Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on July 21, 2022, 08:49:57 am

Title: Use of relay on input?
Post by: joesatch on July 21, 2022, 08:49:57 am

Would there be any issue using a relay like this to select two different preamp signal paths? i would be using coax to and from the relay. Anyone done this?

Title: Re: Use of relay on input?
Post by: pdf64 on July 21, 2022, 09:12:53 am

Input circuits tend to be rather sensitive. Why not select channels later in the signal path?

Title: Re: Use of relay on input?
Post by: joesatch on July 21, 2022, 09:15:27 am

Because none of the circuits are shared (besides the 1M to ground at input)

Title: Re: Use of relay on input?
Post by: sluckey on July 21, 2022, 09:26:50 am

I suggest using the other half of the relay to ground the unused grid. Also I would use two 1M (R1) grid leaks, one for each tube.

Title: Re: Use of relay on input?
Post by: joesatch on July 21, 2022, 09:36:03 am

Exactly what i was gonna do as i see Friedman does that to eliminate channel bleed, he uses the other half of the relay to ground the other channel's grid. So put the relay before the 1M grid leaks and use two of them?

one issue i see with this is i would need to implement an additional relay to switch the channel to output.  I'm thinking instead of grounding the the other half of the relay to use the other half to switch each channels MV to output.  Will having the input grids and MV outputs on the same relay in proximity cause issues?

Title: Re: Use of relay on input?
Post by: pdf64 on July 21, 2022, 11:00:22 am

Quote from: sluckey on July 21, 2022, 09:26:50 am

I suggest using the other half of the relay to ground the unused grid. Also I would use two 1M (R1) grid leaks, one for each tube.

To avoid unnecessary stuff at the input, that could be done further along the signal path.

If a dpdt relay, the other section could select which channel output is used.

Title: Re: Use of relay on input?
Post by: EL34 on July 21, 2022, 11:25:18 am

If you look at my hot switch mod, you can see how to ground the signal that is not being used
https://el34world.com/projects/hotswitch.htm

Title: Re: Use of relay on input?
Post by: joesatch on July 21, 2022, 11:30:55 am

At some point a mechanical switch comes into play when using relays. What i will do is use a standard DPDT switch (front panel thats also linked to a footswitch) The DPDT will switch the input as well as activate a relay to switch the output and mute the other channel's signal at the respective gain pot.

Title: Re: Use of relay on input?
Post by: sluckey on July 21, 2022, 11:40:52 am

Often times it's not necessary to switch the outputs because the outputs are simply combined using passive mixing resistors (such as my Supro, Phoenix, or AC-15), or maybe even feeding each preamp output to different sides of a LTP PI (see my Dual Lite).