Cap can question

[chocopower]

Hello!!

Finally i open the cap can in my Pro-reverb clon (Guyatone GA-1050) and found this....



I would like to know if it is in Blackface specs... Pre-owner change some Silverface values (P.I., tone stack...) but i think all is original in the filters caps...

best regards from Spain!!!

[MWJB]

Resistor & cap vallues are near enough, Fender used 1K in place of the 1.7K that you have but I don't think that 700ohms is going to make or break the amp.

One query - your diagram shows the 100uf caps as being in parallel/aligned in the same polarity ("+" to "+")? Now if there is a jumper wire running diagonally from one cap's "+" to the other cap's "-" (but not the terminals that connect to B+ or ground) then everything is OK. Check continuity from the junctions of the 100ufs & 220Ks from bottom left to top right on your diagram, you should have 0ohms.

[chocopower]

upsss.... you are on right.... the 1,7k resistor is a "bug"... it is really a 1k.

Weeks ago you help me whith this mod:

http://music-electronics-forum.com/s...ead.php?t=2713

could you explain it, (again), in my cap can lyout? (just for dumb techs... )


This week iŽll chek the cap polarity.... It,s is really strange....

best regards from spain!!!!!

[MWJB]

Replace the 1K with a 4.7K 3W. Replace the original 4.7K with a value that will result in 220-230v on the 12AX7 plates in V1...I'd guess at 27K for a starting point.

With a 12AX7 phase inverter this will give you more of a pre-blackface vibe. If you need to restore the BF style "chime" to the reverb channel replace the cathode cap & resistor at V2, pin 3 with 2.7K/15uf or 3.3K/10uf (depending on how easy it is for you to find a 15uf cap).

100uf filters - There's no reason why the caps can't be fitted with "+" to "+" AS LONG AS THERE IS THE JUMPER that I mentioned (missing from your diagram but should be in the actual amp). Fender arranged them in a more logical sequence but schematically they are the same thing. I often fit series filter caps the way that they are shown in your diagram, if there are restrictions in the amount of space/eyelets that you have to play with.

[chocopower]

...Replace the original 4.7K with a value that will result in 220-230v on the 12AX7 plates in V1...I'd guess at 27K for a starting point....

How should i check this voltage?
Pin 1 and Pin 6 to ground?

[MWJB]

That's right, if you end up with a little less than 220v I wouldn't panic. If you get much less than 200v (I doubt that you will as preamps draw very little current and you need a big jump in resistor value to get a significant change) have a listen first then decide if you want to go up in voltage. Likewise if you get up to 240v, that's still ball park.

[chocopower]

more questions about this mod in the right post....

http://music-electronics-forum.com/s...ead.php?t=2713