Gibson GA45-RVT Saturn PT replacement.

[BigE]

Hey Fella's, i got in a GA45-RVT in for a restore.  Somebody has put in a VERY non orig PT.  I am going to put in a correct replacement.  Question is which one would be better?  022756 (twin)  or 022798 (Bassman).  the Twin looks to be almost exactly the same except for the absence of the center tap on the Heater secondary.

which would be better?

Schematic:  http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/gibsonamps/ga45rvt.pdf

[Willabe]

I don't think either will work as a direct replacement match.

That amp's B+ is choke loaded (L1) and that drops the B+ well below a cap loaded B+ PSU. Schemo shows 453vdc on the 6L6 plates but that's after the choke. And a choke loaded PSU will sound and have a different touch response than with a cap loaded PSU.

Now it still might sound and play fine to you or your client, maybe you'd like it better, but you'll loose some, maybe not a lot, but you will loose some of output power because of the voltage drop. Depends on how much the B+ drop is, might be a little darker sounding, little less clean head room before it starts to break up, so.......       

Also you'd have to add up the heater current draw to make sure either PT would handle the current draw. The twin probable would but there's still the B+ issue.

You could look over at mercury magnetics and see if they have a clone PT for that model? But it will be pricy.


                  Brad          

  

[PRR]

> is choke loaded (L1) and that drops the B+

Cap-loaded: C38B near the diodes.

Also 360VAC into 453VDC is reasonable for a low-price cap-input rectifier.

Bassman PT is for-sure the nearest electrical fit you will find at a popular price.

[Willabe]

Cap-loaded: C38B near the diodes.

Oh.....           Yes I see now, I missed the  -Y-  PSU connection going to/splitting off of to the OT/power tubes and the small tubes.  Just looked at it again and it is cap loaded but choke does supply the whole amp.     

Thank you PRR and I'm sorry Big E for my mistake.


            Brad    

[PRR]

> I missed the....

I stared at it a while before I felt sure. Not the neatest drawing, and has been through a bad Xeroxing.

> but choke does supply the whole amp.

Yes. As you say, that affects dynamics (and weight!). Probably drops a few Volts too.

But replacing the offending PT with another of ample current and very-near Voltage "should" work.

The 325V rating on 022798 is significantly lower than the 360V on the Gibson plan.

Yet I once got a "Bassman PT" from another place (Weber), used the lower-volt taps, got 395V from a 108V wall-voltage. So I *thought* that proper wall voltage and the higher-V taps would get into the zone.

IMHO, it would be wiser to go low. The Gibson isn't getting any younger. And 50W tube-amps are readily available new, 100+W tranny-amps are very cheap. I'd rather see it cruising 10% down from gut-ripping power than have it roaring 100% and burning up. (I actually used the Weber to down-rate a burnt-PT Ampeg; better 20W-30W with student-proof reliability than another 60W $120 flame-out.)

https://taweber.powweb.com/store/magnetic.htm
W022798 125P5D 125P7D US voltages Power Transformer for Tweed Bandmaster, Tweed Bassman,....
https://taweber.powweb.com/store/022798sch.jpg

720VCT, which is your 360VAC each side. Plenty of 6V power, a 5V to ignore, and a 45V (not 50V) tap for bias. (Reduce R61 to 1K; increase R59 as needed to get ~~50V bias and trim for happy idle current.)

Check Hoffman's specs for similar parts: you sure will get it faster.