Jumping channels on 1950s Valco

[rebeat]

I’ve got an old 1950s Supro Comet (Chicago 51) with a 10” field coil speaker that sounds lovely when cranked. There are 3 inputs on back (2x instrument, 1x microphone). Can the inputs be safely jumped to bring about earlier distortion (for quieter home use)? I’ve tried to find info about this and so far nothing…Thanks!


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[Partscaster]

I found this, fwiw,

"Some of the Valco/Supro amps had just three parallel inputs going into the very same preamp stage. Only the input impedance is different. Jumpering would make no tonal difference."

[mozz]

No.


Actually we need to know what model number it is. Some have all 3 inputs going to the same tube so it would do nothing a single input wouldn't do. On the other hand, if it moves 2 of the inputs to another tube, chances are it would be out of phase and sound like crap. Unless it was a higher end model it won't work, you can try without hurting anything. Most Supros with multiple inputs are really known to suck tone due to 100k resistors going to ground between the channels, that really loads down the guitar.

I have a prewar Supro with the field coil speaker, they do much much better with a modern speaker but you have to mod the power supply with a resistor in place of the field coil resistance.

[mozz]

If it's the model with a 6SQ7, 6SC7, 5Y3, 6V6,6V6, you can put 2 dummy plugs into jack 2 and 3, that will lift the tone sucking 47k resistors and the amp should get much louder. Schematic below.

[rebeat]

Thanks for the replies! Not sure how to find out the model number; it only displays Chicago 51 on the back. It looks exactly like this one:

https://reverb.com/item/64172376-supro- ... t=64172376

Thanks for the advice about using a different speaker! What I’d really love to do is install a amplifier out jack that would allow me to plug into my universal audio ox box to record it without using the field coil speaker. Does anyone have any experience doing something like that? I suppose it would be the same thing in order to use an external speaker cabinet…


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[mozz]

Probably made by Valco or National. Sometimes there is a small metal plate about 1/4" high by 1/2" with a 4 or 5 digit code, that would be Valco's model number. I always thought the 51 was a postal zone code (before zip codes). Is there 1 6v6 or a pair of them?

[rebeat]

Probably made by Valco or National. Sometimes there is a small metal plate about 1/4" high by 1/2" with a 4 or 5 digit code, that would be Valco's model number. I always thought the 51 was a postal zone code (before zip codes). Is there 1 6v6 or a pair of them?

Can’t find a small metal plate with anything that looks like a number unfortunately…The amp has the following tubes:

- [ ] 6V6GT
- [ ] 5Y3GT
- [ ] 6SL7GT

Thanks!


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[andrewsrea]

@rebeat :

I repaired one of these in the 90's (actually had to rewind the field coil of the Jensen that was in it), although it may have been from the 40's and it was branded 'Premier' (the drum company). It is the same as the Gretsch Comet and many other brandings, which is like a Fender Champ, except the gain stages of the 6SL7GT are in series and not parallel (which provides more gain). As @mozz said, jumping doesn't achieve anything of substance and may likely introduce noise, as there is no dedicated resistor which provides reference to the 1st stage's control grid. It uses the other input's swamp resistor for that purpose, so it wasn't even designed well for 2 instruments at once.

Your amp could be changed with a non-destructive external mod, using a 1/4" plug, two 1/4" jacks, an enclosure box or a plate and a 'on-on-on' SP3T switch. You could wire it to be the internal speaker, or the ox box, or both.

[rebeat]

[mention]rebeat[/mention] :

I repaired one of these in the 90's (actually had to rewind the field coil of the Jensen that was in it), although it may have been from the 40's and it was branded 'Premier' (the drum company). It is the same as the Gretsch Comet and many other brandings, which is like a Fender Champ, except the gain stages of the 6SL7GT are in series and not parallel (which provides more gain). As [mention]mozz[/mention] said, jumping doesn't achieve anything of substance and may likely introduce noise, as there is no dedicated resistor which provides reference to the 1st stage's control grid. It uses the other input's swamp resistor for that purpose, so it wasn't even designed well for 2 instruments at once.

Your amp could be changed with a non-destructive external mod, using a 1/4" plug, two 1/4" jacks, an enclosure box or a plate and a 'on-on-on' SP3T switch. You could wire it to be the internal speaker, or the ox box, or both.

