Help Needed with a New VTA-70

[CoolOhm]

Hello Fellow AK'ers:

Well I finally went out and got myself a tube amp! I have never heard one before and the curiosity got the better of me. Had my heart set on an Dyna ST-35 but this VTA-70 came up and the price was right... Looking for some opinions and advice prior to powering it up and giving it a listen.

This was allegedly built one year ago by a well reputed tech. in Toronto, but I want to check some things out before I plug it in and try it out. My idea here is to really make sure all is right so that it sounds as good as it can the first time (I want to hear a difference) - if that makes any sense...

So I will check and adjust the bias on the tubes but before I do here are some concerns... Pictures are posted below.

Output Tubes
They are brand new Ruby EL34B-STR. My concern is that googling this tube brings up only discussion from the guitar amp world. Crickets from the audio world. Are these even worth trying given my objective here?

Coupling Caps
The only schematic I can find on this circuit is here. This shows the coupling capacitors as 0.22uF, however inspection of mine shows 0.33uF! Again, are these even worth trying given my objective here? Every one of these amps I see on the net seem to use Russian PIOs in this position. I am not so concerned really about the composition but the value of the 0.33 versus 0.22 is concerning.

Driver Tubes
In Bob Latino's installation manual for this board, the three driver tubes are described as 12AU7 (or 12BH7/5963/5814/6189). However inspection of mine shows three brand spanking new Tung-Sol 6NS7 GTBs. Hmmm. Again, this seems to be a deviation from the design and therefore suspect.

System
I will be using this amp (initially) with my Carver solid state C-1 preamp in my main system which uses some older Heresys with upgraded ALK networks.

Here some pictures below... If you do recommend replacing any of these bits please suggest a source (I prefer Canadian dealers to avoid customs and brokerage issues whenever possible, but this a secondary concern).

Any help much appreciated!

 

[ickyfrog]

I have an VTA ST-70 and love it. More info about your amp here: http://www.tubes4hifi.com/bob.htm#ST70

Tonight I'll check, I did assemble this kit 6 months ago and got the upgraded Russian caps with it. Also I have a complete tube list at home for the VTA 70.

If you care to email Bob Latino by following the link above he is always fast to answer questions about his amps.

My kit came with the EL34 power tubes as standard and I'm about to upgrade to Gold Lion KT66's. Bob says he really likes GL KT66s in the VT-70.

 

[ickyfrog]

Here is the inside of the amp I built. Do your caps look the same? These are the ST70 upgrade caps. Also, the layout should look similar if it was indeed assembled by a tech. Should look neat and organized. Also,the tubes and caps need about 100-200 hours of runtime to start to sound their best.

https://ibb.co/4gs9qhC

 

[bberkom]

Those output tubes should work fine, even if they may not be that popular for hifi use. You can always swap them later, but you gotta start somewhere.

As far as the driver tubes, there is a revised board available for the ST-70 from http://www.tubes4hifi.com/ST70.htm (no affil, scroll down until you see the octal driver board) that uses the 6SN7 drivers. I bet that's what you have.

For the coupling caps, the difference in .22 vs .33 is not something I would worry about too much as far as basic functionality, but it could have some small effect on the frequency response. Others with more knowledge may be able to provide more detail about what specific effects that change might have, but I would still fire it up and give it a listen.

 

[eickmewg]

Have fun with your VTA ST70. Now to your concerns:
Power tubes? No problem whatsoever, but many other fine options exist.
Coupling caps? No problem whatsoever.
Octal driver board? No problem whatsoever. I had an octal driver board in my VTA ST120 that worked great. Many hold the opinion that 6SN7 GTB's are wonderful driver tubes. Only possible problem could be if the Tung Sol's are noisy and many other fine options exist.
SS preamp? No problem whatsoever.

 

[CoolOhm]

Awesome! Thanks guys. I'll just check the bias and hook it up then and let this bad boy rip! The handy bias pins on the front are blanked off (presumably because of the octal board allowing independent bias adjustment of each tube). Does any one know which socket pins I need to probe with my DVM to measure the bias? I have Bob's adjustmentr procedure.

Best regards,

D

 

[CoolOhm]

Just a quick update...

Because this amp has no front sockets adjusting the bias was a hassle. It took me a couple hours (including removing the bottom cover and running four test leads out the front). The bias was way off.

The next night I installed it and drove it through its paces... I was floored! I was expecting and improvement in sound and it delivered. I poured myself some whiskey and started listening some more. I couldn't stop listening to it! It was quite an experience. I am really pleased I took this plunge.

