Mid 60's Valco (Supro) S6450 "Royal Reverb" schematic needed

[rrozell]

Brian,
I did pull the section of circuitry you are referencing apart and replaced the .01 cap. The resistors all matched the color code markings. I even pulled the reverb pot off the case to make sure there was no short to ground there. I did not check the wire going to the 12ax7. Will check that. 40-50 year old aluminum wire.
I got some of my parts today. On the Gretsch 6162 schematic in the tone circuit is the "500" cap 500 picofarads? Like a dork, I've been looking all over for a 500 mF disc capacitor...
I did download the pinouts for the tubes. I think I get the 12ax7 as a preamp because 1 and 2 seem to work. I don't get the 6973, but I'm not sure it's aproblem.
I did realize after drawing it up that the two wires to all tubes from below must be grounds, but why two? Floating ground references?
I'll keep trying...
Gracias,
RR

[Glasgow]

RR,

The cap is most likely 500pF in a tone stack.

Regarding the tubes, when you say you didn't get the 6973, did that mean you didn't get a copy of the data sheet or you didn't understand?

I believe the two (or three) pins you are talking about going to ground are actually the heater connections. You see, the majority of vacuum tubes rely on heat to emit electrons from the cathode. Older tubes tended to have directly heated cathodes (called the filament) whereas the 'modern' tubes tend to have indirectly heated cathodes with an element called the heater.

Most tube numbers encode the voltage for the filament or heater as the first part of the number. 12ax7 requires 12V, 6L6 takes 6.3V and a 5U4 takes 5V. The common scenario in an audio amp is to run the 12V heater tubes from a 6.3V supply by running the two sides of the split heater in parallel. On your 12ax7s, you will see on side of the 6.3V heater supply going to pin 9 and the other side going to the connected pins 4 and 5. All of these connections could read any number of voltages referenced to ground, but you should see something close to 6.3V AC across pins 9 and 4 (or 5) on your 12ax7s and pins 4 and 5 on your 6973s.

Good luck,

Brian

[rrozell]

Brian,
I told you I was a vacuum tube circuit novice. ;>(
I did order a book on Amazon to help me fill in the blanks. The heater connections are not shown on the schematics, so I guess they are implied and that confused me.
I don't understand the whole triode thing with the 6973 and how it's supposed to work in this reverb circuit. I know tubes can amplify, rectify, act as switches, diodes, etc..but I'm fuzzy on the details...so far...
RR

[rrozell]

Brian,
Progress! Thank you. If I jumper from the volume pot wiper to pin 2 of the 3rd 12ax7, then I get an amplified signal through the speakers. The signal is still weak in comparison to the working channel 1. If I jumper from the wiper of ch1 volume pot to the same pin on the 3rd 12ax7, then I get a much louder signal. This tells me I have some problems with the input circuitry of CH2 leading up to the ch2 volume pot. If I jumper from the other side of the 270k resistor in the voltage divider after the CH2 volume pot, then there's no signal there. I clipped the leads to the reverb intensity pot and there's still no signal after the 270k resistor. I rang out that resistor when I tested that whole section and replaced the .01 cap. How can a resistor be bad when it reads its rated resistance?
RR

[Glasgow]

RR,

I hope I didn't come across the wrong way; I'm a novice who has been messing around with tube amps for about 10 years. There are plenty of people who are *way* more sophisticated than me here at ampage.

Regarding the signal loss after the 270K resistor, I'm not exactly sure what is happening. This is with the reverb switch open right? Is the signal gone or just very weak here?

A good tube amp book will get you a long way down the road of understanding the basics of this technology. Which one are you getting?

Brian

[Glasgow]

RR,

What's with the 3K3 R to ground? Is that really there in the circuit?
If so, detach it from ground and see if that doesn't help the situation.

Brian

[Glasgow]

I think your hunch about the Gretch 6162 is good and that schematic could be a helpful tool in understanding this amp.

I'm looking at a copy and the channel on the top of the page looks very similar to what you drew. This one uses two sides of a 12ax7 instead of the 6973 to drive the reverb, but you can ignore that for now.

The 3K3 I suggested you lift is a 68K in the Gretch. Both seem small as they are the ground resistor in a voltage divider that attenuates the signal flowing into the next amp stage. In the Gretch, it looks like it also acts as the grid leak for the following stage, but your drawing has that 7M to ground, so you should be Ok lifting it. The 6973 should be able to leak through the 100K and 500K pot combination.

Brian

[Rhodesplyr]

I spent hours looking for a Supro amp schematic a couple of months ago, and I finally found it under the Wards Airline brand, as in Montgomery Wards. The amp had no identifying marks on it other than the name Supro.

Scroll through a few of these under Airline/Wards:

Bargain Bin Heaven Amp Schematics - Airline Garnet Gretsch Hagstrom Kalamazoo Kay Magnatone Selmer Silvertone Supro

See if one of them matches it. The Supro I worked on used a single 6V6 as a reverb driver, so it sounds like they may be related.

[rrozell]

Glascow,
Are you the same person as "Enzo"? I thought I was threading with the same person and didn't notice the name change. I apologize for any confusion or assumptions I may have made in the thread. Maybe Enzo doesn't like Cowboys fans. ;>)
The book came in: "The guitar amp handbook", by Dave Hunter. It got good reviews on Amazon.

RR

[rrozell]

No problem, man. You know way more than I do. I'm more of a computer geek or "digital man" if you will...isn't that a Rush song?
I can build a mean overclocked computer.
RR

[rrozell]

Brian,
The 3.3k does seem strange. Nice catch. The more I look at this amp and the similar schematics, the more I think someone modded this amp. The guy who gave me this amp said they had the speakers reconed for bass. 10" speakers for a bass guitar? I wonder if they didn't make other changes...
I will lift the resistor and see what happens....
RR

[Glasgow]

I am not Enzo. He is one of the top posters here and is very knowledgeable. I am originally from Texas. ;-)

Brian

[rrozell]

Well, "Howdy Pardner" ! I grew up in Houston. Born in San Antonio.
I found a web site where a few people were rating/critique-ing this amp. I'ts one of the few places on the Internet where this amp was directly referenced.
One of the guys said he replaced the same 3.3k with a 33k and the channel came alive. He also used the 6162 Gretch as a reference and the 68k there.
That doesn't explain why my channel 2 was AWOL, but it's a good place to start. I'm of the opinion I have multiple problems at this point. Divide and Conquer!
Thank you.
RR

[Glasgow]

I also grew up in Houston. :-)

Hey man, you might have several problems, but you at least came to the right forum to help you get through 'em.

Good luck,
Brian

[Enzo]

you guys seemed to be getting where you wanted to go without me, so I withdrew. I'm feeling low energy the last couple days... And as to Cowboys fans, I don;t withhold my assistance just because someone is defective.