seen at 5 watts and any frequency above 20Hz. The sound is excellent, this is the amp that I use daily.

Hammond 125CSE:
These have been the best choice in low cost transformers. Best sound is achieved at 70mA. This transformer makes about 1 watt more above 100Hz than the Electra-Print. Specs at 1 watt: frequency response is 11.7Hz to 42.9KHz with 50mA, and 10.4Hz to 44.2KHz with 70 mA. Distortion is .65% with 70mA (mostly 2nd harmonic). Distortion is 2.6% at 6.0 watts.
Low frequency effects at 5 watts: Visible saturation can be seen on the scope for any frequency below 77 Hz at 50 mA, 64Hz at 80mA. clipping was observed at any frequency below130Hz with 80mA, but none was noted at 50 mA.
Low frequency distortion tests at 1 watt: 100Hz lowest distortion was 1.01% with 63 Ma. At 36 HZ .66% occurs with 58mA.

Edcor XSE=15-8-5K
These are a new player in low cost transformers. Best sound is achieved at 80mA. This transformer makes about 2.5 watts more power than the Electra-Print. Specs at 1 watt: frequency response is8.3Hz to 45.9KHz with 50mA, and 16.6Hz to 45.6KHz with 80 mA. Distortion is .75% with 80mA (mostly 2nd harmonic). Distortion is 2.6% at 7.2 watts.
Low frequency effects at 5 watts: Visible saturation can be seen on the scope for any frequency below 74 Hz at 50 mA, 61Hz at 80mA. No clipping was observed at any frequency below130Hz with 80mA, and none was noted at 50 mA.
Low frequency distortion tests at 1 watt: 100Hz lowest distortion was 1.41% with 57 Ma. At 36 HZ .61% occurs with 44mA

Triode Electronics TF103-48
This transformer is about half the size of the others tested.
Specs at 1 W: frequency response 22.1HZ to 18KHz, at 50mA. and 18,9 HZ to 17.3KHz at 40mA. THe distortion is .74% at 50Ma and .65% at 54 mA. Visible saturation occurs at 99Hz at 50Ma.


The Electra prints obviously are not afraid of any of these transformers, I listened to the Edcors, and the Hammond for a while and I find them very similar in specs. The Edcors like more current than the others. The little Triode unit was made to be used in a guitar amp. It would work well there. It is jus too small to be really usefull. If The Hammond and The Edcor were the same price, I would have to go with the Edcor. The fact that it is almost half price seals the deal. I would have no problem using them in a low cost amp.

The Sound tests showed what they usually do. The bigger the transformer, tbe better the sound. The Edcor and the Hammond are very similar sounding transformers.

[leadbelly]

Thanks for the great news! Now I'm eager to see my 25 watters arrive!

[tubelab.com]

There was some information missing from the previous post, and some poor typing. Hey it was late and I was trying to type, watch TV (you don't need sound to watch football) and listen to the amp at the same time.

The Edcor unit showed some anomalies in testing:

I adjusted the amps bias current for lowest distortion at several different frequencies. The Edcor transformer wanted less and less current as the frequency was decreased. This effect was not near as severe with the Hammond transformer. Both Edcor transformers exhibited this effect.

The Edcor transformer produced considerably more power than any of the other transformers tested. I got 7.5 watts at the onset of clipping with the Edcor, 6.1 watts with the Hammond, 6.0 watts with the Electra-Print, and 5.2 watts with the Triode unit. This indicates that it is a really efficient transformer, or that it is a lower impedance than 5K ohms (more likely). I will test this in the next few days.

Despite the low frequency anomalies I liked the bass better with the Edcor transformer. I believe that the Edcor unit sounded better, but there was a 1/2 hour setup time in between tests that makes it hard to tell. In fact the Edcor and the Hammond sounded very similar. Both sounded far superior th the Triode unit which was never meant to be used for HiFi. Neither sounded as good as the Electra-Print which costs (and weighs) 5 times as much although the differences were subtle.

All listening tests were done at 70mA of bias current (the Triode unit was tested at 50mA), which was the most that I wanted to run for a continuous (2 hours) test, since I was pulling 180mA (2 X 70 + 40) from a 120mA power transformer. I did not notice any saturation type distortion when listening to a bass guitar solo at full volume. The clipping that occasionally occurred did not sound very obvious (observed on scope).

The Edcor transformer has a UL tap which opens up some more possibilities for designing a low cost amp. I guess I will have to re-visit the 6AV5 torture testing, and maybe try out some of the other popular pentode tubes.

This transformer may respond well to cathode feedback. The Hammond 125CSE sounds remarkably better with the secondary wired in series with the output tubes cathode. I can't do this with the existing TubalabSE board, so I didn't try it yet.

I would consider this a great transformer for under $20. This one has convinced me to try some of their other transformers. It would be cool if they made a 200 watt SE transformer.

[kevinkr]

Hi Tubelab,
Very thorough work as usual, football game not withstanding and the results are encouraging. I am thinking about getting a pair of the 25W 5K types for a new 45 amplifier design I am working on, I think based on your comments I am willing to take the plunge and order some as soon as funds permit..

I have heard good things about output transformers made by Transcendar as well. Anyone using theirs?

Kevin

[tubelab.com]

I bought two of the 300B transformers that he (Transcendar) is selling on Ebay. I put them in a TubelabSE amp and tested them against the Electra-Prints and One Electron UBT-3. I did not do a thorough lab test, just listening tests with about 5 people. Most people could not distinguish the Transcendar and the UBT-3. Most people could pick out the Electra-Print, although one person said that it did not sound as good as the other two. The Electra-Print is 5K ohm, and the other two are 3K.

Based on these results I purchased 10 more pairs of the Transcendar transformers. Many of these have been assembled into amplifiers and found new homes with happy owners.

If you ever saw the 833A amplifier experiments on my web site, the sample transformer that I tested was custom made by Transcendar. It did not quite work as well as expected. The 3db point was 17 KHz, which is not quite good enough for a high end, high dollar amplifier. It would make a wicked, though impractical, guitar amp. I have been told by several experienced transformer people that a 100 watt SE transformer design is not easy!

I have frequency response and distortion FFT plots for all of last nights experiments that I will put on the web site when I get time. Probably next weekend. I plan to test them further with other tubes.

If you look at the Edcor transformers the "15 watt" transformer that I tested is very close to the same size as the Hammond 125CSE. It (Edcor) has 2 more laminations in the core. The 125CSE is rated by Hammond for 8 watts. The Edcor is $18 and the Hammond is $28. A serious advantage for the Edcor.

The Edcor "25 watt" transformer looks to be similar in size to the Hammond 125DSE, which is rated for 10 watts. The price difference is $4.50. I believe that the hammonds are conservatively rated, while the Edcors may be optimistic.

I am thinking about getting a pair of the 25W 5K types for a new 45 amplifier design I am working on

Are you refering to the CXSE25-8-5K which costs $75 each? The TubelabSE that I used for last nights experiments was originally designed for 45's, that is why it uses 5K transformers. After hearing a friends 45 amp, I had to have those transformers. Jack (Electra-Print) made me those for $100 each. I know that his prices have gone up since then, but they might still be reasonable. These are the best sounding transformers that I have tried. Jack knows these as the "Albert Pagan" design. Basically a 5K primary with a single 8 ohm secondary.