Building a Guitar Amp Out of an Old PA Head - It Seemed Like a Good Idea...

[bob p]

We've all done it at least once -- you know, thought about rebuilding one of those old PA amplifier heads and turning it into one of the classic amplifier circuits for next to nothing in cost?

Well, the idea of building a new old TV-Front octal-based 5C5 Pro out of an old RCA PA amp seemed like a great idea. The amp head was selling for next to nothing on eBay, and it looked like it was 100% original. The idea of building an octal-based pro with original 1950s era iron, 1950s era caps and resistors, and original RCA-branded tubes for practically no cost seemed just too good an opportunity to resist.

Sure the amp was a little rough, but everything was there, and I wasn't too worried about cosmetics anyway. I was thinking, the uglier it looks, the better -- this whole idea behind this project was to build a kick-ass "sleeper." The only thing that looked really BAD was the power cord. All of the iron and tubes looked like original RCA parts, so it looked like my downside was limited:











... continued ...

[bob p]

Yeah, it seemed like a good idea at the time. I bought the amp for all of about $40 plus $20 in shipping. It was a little rough around the edges, but it looked like all of the parts were there, and I figured that if all that I got out of the deal was a decent set of tubes or a decent set of iron and a chassis, it was a low risk venture and the worst that could happen would be that I'd probably break even. So I took the plunge.

I won the auction, paid the seller via PayPal, and he shipped the amp to me via UPS Ground. Everything looked like it was going great ... at least until the amp arrived. Then things took a turn for the worse:



The box that the amp came in looked kind of small. And those evenly spaced puncture holes on the outside of the box were a bad sign. It looked the shafts on the pots had poked through the cardboard. When I lifted the package, there was a dreadful rattle, and clear sand started pouring out of the corners of the box. It didn't look like there was any padding at all between the amp and the outside world:



The cage had a hole in it that wasn't in the auction photos, and it looked like the hole had been made by something on the inside of the amp that was trying to get out!



The sides of the amp had been bashed in, and the microphone's XLR connectors were broken off of their sockets. Yes, the rocket surgeon who had shipped the amp decided to ship it with the XLR connectors plugged into the sockets. As you would expect, without any padding the sockets were broken off of their mounts:

[bob p]

And now for the worst part -- the amp had been shipped in a cardboard box with no padding, and the power tranny had broken loose from its bolt-down mounts. It had been swinging loose inside of the chassis, tethered only by the wires. During shipment it swung about enough to completely pulverize the glass envelopes on the 5V4 rectifier tube and both 6L6. All of the glass was ground into powder during transit, and not one shard of glass remained. After digging through the box, all that I could find were a couple of dead tube carcasses.








When I contacted the seller about the amp, he complained that UPS has been destroying everything that he ships!!! I told him that I had already contacted UPS, and they told me that the amplfier was not packed according to their minimal standards, and they would not honor a claim for damages.

The whole idea of building an amp out of a PA head seemed like a good idea at the time. Now I've got another junk project for the back burner.

[hasserl]

Oh shit! That's too bad. Bob, I have a similiar RCA amp, but it uses a 12AX7 and a 6AU6 in the preamp with a 6SL7 for the PI and two 6L6's for power. I ended up just gutting it and rewiring it similiar to a Matchless Clubman with the 12AX7 paralled into the tone controls than into the 6AU6 than on to the LTPI, it turned out very good and sounds awesome. The iron is very nice Stancor. THis is about my favorite amp, and I've got quite a few old ones and new builds. And I get quite a few across my bench. But this RCA is a match for any of them.



http://members.dslextreme.com/users/...lubmaster.html

[bob p]

Wow. I didn't expect that someone would have such a similar amp!

