Audio Zone amp

Here's a schematic I drew of an  amp made by a local builder. It's pretty much derived from  a Fender Princeton and some of the values here may be not exactly correct especially in the tone stack as I did not have the time to do a full autopsy. It's presented here for reference. The bias network, power section, phase inverter and second gain stage are all accurate to the sample I had with the exception of the raw bias dropping resistor which I believe is 100k. The AC bias voltage from the power transformer is substantial.

This amp was built with a second hand circuit board and the execution left much to be desired. Had it been mine to choose I would have added an additional filter capacitor to the high voltage string down near the first gain stage ala Princeton.

As it was all I was asked to do was to make it work, which took an inordinate amount of effort for such a simple amp. In retrospect it would have been far better and more efficient to make a clean circuit board and just go ahead and replace everything and make a proper job of it.

Making it work proved to be a struggle and after nearly everything had been gone over with a fine tooth comb, the fault turned out to be an intermittent power supply capacitor-a JJ dual can number.

That did it for me. No more JJ capacitors are going in anything I touch.

The Hammond power transformer delivers the goods but the gain on this amp is minimal. It makes a nice 8w amp for very small venues like a practice amp. I am going to try my Boosta grande and see if that gets a little more effort out of it.

Either way, I'm done with it.

Making a jack jig for Fender amps.

If you've ever tried to replace the jacks on a Fender amplifier you know that it's tight working inside  that area particularly next to the chassis wall.

Here's a handy time saver that makes me wonder why I didn't do this earlier.

Get yourself a flat piece of aluminum. I used the cover for a discarded project box. Bend a right angle in it for stability's sake.

Lay out and drill 2 5/16 diameter holes that are 7/8 center to center.

Put your two new jacks in as shown. Install and solder the two 68k resistors and the 1 meg resistor as shown in my shop sketch.

Install and go have a coffee with all the time you just saved.

I Am The Bassman, part three.

I had a chance to put in some quality time with the Bassman ish project this morning.

In no particular order I got the capacitor board wired and mounted, ran wiring inside where it will be terminated, and I think installed the capacitor cake pan for the last time.

I've also started on the internal wiring and I think I'm about 70 per cent complete there.

Some might ask why I've used the waxed cambric pushback wire from Stewmac for everything including the filament wiring and that's because I like it. It conforms nicely, is nicely stranded and tinned, and according to published information, 22 gauge wire-which is what this is-has a reasonable current carrying capacity.

According to published specifications for chassis wiring this is rated at 7 amps and the total computed filament load is 3 amps, which means I've got more than enough capacity here especially as it is not tightly bundled.

You can't see it here but I do have the Alpha pots mounted and when everything's finished I'll add a brass rod for a bus bar ground.

The intention is that when it's ready to go and has proved its worth, then I will dismount the Bassman 50 chassis, swap over the vintage faceplate and other items and then figure out what to do with the carcass. I figure it'll get broken up and sold for parts.

UPDATE: I did enough wiring to test the voltages and the power transformer got kinda hot pretty fast. What I'd done was put a link on the underside of the bias board I'd made. Removing that calmed things down quite a bit. The only thing I'm missing now is filament voltage on V1 and V2 for some strange reason. A couple of better sockets ought to cure that.

When I'm testing preamp sockets for filament voltage I use an oldish RCA cleartop 12AU7. It's easy to see when it's properly lit up.

UPDATE 2: 10/9/2017

I got the Bassman knockoff up and running sweetly this weekend. For some reason the negative feedback kills most of the volume and wiring up the speaker jacks was sort of a pain, so I just wired in a simple Switchcraft type 11 non grounding jack. If I get really ambitious I may swap it into my J.D. Newell cabinet and give it a road test this week. And if that works out, I'll follow through with my plans for the Bassman 50. For that amp, I will probably take out the Super Reverb power transformer that's in it, use the Fender choke and output transformer in the Bassman knockoff and tidy up the cosmetics.

I got a 100w soldering iron from Hobby Lobby today for doing chassis grounds and it's a peach. I'll tidy up the grounds and swap over the blackface cosmetics when all is in order.

Looks ok, dunnit?

More Weird Bassman Stuff

While I've been working on the I Am The Bassman project as yet to be named-I'm considering to call it a Cowpoke 40 or The Walrus with a nod to the beatle folks.....where was I ?

Oh yes. I've had an AB165 blackface Bassman in the shop for lots of bad noises and I guess it finally expired on the guy because it made barely a sound on the shop speaker.

On examination nothing seemed especially wrong but the more I fooled with it the worse it got.

I chased the  noise part and fought it to a standstill but the volume was still faint. After consulting the usual savants I put it aside to let my subconscious tackle the problem.

