Is this power transformer up to the task?

[makinrose]

Hi everyone.  I'm looking for some input.  I had a custom-built amp come in the shop that essentially is one channel a 60's Fender Bandmaster with a three-tube spring reverb unit in front.  The transformers are a Super Reverb clone for the power and Bassman clone for the output.   The thing I'm concerned about is current draw on the high voltage secondary since the amp is now powering a 6K6 power tube in the reverb portion as well as the 6L6GCs.   

The power transformer has rating of 300 ma for the 325-0-325 secondary.  In a Super Reverb with a 5U4GB about 248 ma would be drawn leaving a healthy safely margin.  My best calculation is that this circuit would draw about 270 ma.  Is that enough of a safety margin? The 6k6 is drawing about 30 ma.  Should I be concerned?

[Sherlok Ohms]

Hello Mr. Makinrose

Since your concern is primarily regarding the reverb driver tube, might I suggest we look just at that?

The original Reverb unit with the 6W6 or 6V6 uses a 1k-ohm for bias, with +24V indicated on the schematic. Simple Ohm's Law says there is 24mA through the tube.

On the later reverb circuits using a 12AT7 with its triodes wired in parallel, has 2,200-ohms for bias and shows just +8.4V. This works out to 3.8mA which is one-sixth of the 6V6 current.

Overall, I do not believe that the difference of approximately 18mA will be significant to neither the load on the power transformer nor to the overall output power or perceived loudness of the amp based on the notion that both the Super Reverb and Bandmaster Reverb are 40W amplifiers.

We have no way of knowing what the actual specifications are for the original Fender transformers - at least for the power transformers - but we can make educated guesses.

My guess is that you have nothing to worry about.

Oh, you mention the OT as being 6k6. At 80W peak output 220mA is needed, which on a DC-basis would be 155mA.

Cheerio

[makinrose]

Hello Mr. Makinrose

Since your concern is primarily regarding the reverb driver tube, might I suggest we look just at that?

The original Reverb unit with the 6W6 or 6V6 uses a 1k-ohm for bias, with +24V indicated on the schematic. Simple Ohm's Law says there is 24mA through the tube.

On the later reverb circuits using a 12AT7 with its triodes wired in parallel, has 2,200-ohms for bias and shows just +8.4V. This works out to 3.8mA which is one-sixth of the 6V6 current.

Overall, I do not believe that the difference of approximately 18mA will be significant to neither the load on the power transformer nor  to the overall output power or perceived loudness of the amp based on the notion that both the Super Reverb and Bandmaster Reverb are 40W amplifiers.

We have no way of knowing what the actual specifications are for the original Fender transformers - at least for the power transformers - but we can make educated guesses.

My guess is that you have nothing to worry about.

Oh, you mention the OT as being 6k6. At 80W peak output 220mA is needed, which on a DC-basis would be 155mA.

Cheerio

Thank you kindly for the reply! My guess is also that isn't enough to be worried about too but I want to be sure so the customer can rest easy.  The output transformer is 4 K so current draw.  With the 4K primary on the OT do you think there is a large enough safety margin?

[Sherlok Ohms]

Greetings

Perusing he Hammond replacement transformers for guitar amps, specifically the Super Reverb and the Bassman, I find that the 290D2X is listed as a suitable PT for both. This is rated at 300mA for the plate winding, just as you mention above. Might this be the device in question?

You also mention an OT of 6k6 in the first post, but a correction of 4k as the Bassman OT. Even though the Bassman OT would be "wrong" in a Super or bandmaster, it seems compatible with at least the Hammond PT clone.

Cheerio

[makinrose]

Greetings

Perusing he Hammond replacement transformers for guitar amps, specifically the Super Reverb and the Bassman, I find that the 290D2X is listed as a suitable PT for both. This is rated at 300mA for the plate winding, just as you mention above. Might this be the device in question?

You also mention an OT of 6k6 in the first post, but a correction of 4k as the Bassman OT. Even though the Bassman OT would be "wrong" in a Super or bandmaster, it seems compatible with at least the Hammond PT clone.

Cheerio

A 6K6 is the power tube type in the tube reverb circuit.  Fender used 4K for the Super, Vibroverb and Bassman---anything Black or Silverface during the 60's and 70's with a 40W output rating.   The Bandmaster and Pro used a smaller OT unit, but it was also usually 4K during those years.

The transformers in question are all Mercury Magnetics designed for the Vibroverb.