Pro Reverb Voltages

[bobloblaws]

I just put new 6L6's and a new GZ34 rectifier in a 1972 Pro Reverb. The GZ34 replaces a weak 5U4GB. The tube chart indicates a 5U4 rectifier but does not specify a circuit number. As far as I can tell, for all intents and purposes the circuit is the AB668 as it is a 1972 silverface with no mid knob. Anyway, those details may be neither here nor there. What I'm concerned about is the voltage readings I am getting for B+, plate, screen etc. The voltages seems to continually fluctuate, from between 1 or 2 volts to as much as + or - 5V, making it hard to determine if the screen voltage is below the plate voltage. So, two questions. 1. Does this indicate a potential problem with the rectifier or possibly the power transformer? 2. If I know the screen resistors are good can I safely assume the screen voltage is below the plate voltage at a any given time? The 6L6's that were replaced show evidence of excessive heat. I initially assumed they were biased too hot but now I'm wondering if the screen voltage might have been a problem. The original 1 watt screen resistors appear to be fine, they read a little higher than 470 ohms but within tolerance.

B.

[Justin Thomas]

My first suspect would be, how stable is the voltage at your outlet? How fast does it fluctuate? And in the big scheme, it is only maybe 1.5% at the worst. Are you moving your hands and arms around as you measure? Are your meter leads still? Are appliances in your home cycling on and off? For the PT: is it getting hot - like, REALLY hot? And what brand of GZ34? Some have earned a bad reputation for n o t being able to hang in a real GZ34 circuit...

There are others with vastly more knowledge than I, so I won't say any more than, as long as it's not going out of control, and isn't making funny noises, I wouldn't think it's an issue.

Justin

[pdf64]

Why are you using a different tube type to that indicated on the tube chart? Compared to a 5U4, a GZ34 will raise and firm up HT voltage above that intended by the amp's designer. I suggest a rather more solid rationale is required before doing that.

Why are you concerned about the plate and g2 voltages being a few V this way or that at idle?

Just to emphasise, AT IDLE. As all that matters at idle is that plate and g2 voltages and dissipations are reasonable and within the tube type's limiting values.
So, is the amp at idle, or passing signal? If you think it's idling, have you scoped for oscillation?
As putting a meter probe on to a plate can induce oscillation, especially if the probe leads are not well away from earlier stages.
If you've not got a scope, the potential for power amp oscillation can be eliminated, without affecting the power tube operating conditions, by removing the phase splitter tube V6.

It's a good idea to ensure that probe tips are sharp, so that they break through any surface gunk and so that they don't slip off the terminal under test. Use a fine file to sharpen them if not. Probes are consumables, they will fail eventually, they aren't family heirlooms.

[Mick Bailey]

The voltages could be up to 30v higher with a GZ34. What are your actual voltages?

The screen resistors don't drop a lot of voltage when the amp is idling because the screen current is related to the amp's output. When the power tubes are driven harder the screens pull more current and there's a greater voltage drop across the screen resistors. Many amps only read a few volts difference between plate and screen at idle. If a screen resistor fails on its own you just lose conduction with the associated tube and it fails safe. I wouldn't be concerned about the resistors unless they're scorched or seriously out on value. The tubes showing signs of running hot may have been down to biasing and nothing more.

[Chuck H]

Have you monitored bias voltage? If the bias caps are old the bias condition may be fluctuating, causing current, and therefor standing voltage, to drift. Same applies to new tubes in my experience. They may need to burn awhile before they stabilize.

[bobloblaws]

Why are you using a different tube type to that indicated on the tube chart? Compared to a 5U4, a GZ34 will raise and firm up HT voltage above that intended by the amp's designer. I suggest a rather more solid rationale is required before doing that.

The amp belongs to a friend and he brought me the chassis only to work on. The B+ was low (around 430V) My tube tester showed the 5U4 as being weak so I decided to try swapping out the rectifier. I initially assumed I could rely on the AB668 schematic so I ordered a GZ34. At that point I was not aware that the GZ34 and 5U4 are not strictly interchangeable.

Why are you concerned about the plate and g2 voltages being a few V this way or that at idle?

Just to emphasise, AT IDLE. As all that matters at idle is that plate and g2 voltages and dissipations are reasonable and within the tube type's limiting values.
So, is the amp at idle, or passing signal? If you think it's idling, have you scoped for oscillation?
As putting a meter probe on to a plate can induce oscillation, especially if the probe leads are not well away from earlier stages.
If you've not got a scope, the potential for power amp oscillation can be eliminated, without affecting the power tube operating conditions, by removing the phase splitter tube V6.

My understanding is that the screen grid voltage needs to be slightly below plate voltage. In the past I've used my DMM to verify this.

It's a good idea to ensure that probe tips are sharp, so that they break through any surface gunk and so that they don't slip off the terminal under test. Use a fine file to sharpen them if not. Probes are consumables, they will fail eventually, they aren't family heirlooms.

The probes I usually use have clips, not tips/points.

[Jazz P Bass]

What was the original complaint?

Not knocking tube testers but you have the very best tube tester right in front of you.
The amp itself.

I would reinstall the 5U4 and measure the B+ voltage with and without tubes.

[bobloblaws]

What was the original complaint?

Not knocking tube testers but you have the very best tube tester right in front of you.
The amp itself.

I would reinstall the 5U4 and measure the B+ voltage with and without tubes.

