Vibrolux voltage measurements

[jbever]

Just got a late '70's Vibrolux, blackfaced to some extent, I took voltage measurements tonight and some of them seem materially off. There is a 12AT7 in V6. The transformers appear to be original. Any input would be appreciated. Thanks
Jerry

Order of info: Pin, AA270 layout reading, actual reading
These are the wires coming up from the cans on the bottom of the chassis to the left top of the board looking from the back of the amp.

Red Wire: 410 447
Yellow Wire: 408 443
Brown Wire: 310 420

V1
Pin 1 200 247
Pin 3 1.6 1.6
Pin 6 200 264
Pin 8 1.9 1.7

V2
Pin 1 200 228
Pin 3 1.5 1.7
Pin 6 200 223
Pin 8 1.5 1.7

V3
Pin 1 425 436
Pin 8 8.5 9.3

V4
Pin 1 200 240
Pin 3 1.7 2.1
Pin 6 200 241

V5
Pin 1 300 432
Pin 3 2.0 2.1
Pin 6 360 346
Pin 8 18 6

V6
Pin 1 270 245
Pin 3 100 85
Pin 6 270 234

V7
Pin 3 440 444
Pin 4 455 444

V8
Pin 3 460 444
Pin 4 455 444

[TheAmpNerd]

Looks close enough for me.

The scematic says + or - 20 percent.
These are close enough.

If it was blackfaced then go check the bf voltage spec.

yes, V6 has traditionally been a 12AT7.
Same with v3, should be a 12AT7.

What are you worried about? Does the amp
sound like shit or something?

What is the bias set point.

What are the value of the resistors in the
power supply cap section?

Thinking this should explain some of the
Higher voltages int he pre amp tubes.
Also check the voltage divider resisters, though
and the v1 tube....might be going out.

But your main pre amp tube, V2, voltages are fine.

[ChrisGS]

A buddy of mine has two Vibrolux Reverbs. One is a Blackface (1966?)which has about 440V after the rectifier(GZ34), and the other one is a Silverface with the drip edge trim, which has about 460V after the rectifier (5U4). The Silver face one has a power transformer with extra tap wires capped-off for different wall voltages.

I think that I like the one with the lower voltages.

[Blue Strat]

Jerry, didn't you say this amp was running a 5AR4/GZ34 instead of the stock 5U4GB? If so, that explains the higher than stock readings.

[jbever]

Ah .. yes. Thanks for the info and I will check out what is mentioned. Mike is right, in a previous conversation I did mention that the amp was running a 5AR4. Thanks Mike, I didn't even think of that.

I was checking voltages because the Vibrato channel has less volume when set on the same volume knob number than the normal channel and I was curious to see why? I did replace the vibrato channel volume pot, with a 1 meg Audio pot and checked my wiring. The tone sounds good just a drastic drop in volume.

I noticed the differences between V1 and V2 actuals. Is this demonstrating the difference in volume? Thanks
Jerry

[Blue Strat]

Ah .. yes. Thanks for the info and I will check out what is mentioned. Mike is right, in a previous conversation I did mention that the amp was running a 5AR4. Thanks Mike, I didn't even think of that.

I was checking voltages because the Vibrato channel has less volume when set on the same volume knob number than the normal channel and I was curious to see why? I did replace the vibrato channel volume pot, with a 1 meg Audio pot and checked my wiring. The tone sounds good just a drastic drop in volume.

I noticed the differences between V1 and V2 actuals. Is this demonstrating the difference in volume? Thanks
Jerry

Were the volumes between the 2 channels more compatible before the pot change? If so, the original pots were likely of a different taper (linear or something a little different than the standard audio taper) and "come on" more quickly than the new pot you installed (was there a particular reason that you changed pots?).

A way to prove this is to measure the resistance of the pots from the wiper (center terminal) to ground and compare them. You'll probably get better meter readings if you disconnect the terminal opposite the grounded leg.

[jbever]

The vibrato channel volume pot was very loose as you turned it, so I decided to change it. I'll measure them and see what I get. Thanks a bunch.

[TheAmpNerd]

Swap tubes of V1 and V2....any difference?
Any measured difference?

I don't think the rectifier would increase voltages
on some tubes while decreasing the voltages on other
tubes. What are the values of the voltages drop resistors
in the dog house.

Also, if you swap the the pre amp signal wires does that
make a difference in volume? I.E., change the two wires around
at the 200K channel mix resistors...(this feels the .001 cap at
the PI).


Write it all down as you go though it too. Use as a reference.

Let us know what you find out.

[Blue Strat]

I'd be willing to bet that the V6 plate readings (the only ones with the reported lower voltages) is an error in transcription.

A 5AR4 will indeed increase plate voltages EVERYWHERE in the amp.