1954 Guild Masteramp

[Steve_P]

Sooooo. after getting the bug to own an original Masteramp, what turns up at GC Hollywood but a 1954 Guild Masteramp 120. I figure that somebody has to take one for the team and drag it to the upcoming Guild factory tour. I buy it and have it shipped.
So Tuesday, I went over to my local GC and...

It doesn't work. The on/off switch does not work, the pilot light (green!) and 2 12ax7 preamp tube heaters are on the entire time it is plugged in. The heaters for the 6sn7 oscillator tube and the two 50L6 power tubes and the 35z5 rectifier tube do not fire up at all. There was replacement of the filtercaps with a tiewrapped mess of individual caps dangling from wires and insulated with a piece of foam rubber insulating it from the rest of the circuit. Um, foam rubber. Are you fucking for real? It's sitting there like a farm dog waiting for a supper that never comes.

I talk to the assistant manager. I talk to the manager. I take the back off and see that Hollywood has "fixed" it and has a bag of spare parts to prove it. Talk to the "Vintage Acquisition Manager" on the phone. At this point, I'm told that I have three options - return it, have it repaired (again on their dime) or get a discount and repair it myself. I tell Hollywood that if this were on ebay, it would sell for 250 bucks as a project amp. Personally, I would rather get the discount and fix it myself, because I'm not laboring under the impression that everything is a #@%*!! Fender.
"Ok, Mr Phillips, we'll have it repaired."

Right.

Called the store today and was asked if Hollywood had emailed me(?)
Ahhhh, no. So, I assume that not a damned thing was done with this so far, which leaves me with a quandary. Wait for them to "fix" it or return it, knowing that it might be a very long time before another first year amp shows up.

[sluckey]

Just venting, or do you want to try to fix it?

[Steve_P]

Considering that I could have done a better job as a hobbyist, yeah, I'd rather they they just let me fix it.

Part of it is I haven't seen one of these up close before and I learned quite a bit just having my hands on it. It came from the factory with an isolation transformer and a couple of different filament windings, so it wasn't the deathtrap that series string amps usually were.

[sluckey]

OK. Post a schematic and your symptoms and let's see if we have any ideas.

[Steve_P]

OK. Post a schematic and your symptoms and let's see if we have any ideas.

Part of the issue is I don't have the amp to draw a schemo...

[Slimtim]

  yeah i'm not seeing any online either.other models but not that one.guitar center?idk might want to get the refund.

[PRR]

How much did you pay?

I know how long ago 1954 was. I would never assume a Guitar Center would know jack-squat about such an amp. If the price can be tolerable, do the right thing, restore it with love.

> 50L6 power tubes and the 35z5 rectifier

Sure sounds like a transformerless Death Trap. IMHO, GC should not handle such things; neither should you until you have an Isolation Transformer.

[John]

I'm wondering if someone didn't install the iso transformer, but neglected to change out the power tubes. Or installed the PT wrong. Or pulled the 6L6s and put back the "original 50L6 from factory!" (marketing)

Anyway, here is an article FWIW
http://www.premierguitar.com/articles/Guild_Amps_of_the_1950s_

and a blog about an amp that looks similar to yours
http://gretschpages.com/forum/other-amps/1954-guild-masteramp/34297/page1/

If it were me, I'd make GC an offer for that as/is not working amp, and dive into the project. Sounds like it'd be fun!

[Steve_P]

I had my hands on it in the store. It does have a factory transformer and it did have the proper tubes in it. When you consider how local a company Guild was at the time, and how significant portions of NYC were dc wall powered, it makes sense that they would use 50L6's.

[Steve_P]

Here's a 55 Masteramp - almost the same, but no transformer. The amp in limbo has white knobs and a 54 Jensen in it.The Guitar Center amp is the one pictured below.

[PRR]

> It does have a factory transformer

It has an OUTput transformer. Does it have a POWER transformer??

As it is not working right (and maybe is not in your hands?), there's no good external test. If it worked good, you'd put one hand on a water-pipe or metal outlet cover, the other on exposed chassis. Try the 2-prong plug one way then the other. If you die, it's a Death Trap. If you feel a YOW! tingle, it is "safe" by 1955 Code but the leakage would not be acceptable today.

FWIW: one 50L6 can do 2 Watts, a pair a bit over 4 Watts. Working well, it will rock the house but not the club. The later 6V6 version "should" be a lot stronger.

[Steve_P]

> It does have a factory transformer

It has an OUTput transformer. Does it have a POWER transformer??

I mentioned that it did have a isolation transformer already installed. I should have taken pics with the camera when I had the opportunity. The output transformer was on the backside of the amp to the left. This tranny had leads to the amp. The power transformer was on the backside of the amp bolted into a rectangular cutout in the chassis. This transformer had leads to the 35z5 and another set of leads that powered the 12ax7's.
From the age of the transformer and the positioning of the original components that were still installed, I'm assuming that it was either done by whoever built the thing.

Also, the guy that fubared the repair job was still using one of those ancient capacitors as a tiepoint.

[griehund]

> It does have a factory transformer

It has an OUTput transformer. Does it have a POWER transformer??

I mentioned that it did have a isolation transformer already installed. I should have taken pics with the camera when I had the opportunity. The output transformer was on the backside of the amp to the left. This tranny had leads to the amp. The power transformer was on the backside of the amp bolted into a rectangular cutout in the chassis. This transformer had leads to the 35z5 and another set of leads that powered the 12ax7's.
From the age of the transformer and the positioning of the original components that were still installed, I'm assuming that it was either done by whoever built the thing.

