Fender Deluxe Reverb DIY Kit: Distorted Signal and Noise in V7 and V8

[SirBlackmore]

Hello everyone,

For a while now, I’ve been building the Blackface 22 kit from Tube Amp Doctor, which corresponds to a Fender Deluxe Reverb (AB763).

When it came time to adjust the BIAS and plug in the guitar to test, I noticed that the sound was unpleasant and somewhat distorted. So, I started taking measurements to locate the issue, tracing through the Vibrato channel.

I input a 1 kHz sine wave signal at the vibrato input. In the output of the preamp tubes, the measurements with the oscilloscope appear correct, yielding a sine wave at their output. But it’s at the power tubes where I see the problem, and I hope someone can guide me on where the issue might be.

The image attached is the measurement at the output (pin 3) of V7 and V8 (6V6GT):


The yellow signal corresponds to the signal at the input of the amplifier's vibrato channel. The blue signal is at pin 3 of V7 (left image) and V8 (right image). In the output signal, I notice the wave is distorted and also has noise.

I’ve measured these voltages at V7 and V8:

V7
Pin 3: 442 V DC
Pin 4: 441 V DC
Pin 5: -60.70 V DC
Pin 2 to 7: 6.22 V AC

V8
Pin 3: 442 V DC
Pin 4: 441 V DC
Pin 5: -60.70 V DC
Pin 2 to 7: 6.22 V AC

BIAS pot is turned fully counterclockwise.

I’d like to ask:
- Where could the problem be coming from?
- I also notice that in V7 (left), the output signal has (almost) the opposite phase of the input signal. In V8 (right), the
phase is almost the same as the input. Are these phase differences significant? Could they be caused by the same issue?
- Is there any other measurement or test you would recommend? Right now, I don’t have other 6V6s to test, so I’d like
to try other possible causes first.

I hope someone can help me with this. Thank you very much.I hope someone can help me with this. Thank you very much.

 

[Gerald Jansen]

I can not directly help you. But may I ask if this is the circuit you are dealing with:
https://www.tubeampdoctor.com/media/pdf/76/cf/64/full_documentation_bf-22-reverb_18-02-21.pdf

 

[SirBlackmore]

I can not directly help you. But may I ask if this is the circuit you are dealing with:
https://www.tubeampdoctor.com/media/pdf/76/cf/64/full_documentation_bf-22-reverb_18-02-21.pdf

No worries! Yes it is.

 

[CJ]

Your bias is too negative,

Especially if using modern tubes like JJ or EHX Svetlana or Sovtek.

You want about minus 45 for today's tubes, they bias in at close to EL34 specs nowadays for reasons unknown but I bet it is grid spacing.

Measure DCR of output xfmr from CT to each leg. Fire it up and measure V drop across each leg, then get out the calculator unless you already have 1 ohm R's on the cathodes.

If RCA black plates then keep them at minus 52.

Wait, did you say DeLuxe or Twin? Me bad. Disregard everything. Please refer question to qualified service technician. No user serviceable parts inside.

Drop bias to 6V6 current specs and your plate voltage will come down to 410 where it should be. If your power transformer starts to ooze black tar after 3 hours of hard core then your bias is perfect.

Wait a minutes, doesn't Blackmore use Engle amps?

 

[Alva Goldbook]

You see that 1nF cap going into the phase inverter? Try replacing it with a 500pF cap and see if things improve. Report back if things sound any better in terms of the distortion. It may or may not improve things.

This problem occurs most often in Princeton Reverbs because of the cathodyne PI. It happens less often on LTP PI's like you have here, but what can happen is if the output coupling caps on the PI are too large, the opposite phases can shift to such a degree that they may start to cancel each other out and cause a really nasty blocking distortion. It's most noticeable on bass notes.

The 100nF output coupling caps for the phase inverter was sort of a "screw it, make it a randomly big value" decision by Fender. These days we have fancy frequency calculators. Later Fender fixed this by putting a 1nF or 500pF input coupling cap, but the issue was always the 100nF coupling caps at the output. Honestly a value as low as 22nF will pass all the bass frequencies needed for guitar. 100nF will pass stuff down to 10Hz, which is well below the human hearing range.

So, I don't know if this is the issue or not. I may be barking up the wrong tree, but if this is the issue, reducing the 1nF cap to 500pF should help improve it. If not, it's something else and we can investigate to see what else could be causing this.

 

[musipol]

My vote is the bias, as mentioned by CJ.

 

[Nele]

The schematic actually says you should adjust the bias to -43.3V...
You can use CJ's method for measuring. You should have 5.4V between center (red wire) and each leg (pin 3 on the output tube sockets). See the schematic...

 

[Alva Goldbook]

The schematic actually says you should adjust the bias to -43.3V...
You can use CJ's method for measuring. You should have 5.4V between center (red wire) and each leg (pin 3 on the output tube sockets). See the schematic...

This is not correct.

 

[Alva Goldbook]

The most accurate way to measure bias is the transformer shunt method.

With the amp off and the voltage completely drained down to 0.0 volts, attach meter from output transformer center tap to the plate of each output tube. Measure the resistance and write it down. This is the resistance of the output transformer winding. It can be anywhere from 40 to 150 ohms typically. Make sure you write down the precise resistance, as in 42.5 ohms or whatever it is.

Then leaving your meter in place, measure the voltage drop across this resistance. Write it down.

These two numbers will get you the plate current. Just punch the resistance and voltage drop into an Ohms Law calculator.

Then measure plate voltage. I just leave the meter on the output transformer CT and put the other to ground.

Plate current x plate voltage = watts.

6v6’s are 12 watt tubes. JJ6v6’s are 14 watts. You want 70% dissipation.

12 x 0.7 = 8.4 watts per tube.

In Blackface designs I often prefer to go to 60-65% dissipation. Tremolo can sound more lush and full this way.

 

[Nele]

This is not correct.

Don't shoot the messenger... Just quoting the notes as printed on the schematic.