Using a ST-70 with VTA70 board and PAS Preamp

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Using a ST-70 with VTA70 board and PAS Preamp

Postby joeztech » Wed Sep 24, 2014 6:30 pm

I imagine that at least some of you have used the Tubes4HiFi VTA70 board in your ST-70. In examining the VTA70 input circuit I see that a 270k ohm resistor is used as grid leak resistor along with a series 10k ohm ahead of that at the input jack. I am using my venerable PAS-3X preamp to drive my ST-70 which currently is operating with 6GH8A driver and phase inverter stages. The combination works quite well.

I am wondering about the difference in the input impedance of the 470k ohm grid leak resistor versus the 270k grid leak resistor used in the VTA70. Will there be a mismatch between the PAS-3X and the ST-70 if I change to the VTA70 board? If so are there recommended changes to the PAS to compensate?

Joe
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Re: Using a ST-70 with VTA70 board and PAS Preamp

Postby Geek » Wed Sep 24, 2014 9:34 pm

Bah, don't worry about it, mate!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Re: Using a ST-70 with VTA70 board and PAS Preamp

Postby petercapo » Sun Sep 28, 2014 4:37 pm

PAS-3X in its original configuration should be good with amplifier Z-in down to 100K-ohms.

Regards,
Peter
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Re: Using a ST-70 with VTA70 board and PAS Preamp

Postby joeztech » Sun Sep 28, 2014 8:15 pm

I just finished modifying the input circuit of the ST-70 using Van Alstine's band-limiting filters. The bass response does sound more defined and solid now. He states that his modifications are good for use with low impedance preamps. According to the specifications on the PAS-3X its output impedance is 1k ohms and the terminating impedance should be 100k ohms or higher. So the Van Alstine modifications should be fine with the PAS too.

It certainly sounds better. I just finished listening to some rapidly moving bass notes in a jazz piece from a performance at Lincoln Center carried by a local FM station and the bass was flawless, plenty of punch but no muddiness at all. The PAS does have new high quality glass boards from Tubes4HiFi with metal film resistors and polypropylene capacitors. The ST-70 does too now and its PC board is still the phenolic type, but metal film resistors and polypropylene capacitors. The coupling caps to the 6CA7 grids are now .68mFd with .1mFd caps in parallel for now. I plan to up those to 1mFd polypropylene caps when I send my next order to Mouser. The ST-70 PC board has been modified to use 6GH8A tubes and that is how it is operating now.

Thanks to everyone!

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