Heavily Modded Bassman Clone Schematic

[wittyjeff]

I'm working on the schematic portion of planning a Bassman clone based on the AA864 model but adding a number of switchable mods.  The idea is to end up with an amp that can perform very similarly to a stock AA864 but has a lot of versatility when desired.  I've attached my schematic, a document of notes and thoughts on it, and what it might look like.  (I'll need to do the wiring layout once I have the schematic finalized).
Thanks in advance for your comments and corrections!

[wittyjeff]

...any thoughts?

[shooter]

a Bassman clone based on the AA864 model but adding a number of switchable mods. 

my thought, build the 1st 1/2 your statement, test for a week, then complete the 2nd 1/2 (you can drill holes ahead of time :).  That way you've already 1/2 split in case there's issues

[HotBluePlates]

...any thoughts?

The first item in your Design Notes mention using a salvaged power transformer with a "550V @ 110mA" high voltage winding. I would strongly suggest you save up the $120 to get a Classictone Bassman PT (or better the Classictone Super Reverb PT), because you will have >>$120 worth of pain trying to make that transformer work for you if it doesn't have a center-tap.

     -  Is there a center-tap on the high voltage winding? If no, then 550v * 1.414 = ~780vdc out of it, where the stock AA864 Bassman has 420vdc. No good way to lose an excess 300v.  If this transformer doesn't have a center-tap, I personally wouldn't take it if it were offered to me free, because it would be too much hassle to try to use in a workable circuit.

     - If the high-voltage winding does have a center-tap, then it's a 275-0-275v transformer, for 275v * 1.414 = 389vdc.  This is okay, except the Bassman PT from Classictone is rated for 240ma.  Your PT can perhaps power a pair of 6V6s in a Princeton Reverb, but not a Bassman.

I do think the Mid pot is a good addition.  You might even consider bumping it up to 25kΩ.

I would skip the soft-start switch/resistor.  As long as your filter caps are rated for more than the unloaded voltage of the power transformer you use (with that Classictone Bassman PT is would be 330v * 1.414 = 467v, so at least 500vdc), you just turn the amp on & you'll be fine.

I'll have to let others guide you on the effects loop.  I don't have enough experience to know a really good active loop, and would otherwise argue a loop is not useful in this amp (except for the line-out, preamp-in scenarios you outlined).

[pdf64]

...
     - If the high-voltage winding does have a center-tap, then it's a 275-0-275v transformer, for 275v * 1.414 = 389vdc.  This is okay, except the Bassman PT from Classictone is rated for 240ma.  Your PT can perhaps power a pair of 6V6s in a Princeton Reverb, but not a Bassman.
...

For the OT given, the lower HT will work toward reducing the HT current draw; though even then, 110mA may be pushing it too far.
There again, I've never understood how 50W Marshall type PTs get away with a 150mA rating on their HT winding 

[wittyjeff]

Great replies guys!  Thanks for the help.  Definitely a good suggestion to build the pre-amp first and test.  As for the PT... you are right.  What's another 120 bucks in a build like this.  Might as well prevent $500 worth of trouble by getting the correct PT in the first place!
Just for background, sorry I forgot to mention that the PT is indeed center-tapped.  The actuals coming out of the HT (at 120V going in) are 630V across the HT Secondaries and of course, 315 across the HT and Center-Tap.  I just don't think 315V is going to result in a Bassman that exactly resembles a Bassman.  ...and you're right, shedding 210V is more trouble than 120 bucks' worth.
Thanks again for your advice.

[sluckey]

315-0-315 will give you 445Vdc unloaded. No need to shed anything. Voltage wise, that's a good candidate. But 110mA current is not enough for 6L6s. Would work fine for 6V6s though.

Filament voltage is fine just as is.

Lamp dimmer for the pilot lamp? Really!

[HotBluePlates]

... sorry I forgot to mention that the PT is indeed center-tapped.  ... 630V across the HT Secondaries and of course, 315 across the HT and Center-Tap.  I just don't think 315V is going to result in a Bassman that exactly resembles a Bassman.  ...

315-0-315 will give you 445Vdc unloaded. No need to shed anything. Voltage wise, that's a good candidate. But 110mA current is not enough for 6L6s. Would work fine for 6V6s though. ...

6V6 output tubes and a lower target power output is something worthy of consideration.  An awful lot of folks wind up getting/building a 40w amp, then struggle playing at home because they can't run it loud enough to distort (that complaint comes up a lot, even with 20w amps).