CHAMP AA764 TMB with diode rectifier

[tyru007]

Just built this amp.  Came out great.  A little too much bass with a 12" speaker.  Going to a .68uf on the second preamp helps to cut the low frequency at the higher volumes.  I built this with a second octal socket so I could go parallel 6V6s or go with a EL34 or 6L6 for more volume.

[Geezer]

Nice touch with the "power" & "play" indicator lights!

Also, the adjustable bias is nice....here's another way to do it that may give a bit more "swing" for other tube types>>> http://www.ax84.com/static/sel/AX84_SEL_Options_080729.pdf

A couple of things stand out:
1 - no grid reference resistor on the input of the 6V6 (220k?).
2 - the NFB will change each time you change the impedance switch. The 2.7k should be connected directly to the 4Ω tap.

Once again, VERY nice build. I am really into SE amps right now, & my next one will have the full TMB stack with a parallel pair of big, powerful tubes...haven't decided which ones yet, but I'm lovin' the two parallel 6L6GC units I've built! Maybe KT66's(?)

[tyru007]

Thanks for the feedback

Added the 220K to ground.
I haven't really used the NFB circuit that much.  I may play around with the NFB resistor or remove it altogether.  I built this as a head and connect it to a single 8ohm 12" speaker (G12H30) in a 1/12 cab.

I added a revised drawing of my champ with actual voltage readings along with a schematic for a duo champ, which is actually how my champ will be wired but I havn't tried the second tube socket yet.

Note that I had to add a 1R resistor to the filament circuit to drop the voltage to 6.4VAC due to the oversized PT.  I'll Probably need to added a second 1R in parallel if I add a second tube.

If I want to try a EL34, do I just tie pin 1 to pin 8 of the octal sockets?

Do you have a schematic showing how you connected the power tubes in parallel?

[Geezer]

A better way to drop filament vontage is to use 2x diodes back to back (parallel, each anode connected to the other's cathode) then run in series with one of the filament supply lines...that will drop ~0.7v. If you need more dropped, then just add another diode set to the other filament line. (Drawing attached) Any common diodes will work (1N4001, etc) just so long as they can handle the current draw of your heaters.

See this thread.

http://www.el34world.com/Forum/index.php?topic=314.0

[tyru007]

I use LED diodes across the 6.3 VAC for Power and Play indicators.  Should I also add a diode in the opposite direction as the LED to balance the power draw on the AC power?

Most of the common diodes (like the 1n4001) are only rated 1A.  Would I need something more like a 1N5401, which is rated at 3A?

[sluckey]

I'd use 50V/10A diodes.

[tyru007]

10A?  Why?  I can't even find a diode in an axial package at this rating.

[sluckey]

Yeah, I guess 10A might be overrated for a 2 tube champ, or even a 3 tube champ. You did mention 6L6s and EL34s though. 3A should be OK for a stock amp. I definitely would not want to use a 1A diode.

RadioShack has a 4-pack of 50V/6A axial diodes for $2.49 (Catalog #: 276-1661 ). 10A axials should be available too, just gotta look in solid state places.

[DummyLoad]

http://www.mouser.com/Search/Refine.aspx?Keyword=FFPF10U150S

[tyru007]

Filament power for a 6L6 is 0.9A, EL34 = 1.5A, tow 6v6s = .9A  each 12AX7 = 0.3,  max should be about 1.8A.

As the 6.3VAC power supply is rated at 4A at the most, and the diodes are on active 1/2 the cycle, seems like a 3A diode would be more than enough.

If I add two diodes to one side of the filament power, won't this unbalance the power and cause hum issues?

[sluckey]

Note that I had to add a 1R resistor to the filament circuit to drop the voltage to 6.4VAC due to the oversized PT.

6.4VAC is plenty close enough for a cheap guitar amp. Heck, that's even close enough for a booteak amp! I've never been too concerned with heater voltage in receiving tubes.

[tyru007]

Thought I would post the final version of my Uber-Champ.  I tried using two parallel octals with 6V6s and I just could not get as nice a sound as I get with a single.  The amp can take a any SE octal up to 12 watts including 6V6, EL34, 6L6, KT66, KT77, etc.  It's diode rectified with a Boost switch and NFB Switch.  I mounted the Bias Pot and TP to chassis so that I can switch tubes easily.  Very clean sounding amp with some nice overdrive starting about 1/2 volume.  The Mid pot is a 50K audio, which works as a normal 0-15K pot unitl about 1/2 then some RAW controls to full mid.  Great first amp to build.

[Geezer]

I tried using two parallel octals with 6V6s and I just could not get as nice a sound as I get with a single

Did you change the OT impedance when you changed the # of tubes, or just use the same OT tap?

BTW, very nice looking board!

[tyru007]

I used the 2500 tap on the Hammond 125ESE with the two 6V6. 
Also tried the 5000, didn't seem to make a lot of difference.