65 Vibro champ voltage check?

[clinkous]

Hello everyone,

I just did a cap job on 65 Vibrochamp and have a few questions on the voltages I'm getting.

All the tubes and transformers I believe to be original and working.

I put in original value Sprague electrolytics and replaced the V3 cathode resistor with a 3 watt. The old one was burned and reading around 200k. Replaced with a 470k. The cap can is a 40/20/20.

B+ at the cap can reads:

Main to OT 415V
Screen 405
V1 373

V1

1 228V
2 0V
3 1.98V
4 All heaters 6.9V
5
6 233V
7 0v
8 1.90V

V2 All variable except

6 403V
8 0.01V

Is this because of the tremolo? I confess I don't understand the tremolo circuit just yet. All controls were set to zero. Should I disconnect something to read V2 voltages?

[B]V3[B]

3 399V
4 403V Why is this higher than the plate?
5 .50
8 23.5V

The PT secondary is 356V on each side.

Wall voltage is 122.6V

The schematic shows much lower voltages but I've read that 400V B+ is not uncommon. However the schematic shows the screen voltage to be lower than the plate voltage and I don't understand why mine is higher.

Haven't played the amp yet but before the cap job it had farty bass especially playing the low E with the B and especially the low E/Db. Did the same through my other known good VC speaker. Swapped all tubes with known good ones and it made no difference. Problem might be solved as all caps were original.

Wanted to run these voltages by you before I played it to see if anything looked wrong right off the bat.

Thanks
Chris

[The Dude]

First check: Does it have a 5Y3 installed, or has someone substituted something else?

Also noticed that you have around 50mv bias. Try upping the cathode resistor value maybe to 1K. But again, check the rectifier tube number first.

[Jazz P Bass]

If you want to resolve the farty bass response, change V1 cathode bypass capacitor to a lower value.
10uf or 4.7uf.

[clinkous]

The rectifier is a RCA 5Y3. I tried a new Sovtek 5Y3 and got 423V B+ and 26V on pin 8. I ran the numbers by you guys in an earlier post (I'll find them again) and the PT number was 606540 so I believe it's the original.

I know these aren't gonna shake my pants leg with the bass, I just noticed that this one was much weaker than my SF VC when played through the same speaker. I assumed that the cap job would fix it and might have.

I don't know if there is a problem with the amp or not at this point, I just wanted to get some opinion of the voltages to see if anything looked way off. 6V6 isn't red plating and nothing seems hot. I'll play it this evening.

Thanks
Chris

[clinkous]

The amp plays fine now. I'm just trying to figure out for my own knowledge why the screen voltage is higher than the plate. I'll figure it out. I guess the wall voltage is the reason for the overall increase over schematic.

I believe I will increase the cathode resistor and put a 50V cap there as well.

Thanks for the replies.

Chris

[nsubulysses]

Are you able to drive the amp into a dummy load? I bet if you do the screen voltage will drop below the plate voltage.

All voltages will be higher because of higher wall voltage as you said. Imagine the PT, it is a ratio. I don't know what the ratio is but 5V more on the primary side might make 40V more on the secondary side.

[clinkous]

No dummy load available but if it doesn't seem like a problem to anyone then I'll play it for a while and if something burns up I'll have a REAL problem to fix!

[g1]

Also noticed that you have around 50mv bias. Try upping the cathode resistor value maybe to 1K.

6V6 isn't red plating and nothing seems hot.

It may not be red-plating, but you have a 14W tube idling at 23W. Not good for longevity. It may also be partly responsible for the perceived volume difference compared to the SF.

[clinkous]

I'll try to figure the correct resistor to bring the current down.

[Enzo]

With 24v on the cathode and 470 ohms, you have roughly 50ma tube current. Dropping from 415 to 399 at the plate, and 50ma current, we have roughly 300 ohms resistance in the transformer primary. You have 10v drop across the 1k resistor in the B+ like the schematic wants. So the reason you have a higher voltage on the screen is that the 10ma current the screens and all preamp tubes draw through that 1k resistor drops less voltage than 50ma through a 300 ohm resistance. It isn't something "wrong."

This is a class A amp, so it idles at full current. I figure 50ma through the tube, for roughly 18w.

[g1]

This is a class A amp, so it idles at full current. I figure 50ma through the tube, for roughly 18w.

Yes, I had neglected the cathode voltage when I said 23W. From his numbers it appears to be about 19W less screen current, so your 18W sounds right.
Still suggest 14W for target current.

[clinkous]

Thanks Enzo and g1. I put a 1k in and measured 400V between pins 3 and 8 and 29.6V across the 1k. Weber bias calc shows 11W. I need to put a 50V cap in there I guess. The tube is a 6v6gta Realistic lifetime with Japan stamped on it. Is there a compelling reason to push it closer to 14W dissipation?

[The Dude]

If the amp sounds good, I would leave the 1K. Tube life will be much better.

[clinkous]

Sounds great actually. I believe I will.

Thanks everyone.
Chris