AA764 Vibro Champ PT

[plexi50]

I have a early 1965 AA764 Vibro Champ. Noticed the bias was quit high @ +54.4 volts. Checked the PT secondary voltages and was surprised to find 377-0-377 evenly on each red lead. 315 VAC is what i should have. Any idea's as to what happened to the PT? Filament voltage leads are 3.50 VAC each. PT is original. Pic's on the way.

[mresistor]

What is your input AC voltage? Since USA AC voltage can be as high as 126VAC then if your wall voltage is higher than 117 or 120 VAC then your PT secondary voltage is going to be much higher .  I think if you look at the notes on the AA764 schematic the tolerance is +- 20%.  I wouldn't worry much about 377 VDC on the secondaries.   I would adjust the 6V6 cathode resistor to get the idle current back down to reasonable. The filament VAC is a tad high. 

[plexi50]

What is your input AC voltage? Since USA AC voltage can be as high as 126VAC then if your wall voltage is higher than 117 or 120 VAC then your PT secondary voltage is going to be much higher .  I think if you look at the notes on the AA764 schematic the tolerance is +- 20%.  I wouldn't worry much about 377 VDC on the secondaries.   I would adjust the 6V6 cathode resistor to get the idle current back down to reasonable.

122.8 VAC from the wall.

[plexi50]

Schematic say's bias should be +21VDC

[mresistor]

What does the cathode resistor read in ohms? And the cc resistors usually drift up. And what is the voltage dropped across that resistor?

[mresistor]

After seeing the pictures I'd say that the orange mallory cathode bypass capacitors need to be replaced. Have you tested them?

[PRR]

What DC voltages are on ALL FOUR critical 6V6 pins? K, G1, G2, P?

What does your meter say about a fresh "1.5V" or "9V" battery?

I don't think the cathode caps are involved, but the grid coupling cap sure can be.

[mresistor]

Yes but PRR  those bypass caps are as dry as a popcorn you know. 

[plexi50]

After seeing the pictures I'd say that the orange mallory cathode bypass capacitors need to be replaced. Have you tested them?

All capacitors are new Sprauge. 470 ohm resistor & 25/25 vdc bias cap. I always use the original sleeves if there in an amp.

[plexi50]

I just tacked in a 680 ohm resistor and 50/50 cap and bias shot up to 65 vdc.

[mresistor]

Read through this thread.   https://el34world.com/Forum/index.php?topic=26475.0

[plexi50]

What DC voltages are on ALL FOUR critical 6V6 pins? K, G1, G2, P?

What does your meter say about a fresh "1.5V" or "9V" battery?

I don't think the cathode caps are involved, but the grid coupling cap sure can be.

Meter is good. Have to change and check 1.5 volt batteries every day. Plate voltage settles at 280vdc after 2 minutes. Screen is 295vdc. I have the tremelo knobs in the off position. The cathode 470 again is getting smokin hot.+54vdc again.  I am going to replace the PS Cap Can. I knew it might be a major factor but held off until all new cathode caps were in place. No Hum.

[plexi50]

I put a 150 ohm 5 watt resistor for the bias and i now have +20 vdc. Plate voltage 260. Screen 280 vdc. Grid +8 vdc and fluxuating a little. The higher wall voltage does make a difference. After i put a new cap can in it i'm sure i will see a difference even though the amp is silent with no humming. The PS is the heart of the B+ rail.

[sluckey]

I put a 150 ohm 5 watt resistor for the bias

Wrong!

Grid +8 vdc and fluxuating a little.

Grid should be ZERO VOLTS. No exceptions.

[plexi50]

I put a 150 ohm 5 watt resistor for the bias

Wrong!

Grid +8 vdc and fluxuating a little.

Grid should be ZERO VOLTS. No exceptions.

I FOUND THE PROBLEM! Ground wire laying above the solder tab. Pic

[plexi50]

Loose ground wire re-soldered. Grid is zero volts. Plate 374vdc. Screen 376vdc. Bias +22.5. Bias resistor is 470 ohm with 25/25vdc cap. I love it when it turns out to be something right in your face and you don't see it until picking around with a wood pencil stick.

[66Strat]

Plate dissipation at idle is over 16 watts. Need to figure out a way to cool it off. B+ is too high IMO.

[CO_Hoya]

See this thread for a discussion of reducing 6V6 dissipation in a VibroChamp via lowering screen voltage vs. increasing cathode resistor value.

https://el34world.com/Forum/index.php?topic=29154

[plexi50]

See this thread for a discussion of reducing 6V6 dissipation in a VibroChamp via lowering screen voltage vs. increasing cathode resistor value.

https://el34world.com/Forum/index.php?topic=29154

That is a really a great thread and very well explained. Thanks for posting it CO_HOYA.

[HotBluePlates]

I have a early 1965 AA764 Vibro Champ. ... Checked the PT secondary voltages and was surprised to find 377-0-377 evenly on each red lead. 315 VAC is what i should have. ... Filament voltage leads are 3.50 VAC each. PT is original. ...

See my measurements of a 1965 Vibro Champ here.

Most of your voltage issues probably have settled down.  However, my amp did not land near schematic voltages until I set variac voltage to ~107vac.  That's because the Champ/Princeton power transformer (sized for the 2x 6V6 Princeton amps) is under-loaded in the Champ application.

[plexi50]

I was surprised at first to see how much voltage the PT was putting out. More of a drastic difference than most any other amps i have seen. The loose ground wire from the cathode and grid surprised me as well. That's the fun part about doing this. Some things stand right out and others hide under other wires as this one did. It all worked out fine. I wonder if anyone has made a plugin mini variac to drop the wall voltage 10 volts. No ones going to lug around a full sized variac. You would still need a transformer in the mix.

[sluckey]

I wonder if anyone has made a plugin mini variac to drop the wall voltage 10 volts. No ones going to lug around a full sized variac.

Look at page 2 of my amp scrapbook.

[plexi50]

I wonder if anyone has made a plugin mini variac to drop the wall voltage 10 volts. No ones going to lug around a full sized variac.

Look at page 2 of my amp scrapbook.

Very cool! That's would be the perfect solution. Great info. Thanks!

[sluckey]

Here's another pic of the Buckaroo...

[CO_Hoya]

Because I already owned it, I simply installed a power resistor on the AC L going into my SF Champ to knock down high voltages (link to thread).

I selected a resistor at 33 ohms, 25W to reduce the nominal 6.3VAC / 5.0VAC down near spec.

Seems to work well in this application. I measure 11.4 VAC drop across the resistor with 124VAC from the wall.

[plexi50]

Because I already owned it, I simply installed a power resistor on the AC L going into my SF Champ to knock down high voltages (link to thread).

I selected a resistor at 33 ohms, 25W to reduce the nominal 6.3VAC / 5.0VAC down near spec.

Seems to work well in this application. I measure 11.4 VAC drop across the resistor with 124VAC from the wall.

The link is not working. Edit: It's working now.