First Time Biasing my Original '68 Fender Deluxe Reverb

[g1]

Also,

My previous voltages were 5v too high so today I adjusted the grid bias again to lower the plate voltage and current. I dropped it more in line with the schematic voltages and got my plate voltages to 416.5vdc, which gave me about 19.5-20ma, in line with recommendations here.

Idle current and plate voltages are inverse, increasing one decreases the other. So if you decreased your idle current, your plate voltage and B+ should have increased.
I thought your idle current was higher than 20mA before, so I'm not sure how you could have decreased idle current and B+ voltage at the same time. You should recheck your numbers.

[Chuck H]

I've wondered at times why no one has ever used the zener across a cathode resistor thing to reduce "working" volts in amps like the DR. Just add a cathode resistor that biases 21V or 22V and put a 20V zener across it to fix the voltage. You could just use the zener, but resistors are good, durable buffers for such a circuit. So if you have 430V and add such a circuit your "working voltage" (the figure that matters to the tube) is now 410V. Take up the bias slack with the existing bias circuit, perhaps slightly modified to get a lower -voltage. Easy Peazy.

[Perkinsman]

My mistake, I rechecked today and you are correct, my plate voltage went up to 431.5v. The schematic states that plate voltage should be 415v, but because of this inverse relationship, I had to increase it in order to get the current down. +/- 20% would allow from 373.5 to 456.5, so I'm good! Thanks for the assistance, my amp sounds a little quieter but it's got a great blackface tone!

[pdf64]

Whenever you put an amp on the bench to check its operation, the first measurements to be made should be the mains and heater Vac; the ratio between the actual mains voltage and the nominal voltage that the transformer's primary was intended for will be applied to all secondary voltages.
If the precise intended primary voltage is unknown (eg USA may be 110, 115, 117, 120), the reciprocal of the ratio between the actual heater voltage (with tubes fitted!) and its nominal, when multiplied by the actual mains voltage, will provide an accurate indication.

[Perkinsman]

Whenever you put an amp on the bench to check its operation, the first measurements to be made should be the mains and heater Vac; the ratio between the actual mains voltage and the nominal voltage that the transformer's primary was intended for will be applied to all secondary voltages.
If the precise intended primary voltage is unknown (eg USA may be 110, 115, 117, 120), the reciprocal of the ratio between the actual heater voltage (with tubes fitted!) and its nominal, when multiplied by the actual mains voltage, will provide an accurate indication.

Is the reason for that to understand why the measured voltages may be different than the schematic? I noticed on the AB763 schematic that there wasn't a stated mains voltage. My home voltage tested at 120.5VAC, which I'm sure was a bit higher. Btw, I liked your song, "Unlucky in Love", nice job!

[pdf64]

Unfortunately the intended mains voltage isn't always noted on every vintage Fender schematic or layout (might be on either, both or neither); however, by checking others contemporary to that of interest, we can be reasonably certain what it was. In this case, where noted, from the late 50s to the late 60s, most all Fender have 117Vac as the nominal mains, eg see the non reverb Deluxe AB763 https://el34world.com/charts/Schemat...763_layout.pdf

120.5/117 indicates that all PT secondary voltages (and directly related Vdc) will be about 3% high, all else being equal; hence expect ~433Vdc at the standby switch / reservoir cap / OT CT HT node.

The high idle current that your power tubes were pulling will have dragged that down, hence you measured ~425Vdc; but of course your mains may not have been 120.5 at the time of that measurement, hence the importance of checking it each time other measurements are being made.

Thanks for the kind words

[67super]

First post here so Hello to all.

One thing I've done over the years to lower the plate voltage in vintage (or any) amp is to substitute a 5R4 rectifier. In the DR for example it has a 5AR4 rectifier which has little DCV drop across it. By substituting the 5R4 you effectively lower the plate voltage by around 20VDC IME. The DC drop across the 5R4 is a lot higher than the 5AR4 so you will get sag but I generally like the effect on my amps. I use them in PR and DR circuits all the time. Since they are not used much you can purchase NOS ones very cheaply.