Choke Placement?

[apehead]

Hello,

Here is a schematic link for a Peavey Butcher:

Peavey Butcher Schematic

I want to make some mods to it, a choke from Mercury Magnetics being one of them. As you can see, this amp does not have a choke installed. Mercury does not provide installation instructions. This schematic is a confusing one, and I am not sure which resistor should be replaced. Please help.

[J Martin]

Place the choke between the terminal marked "B+" (not B++) and the terminal marked "7".

[apehead]

Zeroing In

O.K. this is what I'm thinking:

between the output of the diode bridge and the pos. leg of C23

Between R49 and the "standby" switch, S1 (although that only affects screen voltages, not the plate)

and finally, after C23. (I don't think this will make much difference than the first one, I think they are electrically the same)

[apehead]

Place the choke between the terminal marked "B+" (not B++) and the terminal marked "7".

It's O.K. that it doesn't affect the plate? (Not challenging you, genuinely curious)

[J Martin]

It can affect the plates if you want it to, but you will need a much larger choke since it will need to pass all the current in the amp. In a push pull amp a choke from the screen node on forward is usually good enough. If you were building a single ended amp or an ultralinear amp, the leading choke would be advisable. I didn't look to see what tubes are used, but you would need a 300 to 400 Ma. choke if used right after the rectifiers. It would be as big as your output transformer. Go to the Hammond Manufacturing site where you can see diagrams and drawings of various chokes.

[Jag]

The "traditional" spot would be between the standby switch and the top of C21/R47 and replace R49 with a straight wire.

Unless you want to significantly lower B+ you wouldn't put it between the rectifier output and C23.

Generally the choke is not meant to affect the plate.

Take a look at the Twin Reverb AB763 schematic.

[apehead]

Would it be O.K. to leave R49 in? I think the F.P. stands for "Fault Protection". Just a few weeks ago, my buddy kept blowing fault protection resistors in his Hiwatt clone when he had hooked up the bias wrong (no neg. grid bias on P.A. section!!), and since I will soon be screwing with the bias circuit (adjustable bias mod.), it would really give me a warm and fuzzy to keep it where it is!

[apehead]

Also, on a different note, I've never worked with 6L6-GC's, but isn't 500v a little high for screen grids when plate voltage is only 510v?

[Enzo]

FP means "flame proof." This is a common designation. "Fault protection" is something you or someone made up. The resistor has nothing to do with the bias, and other than that you could burn it out, it is not there for protection.

The FP resistor type will not catch fire if it heats to the point of burnout. Replace R49 with the choke. You could leave R49 in series with the choke, but I wouldn't.

[apehead]

Thanks for clearing that up. Should make putting the choke in a lot easier.

[Jag]

Replace R49 with the choke

Doh!!! Leave it to me to make it more complicated than it needs to be. I'm recovering from surgery so drugs is my excuse.

[apehead]

Help!!

I replaced R49 with the choke, now the amp is really, really quiet and sounds flat. What happened?

Resistance measures .2 ohms across each solder joint. Visually checked all other connections.

[Enzo]

I doubt the choke has anything to do with it. Do you have B+ on both ends of it? If so, it is doing its thing.

It is more likely you mis-wired something, broke a connection somewhere or otherwise did some mis-deen while doing the work.

Rule of thumb is that any time a new symptom occurs after you did some work on an amp, the new problem is a result of somehting you did. happens to us all.