vintage PA amp conversion

[joshester]

This looks good but seems like this is the AU 35, I've got the 33. You got anything for the 33?

[g1]

Here is a zip of the djvu file. You will need a free djvu viewer. I use Document Express DJVU plug in.

[olddawg]

I always say this, but it is coming from my experience in converting a few old amps. My suggestion is to gut it, replace all of the tube sockets first (nothing worse than 60 year old brittle and oxidized, intermittent tube sockets), then use a tag board or turret board. It is only a few dollars more expense to do it right and can save you a lot of time and headaches. There is usually room for modern filter caps on the board. You have plenty of room under the chassis. Consider using a diode rectifier. If it has a pilot lamp off of the filament supply, get rid of it to have more heater current available. Just my 2 cents.

[bluto]

Depends on the amp, if I'd gone with a diode rectifier on my most recent (last night ;-) PA conversion the B+ would have been waaay too high. I stuck with point-to-point wiring, added one terminal strip and reused the old time octal phono socket for another...still need to wire up one more tube for ch. 2 but think I can do it as is. I actually enjoyed the challenge of making point-to-point work...but won't need to an my AU-32, the AU series were built with tag boards by Stromberg. FWIW this is what the underside of my Lafayette looks like currently...

[olddawg]

Depends on the amp, if I'd gone with a diode rectifier on my most recent (last night ;-) PA conversion the B+ would have been waaay too high. I stuck with point-to-point wiring, added one terminal strip and reused the old time octal phono socket for another...still need to wire up one more tube for ch. 2 but think I can do it as is. I actually enjoyed the challenge of making point-to-point work...but won't need to an my AU-32, the AU series were built with tag boards by Stromberg. FWIW this is what the underside of my Lafayette looks like currently...

[ATTACH=CONFIG]28630[/ATTACH]

To each his own. I just prefer nice new clean ceramic sockets and if the tubes are weak I tend to change to 12AX7s and use a more modern/conventional design. But that's me. It usually isn't a problem to lower the B+ with a zener diode or 2. And... It does look like your chassis is in clean good shape. I've seen people go insane with old Bakelite socket intermitancies.

[silverfox]

How much experience do you have?

srinva, I started learning tube amp theory, design and construction about 2 years ago. My first project was very similar to what you have there. A McGohan 35 watt PA to guitar amp conversion. I started by using the phono input to see how it sounded but could never get complete satisfaction from the sound. I then began to tinker with it and in the end gutted it and used some parts to build other amps.

Depending on your level of experience constructing circuits, particularly point to point wiring from schematic and soldering close in connections that may require substantial heat, you may want to start by replacing some of the critical capacitors such as the power supply caps. Consider the chassis as a lab. On the other hand you may have substantial soldering experience as well as the prerequisite saftey knowledge required to work on tube amps. Please let us know if you have worked on high voltage tube circuits in the past.

In any event, the can caps may be good enough initially to experiment with but for sure I'd replace the wax covered tubular caps by cutting the leads at both ends one cap at a time and patching in the substitute part. If you aren't going to recap the whole amp, check for a fuse and if there isn't one try to find a fusing system such as a power strip to protect the circuit.

If you have limited experience, after replacing the caps, you might try modifying the tone stack to gain more. There are plenty of people here that could suggest tone stack circuits for this amp.

Good luck,

Silverfox.

[Jazz P Bass]

Here is the AU-33 schematic.

[bluto]

To each his own. I just prefer nice new clean ceramic sockets and if the tubes are weak I tend to change to 12AX7s and use a more modern/conventional design. But that's me. It usually isn't a problem to lower the B+ with a zener diode or 2. And... It does look like your chassis is in clean good shape. I've seen people go insane with old Bakelite socket intermitancies.

I should have been more specific, this amp uses a 5Y3 (60 volt drop) so converting to a SS rectifier would have bumped the B+ up a LOT...the AU-32 I'm doing next does too so I'm planning to stick with the 5Y3 on that one too.

[highfigh]

Late to the party, but I recently acquired an AU 33, in good condition. The store owner said his service tech "did a bunch of work on it", but all I saw was fresh solder on the multi-segment caps and the piece of zip cord he soldered onto the 8 Ohm tap on one of the speaker output jacks- didn't even bother to solder a 1/4" jack onto it.

Never having heard one of these, I didn't do much before trying it, although the octal output sockets aren't terribly user-friendly, so they didn't have much chance of staying. I did decide that it should be as usable as possible, so I installed 1/4" jacks for Mic 2 and 3, as well as a 1/4" jack for the output. Since I can install two jacks on a cabinet or make a good Y-cord to allow connecting two cabinets, I went with one jack. I do, however, want to be able to play through just abut any cabinet so, rather than use two jacks (or more), I installed a Marshall-style impedance selector in the other octal location. I can now connect it to a 4, 8 or 16 Ohm cabinet and not worry about whether the impedance is correct. I replaced the rest of the electrolytics and the power cord while I was at it and am using it with an old Ampeg VT-212 cabinet, loaded with 1960s Jensen C12R speakers that I bought when I worked at a music store. They were in rough shape- several tears in each cone, but with the volume control at 30/100, it had a real 1950s sound to it, mainly because of the sound made due to the tears.

I didn't want to push it too hard with torn cones, so I tackled those last weekend by cutting strips of blue paper Shop Rags towels, putting a coat of foam speaker edge sealer on the backside of the tears, coating the strips with the same sealer and laying them onto the tears. I used an acid brush to make good contact and let them dry. I added a coat of sealer to the backside of the paper and made sure the edges were sealed, then put a thin coat on the front, to make sure the edges of the tears were sealed and making good contact. I let that dry, then put a coat of TiteBond Original Wood Glue on the backside, to seal and to add a bit of stiffness. I had heard, from someone who repaired speakers, that spraying clear matte Krylon on paper cones seals them and makes them a bit stiffer, but mine looked pretty bad, so I first sprayed them with flat black Krylon. This left some nubs that were rough, so I used a green ScotchBrite pad to smooth them before the next coat, which was dark gray Krylon primer (two coats). They now have a uniform color, the tears are barely visible from 10 feet and best of all, they sound much better. I think they're a good match to the Stromberg-Carlson- with the bass control all the way down, the treble almost maxed and the volume between ~30-40, it really gets up and goes! Easily enough for small clubs without a mic. The attack is pretty hard, though- not used to that, with a tweed Bassman, 1930s Oahu and Magnatone 210. I put my Boss SD-1 in front of it and it tamed the attack very nicely. The SD-1 is modded- mainly to remove the fizzy top end, a different cap to improve the low end and a different resistor in one gain stage, to increase gain.

I worked at that music store 40 years ago and don't remember ANYONE ever saying a word about these. I'm going to look for more.

For those who are looking for a schematic- did you look inside? Mine has one taped to the inside of the bottom pan. It's really, really hard to read, though.