johnheath

Guitar amp switching
« on: December 03, 2019, 03:55:59 AM »
Hi all...

In the studio we have been discussing how to switch between two separate guitar amp head and the following two separate speaker cabs.

I drew a simple schematic on a solution what I believe might work.

A concern is that when playing one of the amps the signal would be sent to the output of the other amp... so I was thinking of the diodes.

So the simple question is would this schematic work... with or without the diodes. And if the diodes is necessary what types are recommended?


Best regards

// John
Nothing is impossible - It just takes some more time


Re: Guitar amp switching
« Reply #1 on: December 03, 2019, 06:42:03 AM »
No, thats not going to do what you want at all ,

A tube or valve amp needs a load at all times , if you drive input signal with no load you stand a good chance of creating very high voltages across the transformer primary  , these could punch through the insulation in the windings and your amp will die horribly .

Your basically on the right track with the two sets of DPDT switches ,  but you still need to switch in a load resistance equivalent to the speaker  on the unused amp for safety , on the input side its best if you connect grid to ground on the unused amp also .   Hopefully theres enough there for you to have another go , I could just draw the circuit  out for you , but I think your better served going back to the drawing board so to speak . Your load resistors will of course have to be able to withstand the full wattage/heat dissipation of either amp ,  any other questions you have dont hesitate to ask .




johnheath

Re: Guitar amp switching
« Reply #2 on: December 03, 2019, 08:04:23 AM »
Thank you Tubetec

I was considering the output load necessary from the beginning but according to the schematic either amp will have a speaker cab loaded... the amp not connected will not have any signal and supposedly no output signal?

Or am I missing soemthing?

best regards

// John
Nothing is impossible - It just takes some more time

dmp

Re: Guitar amp switching
« Reply #3 on: December 03, 2019, 09:29:39 AM »
You cannot use diodes like that because a speaker signal is alternating.  How will current return to the the amp output transformer?
I agree your schematic could be a problem with tube heads. You can't use a MBB switch because the two amps would see each other temporarily, while BBM will disconnect each amp from a speaker momentarily. Not only would it be bad for the amp, but may give a loud pop. You'd have to use it in standby.

Something here work?
https://www.radialeng.com/product_category/switchers

It would not be what you describe exactly, but the cab-link would allow you to switch one amp between two cabs.

squarewave

Re: Guitar amp switching
« Reply #4 on: December 03, 2019, 10:01:04 AM »
As others have said, your diode arrangement will not work at all. And you cannot switch the load with the amp actually on. It could easily at least destroy the power tubes. I know from experience.

johnheath

Re: Guitar amp switching
« Reply #5 on: December 03, 2019, 10:33:38 AM »
Thank you guys

So... not even in standby?

I have scratching my head around the ... well, possible switching?

So leaving the diode... standby mode... BBM-switches and adding load to the output transformer and shorting the input on the unused amp. Could it work?

Best regards

// John
Nothing is impossible - It just takes some more time

CJ

Re: Guitar amp switching
« Reply #6 on: December 03, 2019, 03:23:11 PM »
what kind of amps and speakers?
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

squarewave

Re: Guitar amp switching
« Reply #7 on: December 03, 2019, 07:14:05 PM »
That's not gonna work either. There's no point in putting load resistors behind switching contacts anywhere. Not sure how you could put a dummy load in there at all actually. You could do 100 ohms hard-wired on each out. That and switching only on standby might stop something from blowing up but those resistors are going to dissipate a lot of heat. Also I don't understand your switching at all. Why do you need 4 pole switches? Simple DPDT will do fine for both swithing amps and speakers.

CJ

Re: Guitar amp switching
« Reply #8 on: December 04, 2019, 12:56:52 AM »
wire a 620 ohm 2 watt resistor across each 1/4" speaker jack,

use a make before break relay and you are done.

switching the amps is easy, just make an A/B box, or buy one.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

johnheath

Re: Guitar amp switching
« Reply #9 on: December 04, 2019, 06:03:45 AM »
wire a 620 ohm 2 watt resistor across each 1/4" speaker jack,

use a make before break relay and you are done.

switching the amps is easy, just make an A/B box, or buy one.


Sounds interesting CJ

The A/B box and 620ohm resistor across the jacks would be no problems, but the relay is a question mark for me.

