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

[rrozell]

...I checked the reverb further. I have a good strong signal going into the tank (on a scope) coming out the signal looks strange, but it probably should...where the tank signal comes back into thye amp there was a 27k resistor to ground on the back side of the .02 coupling capacitor that was not on the schematic for the G6162. I clipped the resistor and that seems to have improved things. I have a better reverb now. The duration is very short and subtle...actually how I like it, but not like the springy reverb of today's amps. Interestingly enough the tank out of my little Fender does not really work. I guess the circuitry has to match the tank? I'm happy with the amp the way it is now, although it would be nice to have more dramatic reverb.
I will replace the remaining tube sockets and old capacitors and perhaps replace the 47k resistor with a 68k to see what that does. Any way to crank up the preamp tubes? The book I bought kinda sucks...
RR

[rrozell]

Tom,
That's good point. I checked the old 12ax7s out of this amp in my tube driver and they worked just as well as the newer tubes. Good old American RCA tubes are reputedly of better quality that the newer ones made in Russia or China or wherever. Or so I've read...
RR

[pbjbryan]

Just FYI, I have a '68 Supro Royal Reverb which is a Valco model 6650TR, and it also has (had) a weak reverb level, so I think this may be characteristic of these amps. The 6650R uses a 12AX7 for the reverb driver, which is a poor choice, and passively sums the reverb recovery with the output of the passive tone circuit being fed from the input of the reverb driver. I've replaced the 12AX7 circuit with a 12AT7 circuit, and added a recovery amp, and it's much better. The return's high-pass filter was also set too high, so dropping it provided better level and more meat in the reverb signal.

Btw, these amps have a lot of attenuation in their gain staging, which you can reduce for more gain. Another mod is to replace the divider in front of the last 12AX7 (the splitter) with a master volume. I've also added an input join/cascade switch that provides three gain sets including boost at the flick of a switch, and a splitter balance trimmer to adjust the relative drive to the output tubes. This is really useful for adjusting the overdrive harmonics and output power. You can also use it to switch/blend the amp from class-AB to class-A operation.

These amps are pretty noisy, mostly because the grounding is shabby and the heaters use unfiltered AC, but this can be fixed pretty easily. I eliminated most of the noise by by switching the heaters to a filtered DC supply (added a bridge and a few caps), and moved several grounds around. The tremolo level pot was grounded near the power supply (just because it was convenient), so moving it to the preamp area got rid of a lot of hum. The grounds for the main supply caps are tied to the chassis, but it's better to tie them directly to the rectifier's return at the transformer center tap.

These amps have a really unique sound, I think mostly because of the 6973 output tubes. The metallic clang tones of Houses of the Holy and Physical Gaffiti (Dancing Days, The Wanton Song) are really easy to get with this amp. Page has heavilly modded his Supros, and I now know why. They're worth the effort.

[rrozell]

pbjbryan,
I really appreciate the information you just posted, although some of it goes over my head. I realized that I need to sit down and draw this amp out on a professional looking schematic so it can be better analyzed and others out there might benefit. It would have been much easier to repair channel 2 if I had had one. I need to draw it out and really think about what I want any mods to accomplish. I also don't want to do anything to diminish the value of it as a vintage amp. You are absolutely right. There is much potential there. I have caught the bug of learning about vintage tube amps and their circuitry. I think I want to build my own amp or tube pre-amp just to learn more.
I learned to play guitar on a tiny old (1970's) Fender Champ that my Dad bought me.
I loved the sound of that thing cranked all the way up. I have a Fender Blues Jr. and it's ok, but it's not the same. It sounds sort of 'sterile' or something. Maybe I'll modify that one as well after I have learned more.
I never knew Page modified his amps. He was always hush, hush about his equipment...or at least his amps. He would never say how he got that sound on "Trampled Under Foot". He said it was "sort of a reverse wah" or something like that...
Cheers,
RR

[pbjbryan]

You might want to checkout the older Valco 510-149 (http://www.schematicheaven.com/bargainbin/valco_510-149.pdf) schem to see how close it matches your reverb section. The newer Valco S6650TR (http://www.schematicheaven.com/bargainbin/valco_6650tr.pdf) is very similar to several other Supro reverb/trem amps, and might match many of the other circuits in your amp. The circuits using the 6EU7s in the 510-149 schem are probably using the 12AX7 versions from the S6650TR in your S6450 (pins are different!).

