Help needed with a 1960 Gibson GA-18T

[rdrzman]

Hi Guys,

I am new here and I need some help. I recently purchased a 1960 Gibson GA-18T tube amp. It is in decent shape but is missing the main powersupply transformer.

Does anyone know what powersupply can be used and where to get one?

Thanks,

Farel

[Jazz P Bass]

What output tubes are in it?
(2) 6BQ5?
(2) 6V6?
Either way a Fender Princeton power transformer should fit the bill.
Here is a cool link about a repair of a GA-18T
Link: http://www.netads.com/~meo/Guitar/Amps/Gibson/GA18T/

[olddawg]

If it has 6BQ5 (EL84) output tubes you might want to consider a PT for a 18 watt Marshall clone.

[Chuck H]

IME Gibson amps often run low(ish) voltages when compared to other amps of the same era. IIRC I saw a thread where a GA18T had about 305V on the powr tube plates. It was also reported that a rectifier swap raised the voltage to about 348Vp and the amp sounded better. So, there you go. Between 305V and 348V should give good results. Duncan PSUII (free download) is a good tool for making more finite spec determinations. Be sure to plug in all the current loads.

[Jazz P Bass]

At 110Volts these amps where rated at 12-13 watts.
So if a "higher" than factory B+ is used that may challenge that wimpy output transformer Gibson installed.
Just a thought.

[rdrzman]

What output tubes are in it?
(2) 6BQ5?
(2) 6V6?
Either way a Fender Princeton power transformer should fit the bill.
Here is a cool link about a repair of a GA-18T
Link: Gibson GA-18T Explorer

It has the (2) 6v6 tubes in it. According to the voltage chart on the Sch the 6v6's use 292 volts at pin 4. The 5y3 tube uses 280AC at pin 4/6 and 305 volts at pin 8. Would the princeton still work?

[Jazz P Bass]

The lowest B+ that I know of on old Fenders was the Princeton "white knob". 125P1A / 315V's
MM has a FBPP that would fit the bill.
http://www.guitarpartssite.com/Mercu...-p/fbpp-mm.htm
Or you could buy something less costly & zener down the B+ voltage to match the Gibson specs.

[Chuck H]

The Hammond 270dx should provide around 300Vp. RadioDaze has them for $56

http://www.radiosupply.com/product/15120.aspx

[Gingertube]

HI,
My first post to this forum, just registered today. My day job is Electronic Design Engineer and I have many years experience designing, building, modifying and restoring Tube Guitar and HiFi Amps.
The schematic for the GA-18T can be found here:
http://www.gibson.com/Files/schematics/GA-18T%20Amp.pdf

It doesn't have much info but the rectifier tube is a 6CA4 (EZ81).

A "smidge" of reverse Engineering from that schematic.

Power Transformer has a single high voltage winding (centre tapped) and a single heater supply.

Heater Supply (6.3V) needs to be capable of about 3 Amps and the High Voltage Supply needs to be capable of around 100mA and will be around 250-0-250 Volts

That should be enough info for you to purchase an Edcor or Hammond or whatever power tranny to get you going.

Hope this helps.

Cheers,
Ian

[Chuck H]

Gingertube, there is another version of this amp model. The rectifier in rdzman's amp is a 5y3 and the power tubes are 6V6's so there will be a greater voltage drop AND a 5V secondary is required. Running a SPICE model shows that a 295-0-295 HV secondary will produce vey close the the original voltage. The Hammond model I suggested has a 275-0-275 secondary, BUT... It has a 115V primary. Since modern wall voltages (in the USA) are closer to 125V the end result is very close.

[Gingertube]

Ah , thanks for that Chuck H. I worried that I may have spec'ed too low a voltage secondary and was going to edit my post this morning anyway.

Given 6V6 Outputs and a 5Y3 recifier then a Hammond 270DX will suit admirably -
"PSU Designer 2" from Duncan Amps is very useful - I use it a lot for HiFi Amp Design.

There may have been a temptation to go for a higher voltage again (Hammond 272BX) BUT I always suggest its best to try to keep close to the original when doing restorations.


Cheers,
Ian