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Old 24th March 2004, 04:29 AM   #1
edsmith is offline edsmith  Australia
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Question Stupid question about capacitors

With capacitors, which voltage rating and type should I buy?
Greencaps? 100V?

They seem to vary throughout the circuit. Am I meant to assume the ones that arent marked a certain voltage tolerance?

Schematic:
www.drtube.com/schematics/marshall/1987pljp.gif

Board picture:
http://www.sonicdeli.com/ThunderTweakWeb/boards.htm
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Old 24th March 2004, 06:24 AM   #2
TG is offline TG  Ukraine
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AFAIK Marshall normally uses 400V MKTs for 1nF and higher and 500V disk ceramics for lower than 1nF.
Electrolytics are usually 500V (sometimes 450V for the input stages).

Good luck!
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Old 24th March 2004, 06:47 AM   #3
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The best way would be to understand the circuit and what voltages to expect between whatever nodes... but failing that, well, I can't even say because there are NO voltages listed after the power transformer primary!

In general, coupling caps need to handle full B+ voltage for at least as long as the tubes heat up, and usually a safety allowance is made, specifically, using the next highest voltage type for it. Say you have 250V across a capacitor in normal operation and a B+ of 400V. Well when the tubes are cold during startup, you'll probably get 450...maybe 500V. So a 400V cap would probably work (most are rated for 150% of ratings for a few minutes) but 600V would be better.

Electrolytics cost, and also can have odd values when ran at voltages much lower than their rating, so these are usually chosen much closer... typically you'd grab one for the maximum voltage seen. In the above case you'd want 450V (500V surge rated is typical here).
'Lytics are often also used for cathode bypasses...these can be as low as 6.3V but that obviously depends on the tube above it (a big healthy triode might run upwards of 100V in a cathode-biased configuration ). Check the bias points of whatever.

Signal caps such as on the tone circuit, bypassing series signal resistors (to "speed up" the connection, essentially) and stuff in NFB loops usually don't have to handle a DC offset (long post here...I forget if your schem shows coupling caps before the tone circuits) so can be relatively small units.

Tim
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Old 24th March 2004, 07:22 AM   #4
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It seems most resistor values are 1watt so I am assuming it's limited to 300-400 volt?

I would go and simulate, or ask someone in the know how the voltage paves out for the begining of the circuit into the first tube and take a shot at overall load to calculate a proper choke because there apparently is no PS choke ratings...

Usually there is another component list with a schematic, where is it!?

If I was going to build it with so much information missing

id use 2watt resistors for the whole lot and at least 600volt capacitors for every single one.

I can't see how an ECC83/EL34 schemat would ever see 600v... :P

If you just use 600volt caps and it pans out half or 3/4 the 600volt at least you can be sure they'll last longer by running easier.

Eventually bringing it all up on variac while checking voltages along the way making sure it's all OK.

I'm also assuming C connector goes to the other C connector..

There is no where it says where B goes to or A around the output tranny...
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Old 24th March 2004, 01:11 PM   #5
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Hi Layberinthius

Quote:
I can't see how an ECC83/EL34 schemat would ever see 600v...
Maximum screen voltage for an EL34 used in PP pentode mode = 425V. The power supply voltages shouldn't be much higher than that. 500V caps in the PS (50uF) should be about right. The CT of the output tranny should be about 440V, my guess.

Quote:
There is no where it says where B goes to or A around the output tranny...
A is from the junction of the fuse and ps choke.
B is from the other side of the choke in the PS, then A goes to CT of output tranny, B goes to EL34 screens.

Cheers
Wayne

P.S. Those power supply supply voltages are ONLY my best guesses!
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Old 25th March 2004, 07:15 AM   #6
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Hi Layberinthius,
You said
Quote:
I can't see how an ECC83/EL34 schemat would ever see 600v...
Well,there ARE some EL34-based amps with a B+ of 1000V.
These amps were made by an Italian brand named Geloso back in the '50's.The Geloso brand was famous mainly because their very good-quality RF equipment,but they also produced a wide variety of public address audio amps.
Sorry for not being able to upload a Geloso schemo (though I have some of them),but if you're interested you'll might want to "google" a little.The Geloso amps benefit more or less from the RF expertise and tricks of those old foxes,making them an interesting material for study.
Ahm...only a small detail...of course,those amps were working with those GOOOOOD NOS samples of EL34 or 6CA7,wich could withstand such voltages....but,please,if you intend cloning a Geloso amp,don't even think about using some modern EL34's,unless you want some melted anodes and a spectacular firework!
Have fun,
Le Basseur
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Old 25th March 2004, 08:17 AM   #7
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Quote:
Well,there ARE some EL34-based amps with a B+ of 1000V. These amps were made by an Italian brand named Geloso back in the '50's.The Geloso brand was famous mainly because their very good-quality RF equipment,but they also produced a wide variety of public address audio amps. Sorry for not being able to upload a Geloso schemo (though I have some of them),but if you're interested you'll might want to "google" a little.The Geloso amps benefit more or less from the RF expertise and tricks of those old foxes,making them an interesting material for study. Ahm...only a small detail...of course,those amps were working with those GOOOOOD NOS samples of EL34 or 6CA7,wich could withstand such voltages....but,please,if you intend cloning a Geloso amp,don't even think about using some modern EL34's,unless you want some melted anodes and a spectacular firework! Have fun, Le Basseur
While I find your logo is quite funny, my logo has a love streak to it..Kinkless are my favouritte and I can't see at anytime in the near future of me building an EL34 amp

Maybe if I get a vehicle I will want to have those nice tubes in my boot but other than that, nope

It is history like this which makes me smile with Mystery and Explorer's hope for what he/she might find.

The same thing which keeps me interested in valves are stories like these.

I am actually having an affair with D. T. N. Williamson, Menno
Vanderveen and Plitron...oh and KT :P

Sucking up all my cash

Until it's all over, My hands are tied to the ship.

Quote:
Hi Layberinthius quote: I can't see how an ECC83/EL34 schemat would ever see 600v... Maximum screen voltage for an EL34 used in PP pentode mode = 425V. The power supply voltages shouldn't be much higher than that. 500V caps in the PS (50uF) should be about right. The CT of the output tranny should be about 440V, my guess.
Maybe I should have said:
- I can't see how this ECC83/EL34 schemat would ever see 600v...

The main reason why I shot at 600volt was to leave headroom for spikes during powerup and during power fluctuations ...

PSUD has shown me with a poorly designed power supply you can see in excess of 700volts on a psu designed for 400... for 1/2 of a second even..
Cheers :P
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Old 25th March 2004, 11:51 AM   #8
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Quote:
PSUD has shown me with a poorly designed power supply you can see in excess of 700volts on a psu designed for 400... for 1/2 of a second even..
Yep! Depends on Power Transformer regulation and voltage. Most good pwr trannys have 5-10% regulation.

I said:
Quote:
Maximum screen voltage for an EL34 used in PP pentode mode = 425V. The power supply voltages shouldn't be much higher than that.
Only because of the voltage drop across the choke. I could do a sim but without knowing the pwr tranny voltage, current and regulation I'd still be guessing. Now if I were to put in a known transformer (start from scratch) ...
I believe you could go to 475V (500V supply max but that would be good ol' NOS) on the screens in Ultralinear or 450V triode mode. Back in the good ol' days you could put 800V on the plates and get 100W from a pair! A friend of my father built one 40 some years ago. It was mono back then, he was a Ham radio operator so he knew his stuff when it came to high voltage.

I don't know if the original poster actually has that amp or just wants to build one using the schematic.

Cheers
Wayne
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