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

[rrozell]

Greetings! I have an old Supro S6450 "Royal Reverb" tube amp that I'm trying to repair and cannot find a schematic anywhere. I think it was made in the mid 1960's, but I cannot be sure. It has two channels, reverb and tremolo, (3) 12ax7's, (3)6973's, (1)6eu7 and (1)5R4 tube. Channel 1 works, but channel 2, the reverb and tremolo does not...apparently. I think the reverb and tremolo are associated with channel 2 on this amp and therein lies the problem with that. I have swapped out all of the tubes and that does not solve the problem. The power section obviously works since I get sound out of Ch1. I have replaced several tube sockets which did not hold those tubes in place very well and several old cardboard capacitors in the channel 2 circuitry and this got rid of some static noise, but did not solve the problem. The input jacks and potentiometers all test ok as does the reverb tank (used an ohmeter). All of the tube bias voltages are there, and I get a signal out of the CH2 12ax7 preamp tube (checked it with an oscilloscope). I think the signal for CH2 is getting lost somewhere in the input or tone circuitry or possibly in the reverb or tremolo section. I get no reverb or tremolo in CH1, but I'm not sure I'm supposed to. I can hear a very faint sound of my guitar coming through the speakers when plugged into CH2 and some hum when I turn the volume all the way up, but no amplified signal.
The closest schematic I can find is a Gretsch G6162 amp which I understand was a rebranded Valco, but it has a 12ax7 instead of a 6eu7 in the power section and (4) 12ax7s in the preamp/reverb/tremolo section instead of (3) 12ax7's and a 6973. I would appreciate any suggestions or help of any kind, particularly a schematic but I've lost hope of finding one of those. I also suspect this amp is rare since I can find nothing about it anywhere on the Internet, so I'm interested in any history I can find. I'll try and attach some pics.
Best Regards,
Russ Rozell

[Enzo]

As you said, the power amp works, so we can ignore that part of any potential schematics - besides, the main differece between 12AX7 and 6EU7 is the heater and pinout. people often rewire 6EU7 sockets to take 12AX7s.

So you are left with a preamp with three 12AX7s and a reverb driver. I can;t imagine the 6973 doing anything else but driving a reverb up there.

SO you have a preamp with one channel not passing sound.

How close now is the Valco 6650TR?

In any case we have three 12AX7s and a 6973. Verify the 6973 as reverb driver, then I bet one whole 12AX7 is the trem circuit, leaving two 12AX7s as the whole circuit.

Get out a pad of paper, put the tubes on the page, and draw out a schematic from the unit.

[rrozell]

Enzo,
Thank you for your reply. The upper 6973 is indeed the driver for the reverb.
I have no signal going into pin 3 of that tube. Pin 9 of that 6973 goes out to the winding of a small transformer which is in the reverb tank circuit.
The Valco 6650 is also similar to my amp, but it has more preamp tubes.
The Gretsch G6162 has the same number or preamp tubes, but no tremolo.
The V2 12ax7 on both of those schematics for CH2 appears identical to the one for CH2 on my amp. I have signal going into pin 7 of that tube and coming out of pin 6 and up to the controls section of CH2 (Volume and Tone).
Pin 2 on the other half of that same 12ax7 has no signal and this goes over to pin 3 on the 6973 which feeds the reverb. I have swapped out that tube with several other known good ones and this doesn't change anything. The socket also seems ok, but I will change that later if/when I get the CH2problem fixed. That's as far as I've made it. I suspect I have an open resistor up in the controls circuit, but I have to cut one leg of the suspect ones to really test with a meter. I have an degree in digital electronics, but I'm not really a bench tech, nor do I know tube circuitry very well at all. I don't know if I could draw this all out with the time constraints in my life right now, but I'm willing to try if I have to. Thanks for any more help you can offer.
Regards,
Russ R.

[Enzo]

Your wording is unclear to me. Is the reverb channel silent? Or does it work but the reverb effect itself does not? I have the assumption that the reverb channel is not passing signal.

If the channel doesn;t pass signal, then it is not surprising that the reverb circuit is not getting any signal from it.

You have signal leaving the plate of one tube and running to the controls. OK, is ther signal on the controls? Is there signal on top of the volume control? Is ther signal on the wiper of the volume control? Does the wiper of that control then run back to the grid of the next tube stage?

