Judybox amps

[Smokebreak]

Ignore class issues for a moment.

If the amps were running 275V on the plate, and the class A short data calls for 250, keep in mind that's 250 "effective" plate volts-- thus to cathode. If it's cathode biased, there's your missing 25ish volts, right?


As far as class stuff goes for power reduction, you've got a much better chance of getting what you want by leaving the cathode bias resistor unbypassed. Whether you separate the cathodes or not is up to you, but done that way, you'll be fine with a 30W speaker.

Ah yes I see effective plate voltage is a bit less in cathode bias, but I'm not understanding the pros/cons with calculated reduced output, with regard to lower voltages. Also, how does separating the cathodes affect things here?

[Phil_S]

Smoke: What you got sounds perfect for a 5B6. I suppose if you bias it right you could get it running Class A or nearly A.

Phil, that looks dirty, and I like it. I'm still very shady on how I would accomplish Class A in PP, especially running 300V+ on the 6L6s. How would you bias them?

You've gotta do the calculations and see if you can do it. I'm not good at this sort of thing. My gut tells me (see how I waffled above) it will be AB. Who really cares about class of operation? Build it. Tweak for the tone you like.

Look at RC-30, page 353. You can find it on line if you don't have it. It says for PP Class A for 2 tubes, Va=270, Vg2=270, Vg1= -17.5, zero-signal plate current = 134mA, plate to plate load resistance = 5000. 2 tubes will give you 17.5W Max. You might find another Class A load line.

I guessed you might be close enough to do it. <shrug>

[matt h]

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[martin manning]

As for separating the cathodes or not... when they're bypassed, it doesn't matter much. If you're going to leave them unbypassed, you get two effects working against each other, bias drift (creep!) under load and local nfb.

You won't get bias drift unless the cathode resistor(s) are bypassed with a cap. There has to be something to charge up over time to raise the average cathode voltage above the quiescent level.

[matt h]

(deleted)

[martin manning]

Separate or shared cathode resistors, un-bypassed. See here: https://tubeamparchive.com/viewtopic.ph ... highlight=

[matt h]

(deleted)

[martin manning]

Yes, but notice with the bypass cap included the loops migrate downward, which is what I thought you meant by "bias drift." In the time-based plot that shows a cathode voltage trace you can see it drifting up.

[matt h]

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[Smokebreak]

I'm gonna go with 6V6s
I imagine I can try Class A PP with those too

So if I'm running 8ohm speaker 8K primary ~370V, to run 250V plates I'll need to use 16ohm tap to get around 4K? Do I have that right?

Edit: my bad , I was reading the data sheets incorrectly. It's looks like 10k primary for 250V

[Smokebreak]

I'm building a switchable 1,2,3 stage thing, no tone stack, cut control, variable dump before PI. 6V6 cathode biased. Inspired by Rocket, Monza, AC30, and the desire to have at least 2 switches on every amp:roll:

I'm calling it the Hot Dog because I'm using lips and b-hole parts..random sockets, twisted pairs, eyelets, junk bin parts. I'm also gonna use a valve rectifier for the first time in a long, long time.

[M Fowler]

Damn it, now I'm switching to red board with blue caps

[Smokebreak]

Haha I just saw that. Lookout, I've got 1 red board left that's probably up next

[M Fowler]

I've got some green boards too I'll have to use yellow caps for that John Deere look.

[Smokebreak]

I got this amp assembled in the Judybox with attached schematic. It's switchable to run 1, 2, or 3 stages. No tone stack, cut control as tone, type 2 KF(I think, the really simple one) MV, no NFB. 22K tail LTP with 6V6s in cathode bias. 320V plates.

It's really raw, and a bit too ragged for me. I really just wanted to try out the switching and experiment with no tone stack/different tones stacks and NFB/no NFB.

I just put in a Bandmaster 6G7 tonestack, and 820r/100r NFB and now we're gettin somewhere.
I'm not entirely sold on the 6G7 stack though, and am going to put in another one. Maybe I'm just not used to it, but it seems a little muddled.

Any suggestions on a stack that would work nicely with this circuit, or your fave 2 knob? I'd like to keep it 2 control, but will drill if I need to. I very well may just put in a Fender or Marshall, with mid on a trimpot, then fix it.