Thanks for the detailed answer! Makes sense…

So from what you describe re: output jack: cut the speaker wire and solder a plug on the speaker end and a jack on the amp end basically?

Just wasn’t sure if the amp needed to see anything special because of the original field coil speaker…Hard to read the speaker stats but is it safe to assume the amp wants to see a 8ohm load?


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[andrewsrea]

So from what you describe re: output jack: cut the speaker wire and solder a plug on the speaker end and a jack on the amp end basically?

Just wasn’t sure if the amp needed to see anything special because of the original field coil speaker…Hard to read the speaker stats but is it safe to assume the amp wants to see a 8ohm load?

Yes, that amp wants to see an 8 ohm speaker and those output transformers are not as robust as other brands, so a mismatch is not recommended.

I would:
1.) Buy an enclosure (https://www.tubesandmore.com/products/c ... -199-x-122), (3) British style jacks ( (2) 1/4" Switchcraft plugs ( (1) on-on switch (
2.) Plan ahead on where to fit the jacks and switch on the enclosure, then drill (3) 1/2" holes for the jacks and a hole (1/4"?) for the switch.
3.) Install the components and solder the (-) sleeves [tab closest to the mouth of the jack] together with some wire.
4.) Identify one jack as 'input' and solder that (+) tab [farthest from the jack mouth] to the common lug of the switch.
5.) Solder one lug of the switch to the tip tab of an output jack and the other switch lug to the tip tab of the other output jack.
6.) Unsolder the leads to the speaker (i.e.: off of the speaker), identifying which is (+) and which is (-).
7.) Solder those leads to a 1/4" Switchcraft plug, with (+) to the center pin [tip] and (-) to the long tab [sleeve].
8.) Plug that into the 'input' jack of the enclosure.
9.) Solder wire to the speaker and the 1/4" jack, making sure the tip is (+) and the sleeve is (-) and then plug that inton one of the output jacks.
10.) Connect the remaining output jack to the 8 ohm extension speaker.
11.) The toggle will allow you to switch between the internal and external speaker.

Or, you can just take the speaker leads and wire them to a 1/4" jack and operate the amp with an extension speaker.

DO NOT remove the field coil wires. Leave them connected.

[rebeat]

So from what you describe re: output jack: cut the speaker wire and solder a plug on the speaker end and a jack on the amp end basically?

Just wasn’t sure if the amp needed to see anything special because of the original field coil speaker…Hard to read the speaker stats but is it safe to assume the amp wants to see a 8ohm load?

Yes, that amp wants to see an 8 ohm speaker and those output transformers are not as robust as other brands, so a mismatch is not recommended.

I would:
1.) Buy an enclosure (https://www.tubesandmore.com/products/c ... -199-x-122), (3) British style jacks ( (2) 1/4" Switchcraft plugs ( (1) on-on switch (
2.) Plan ahead on where to fit the jacks and switch on the enclosure, then drill (3) 1/2" holes for the jacks and a hole (1/4"?) for the switch.
3.) Install the components and solder the (-) sleeves [tab closest to the mouth of the jack] together with some wire.
4.) Identify one jack as 'input' and solder that (+) tab [farthest from the jack mouth] to the common lug of the switch.
5.) Solder one lug of the switch to the tip tab of an output jack and the other switch lug to the tip tab of the other output jack.
6.) Unsolder the leads to the speaker (i.e.: off of the speaker), identifying which is (+) and which is (-).
7.) Solder those leads to a 1/4" Switchcraft plug, with (+) to the center pin [tip] and (-) to the long tab [sleeve].
8.) Plug that into the 'input' jack of the enclosure.
9.) Solder wire to the speaker and the 1/4" jack, making sure the tip is (+) and the sleeve is (-) and then plug that inton one of the output jacks.
10.) Connect the remaining output jack to the 8 ohm extension speaker.
11.) The toggle will allow you to switch between the internal and external speaker.

Or, you can just take the speaker leads and wire them to a 1/4" jack and operate the amp with an extension speaker.

DO NOT remove the field coil wires. Leave them connected.

Thanks so much for the detailed directions! Can’t wait to try this…


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