Power Supply Surprise...Maestro(Gibson) GA-1RT W/Pics

[BrianS]

Oh my, my amp's got issues...

So, I bought this little amp strictly for resale (see separate post).  The amp looks to be in very good condition when I pick it up.  Strangely, the rectifier tube is missing.  Without going into half a page of detail, I have reason to believe this amp sat neglected for most of its life.  Well, I believe I was wrong on that count.

First off, I opened it up to take a peak at the guts.  Right away I see that the filter caps have been replaced at some point.  Fine.  Also, there is an unused tap on the power transformer just hanging there.  Well, that might be normal.  Anyway, I just want to play the damn thing, so I slide the chassis back into the cab, plug it into the leak light & fire it up.  Forgot to mention, I put an old-stock rectifier tube in the socket...

The amp lights up & I'm getting some decent tones out of it.  Crank it up & its a little muddy, but hey, I'm not expecting tonal bliss out of this thing.  Tremolo doesn't work, either.  After a few minutes, I start getting some hissing and crackling.  I take a look at the 6BM8 tube, and I see some redplating.  I shut it down & get on with life.

Now, this morning I open it up to take some voltage measurements & poke around a little.  I fire the amp up & find that I have about 392 volts on the pentode plate where there should be 275; 250 volts on the plate of the triode half where there should be 130 volts.  After a minute or two, I start getting the hiss/crackle sound, so I shut it down.  I take the tubes out & measure again...around 480 volts of B+.  The filter caps are only rated up to 450v.

Now my thinking cap is on.  How can my B+ be so high?  Well, once I really start looking, it starts to look like like maybe the power transformer was replaced at some point AND I realize that there are DIODES bridging some pins across the rectifier tube socket.  That's why there was no rectifier tube, someone had built a solid state rectifier into this thing.  So, I take the rectifier tube out & sure enough, I still have 480v of B+.

My hopes of selling this amp as is are dashed.  The upside is that I can do some "fun" work on this thing to get it playing right.

Anyway, here are some gut shots.  Any advice will be appreciated.   

[terminalgs]

If green wires are for the filaments (how could they not be?) The green/yellow is the center tap.

This should be grounded for maximum hum canceling. Make sure the filaments don't have some other reference to ground, if they do I'd disconnect and try this CT.

Is this amp cathode biased push-pull? If so, I'd connect the filament CT to shared cathodes on power tubes.

[echuta13]

Well those diodes are new and were probably there as a replacement for the rectifier tube?  Maybe they're part of the cause of the overly high voltages?  I haven't seen the backside of too many GA-1RT's...  but most of them have a black power transformer.  Maybe this one is a replacement?  Any markings/stamps on the tranny?

[Jack_Hester]

If the overall amp is in good shape, shouldn't be too hard to take it back close to stock.  Resale should be better, if that's still your intention.

Jack

[eleventeen]

Couple of drive-by observations:
 
In my experience, your PT looks like a Triad (grey end bells) which is way, way better than what went into low end Gibsons/Maestros. It does not appear to me that different holes were drilled for mounting it...but perhaps the replacement tranny (what we are both surmising) has a lot higher output volts than what was desired. Maybe I'm wrong. Seems odd to see such a high-end tranny in there, yet, I *do* note that the wires do seem to come out (and thru the chassis) in just the right place.

Obviously the e-caps have been changed around, we do not find wires soldered to stub-ends of such caps in OEM config.

But something is weird what what you have there. We have your two red wires going to pins 4 & 6 of the 5Y3, and a few of the unused pins are used as tie points. Fine. The diode cathodes junction on pin 2. OK. It appears to me that nothing was ever soldered to pin 8 on the 5Y3 socket. How can that be? How could the 5 volt filament (pins 2 & 8) have been powered with the original tube there?

Do you want to back to tube rectifier?

I'd remove that big black death cap from the fuseholder to ground.

[BrianS]

Well, I'm learning a thing or two here & there:

It appears to me that nothing was ever soldered to pin 8 on the 5Y3 socket. How can that be? How could the 5 volt filament (pins 2 & 8) have been powered with the original tube there?

Edit:  After looking again, I get what you are saying here.

Well, ah, here's the deal:  there are no 5v filament windings on the transformer.  Maybe whomever "fixed" it, had this transformer laying around & plugged it in there, basically bypassing the rectifier tube & putting in the diodes to replace it.  I've got 700VAC between the high voltage windings.  Normal for a higher wattage amp, right?  But for this amp?

