Tuesday, January 22, 2013

I drove a couple towns over and picked up a Gates Studioette M-5381 yesterday. Let's have a look.

Solid logo, I haven't yet put a fine point on internal date codes, but the manual was printed in 1962.

Only 24 inches wide, seated in my work chair. I'm going to have to make a better place for it & will follow up with a better front panel shot later.

Recessed VU meter, quite modernized styling over the RCA BC-3C from the same era.

The front panel of this unit has escaped the rough hand of modification. There are some additional holes on the sides, but for once the face is left alone (which reminds me, I really need to post pictures of the basket case RCA).

This is where the serial would go these days, curious if there are identifying marks inside that would point to record with the manufacturer (not that it would do me any good).

GUTS. Hinged front is the control surface. Moving into the chassis, lower left is a pair of microphone preamplifiers, each with input and output transformer (albeit somewhat puny ones for the era, no need for 5 hz on AM radio, thanks) and room for one more. Shifting right we see a solitary UTC A-21 doing duty on line level inputs, keeping those voltage regulation tubes company. Continuing on to the right there is the program amplifier with three pentodes and a dual triode, the circular transformer being input and the larger rectangular transformer being output.

Shifting to the back row, at far left there is the monitor booster amplifier, 150 ohm balanced line input to 12AX7 pushing... a push pull 6BQ5 monitor amplifier which wisely has its output iron bolted to something more substantial than phenolic PC board, and on to the power supply.

Here's a closer look at the power supply, nothing unusual: mains transformer, rectifier tube, a couple of chokes. The output transformer for the amplifier stage is in close proximity, which will confine power hungry pulls on the B+ well away from high gain stages. Those two relays will probably go unused. I haven't seen a power switch on this unit yet, I will certainly be paying this section some scrutiny when update time rolls around.

Staring too long at this may make me delay diving right in. No! That's the wrong angle, this looks like meditative fun to sort and resort. Wind and dive.

Conductive plastic or mechanical fortitude? I know which direction my preferences lie.

I found the maker's mark in the Daven cap interesting enough to photograph, given the amount of scuffing (original as far as I can tell).

Apparently the #3 gap is standard from factory, designed in room for expansion. Who knows what will take place there in the future? Since I have the manual, and having roughly 52 hours notice of collecting this device spent some time fruitlessly searching for a digital copy (of which I could find none) I intend on scanning up a pdf and sharing that with the digital universe once I get a chance.

Here's a closer look at the preamps.

A truncated shot of the program amplifier (I think I'd term this summing section, but since it was named from the factory I'll show it a little respect). Observe the dirt build up from the right end..

Closer look at the monitor booster.

Yes, some clean up is in order.


The fan was added post factory, it would have been a good idea to put a filter on it. After seeing this I'm pretty sure the fan is getting the heave, I plan on undervolting the mains a couple percent, which should rein in the heat a bit. Honestly, having looked at the underside of the lid I don't think excess heat was ever really an issue.

Exposed tabs! SO much nicer than having to work through a punch hole in the chassis, this one's a keeper for sure.


krivx said...

I'm sure you bought this with the intent of getting it into a working state, but I would suggest looking into potential replacements for the rotary switch in image no. 9.

I have a couple of precision low-level resistance decade boxes with similar overbuilt switches, the contact resistance on those can be as low as 1 milliohm or so (!). Finding switches with such low resistances is now very difficult. It could be worth quite a bit to a metrology lab.

crochambeau said...

Daven stepped attenuators remain fairly commonplace on that auction site, though you make a good point. Is this caliber of hardware no longer made, or is it just at a price point that is unattainable to even labs?

I do have a few multisection proper switches (complete with detents) of the same design that would not require destructive disassembly to modify into switch operations..

Is 1 milliohm your measurement? I'd love to see pics of your test gear if so. I'm a bit primitive on that front. Inspiration to see about fixing up a bridge or some such. Cheers!

MKULTRA said...

Slam the input with an OD-1 driven Les Paul and drive a 2-10"s Tremolux cabinet from the 6BQ5's.
Goose the levels right and proper mind you.

Something tells me there is some Fripp to be had.

Great score.

krivx said...

I put some pictures of my resistance box here http://www.eevblog.com/forum/projects/h-w-sullivan-dual-dial-non-reactive-resistance-box-restoration/msg131385/#msg131385 I keep meaning to start blogging any audio or test gear work but finding the time lately has been difficult.

I haven't measured the milliohm ranges myself, I've mostly just used the upper ranges the few times I've used it. I don't really have gear that can do milliohm ranges very well. I want to try one of these though:

pearshapedhuman said...

Thanks for the sexy exposé. I don't think I've ever seen that kind of radial film cap before. How nice that it is complete and un-fucked with. It almost looks like you could just plug it in and go. Naw, that would be too easy.

krivx said...

I looked up Daven attenuators - they seem like really great parts even if they can't be re-purposed as fancy switches. More stuff to keep an eye out for...

crochambeau said...

Yeah, the seller and I talked about possibly just bringing it up on a variac to see what happened, he'd never attempted it. Once I saw the bed of grit pulled in by the fan I decided clean-up first, THEN variac. I'd also like to see the underside of those can caps before courting smoke..

Looks like a couple of my replies were lost to email (I thought that worked, dammit).

MKULTRA: Thanks for adding even MORE stuff to my radar list, I have exactly zero of the items you use in your example, guess I'll have to just use my Strat & a 2x15.

krivx: Wow, that decade box is beautiful. I've some slip gaps in my blogging for sure, but with your quality in pictures you don't even need to say much...

I'll have to check out that precision adapter, I've a thing or two I could road test it on. Thanks for the tip!

and: yeah, Davens are great, they proliferate in rotary broadcast mixers, even ones as recent as the early 1980s Autogram I'm using.