Tube Amp Power Switching

Hello everybody,

I have an 18w tube amp with a power transformer that has 3 terminals on the secondary: 0v, 190v and 275v.
Is it ok to use an SPDT toggle switch across the 190v and 275v terminals to select between 9 and 18w?
Something like this: https://www.banzaimusic.com/search.php?mode=search&page=1&keep_https=yes

Does the amp need to be powered down to use the switch or can I toggle between 9 and 18w with the amp live?

Thanks in advance!
Cheers
Sono

Try it.

I suspect it may POP and scare the audience. And since you are switching high voltage DC, the switch won't last forever. Power-down switching is kinder for everybody.

Your link can't work--- it goes to Banzai Search but the search-term isn't in the URL so we get ALL the products....

PRR said:

And since you are switching high voltage DC, the switch won't last forever. Power-down switching is kinder for everybody.

Click to expand...

Actually no. I'd be switching AC... although I agree that powering down is always kinder for everyone, but I have seen rotary switches used like that. Hence my great idea
They were BBMs and had more than 2 positions and allowed the user to select between 4, 18 and 36 watts without powering down and without pops.
So I was wondering if a SPDT toggle switch would behave in the same way.

PRR said:

Your link can't work--- it goes to Banzai Search but the search-term isn't in the URL so we get ALL the products....

Click to expand...

Oooops sorry. This one should work: https://www.banzaimusic.com/Carling-SPDT-Toggle-Switch.html

Cheers
Sono

It will probably work , but you may have to compromise between different values of load depending on which Ht voltage you have and  variations due to bias . I think it might be  best to optimise the high voltage  mode and live with slightly less effeciency in the lower output mode due to mismatch and wrong bias resistor etc.

Ive never tried to make an amp with variable HT , I think PRR is right it will pop when the valves are hot switched .I've tried triode/pentode switching with an adjustment in bias resistor but again its all a bunch of compromises ,and theres a large 'BOOOF' through the speaker if the amp isnt in standby .

Maybe if you made it so on the low voltage mode your amp ran class a cathode biased ,then when you go to high voltage mode you could add extra grid bias to keep the valves within safe operating parameters . One way of avoiding or reducing  pop could be to use a three position toggle , Hi -standby-lo ,(on off on)now your amp is automatically muted when you change modes . a second pole on the switch could cut in a subsidiary neg voltage to the grids in hi mode or just ground the lower ends of the grid leak resistors for normal cathode bias.

A three pole rotary switch could allow several positions varying from cathode bias class A ,to  fixed bias class a/b  ,and hi or lo ht voltages , still gonna send a big thump through the speaker though I think . Mode switches are probably best on the rear panel of amps ,that way a big Bloop that takes out front of houses bass bins is much less likely in a live set up .I'll probably try hybrid cathode/fixed bias switching on a project soon , KLD's Mojo12 kit is on its way to me

I think it's what Orange did on the tiny terror. (And other amps but tiny terror schematic is easy to find).

One switch in a sp3t ON-OFF-ON type. It'a labelled "7w-standby- 15w".

So you switch between two taps of the transformer and stand by. It never popped.

Thomas

Well spotted Ray ,
its exactly how they did it , its funny you mention it because just the other day I was looking at the tiny terror schematic ,I didnt pick up on the detail of the extra tap on the mains tx ,but the three position standby must have clicked in my brain 

Im starting to like the idea of a tap on the ht winding for output power control now , maybe the mismatch between hi and low mode isnt so bad as triode /pentode ,and halving the ht voltage will most likely increase valve opperating life ,where  over volting /bias and mismatch  in triode mode is liable to be detremental to valve life .

We stock the three position toggle at work ,its standard on small generators for 110v-off-220v.
Id say almost any tool hire /repair place will have them  on the shelf .

Those switches come in 3 and 4 pole versions too , could allow you to shift several parameters of the output stage in one go.

Wouldn't a Break Before Make SPDT toggle switch do the same?

Is there a way of determining if a switch is BBM (apart from reading the specs of course...). I ask cos I got a few spare ones in my workshop...

Cheers
Sono

If you use a SPDT switch then  you need a Break Before Make switch. Otherwise you will be shorting out the winding on the transformer when switching.
As suggested, best to use a three position switch, On-Off-On.

So are you saying there are no BBM toggle switches?

Is there a way of determining if a switch is BBM or MBB?

Thanks
Sono

Glad i could help!

To check if a switch is BBM or MBB, i would try the attached schematic maybe?

If the led blinks when you switch, then it should be of shorting type (MBB).

Although, i'm not sure it would work, the blink might be too short (maybe add a small cap?)

Or you could use a DSO.

Thomas

sonolink said:

So are you saying there are no BBM toggle switches?

Is there a way of determining if a switch is BBM or MBB?

Thanks
Sono

Click to expand...

I was thinking more along the lines of rotary type switches rather than toggles with my comment.
I haven't seen a SPST toggle switch that *wasn't* break before make.

totoxraymond said:

To check if a switch is BBM or MBB, i would try the attached schematic maybe?

If the led blinks when you switch, then it should be of shorting type (MBB).

Although, i'm not sure it would work, the blink might be too short (maybe add a small cap?)

Or you could use a DSO.

Click to expand...

Thanks a lot Thomas!!
What do you mean with DSO?

Walrus said:

I was thinking more along the lines of rotary type switches rather than toggles with my comment.
I haven't seen a SPST toggle switch that *wasn't* break before make.

Click to expand...

I'm pretty sure all toggle switches are BBM but I wanted to be sure. If you check my previous posts you'll see that I mentioned rotary switches before:

sonolink said:

[...]but I have seen rotary switches used like that. Hence my great idea
They were BBMs and had more than 2 positions and allowed the user to select between 4, 18 and 36 watts without powering down and without pops.
So I was wondering if a SPDT toggle switch would behave in the same way

Click to expand...

meaning breaking before making
So if toggle switches are BBM I should be fine using a SPDT right?

Cheers
Sono

sonolink said:

Thanks a lot Thomas!!
What do you mean with DSO?

Click to expand...

Digital Storage Oscilloscope.

Put a voltage on Common pin, resistors on both other pins. Monitor those pins and set the scope to get a picture when triggered. This way, you could actually see when poles change value.

If pin 1 is initally +5V and gets down to 0 before pin 2 gets up to +5V: BBM
If both pins are at +5V for a msec or so : MBB

But if (like me) you don't have a DSO, it would be a bit expensive...

I have a DSO 
Thanks a lot for the tip. I'll try it!
Cheers
Sono