My amp is the MI-12155 / SA-32A. Its a 30W 2x6L6 amp that uses a 6SC7 for the phono stage, a 6SJ7 pentode for the gain stage, and a 6SL7GT for the PI. It uses RCA 6L6G for the power section, and the original schematic shows the plate voltages to be 470 VDC. That is pretty high for a 6L6G!!! Maybe at the time that RCA was building the amp, they had already started building to the improved 6L6GC spec and they just weren't telling anybody.

As it turns out, the amp actually worked after I reformed the caps in the chassis and popped in a pair of 6L6GC that I borrowed from another amp. (I've built a custom SS rectifier that's in series with a current limiting resistor that plugs into the rectifier socket. Its my "automatic" cap reformer).

The amp still has the original Stancor choke and OT. It looks like the PT was blown, and some cobbler replaced the 575-0-575 Stancor and the choke input filter, and put a 400-0-400 200mA Thordarson in its place with a cap input filter. The Thordarson is a nice tranny, but the voltage is rather low in relation to what's need to balance the PI -- even after switching over to a cap input filter the PI voltages were low enough that the PI isn't balanced and the amp sounds bad. I presume that the amp never sounded quite right after the repair (no surprises there), and that's why it was shelved. If the repair guy had only rebalanced the PI, the amp probably would have sounded fine.

Right now the amp also has a bad cathode bypass cap in the output section, which couples some noise from the heaters into the cathode. I should say lots of noise, actually. My 1 KHz test signal looks like noise riding on top of the big 60 Hz waveform!

I'm really quite surprised that it works as well as it does, having all original tubes, caps and resistors. I haven't decided exactly what I'll do with this amp yet. I'm still thinking about building something along the lines of a Tweed Pro, or maybe 'wrecking it. Its such a rats nest of wires with cracked insulation that I think I'm just going to gut all of the wiring and start over with a clean slate. What I really need is a killer 6-knob circuit. I'm open to suggestions. Right now the faceplate says, "Mic1, Mic2, Phono, Bass, Treble, On/Off."

Its too bad that some people are such Bozos about shipping. The seller took as much care in packaging the amp as he would have taken in packing a sponge for shipment across the country. Some folks just don't get it. I have to admit, it really made me sad to see a couple of old RCA 6L6G power tubes die such a meaningless death.

[Jag]

Wow, sorry you had such hassle with it. At least it sounds like you'll come out okay.

That reminds me, I bought an old Bogen that looks alot like your RCA before it was shipped. I've also got an old Bell. I got them both about three years ago on the Bay for less than $40 each. I should get around to doing something with them.

[Steve Conner]

Arg, that sucks Bob. It's a shame to see a nice old amp get splattered so messily by poor packaging But it's good to see that you managed to salvage it. If the wires had got ripped out of the PT, that would have been really bad...

If it helps, I have a pair of those same RCA 6L6s, and they sound great. :P I actually pulled them from some giant hulk of industrial machinery that was getting scrapped, I guess they were part of some servo circuit or something.

[hasserl]

Mine came originally with the metal 6L6's, and rated at 15 watts. It has the original potted power transformer and choke input, I did not change that to cap input, though I did flirt with it but returned it to choke input. The voltage is around 370 - 380 IIRC. A little low for the 6L6GC's, but it let's it run very much toward class A. It's putting out around 22 watts clean, which gives plenty of volume for places I play at.

They really are a rat's nest inside, but PTP wiring tends to look that way no matter what. Still, it can be cleaned up. I went the route of gutting it and doing my own thing rather than just rewire the input for high impedance and leaving the rest alone. The original design may have sounded okay, but I'm really happy with what I ended up with.

[cgiff]

Rough Handling

Bob, Sucks that the shipper was so dumb to package the amp the way he did. However for a transformer to break away from the chassis, this amp was dropped likely by the shippers. I worked for a company that occasionaly ( too often) used a 3 letter shipper for commercial products that were properly packaged and even then we recieved materials returned with shipping damage. Half blame to the individual you purchased the amp from the other half the shipping company. Other than the unfortunate damage that was a very good buy.
cgiff

[bob p]

If it helps, I have a pair of those same RCA 6L6s, and they sound great. I actually pulled them from some giant hulk of industrial machinery that was getting scrapped, I guess they were part of some servo circuit or something.

