1959 GA-20T

[Thndr]

I replaced the caps before it received any voltage, a mistake I made during that process was to think the guy before me had them installed properly. During initial startup using a variac bringing everything up 20 volts at a time, at 80 volts the 2 amp fuse blew. This caused me to reexamine the caps, and correct its installation.. hind site being 20/20 I should have payed closer attention to what the other guy had installed, I am wiser for it now. The same new caps that saw 80 volts are still in it. Funny thing is when grounding the caps out Ive never seen a spark like I was told they would do.

yep it was a typo.

I'll get another 5y3 on order tomorrow and check socket out for burns and such.

I could sure use some suggestions on startup. I have a variac, and two meters, digital and analog. With safety always first of course, should I start it up without tubes, checking voltages at the socket of the 5y3 first to insure the tranformer is supplying proper voltage, then put the new tube in with out the others..

on a side note.. 52 Bill.. My real name is Bill and I am 52..

Thanks for your help.

Thndr

[Thndr]

Awesome, I'm an old blues player always searching for tone, plus I like the possibility of bringing this old amp back to life.
My schematic reading has improved some, in the beginning it was a bit overwhelming, now not so much. It helps to read up on it, which I am doing. so much to learn.

Thanks for the reply man.
Thndr

[Enzo]

Let's advance your variac technique a step. You mention you raised the mains voltage up to 80 volts then the fuse blew. It is important to use a current mneter along with the variac. The reason we want to bring something up slowly on a variac is to help prevent the circuit from being damaged. But if we still blow a fuse, then the circuit faced excess currents. You need to insert an AC current meter in series with the mains hot lead. You can set up your hand meter on a high current scale and clip it in series, but in a shop, you ought to make a metered outlet. A box with a meter on it and an outlet, with a power cord. PLug the cord into the wall, then the unit you are servicing into the box. The meter watches current. As you turn up teh dial, watch the current. This allows you to turn it right back down if the current starts to ramp up. If the amp is already trying to conduct hard at only 30-40 volts of mains, then there is no point in continuing on to 80v.

[soundmasterg]

Let's advance your variac technique a step. You mention you raised the mains voltage up to 80 volts then the fuse blew. It is important to use a current mneter along with the variac. The reason we want to bring something up slowly on a variac is to help prevent the circuit from being damaged. But if we still blow a fuse, then the circuit faced excess currents. You need to insert an AC current meter in series with the mains hot lead. You can set up your hand meter on a high current scale and clip it in series, but in a shop, you ought to make a metered outlet. A box with a meter on it and an outlet, with a power cord. PLug the cord into the wall, then the unit you are servicing into the box. The meter watches current. As you turn up teh dial, watch the current. This allows you to turn it right back down if the current starts to ramp up. If the amp is already trying to conduct hard at only 30-40 volts of mains, then there is no point in continuing on to 80v.

Thndr, on a similar subject to the above, you may also want to consider inserting a light bulb current limiter on the AC line too. I made a box that does this, or if I flick a switch, will do what Enzo suggested. This way you see a visual representation of when there is excess current draw and the light bulb will give you enough time to back off or shut the device under test down and fix the problem.

Enzo, for those of us who do use a light bulb current limiter like I mentioned above, do you have any idea how to make something like that work when they get rid of our incadescent light bulbs? I stocked up on 100 watt bulbs just in case but they won't last forever....

Greg

[52 Bill]

on a side note.. 52 Bill.. My real name is Bill and I am 52..

That's weird, my real name is Thunder...

[Thndr]

I thought as much

[tubeswell]

A safe way of measuring current is with a power resistor (in series with the current draw) and a voltmeter (across the resistor). (Current = Voltage/Resistance)

[Thndr]

Thanks for the input Enzo and Greg, I have a clip on Amp meter when I fire it up next weekend.

Today was a cleaning day on the old GA20T, pulled the defective 5y3 and cleaned the socket and surrounding areas with Goof Off, didnt see any burnt areas or cracks using my headset, put my meter on and to check for any cross talk none noted. I pulled the first 10k 1 watt from the B+ (the one coming from the heated cathode) along with all the excess solder (it appears the previous owner got tired of working on it). The resistor was bulging and open. Checked the second 10K 1w resistor it ohms within spec. The 200ohm 10w resistor arrived Thursday, I'll install it once the cleaning is completed. Noticed some loose solder, that could create an issue, looking for more.