The Valco amps were kind of pieced together from common building blocks, so different amps often share many of the same circuit sections, usually with the same or very similar part values. The tremolo, EQ, and certainly the 12AX7/dual-6973 output section are like this.

The tremolo is interesting because it modulates the bias of one of the preamp tubes (cool effect because it adds lots of color), so if this isn't working properly, you get no sound. I think the 5K pot on the 12AX7 cathode in your diagram might be the trem level adjust because that's where it would connect.

One other tip, before you make any changes/fixes, try swapping tubes from sections that you know work into the sections that don't work. Sometimes that's all it takes. I've "fixed" blown amps on stage by swapping an unused input or tremolo tube into a blown driver tube socket. (Obviously they need to be the same type of tube, or flames will shoot out of the ends of your nose hairs!)

[rrozell]

PBJ,
Sorry Ive been slow to repond. Offline for a couple of days. I have previously scoured the Internet and I have the schematics you listed already printed out. The closest one to my amp is the Gretsch 6162. Much of the circuitry and component values are virtually identical except for the 6973 in the preamp section hooked up to the reverb circuitry instead of a 12ax7. I have not opened up the power output section to compare since that part of my amp has always been working. As you've said, there are clearly sections of circuitry copied from amp model to amp model. Supro, Valco, Gretsch, etc...I ordered an electronics drafting template today so I can draw this thing out properly instead of rendering in my 'chicken scratch'.
The 5k pot in the upper left corner of my drawing is incorrect. That's what I changed to get CH2 working. The wiper's actually now correctly wired into the feed for the tone filter and the other side goes through a series of coupling caps to the final stage 12ax7 which feeds the power output section. It's really the second half of a double ganged 500k pot that is CH2 volume. If you look at the upper channel on the G6162 drawing, you will see two 500k pots and then the last 500k for tone. The drawing does not depict that the 1st two 500k pots are actually one control, but they are in my amp. You are correct in that it looks like the reverb is an integral part of the CH2 chain. That's why the reverb being so weak is concerning, but from what you've said that might be common and not necessarily mean something else is malfunctioning. The dual ganged volume pot looked strange to me since there wasn't one on CH1. I thought someone modified this amp, but others have explained that was a common practice of the day. Two very different channels in 1 amp. Someone did make some changes that killed CH2 (I swear it was like that. I didn't do it!). I thought changing the bias with a pot was some kind of overdrive or something, but it was just soldered to the wrong landing.
Cheers RR

[rrozell]

I also want to say thanks again for all of the help/Input/feedback. You guys are awesome! ;>)
I bought a couple of the Mil spec 5751 tubes that are direct replacements for 12ax7's.
They are supposed to saturate more easily and be of more stringent specs. Anyone have any experience and or opinions?
RR

[pbjbryan]

Cool, I'm glad you've got it sorted. The dual pots are in my amp's ch-2 also. One intersting aspect of this is the first pot (on mine at least) is a linear taper, while the second is an audio taper.

It sounds like someone's already made some mods to that amp. One of the more "common" ones that may have been made on yours already was to change the divider that feeds the dry signal around the reverb circuit (from the wiper of ch-2's first volume pot). This boosts the channel's gain, but sacrifices reverb level. One solution to this is to redesign the reverb circuit to include a recovery amp.

Another way to boost the gain without messing with the front-end gain staging is to replace the 270K/100K divider feeding V5 (on the 6162 schem) with a master volume pot. This will give you another 12dB of gain at the top of the pot.

Another mod is to replace the 270K/12K divider between the stages of V5 with a pot to balance the output section (or unbalance it for class-A operation with a lot of extra gain - vey cool sound).

[Prairie Dawg]

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.

