Dual Power Supplies?

[Justin Thomas]

Nope, didn't sleep well.

Clear on the jacks. So if I use those, my OTs & tubers won't blow up? Even with all the other voltages appied?

Justin

[Chuck H]

Nope, didn't sleep well.

Clear on the jacks. So if I use those, my OTs & tubers won't blow up? Even with all the other voltages appied?

Justin

Nope. They won't blow up. And they won't conduct and make noise. They'll just sit there with voltage on them not doing anything. No cathode connection = no current. No current = no potential for damage.

[Helmholtz]

....and no tube current means no OT current.

[Chuck H]

....and no tube current means no OT current.

Right. After all, we test amps for "unloaded" conditions with no tubes in them all the time. No OT's are blowing up because of it.

[Justin Thomas]

Hooray! Okay, that sounds a LOT easier & cheaper! I'll give it a shot.

Good thing I make my own front playes, covering up all those rendered-unnecessary holes I drilled last night!

Justin

[Chuck H]

Hooray! Okay, that sounds a LOT easier & cheaper! I'll give it a shot.

Good thing I make my own front playes, covering up all those rendered-unnecessary holes I drilled last night!

Justin

Yupper. I've covered up "design change" holes many times. I'm going to do that with the project on my bench now in fact. There are plenty of examples by respected manufacturers that do it too either because they change a model and still have a bunch of the old chassis or because they use the same chassis for more than one model.

Now you can have a single standby switch. Or no standby switch. Many posters here have determined there's no real advantage to them for guitar amps. Well, other than actually using them to put the amp on standby so the filaments remain hot rather than turning the amp off. But as for the standby switch making for a gentle startup or shut down there are several threads here where it's been pegged as a misnomer.

[g1]

So lemme get this straight. If the cathodes are ungrounded, I can leave all the other stuff (voltages - plate, screen, heater, bias) running, AND not blow up my OT if I don't have a speaker plugged in?

(No details right now, just "doofus-level concept) but, am I hearing this concept right?

Justin

That's how the Hughes & Kettner I referred to in post #21 does it. There's a schematic link there.

[Chuck H]

That's how the Hughes & Kettner I referred to in post #21 does it. There's a schematic link there.

I was late getting to this thread so I "skimmed" (though that's bitten me more than once). But yeah. That's the idea. The TRS jacks would act as an 'auto cathode lift standby'.

[HaroldBrooks]

There's a sort of good idea. You would have a bit of a rise in B+ for the output tube pair left running. I'll leave it to you whether you make a more complicated compensation for that when the amp's running one channel only. I'd say don't sweat it, chances are pretty good the bias current won't go way out of whack, but it is bound to rise a chunk, when you go single channel.

I can see shutting off filament current to the "sleeping" output tubes too, if you're worried about cathode poisoning which may take place in tubes left filament on with no B+ if left that way for extended time.

I might be wrong here, but you could also use a tougher EL84M or I believe a 7189 if you were concerned about the voltage and current draw rising when one side was cut out. That's actually what I am doing with my stereo amp, as I just yank the one side tubes and play a single side sometimes. Hope that is not screwing things up further ! But I have been getting away with it so far, and my caps are overrated for any rise in B+ as a result of pulling the unused side's tubes.

A swtich like Justin is looking at would be better, particularly with as Leo Suggested killing the filament connections as well. I like the idea Justin has overall, and may incorporate it myself instead of doing it the primitive way.

EL84M or 7189 are supposed to handle 400v on the plates, and 13.5 watts or so plate dissipation. They are the same pin outs. I saw something about an IC connection, but this didn't effect anything in my amp at least.

[Justin Thomas]

I am trying to stick with what I have on-hand for tubes & other pricey parts here. I'm not ruling out 7189s or the original 6973s at some point, but with EL84s & acouple 6V6s I can use tubes I have on hand.

Justin

[HaroldBrooks]

I am trying to stick with what I have on-hand for tubes & other pricey parts here. I'm not ruling out 7189s or the original 6973s at some point, but with EL84s & a couple 6V6s I can use tubes I have on hand.

Justin

Sure, you probably won't need them. I have another amp with ordinary EL84s and I am running the plates at 383vdc and the screens at 350vdc, and no red plating and no screens glowing (1k screen grid resistors and a reasonable 10 watt per tube plate dissipation), even with a highly boosted signal and horrific distortion. If you get a good set of old EL84s, those tubes can take it I find. Some old Hi-Fi schematics show those types of voltages, even though the spec sheet limits are less.

I like EL84s, they quickly have become my favorite smaller tube, better than 6V6s or 6973s, and least the way work in the Zenith Hi-Fi amps I've converted so far (might be the particulars of those circuits). Good choice for a modest power amp.

Good luck with your build and I think your making good choices, and I will now copy your one amp cutout scheme, so thanks for the good ideas Justin.

[Enzo]

It isn't the voltage that makes plates red hot, it is the current.

[HaroldBrooks]

It isn't the voltage that makes plates red hot, it is the current.

Right, thanks Enzo, that's my understanding too.

Once I raised the screen grid voltage by dropping the power supply node resistor, and pdf64 jumped in and reminded me that it might change the tube plate current and upset the bias a bit. I hadn't the experience to make that association yet, but he was correct and it now measured high, so I added some cathode resistance and all was back to normal.

[Justin Thomas]

Well,

I'm glad I checked before I got TOO far along in this build. One of my OTs had an open winding on one half of the primary. I dissected it & found the break, in the second layer down. Me being me, I kept unwinding out of curiosity or boredom. There was another break in the other half of the winding, in the 3rd layer from the middle, even though that half of the winding had checked decent with my multimeter. The 2nd break had a little tiny hole punched through the insulating paper, surrounded by a corrosion-green-tinged ring, very tiny. I ran across a few other green rings, though no further breaks.

I'd taken trannies apart before, but never this methodically. Though I think with some minimal equipment investment I could have rewound it. Oh well.

So I'll work on one half first & gank another OT from one of my other 5 chassis I have in the attic.

Justin

[Justin Thomas]

Well, half is working now & showing great potential, after fixing a gross wiring error where I fed the cathode side of the cathodyne back into the volume control, forgot to ground the preamp filter caps, & didn't solder the 6U8A screen! DUMBIS!

I gotta get my pot wiring theory down.

Jusrin