Quick question about attenuator switching

[ricothetroll]

Hi,
I built a dummy load acoording to Aiken's principle ( http://www.aikenamps.com/spkrload.html , without R2 ) to be used in parallel to my "real" speaker, a Celestion G12M Greenback Heritage. My goal is to be able to switch ON/OFF it with relays, to use the 8R output without the dummy load on the clean channel and the 4R with the dummy load in parallel output on the Drive channel.
My question is : would it be a problem to switch it while playing (voltage/current surge, etc...) ?
My amp is an AC15 adaptation built in a Fender Hot Rod Deluxe.
Thanx in advance !
Best regards.
Eric

[R.G.]

On a tube amp, it's best to never leave the output open, even for the time between switches opening and closing. So you want "make before break" switching, so that you switch in the attenuator and have the contacts make solidly for at least 50ms or so before the contacts open on the real speakers; and the reverse when switching back, being sure the switching contacts make contact in parallel before breaking.

[ricothetroll]

Thanx a lot for your answer RG ! 

[ricothetroll]

Just a precision :  the 8R and 4R taps are on the same coil of the output trafo, so doing a "make before break" xwould result in shorting the output transformer between the 8R and 4R taps. is it ok to do it for a short time ? Is it ok to do it permanently ?
Here you can see the trafo on the schematic of the HRD :
http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/fenderamps/hotrod_deluxe.pdf
Best regards.
Eric

[R.G.]

Tube amps can be shorted for some period of time - perhaps indefinite! - with no harm. This is the exact opposite of solid state amps. Solid state amps can usually be run open-load with no damage, but shorts are dangerous to SS amps.

What's important with tube amp output stages is to never leave the output open.

There is another possibility. You can put a 220-470 ohm resistor of 5-10W rating on the output, perhaps the 16 ohm output, and open the speaker line for short periods with little chance of harm. The always-there resistor keeps an open circuit from existing for the short time of switching. This is generally OK, although high resistance is not always good.

[ricothetroll]

My intend is to short the 4R tap and the 8R tap, like this :

Then I'm sure there's no chance the amp ever runs unloaded...

[PRR]

> intend is to short the 4R tap and the 8R tap, like this :

A short across the 4 and 8 taps is a DEAD short on the amp.

No output!

(From 4 to 8 is a 0.7 ohm winding. You could connect a 0.7ohm load and get good output. If you connect a short 4 to 8 you get essentially zero output anywhere. {Deeper: the 4-8 portion probably has 0.4 ohms self-resistance. So a dead short only eats half the total amp power, some will still be available on the 0-4 or 0-8 connection.})

If this is a cathode-bias AC15-sh power amp, the tubes and PT will survive dead-short all day. (OT may be strained.)

(High power amps are safer shorted than open. HIGH-power amps can be killed with a short but it takes seconds or hours, not a clack. Un-loaded tube OTs can be ZAPed in an instant.)

Since relays don't come make-before-break, and you apparently want an impedance shift, while playing, it is not a "quick question". This stuff is my Sodoku and I'm not seeing an obvious answer.

[ricothetroll]

Hi Paul,

Yes, I actually thought it would be a quick answer Now I realize I over simplified the problem...

I also realize that the idea of shorting taps of the output trafo was... hem... stupid ! 

Thanx for your explanations ! I think I'll do a "make before break" relay using two relays with a delayed control voltage for the "break" one, combined with RG's idea of puting a fixed safety resistor of 100-200R between the 8R tap and ground.

Best regards.

Eric

[ricothetroll]

Doing some further researches I found this :
http://web.archive.org/web/20030718021444/http://aikenamps.com/HeadSwitch.pdf
Aiken's using 470R for protecting both head's output transformer against open circuit and simple switching with relays, as RG suggested. I'll give that a try (if he did it, that should be safe enough   )
Thanx again for your help !
Best regards
Eric

[wavley]

Tube amps can be shorted for some period of time - perhaps indefinite! - with no harm. This is the exact opposite of solid state amps. Solid state amps can usually be run open-load with no damage, but shorts are dangerous to SS amps.

What's important with tube amp output stages is to never leave the output open.

There is another possibility. You can put a 220-470 ohm resistor of 5-10W rating on the output, perhaps the 16 ohm output, and open the speaker line for short periods with little chance of harm. The always-there resistor keeps an open circuit from existing for the short time of switching. This is generally OK, although high resistance is not always good.

Traynor amps use a 180 ohm 15watt on their 8 ohm outputs as a confirmation of another "What R.G. said"  I wouldn't try to run it all day like that, but it's certainly nice if you have an idiot moment and forget to plug in your speaker cable.

[ricothetroll]

Traynor amps use a 180 ohm 15watt on their 8 ohm outputs as a confirmation of another "What R.G. said"  I wouldn't try to run it all day like that, but it's certainly nice if you have an idiot moment and forget to plug in your speaker cable.

Thanx !