Both you guys are in the eastern division doghouse then. I've been a Giants fan since the late sixties-grew up in Jersey-and that was when winning 3 games in a season was a big deal, even though the season was shorter.

[pbjbryan]

I also want to say thanks again for all of the help/Input/feedback. You guys are awesome! ;>)
I bought a couple of the Mil spec 5751 tubes that are direct replacements for 12ax7's.
They are supposed to saturate more easily and be of more stringent specs. Anyone have any experience and or opinions?
RR

I tried a couple of NOS GE & Sylvania JAN 5751s I got from an old hi-fi buff in a few different positions, and found them harsher than the original RCAs. But that's a very small sample set, and your mileage *will* vary. I'd though they'd be cleaner, but they were just the opposite.

Give them a try and let us know how they sound. The 5751 is good for preamp stages because it's more durable (less microphonics), and since it has less gain, doesn't overdrive later stages as much.

Btw, The Supro builds its tone through several stages, and using less edgy tubes makes the tone richer instead of harsher. Each stage gets a change to add more predominantly lower-order harmonics, and the interstage filtering cleans up the upper nasty harmonics, which gives the final tone some real depth. I find less edgy tubes provide better touch, and give that "one knob" response that makes playing a tube amp so great. The edginess of different tubes, even of the same type, varies greatly, and is unrelated to its overall gain or microphonic tendencies. A crappy preamp tube might be perfect in a later stage.

Your ears are the best judge.

[rrozell]

Thanks for the tip. I did find one with some similar circuitry. The Wards GVC-9025a is the only other amp I have found using a 6973 for the reverb driver. The rest of the input circuitry and power section are disimilar, however. I also went through Selmer and didn't find anything. Same thing with Sears Silvertone.
RR

[pbjbryan]

The GVC-9052 (http://www.el34world.com/charts/Schematics/Wards_Airline_GVC-9052.pdf) also has a 6973 reverb driver, with 12AX7 preamp tubes. The output is 6L6, and it has a diode ps bridge. Otherwise, it shares a lot of the circuits from the Gretch 6162 and Valco 6650TR. One interesting thing about the 9052 is that the typical single eq control is split into separate bass & treble controls (same circuit otherwise).

[rrozell]

6450 repair/refurb status

Well, I've come a long way since I started this thread. Channel 2 now works even though the reverb is weak. I have played the amp several times at length without any problems. Most of the noise and static went away when I grounded the amp and replaced several wallowed out tube sockets. I've got most of the parts I need to upgrade old or worn components. I ordered some stuff from a place called "Radio Daze" and they only sent me a couple of pieces and said the rest was out of stock and they cancelled the rest of the order. Not cool. If they are no longer a viable distributor, then they need to pull that stuff off their web site. Perhaps a sign of the times. I lost a couple of weeks waiting for that order. Mouser and Jameco sent me the stuff I ordered, but it took several weeks and they charge too much for shipping.
$20 bucks for stuff that weighs an ounce or so? They also do not stock parts geared for vintage amps, just commercial parts. I've had the best luck with Tube Depot. They have most of the stuff I need/want, it comes in a couple of days even when I do not pay for overnight and they don't overcharge for shippng.
I've now replaced most of the old cardboard capacitors and some old plastic caps on the bias circuits for several tubes. Pic attached. No real changes in tone or functionality, so presumably they were still working after 40+ years. I opened up the power section for the first time and there's a big cap in there leaking ooze. I'm waiting on the large can type caps to come in from Tube depot so I can replace all the high voltage power supply smoothing caps. The scope showed lots of ripple when I was working on CH2. I still have to draw this thing out when I get more time.
Best,
Russ R.

[Glasgow]

Russ,
It has been a while since I've dealt with them, but Antique Electronics Supply was always a great place to 'shop' for me.

Brian

[Habile]

schematic for Royal supro

Greetings! I have an old Supro S6450 "Royal Reverb" tube amp that I'm trying to repair and cannot find a schematic anywhere. Russ Rozell

I've got a print ! I traced it all by myself and it's on Autocad file.