GA-15 RVT rebuild

[J A]

Hi all
I'm new to tube amps but I've got reasonable related experience and am putting in the time to research and learn before I start running current. I do understand the risks associated with high-voltage circuits and the ways to safely work on tube amps but I'm eager for any information or advice that enhances my well-being. :)
My current project is a partially-rebuilt GA-15 that I want to start clean and rebuild with new fiberboard and a few new components. I'd like to keep as much original stuff as possible but make improvements and mods.
The attached schematic is the one that most closely fits what's actually in the amp. Also attached is a pic of the amp in current condition.
The power choke is missing ( I have a replacement). The filter caps are gone and I'm ordering proper replacements. The phase inverter transformer is missing and someone has started a cathodyne circuit. There's no indication that the power transformer, output transformer and reverb transformer are bad but I'll have a better idea when I get things further along.
My goals are:
- Rebuild power section to original specs, including replacing choke.
- Remove and replace fiberboard and clean up and re-wire pre-amp section components following the original schematics (pretty much).
- Re-run heater wires and clean up messy lead dress wherever possible.
- Redesign and rebuild the cathodyne PI circuit.
- Maybe add switchable negative feedback circuit and/or variable cathode resistor circuit and/or gain stage mod.
- Optimize the voicing with tweaks to the tone stack/coupling caps, etc.

Two main questions, I guess... Is the layout in the attached PDF on the right track in terms of the power section/cathodyne rebuild (I haven't finished laying out the pre-amp section)...and...aside from specific components that I know I'll need, can I get suggestions for resistor/capacitor values/voltages/wattages that I should have on hand to play around with. I'm ordering filter caps and a few other things and I'd like to get as much as possible on one order before I get started.

Thanks in advance for any advice you can offer!  :)

[sluckey]

I would use the schematic to create a parts list. Then compare the list to the parts I have on hand. Finally, order everything I don't already have. Since you want to keep things as original as you can, you may want to buy an interstage transformer. You can find them here...

     https://www.tubesandmore.com/products/transformers_chokes?filters=Type%3DAudio%20Interstage

Oh yeah, flip D1 around.

[PRR]

Tremolating a cathodyne is difficult because of impedance issues.

Your layout has both power tube grid resistors going to ground, but they want to go together to a trem-pot.

Agree that for this project, an interstage transformer is probably the way to go.

[J A]

Thanks!! Just the sort of feedback I was hoping to get.
I'll look into the tranny. I had been figuring that since the cathodyne circuit was mostly there, I'd follow through on that but if I can figure out the right replacement, the original PI circuit makes more sense.

[J A]

Regarding the replacement PI transformer... There are 2 that seem to fit the bill and have apparently been used as replacements in GA-15 projects.
Opinions as to which might be more correct?
https://www.tubesandmore.com/products/transformer-audio-interstage-10-ma
https://www.tubesandmore.com/products/transformer-hammond-audio-interstage-5-watt
Thanks!

[PRR]

The cheaper one.

[shooter]

fwiw
2 of the Gibson's I fixed with smoked IS trannies, I upped the cathode R for the tube that "drove" the tranny, remember it can only handle 10mA.  most 12__7s are only set for couple mA at idle, but...
here's the schematic I'm looking at
https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga15rvt.pdf

I would change the 1k cathode R to 1.5k - 2.2k for experimenting, then final tweak as you get valid data.  maybe add grid R's to the PA tubes also 

[J A]

The cheaper one.

There's also this one - http://triodeelectronics.com/40-18093.html - that seems to be a better version of the same thing(?). It's listed as a replacement for Fender Musicmaster which specifies the Stancor A53-C which is the stated equivalent of cheaper one.
I'm happy to save the bucks but will there be better sound quality/durability from the ClassicTone?
Thanks again to everyone for the help!!!

[PRR]

The "for Musicmaster Bass" may be a shade better for bass. (Even so, the MM is a joke of a bass amp.)

Gibson would not have spent 3X as much when a $17 part existed. I think it is more in keeping with the original design.

Both amplifiers use 12AX7/6EU7 to drive the IT. There's no way these tubes can deliver 10mA. Probably working nearer 2mA. 

[shooter]

There's no way

I'll agree under normal operating conditions they shouldn't smoke, but the 2 I've replaced were smoked, even had that funky burnt enamel smell 
one also had a smoked OT also.  So are they from hard fail in the PA or IT driver then?
never did "investigate" just rebuild 

from notes one;
fluke monitoring Rk, noted 5-6mA (calculated) swings dimed (dated 2015 - too far back for recall :)

[vampwizzard]

Impulse response acts funny in transformers.. imagine jamming the thing with inrush constantly. Makes sense that you could smoke IT transformers when old components start to drift.

As far as tone shaping goes, start with the regular circuit (including tone suck) and then evaluate what you want to do. A lot of youtube amp guys bypass it and it may work for them.. just realize they were designed to work with gibson branded guitars.

Looking forward to updates on this one. Great amp.

[J A]

I haven't started the rebuild yet but I have all the parts ( I hope) either in hand or on the way.
A quick and dirty check of the power transformer shows that the voltages are in line with 120 VAC input - about 7.5 volts on the filament leads and around 270 on each side of the secondary. It looks like I can adjust the voltage dropping resistors and power up.
The reverb transformer reads about 600 ohms and 70 ohms with no evidence of shorting to ground. Don't know what the resistance values should be but maybe its ok.
The output transformer may be a different story? I don't have specs to compare DC resistance readings but I'm getting 300 ohms across the primary and around 150 to the center tap. I'm getting no measurable resistance across the secondary but I'm not getting any shorting to ground. I'm using a pretty lame multimeter for this so I can't get really accurate readings but the readings seem low. I connected a speaker to the secondary and touched a battery to the primary and got a little sound so it's working to some degree and maybe it's fine.
If I have to change out the OT, any suggestions?

[shooter]

the readings seem low

IF none of the secondary wires shows ohms to ANY of the primary, or the iron, probably ok.
the OT is a step down transformer, you want to take ALL the primary voltage and convert it to current at the speaker so you should have low ohms.  when I get a chance i'll ohm a secondary n see

[J A]

IF none of the secondary wires shows ohms to ANY of the primary, or the iron, probably ok.

That's what I was thinking...There's a lot I don't know but I know that windings need to avoid talking to one another. :)
Thanks!