Getting Inside the Reverb Deluxe

Just before I moved from California to Sweden, I sold my Fender Bandmaster and picked up a more train friendly amp, the Reverb Deluxe Reissue. It’s a fantastic amp with a nice clean sound. I also picked up a dual voltage transformer so that I could plug it into the 220V mains here in Europe. Well, fast forward 6 months later and I finally got around to it. The folks at ClassicTone claimed that the transformer would not fit the reissue, but I had a feeling that it I could get it to work.

The first step was to measure the voltages of the existing transformer, then check the output of the dual voltage transformer.

Checking the voltages on the stock transformer.

The 660 volt secondary voltages matched up fine, so did the 6V heater taps and the 5Volt Taps. However, the AV bias voltage is 50V on the new transformer, while the Reissue’s stock transformer is 35 Volts.

I proceeded with the install and decided to test my circuit voltages after the procedure. The new transformer didn’t align perfectly, but it was pretty close. I had to drill 2 small holes to align the mount points. It could have been MUCH worse! Here you can see the new tranny with washers to hold it in place.

The amp comes on, but it’s fairly quiet and I noticed that the bias current is very low, Because I don’t have a bias probe, I measure the voltage drop across each side of the 220 ohm output transformer to determine the bias current. Before, the drop was 3.5 volts, now it is .5 volts, even with the bias adjustment turned all the way up. The B+ voltage coming off the rectifier tube was 443 (before it was something like 396).

After measuring, my bias supply (C-) was too hot for the circuit as expected.. (50 volts AC from the red/blue tap on the transformer, after the diode is -67 V DC, however the schematic calls for -49VDC). The new transformer is imposing too much negative voltage on the input signal to the tube. So I need a voltage divider to bring the bias voltage down to the spec range.

They don’t have a proper electronics shop here in southern Sweden, and mail order is like going back to the stone ages, so i found a 10k resistor in my project box and put it in series with the bias adjust pot, effectively making the max resistance 20k on the pot. Now the voltage at TP36 (the wiper on the pot)  runs between -35V and -50V, thus bringing the voltage across the output transformer to 3.5 – 4.5 volts, creating around around 20ma of bias current. This is acceptable and the amp is sounding like it should again!

Next, carefully studied the transformer primary portion of the schematic and wired a DPDT switch so that I could easily switch the supply voltage for traveling between North America and Europe. Here is the result!

Dual Voltage Switch!

While I had the amp open, I read about a few more mods and decided to clip the bright cap on channel 2 (the original Blackface did not have this and the Channel 2 is extremely bright!). I just lifted the leg of the capacitor off the board so that it would be very easy to reverse this mod.

Bypass the bright cap. Much better!

Lastly I did the Fritz mod as described here, allowing reverb and tremolo to be used on both channels. It was a very easy mod that gives a lot more versatility to the amp. I still think channel 1 sounds fuller and it’s really nice to have reverb on it now.

Fritz Mod. Add reverb and Tremolo to CH1!

I’m super happy with this amp. I plan to do a few of the tone circuit mods as described here in the future, but for now, the amp sounds great. Here is the DRRI with it’s perfect match, the AC15 for an incredible stereo blend!

Sweet Stereo!

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