The number oif tubes doesn't much matter, they all work about the same. Without me pulling up the files again, are not the tone and volume controls circuits about the same in each case? A circuit in one amp might use one side of a 12AX7 for the input, then to tone controls and volume then back to the other side of the same 12AX7. A different amp might do exactly the same thing but the two triodes could be in separate 12AX7s. I doesn;t matter in terms of troubleshooting.

Imagine you have a set of 26 encyclopedia volumes - one for each letter - arrangemt in a row on a bookshelf. Does it matter if it is two long shelves or three short ones? You still can run down the row/rows of books to find what you want. A circuit flows triode to triode, which glass bulb they are in doesn;t matter much.

If you have no signal on a grid pin, then don;t expect any on the plate either.

[rrozell]

Enzo,
Thanks again for helping a tube circuit impaired novice like me. You are a scholar and a gentleman. Part of my miscommunication is that I am unfamiliar with an amp that has a "reverb channel". Every 2 channel amp I have ever used has reverb and/or tremolo work for both channels as does my Carvin 2 channel I bought in the 80's. That's why I was trying to get a schematic. To answer your question: Channel 1 works, I can crank up the volume and the tone control works. There is no reverb or tremolo effect on CH1 or they do not work. Not sure which. CH2 does not work at all. No signal, volume, tone tremolo or reverb...nothing but a very distant sound of the guitar. Like something's shorted.
Your description of the series of books is helpful. I may be wrong, but I'm thinking in terms of inputs and outputs for each side of the preamp tubes.
On the 12AX7's pins 1 and 6 appear to be outputs, 2and 7 inputs, and 3 and 8 bias voltage pins.
I was up until 1 am last night trying to trace out this convoluted routing of wiring and components, but I think it's as follows:

1st 12ax7: 1 side ch1 preamp and the other side tremolo (speed?).

2nd 12ax7: 1 side CH2 preamp and the other side tremolo(intensity?)


3rd 12ax7: 1 side drives output to the power section and the other side goes up to reverb and or tone circuitry on CH2

the 6973 is definitely reverb

I can trace signal through CH2 input jack, down to the 2nd 12ax7 and back up through the volume pot and down to a series of resistors and caps that lead to the reverb intensity pot. I lose the signal somewhere in that series of resistors and caps. I was up until 1am clipping resistor leads and wires trying to find the bad part, but they all test ok with an ohmmeter...or at least the caps show open...I know that's not the best test...I think there's something right in front of my face that I'm missing and I'll feel stupid if/when I find it.
I'm diving back in tonight. Am I insane? ;>)
One thing that's different from the other channel and all the schematics of similar amps is the CH2 volume is dual ganged (two pots stacked). The volume side passes signal. The other side looks like it runs to the bias circuit of the 2nd 12AX7 (pin 8) and another resistor/cap circuit. Overdrive of some sort? I'm wondering if someone didn't modify this amp at some time in the past.
I don't want to waste any more of your time, I may have to draw this thing out to figure it out. I ordered parts to replace all the cardboard caps in this amp and the electrolytics. Many web sites say to go ahead and replace them all in radios and amps that are 40 years old, but finding 450v caps and high volt disc caps ain't as easy as running down to Radio Shack anymore. Jameco wanted like $25 bucks to 2nd day air a tiny little package of parts to me (thieves!), so they are coming by mule. I may just start replacing parts where I'm losing signal when the stuff comes in if I can't figure it out. Pathetic...
Signing off for now. Thanks again, man.
Cheers,
Russ R.

[Enzo]

Look at many Fender amps, there will be a NORMAL channel with just basic controls, and a VIBRATO channel which has the trem and reverb added. other amps do apply reverb to all channels. It's up to the designer.

Actually if you look at teh Vaclo 6650 drawing, the one channel across the top has a dual volume control. I would bet your amp circuit is similar in that regard.


Mouser probably has a better selection and pricing than Jameco, at least on this stuff. I do use Jameco now and then, but usually for specific items. And a visit to Antique Electronic Supply will find you a wide selection of parts specifically used in amps - including high voltage caps that are less common elsewhere.

2nd day air is always expensive. UPS shipping takes up to 4 days all the way across the USA, but if your order is small, consider the post office. First class mail usually takes 2 days across the country, and it is cheap.

