Bob Latino ST-70 Kit Project

[1MOR]

I have completed a few upgrades. The driver tubes are now NOS Mullard CV 4024s with a RCA black plate in the center position. Jim McShane sent me a quad of reissue GL KT66s, which now have about 30 hours run time. I have replaced the Chinese rectifier with a Mullard GZ34 sourced from the auction site.

Like Bob Latino, Jim McShane was a pleasure to deal with. The GL KT66s are a worthy upgrade from the Chinese EL34Bs.

Glad to hear that my experience with this amp matches the other ST70s and ST120s that have been referenced here.

I guess its time to upgrade the ICs!

 

[kaplang]

Let me jump in here and mention that about 6 months after assembling my ST70 from Bob Latino it started blowing the master fuse for no apparent reason. It was working great before the problem and just started all of a sudden. I have to say that Bob was great and stood behind his product all the way. We spent some time on the phone trying to trouble shoot and he decided to send me some new coupling caps. It was not actually determined that new caps would solve my problem but it did and the parts were supplied under warranty. It has been another 6 months and it is still going strong. Thanks Bob! :thmbsp:

 

[Bob01605]

Let me jump in here and mention that about 6 months after assembling my ST70 from Bob Latino it started blowing the master fuse for no apparent reason. It was working great before the problem and just started all of a sudden. I have to say that Bob was great and stood behind his product all the way. We spent some time on the phone trying to trouble shoot and he decided to send me some new coupling caps. It was not actually determined that new caps would solve my problem but it did and the parts were supplied under warranty. It has been another 6 months and it is still going strong. Thanks Bob! :thmbsp:

Hi George,

Those coupling caps are 630 volt rated and they (in an ST-70) look at maybe 275 volts on one side and about NEGATIVE 35-40 volts on the other side. That's a voltage swing of a little over 300 volts and a 630 volt rated main coupling cap *should* be able to handle that with no problem - BUT - stuff does happen sometimes that no one can predict. Fortunately when your coupling cap "bit the dust", it only blew the fuse and didn't take out anything else in the amp.

Best regards,

Bob Latino

 

[yungin's-dad]

Dang 1MOR! Beautiful work, beautiful kit, beautiful amp!!! If my skills were a little sharper I'd attempt one of these myself. Good to hear people like Bob are still out there. Maybe some day I'll try my hand at one of his kits.

Great job!!!

Ron

 

[Isotonic]

I have lusted after one of these for a while. Great thread. Love the images. I agree that this is one of the best values in audio.

 

[Keith C]

If he's going to go to the bother of building an amp kit, why a Stereo 70 when there are so many Stereo 70s that still turn up cheap, and the design has so many issues that could be corrected by going to another design.

 

[LinuxGuru]

Let me jump in here and mention that about 6 months after assembling my ST70 from Bob Latino it started blowing the master fuse for no apparent reason. It was working great before the problem and just started all of a sudden. I have to say that Bob was great and stood behind his product all the way. We spent some time on the phone trying to trouble shoot and he decided to send me some new coupling caps. It was not actually determined that new caps would solve my problem but it did and the parts were supplied under warranty. It has been another 6 months and it is still going strong. Thanks Bob! :thmbsp:

As I have seen on photo, you are using Russian-made K40Y-9 paper-in-oil coupling caps (to output tubes), which are now about decade old. Although majority of them still working OK, their shelf lifetime (stated by manufacturer) is over long time ago. If they started to leak, BIAS voltage will drop and output tubes will overheat. If you are keen to use NOS Russian stuff, please consider FT-3 teflon caps, they are almost eternal.

PS. The blowing fuse may also mean bad power supply filter cap, or bad rectifier tube (e.g. arcing between cathode and anode).

 

[Bob01605]

LinuxGuru,

The original poster > 1MOR is using Russian PIO caps as seen in the photo. The cap that failed, however, was a .22 uF @ 630 volt SBE 716P new production cap from poster > kaplang. Again, it was NOT a Russian PIO cap that failed but a new production cap.

I test every one of the Russian PIO caps, hit them with some voltage and then put them in matched sets of four. I have not as yet had a failure of one of the Russian PIO caps. The Russian caps are also 1000 volt rated. I understand your liking Russian FT-3 teflon caps. They are excellent sounding caps. Unfortunately the .22 uF @ 600 volt Russian FT-3 teflon cap at about 3 1/8 inches long and 1 1/4 inches in diameter is just physically too large to fit into the area available on the VTA driver board.

To Keith C Re: > "If he's going to go to the bother of building an amp kit, why a Stereo 70 when there are so many Stereo 70s that still turn up cheap, and the design has so many issues that could be corrected by going to another design."

I believe with your statement above that you are working under the assumption that this amp kit uses the original circuit design and original spec components. The design philosophy of this amp kit was to make the amp LOOK LIKE an older original ST-70 amp yet have a modern circuit and upgraded components so that it would SOUND LIKE a modern amp.

Bob Latino

 

[guiller]

Congratulations for your successful project, and thanks for the pics!

 

[erlingt]

I ordred one too

Nice project. Inspired by this, I ordered a ST-70 at Bob Latino today.

So now it's just waiting for the packet to arrive in Denmark

 

[1MOR]

Nice project. Inspired by this, I ordered a ST-70 at Bob Latino today.

So now it's just waiting for the packet to arrive in Denmark

This is what AK is all about, sharing and inspiring! Take your time, check everything twice. I measured each of the resistors before stuffing into the board or mounting PTP. Pick up some extra fuses, that way you will not need them!

