Gibson BR-9 repair and modifications

[beatnik]

Hi everybody

I have been working on an old Gibson BR-9 .

Amp arrived in very bad condition. Speaker torn apart and the interstage transformer used as phase inverter was blown.

To keep the repair cost effective I have decided not restoring the amp to original specs but working out some mods instead

First of all I have replaced the field coil speaker with a permanent magnet, Jensen P8r alnico 8". In place of the inductance which is built into the speaker I have used a 1K 15W resistor

I have replaced all the capacitors in the amp and checked resistors didn't drift in value

In place of the damaged interstage transformer I decided to fit a tube phase inverter

I had a few 6SL7 laying around so I used one of these to keep the amp all octal tubes

I wired up the tube using schematics for other Gibson amps with 6SL7 in phase inverter as a reference.

Amplifier works and voltages seem ok but with the gain control about 50% it starts motor boating

I have triple checked my wiring and everything seems fine, maybe it's something wrong with the way the 6SL7 is biased ?

I have attached a schematic of how I got the amp wired up at the moment. If someone can give it a quick look and spot any mistakes or suggest changes, it would be very helpful. At the moment the amp is kind usable at lower gain but it cannot be cranked up as it gets too noisy

[beatnik]

these are the schematic of the Gibson BR-9 and the GA-6 there i took the 6SL7 phase inverter circuit

[Enzo]

The country players tend to favor the BR5-49.


Try giving V1 its own B+ node.

Also you have added a gain stage to the original design.

You know you can easily find the interstage transformers, Hammond makes them, Mouser stocks them:
http://www.mouser.com/ds/2/177/5C_124_126-32259.pdf

Allied used to sell one in their own brand, but I no longer see it.


I recently helped a friend with an amp using that same phase inverter, we went nuts trying to stop oscillation (motorboating). Ultimately we found the balance in the phase inverter, your R5 and R13, was very sensitive. One of few situations where the part values made much difference.

[beatnik]

Hi Enzo,

thanks for your reply

I had the amp on the bench so I quickly tried adding a separate psu node for V1 as you suggested

I have used the same values as V2 , 47K and 8uF to ground. They both connect to the output transformer centre tap

Unfortunately this didn't sort out the noise issue.

Basically it seems that the amp has far too much gain, with the volume knob at 3 there is already a good amount of distortion and past 4-5 the noise kicks in

I should try the other tweak you suggested changing the values of R5 / R13

Actually I am not sure of the value of R5, because it was hard to read on the Gibson schematic

What resistance value do you suggest for R5 ?

[Enzo]

I have no idea the value, just suggested it COULD be involved. The experiment might be replace the 2.7k with a 5k pot and dial it around.

8uf caps???

Too much gain? That was my remark about the extra gain stage. Here is a test. Leave the input grid grounded by the jack, and feed a signal direct to the top of the volume pot. That bypasses the first gain stage, work now? For that matter, with nothing plugged in, if you crank the volume, does it go unstable?

[beatnik]

I had abandoned this project and it has been sitting on the shelf for a few months. Today I was into spring cleaning mode and decided to give it another go

The amp starts motorboating if I crank the volume, even without any signal inserted

I have done the test Enzo suggested - inserting the input signal directly on the gain pot, bypassing the first gain stage. This way the amplifier works fine, so the issue is probably caused by too much gain in the preamp stage

I have tried adding a 1M resistor before the gain pot, and this eliminated the motorboating issue, the amplifier doesn't become unstable if I crank the gain, but now I have some high frequency squeals that change as I turn the gain pot. The gain pot also makes a click noise when it reaches maximum.

I have attached an updated schematic of the amp in its current state. V1 has its separate B+ node, and I also added a 1M grid leak resistor on V1

I believe there might be a problem with oscillation, but I don't know how to troubleshoot this. Do you have any suggestions ?

[beatnik]

Today I have tried a few tweaks to trying isolate the oscillation problem, with no results

I have decided that at this point it would be better to just rebuild the entire amp, so I have stripped off all the components and ready to start from scratch

I would like to make sure that the positioning of the tubes would work, at least in theory. I am not entirely sure about the position of the output transformer, and the two output tubes being spaced away from each other

Would be great if someone can take a look at the picture and give me some opinion

[g1]

You are going to rebuild the amp as a BR-9, or as something else?
If it is staying the same, you do not have to move anything around. There are plenty of these amps that do not motorboat. It was not caused by any flaw in the design or hardware placement, more likely just a defective component (or one or more of the modifications).

[beatnik]

Well, to be honest since the interstage transformer is shot and the field coil speaker is dead too, I will abandon the idea of restoring the BR-9 circuit

I've had a look at more Gibson schematics and the BR-6 circuit seems a good candidate. The power supply and output stage are basically the same, and the preamp is 6SL7 and 6SN7 which is exactly what I have been trying to use.

There is also a version with a 6SJ7 preamp and I have a few of these lying around, maybe I can experiment with different preamp configurations and see what it sounds best.

[beatnik]

I have been looking at the BR-6 schematic and was wondering what is the purpose of the 500K resistors from plates of the output tubes to plates of the phase inverter ?

[xtian]

Looks like local negative feedback to me.

[beatnik]

I did not use these feedback resistors on my first attempt, maybe that was one of the problems ?

Anyway, I have started rebuilding the amp following the Gibson Br-6 schematic.

I have noticed that these very old Gibson amps tend to have less filtering stages than later models. I was wondering if adding a separate B+ node for the phase inverter plates could be an improvement ?

Since I am not using the original field coil speaker and I am going to replace the inductance with a resistor, are there any improvements that I can make to the power supplies ?