What could it be? I mean, there are only 11 parts in the entire amp. I spliced in another output transformer with no progress made. Same with the power supply diodes and validated the entire bias supply.

This morning I was thinking as I was validating every component and connection, what if those nice looking Sprague Atoms in the power supply were the problem? I mean, they passed ESR, and the voltages were correct in all the right places.

Sure enough. I installed a set of new F&T power supply capacitors and dropping resistors and when I warmed it up and threw the standby switch I was greeted with that nice thump you get when everything's working right.

So the first thing I did was I called the owner and told him he was going to get his amp back. He'd left me a message but I had my phone turned off most of the weekend so I could get some work done.

I should turn my phone off more frequently, I think.

UPDATE: As it turned out the cure was far more elusive than I had thought. This amp was a late 1965 build but it was sort of a hybrid that had been hacked, the end result being that it hummed and was low on volume. Why was I kidding myself?

So I decided to nut up and build it back to print. Starting at one side and working my way across to the other it was rebuilt to proper AB165 configuration, the connections were tested, and powering it up proved the point.

Sometimes you just gotta start from scratch, erase all the previous mistakes and hacks, and get a new baseline of known good parts.

It was all good drill for the AB165-ish project I'm working on at present..

I Am The Bassman, part 2.

I've done a little more work on the I Am The Bassman since we spoke, which included building and populating a proper Fender style eyelet board and doing some preliminary wiring of the things other than the controls and the power supply capacitor array.

For that I will have to buy a capacitor board and backing board as well as a cake pan to finish it off. The plan is to use the old Bassman 50 faceplate to mock things up and then change over the vintage Bassman faceplate I have upstairs.

I know that people say to me "Whyncha just buy da boards from somebody already made and save yourself a lotta work? Jeez."

Well, there are a few reasons.

First of all I like to have total control over my work product, and as you can see I do a pretty decent job of cooking up main boards and a sorta custom bias supply board because this amp will use a tube rectifier.

Second of all I get it when I want it, don't have to wait around for the post office, and I get exactly what I decide I want.

I've got the tooling to make boards and the know-how thanks to Jeff Gehring. I've also got a pretty good American made 1936 vintage Walker Turner  drill press, a Dake arbor press for staking eyelets, and some suitable anvil type stuff. I've just about used up all my stock of board stuff so it's time to get some more vulcanized fiberboard from Mojotone or whoever has it.

I did purchase a board one time from an unnamed source professional courtesy forbids me to mention by name because I was feeling lazy and the price seemed right. When it arrived the board was nice until I got it half way out of the envelope and half the eyelets were missing. So I had to pay for postage back to the seller because I think they didn't really believe me. Had it been I who was making boards and offering them to the trade I woulda just nutted up and sent out another one but that's just me and how I roll.

Between paying postage both ways I could have made it myself for less money and if I really had anything to squawk about I could find the guy in the bathroom mirror.

Either way, it's better to make things yourself if you can. It rules out such dismal considerations. I believe it is called good process control.

Anyway that is my story on making eyelet boards.

Mesa Boogie Mark II power supply wiring.

Here's an excerpt from the schematic diagram for a Mesa MK II that I recently brought back from the dead. It had a damaged power cord that needed replacing and as I got to digging into the schematic and mapping out the wiring scheme I uncovered something that bothered me.

As I understand it, the correct way of wiring a three conductor cord to an amplifier is as follows. The black lead from the power cord (hot) goes to the tip of the fuse, then to the switch, and then into  the power transformer. The other primary lead is connected to the white, or neutral, wire.

No wire nuts here chillun. This ain't residential. Do a good wraparound splice with an inch of bare wire on either lead, solder well. and use two courses of heat shrink tubing.

The white, or neutral is connected to the power cord and out we go to the plug.

It has always been my understanding that the first thing the hot lead should see is the tip of the fuse holder. That way, if the fuse blows for any reason whatsoever, the hot lead is disconnected.

If, on the other hand, the fuse is on the neutral side, then potential will always be present in the primary side of the power transformer and that means danger of electrical shock lurks therein.

Needless to say, I revised it and removed all the excess trash-a line reverse switch, a thermistor, and the so called death cap. I disabled the courtesy outlet and used the existing hole in the panel for a SPST switch so that the owner can switch off the fan if he is recording.

It now looks like this. The fan will not function until power is applied to the power transformer primary through the appropriately placed fuse and switch.

And of course the "No adjustable bias! Use only Mesa tubes! Let us handle this! Begone!" business was something they did not think was that important back in 1980 when this thing was built.

Note the nifty bias adjust pot. You can always incorporate this feature into any amp you like if it does not have an adjustable bias.


Comments?

Silkyn Super 50 schematics redux.

Folks, in case you found a broken link I herewith have added copies of the Super 50 schematics.