B+ was low which jibed with what the tester indicated. I will try the 5U4 again with the new tubes.

Edit: Indeed, popped the old 5U4 back in and the B+ is right around 465V. Why would you want to measure without the tubes?

[bobloblaws]

Have you monitored bias voltage? If the bias caps are old the bias condition may be fluctuating, causing current, and therefor standing voltage, to drift. Same applies to new tubes in my experience. They may need to burn awhile before they stabilize.

Bias cap was replaced recently. Maybe the new tubes then.

[J M Fahey]

You-don't-have-a-problem. Period.

Like Enzo always says: Fender amp voltages (and most other amps too) can vary up to 20% and still be normal.

Don't worry about 1.5%

And in any case, what was the actual *functional* problem/complaint?

[bobloblaws]

You-don't-have-a-problem. Period.

Like Enzo always says: Fender amp voltages (and most other amps too) can vary up to 20% and still be normal.

Don't worry about 1.5%

And in any case, what was the actual *functional* problem/complaint?

Maybe you misunderstood, my issue was the voltage not being stable enough to get a clear picture of whether the screen voltage was lower than the plate voltage. Other posters have enlightened me as to not being concerned with those voltages at idle.

The amp was brought to me for a non functioning tremolo and a general checkup. I noticed that the power tubes showed evidence of excessive heat and the bias was way out of wack. Got it sorted now, thanks all.

[mozz]

Screen voltage will not always be lower than the plate voltage, so forget checking for that as any sign of a problem. I am seeing this for model numbers.

1964 -1967 blackface circuits AA165, AB668
1968 -1982 silverface circuits AA1069, AA1009, AA270
There will be a chassis number like
Pro Reverb AA165 (blackface)
A00100 to A02700 - 1965
A02700 to A09200 - 1966
A07000 to A10200 - 1967

Pro Reverb AA1265, AB668, AA1069, AA270 (silverface)
A10000 to A10500 - 1967
A10500 to A12000 - 1968
A12000 to A13300 - 1969
A13400 to A14500 - 1970
A14500 to A15000 - 1971
A15000 to A15600 - 1972
A15600 to A17200 - 1973
A17200 to A19700 - 1974
A19700 to A20000 - 1975
A20000 to A21500 - 1976

Make sure you have the correct model number and schematic, you may also have something in between with parts of one model and parts of another.

Some Fender power transformers will take either the 2amp or 3 amp rectifier tubes, other transformer numbers were meant for the 2 amp version only. A google search will tell you if you find the number stamped on the tranny. You can't always go by any Fender tube chart pasted on the cabinet, they were often misused or mislabeled.

[bobloblaws]

Screen voltage will not always be lower than the plate voltage, so forget checking for that as any sign of a problem.

I wonder where I got that notion then? I thought that was the purpose of the screen resistors, to ensure the screen voltage was slightly lower than the plate voltage. I was also told that it's a good idea to replace the 1 watt screen resistors with 2 or even 3 watt because a failure would be bad news for the tube(s). While we're on the subject, I also have a note suggesting that increasing the resistance to a certain degree, from 470 ohm to 560 ohm for example, can improve the amp's tone.

Make sure you have the correct model number and schematic, you may also have something in between with parts of one model and parts of another.

Some Fender power transformers will take either the 2amp or 3 amp rectifier tubes, other transformer numbers were meant for the 2 amp version only. A google search will tell you if you find the number stamped on the tranny. You can't always go by any Fender tube chart pasted on the cabinet, they were often misused or mislabeled.

Thanks, yeah my friend and I were emailing back and forth trying to figure out what was up since in addition to the GZ34 rectifier the AB668 schematic also showed an 8 ohm load and he measured 4 ohm for this amp. I found this page Fender pro reverb questions | Page 2 | Telecaster Guitar Forum (message #36) where it mentions 022848 on the OT indicates 4 ohm. His OT has the 022848 number so we figured in this case at some point Fender changed the OT's and also at some point changed the rectifier to 5U4 and updated the tube chart with the 5U4 and simply removed the reference to the AB668 circuit number/schematic. At that point I trusted the tube chart in terms of the 5U4 but I'll double check on the transformer heater current.

[The Dude]

FWIW: It's usually tube issues that cause screen resistor failure- not the other way around. If a screen resistor fails, the tube is going to conduct very very little, if at all, so there's little chance of harm to the tube.

[jmaf]

I wonder where I got that notion then? I thought that was the purpose of the screen resistors, to ensure the screen voltage was slightly lower than the plate voltage. I was also told that it's a good idea to replace the 1 watt screen resistors with 2 or even 3 watt because a failure would be bad news for the tube(s).

The screen grid is a very fragile structure. The purpose of those resistors is to protect the screen grid from excessive dissipation (literally burnup).

The screen is more positive than the plate about half the time when you're playing.

Remember, the screen grid is tied to the +B2 supply, it does not swing very much - but the plate does swing, a lot.

Every time the plate swings on the negative direction, the screen will be more positive than the plate. Sometimes hundreds of volts more positive. During these transients those resistors keep the screen from burning up, especially on pentodes where the screen takes one hell of a beating. Higher power resistors add reliability. Since factories usually save on resistors installing cheap and minimal power components, one good idea is to change these resistors for higher power versions.

If the screen resistor fails nothing happens to the tubes, but the amp may fail on stage which is why good techs use higher power ones.