Anyone use the Mojo PT for Tweed Champ??

[Ringwraith]

Pin 1 & 2 on the 6V6 are joined & grounded to the lug near the 12AX7.
There was a bit of wire sticking out the back of pin 1 so it looks like it's part of the cathode resistor wire which is connected to pin 8 only.

I just measured the 470 Ohm cathode resistor & it's measuring 750ohms!!
Not sure how it could be that far out of whack but I'll order some 470's &
check the current again. With 750 ohm, this would mean the amp is under biased right??

Sean

[MWJB]

Yes, a 750ohm resistor would account for the low bias. It would also raise your B+, this will come down when you fit a 470ohm. Don't muck about with 1W resistors, use a 5W, use a 50v rated cap and keep the cap physically away from the resistor.

[Ringwraith]

So I'm digging through my supplies looking for a 470 ohm & of course I don't have one. I'm about to make a couple orders from the suppliers & I'm thinking this sucks because I'm going to have to wait to get this champ going again! I rigged up another resistor in parallel to get me about 490ohms & noticed a huge improvement!!! 2 seconds later the post guy's at my door with a package from CE Dist. ;-) I don't remember ordering 470 ohm resistors last week but there they were, a 5 pack of 2W Metal Oxide sitting in the box!! So I popped it in & again, the amp is sounding much better! Fuller, rounder with more guts maybe a tad louder. Basically all the things a hotter, or in this case properly biased amp brings! lol The original resistor must have been out since day one or pretty soon after because the amp never sounded this good! I actually built it about 2 years ago & have never really been impressed with it until today! Only got to play it for a bit but am looking forward to tomorrow when I can really wined it out!

Here are some measurements & as you said, the voltages did change a lot...

EH 6V6
B+ from 391 to 367
Plate 385 to 361
Cathode 25 to 19
Current 30ma to 39.6ma
12AX7 Plates 198 to 165

Thanks for all you help MWJB!!

Cheers
Sean

[black_labb]

not sure about a champ, but most tweed era amps tend to have the plate voltage on the 12ax7's closer to 150v or so from memory. 198v could be part of the reason for the fizzy top end. lower voltages should give a "browner" sound (whatever that means )

[MWJB]

Most tweed amps (late 50's) might have more like 180-235vdc on the plates of a 12AX7 in V1. The bigger amps typically used 12AY7 which have a higher current draw and return less plate voltage than a 12AX7 in the same circuit.

As a rule of thumb, in a Fender preamp configuration (100K plate/1.5K per cathode) a 12AX7 shows around 2/3 the voltage applied to the plate resistor, a 12AY shows around half the voltage.

[black_labb]

Most tweed amps (late 50's) might have more like 180-235vdc on the plates of a 12AX7 in V1. The bigger amps typically used 12AY7 which have a higher current draw and return less plate voltage than a 12AX7 in the same circuit.

As a rule of thumb, in a Fender preamp configuration (100K plate/1.5K per cathode) a 12AX7 shows around 2/3 the voltage applied to the plate resistor, a 12AY shows around half the voltage.

good to know, thanks.
to the op, sorry for the confusion