I picked up more 12AT7s to try in the center tube position. The early Amperex 7062 tube is impressive. I have a Holland Bugle Boy with an early 60s code working now and so far it has been outstanding. It has a strong getter flash at turn on, so I suspect it may be a Mullard.

 

[WhiteSE]

Well, maybe one of these could be my first DIY amp...I hope Bob L. doesnt hate the phone calls to come...lol

 

[vincei]

Very nice! There is certainly a great satisfaction by building it yourself as you know what exactly went into it and of course, you can also enjoy the sonic results of all your hard work!

 

[JHoman]

This is what AK is all about, sharing and inspiring! Take your time, check everything twice. I measured each of the resistors before stuffing into the board or mounting PTP. Pick up some extra fuses, that way you will not need them!

I picked up more 12AT7s to try in the center tube position. The early Amperex 7062 tube is impressive. I have a Holland Bugle Boy with an early 60s code working now and so far it has been outstanding. It has a strong getter flash at turn on, so I suspect it may be a Mullard.

I replaced my Mullard 4024 center tube with a Telefunken 12AT7, sweet sound. Also running Ruby KT88s and a 5U4 GA rectifier. I love my ST120!:thmbsp:

 

[wallacefl]

Yeah, those are nice, I just wish that there were other chassis options besides the original...I hate the power switch on the back and the inputs on the front, plus the big octal sockets that were meant to drive a preamp setup gaping at you from the front....nope...don't like that at all...

Which is why my two original ST-70's sit in a closet...at least my VTA-70i has the inputs on the proper side, the back...too bad the power switch on it isn't in the front...it's no fun reaching across a hot amplifier in the stand and fumbling to get to the power switch....I've scorched my arms more than a few times doing just that...

If I were to rebuild my ST-70's I would also go with the Tube4HiFi VTA board, I have heard what a fresh front end and 7199's sound like compared to even my VTA-70i's 6GH8A and it's improved over ST-70 power supply, the stock ST-70 is just not in the same league as the 70i or the better boards....which is another reason why my ST-70's sit in a closet....

You did a nice job, and I'm sure you'll enjoy the heck out of them!

.

agree with the above ...they need another chassis option for a more modern look...a great example is one done by "Wardweb" on Diy tube and the Martin Logan owners site...centering the rectifier tube and placing the can capacitor under the chassis really helps with the looks...I would pay an extra $200 for this as an option chassis...BTW I heard the VTA board on a friends rebuild and

 

[Bob01605]

"Originally Posted by RickB
Yeah, those are nice, I just wish that there were other chassis options besides the original...I hate the power switch on the back and the inputs on the front, plus the big octal sockets that were meant to drive a preamp setup gaping at you from the front....nope...don't like that at all..."

To RickB and Wallacefl,

First of all I do respect everyone's opinion and their right to give that opinion regarding the design concept of the Dynaco VTA ST-70 kit. The general design concept was to have the amp LOOK LIKE an original Dynaco ST-70 amp yet SOUND LIKE a modern tube amplifier without the design compromises that were made on the original amp that affected the amp's sound quality. Something on the order of a redesigned '57 Chevy that still looks like a '57 Chevy but with a modern motor.

Re: the tube sockets on the front of the amp. Two pins on each socket are used as bias measuring points. If you remove the sockets then you still need two bias measuring points on the front face of the amp. Some use a backing board here with two or sometimes three (an extra one for ground) "test pin" outlets. It is less costly and just much simpler to put a tube socket here than to have separate backing boards and two or three test pin outlets.

Re: "Why don't you have an IEC connector instead of a captured cord". The power transformer is "sunk into" the chassis and you only have about 5/8 of an inch of space between the chassis and the sunk in portion of the power transformer. Most IEC connectors require 3/4 of an inch into the chassis (not enough space). Move the power transformer forward a little ? possibly. Move the IEC further to left or right ? Now you move into space occupied by the power switch or fuse post. The captured cord works fine here as it did on 350,000 of the original ST-70's.

Re: "I don't like the power switch on the back and inputs on the front". You CAN remove and bypass the STEREO/MONO switch on the front and install the AC power switch in its place. You then run the AC wiring around one side of the amp to your new power switch location. I recommend the left side because then the AC wiring won't cross in front of the two RCA input connectors. You can also plug the ST-70 into a switched power strip that is near the front of the amp and use the power strip to turn the amp off and on. Re: Inputs > You could drill two holes on the back face of the amp and rewire the inputs to the rear but an easier solution is to just use right angle connectors on the front face of the amp and run the RCA interconnects around the side of the amp.

Probably no one can design an amp that would be acceptable to all users and usually compromises have to be made at some point.

Bob Latino

 

[erlingt]

One big box from the US arrived today and everything looks fine

 

[131east23]

Ahhhhh... I love the look of these amps. I am slowly saving for one of these kits. It's what my Cornwalls tell me they want... Maybe around early winter I'll be hitting Mr. Latino up for one of these. Very inspiring!

 

[erlingt]

Finaly, I have finished assembling the ST-70 kit. It was a pleasure to build, and it is a pleasure to listen to.

A few pictures of the product:

 

[Wolf012558]

Nice photos ... I also have one of Bob's ST-70 amps that I built from a kit a couple of years ago. It still works great. I notice on one of your photos that you have the stepped attenuator volume control that Bob now offers as an option with the amp kits. I was thinking of adding this to my ST-70 amp? Does the stepped volume control seem to work well with this amp? CharlieW