What voltage should these caps be?

[bluzmn]

I'm getting ready to order some parts, but I can't see the voltage markings on a couple of capacitors in the chassis of an amp. I'm attaching the schematic; the caps I'm referring to are C4 and C5. Could anyone hazard a guess as to what voltage these caps should be? And I'm wondering; is the way these caps are used in this circuit what is known as "grid leak bias"?
Thanks for your help.

[MWJB]

C4 - 25v will do, 50v or 63v might be just as easy to find, use 63v prefereably 100v at C5. These are cathode bypass caps, used in cathode biased stages. C6 is the grid leak cap.

[Tilman]

Hi,

I have never seen a filter choke in that position before. Is the unit really build this way ?
The safety ground at the main input is missing in the schematic. Don#t forget to install one.
Tilman

[defaced]

If my thinking is right, it'll do the same thing as if it were on the positive end of the caps, but it'll lower the choke core to amp chassis voltage difference, preventing arc over. http://music-electronics-forum.com/t13022/

[J M Fahey]

What a *cheap* minded designer they had.
He'll do anything to save 2 cents.
Beware the schematic has errors, too.
1) C5 does not need to be rated 100V; 16V will be ample, because it receives , as stated, 1.3V DC.
Those "80V" also printed by it are a typo, check that the *plate* voltage is also 80V !!!!
Besides, Pins 3 and 5 can't be at different voltages, since they are connected directly. Oh well.
2) C4 is drawn with the wrong polarity, it's receiving a negative voltage relative to chassis.
It appears because:
3) The designer saved an "expensive", maybe "wirewound" cathode resistor in the 6L6, and an "exotic", even "more expensive", electrolytic capacitor bypassing it, bay making the cheap cheat of using the choke's DC resistance do its job.
That way, the full amp current goes through it, creating magically 7,5V.
Look Ma !! no hands !!
Problem is, those would be positive 7.5V and last time I checked, tubes needed negative bias voltage.
Our intrepid designer grounded V3's cathode, making it the chassis reference point, and those 7.5V now are negative with respect to it.
Since ripple (bad hum) voltage appears on the left side of the choke, it's filtered thanks to R11 and C4 (which is drawn, as said, with wrong polarity).
I guess in the long run he saved nothing and ended with a kludgy amp.
4) He saved another cathode resistor and decoupling electrolytic by wiring V1 as a space charge biased tube.
Personally I would rewire the amp to normal practice, including:
5) R4-> 1M
6) V1 Pin2 -> to ground through a 1K resistor; if more gain necessary (doubt so) bypass it with 22uF or something you like.
7) Major surgery:
Wire the PSU as a normal one, meaning C1 & C2 negatives to ground, separate both positives and wire the choke between them, remove R11 and C4, and last but not least, you'll need to ground V3's cathode through a suitable resistor and bypassing capacitor.
I'm sure somebody more experienced with tube amps than me will suggest suitable values.
Hint: measure (and post) the DC resistance of the choke, you'll probably need something close to that, we already know it makes 7.5V appear at idle.
The amp will sound like a normal one; I'm sure the original one was farty, hummy and a very poor performer.
Well, maybe I forgot something else, don't know.

[bluzmn]

JM Fahey, I don't understand...

I want to be sure that I understand what you're saying. Are you saying that the voltage at pin 5 of the 6L6 is -7.5v in spite of the inept design, or are you saying that the voltage is not -7.5v because of the inept design? In other words, will the amp work with the polarity of the 10mfd capacitor as pictured in the schematic? And, are you suggesting that the choke should be wired like the one in this schemo?MascoMA17schem.pdf

[J M Fahey]

Hi.
I'm saying this voltage on pin 5 *is* -7.5V and that is what the designer wanted. So far, good.
I'm also saying C4 is drawn backwards. Maybe it's wired correctly !!
Please check, as well as confirming that the bias voltage is negative with respect to chassis, as well as its value.
I assume the amp is working and in its original state.
I suggest you rewire that amp as a more conventional one, including using the choke as in the example you post.
A concern has been raised in an earlier post as a possible poor insulation between choke winding and core, I suggest that is the choke is so poorly built, it's a hazard anyway, junk it and get a good one.
Also, maybe it works with no problem, so take the least effort path first.
Worst case, strip most components and build a better circuit there, using its chassis and iron (the most expensive part), maybe sockets, not much more.
Remember resistors will probably be noisy, out of value carbon composition, oil capacitors leaky, do not even speak about electrolytics !!!
You may end with a trusty good sounding little Blues or Harmonica amp.
Good luck.

[bluzmn]

Thank you all for your input.

[dai h.]

If my thinking is right, it'll do the same thing as if it were on the positive end of the caps, but it'll lower the choke core to amp chassis voltage difference, preventing arc over. http://music-electronics-forum.com/t13022/

I remember reading on a japanese site someone suggesting this for that very reason (choke voltage rating) so, that's probably it.