Old 2xEL34 PA Amp - what should I build?

[SixStringing]

Hello all.

I've been looking at rounding out my small collection (ie. two) of amps and stumbled upon an old PA mixer/amp (appears to have been from a church). It's a Northern Electric PA35A and looks like it would make a decent foundation for a really good guitar amp.

It's set up with 5x12AX7s, 2x6CA7s/EL34s, a 5AR4/GZ34 and 2x6AU6s(?). Iron seems beefy and it weighs a ton! I hooked up a speaker and tested with a microphone and it actually works w/ very little hum.

Power transformer output measures 725V AC (end-to-end, no load), and the output of the rectifier tube is at 375V DC. There are 2x100ohm resistors (current limiters?) on each leg of the PT output going into the rectifier tube. Other than the transformers, tubes/sockets, I pretty much intend to start from scratch. There are a lot of things I won't need - input transformers, the 7-pin 6AU6 sockets (maybe?).

My preferred tone seems to live in the neighborhood of just-beyond-the-edge-of-breakup. Sometimes a bit under, sometimes a bit over. I don't feel the need for huge clean headroom, but some, or at least the option for some, would be nice. Master volume would be good, as would an effects loop. I seem to prefer a simple 'interactive' style tone stack. I also appreciate a high-cut at the power-amp stage.

So now the question is what to build?

My initial thoughts are the 93 Matchless Clubman 35 that's floating around, and maybe even incorporating one of the 6AU6s in place of the 93's 6SH7.

I'm curious to hear what others might put together if confronted with this beast.

Thanks,

/Dave

[nuke]

Looks like a great chassis to mod up!

Decent looking iron and lots of space to play in and it looks pretty cool too.

[trobbins]

The amp looks to be very well made, and may well have few if any faults. Have you been able to locate a schematic for it?

I'd recommend some caution and time delay before any knee jerk stripping of all wiring. You may well find that some of the circuitry is very similar to some classic guitar amp circuitry - that may take some time, but imho would be well worth the effort to carefully assess and compare, as it may give a low risk way to make simple modifications and then listen to the results - a sort of learning curve.

It may also cover any risk that the output transformer is not 'typical', as it could have quirky speaker side impedance windings, and the primary side could be uncommon (such as cathode coupled winding). You may need to make some measurements to confirm the impedance ratios - best to know what you have for starters.

It doesn't really matter, but I was intrigued by the grounded wire looping one of the coupling caps to the output stage - that would indicate some effort to balance the high frequency response that could be related to feedback stability.

Ciao, Tim

[nuke]

Looks like it might be ultra-linear. But I can’t tell where the g2 leads are going. Interesting resistor from the g2 of the 6CA7 to the pentode driver.

Would be an interesting schematic to trace out.

The input is balanced transformer coupled, so you might be able to bypass an input transformer and plug your guitar in there just to see what does.

[maxkracht]

Very cool looking amp. I'm also interested in that coil of wire. Didn't know if it was meant as a cap, shielding, or both.

[TUBEDUDE]

Those input transformers sell for ~$1k a pair in the audiophile world.

[nuke]

Those input transformers sell for ~$1k a pair in the audiophile world.

Yup, they sure do. I hadn't looked those up till you mentioned it, then the light went on, "Northern Electric" and "Western Electric" and those are very good quality plug-in line transformers.

Looks like the OP scored the trifecta. Heck, that would pay for the amp, the mods and maybe a new gee-tar to plug into it.

[SixStringing]

Those input transformers sell for ~$1k a pair in the audiophile world.

Yup, they sure do. I hadn't looked those up till you mentioned it, then the light went on, "Northern Electric" and "Western Electric" and those are very good quality plug-in line transformers.

Looks like the OP scored the trifecta. Heck, that would pay for the amp, the mods and maybe a new gee-tar to plug into it.

Using an image I found online, I googled those numbers printed on those cans prior to buying the amp as I had no idea what they were - lucky indeed!!

I've been scouring for a schematic and have only found drawings for their more common items (radios, telephone equipment). There's a Canadian Science and Technology Museum in Ottawa with an extensive (and searchable) library/archive of documents - it looks like they may have a binder with the service manual. I've emailed them about getting a scan of the schematic, so fingers crossed.

And yes, I'll heed the advice of caution and delay. I'm a newbie to the inner workings of this old-school tech (my dad was a TV repairman so some of this is in my blood), and I'm definitely hoping to use this thing as an opportunity to learn. Lots of material to read up on, and I'm sure I'll have a lot of questions.

