I have a large quantity of the guitar amp specific open frame Push Pull OPT's seen here, still in their original 8 pack boxes from 1990. These are the same transformers seen in my "under 100 WPC" tube amp experiments dating back to the early 2000's. Originally made by Schumaker Electric for ADA Signal Processors for guitar amp duty, they are rated for "80 VA from 82 Hz to 5000 Hz 6600 ohms to 0-4-8-16." They work well at 3300 ohms to 0-2-4-8, which is how I usually run them.
It is pretty hard to make an OPT that only goes to 5KHz, so I have tested them in HiFi applications. They were used in my 300Beast, a zero feedback push pull amp that used 300B tubes. Here they went from 25 Hz to 18.5 KHz at 10 watts with some saturation seen at 28 watts below 30 Hz. They do better when driven with low impedance sweep tubes, and slightly worse in an SPP amp with EL84's that really want an 8000 ohm load. They are a standard size as I have found some end bells from other transformers, that fit.
Stuffing over 100 watts through one for guitar use seems OK, but don't play bass through one beyond about 35 watts. I will sell small quantities to interested builders. for $30 each. They weigh a little over 5 pounds each so shipping isn't cheap and international shipping is probably not viable. Up to 4 transformers will go into a medium flat rate box for delivery in the US for $19
For those that shop hamfests, I have been selling these at the Dayton Hamfest for years, but at a higher price. I will be there again this year, so if you want these at the Hamfest, mention this forum to get them at $30.
It is pretty hard to make an OPT that only goes to 5KHz, so I have tested them in HiFi applications. They were used in my 300Beast, a zero feedback push pull amp that used 300B tubes. Here they went from 25 Hz to 18.5 KHz at 10 watts with some saturation seen at 28 watts below 30 Hz. They do better when driven with low impedance sweep tubes, and slightly worse in an SPP amp with EL84's that really want an 8000 ohm load. They are a standard size as I have found some end bells from other transformers, that fit.
Stuffing over 100 watts through one for guitar use seems OK, but don't play bass through one beyond about 35 watts. I will sell small quantities to interested builders. for $30 each. They weigh a little over 5 pounds each so shipping isn't cheap and international shipping is probably not viable. Up to 4 transformers will go into a medium flat rate box for delivery in the US for $19
For those that shop hamfests, I have been selling these at the Dayton Hamfest for years, but at a higher price. I will be there again this year, so if you want these at the Hamfest, mention this forum to get them at $30.
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In response to some questions, the wiring follows the typical color scheme seen in vintage push pull OPT's. Plate and B+ is on the right side in the pictures. Red is B+, blue and brown are the plate leads. Speaker connections are on the left. Black is 0 ohm or speaker common (typically grounded). The rest go in order, yellow is 4 ohm, green with yellow stripe is 8 ohm, and green with no stripe is 16 ohm. The transformer is rated for 6600 ohms at "80 VA" into 0-4-8-16 ohms. It can be used as a 3300 ohm to 0-2-4-8 ohm OPT. The hookup is the same and the ratios do not change.