So on my todo list is to install some front sockets to facilitate future bias checking. I will replace some output sockets as well because at least one of them looks original and one of the small connector plates gets pushed out by one of the pins upon tube insertion. At the same time I am considering installing the pentode-triode switches.

Are there any other highly recommended improvements I should make on this first pass?

Best regards,

David

 

[ickyfrog]

6 months ago I assembled a VTA-70.Recently I replaced the stock rectifier tube in my VTA-70 with a Genalex Gold Lion GZ34 and I was shocked by the improvement. Shocked!
Next upgrade for me is 4 Genalex Gold Lion KT66 power tubes. Just ordered em. Bob Latino recommended these tubes for this amp
Keep in mind new tubes will improve during the first 100 hours or more

 

[bberkom]

Installing some external bias test points is a big convenience. It’s doable setting the bias from underneath the amp as you found, but it’s so much easier and safer if you can wire up some leads to the front sockets. Glad you’re enjoying.

 

[airtime]

I'm "assuming" you did not build this kit?? The bias points should be located in the front sockets.

As for the board you mentioned you are using three 6sn7's. Again I'm assuming someone put that "upgrade" board in??? or are those adapters? I'm assuming the 0.22-0.33uf cap difference is from that board change out.

If it's a "stock", 6SN7 driver board and it was built as intended then basically you're DONE. No more upgrades and tweaks needed as long as you have the available capacitor upgrade Bob recommends. Post a pic of the driver board. Let's see if you have the PIO coupling caps and upgraded uf electrolytics.


. The GL KT66's and GZ34 tubes are great with that amp. I'm not familiar which 6SN7 are best so ask around. If you are/can run the small 9 pin tubes then use a EH 12BH7 in the center position and two TungSol 12AU7's on the outer positions.

Also if someone else built the kit do check which ohm speaker wire was chosen to be used.

 

[arts]

Output Tubes
They are brand new Ruby EL34B-STR. My concern is that googling this tube brings up only discussion from the guitar amp world. Crickets from the audio world. Are these even worth trying given my objective here?

The Ruby EL34B-STR is an excellent tube,both in terms of reliability and sound. I have used it in musical instrument and home audio,and have been impressed with it's performance in both applications.It easily handles higher voltage and output situations.
Well made,relatively inexpensive (Chinese) and available at most music stores.

And in my case,an added bonus is that unlike many other brands of EL34,these don't rattle when they receive some mechanical vibration.This mostly concerns usage in MI amplifiers,but I have had some brands that were constructed so ''loosely'' that they were making mechanical noise in home equipment.Not very pleasant for quiet listening!

 

[CoolOhm]

Hi Guys:

Thanks for the input.

Here are the pics... https://imgur.com/gallery/lQkXNXY.

ikifrog: my rectifier is the Genalex GZ34 (don't see any Golf Lion markings though), does this look like yours?

arts: good to hear you like the Rubys. This amp sounds great to me so maybe I'll just live with these for a while.

airtime: you will note from the pics that is the standard octal driver board from tubes4hifi.com. No adapters in there. I kinda wish this was the noval version though because I am noticing 6SN7s are pretty pricey. The other board seems to accept a great variety of tubes, all of which seem more affordable. You will also note that I don't have the PIO upgrade either. But I do think those coupling caps are at least decent quality films.

This amp is basically a set of original OPTs with a new chassis, new power xfo, and new octal driver board. Bob Latino says the board is installed wrong because it should have been underhung. I do have a hum on the right channel unaffected by the gain setting on my preamp or input source selected, sounds like 60 Hz transformer hum. It could be related to the board being above the chassis but I doubt it. I am guessing there is a flaky pin connection so I will wait and see what happens with new sockets. I will install new sockets and move two of the better-looking originals to the front. For my first pass maybe that's it... New sockets and maybe PIO caps.

Thanks again!

Best regards,

David

 

[crispycircuit]

I'd pull the preamp RCA plugs and make up a pair of RCA plugs that are shorted, fire up the amp to see if it is the amp humming? Shorting the input won't hurt a thing. If your amp has a high input impedance that will cause hum. A few things to check

 

[CoolOhm]

Hi guys:

I bought these today locally...

Any good?



Seller tested these and says they are stronger than typical spec for new. Two are matched. Going to put the matched ones in the left and right with the 3rd in center position...

 

[BillWojo]

Those chrome top Sylvanias are nice mil spec tubes and should sound very nice. 6SN7 tubes were designed as a very linear preamp tube and are very popular, you will see them in some high end tube amp builds.

BillWojo

 

[eickmewg]

I will second BillWojo's assessment of those Sylvania 6SN7 tubes. They should be very nice. If I were you, I would not lament the loss of the noval driver board. What you have is a nice setup.