Tube PA Amp

[CJ]

somebody came in with a Knight 60 watt stereo amp chassis, transformers but no tubes,

wanted 100 bucks so i took it,

so we decided to turn it into a power amp for the band's vocals,

kind of a fresh alternative to Class D , DSP processing, self powered speakers on a stick,

got some 6550's instead of EL37's, Sylvania metal base, a GE and a GEC that we had floating around, seem to match up alright,

dual rectifier , using 5AW4's instead of GZ34's since we had them, 750 ma yahoo!

 

[CJ]

turned it into a Dynaco 70, fixed bias instead of cathode, more power and the pwr xfmr won't drip anymore,

threw out the pc boards from the Knight, read that they crack,

went P to P with some USSR oil coupling caps,

got some new filter cans in today, not in this pic,

525 vdc which barely covers the 500 B+ but they are after a 650 volt cap and choke so not much ripple current,

 

[CJ]

here is one side of the Dynaco circuit, pentode input, disimilar sections

F/B comes from screen tap and spk sec,

B+ via the 330K to the first cathode is strange,

 

[CJ]

i love this dual rect circuit, never seen it before,

takes all the stress off the plates as far as having peak AC volts opposite each other,

and doubles current capacity, using the 750 ma 5AW4's will mean a pretty stiff supply from the vac rects.

series heaters means 10 VAC instead of 5,

so it is a Knight pwr supply and a Dynaco amp circuit,

tried it out at practice, very quiet, sounds nice! we don't play loud so it has plenty of volume,

passive speaks means, no ground loops or extra cords floating around on stage,

 

[fazer]

Nice potted transformers. Worth 100 for the chassis any day. Nice work .

 

[sahib]

Yes, you got a chassis for 100 bucks and the transformers thrown in. Well done CJ.

 

[trobbins]

7.6k PP may be a bit high, but no big deal

 

[CJ]

Tried it out with some cheapazz Soundtech 12" 100 watt pa speaks and it sounds decent, I can get the voice coils to spazz out with high bass volume so enough juice to fill a small room , it is quiet and gets that warm tube sound, the vocalist us happy and you can warm a ham sandwich on the lid during set breaks.