Is a relay necessary or can it be fixed with a "normal" switch?

The amps are home brew 45W tube amps and likewise the speaker cabs.

Best regard

// John
Nothing is impossible - It just takes some more time


johnheath

Re: Guitar amp switching
« Reply #10 on: December 04, 2019, 06:07:14 AM »
That's not gonna work either. There's no point in putting load resistors behind switching contacts anywhere. Not sure how you could put a dummy load in there at all actually. You could do 100 ohms hard-wired on each out. That and switching only on standby might stop something from blowing up but those resistors are going to dissipate a lot of heat. Also I don't understand your switching at all. Why do you need 4 pole switches? Simple DPDT will do fine for both swithing amps and speakers.

Thank you Sir

Well, the 4PDT switches were just for bringing along the neg. but as you say they are probably not needed.

The dummy loads were aimed at having a load on the unused amp head at all time...

Best regards

// John
Nothing is impossible - It just takes some more time

CJ

Re: Guitar amp switching
« Reply #11 on: December 04, 2019, 06:12:16 AM »
you do not need a relay if you are not switching in the middle of a song, 
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Re: Guitar amp switching
« Reply #12 on: December 04, 2019, 09:59:58 AM »
A tube amp has no problem with a temporary low or shorted load ,output voltage drops off but it wont damage anything . I dont see any need to be switching the ground connection , but dual pole will allow you switch two seperate loads resistors . Some heating elements like you find in a cooker hob make a great speaker equivalent load ,  it will easily dissipate anything you can throw at it  ,up  to maybe a KW , could also double up as a 'silent record' where you just DI whats appearing across the dummy load .

Relay might be useful in cutting down meters of cable depending on how stuff is placed .

johnheath

Re: Guitar amp switching
« Reply #13 on: December 04, 2019, 10:20:03 AM »
A tube amp has no problem with a temporary low or shorted load ,output voltage drops off but it wont damage anything . I dont see any need to be switching the ground connection , but dual pole will allow you switch two seperate loads resistors . Some heating elements like you find in a cooker hob make a great speaker equivalent load ,  it will easily dissipate anything you can throw at it  ,up  to maybe a KW , could also double up as a 'silent record' where you just DI whats appearing across the dummy load .

Relay might be useful in cutting down meters of cable depending on how stuff is placed .

Thank you sir

 
I have been fiddling with the switches back and forth and as suggested before I have downsized them to three DPDT switches. Also adding facts from other comments.

I mean... If you don't send a signal into a tube amp is there really a need for a dummy load. And if using a dummy load and if the user switches amp and still plays a few second... he or she would most likely hear that it is silent and stop doing so... without a n incoming signal how much heat dissipation will there be? I don't know but merely kindly asking :)

Best regards

// John

Nothing is impossible - It just takes some more time

squarewave

Re: Guitar amp switching
« Reply #14 on: December 04, 2019, 11:41:32 AM »
I have been fiddling with the switches back and forth and as suggested before I have downsized them to three DPDT switches. Also adding facts from other comments.
Much better.

I mean... If you don't send a signal into a tube amp is there really a need for a dummy load. And if using a dummy load and if the user switches amp and still plays a few second... he or she would most likely hear that it is silent and stop doing so... without a n incoming signal how much heat dissipation will there be? I don't know but merely kindly asking :)
The heat dissipation is from when your playing since the 620R will always be in.

As for whether or not you really need the dummy load, I would say "yes". There's a decent amount of power there. That can be deceiving. Removing the load even for the instant it takes for the switch to transition from one contact to the other is an eternity for electricity (electrons are fast). If the output signal is near silent, then no, a dummy load probably isn't necessary. In theory, a concern might be that if the amp is a little unstable to begin with because it's a super high gain / no feedback monster, removing the load could cause some oscillation and then boom you wipe out your tubes or worse.

I once made a dummy load that naively didn't have a load at all at the first position (meaning the load was out). I was switching it while testing with signal and when I got to that no-load position, the power tubes emitted blue flashes. It was magnificent! Until I realized the tubes were fried.

