Biasing the Albion TGT-50

An Albion TGT50 head came my way for some routine service and it occurred to me that it would be worth checking the output tube bias to make sure it was reasonably close to what it should be.

Of course I couldn't find any service information readily to hand. So I opened the small hatch on the rear of the chassis and saw what looked pretty close to something I've seen on Marshalls and on a Randall head I used to own-a pair of potentiometers and three pins. I then removed the chassis for a general lookover and it seemed like a pretty conventional 50w channel switching head. Nothing was remarkable except that the tube sockets are chassis mounted and wired to the board.

A nice touch, and one that makes service people happy, if you followed my adventures with hard wiring a Blackstar 100w head recently because the board had burned up.

After using my bias probes and measuring the plate voltage at about 470v DC I adjusted the pots and got the results I needed. I then went back to the original setup, set my multimeter to millivolts and measured the left and right pins with the ground probe to the center pin.

The results were quite similar which led me to believe that the designer of this amp, Steve Grindrod,  used the same setup that Marshall used-not surprisingly when you figure that he worked for Marshall for a while. I figure I was reading across a 1 ohm resistor in the cathode string for each power tube.

Or, of course, you can take the better path and check bias with a pair of probes and a couple of dedicated multimeters. It'll give you a chance to look around inside while you're visiting.

One Day In 1970 Or, How We Fixed Your Telephone

I was having a discussion at the Other Place with a few fellows on the subject of fixing electronics, namely guitar amplifiers, sans a digital multimeter or a scope. So this was rolling around inside my head and maybe it was the story in the AARP bulletin today on the subject of memory loss that had me casting about for something to prove that I'm not dead yet.

So anyway, back in 1970 I was working for New Jersey Bell Telephone Company, better known as Ma Bell. Mother was very picky about a lot of things and when she'd settled on something she liked, nothing else would ever do and no off the shelf stuff either. Mother loved MacMillan Ring Free motor oil and that's all we ever used. Macmillan's only claim to fame was that they'd sponsored the Skoda racing team in the fifties.  The supply crib was well stocked with goodies like Ma Bell bandaids and aspirin, spray paint (olive drab, safety yellow, and black) and other interesting things. They had the best damn first aid kits you ever saw, complete with ampules of antiseptic. They were packed ten to a box with a glass ampule inside a plastic tube with a cap on one end. Pull off the cap, crush the ampule, squeeze the plastic tube and anoint thy self, brother....where was I? Oh, I remember.

When there weren't any big projects going on the repair crew would be sent out to clear defects. What that meant was, there was a defect on a particular pair of wires that the installers and frame men couldn't fix, so they'd bypass the pair (spare pairs, doncha know?) and sooner or later the splicing gang would have it.

The process consisted of going to the frame (usually Somerville), locating the pair, and blasting it with the breakdown set, which was a zapper that would send a jolt down the wire and maybe dry out the fault, but usually ended up fusing the pair at the site of the problem. So we'd read the resistance which would tell us how many miles away it was, and attach a tone generator to the pair. We knew from the print what bundle it was in in the cable.

Off we'd go in the truck watching the odometer and when we got close, a pole with a probe was stuck out the window. The probe was connected to a small amplifier and when we reached the site of the defect the tone would quit. We'd do a little more walking around until we had it located pretty close and then we'd put the bucket up to the cable and open it up to determine the final location. A slack puller on the wire rope could pull enough slack into it that we could dig around inside the cable with a probe until we found the pair. This took a little bit of doing since a 2,200 pair cable is not an easy thing to wrestle with.

Once the pair was located we'd fix it with a splice, put everything back together and install a splice case. We'd wait for a little bit until the air pressure came up, check the case with soap solution for leaks and then head to the diner for coffee.

Tweed Super On The Cheap

While trolling my usual places on fleabay (the "Why pay more?" store), I ran across a trio of partially built tweed amps that consisted of Weber chasses, pots, knobs, and Mercury Magnetics iron but not much else. There were a 5E7 Pro, a 5F6A Bassman, and a high power tweed twin.

Since I'd already built a couple of 5E7 Bandmaster kits I opted to snag the 5E5 Pro and build it as a 5F4 Super. All three of these amps are pretty much the same with some very minor differences.

Once the partially constructed chassis arrived I made an eyelet board for it and loaded it with F&T electrolytics, Mojo Dijon capacitors and mostly metal film resistors. At about the same time I bargained with Dirk Newton for a pair of Weber Ferromax 10 inch speakers and contracted with J.D. Newell for a lacquered tweed cabinet.

Looking through my stash I came up with a trio of new old stock Cinch shielded 9 pin sockets for the preamp tubes and a trio of new-ish Chinese ceramic top loading sockets. For glassware I used new old stock RCA preamp tubes, blackplate power tubes, and a Mullard 5AR4 rectifier tube.

The chassis went together easily as this was my third one, and everything else went together when the cabinet arrived, and it is a beauty. I mostly wired it with 22 gauge pre tinned cloth pushback wire from Stewart MacDonald.

There's a lot of controversy about  what sort of wire and gauge to use for general hookup and filament service but I think it is a relatively simple matter to determine what the current load is and wire accordingly. For instance the total current draw for the preamp and power tubes is about three amps total once the initial inrush is over. At that point we see that 21 awg chassis mounted wire maximum load is 9 amps so that should be well above the margin of safety. Some folks think that they have to use 18 gauge single strand for the filament wiring and there's simply no need for anything that heavy.



Here's the breakdown on prices which includes shipping:

$375.00 chassis
$260.00 cabinet
$  75.00 speakers

Glassware and other components were from stock

Peavey weirdness now morphing into Blunt Instruments

UPDATE: I think I've found an appropriate name for the amps I build, I checked TESS and nobody's sitting on the trademark, so henceforth and forevermore, anything I build will be yclept a "Blunt Instrument". You heard it here first.



Last week as I considered the state my monster amp project was in I decided to plunge for power and output  transformers for the Super Bee.

There are two potential sources for this class of iron, and a possibility of a third. There is, of course, the ubiquitous offerings from Mercury Magnetics about which more anon, or the offerings from Mojotone which are manufactured by Heyboer. If Classic Tone made these transformers I would have shopped there but alas! they do not.

The net total was $117.42 for the Mojo 753 power transformer, and $109.70 for the Mojo 756 output transformer-which came with multiple taps by the way. Shipping was another $20 or so. Mercury would have run me $425 plus shipping so it was a no brainer.  I suppose I could have used a Twin Reverb power transformer which I have as well as a Twin Reverb output transformer and then had to come up with a 5v filament transformer to run a rectifier tube which would have saved some dough but would not be close to the original setup. This amp has to look and sound like something Fender built. If I wanted to hunt til I'm old and gray I could possibly have turned up an original p/n 7993 Triad power transformer and a 45268 output transformer but that is not a likely proposition. I could die waiting.

The transformers were installed with blue Loctite on the screws. I've seen too many get loosened up with age.

I've selected a new old stock Mullard GZ34  for rectifier duties and RCA black plate 6L6GCs for power section duties. I think Uncle Boris over at Sovtek can do the  rest. There is also a Dynaco socket mod for protecting the rectifier tube which I plan on incorporating- to power 4 6L6GCs is asking a lot of a 5AR4.

With the board built, all I have to do now is build a bias board and a cap board, install some controls and start stitching. The biggest unknown is fitment and speaker selection. That will come later.



On the top you can see my layout. This was possible only because a friend owns a high power tweed twin and I was able to lay out the components in the original way. By comparison there is a shot of a recently built tweed twin replica and the output transformer and cake pan are reversed.