Steve, it would really help if you sent them my way.

[bob p]

They really are a rat's nest inside, but PTP wiring tends to look that way no matter what. Still, it can be cleaned up. I went the route of gutting it and doing my own thing rather than just rewire the input for high impedance and leaving the rest alone. The original design may have sounded okay, but I'm really happy with what I ended up with.

I took the time to trace the circuit just to see what I was dealing with. It was a MAJOR PITA -- moreso than tracing most PTP stuff, as some of the RCA wiring is just abominable. I have to hand it to RCA quality control though -- those red QC dots on every original solder junction made it easy to tell where the amp was original and where it had been modified.

When I tinkered with it, I just plugged into the 1/4 ich phono jack (which skips the first gain stage). The signal gain was pretty anemic. You definitely need the added gain of the MIC stages to bring the signal level up. I didn't bother with that, as the MIC inputs were already trashed and I didn't feel like rewiring them.

Because so many of the amp's original wires have cracked insulation, the amp is going to require a complete rewire. If I have to go that far, I don't think its worthwhile to preserve the original circuit. Most people seem to think that the old PA amp circuits don't sound that good for guitar even when they're working. So I guess I'll just rip everything out and start over.

One thing that I would like to do is to implement a circuit that can take advantage of the original faceplate markings, or at least have functions that resemble the faceplate markings. The faceplate is actually in excellent condition, and just looks so cool.

I'm thinking that MIC1 and MIC2 could be used for two volume controls, the PHONO input could be presence (it begins with "P"), then I'd have bass and treble controls, and an On/Standby/Off switch. Maybe I could do something like a 6L6 Tweed Super with the amp. The voltages might be right, as the PT is a 400-0-400 200mA Thordarson, and I also have a spare 375-0-375 150mA Thordarson lying around.

OTOH, I guess I could just abandon the faceplate and build a Komet into the chassis. Nobody would expect an amp that looks like this to sound like a Trainwreck. Another obvious choice would be the Tweed Bassman circuit.

BTW, @hasserl, thanks for your photos. I had been wondering how white chicken-head knobs would look with the amp. One other thing that I'm looking for is info about the mounting screws that hold the cage onto the sidewalls. My amp doens't have the screws. What kind of mounting system does your amp use?

[hasserl]

I don't recall what was there originally. I think I used self tapping sheet metal screws to attach the cage to the sides.

[stokes]

It sucks that someone would ship so carelessly,but on the bright side those trannies are worth the cost.I've built a couple of PA conversions over the years and have always found it easier to just gut and start from scratch around the iron.I had a Chicago/Webster that was in good enough shape to use as is,and it really didnt sound good for guitar,ended up using just the iron in a 5E3 type.As for the mounting screws for the cage,any units I've seen from that era used the self tapper screws.Good luck with the project,Bob those trannies will do you well,love those Stancor and Thoradsons.

[Rob Mercure]

Beware the NFB gremlins

Hey Bob,

Just a "quick" note: While PA amps make great guitar amps conversions - I've done a few and you and I have privately discussed some of these - I've found more "issues" in converting the all octal, late 1940s, "pre 5881" amps than earlier or later designs.

About 10 years ago I purchased about 10 years worth of Radio and Television News magazines (which had subsumed several other pre WWII engineering electronics mags) roughly from 1947-1956. In reading and rereading these magazines and studying the circuitry I've found this to be the "golden era" of negative feedback! This is right at the point in time when the "Williamson" amp design(s) started becoming the norm but there were still many other designs being floated around and some of these had multiple and nested NFB loops. Of course the philosophy was to create a better audio amplifier but, as far as PA amps go, part of the rationale was for the manufacturer to be able to use a cheaper output transformer. So many of these amps had various feedback windings on the OT often with all or part of the output tube cathode current flowing through a winding (used to great success - and really a philosophically distinct concept - by MacIntosh) or all/part of the PI cathode current flowing or all/part of one of the preamp stages flowing through a winding, etc., etc. I remember one amp in particular that first used "cross coupled" input stages then global NFB from a small winding to one of the preamp cathodes and then a small capacitor that coupled the plate of a post PI driver tube to the plate of the driven output tube!

