Question for tech gurus: Pulling tubes

[boobtube21]

I've got a question (or two or three):

What happens if I pull both the power tubes on my Vibrolux, and just sit and play through it? No volume of course, but will it hurt the rest of the amp?

Or, say it was my SF Twin, and I only wanted to use the line out...no sound through the speaker. Can I pull all 4 power tubes, plug the line out into an appropriate device, and jam away? No output tranny/other damage?

Ok, how about on an amp with a field coil speaker? Same thing?

Thanks!

[Mesotech]

The simple answer is yes. You can pull the power tubes without harming it.

When you think about it, when a builder first builds an amp, he typically fires it up with no tubes in it at all to test voltages and such. Then he'll put in the preamp tubes and inject a signal into the amp so that he can follow the signal through the circuit with an O-scope to ensure it's doing what it's supposed to do. One of the final things he'll do is put in the power tubes to go over that part of the circuit (probably with a dummy load output instead of a speaker, then once again with the speaker connected). All of this testing and burn in time without power tubes doesn't harm a thing, and makes troubleshooting and testing quiet.

There are "some" amps that will allow you to plug into a "line out" on the back, and it will effectively disconnect the power section of the amp when you connect the cord. So, no sound will come out of the speaker when using the line out. If you have an amp like that, you don't need to connect the speaker(s) while using it that way. There are some other amps that have a line out and you can use it at the same time as the speakers, but if you put the amp on standby, the preamp (and line out) still work normally, but you don't get any sound from the speakers.

You ask about the output tranny damage if you pull the tubes, there won't be any. With no input voltages coming from the tubes, the tranny is sitting as if it were powered off. Zero volts in yeilds zero volts out.

Disconnecting the speaker to obtain silence is NOT a good thing though, and should be avoided. In this case, the power tubes are pushing the output tranny, but there's no load on the other side, which causes all of that energy to be fed back into the tubes in the wrong direction. This can destroy the tubes. It also will cause the output tranny to overheat and destroy it as well. So... Pulling the power tubes = OK; pulling the speaker = notsogood...

I don't know the answer to the question as it relates to feild coil speakers.

[boobtube21]

Thanks Mesotech, that's what I was looking for.

I'm actually trying to get a line out hooked up on my Hammond spinet, and no one in the organ commnuity seems to be able to agree what the best procedure is. There is a "radio phono" jack in there (so you can plug in your favorite Nat King Cole record and play along) and you can use it but you still get sound through the organ's internal speaker (crap). It is a field coil speaker though, wish I knew more about those. I might just try pulling the 6v6's for a while and see if anything seems to get excessively hot.

Maybe I can get a standby switch hooked up on this thing, that would probably do the trick...hmmm....

Thanks again for the reply!

[steve25]

If you pull all of your power tubes from your Twin, the heater voltage on you preamp tubes will likly soar to over 7 volts which will not be good for them!
Ans yes you can pull the output tube out of an amp with a field coil speaker with no harm if you have such an old beast!

[steve25]

If you want a low power 1 watt amp you can use your Twins reverb send output feed to drive an 8 ohm speaker, as that is what that whole stock reverb drive circuit is, a 1 watt single ended amp, or a 1/3 power champ.

[boobtube21]

Thanks Steve, that's a good point. So what you're describing with the heater voltage on the Twin...is that likely to occur in most amps when pulling the power tubes? Or is it just more likely in a Twin because of what 4 6L6GC's draw?

I'd post a schem of this thing but the only internet version I can find is a 2-piece bitmap (left and right). It's huge and pretty complicated.

After pulling the 2 6V6's in this amp, the tubes left over would be:

(1) 5U4GB
(2) 12AU7
(1) 12AX7
(2) 6AU6
(2) 6C4

I'm thinking whatever voltage rise you'd get by pulling the power section in this circuit, if absorbed by everything else, can't be as much as what the 6 12AX7's in a Twin would get hit with after pulling 4 6L6GC's. But then again I'm not a tech ;)

[steve25]

Find your self some used and shot ( but not shorted) 6V6s and with a jewlers file cut off pin 3. then they will still provide the needed heater load fo the other tubes to live!

[boobtube21]

Hey, now there's an idea. So far it's been working great though. I did turn off the lights to see if the others would glow any brighter with the 6V6's pulled, didnt' seem like they did. Not sure if that's the test but either way I jammed for about an hour last night and no smoke! Thanks guys!

[limbe]

I have gotten into the practice when building amps with one or two 6V6s to wire a 8 ohm power resistor on the output jack for the speaker so it is connected to the amps output when there is no plug inserted in the output jack.When the loudspeaker is connected to the output jack it takes the resistor out of the circuit.That way I can do all the measurements I want to in complete silence.It also is a safeguard if I forget to connect the speaker.The 6V6 draws half the heater current than the 6L6 does.I have never had any troubles with this metod.If you do this use the chassie as heatsink for the high watt resistor

[Kap'n]

Find your self some used and shot ( but not shorted) 6V6s and with a jewlers file cut off pin 3. then they will still provide the needed heater load fo the other tubes to live!

That's a great idea.

On a same idea, if you have any of those crappy black glass Russian "6V6" firecracker tubes from the 90's floating around, just chop the pin off on one of those. The filament at least is useful.