Supro/Valco Questions

mr.d.bluster said:

It seems like there are quite a few amp-savvy folks on this forum, and I hope some of you will be able and willing to help me out with a few questions. I have a circa 1960 Valco/Supro Spectator 5W amp with 12AX7, 6V6GT, and 5Y3GT tubes. The schematic for the original '50s version of this amp, having a single 6SL7 preamp tube, is HERE.

However, mine is a later '60s revision with a single 12AX7*. I haven't been able to find the schematic for this later version of the 1614, but the Valco/Gretsch 6150 (below) seems very close. The tubes are the same, and both have three inputs, but mine also has a tone control. Not sure what else may be the same and/or different from mine.

So, here are my questions:

1. This amp came to me with a Realistic speaker, not the original. I replaced it with a 4 ohm Weber signature alnico blue. The amp is working and sounds very good, but I wonder it that's the right impedance for this amp. Is there a way to determine the rating by measuring the current or voltage through the speaker leads? Will it hurt any of the components to use a lower than spec'd impedance? I do have an 8 inch Jensen 8 ohm ceramic that will fit. I might eventually go to a dual output transformer, but in the meantime…?

Most of American single-ended amps had 4 ohm speakers, you should be safe.

2. It looks like there's a death cap between the fuse and the power transformer. Should I just snip this out?

Yes!

3. I want to install a 3-prong power cord. Any caveats? I believe the switch and fuse should be in series on the hot lead?

That's correct. Ground should have a ring terminal connected to a brass machine screw and nut, or a cleaned transformer bolt.

4. Can I substitute a 12AT7 tube (actually a 6105) for the 12AX7 without harming the amp? [Is this relevant to the type of bias?] This is not important, but I'm just wondering how it might sound.

A 12at7 will reduce the gain and give it a cleaner sound, but a 12ay7 would sound a bit better.

I should mention that I have just enough electrical knowledge to be dangerous around tube amps. I know that residual voltages need to be discharged, but I don't know how to do it, or how to check to make sure it's done. I've opened the chassis on this amp once, just to take a look, but only after it had not been turned on for several months.

Discharge the filtercaps with a screwdriver to drain off any residual charge.

Thanks for any advice you might be able to provide.

*(At the same time, bias was changed from cathode to grid leak bias. These changes are discussed in posts on the TDPRI forum. Mine is definitely the 12AX7 version, as that's how the socket is labeled on the chassis).

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