SE 300B amplifier

[popa marius]

Hi guys, I tested a new SE amplifier with 300b and a mu folower type driver with D3a and EL802, DC final tube filament supply, anode voltage 420v, bias current 85mA, OT impendance-5.2K/8 ohm and a servobias system for bias regulation. All transformers are built by me and soon I will start building the chassis

 

[Rod Coleman]

I like the protection cage for the 300B.

 

[grataku]

That sounds interesting! Will you post the schematic eventually? What kind of PS did you go with?

Rod,
I am still interested in your cap multiplier PS. Emailed you a few months back.

 

[popa marius]

This is the designed and verified scheme, driver distortion is less than 0.2%/70v a.c. required for 300b tube and amplifier output power is 7w with a THD <3%. Operating stability is ensured by the correction loop of the servobias system and by stabilizing the anode voltage

 

[drjlo]

All transformers are built by me

Meh, another 300B SE..until I saw that. Now you got my attention

 

[tubelectron]

All transformers are built by me

This is very courageous, and require adequat equipment to do so ! Congratulations...

T

 

[popa marius]

Thanks, these are network transformers, shock coil and OT

 

[grataku]

What kind of core material are you using?

 

[popa marius]

The output transformers have oriented grain core, 6P+4S+KFB, L=23 H, Llk=5.1mH R=200 ohm, 18Hz/-1dB-52.6Khz/-1dB. For the network transformers I chose the solution to divide the power part on three transformers distributed more easily on the chassis and the shock coil has 5H/320mA/40 ohm

 

[zoom777]

Very impressive work! Would you be showing your power supply design too?
Thanks.

 

[Kay Pirinha]

This is the designed and verified scheme, driver distortion is less than 0.2%/70v a.c. required for 300b tube and amplifier output power is 7w with a THD <3%. Operating stability is ensured by the correction loop of the servobias system and by stabilizing the anode voltage

With some properly implemented GNFB, THD could easily be decreased even more.

Best regards!

 

[daanve]

Lower the 220k grid resistor to 50k when applying fixed bias.

 

[popa marius]

Thanks for the suggestion

 

[daanve]

WE 300B datasheet:

 

[popa marius]

zoom777
Power supply

 

[grataku]

very nice! What is the BC transistor?

 

[Kay Pirinha]

I guess any small signal transistor will serve well as current limiter.

Best regards!

 

[popa marius]

grataku
Any general-purpose transistor with 30v Uc-e voltage

 

[zdenoeddie]

I assume, that 0R68 resistor in filament supply is used for setting current limit, but according Ube=0,7V/0R68 is only a little above 1A, not to few for 300B (1,2A nominal)?
I just building 300B SE too, so it is nice circuitry of filament supply.

 

[popa marius]

The Ube voltage can be 0.6v-0.7v, therefore the resistor can be chosen experimentally so that the voltage on the filament of the 300b tube is 5v