Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: turtle441 on August 21, 2019, 08:12:50 am

Title: Question about channel switching strategies
Post by: turtle441 on August 21, 2019, 08:12:50 am

I'm slowly doing to planning to move towards some form of a low-wattage Soldano-style amp.  (Not sure if I'll go the EL91 route as was done recently here or just run the pre-amp into SE poweramp with an EL34 or 6L6.)  Regardless, I've been slowly wrapping my mind around the ins and outs of the schematic.  In the Soldano SLO and x88, it appears he used relay switching, which then turned on and off the voltage to vactrols to switch behind the 3 channels.  That seems like an extra step.  Is there something that I'm missing to why plan old relays weren't used?

I've only built one amp with channel switching, but it worked well without any popping.  Although, admittedly, it was much lower gain than a SLO.  I'm sure there's a reason that switching was done in that way.  I just don't get what it is.

Title: Re: Question about channel switching strategies
Post by: d95err on August 21, 2019, 09:54:15 am

SLO’s don’t use relays, as far as I can recall.

The benefit of Vactrols is that the change in resustance is relatively slow, resulting in a smooth switch with no pops or clicks.

Title: Re: Question about channel switching strategies
Post by: nandrewjackson on September 01, 2019, 08:15:43 pm

I'd say it's the layout of relays that will add noise with all of the high gain going on. With vactrols, certain parts of the circuit are muted out by the high resistance,  or shorted to ground by the low resistance,  effectively eliminating whichever part of the circuit is being switched out. 
With only relays, both channels,  high gain and low gain are still active,  and both signals have to be wired over to the relay, with their leads only mm's apart, sending and receiving each other's  signals.


Vactrols can be placed into the circuit without added extra spaghetti   except for the small DCV control line for the LED half of them, so the audio signal path can stay nice and tidy.

Title: Re: Question about channel switching strategies
Post by: nandrewjackson on September 01, 2019, 08:20:04 pm

https://el34world.com/Forum/index.php?topic=24427.msg262881#msg262881






Here's my build blog post that includes my vactrol switching system I came up with.  It's super complicated but it works.  :l2:

Title: Re: Question about channel switching strategies
Post by: turtle441 on September 03, 2019, 08:19:07 am

Thanks.  That makes a lot of sense when you look at it that way.