[rrozell]

If CH2 is specifically the reverb/tremolo channel, as it seems to be, then would you consider it safe to presume that the problem is not in that circuitry if I turn the reverb and tremolo controls all the way down and the signal does not come back? The reverb pot even has an off switch built in.
The reverb tank is very old and made of cardboard. It ohms out at 260 ohms, but I don't know if that means its good just because there's continuity. Can you jumper out a reverb tank to eliminate it from the list of possible problems?

I got some of the electrolytic caps I ordered from Mouser today. I replaced a 20mF 450v cap that smooths out the DC voltage running up to the controls section and my weak signal got louder. Not much but noticeably louder. I also soldered all the parts I had disconnected back together, so maybe I just reheated a 40 year old cold solder joint and that's what helped. The whole amp looks like cold solder joints to me because they don't shine like a new one does. I'll wait for the 450 volt Mylar caps that I need to replace all of the cardboard caps with, and go from there. I'm still losing the signal at the same place. I'm wondering if it would be worthwhile to go through and heat up all the solder joints on those terminal strips.
Jimmy Page supposedly recorded much of the 1st two Led Zeppelin albums using an old Telecaster and several Supro amps distance miked. I really want to hear what this amp sounds like with my Chandler tube driver in the chain on CH2.

[Enzo]

COnceivably, if the trem circuit has failed in the maximum diminish point of its cycle, it could drop your signal level hugely, but my spider sense is not tingling that direction. And pretty much nothing the reverb does will kill the signal through the channel.

[rrozell]

Thanks Enzo.
I'm leaning in the direction of a leaky or popped cap in the ch2 input or tone circuitry...or maybe an open resistor or cold solder joint. I didn't tell you that if I put the scope on the tremolo pot wiper, I get an oscillating signal. Not sure if it's supposed to look like that or not, but if I crank on the intensity pot, then the oscillation's frequency changes. I kinda think the reverb and tremolo works.
I'm going to focus where it seems the signal disappears and see what I can find. Definitely replace all the old cardboard caps when I get all my parts as well.
You have a ton of posts in 3 years. You must have a real passion for electronics and/or music. How are things in Michigan? At least the Lions won.
I'm in DC, but not a Redskins fan. Grew up in Texas...Cowboys fan...
Cheerio,
Russ

[Enzo]

Oops, you just blew it. I have been in Michigan since 1965, but I am no Lions fan. I grew up in Washington DC, I AM a Redskins fan, and of course Dallas is the arch enemy.

[rrozell]

I don't think either team has much to brag about right now.
Not a good time for Redskins fans, but at least they have won a playoff game in the last 10 years.
;>)

[rrozell]

...are the Cowboys the arch-enemy or is Dan Snyder?...just like Jerry Jones...arrgghhh!!!...

[Enzo]

Long before either of those men...

[rrozell]

Enzo,
Well I'm officially stumped. I tried to draw this thing out as best I could for CH2 and it's attached. It's not pretty. I'm using a little metronome-like drum machine since I don't have a signal generator as a signal going into the amp while I'm working. I can follow the signal with a scope to the point where it splits off to the reverb circuitry, the 3rd 12ax7 and the 6973. I have a very weak signal going into pin 2 of the 3rd 12ax7. Strong signal at pin 3 of the 6973, but nothing coming out that I can see. Nor do I see anything on pin 1 of the 3rd 12ax7, just the high DC voltage. There's a switch on the bottom of the reverb pot that opens that circuit when the pot is all the way down, so I'm presuming that eliminates the reverb. The switch does work.
When plugged into CH1, I have signal on both wipers of the volume and tone pots. When plugged into CH2 I have signal on the volume wiper, but not the tone wiper. I tried replacing most of the caps in the CH2 tone filter circuit, but that didn't help. I have 1 VDC on the CH2 tone wiper where I expect signal.
Do you have any ideas?
RR

[Glasgow]

RR,

It feels like either one of the components between the 500K pot and pin 2 of the 3rd 12ax7 is bad -or- (doubtful because it looks like you can switch it out) the reverb return is 180 deg. out of phase and canceling the signal between the intensity pot and the 270K resistor.

Can you jumper from the wiper of the 500K pot right to pin 2 and get a good result? If so, keep one side on pin 2 and move the jumper part-by-part along that voltage divider and see if you can find a place where the signal goes south.

FWIW, you might consider checking some web pages for the pin outs on the 12ax7 and the 6973. I think that could really help you understand the circuits in your amp. Here is an example: 12ax7

Good luck,
Brian