About 20 years ago I bought 200 surplus transformers to make guitar amps. The transformers were made by Schumaker for ADA. ADA was a well known guitar effects maker in the 80's. They had ordered a large quantity of OPT's for a tube amp that apparently was never made. ADA went out of business (due to a fire ?) some time in the 90's I think. The transformers were all over Ebay in the early 90's. There was a single page spec sheet that said "6600 ohms CT to 0-4-8-16 ohms @80VA. Frequency response 80 Hz to 12KHz". The rest of the page was mechanical and wiring data. The size is about 5 pounds.
I got one sample before buying them which looked familiar. I owned (and still have) a Schumaker automotive battery charger and the transformer inside it has the same core and construction.
I connected the sample transformer into a Bandmaster and it sounded better than the stock Fender OPT so I bought 200. I used them to make guitar amps with 6L6GC's on 430 volts, and later Sovtek 6550WA's with a 2, 4 or 8 ohm load to reflect 3300 ohms to the tubes. I never measured anything but power output on any of those amps, but they were well liked.
I took one of the OPT's apart and found zero interleaving. One half primary was wound completely, then the entire multi tapped secondary, then the second half primary was wound with thicker wire to offset the longer winding diameter. All insulation was paper. There were no end bells, just an open frame paper core transformer.
I made a KT88 HiFi amp with a pair of these OPT's. I ran the tubes on 400 volts and the 6600 ohm configuration for about 35 watts per channel. Adjustable GNFB was used. There was an unlabeled knob that applied from zero to about 12db of feedback and reduced the gain by a similar amount as the feedback was turned down. The amp sounded far better than it's OPT dictated. I had measured specs somewhere, but I don't know where right now.
I used these OPT's to breadboard and test a P-P 300B amp about 15 years ago intending to replace them with a set of 6600 ohm vintage UTC's that I have. The 300B amp was just an experiment to see how it would work, but it sounded awesome, very dynamic. It was all made with junk box parts, but it just worked. This was a tube amp that got drums and percussion RIGHT....Any attempt to upgrade that amp just messed it up. The UTC's made it sound harsh.....So I made a chassis for it and left it the way I built it. It's the only amp from the 90's that I still have.
I have used these transformers in several amps with varying degrees of success over the years. I use them for all initial experiments where something could go wrong, like running 13GB5's on 650 volts! I set one OPT on fire several years ago when the load resistor blew at full power. So I bought a pair of 500 watt 8 ohm resistors.
I have never tried to measure the primary inductance, but I suspect it's too low to support 20 Hz operation with a tube like a 6L6GC at 6600 ohms, at any power level.
Core saturation appears at different frequencies and power levels depending on load and the driving tube impedance. The best results occur with a low impedance tube like a 300B or a sweep tube with local feedback around it and the 3300 ohm connection. The 300B amp (6600 ohms) made 25 WPC all the way down to 20 Hz. A 6HJ5 sweep tube amp (3300 ohms) with local feedback would make 60 watts down to 30Hz and 35 watts at 20 Hz. It also makes 125 watts at 1 KHz. An SPP using EL84's at 6600 ohms and 8db of GNFB would make 25 watts at 1 KHz, but saturate full power below 60 Hz. Power at 20 Hz is only 6 watts. The SAME amp with 6CW5's in it and a 3300 ohm connection made the same 25 watts at 1 KHz, but could keep tat power level all the way down to 30 Hz.
In my playbook, there is, but I like to experiment. If you have them, then what do you have to lose except time. But as with anything in life, we trade time for knowledge.
The going price seems to be about $3 now....we talk too much, and there are people with too much money that listen!
I wish I can find something like that!! I experimented with two Classic Tone OT, one is for Bandmaster, the other for Marshall Plexi 50. I even put switch to switch the OT from one to the other. They sounded the same. I went back and fore many many times and still can't tell the difference and different volume.
Both the Classic Tone Marshall and the Marshall 100W OT have only two section primary. But they do have two section secondary also. I guess it's normal for guitar amp.
Problem is I don't have a pair of identical OT, they are so different. I don't have any plan to build another guitar amp, so I really don't want to spend the money to buy OTs just to switch them out for Lundahl later. Also, guitar OT doesn't have UL tap either.
Ha ha, I wonder what will the power amp of the guitar amp sounds like using as stereo amp. The power amp section is just a simple minded Class AB push pull amp with LTP driving the power tubes. That won't be too hard for me to wire two guitar amp to try that out!!!
I am Looking for @ Power OUTput Transformers to Accomodate * KT88 in PUSH Pull Mode.
Please Contact Francis Jansz Ph:0403 146 207 E:Mail: fsj@iprimus.com.au
Please Contact Francis Jansz Ph:0403 146 207 E:Mail: fsj@iprimus.com.au
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