Lectrolab amp - schematic help

[johnnyreece]

So, I recently purchased a vintage Lectrolab amp (I posted about it in Garage Talk: https://ampgarage.com/forum/viewtopic.php?f=7&t=34844). Unfortunately, there's little information about them on the internet, and what is there doesn't include this early model. The main problem is, someone has been in it before me and so I'm not sure what the original circuit looked like. I've traced the schematic, and some of it makes little sense (this is unfinished: The power section isn't added yet, but it's pretty standard). I've marked the schematic with my question areas. The two in red I really don't understand...I don't know why there's a cap to ground off the preamp plate, and the tone control doesn't make sense to me (and, it doesn't work). The area in blue is fine, but I don't understand why they've put two 100k resistors in series. Is there any reason to have two (identical) inputs going to separate grids when the plates and cathodes are merged? I'm not sure what tube *should* be there. It was wired for the 6S_7 dual triode, so I pulled what was in there, and it measured as a weak 6SN7. Replacing with a new 6SN7 sounds fine, but a 6SL7 makes it shriek with feedback. The snubber cap on the schematic I was unable to get the value of. It's domino-style cap buried at the bottom, and I can't see any of the colors on the dots, but I'll try to figure it out. I appreciate any insight you all may have!

Edit: Disregard the question about the top left components...After some research, those help decouple that stage from other stages on the same power node. Still confused about the tone control, though...

[Stevem]

That resistor in the blue circle should likely be 10k, or maybe even 1k, but. Certainly not 100K!

Also the near 20K ohm preamp cathode resistors make no sense to me either!

The first preamp tube to me should be a 6SL7 if the amp where made for guitar and not HI Fi use.

If it where me and since you do not have a schematic. I would just gut it and rebuilt it into a Silvertone model 1301.
It couldn’t cost you more then 40 bucks in parts!

[sluckey]

The tone control circuit doesn't look right. When the pot wiper is turned down to ground, all signal will be killed.

The plate circuit for the preamp circuit was used by several Valco and Supro amps. I don't know what the idea behind this circuit is. Here's one example that I know works well...
https://el34world.com/charts/Schematics ... _s6424.pdf

The two inputs are simply being mixed together at the plates. This was a common way to mix mic inputs in low cost PA amps. Even the Marshall 18watt amp mixed the two normal inputs using this same method.
https://el34world.com/charts/Schematics ... _schem.pdf

[johnnyreece]

Thanks for taking a look! The preamp cathodes are 1,950 ohm (they're actually two 3k9 in parallel). As far as the 100k, here's the website where I found the description for that (it's the last two components discussed on the page): https://www.frostburg.edu/personal/latt ... audio.html In my other thread in General Discussion, I linked to a similar Valco model that did have a 6SC7, and I believe I've found a similar circuit snippet. I've attached a gut shot of that, circling the components involved. It, too, seems to have linked the plates together.

All said, I may just gut it, although I have replaced all the caps (except a ceramic one in the non-functioning tone circuit), and it *is* working (again, except for the tone circuit). The tone circuit seems to have no effect at all. I'll try to check it closer. It may be that it isn't connected to ground, and that's why it does nothing.

[johnnyreece]

Okay, looking through some old Valco amps, I found a tone circuit that seems as if it might work. I've modified the schematic accordingly, and removed the 56k resistor. Does this seem like it will work/not explode?

[imjonwain]

Neat amp! I used a have a small mystery amp that was either a lectrolab or melody, maybe teisco ( i never figured it out) that I traced out.

it was 2x6AV6 into a 6AR5 with a 6X4 rectifier.

[Stevem]

That 1 meg resistor and that cap to ground between the plate of the driver tube and the output tube bothers me, as I can't see a need for it!

[johnnyreece]

Neat amp! I used a have a small mystery amp that was either a lectrolab or melody, maybe teisco ( i never figured it out) that I traced out.

it was 2x6AV6 into a 6AR5 with a 6X4 rectifier.

Thanks! Yeah, half the fun is trying to figure out what these folks who were WAAAAAY smarter than me were doing.

That 1 meg resistor and that cap to ground between the plate of the driver tube and the output tube bothers me, as I can't see a need for it!

It's a form of negative feedback, from what I read. I did find an error after looking at my schematic, though. It was the placement of the 100k resistor in the tone circuit. As I have updated the schematic with the power supply, I reattached both the original and one with modified tone control. Any thoughts on the revised tone control, and whether the omission of the 56k resistor is a big deal? I'm starting to wonder if the .01uF cap is bad, since the tone control has no effect. It's a ceramic disk, which I usually don't mess with on a recap, but I suppose one should never assume anything. Thanks for taking a look!

[johnnyreece]

Well, I went ahead and wrapped this up. I settled on a simpler tone circuit from a different Lectrolab amp, which allowed me to clip the 100k and 56k resistors. It still doesn't do much, but oh well...I did find, after all was said and done, that I could use either a 6SN7 or 6SL7. As expected, the 6SN7 stays pretty clean up until 8-9 on the dial. The 6SL7 stays pretty clean, too, but the onset of distortion starts a little earlier, and gets a little rowdier. No surprises there. I took a couple snapshots for posterity, and I added my updated schematic. Not a bad sounding little guy, at all! Has a nice shimmer to it.