As far as I can tell, I either need a new power transformer with the proper windings, or I could use one of the filament transformers that Hoffman sells.  Problem with that is I just don't have the room on the chassis for it.

I'd remove that big black death cap from the fuseholder to ground.

Certainly doing that.

[eleventeen]

"As far as I can tell, I either need a new power transformer with the proper windings, or I could use one of the filament transformers that Hoffman sells.  Problem with that is I just don't have the room on the chassis for it"

Hate to see you spend money on a junkbox part. You could (without changing tranny) either go to a 6X5 tube (octal, 6.3 volt heater, same the other tubes,  indirectly heated cathode, so your B+ does not show up on the other fils) ----cheap tube----or, stay largely how you are but calculate a resistor that would drop maybe 100-150 volts from your B+. Assuming the amp draws 30 mils, a 5K resistor would knock out 150 volts and dissipate 4.5 watts meaning you'd need at least a 7 and better, a 10 watt res in there. Keep the diodes, no rectifier tube. Easily the cheapest solution.

The mystery remains: How could a 5Y3 *ever* have worked in that socket (which would appear original) without residual solder on pin 8? Enquiring minds, and all that.

If I go to the GA-1 RT schematic in Doug's library...I notice that on the tube voltage chart, it shows 260 VAC on each of pins 4 & 6 (of the 5Y3) implying that your tranny is indeed overvoltaged (you read 700, should be ~~520 or 260-0-260) but oddly...the chart shows 285 volts DC on pin 2...but nada on pin 8!!

LOL. I can assure you, those two pins will read the same DC volts on a functioning 5Y3!  

[BrianS]

After several search inquiries, I was able to find a series of pics of this amp that shows the guts, as well as the original transformers with part numbers.  Here's a link:

http://smg.photobucket.com/user/goodstone/media/Gibson%20GA1RT%20Maestro/DSCN1627.jpg.html?sort=4&o=0

This was a godsend for me, as everything makes sense after looking at this pic. 

If I'm right, I could get a replacement PT by Merc. Magnetics for $108 (http://www.beyondeleven.com/Mercury-Magnetics-GA5-PW-5V-p/ga5-pw-5v.htm).  Ouch.  Anyone know a less expensive source?

Now the question is, is it worth it?  I guess I might as well give up how much I paid for the amp:  $100.  So, new cord; new PT; possibly new 6BM8 tube; maybe a couple resistors...I don't know.

I don't really like the idea of any work arounds.  I'd like to get the amp as close to original as possible, so I guess the new PT is the best option.  Would like to find one cheaper, though...

It seems that many of these amps were gutted of their "reverb" at some point.  There's only two on the 'bay right now & they are asking exorbitant prices for them...and they need work!  Well, I'm just thinking out loud at this point.

[sluckey]

Since you bought it strictly for resale, why not just put it on eBay as is? You're sure to turn a profit and you won't be out any time.

[eleventeen]

http://www.ebay.com/itm/Merit-Tube-Power-Transformer-P-3148-260-0-260V-at-90ma-6-3V-at-4-7A-/281262801844?pt=Vintage_Electronics_R2&hash=item417c91c3b4

This one looks a little thicker in the laminations than what you have. Despite the description, it does not appear to have a separate 5v winding.

Otherwise, I agree w/Sluckey, all you can do is to dig a deeper hole money-wise and it's not going to be stock. Off it.

[BrianS]

Since you bought it strictly for resale, why not just put it on eBay as is? You're sure to turn a profit and you won't be out any time.

You are right, but the tech in me is starting to take over.  Since the power supply is all f-ed up & the trem does not work, I think I am going to get this little guy up and running properly again.

I found an old transformer labeled GA 5 for sale for cheap on Ebay.  It appears to be the correct transformer & will cost me $20, so I'm going to take the chance. 

I'm definitely going to document the process.  There is so little info on these amps, hopefully someone, someday might find the info useful.  I know I was very grateful to find those gut shots last night.

[Steve_P]

I'll dig mine out and take some gut shots. Unfortunately, the alligator clips on down was removed, so I can't help with that, but I can give you parts numbers and voltages.

[BrianS]

SteveP, that would be awesome!

I was outbid on a transformer I thought would work, so I'm still looking.  I taught a "Guitar Maintenance" class this past week & one of my students is going to check with an old amp tech to see if he might have something.

I also found a current manufacture transformer that will work voltage wise, but the dimensions are not quite the same & I'd have to drill a couple holes in the chassis to make it work.  If that is what I have to do, I will.

Thanks!