Also, the guy that fubared the repair job was still using one of those ancient capacitors as a tiepoint.

Steve, you know you want to fix it.  Make them an offer they can't refuse.  See you in NH. 

[Steve_P]

Just talked to the assistant manager. Nobody has done anything with it so far. See ya in a couple!

[HotBluePlates]

How did they get the B voltage up on this amp?  Did they use a voltage doubler?

Nope, just the 35Z5.

110vac * 1.414 = 155.5vdc. The maximum plate voltage rating of the 50L6 is 200vdc. The tube was designed from scratch to run from rectified wall voltage.

[Steve_P]

Just to check in, out of town for the Guild factory tour.

The amp was bone stock until GC fixed it. They decided to give me a partial refund, because I would rather fix the thing for the knowledge I would gain. The filaments on the 12ax7s are running on a different circuit than the filaments on the 50L6 and 35z5. Early Guild amps are all over the map. I have a little 12AB5 single-ended amp here from 1956 and I think that it has more than a passing resemblance to a Webcor tape recorder power supply. The can cap on that amp has pins and plugs into an octal tube socket.

I am picking up a 100J tomorrow. Look at the paralleled phase inverter on that beast. 

[Steve_P]

No, it does not. Wondering if the dc out of the NYC wall sockets made full-wave not necessary. I was  talking to one of the long time sales guys here and he said that the first Guild products were, in fact, amps. That kind of makes sense because Al Dronge was an accordion dealer when he started out.

[PRR]

> Wondering if the dc out of the NYC wall sockets made full-wave not necessary.

If you "know" it is DC, you don't need a rectifier at all.

With no power transformer, half-wave is the only scheme which allows circuit common to also be the wall's grounded-conductor potential. (Yes, I see you have a PT, but it is surely derived from a no-PT model.)

BTW: another use for "AC/DC" radios and amplifiers is farms off the power grid. Windmills, water wheels, or engines charged batteries which ran lights and other appliances. Early small models were 32V, but later 110V systems were used in larger installations.

[Steve_P]

> Wondering if the dc out of the NYC wall sockets made full-wave not necessary.

If you "know" it is DC, you don't need a rectifier at all.

With no power transformer, half-wave is the only scheme which allows circuit common to also be the wall's grounded-conductor potential. (Yes, I see you have a PT, but it is surely derived from a no-PT model.)

BTW: another use for "AC/DC" radios and amplifiers is farms off the power grid. Windmills, water wheels, or engines charged batteries which ran lights and other appliances. Early small models were 32V, but later 110V systems were used in larger installations.

How much ripple is there in that (alleged) wall current? how many ac/dc amps were made without rectifiers? Heard from a longtime Guild sales rep that the first Guild products were amps, not guitars. I have no way of checking that, but I was given a lead or two by Mark Dronge that might provide some insight into Guild amps.
<edit>

[HotBluePlates]

... how many ac/dc amps were made without rectifiers? ...

Is this a trick question?

Answer: None, because you have to have a rectifier for the possible a.c. from the wall.

The orientation of the rectifier is such that if the wall is d.c. instead of a.c., the d.c. passes straight through to the filter cap if the cap voltage ever drops below wall voltage.

Hence the "a.c./d.c." designation; the amp doesn't care which is supplied.

[Steve_P]

It was a rhetorical question, actually, posted in haste, on my phone, during a three day party and under the influence of beer and some superlative guitars.

[Platefire]

I've always avoided the "AC/DC" amps because of the live chassis issue but wonder if they actually sound good regarding guitar tone and response? Never heard anyone raving about them that I remember. I love the looks of this old Guild. I once had a Guild Starfire 335 type Guitar that I wish I had held on to. Platefire

[Steve_P]

Just got it home. Like PRR explained, it should only be 6-8 watts. It has a Radiokraftsman 12 inch speaker from the 8th week of 53. Wire window screen grill "cloth" with the Guild logo flocked on.

Son is over, so willl post pics later!

[Steve_P]

Some pics, more later.

[Steve_P]

[HotBluePlates]

"274" on the speaker indicates it was made by RCA.

The crude positioning of the PT core into the too-square hole, along with the fact that the endbell which should be facing into the hole in the chassis is on the outside, tells me the PT was not installed at the factory. The mix of plastic and cloth insulation in the one transformer also seems odd.

All that said, you may simple have to re-verify everything that doesn't look uniformly old. If the PT only preformed isolation, then everything should work. Since not everything works, you should probably compare to what a properly-installed isolation PT would look like.

[Steve_P]

Thanks for the correction on the speaker code.

All valid points. Today is the first chance I have had to get to it and it has been messed with considerably. Why the tech in Hollywood didn't bother to order a cancap and save himself some trouble, I don't know. The cap that was in it was 10/10/20/+100@200v. No serial numbers on the OT. After I finish drawing the layout and schemo, I'll disassemble it and see what's cooking.

[Steve_P]

Thread and gutshots on the Guild board of a pristine Masteramp 66 with an added isolation transformer. I bought a none working amp that looks identical from Kummers Vintage. It uses a similar chassis, but it has 6L6s and the rectofier and B+ bolted to the cabinet, connected by an umbilical. If it's a conversion, it used the same vintage and brand components as the rest of the amp.