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

[Glasgow]

Good find RR.

Enjoy your new obsession!

Brian

[rrozell]

Brian,
I will. I would never have found it without you guys helping me. Is it possible to add a 3 wire grounded power cord to these two prong amps?
RR

[Glasgow]

Originally Posted by rrozell

Is it possible to add a 3 wire grounded power cord to these two prong amps?

Not only possible, but highly recommended!

I believe the best practice would be the following:
* Remove any circuitry on the primary side of your PT including ground switches, caps, etc.

* Connect the black (hot) wire from your new cord to your fuse, then to your power switch, then to one side of your PT primary.

* Connect the white (neutral) wire directly to the other side of the PT primary.

* Connect the green (ground) wire directly to your chassis. This could be mechanically through a bolt/nut or soldered to a lug. Make sure you have a great connection! You may find a high wattage iron helpful if you try to solder to your chassis.

Brian

[rrozell]

Brian,
I take it I should not reconnect the caps or other circuitry on the hot side of the primary transformer?...other than the fuse, of course...?
RR

[Glasgow]

Fuse and power switch.

Brian

[Enzo]

Fuse and switch. NO grounding cap, no ground polarity switch.

DO not assume your reverb pan is bad. FIND OUT. Measure resistance across the jack at each end. 99.99% of the time, it will either be open (bad) or have continuity. (good) The resistance won;t be wrong, it will be open or good. Now the pan could be the wrong type for the amp, but that won;t be the pan's fault.

Turn the amp on, pull the cable out of the OUTPUT jack of the pan, turn the reverb up half way, and touch the tip of the plug with your finger. A loud hum should result. If so, the recovery circuit is working. If nothing, pull out the other plug and touch it. If that one makes the hum, then the wires were backwards, so fix that and see if it works. The one that hums goes in the jack marked OUTPUT.

[rrozell]

Enzo,
The reverb tank is hooked up correctly, but I checked it as you suggested. The coax cable goes to the output jack. There's hum on the coax if I put my finger on the jack.The other footswitch cable is a single conductor. The output jack of the tank reads like 200 ohms. I was going to pull the little reverb tank out of my Blues Junior as a test before I ordered another tank. When I turn on the reverb pot and the built in switch goes closed to engage the reverb circuitry, I lose signal level from CH2. Not a lot, but it definitely drops. I just noticed there's no bias voltage on that 6973. Pin 7 has no volts. I could swear it was like 18 VDC when I started working on this. Gotta check that cap. Man, this amp has lots of issues, but I'm almost there.
I'll cut out the giant .01 cap on the 120v and do away with the reversing switch which is not needed. The 3rd wire ground cut out lots of noise and added safety.
I'm thinking about upping the ground resistor going into the 3rd 12ax7 from 47k (originally 3.3k) to 68k as it is in the Gretsch 6162. Waddaya think?
I read lots of opinions about keeping the old tubes in there for "vintage" sake. Others say pop in all new tubes. Opinions? I'm saving all the old parts regardless.
RR

[Enzo]

In my shop we have a rule:

Fix it before you start to modify it. Make it work right before you start pplaying with changing values of parts. Once it is as good as it gets, then you can start top play around in it. If that resistor change makes the reverb more livable then so be it, just make sure everything around it is 100% first.

SOunds like the output end of your pan is OK, check the input end as well though. If your reverb is timid, turn the reverb up midway, then rock the amp to crash the reverb spri8ngs, or brush them with your fingers. Does the reverb spring noise come out strong, or still timid? if the noise is strong, then your drive circuit maybe weak or faulty. If the noise is weak, then your recovery circuit is bad in some way.

Old tubes for vintage sake? What nonsense. Y0u want to keep old worn out crappy noisy tubes in an amp for old times sake? DOn;t forget to leave the original strings on your guitar too.

It will be great to have a pile of original old crappy tubes in the bottom of the amp when you go to sell it to a collector, but nice fresh new WORKING tubes in it also demonstrates that the amp works. And the bonus is that you can use the amp between now and the time you sell it.

[Tom Phillips]

Originally Posted by rrozell

...I read lots of opinions about keeping the old tubes in there for "vintage" sake. Others say pop in all new tubes. Opinions?....
RR

My philosophy is to replace tubes only if they are bad (weak, noisy or just not correct for the circuit) Some vintage tubes will outlast several sets of "new" tubes. The derision is made on an individual tube by tube basis.

Many times I have had a customer bring in an old vintage amp they just acquired. They say, "it's not working and I already replaced all the old tubes."
A real waist if they didn't save the old tubes. If they did save them, they were often better than the new tubes and we put them back in the amp if they were good but not just because they were vintage.

Regards,
Tom

[rrozell]

It's not the tank. I tried the one from another amp and same symptoms. I can barely hear a very, very slight delay with the reverb all the way up. I don't think the 6973 is doing its job of amplifying the reverb signal. Put the newer tubes back in. You're right old tubes makes no sense. Ithink some people are more concerned with restoration than actually using the amp. I want to use it.
If I put a tube driver preamp in the chain before the amp, this thing just screams and I can't even crank it up in my condo. Running the guitar straight in is very nice as well. It has that Stevie Ray Vaughn kind of clean bluesy tone
RR

[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]

Originally Posted by 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.

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]

Originally Posted by 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

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]

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