KT88 tubes on ST70 Classic design with PC3 And 60W EDCOR OPTs

by nraghunadh Wed Jun 19, 2013 2:23 pm

nraghunadh: Posts : 6
Join date : 2012-10-05

Post n°1

KT88 tubes on ST70 Classic design with PC3 And 60W EDCOR OPTs

by nraghunadh Wed Jun 19, 2013 2:23 pm

Hi Bob,
Can i use KT88 tubes directly on My ST70 Which i have made recently
with Original PC3 driver board and 7199 tubes, EDCOR CXPP60-8-4.2K OPTs.
I am using custom made EI core Power Transformer ratings with
360-0-55-360 @ 500mA
6.3v @ 5Amps (heaters)
5v @ 4Amps (rectifier heaters)
Rectifier tube i am using GZ34 with my own custom made power supply board.
But i did not changed any component values in the circuit.
Can you please advise me is it possible KT88 to use in this condition?
If it is possible what changes need to be done before use the KT88.
right now i am using RCA 6ca7/EL34 NOS tubes
Thanks,
Raghunadh

by **Bob Latino** Wed Jun 19, 2013 2:53 pm

Bob Latino

Bob Latino: Admin; Posts : 3276
Join date : 2008-11-26
Location : Massachusetts

Post n°2

Re: KT88 tubes on ST70 Classic design with PC3 And 60W EDCOR OPTs

by Bob Latino Wed Jun 19, 2013 2:53 pm

Yes - You can probably use KT88's in your custom ST-70. Your power transformer has two 6.3 VAC @ 5 amp lines which should be able to take the extra current draw of a KT88 output tube. One thing with respect to biasing the KT88 output tubes in your ST-70 ... You may have to change out the two 10K ohm main bias resistors on the 7 lug terminal strip to a LOWER value in order to get KT88 output tubes to bias near the center of the two bias pot's range.

Just to be clear to others with a STOCK Dynaco PA-060 power transformer on their amp ... Do not use KT88 output tubes in your original ST-70. The original Dynaco PA-060 power transformer is undersized for the amp to begin with and tends to run pretty warm just using the standard EL34 output tubes.

Bob

by nraghunadh Wed Jun 19, 2013 3:25 pm

nraghunadh

nraghunadh: Posts : 6
Join date : 2012-10-05

Post n°3

Re: KT88 tubes on ST70 Classic design with PC3 And 60W EDCOR OPTs

by nraghunadh Wed Jun 19, 2013 3:25 pm

Bob Latino wrote:
Yes - You can probably use KT88's in your custom ST-70. Your power transformer has two 6.3 VAC @ 5 amp lines which should be able to take the extra current draw of a KT88 output tube. One thing with respect to biasing the KT88 output tubes in your ST-70 ... You may have to change out the two 10K ohm main bias resistors on the 7 lug terminal strip to a LOWER value in order to get KT88 output tubes to bias near the center of the two bias pot's range.

Just to be clear to others with a STOCK Dynaco PA-060 power transformer on their amp ... Do not use KT88 output tubes in your original ST-70. The original Dynaco PA-060 power transformer is undersized for the amp to begin with and tends to run pretty warm just using the standard EL34 output tubes.

Bob

Thank you very much Bob, I will follow your advice and do it soon. I am posting my power supply board pics.

Thank you,
Raghunadh