How well these circuits worked to our ears is hard to say as most have, as you've experienced, crumbling insulation, dried up electros, and runny/gooey/flowed waxed paper capacitors as well as other age related issues. But when these amps were gutted and a new circuit installed - most of which are Williamson derivatives - the amps really sounded terrible as the response of the OT/amp was heavily dependent upon use of the OT feedback winding(s)! While I did make one or two work, in most cases I replaced the OT with something suitable from the junk box (helps to have a healthy, if heavy, junk box). If my memory serves - it's been a while - one of the "symptoms" was a prounounced "hump" in the response around 6-800 Hz.

Roughly speaking the introduction of the 5881 by TS in 1950 - and its adoption by the amplifier manufacturers over the next few years - seemed to coincide with the "standardization" of the Williamson derived topology and spell the end of the NFB "mania." (or, at least, to a large extent). So any PA amplifier using a 6L6/A, 807, 6AR6, or triode outputs, etc., I treat with suspicion.

Now this may have absolutely no bearing on your particular amp but as you diagram out the schemo you might want to take a careful look at the OT secondaries - it's easy to miss a feedback winding and mistake it for a 10/25/35/50/70/100 volt "constant voltage" line (were their others? these are the ones I've noticed) and wind up with an amp that "mysteriously" sounds like iguana poop when you've adequately built a proven circuit into the chassis (ask me how I know <grin>).

Back when I had more "raw monkey energy" I always intended to characterize at least one of these OTS and see if I could use the "hump" energy to deflect a meteor or perhaps something a bit more modest. But in the end when I'd spent a few nights soldering non- stop I was simply interested coming up with a reasonably good sounding amplifier and I often did a couple or three OT swaps to find my "match."

For the record one of these - a handy reach right now - was a Bogen "Challenger Amplifier" Model CH30 (which had the designation CG30 carefully silk screened over changing the model number) made in 1947, or so - I wound up sticking in an old Baldwin organ OT - never could get a decent sound out of the original OT.

Rob

[bob p]

thanks for your help Rob. As it turns out, i was able to get a real RCA service manual for this item, so I don't have to rely on my circuit tracings.

Murphy's Law being what it is, I couldn't get hold of the schematic, so I had to trace the circuit myself. It was a real bear, as the RCA PTP wiring shoves way too many wires in one place. With the wires being as old as they were it was impossible to trace them by the faded colors. And if you had to manipulate them they crumbled. Tracing the circuit was in some ways a destructive process. Of course, a couple of days after I had finally finished somebody kindly offered me a schematic. Aargh. I've attached it below.

You're right, the OT on the PA amp is pretty inexpensive looking compared to the Williamson style OT. But then that's a beautiful thing if you're wanting to build something like an old Tweed Pro instead of a HiFi amp. Its certainly not a cathode feedback design, and its definitely not a Williamson design -- its way too small, and it predates the Williamsons. In general, I'd be shy about trying to Williamsonize an amp that didn't have the right OT. One thing that Williamson did was to introduce his own design for the OT which was revolutionary at the time. Without an OT that's wound with phase compensation in mind, you'd probably end up with an oscillation monster with a traditional Williamson deisgn. Essentially, with iron of that era, if it wasn't designed for a Williamson style amp, its probably not going to fare too well in that sort of application.

Interesting that you mention the old Bogen Challenger. I always thought that the maroon and silver amp does had a good look to it. I'll keep my eye on the ouput transformer just in case its something that causes me trouble. Thanks for the tip.