"Half Power" switch problem

[Valvehead]

So put the unit on a current meter. What is the mains current draw. Is it a steady highish current just waiting for a "straw that broke the camel's back" to pop a fuse? or is it sitting at a reasonable common lowish reading and spikes hard when switch is thrown.

COuld be something about the switch. It may not MEASURE shorted, but it could have a flake of something inside so it arcs in one position. You could take the wires off the switch and either tack in a different switch or just simply tack solder the resistors to the pin 6 wire to hard wire pentode mode. Does it still behave this way?

i have an ac ammeter on the way along with a new switch to try .

Its now popping the fuse in either mode , when the amp is clicked on . Its very random , last night i powered it up at least 20 times and it wouldnt blow fuse . I did try a new gz34 tube .

[Valvehead]

Did the amp ever work, or is it a new build? In other words, is it possible something could be wired incorrectly?

Yes , its a Louis electric . The mod was done by a reputable shop .

[Chuck H]

What's the idle current in either mode Enzo already asked this though he wanted a measurement at the fuse. You can test for idle current without an ammeter by:

1) powering down
2) drain the filters
3) measure the resistance across each half of the OT primary
4) power up and switch out of standby
5) measure the voltage drop across each OT primary

Divide the voltage drop by the measured resistance to find the current through each half of the primary.

[Valvehead]

What's the idle current in either mode Enzo already asked this though he wanted a measurement at the fuse. You can test for idle current without an ammeter by:

1) powering down
2) drain the filters
3) measure the resistance across each half of the OT primary
4) power up and switch out of standby
5) measure the voltage drop across each OT primary

Divide the voltage drop by the measured resistance to find the current through each half of the primary.

100 ohms

plates 422v

CT 427v

5v drop

= .05 A ?

[Jon Snell]

Looking at this issue from a different angle.

I would think it obvious if the switch was arcing but if switching inductive DC, the switch rating is paramount.
Is your switch a Double Pole, Double Throw, Break before Make type or better still, Centre Off? If not there is every possibility that you are observing a momentary short circuit.

Most switches are rated at 250v A/C only! The highest DC rating I can find is 125v so the extra break point in the centre will help to stop any arc and being DC, there is no snubber to quench any possible arc.
Just an observation.

[Chuck H]

100 ohms

plates 422v

CT 427v

5v drop

= .05 A ?

Yep. That's correct. And thank you.

That's one tube. How about the other? 21W is pretty damn hot for an amp that's not class A, but I don't think it's the problem. The switch could certainly be at fault as Jon just pointed out. Most switches are on borrowed time in that environment. The switch has been suspect the whole time by you and others anyway.

Please do an independent reading for both halves of the OT primary. They're rarely exactly the same and, to be honest, the readings of exactly 100 ohms and exactly 5V are strangely round and convenient. For low ohm readings be sure to take the meters null figure into account.

[Jon Snell]

Yep. That's correct. And thank you.

That's one tube. How about the other? 21W is pretty damn hot for an amp that's not class A, but I don't think it's the problem. The switch could certainly be at fault as Jon just pointed out. Most switches are on borrowed time in that environment. The switch has been suspect the whole time by you and others anyway.

Please do an independent reading for both halves of the OT primary. They're rarely exactly the same and, to be honest, the readings of exactly 100 ohms and exactly 5V are strangely round and convenient. For low ohm readings be sure to take the meters null figure into account.

I quite agree that the figures are very convenient but not very accurate as a proper reading.
I always fit a 10Ohm 2W resistor in series between the cathode of each valve and ground, (if you have the facility to balance the bias), otherwise a 10Ohm 2W from the cathodes to ground.
Don't forget to take a few milliamps off, to allow for the screen grid current.

[Helmholtz]

Do we know meanwhile how the amp behaves when hard-wired to pentode mode?

Can you post pictures of the switch and socket wiring?

[Enzo]

Do we know meanwhile how the amp behaves when hard-wired to pentode mode?

Me too, I suggest it in #14, but he apparently wasn't interested.

[Valvehead]

Yep. That's correct. And thank you.

That's one tube. How about the other? 21W is pretty damn hot for an amp that's not class A, but I don't think it's the problem. The switch could certainly be at fault as Jon just pointed out. Most switches are on borrowed time in that environment. The switch has been suspect the whole time by you and others anyway.

Please do an independent reading for both halves of the OT primary. They're rarely exactly the same and, to be honest, the readings of exactly 100 ohms and exactly 5V are strangely round and convenient. For low ohm readings be sure to take the meters null figure into account.

sorry forgot 1 side , it is 100 ohms and 79 ohms

[Valvehead]

Me too, I suggest it in #14, but he apparently wasn't interested.

sorry--havent got to that yet. Ill try to make time tonight

heres a pic of the switch .

[Chuck H]

That tiny switch?!? The guy wasn't even trying to get a switch that would handle the voltage. I'll wager it's bad. Maybe only under high voltage conditions and not on your meter though. I looked today for a switch that would actually be rated sufficiently for a triode/pentode circuit and Mouser just straight up doesn't carry one. And I recall the one and only time I ever wired up a triode/pentode circuit I couldn't find what "I" thought would be an appropriate switch, so I just used the highest rated switch Mouser had that wasn't some fifty dollar Honeywell.

Replace that thing with the highest voltage rated standard switch you can get. And...

Jon talked about buffer circuits for the switching. Not sure what he has in mind, but maybe he'll elaborate on it.

[The Dude]

^^^^^ Agree. That switch isn't near large enough for switching plate and screen voltages.

[Leo_Gnardo]

That tiny switch?!? The guy wasn't even trying to get a switch that would handle the voltage.

Agree with you & The Dude. Not only is that dinky switch not up to the task, the short distance between solder posts & frame is begging for an arc-over.

FWIW I usually use a full size Carling toggle - the one that has big screw-fit tags for wiring up. Part P-H523 at Antique/CE. Take the brass screws out & solder to the terminals. Haven't had one go bad yet.

I notice Mesa's been using a cheap plastic Carling rocker switch for decades. Surprisingly haven't seen a failure with those either. I'd use them but it's a drag drilling rectangular holes...

Someday if I'm feeling spendy I'd like to try one of those $50 Honeywell jobs. Good enough for government work, I'm sure!

[Valvehead]

That tiny switch?!? The guy wasn't even trying to get a switch that would handle the voltage. I'll wager it's bad. Maybe only under high voltage conditions and not on your meter though. I looked today for a switch that would actually be rated sufficiently for a triode/pentode circuit and Mouser just straight up doesn't carry one. And I recall the one and only time I ever wired up a triode/pentode circuit I couldn't find what "I" thought would be an appropriate switch, so I just used the highest rated switch Mouser had that wasn't some fifty dollar Honeywell.

Replace that thing with the highest voltage rated standard switch you can get. And...

Jon talked about buffer circuits for the switching. Not sure what he has in mind, but maybe he'll elaborate on it.

So...this is a popular mod for tube amps......what does everyone use then for a switch ? Ive never done this mod . With 420vdc or whatever on the power tubes...???