Ampeg G15 (Gemini 2 with 6L6 power tubes)

[davohilts]

I've got this amp on the bench and it has a broken cap can base but still wired up. The cap can is 40x3 @500v and looks to be original to the amp. There are two other caps that clearly have been replaced by modern caps. One is comprised of two 100mfd x 350V caps in series with a 220k resistor between them, the other is a Sprague Atom 20mfd x 500v.
Looking at the schematic I can see the 3-40mfd x 500v caps and also a 30mfd x 600 cap and a 40mfd x 500v cap.
What I'm guessing is the pair of 100mfd caps in series with the 220k resistor is an attempt to meet the 600v requirement of the schematic. Am I correct that this results in a 50mfd x 700V cap value and what does the resistor do?
The 20mfd x 500V sprague atom is just the wrong value and should have been 40mfd x 500v by my guess.

I've always understood that by connecting two caps in parallel doubles their value but their max voltage stays the same and is a good practice to follow if you don't have the proper cap value.
I've also understood that by connecting two caps in series you cut their value in half but double the max voltage rating but it isn't recommended for reasons I've never understood. I seem the remember something about using a calculated resistor value between the negative of the first cap and the positive of the second cap to help with stability.

My question is where does one find a 600v x 30mfd capacitor? Can building a series pair with a resistor between them be a workable solution?

I'll attach the schematic i'm working from.
thanks for any insight!
Dave

[52 Bill]

I've also understood that by connecting two caps in series you cut their value in half but double the max voltage rating but it isn't recommended for reasons I've never understood. I seem the remember something about using a calculated resistor value between the negative of the first cap and the positive of the second cap to help with stability.

Series connected filter caps have been used by many manufacturers for lots of years with no real problems, so yours should be fine.

Adding a 220K resistor across (paralleled with) each cap does two things, one it causes the voltage across the two caps to divide in half so that each cap will see one half the voltage applied to it and two the resistance will act as a bleeder when the power is turned off and will cause the filter caps to drain down fairly quickly.

[davohilts]

Series connected filter caps have been used by many manufacturers for lots of years with no real problems, so yours should be fine.

Adding a 220K resistor across (paralleled with) each cap does two things, one it causes the voltage across the two caps to divide in half so that each cap will see one half the voltage applied to it and two the resistance will act as a bleeder when the power is turned off and will cause the filter caps to drain down fairly quickly.

Thanks for the info. So by parallel you mean the resistor is connected from the positive of one cap to the negative of the other where the two caps are in series?

[nsubulysses]

Sound_City_LB_120_MK_IV_(11-73)_Schematic.pdf

Yes you are correct. Sorry this is not a G15 schematic I just grabbed whatever I had easily available. But you'll see an example of this off the bridge rectifier on this Sound City 120 schematic. Two 220uF caps in series with two 220K resistors paralleling them.

They also do the same thing just to the right of that with two 32uF caps in series, with two 220K resistors in parallel, so that's how you would wire it up.

[52 Bill]

Thanks for the info. So by parallel you mean the resistor is connected from the positive of one cap to the negative of the other where the two caps are in series?

No, one resistor for each cap in series, connected from the positive terminal to the negative of the cap.

The two resistors form an equal voltage divider with each leg at 220K for a total of 440K resistance across the high voltage source. That way the high voltage will be equally divided across the two resistors. Let's say the high voltage is 500vdc. With two series connected resistors connected across the high voltage, as long as the two resistors have the same value (220K) the voltage will divide across them equally putting 250vdc across each resistor.

Look up the schematic for nearly any Fender Twin or Bassman, etc. This is the basic input cap setup for the power supply.

[davohilts]

thanks guys! I think I've got it!
I appreciate you taking the time to help me out!
Dave

[audiopete]

Go to fliptops.net for all proper Ampeg parts. I believe this is what you need. Cap Kit for Ampeg Gemini II, G15 V.1 - Product Details No need to custom build caps.

They also have the insulated can caps for putting them in series but mounting them externally. With some of these amps, they have two can caps in series to make the voltage rating (Ampeg and Garnets come to mind). That means one of those caps will have half the voltage of B+ (or whatever node it is) on the outside of the can (negative) and must be sleeved with an insulated collar like cardboard, or you can get shocked.

[davohilts]

Thanks for the tip Audiopete!
I'll check it out.
Dave