Build Log - John Mayer Amp Clone

[sluckey]

The fender amp is using a FWB with that PT. Just copy the fender schematic.

[bam]

Thank you for the quick replies! There is a lot I'm misunderstanding, I'm sure of at least that :)

I've already removed the HI (100W) / LOW (25W) switch from the circuit. So it's not in play at this point. When I connected the center tap RED/YEL to ground, and the two diodes in series on each leg of RED, then combined their output, I read ~ 235 VDC relative to ground. This made sense to me and I knew enough that the use of the CT was the difference but not knowledgable or confident enough to modify it without some input. This would be my first amp build and I appreciate the help.

I believe the position of the switch as shown in the "The Twin" schematic would be the position when in HI power mode. As Martin's modification suggests, in HI the B+ would pass through the choke from the "top" of the 220k Ohm / 220 uF filters (versus the "middle" when in LO power mode.) I suppose I will add the two 220k Ohm / 47uF filters shown in the "The Twin" schematic and modify to then continue into the 220k Ohm / 20 uF filters and possibly adjust the value of the 10W resistors as necessary to match the voltage listed on the JM Wonderland schematic. 

In summary:

• Change the rectifier to match the "The Twin" schematic's full wave bridge.

• Tie the CT RED/YEL to the midpoints of the two 220k Ohm / 220 uF filters.

• Add an additional 220k Ohm / 47 uF filter section (I wasn't clear if these 220k Ohm / 47 uF filters were necessary or if going directly to the 220k Ohm / 20 uF filters following it would be sufficient.)

• Adjust as necessary the resistance prior to the 220k Ohm / 20 uF filters.

I'm not sure about the spec of the choke. Hammond seems to suggest their replacement for the 022707 is rated at 4HY, 50mA. Would I add up the plate currents for the 12AX7s,12AT7 and the grids of the 6L6GCs to determine the total draw? It seems 50mA would be sufficient if that were the case but they also make a 90mA option if it's not. 

Hammond choke options.png

[martin manning]

You're on the right track above. On the choke, a 90mA current rating has been used by Fender on Twins for a long time. The steady state current is probably only half of the 50mA your choke is rated for, but screen current is subject to some very large increases when the power tubes are driven hard. If it's a DR sized unit, I think I would replace it with the standard 90 mA 4H unit.

[bam]

Good point. This Twin has less passing through the choke so I suppose Fender could have made some economy by sizing it to 50mA. For comparison, I popped under a 65 Twin Reverb and noted it has a 036486 which Hammond specs as 4HY,90mA. I'll probably want to add their 194B to my next Mouser order.

Thanks again!

[bam]

Wanted to follow up on this thread. Finished the amp about a month ago and have been getting acquainted with it since.

Ordered a new Hammond choke after my last post. I had initially put it together with the choke I had on hand and it tested out OK. Swapped it out with a 4HY,90mA replacement once it arrived and have been really digging the result. It's a great amp!

Thanks again for the assistance. Really appreciated!