Stereomour II with Specialty AV SP-71

[gmccown1]

Hello,

I built a Stereomour II amplifier and I am extremely fond of it. I thoroughly enjoyed the process and I enjoy the sounds that it provides to my ears. Recently, I have obtained another amplifier and I would like to be able to switch from one amp to the other for a variety of sonic sounds. The caveat, I only have one pair of speakers! I have found the SP-71 by Specialty AV that should solve my problem, I just wanted to get some technical advice to make sure that my Stereomour II will be safe using it with the SP-71 switch box. There is not a whole lot of information or reviews of people using this with tube amps. I have attached pictures of the inside wiring of the box with the resistors(5w100ΩJ) and switches. Anybody else run into this situation? or have a recommended trustworthy switch box that they would recommend? Will the Stereomour II be safe/protected to run with the SP-71? Thanks in advance for your thoughts, recommendations, and technical advice.

[2wo]

That looks like it should work, the resistors are probably there to provide a load to the unused amp. If the amp you are not listening to is switched off, you probably don't need them...John

[Thermioniclife]

Well, the SM2 can be run without a load on the outputs either 0 ohms 8 ohms or 100 ohms. Having no knowledge of your second amp it may have to have a load when it is turned on as in a tube amp with a feedback circuit or a transistor amp that requires a load while turned on. My opinion is to clip off the 100 ohm resistors on the SM2 input and inquire if your second amp requires a load to run safely. Do you intend to run both amps at the same time and switch back and forth? If no then I would lose all 4 of the 100 ohm resistors. ymmv. what s your second amp?

[gmccown1]

Thanks for the responses, my other amp is a Pioneer SA-7500. I only plan on using one amp at time, the other will be shut off while the other is in use.

[denti alligator]

I have this same switch box, which I use with my Stereomour II and an Onkyo home theater receiver. Are you suggesting those resistors might affect the sound of the amp?

[gmccown1]

I’m glad to hear others are using this switch box with their smII!  My main concern is to protect my smII, from “blowing up” or breaking when I run the Pioneer SA-7500. I would only be running one amp at a time. I read one review that said their tube amp did not fair well when paired with this box. Not sure the reason why. I figured I would ask for some technical advice before I pair together. Did you modify your Sp-71? Would the resistors affect the sound?

[denti alligator]

No modding. And I have accidentally left both amps on. Doesn’t make a difference.

[Paul Birkeland]

I read one review that said their tube amp did not fair well when paired with this box. Not sure the reason why.

For certain tube amps that have pentode power tubes, the gain in the output stage is relatively unbounded.  When you run such an amp with no load on the output transformer, the load impedance on those pentode output tubes goes way up, and the amplification factor of the output stage follows suit.  With all of our amps, the output stages use triodes that have relatively low mu, and this just isn't an issue.

[gmccown1]

Thank you for all the responses, you guys have been very helpful and reassuring. I’m gonna wire it up and have a go.