I'll keep y'all posted.

[nuke]

Take your time and trace out what you got there. It's a very well built amplifier, top notch construction for its era, way better than most guitar amps. That's all commercial grade work. Laced wire bundles and well thought out point-to-point construction.

Just from what I can see, it looks like a very standard fixed-bias (the diode for the negative supply is quite apparent) and fixed G2 screen supply, but I can't see where the screen grids are wired.

It is very unusual in using the 6Au6A pentodes to drive the output tubes. That's a very different approach. The 12AX7's are definitely pre-amps and doing the phase splitter. I think you have a phonograph input and 2 or 3 microphone ins.

Trace out the output section where the 6CA7 are wired to the transformers and the 6AU6's that drive them. I'm real curious. If you get stuck, post a photo where you think the wires are going and I'm sure someone will help figure it out.

[TUBEDUDE]

I haven't seen a schematic but it sounds like one of two things is happening. Either the 6AU6 is regulating the screen voltage of the output tube, or it the screen is being driven in enhanced mode. Audiophiles do that for better linearity

[SixStringing]

Yes, there are 3 mic inputs (each with a step-up-transformer), and a Phono channel - 2 RCA jacks, one labeled MAG (for moving coil?) and on XTAL (piezo?).

I tried the guitar through the phono jack - the amp is definitely not voiced for guitar (yet) - it's very low-mid heavy. It does start to sound pretty good with the bass almost at one and the treble cranked. And it's loud - I don't really have enough speaker horsepower (25M greenback) so I couldn't crank it too much. It's also very punchy on the bass. It's also extremely clean. Maybe with more speaker load I could get it to growl - we'll see.

There's barely any hum or hiss, so hopefully that's a good sign.

I've tried to trace out the the phase-inverter and output stages - here's a rough schematic - components at least - no values. When I'm feeling a bit more adventurous I'll try and take voltage readings.

/Dave

[trobbins]

Well done for having a go at a schematic.

R8 and R12 are likely to have the same value, to make that phase splitter stage a cathodyne/concertina/split-load configuration.

The output stage does appear to be a bit uncommon. There should be a main high voltage supply rail (B+) feeding into that circuitry - likely as a centre-tap (CT) to one or more of your windings - an ohm meter should be able to identify windings with a common CT connection. If there are windings connected to the cathode end of the EL34's then they may also have a CT tapping that goes to 0V, or through a resistor to 0V.

You may be able to identify the output transformer connections to speaker terminals, and if that circuitry has a connection to 0V, and any connection back to the primary side such as to R14, and if there are any labels for speakers.

Ciao, Tim

[SixStringing]

Here's a photo of the back - pretty standard speaker impedances.

[dragonbat13]

Might be a decent amp for vocals, lol.

What about just having three inputs reworked for guitar with minimal modifications for some pedal madness.

I mean, it's a nice looking piece the way it is. And I'm usually on the "butcher" side of the fence for stuff like this.

Another idea would be that harmonic vibrato thats in one of fenders amps (I forget which one). I believe it requires at least two preamp tubes on its own.

My wonderland overdrive has five preamp tubes for the internal dumbelator.

[nuke]

Yeah, the phono inputs will have RIAA equalization and won't sound very good with a guitar plugged straight into them.

If you pull one of the input transformers, and trace the pin with a 1M resistor and the coax to the grid of a 12AX7, that would be the spot to tack on a 1/4" jack for guitar input.

It's an unusual output section, I'm still studying the image a little bit. I can see on the 6CA7/EL-34 that pin 8 and 1 are tied together, (suppressor grid and cathode). I can't quite see what they are connected to though.

If they are indeed then going to the output transformer, then they will go to a center-tapped winding, with that center tap going to ground. That's used for cathode coupling for negative feedback. AR does something similar in their hi-fi tube amps. But I see only little wires to the G3/K pins, which might be transformer, or to ground.

The 6CA7 screen grid (G2) do appear to go to the output transformer, and that would indicate ultra-linear output stage.

It's interesting that the plates of the 6AU6A are fed from the 6CA7 screen grids, on the U.L. taps of the output transformer. I would think that functions as a power limiter. As the output section plate current goes up, the available voltage to the 6AU6 is reduced, in turn, reducing the output and limiting the grid drive of the 6CA7.

Clever.

So yeah, this one was designed by people who actually knew what they were doing and also built it very nicely too.

Great score, I think I found the reverb listing. Quite the deal on that!