Back in 1981-82 one musician asked me to repair scarce then in USSR tube amplifier, it was a BEAG guitar amp. He needed it the next day, but output tranny was dead, so I suggested him to try transistor output (Hybrid amps were not known then in USSR).
Here is the original schematics:
http://files7.webfile.ru/529381/beag251.gif
The next morning the modified amp was ready.
And here is my halph of the output stage (I am not sure of resistor values, it was a long time ago), instead of 2 pentodes I made 2 current amplifiers with close to tube sound when overdriven:
To tune it a first, don't connect any load. Put 10 Ohm into fuse sockets, and slowly decreasing R4 and R5 set 1V on each resistor, also output voltage must be as close to 0 as possible. Balance such a way each half of the amp.
Then connect 1/2 of the load (nominal load is 16 Ohm, so use 8 Ohm 50W resistor) from output of one half to the ground and slowly decrease R1 and R2 untill it starts softly clipping on 24V peaks symmetrically. It means 70W of sinusoidal output power on 16 Ohm load, exactly as that BEAG amplifier was specified.
The amp works as a voltage follower untill D1 and D4 diodes are open, after that it is turned into a symmetrical CCS controlled by input signal.
Errors on the schematic: right bottom trimpot is R4, top one is R3, both are 100K.
Point between trimpots R1 and R2 are grounded.
Also, I increased capacitors from phase splitter to output stage. Speaker socket must not be grounded!
After the modification the owner discovered that the amp sounds well also with bass guitar while did not before, it was not a surprise for me, since output transformer was removed.
He left me the pair of brand new Mullard EL34'th, later I donated them to some other musician...
I never heard of any problem with the amp, the power stage was robust like a T-34 tank.
I used Soviet transostors KT814/815 and KT818GM/819GM, I had some selected with beta=100 and 150+ breakdown voltages.
Here is the original schematics:
http://files7.webfile.ru/529381/beag251.gif
The next morning the modified amp was ready.
And here is my halph of the output stage (I am not sure of resistor values, it was a long time ago), instead of 2 pentodes I made 2 current amplifiers with close to tube sound when overdriven:
To tune it a first, don't connect any load. Put 10 Ohm into fuse sockets, and slowly decreasing R4 and R5 set 1V on each resistor, also output voltage must be as close to 0 as possible. Balance such a way each half of the amp.
Then connect 1/2 of the load (nominal load is 16 Ohm, so use 8 Ohm 50W resistor) from output of one half to the ground and slowly decrease R1 and R2 untill it starts softly clipping on 24V peaks symmetrically. It means 70W of sinusoidal output power on 16 Ohm load, exactly as that BEAG amplifier was specified.
The amp works as a voltage follower untill D1 and D4 diodes are open, after that it is turned into a symmetrical CCS controlled by input signal.
Errors on the schematic: right bottom trimpot is R4, top one is R3, both are 100K.
Point between trimpots R1 and R2 are grounded.
Also, I increased capacitors from phase splitter to output stage. Speaker socket must not be grounded!
After the modification the owner discovered that the amp sounds well also with bass guitar while did not before, it was not a surprise for me, since output transformer was removed.
He left me the pair of brand new Mullard EL34'th, later I donated them to some other musician...
I never heard of any problem with the amp, the power stage was robust like a T-34 tank.
I used Soviet transostors KT814/815 and KT818GM/819GM, I had some selected with beta=100 and 150+ breakdown voltages.
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