new power transformer

by kost Thu Aug 22, 2013 6:14 am

kost: Posts : 35
Join date : 2009-05-12
Age : 60
Location : athens

Post n°1

new power transformer

by kost Thu Aug 22, 2013 6:14 am

Hi dears

I just changed the VTA ST 70 old power transformer
with the new one from DYNAKIT PARTS with very good results.
I had also the chance to try the
PSVANE KT88 and i am amazed
with the sound!!!
I am using diodes in place of the rectifier
and the sound is fast with good bass and
dynamics but i noticed that to have
0.50 V for each tube i have to turn the bias pots
all the way down,is this normal???
My next project is to try the new vta
octal pcb BUT i wonder if the new power
transformer can handle both,the octal preamp tubes
and the PSVANE KT88.
Bob,what do you think??

warm regards

by **Bob Latino** Thu Aug 22, 2013 7:29 am

Bob Latino

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

Post n°2

Re: new power transformer

by Bob Latino Thu Aug 22, 2013 7:29 am

You can use KT88's in any VTA boarded ST-70 amp as long as you have an upgraded power transformer (as found in all VTA ST-70 amp kits) OR one of the Dynakitparts upgraded ST-70 power transformers.

The recommendation to bias at .50 VDC per tube may cause biasing problems for KT88 output tubes in a VTA ST-70 because the bias system was set up for biasing EL34, 6L6, KT66 and KT77 output tubes with a 10K main bias resistor. When the newer version CCS driver circuit started being used last year Roy and I started using a 7.5K main bias resistor. The 7.5K resistor better allows biasing KT88 output tubes more near the center of the bias pot's rotation. The solution for you is to pull the 10K resistor (R39) off the bottom of the driver board and replace it with a 7.5K (1/2 watt) resistor. R39 is on the back edge of the driver board between the two 100 uF @ 100 volt bias capacitors.

Bob

by kost Thu Aug 22, 2013 1:28 pm

kost

kost: Posts : 35
Join date : 2009-05-12
Age : 60
Location : athens

Post n°3

Re: new power transformer

by kost Thu Aug 22, 2013 1:28 pm

Hi Bob
thank you for the fast response!!!
Do you think that now the new transformer can handle both,the new VTA PCB with octal preamp tube and the PSVANE KT88?

warm regards

kost

by sKiZo Thu Aug 22, 2013 1:47 pm

sKiZo

sKiZo: Posts : 1530
Join date : 2013-04-01
Location : Michigan USA

Post n°4

Re: new power transformer

by sKiZo Thu Aug 22, 2013 1:47 pm

Which Psvanes? I recently scored a quad of gently used "HiFi" series KT88's for spares. Their entry level tubes, but from the reviews, that just makes for three layers of excellence ...

Which brings to mind the question ... can I combine KT-120's and KT88's without anything ... like ... exploding? The ST120 has individual bias controls, but that does seem a bit of a stretch. Might be a fun experiment if nobody's ever tried it ... where'd I put my helmet and safety goggles? pirat

by **Bob Latino** Thu Aug 22, 2013 2:37 pm

Bob Latino

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

Post n°5

Re: new power transformer

by Bob Latino Thu Aug 22, 2013 2:37 pm

Kost - "Do you think that now the new transformer can handle both,the new VTA PCB with octal preamp tube and the PSVANE KT88?"

Yes - no problem ..

sKiZo - "can I combine KT-120's and KT88's without anything ... like ... exploding?"

The bias system on the VTA ST-120 can handle any KT88, KT120 or 6550 output tube within the range of the bias controls.

Bob

by sKiZo Thu Aug 22, 2013 2:53 pm

sKiZo

sKiZo: Posts : 1530
Join date : 2013-04-01
Location : Michigan USA

Post n°6

Re: new power transformer

by sKiZo Thu Aug 22, 2013 2:53 pm

Thanx Bob ... I know you were experimenting with the KT120's at one time. Ever try that yourself? If the amp will bias up properly and there's no other concerns, I'd think it'd make for a whole new flavor running one of each in each bank ... yet another fun adventure in rolling ...