V4 PCB component repair/updating

[beedoola]

Doh! I actually replaced it, but I didn't take the old one out!

Also, I used 3.3k for R53, that alright?

[Liquids]

Doh! I actually replaced it, but I didn't take the old one out!

Also, I used 3.3k for R53, that alright?

3.3k is fine here. You might see slightly lower voltages downstream comparatively, but with location to location voltage variants, it's negligible--especially for the preamp. It's not a very significant change in value IMO.

[beedoola]

I fired up to the amp, seems to be good, though I noticed that the two power tubes closest to the filter can caps seem to react more to playing that the others. When I play notices, you see those tubes being pushed, they pulse, the others don't. I don't have a bias probe so I'm not sure if they're all getting the same current.

[Liquids]

I've seen my power tubes pulse like that. Not red plating, but some other 'glow' -- when playing pretty loud and on transient 'attack' in the rare times I've been able to crank it. Keep an eye on it!

Are your power tubes matched? If it's only two (and assuming it's an inner or outer pair, not 2 L or 2 R tubes), you might want to track down if they are indeed seeing the proper screen resistance, or if they're not really matched.

In this amp, it's easy to measure bias. If you are safe and know what you are doing, you can measure the voltage across the plate resistors. It's not easy math unless you made them ~10R or 1R, but the voltage across 5R only adds one extra step. So you figure out which resistor to which tube's plate, and measure the voltage carefully on either side of each plate resistor at idle.

Rule out 5% differences given that the resistor's tolerances are within that range--if you didn't replace these resistors, measure each resistors value when the amp has no voltage on it, so you know the spread of resistance since time/use/heat may have caused the values to drift over the years. I think if you see a wide spread (as opposed to a reasonable variation within a range) you'll spot it. BE CAREFUL when doing this. We're talking 500v+ DC on your probes and this measurement is done powered up. Be sure you are practicing all safety precautions.

If you want to be safer and easier on your tubes, you can tweak the bias resistor that is 75k (R49). It's been a while since I did this and I am a little foggy right now, but increasing that resistor to 82k, 91k, maybe 100k should increase bias voltage, causing idle current to go down...a little may go a long way. Hopefully I'm not getting it backwards at the moment.

[beedoola]

I've seen my power tubes pulse like that. Not red plating, but some other 'glow' -- when playing pretty loud and on transient 'attack' in the rare times I've been able to crank it. Keep an eye on it!

Are your power tubes matched? If it's only two (and assuming it's an inner or outer pair, not 2 L or 2 R tubes), you might want to track down if they are indeed seeing the proper screen resistance, or if they're not really matched.

In this amp, it's easy to measure bias. If you are safe and know what you are doing, you can measure the voltage across the plate resistors. It's not easy math unless you made them ~10R or 1R, but the voltage across 5R only adds one extra step. So you figure out which resistor to which tube's plate, and measure the voltage carefully on either side of each plate resistor at idle.

Rule out 5% differences given that the resistor's tolerances are within that range--if you didn't replace these resistors, measure each resistors value when the amp has no voltage on it, so you know the spread of resistance since time/use/heat may have caused the values to drift over the years. I think if you see a wide spread (as opposed to a reasonable variation within a range) you'll spot it. BE CAREFUL when doing this. We're talking 500v+ DC on your probes and this measurement is done powered up. Be sure you are practicing all safety precautions.

If you want to be safer and easier on your tubes, you can tweak the bias resistor that is 75k (R49). It's been a while since I did this and I am a little foggy right now, but increasing that resistor to 82k, 91k, maybe 100k should increase bias voltage, causing idle current to go down...a little may go a long way. Hopefully I'm not getting it backwards at the moment.

I bought a quad set from Tube Depot.

I used 10R for the Plate Resistors, replaced both the Plate and Screen with new resistors.

Are you talking about measuring each side of the Plate Resistor with the Probes? Or the Positive probe to the PR and the Negative probe to ground?

And are you saying measure while the amp is On Standy By or in Operating mode?

I measured the DC when the amp on stand by - black probe to ground and positive to the tube pin side of each plate resistor, I got -.454DC.

I tried briefly with standby off - in operating mode and got 535V DC, I only measured on spot.

[Liquids]

You are measuring voltage difference from one side of each plate resistor to the other side (no ground reference) with the amp ON; no standby--like you were going to play it, but with no signal (turn the volume's down when you do this)

You will get a measure that is the voltage drop across the resistor It will be less than 1V at idle most likely (or...well...).

You are then looking for current - that is, your idle current draw.

Ohms law tells us, Current = Voltage / Resistance.

Once you know the voltage drop, you divide the voltage you get by 10 (since your plate resistors are 10R).

Then you get a current, measured in Amps (it will be a decimal since you are looking for miliamps.

Then you compare this to the next 3 tubes, doing the same.

Ah, yes, tubedepot...a matched quad will be...well, hopefully within 10% here. The high voltages seem to make "matched pairs" significantly less "matched" than originally stated, from most sources.

Once you see how differently they all bias, you can try and swap them around and pair them up differently.

Put the two that draw the most current as a pair (outer or inner pair) and the two that draw the least current as the other pair (outer or inner pair).

Try to balance the overall current draw between the two sides as well, so if you have 4 tubes, each draws a different idle current, 1 being the least 4 being the highest, they are ordered something like:

4 1 2 3
4 and 3 are one pair, 1 and 2 are another pair, and lowest and highest are on the same side to try and equal out the total current draw on "one side" as compared to the "other side" of the PT that has 2 & 3.

Fun?

---

If you use this calculator:

http://www.james-gang.org/jca/Bias.php

Using 540 as the plate voltage, 6L6GC as the tube type...60% at idle is 33ma! IMO this is absolute MAX in this case!!! IMO, off the top of my head, 22-26ma would be much much much better tonally and for lifespan...you'll only have so much room to play with if your tubes all mismatch, but if even one tube is drawing 34 ma+ at idle, I'd definitely change the bias resistor.

[Hangman]

for the most part I agree with Liquids on the biasing. 60% is the max I would set the bias.

generally speaking, I don't find tubes reacting to playing as a bad thing... but if only 2 of the tubes are doing it... It makes me wonder. are both sides of the power amp seeing input signal?

[beedoola]

for the most part I agree with Liquids on the biasing. 60% is the max I would set the bias.

generally speaking, I don't find tubes reacting to playing as a bad thing... but if only 2 of the tubes are doing it... It makes me wonder. are both sides of the power amp seeing input signal?

Two seemed to be doing it more than the others, the others were doing it as well.

I'm a little hesitant (not confident enough) to try measuring the Plate voltage as Liquids suggested. I have a Bias Probe on the way and I'll read the MA for each tube once I get it.

[beedoola]

Is it possible to use Deoxit for the rocker switches? I get a bit of scratchiness when using the Treble rocker in the highest/right position.

[Hangman]

Is it possible to use Deoxit for the rocker switches? I get a bit of scratchiness when using the Treble rocker in the highest/right position.

I would recommend using a cleaning agent that doesn't leave an oily residue like deoxit does. big bath is great stuff (just dont let it get on your black plastic knobs... it will discolor them)

alternately, you can take the switch apart and clean it manually (99% alcohol on a q-tip would work fine in that case)