Bassman/JTM45 ExpressSCH

[glassesnvests]

So a customer requested a Bassman AA864, but had heard that there was a mod out there to change the Bass channel to a JTM45 flavored setting. Found an altered BF Bassman schem that had the JTM mods, supposedly: (http://s101.photobucket.com/user/chocopower/media/BASSMANHYPERON.gif.html)
so I redrew it into ExpressSCH in case anyone wanted it, I couldn't find any like it online.

Please double check me! And I've got some questions...

The customer wants:
1. Power Scaling/Master Volume
-and-
2. An Effects loop. ( I mentioned to him that it was probably unnecessary, but he insists)


1) Are there any schems out there for a fixed bias power scale? Or does Mr. O'Connor have an utter monopoly on these?
2) What are some good FX circuits that I could drop in? Should I spring for buffered or unbuffered? Series or parallel?

An help appreciated!

[TIMBO]

Hi glassesnvests, There is a few mistakes in your schem but this will get better after using the program for a while. It also makes understanding how these things work easier as well.

The JTM45 circuit can be built in that half of the Bassman.
A VVR circuit can be found in the SS section and achieves as well as a PPIMV circuit.
There are many FX circuits around. There is also info on Tubenit's active FX as well as a passive one.

[TIMBO]

I changed a few things to be more like a 45. 

[tubenit]

On board active FX:
http://el34world.com/Forum/index.php?topic=10208.0

Some VVR info about cathode biased amps:
http://el34world.com/Forum/index.php?topic=6899.0

Some VVR info for fixed biased amps under 50 watts:
http://www.hallamplification.com/main.html?src=%2F#2,2

I personally would recommend using a PPIMV instead of VVR on this amp.

With respect, Tubenit

[sluckey]

That .02µF coupling cap for V2 should probably connect to the plates rather than the grids.

[glassesnvests]

Thanks all on your feedback! You dudes are my heroes.

Fixed the grid coupler. It's on the plate now. D'oh.

Debating whether it'll be beneficial to put in the FX loop. A PPIMV is simple enough to not add too many parts, but I feel like putting in an active FX loop, although it may? help the tone, it may not be enough of a justification for the added tube or so.

[tubenit]

I have installed an active FX loop in all 3 of my amps.  It's become a "must have" for me as I use it very much like a master volume.

Take a look at this schematic and think about how the FX functions much like a typical Marshallish master volume.

With respect, Tubenit

[glassesnvests]

Added in the FX loop that tubenit posted back in January: http://el34world.com/Forum/index.php?action=dlattach;topic=10208.0;attach=41300;image

I sketched it in, however,
my only question is where to insert it and how to wire a DPDT to switch it in and out.


Timbo added the PPIMV to the circuit, so I'll keep it in a possible version. Might build without it to test out the functionality of the FX loop as a master.


Thanks!

[tubenit]

Insert it just like I have in the schematic I posted for you between the preamp & LTPI like you are indicating.

Regarding switching the FX in and out.  Look in ARCHIVES on FX loops.  There are numerous illustrations on how to do this using a simple mini-toggle DPDT.

With respect, Tubenit

[ac427v]

Earthquake Engineering--Why can't I think of great names like that?
Looks like a fun project. It looks like Timbo caught diode and cap polarity in the bias circuit. I would use bigger caps on the first power supply node. Two 30uf caps in series is only 15 uf total. If you used two 100 uf/350v volt caps there, the result would be 50 uf at 700 volts. In the bias circuit, I like to use 47 uf at 100 volts, Also 470 ohm/3 watt screen resistors on 6L6 tubes--Doug Hoffman sells them. V1b needs a coupling cap.
I've never used adjustable nfb so I hope hope share your experience with us. I've been toying with fitting it on my one channel deluxe ab763. Let us know how it sounds.
Always fun,
ac427v