Jumping channels on a Supro Super

[Zaxxon]

I recently got an old Supro 1606 and wondered since it has three inputs, if I could jump the channels. It's got two inputs and another labled "switch." The guitar plays through all three, and they all sound the same to me. With every combination of plugging into one channel and connecting the other two, there is an increase of volume and gain. But I also noticed when I plug into the first channel, and plug the short cable into the second and just leave it out hanging, without plugging it into the switch channel, there is the same increase in volume and gain. The amp already overdrives easily, but all of the jumpering results surprised me.

[jeremy]

Re: Jumping channels on a Supro Super

the ones ive seen only have two inputs on top by the knob, where is the other jack? we are talking about the old valco made ones right?

[Zaxxon]

Re: Jumping channels on a Supro Super

Yes, it's one of the Valco ones; it seems like 1962, looking up the serial. I see what you mean, looking at pics online, about the two-input panels.

[Darg1911]

Re: Jumping channels on a Supro Super

Just a guess here but when you plug in to the second jack you are probably, essentially, switching in another resistor (grid to ground resistor on each jack) in series which is increasing the grid to ground resistance. That would allow less signal to bleed to ground and more signal to hit the grid of the input tube.

Also, based on the photo above I don't think the "Switch" graphic applies to the input jack. I think it applies to the black knob at center which, I assume, is a combination volume pot, rotary on/off switch?
I think one of those input jacks is meant as a microphone input based on some of the other models of early Valco/Supro amps. I haven't looked at any schematics for this one though.

[Zaxxon]

Re: Jumping channels on a Supro Super

That's interesting and it's good to know there could be a concrete reason for the effect. Thanks for the explanation and the clarification about the switch label & mic input. Yes, the knob is on/off & volume, which I haven't had before, and really like. I know the increase is not my imagination; I tested it sitting at the same distance from the amp as unjumpered, and my ear is still hurting.

[devastone]

Re: Jumping channels on a Supro Super

Darg is right, that's how the low/high inputs on the old Marshalls worked, I can't remember off the top of my head if the high jack added a resistor in series, or just lifted a resistor all together, but either would have the same effect.

[dystrust]

Re: Jumping channels on a Supro Super

Darg is right, that's how the low/high inputs on the old Marshalls worked, I can't remember off the top of my head if the high jack added a resistor in series, or just lifted a resistor all together, but either would have the same effect.

I can't remember how the 1959/1987 circuits worked, but in the 2203 & 2204, the low input bypasses V1A.

[devastone]

Re: Jumping channels on a Supro Super

Oh yeah, I guess I'm thinking of the hi/low inputs on the 4 input models.

*EDIT* I just looked them up and yes, the high input on the 4 input models puts an extra 68K resistor in series to ground.

[Darg1911]

Re: Jumping channels on a Supro Super

There are several things going on with the 4 input Marshalls like the JTM45's. The "low" inputs for both normal and high/treble "channels" are attenuated by 6db. Also, each input (normal and hi-treble) only uses 1/2 of the first 12AX7. When the inputs are jumpered, both halves of the 12AX7 are used in parallel, not in series/cascade. But when you plug the guitar into the treble "channels" high input, for example, then jumper from the treble "channels" low input into the normal high input (which is a common way of jumpering the amp) you are feeding that normal "channels" high input with a signal that is 6db down. I never found a whole lot of increase in gain doing that even though you are adding in the second triode of the 12AX7. Probably because of that 6db attenuation of the low input. Using a Y cable into both "channels" high inputs is a better way to take advantage of getting the most signal into both halves of V1 but since it's in parallel, it's still not nearly the same as a cascading the two halves of V1. Personally, I like to use an A/B/Y box. Anyway, this if probably off topic.