‘68 Deluxe Reverb AB763 V5 Problem

[Perkinsman]

V5 pin 1 reads 420vdc, should be 210. Pin 3 reads 0vdc, should be 2.1. All the others are working within spec.

[g1]

Is there a problem with the 'vibrato' not working? When you checked those voltages did you have vib. turned on or off?

[Jon Snell]

V5 is a dual triode of the 12a*7 series.
Pin outs are;
1/Anode, 2/Grid, 3/Cathode, 4/Heater, 5/Heater, 6/Anode, 7/Grid, 8/Cathode, 9/Heater.
So pin 1 (Anode) is at full rail meaning no conduction.
Pin 3 (Cathode) is zero; no current flowing.
If the Vibtato is switched off, pin 2 becomes very negative and ensures the readings you have found by turning the valve off.
Vibrato on, means pin 2 will be near zero restoring conduction of the valve.

[Perkinsman]

Pin 2 is -45vdc. Vibrato channel isn’t on, at least it’s not working when I plug the guitar in. The amp didn’t come with a vibrato pedal. Is there a way to turn it onoff to even see whether it works?

[g1]

Put a short across the Vib. pedal jack.

[Jazz P Bass]

"The amp didn’t come with a vibrato pedal. Is there a way to turn it onoff to even see whether it works?"
Take any RCA jack that you may have and short the terminals at the other end.
Or cut back the wire and twist the two ends together.

[Perkinsman]

Ok, I shorted the rca’s and the tremolo works but the voltages on V5, pins 1-3 jumped all around., couldn’t get a stable fixed number, is that normal when the trem is on?

[Randall]

Yes that is expected, the tube is oscillating.

[Perkinsman]

So let me see if I understand....when the vibrato is off, it's normal for pin 1 V5 to be 395vdc, almost 2/3 higher than the voltage that's listed on the schematic....but when it's on, the voltage decreases but you're unable to read it? Why even list the voltage on the schematic if it's expected that the voltage jumps around?

[g1]

So then when the vibrato is off, it's normal for pin 1 V5 to be twice the voltage listed on the schematic?....why list the voltage of V5pin1 as 270v if the voltage jumps around?

Back when this was a current model, they were using analog meters with needle meter movements. They probably gave a more stable reading, especially if the speed was set higher. Did you check the cathode voltage? Does it jump around less?
Sometimes it helps to look at re-issue schematics. With the Deluxe, they no longer give the plate DC voltage, but they do give for the cathode. See attached.

[Perkinsman]

Got it...another question on this amp. I've been getting intermittent distortion, hum and the rectifier tube seems to be getting hotter than usual. I tested it on the tube tester. My tester has 2 tests for the 5AR4, my tube passed the first and failed the second. My question is what is being tested and how can the amp still be working with only 1/2 the rectifier working?

[Enzo]

WIth half the recto tube working, then you get only one side of the AC being rectified. Your hum would increase, ripple distortion would increase.

Kinda like asking how a 2x12 cab could still work if one of the speakers were dead.

[J M Fahey]

So let me see if I understand....when the vibrato is off, it's normal for pin 1 V5 to be 395vdc, almost 2/3 higher than the voltage that's listed on the schematic....but when it's on, the voltage decreases but you're unable to read it? Why even list the voltage on the schematic if it's expected that the voltage jumps around?

Like G1 said, way back then a *needle* meter (VOM or VTVM) was the norm, digitals not even invented yet.
Needle would swing up-down *around* the schematic shown voltage, so it HAS useful diagnostic value.
Those newfangled digital thingies take 1 second to agree on a number to display and by then such number must change again, useless.
I *still* have a needle meter on my workbench and sometimes I prefer it, go figure.
Some expensive digital meters include a strip or bar in the display, below the numbers, to imitate analog meter behaviour.

Here´s one of many, in your case you would see a longer/shorter bar moving all the time, which our brains can easily understand, while a jumble of constantly changing numbers is a mess:

[Jon Snell]

I still use my AVO 8 Mk3, can't beat it to check transformer primaries and the High Ohms range uses a 15volt battery and has successfully shown leakage in nasty NOS PI type and older electrolytic capacitors, where a DVM won't.

[Perkinsman]

To wrap this one up, it was never a problem with V5. Chemically cleaning & replacing the two power tubes & the rectifier did the trick. Thanks very much gents.