Gibson BR-9 restoration

[beatnik]

Hi everybody

I have purchased a Gibson BR-9 in very bad conditions as a restoration project

After a first diagnosis it appears that the interstage transformer driving the single ended pair is broken - one of the two primary windings is open

I have been looking for a replacement and read of other people successfully using a Mercury Magnetics GA-PI

Honestly would really like getting a cheaper option. I have been looking at the Hammond 124 series and I believe these could work as well

But as i don't have a datasheet for the original transformer, I don't know which Hammond to choose from these: Hammond Mfg. - Audio - Tube Driver Transformer - (124 - 126 Series)

I believe these all work but maybe with slightly different tone. Would be great to hear any suggestion in for picking the best one

cheers

[g1]

Have you looked inside to see if it is repairable? It's not all that likely but what have you got to lose?
I had the same problem with an old Yamaha SS amp that used a driver xfrmr. The break in the wire was not far from the end so I got lucky. Did the same with a coil in an antique radio. That one I wasn't so lucky with, several laps of wire around the basement, but I did win.

[beatnik]

Have you looked inside to see if it is repairable? It's not all that likely but what have you got to lose?
I had the same problem with an old Yamaha SS amp that used a driver xfrmr. The break in the wire was not far from the end so I got lucky. Did the same with a coil in an antique radio. That one I wasn't so lucky with, several laps of wire around the basement, but I did win.

I have tried taking apart the transformer. Unfortunately the bad winding was the one closer to the center and in the process i have just torn the transformer apart. I definitely need a replacement now and i can't figure out if any of the Hammonds would work well. Maybe i should go with the Mercury Magnetics ? It is way more expensive though

[beatnik]

The Mercury GA-PI is 2.4 ratio. This is a replica of the TF-1001-D but I don't know if that is actually the model Gibson used in the BR-9 amplifier.

[dkevin]

I think the Mercury transformer (replica of TF-1001-D) is used for the Gibson GA-5 white panel Skylark from the mid-sixties or so. It was copied by Fender for the Musicmaster Bass amp. If it seems to be a possibility for you to use, Triode Electronics sells the Musicmaster transformer PI.

[Leo_Gnardo]

I've used this one for repairs in Gibson & other amps that employ an IT:

https://www.tubesandmore.com/product...terstage-10-ma

Worked well every time, & can't complain about the price.

[beatnik]

yes the TF-1001-D had been used in a number of Gibson amps, including that version of the GA. I have attached a copy of the GA-5 schematic and in fact is very similar to the BR-9, however it uses 6BQ5 / EL84 instead of 6V6 in the power amp

all the BR-9 schematics I could find don't mention the transformer part number, and in my amp was actually an Halldorson, catalogs don't show that part number so i suppose it was custom made ?

I have had a look at the Fender Musicmaster schematic. The secondary stage of the interstage transformer is pretty much the same as the Gibson BR-9 but the primary side is different with that parallel 15K resistor.

I have attached a schematic of the Fender MM too

I really think the Mercury Magnetics TF-1011-D replica would work fine, but the Classictone replica for the Musicmaster is considerably cheaper and I am wondering it would work as good ?

[dkevin]

Just out of curiosity, does your BR-9 use a 6SN7 preamp tube as the commonly available schematic shows?

[beatnik]

Just out of curiosity, does your BR-9 use a 6SN7 preamp tube as the commonly available schematic shows?

yes i have now attached a schematic. this is the same version as mine

[beatnik]

I've used this one for repairs in Gibson & other amps that employ an IT:

https://www.tubesandmore.com/product...terstage-10-ma

Worked well every time, & can't complain about the price.

Interesting. This has the same impedance ratio as the Hammond 124A and 124B

[dkevin]

In your first post you made reference to a "single-ended pair" of output tubes being driven by the PI transformer. There are BR-9's that have single-ended pairs of 6V6's in the output section but they are not driven by a PI transformer. They also do not have a 6SN7 as a preamp tube. They do not match the commonly available schematic. Instead, they use a 6SJ7 for the input. They seem to be closer to the GA-9 in design.
Your version of the BR-9 is a push/pull output section and does use the interstage transfomer as a PI. In my experience, it is far rarer.

[Enzo]

Wouldn't be a need for a phase inverting transformer for a single ended amp. None of the examples posted are single ended.

[beatnik]

I am resurrecting this thread as I finally decided to invest some money and time in restoring this old little amp.

I have decided replacing the hard to find transformer with a tube phase inverter

Tube choose is 6SL7 as I had a handful of these available and fancied the idea of having an all octal amp

The Gibson GA-6 uses a 6SL7 in the phase inverter so I have basically copied that circuit with same values

I have checked with the scope and confirmed phase inverter is working. Output stage works, amp sounds but breaks up very early and at half volume it starts motor boating

This is my first tube amp modification project, I am just joining together schematics but I don't really know what I'm doing, would be great if someone more expert could chime in and suggest a couple of tweaks

Attached there is a schematic of how i currently have the amp wired up, and also the two Gibson schematic I am basically trying to blend together

[Gingertube]

Just some thoughts on your motor boating issue.
1st - a problem with your modification. It has been my experience that the Phase Inverter always wants it's own power supply node (that is decoupled from all other stages). That 47K in the power supply filter - divide that into 10K followed by 39K and add extra 10uF cap from the 10K 39K join to 0V - and use that join to feed the phase inverter.
OH! - you have changed the electrolytic filter caps? An old electrolytic cap can cause motor boating.

Cheers,
Ian

[beatnik]

Hi Ian,

thanks for your reply

When I had modified the amp I had a look at some other gibson schematics and basically all the G- and BR- series amp (with a tube instead of transformer phase inverter) have plate anode resistors of both preamp stages and phase inverter connected to the same power supply node. I have attached a couple of them as reference.

All the capacitors in the amp have been replaced. The choke in the power supply has been substituted with a 1K 15W resistor

I am unsure where the motor boating is coming from but it's definitely gain dependant, it kicks in at a certain spot on the gain pot

I would really appreciate some suggestions on trying things up the next time I open up the amp. The restoration is almost successful, I only need to sort out this motor boating issue in order to use the amp at full gain