Gibson BR-9 transformers info

I have purchased a very old and rusty Gibson BR-9 guitar amp as a restoration project

Found one secondary of the interstage transformer was shot. I tried taking the transformer apart and finding if the fault was near the end of the winding but no luck.

The amp needs a new interstage transformer but I am not sure of what to replace it with. The original in my amp was an Halldorson, probably custom part number

Mercury Magnetics GA-PI which is a replica of the TF-1001-D Gibson used in other amps, like in some version of the GA-5, which has a similar pi and output stage as the BR-9 but uses EL84s instead of 6V6s

The Mercury recreation is 2.4 : 1 ratio and fits the GA-5 amp that's all i know. Some people on forums reported using it successfully for BR-9 as well but the thing is, after overseas shipping and taxes this little transformer is gonna be very expensive at about $120

Hammond 124 series offers a few cheaper options but they all have different ratios than the Mercury GA-PI.  I am thinking the 3 :1 versions could work as well but I am not sure. Can someone please explain me what should i be looking for ?

http://www.hammondmfg.com/124.htm

One more problem, the speaker was also torn apart and because it is a field coil type i decided to just replace it with a pm and put a choke in the power supply

Do you think I could use an Hammond 155J choke ? It's 15H / 30mA / 1.026KOhm / 400V max

I have attached schematic for this amp

> put a choke in the power supply

A field coil has high resistance. You need that much resistance.

Two 6V6 will suck at least 60mA. A 30mA choke is not a first choice.

You do not need 15H here.

Personally I would drop a large 2K resistor in there, then adjust for reasonable plate voltages.

are you going to re-sell it or is it going to be a player?

in other words, do you want a museum restore or something that sounds good?

if it is to be a player, punch a hole in the chassis and stick a tube in there to replace the expensive transformer,

if you want to buy an  innerstage  xfmr, look for 20K primary impedance and about a 1:2 ratio, which would mean 80K on P-P grids, and able to take about 15 volts on the primary, Triad HS-35 would work, but is probably expensive,

Fender used an innerstage in there short lived Musicmaster Bass amp, so you might look for one of those,

PRR said:

> put a choke in the power supply

A field coil has high resistance. You need that much resistance.

Two 6V6 will suck at least 60mA. A 30mA choke is not a first choice.

You do not need 15H here.

Personally I would drop a large 2K resistor in there, then adjust for reasonable plate voltages.

Click to expand...

The original speaker coil measures about 900 Ohm. The choke should match that resistance exactly ?

Current rating around 80mA would be enough ?

Why would you rather use a resistor in place of the coil ? Would this just affect noise performance or also the tone of the amp ?

CJ said:

are you going to re-sell it or is it going to be a player?

in other words, do you want a museum restore or something that sounds good?

if it is to be a player, punch a hole in the chassis and stick a tube in there to replace the expensive transformer,

if you want to buy an  innerstage  xfmr, look for 20K primary impedance and about a 1:2 ratio, which would mean 80K on P-P grids, and able to take about 15 volts on the primary, Triad HS-35 would work, but is probably expensive,

Fender used an innerstage in there short lived Musicmaster Bass amp, so you might look for one of those,

Click to expand...

i am gonna keep this amp so i would definitely prefer tone over authenticity , however the very simple circuit and the unusual interstage transformer seemed kinda different from the other amps i have and this is why i wanted to try preserve the original design as possible

the musicmaster transformer i actually looked into before. classictone makes a replica but i have asked them the specs and the turns ratio is just 1 : 1.35 . do you think this one could actually work ?

but are you also telling me the tone would be better with a tube pi circuit instead of the interstage transformer ?

There's this:

http://triodeelectronics.com/unintrui.html

Probably would work fine.