Driver Comparison

[craigtone]

I am in the process of modding my first ST70 and ordered the VTA driver before I found this site. Plans are to refinish the chassis, VTA driver w/Mullard 12AT7's, 40-80-30-20 quad cap, new choke, Series II-esque bias LED system, Groove Tubes' EL34 LS's, and new jacks. I'll keep everyone posted on the progress with pics, etc.

Anyway, I am curious if anyone has direct experience with the VTA and the diytube driver. I like the flat response and low THD of the diytube and am curious how the VTA "stacks up".

I'm a newbie here, so if this is a faux pas topic, just say the word! Yellow_Light_Colorz_PDT_06

My overall goal is to make a Dolby 5.1 home theater system all tube driven. ST70 for front mains, Mark IV for center, ST70 (or ST35) for rears, and Mark II/III for sub.

[mesherm]

I have used Roy Mottram's VTA driver both for ST70 upgrade and in two home built amps and they all sound great. One advantage is that the power supply filter caps are on the board so you can feed the board from the same B+ as the OPTs. I also usedc that same 80-40-30-20 cap and ended up using the 30 section on the rectifier and bussing the 80-40-20 sections to make a 140 after the choke. The VTA also has onboard bias so you can ditch the stock bias network. I mounted all the resistors and small caps on the top side and all the big caps underneath for a cleaner look. There is also an adjustment for balancing the phase inverter in each channel.
Sounds like a great project.
Have fun!
Mike

[craigtone]

Thanks for the feedback! Just curious why you used the 30 behind the rectifier instead of the 40? I was going to use 40 after the rectifier then join the 80-30-20 for 130.

Since you have built this board, how difficult would it be to modify it to work as one bias pot per channel (vs. 2)? For my bias LED circuit to work, I need to tie the output tube bias' together.

[mesherm]

I used the 30 to be a little easier on the rectifier tube. Some of the new 5AR4s are not the same as the old ones. Also I wanted the option of using other rectifier tubes since I had a sweet Mullard GZ32 "coke bottle" in the tube bin.

If you don't have it already, you might download Duncan Amps Power Supply Designer. It is handy for making designing new PS and making changes.

http://www.duncanamps.com/psud2/index.html

As for the bias, it shouldn't be a problem. Just omit the 4 trimpots and the bias circuit components and do a little jumpering to tie each side together to your proposed bias circuit. I don't have the schematic here at work but it shouldn't be a problem.

[craigtone]

WOW! That is all I can say right now ...

Last night I got home and the rest of the parts from Roy at tubes4hifi to build his ST70 board were here! I got out the soldering iron and populated the board. I used standard orange drops for coupling, gold-plates sockets and a set of Mullard 12AT7's. When I got done with that, I decided to finish it up! I installed it along with a new 40-80-30-20 quad cap. I tied 40 to the recifier, then 80+30=110 to B+, then a 2.2K to 20 to Roy's board (the 2.2K is for higher 120VAC vs. older 110VAC). I got it together and the tubes all biased at 0.40V (40mA). EL34's are just some RCA's that came with the amp. I am thinking of upgrading to some New Production Mullards and a JJ GZ34 later unless someone has a Mullard GZ34 they'd sell me for say $50? Yellow_Light_Colorz_PDT_06

Incredible! I spent about 1.5 hours last night listening to music through an iPod of all things! Still waiting on a GOOD phono preamp! I couldn't keep my eyes open any longer so I went to bed. I got up at 9AM this morning and spent 2.5 hours litening this morning.

The changes from the original Dynaco are incredible. The highs are much more pronounced and detailed. The lows ... well ... what can I say. For kicks, I played "My Humps" and the amp never once sagged or got muddy on the bottom end. I am in audio nirvana and doubt I'll get much done this weekend.

For anyone considering Roy's board, I'd HIGHLY recommend it! The dynamic range is much better and detail and bass are back baby!

OK, enough babbling and back to the music ...