Using a spdt switch to select between triode mode and pentode mode on a guitar amp.

[tubelab.com]

If you can remember to flip a mute switch, then you can remember to flip the standby switch. No voltage, no arc. I had flipped the mode switch at full tilt several times with no ill effects. I flipped it once too many, and ZAP!

I haven't gigged in too many years to remember, I used to jam with my daughter and some of her friends when she lived at home. It was payback to the neighbors, their son played in a death metal band.

[Sherman]

Originally posted by G
I just considered something. Since there is essentially no load connected to the contacts would the switch arc? Have you seen them arc? If you have then conversation over but it would seem to me that with almost no current flow there would be minimal arcing. Maybe I'm looking at it wrong.

I have seen them arc. Both the amps I have them in arc when the mode is switched with power applied (regardless of volume setting). That in itself isn't necessarily a bad thing, every switch will arc to some degree, even the lowly mains power switch arcs. However if the voltage is substantially higher than the switch's rating it will eventually burn the contacts, though that might take hundreds or even thousands of actions.

I'm not trying to discourage you from doing this. I've done it myself and it is an interesting and worthwhile thing to do. It is nice to be able to hear the difference between modes in the same amp. Just be aware of a
potential problem (pun only partially intended ).

[jmartins]

Which switch have a best quality??? with low indutance


tks,
João

[Sherman]

Originally posted by jmartins
Which switch have a best quality??? with low indutance


tks,
João

APEM switches (for SPDT toggles) from Mouser are pretty nice. A little pricey but rated for 400VAC (@10A I think, but several amps anyway).

Even the inexpensive Shin Chin switches are rated for something like 250VAC @ 5A.

I haven't tried to find any high voltage rotary switches but I'm sure others here have sources.

[jmartins]

tks, but I'll need SPDT switch 600VDC
[]´s
Joao

[Sherman]

Originally posted by jmartins
tks, but I'll need SPDT switch 600VDC
[]´s
Joao

With most switches the allowed voltage rises when current falls. (There are limits of course.) Most switches are rated for several amps while your tube circuit will be drawing a fraction of an amp.

If you can find reasonably priced 600V switches go for it. If not then try a lower voltage switch. It probably will be fine as long as you aren't constantly changing modes.

[Johan Potgieter]

Perhaps just one more thought (most have been said).

An output transformer is inductive, though well-damped if the matching is good, but still, especially at high frequencies. It is possible that the switching action, when break-before-make, can generate a pulse that could peak a high anode voltage (referring to the way a car's spark coil works is perhaps way over the top, but something like that).

If there is feedback in the power amplifier the effect could be compounded. One wonders what will be worse: a momentary shorting of the transformer primary (make-before-break), or an equally momentary open screen grid. Perhaps some adventurous member with a digital storage scope could try this and favour us with graphs of the transients!

I am not going into differing stability requirements in feedback amplifiers with triode and pentode output stages, because as far as I know no appreciable feedback is used in guitar amplifiers, if at all (not too familiar with them). But just for completeness sake, this aspect also exists.

Regards.

[Bobken]

Hi Gavin,

I have a DIY-designed & built all-valve guitar amp which, if anything, has rather more 'comprehensive' switching than your proposal here.

As it was difficult in home conditions to overdrive the original output valve I used, to levels which satisfied me from the desired distortion point of view (like serious threats from the other half to abandon me, if I didn't turn it down!), I solved this by 'adding' another output valve to the chassis.

The orig. valve was a 6BQ5/EL84, and the 'weedy' substitute was a 6005, which was supposedly only good for approx. 1.5 watts!

Using their own individual plate, grid, cathode resistors and whatever else etc., and arranging for the changeover via a multipole switch, I could switch directly from using one valve to the other, although this was never intended to be done 'on the fly'.

It all worked out quite well, and I could distort like hell when I wanted to, both by overdriving the pre-amp section (I always could, of course) but after this modification I could also distort the output-stage, similarly, which as you doubtless know will give rather different 'sonic' effects.

Anyway, I had also incorporated a separate 'switchable' tone circuit, and a valve & spring reverb-tank, both of which could readily be switched in or out of use, at will, and on the fly, but in these areas it was 'safer' to do this. The only consequences of switching here was a minor change in overall gain, as the two circuits I designed were not exactly unity gain blocks.

One (almost!) fateful evening, and after a few too many drinks, I stupidly threw this valve 'changeover' switch by mistake whilst powered up, followed by some very interesting and loud noises, which I had thought were the dying moans of my favourite creation.
By sheer good fortune, when I checked it all out I could find no harm having been done, but I don't recommend this to the fainthearted, nor to anyone who is overdue visiting the lavatory!

I have since changed the switch to a latching one, hopefully, to avoid this kind of event in the future, and this does prove how much more resilient valves are than solid-state components.

Actually, I do have a digital storage 'scope, but I am far too scared to try this again, merely in the interests of science!

I would strongly advise that all caution be taken when considering any 'live' switching as suggested, in case you have a similar experience.

Regards,

[richwalters]

A hint for those comtemplating modified triode/pentode/UL or whatever via a flip of a switch is to use a parallel paired output stage with modest B+ to save the switch life. That's the simplest..Design for lowest B+ using parallel tube pairs and the output poke will still be there but also saving the switch. Unfortunately to save the day on an already built 500V B+ amp using a switch it's going to have a very short life and sound "saaaack" awful when operated, unless one blanks off the B+ and signal on change/over.
I know there is a commercial version using a triac configuration around..(I dread to see it)... but is it unreligious to put such a wicked SState device into a tube output stage ?

richj

[Johan Potgieter]

I did not go back to the original question to look whether there is a pressing reason for switching "on the run". It would appear to me though, with all these possibilities for havoc of the real kind and very little good reason, that it is altogether best to go through the small trouble of switching off, switching over and switching on again.

Johan