Re. grainy top-end :
You might try putting grid-stoppers on the ECC88's if they are running 10mA . Try something like 1k carbon-comp ( not carbon film ) & see if anything improves . You may get away with less than that . Small ferrite beads as well, or as an alternative if you haven't got carbon comp in that size.
Are there any cheap blue metal film resistors in the signal loops ( eg. the ECC88 cathode resistor ) ? Please change to something better, carbon films are OK .
tips on refining an amp
-
IslandPink
- Amstrad Tower of Power
- Posts: 10041
- Joined: Tue May 29, 2007 7:01 pm
- Location: Denbigh, N.Wales
#16
"Once you find out ... the Circumstances ; then you can go out"
-
cressy
- Shed dweller
- Posts: 2906
- Joined: Fri Aug 03, 2007 7:07 pm
- Location: the great white space
- Contact:
#17
Ive found some 680r allen bradley carbon comps I can used for the grid stoppers so they'll do that job
there arent any of those blue resistors in there, the cathode resistors are green welwyns in that position.
the anode resistors are some of those square ceramic power resistors though so there will be a better alternative to them available.
tbh the entire collection of resistors is a motley collection of whatever was of the right value in the spares box........
there arent any of those blue resistors in there, the cathode resistors are green welwyns in that position.
the anode resistors are some of those square ceramic power resistors though so there will be a better alternative to them available.
tbh the entire collection of resistors is a motley collection of whatever was of the right value in the spares box........
-
cressy
- Shed dweller
- Posts: 2906
- Joined: Fri Aug 03, 2007 7:07 pm
- Location: the great white space
- Contact:
#18
ok, so after the complete debacle of owston for this amp ive done several things. firstly repaired the dry joints i found and replaced the duff components that were causing the trouble.
i then found the quad of el34s that id forgotten about and had the bright idea to rebias it completely so that it could use either 88's or 34's. as it was the loadline was rubbish for 34s.
i rejigged the loadline for the 88's and came up with a 500r cathode resistor, then did one for the 34's and came up with 380r. both with a 5k load, 350v ht running at 70ma. i plumped for a 470r resistor in the end so that the current through the 34 is abit lower and the 88 a bit higher making the conditions slightly out for either, but close enough.
shoving in the 34s, they sounded alot nicer than the 88s, so now this is abit of a moot point...........
i have now added some grid stoppers to the drivers, 680r allen bradleys which has cleaned up the top end, and changed out the nasty obbligato's for some 0.25uf TAD oil caps. i have not been impressed with obbligato's in any amp ive had them in to be honest.
so next thing is to change out the c/r again for a 380r for the 34s.
i note on some circuits there is a grid stopper on the el34 but values vary. would the 680r allen bradleys do in this position? or am i better off with the often shown 1k5?
its getting there now
i then found the quad of el34s that id forgotten about and had the bright idea to rebias it completely so that it could use either 88's or 34's. as it was the loadline was rubbish for 34s.
i rejigged the loadline for the 88's and came up with a 500r cathode resistor, then did one for the 34's and came up with 380r. both with a 5k load, 350v ht running at 70ma. i plumped for a 470r resistor in the end so that the current through the 34 is abit lower and the 88 a bit higher making the conditions slightly out for either, but close enough.
shoving in the 34s, they sounded alot nicer than the 88s, so now this is abit of a moot point...........
i have now added some grid stoppers to the drivers, 680r allen bradleys which has cleaned up the top end, and changed out the nasty obbligato's for some 0.25uf TAD oil caps. i have not been impressed with obbligato's in any amp ive had them in to be honest.
so next thing is to change out the c/r again for a 380r for the 34s.
i note on some circuits there is a grid stopper on the el34 but values vary. would the 680r allen bradleys do in this position? or am i better off with the often shown 1k5?
its getting there now
-
cressy
- Shed dweller
- Posts: 2906
- Joined: Fri Aug 03, 2007 7:07 pm
- Location: the great white space
- Contact:
#20
i got hold of some suitable resistors to bias it for el34s, so they are going in when i get around to it. too tired today after demolishing a neighbours shed for her. however, thomas has put the seed of an idea in my head for rejigging the amps psu.
i was thinking of removing the gz 34 and replacing it with a couple of ss diodes, 1n4007 or something and using the redundant octal for another el34 as a shunt reg for the driver stage. i will gain about 30 v ht too without the gz34 voltage drop.
i currently have a dropper on the end of the psu to give the required ht for the driver stage, so i was thinking of inserting it after that. i was also thinking of not using a glow tube if i can help it (i dont really want to cut more holes in the top plate if i dont have to) and using a string of zeners.
the driver stages require 10 ma apiece so hopefully it shouldnt pose a problem to add an extra 20ma to the current draw.. that will take it up to 180ma at idle though, and the tx is rated at 200ma
is this sailing abit close to the wind? only 20ma headroom might make for more problems than adding the shunt might cure
subbing the gz34 for diodes will mean that i can safely add more capacitance to the psu too, so maybe this will be more beneficial than a shunt.
who knows? well i suppose somebody does.
any thoughts?
cheers ant
i was thinking of removing the gz 34 and replacing it with a couple of ss diodes, 1n4007 or something and using the redundant octal for another el34 as a shunt reg for the driver stage. i will gain about 30 v ht too without the gz34 voltage drop.
i currently have a dropper on the end of the psu to give the required ht for the driver stage, so i was thinking of inserting it after that. i was also thinking of not using a glow tube if i can help it (i dont really want to cut more holes in the top plate if i dont have to) and using a string of zeners.
the driver stages require 10 ma apiece so hopefully it shouldnt pose a problem to add an extra 20ma to the current draw.. that will take it up to 180ma at idle though, and the tx is rated at 200ma
is this sailing abit close to the wind? only 20ma headroom might make for more problems than adding the shunt might cure
subbing the gz34 for diodes will mean that i can safely add more capacitance to the psu too, so maybe this will be more beneficial than a shunt.
who knows? well i suppose somebody does.
any thoughts?
cheers ant
-
cressy
- Shed dweller
- Posts: 2906
- Joined: Fri Aug 03, 2007 7:07 pm
- Location: the great white space
- Contact:
#21
I cant be arsed to do a regulated psu, and i dont have the time. In between jobs today i put in the correct value c/rs for el34s and all is good. Sounds lovely now. I havent added any stoppers to the 34s but might do if i get time
Maybe......
Maybe......