Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: wittyjeff on August 23, 2020, 10:39:13 pm
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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!
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...any thoughts?
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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
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...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 (http://www.classictone.net/40-18005.html) (or better the Classictone Super Reverb PT (http://www.classictone.net/40-18029.html)), 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).
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- 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.
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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 :w2:
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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.
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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!
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... 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).