Ordered 2 10K 2w resistors, 2 more 20 mf caps and a full set of JJ tubes. The tubes currently in it look over 50 years old... everything should be in by this next Friday.. In the mean time I'm still reading.

I'm probably getting ahead of myself but... Does anyone have a control template for the top side controls? Or someone that provides this service? Not sure what to call that hope that makes sense.. Most of the printing top side is missing.. I don't what to re-invent the wheel on this, I suspect someone is doing this service.

Goof Off in the can worked great on removing cap guts and did not attack the socket or wire insulation. Good product IMHO.

That's all for now, thank for reading my ramblings.
Thndr

[Enzo]

They are getting rid of common 100 watt bulbs, but specialty bulbs remain. Get one of those large globe shaped ones like they use over bathroom mirrors. Or an infrared heat lamp. ROugh service lamps. 100 watt bulbs are not going to go away altogether.

[Thndr]

I have a couple of rough service lamps, I'll use one of them.

Has anyone used the Paul Ruby startup procedure? Would they recommend it? I know this isn't a new build but at this point it's getting pretty close to it.

T

[soundmasterg]

Thanks Enzo! I stocked up on 100 watt incadescants but its nice to know there are other ways when these run out.

Thndr, just because the tubes look 50 years old doesn't mean they aren't still good. Tubes don't go bad with age for the most part as long as the glass stays sealed. The old tubes are going to be better quality and longer lasting than anything made today.

Greg

[Thndr]

Thndr, just because the tubes look 50 years old doesn't mean they aren't still good. Tubes don't go bad with age for the most part as long as the glass stays sealed. The old tubes are going to be better quality and longer lasting than anything made today.

Greg

I've read that, the old tubes won't be going into the trash, they will go on the shelf for later tone experimentation. The main objective of course is to breath life back into the GA20T, and learn a bunch while doing it. The 5Y3 was toast and the price was right for a new set of tubes.

Everyones help over the months has been appreciated, the light bulb current limiter will help me tons.

Regards,
Thndr

[Thndr]

Replaced the two resistor that were bad (10K and 200ohm), new 5y3 installed, ramped up the variac to 90 started seeing and smelling smoke on the 5y3 base. took quick measurements... filament = 330 Plate = 20v .. the base terminal solder was melting.. turned it off. Thinking it could be the 20 elect. cap I changed that and tried again, same melting base terminal.

Attached clip-on amp meter without the light current limiter to filament reading peaked at around 2 at startup then fell to .9, I took measurements at both filaments both read the same. Please correct me if I am wrong but doesnt that suggest a bad base?

What other tests can I conduct? or does it appear to be a bad base? or could it be a bad transformer... I'll try the base first..

oh... it was terminal 8 that the solder was melting on.

also, I felt the transformer it was not hot.

Any help would be appreciated.

Thndr

bizarre..

[Jazz P Bass]

Something tells me that the 5Y3 rectifier is not wired properly.
What I would do is verify, without a tube in the socket, that pins 4 & 6 are the high voltage taps.
They are center tapped. So the center tap should be at chassis ground.
You can then read pin 4 to chassis & also pin 6 to chassis.
Next up would be pins 2 & 8.
That is the 5Vac winding.
Without a rectifier installed, that is what it should read. 5Vac.
If all is as it should be, before attempting to start up the rectifier, disconnect it's output wire to remove any load on it.
That would be the wire that goes to the output transformer & the first 20uf capacitor & the 10K load resistor.

[Thndr]

Something tells me that the 5Y3 rectifier is not wired properly.
What I would do is verify, without a tube in the socket, that pins 4 & 6 are the high voltage taps.
They are center tapped. So the center tap should be at chassis ground.
You can then read pin 4 to chassis & also pin 6 to chassis.
Next up would be pins 2 & 8.
That is the 5Vac winding.
Without a rectifier installed, that is what it should read. 5Vac.
If all is as it should be, before attempting to start up the rectifier, disconnect it's output wire to remove any load on it.
That would be the wire that goes to the output transformer & the first 20uf capacitor & the 10K load resistor.

Thanks Jazz for the reply, here is what I got with no tubes in the amp:

Pins 4 & 6 both read 274 vac

Pin 2 reads between 5 and 3 started at 5 and dropped to 3

Pin 8 0vac

What do you think?

Thanks again
Thndr