You could use this as an excuse to make it also a dummy load. For example, you could do a 50W 4ohm resistor in series with a 25W 8ohm resistor to make the load and then strap your speaker across the 8ohm. That would give you a modest 12dB attenuation (?) and the load would always be in. Get the aluminum cased resistors and mount them inside a die cast aluminum enclosure and bolt them through to an aluminum heat sink (note that the power ratings on those resistors depend on the heat sink they're mounted to).

johnheath

Re: Guitar amp switching
« Reply #15 on: December 04, 2019, 12:52:19 PM »
Much better.
The heat dissipation is from when your playing since the 620R will always be in.

As for whether or not you really need the dummy load, I would say "yes". There's a decent amount of power there. That can be deceiving. Removing the load even for the instant it takes for the switch to transition from one contact to the other is an eternity for electricity (electrons are fast). If the output signal is near silent, then no, a dummy load probably isn't necessary. In theory, a concern might be that if the amp is a little unstable to begin with because it's a super high gain / no feedback monster, removing the load could cause some oscillation and then boom you wipe out your tubes or worse.

I once made a dummy load that naively didn't have a load at all at the first position (meaning the load was out). I was switching it while testing with signal and when I got to that no-load position, the power tubes emitted blue flashes. It was magnificent! Until I realized the tubes were fried.

You could use this as an excuse to make it also a dummy load. For example, you could do a 50W 4ohm resistor in series with a 25W 8ohm resistor to make the load and then strap your speaker across the 8ohm. That would give you a modest 12dB attenuation (?) and the load would always be in. Get the aluminum cased resistors and mount them inside a die cast aluminum enclosure and bolt them through to an aluminum heat sink (note that the power ratings on those resistors depend on the heat sink they're mounted to).


Thank you sir

I will have a look at this as well.

Best regards

// John
Nothing is impossible - It just takes some more time

Re: Guitar amp switching
« Reply #16 on: December 04, 2019, 02:24:50 PM »
I was going to say but Squares got in first , in theory if the input is muted ,there shouldnt be any output signal , instabillity or a microphonic tube or motorboating might cause a signal to appear even when the first grid is grounded though , so always consider an unlikely eventuality just to err on the safe side .

abbey road d enfer

Re: Guitar amp switching
« Reply #17 on: December 05, 2019, 06:55:58 AM »
Thank you guys

So... not even in standby?
That would solve the issue of leaving an amp without a load. Even without signal, an active tube amp becomes unstable and oscillates, which quickly burns the output tubes and the output transformer.

Quote
So leaving the diode...
Forget about diodes in the signal path. The signal is AC, so you need to pass both positive and negative waves, which a single diode cannot do. Using two diodes in anti-parallel mode would offer no separation and anyway would introduce distortion.
You have to uses mechanical switches or relays, period. Make sure that when an amp is not connected to a load, it goes to a dummy load of adequate power rating.

EDIT: I see you have ditched the diodes for a much better solution. However the 620 ohms resistors are no significant load. You need about 8 ohms rated  sufficiently for the most powerful amp.
« Last Edit: December 05, 2019, 07:01:16 AM by abbey road d enfer »
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

johnheath

Re: Guitar amp switching
« Reply #18 on: December 05, 2019, 08:24:53 AM »
That would solve the issue of leaving an amp without a load. Even without signal, an active tube amp becomes unstable and oscillates, which quickly burns the output tubes and the output transformer.
 Forget about diodes in the signal path. The signal is AC, so you need to pass both positive and negative waves, which a single diode cannot do. Using two diodes in anti-parallel mode would offer no separation and anyway would introduce distortion.
You have to uses mechanical switches or relays, period. Make sure that when an amp is not connected to a load, it goes to a dummy load of adequate power rating.

EDIT: I see you have ditched the diodes for a much better solution. However the 620 ohms resistors are no significant load. You need about 8 ohms rated  sufficiently for the most powerful amp.

Thank you sir

So...basically replace the two 620R resistors with larga wattage 8ohm resistors is the significant difference at the moment?

Best regards

// John
Nothing is impossible - It just takes some more time

abbey road d enfer

Re: Guitar amp switching
« Reply #19 on: December 05, 2019, 09:08:07 AM »
So...basically replace the two 620R resistors with larga wattage 8ohm resistors is the significant difference at the moment?
Not really, there's something fishy in the way the output switching works. The amps should feed the common of the switches.
« Last Edit: December 05, 2019, 09:15:40 AM by abbey road d enfer »
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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