Why not. I like studying the old schematics of yesteryear and many of those builds used very high plate voltages. You have to be careful of 2nd grid getting to high. I am pushing the max rating on the 2nd grid a few volts but willing to take the risk. Just a few.
ST-70 Tube Amplifier DIY Kit
- Thread starter wdecho
- Start date
The plates can take a lot of volts. It is the grids that one has to be careful with. I learned this watching the presentation by Roger Modjeski at the Burning Amp Festival 2015 in the year 2015. The video is online.
I recently bought some 1625 tubes on Ebay. Roger at that presentation said he would have no problem running a 807 tube at a 1,000volts with the plate cap. The 1625 is a 807 with a 12volt filament. Just received them yesterday and guys they sure made some pretty tubes back in the day. I do not know if I will ever do anything with the quad I bought but they should be displayed. Gorgeous tubes.
VTA guards their schematic. With the driver board one gets a partial schematic but it is not hard to figure out the rest. The instructions describe the changes needed in the Dynaco schematic for the board to work. Not a lot of changes to the Dynaco ST70 schematic, just a few involving the use of 4 pots for bias and having the filtering caps for the VTA board on the board. VTA actually lowers the bias of the output tubes somewhat.
I checked the Dynaco schematic and it uses a 1K resistor in the FB circuit, a lot of feedback. Much more than the VTA circuit which uses a 6.8K resistor, less FB so I do not believe the problem I am having trying to make this build produce 35 watts is in the FB circuit. So for now I am going to leave it be and be satisfied with what I have. There is always a point in any build when you say enough is enough. I do still believe the problem is in the iron, more so in the OPT's but still possibly the PS transformer. All the mathematic calculation formulas and understanding them whether the PS transformer is capable of running this amp as per a Dynaco ST70 is over my head and at 77 years of age not something I want to get into anymore than I have to. The guy at Antek says it will but then they put a really small PS transformer in the kit originally that I do not believe is anywhere near able to run the kit at Dynaco ST-70 specs. Remember we are talking about a larger transformer I bought that did not come with the kit. All that being said I am leaning towards the problem being oscillation being caused by the toroidal OPT's just because the way the bias starts wandering around all over the place in all the output tubes when trying to raise the bias to 40ma. If I wanted to pursue this further I would hook up a scope while trying to raise the bias to 40ma and see exactly what is happening but for now I am going to be happy with what I have. I have a very nice 25 watt PP EL34 amplifier that test and sounds really good to these old ears.
Many thanks guys for the input and feel free to discuss it further if you wish. This thread was started for the new kit using EI transformers and not the old one. I am supposed to receive it tomorrow and will post my thoughts of the appearance of the iron and chassis later. I believe it will be fun comparing the sound between the two if I am successful at building it. Never know at my age with health and mental ability deteriorating.
@wdecho
Sorry for side-tracking your thread to explore the toroidal kit’s ssues. Good luck with your new classic style St70 build. Please keep us informed.
I hope so too, but with Antek being an importer of Chinese magnetics i do not expect that. Do you have any more information on the output transformers, other than Antek calling them A-470.
Sorry for side-tracking your thread to explore the toroidal kit’s ssues. Good luck with your new classic style St70 build. Please keep us informed.
I hope so too, but with Antek being an importer of Chinese magnetics i do not expect that. Do you have any more information on the output transformers, other than Antek calling them A-470.
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No apology necessary. I am glad you suggested a possible cause. This is what forums are for.
As far as more information on the EI kit, no more than what is on the Ebay listing. Not much to go on other than trusting Antek to provide quality transformers. Counting the transformers I have purchased from them the total including the two kits is 20 according to my completed orders list. They sell really good transformers.
If they are Chinese built I hope and am trusting they are the same quality and specs as the ones sold to the other sellers of Dyanco ST-70 kits. Who really knows who builds the transformers for the other sellers. Three that I know of and of the 3 one has stopped taking orders, one has a 2 month waiting period and the other almost twice the price for their kit verses the other two. This kit, if good iron, is not that hard to build if one buys and stuffs the VTA driver board which is not that hard to do. Only thing left is building the PS circuit and installing the few parts to make the output tubes to work. I have never been a fan of tube board builds. All my other tube builds have been point to point but the board does make the build much easier.
Hey if one just wants a PP EL-34 amplifier there are some really decent Chinese retail offerings for around the same price, if not less, than building any of these kits. But where is the fun in that?
Lavane
Lunatic Member
Not really. A complete schematic is sent with the assembly manual for the VTA ST-70 amplifier kits.
I didn't get one with the Antek kit I ordered with board from tubes4hifi. I also ordered a new board kit from tubes4hifi.com with the 6sn7 tubes already made up and it has been crickets from them. In all fairness they did say it would take a bit to build boards as they are behind. I would have built it myself but the last board from them had zero instructions and I have been building it from pics and all the parts were just thrown in a couple of bags, no markings at all. I had to use the color bands and my meter to check the resistors to figure out where they went. Hey it was good practice for amp building but bad practice for a kit seller. I asked him for the drawings when I ordered a new board and a new made-up board with the 6sn7 tubes, he never responded. I think after I build these and get them working it will be the end of my dynaco amp usage. Will dump them off and move on. I got a bad taste from their poor service and poor response time of never responding back even after spending plenty of money. I also ordered their upgraded caps for the boards.
whoaru99
Epic Member
Help me understand this.
I look at the images in the two links in the first post and the driver boards don't look anything like the VTA/tubes4hifi board I have.
Is the implication you can get the genuine VTA/tubes4hifi board with the Antek kit by special request, or what am I missing?
I bought my first board kit off Ebay where he has store and sent him a question with no response either. I figured it out by the parts list but I would call it poor customer service. The part in question is not on the schematic he sent me. Others have complained as well about not getting a response when contacting him. I assume too busy to answer questions. He does ship in a timely manner. I just bought another kit for this new EI build and it was shipped 2 days after placing order.
I asked the same question on the Dynaco forum and got a quick response from Bob Latino the original designer and previous owner of tube4hifi amplifier. Dan has changed the design from what Bob designed. Dan Willoughby is the guys name that bought out tube4hifi. There are some similarities between the Dynaco ST-70 and the VTA clone but there is so much that is different I would not even call it a Dynaco ST-70. The true Dynaco St-70 is being sold by Dynakit. I did contact them about purchasing a kit and was told at that time it was a 2 month waiting period. I just had another heart attack in March, ambulance, 2 stents implanted and did not want to wait that long. Who knows how long old people have. That and the Antek kit came up for sale.
The VTA amplifier has the appearance of a Dynaco but that is about it as far as I am concerned. More like an improved Dyanco design, in some areas. Some may even say the original Dynaco St-70 has it's roots in the Williamson amplifier.
The reasons I wanted to build a PP EL-34 amplifier is because before this one the only other PP tube amplifier I have is a class A EL84 among my many SE tube amplifiers. That and I love the sound of my SE EL-34 amplifier. I have a SE KT-88, a SE KT77 and a SE 6L6 amplifier and prefer the EL-34 tube better.
I have also built most of Nelson Pass' clones he has published at diyaudio.com. This Antek ST-70 amplifier sounds very similar to one of his PP class A mosfet amplifiers. Nelson has made a career getting a SS amplifier to sound like a tube amplifier. Mosfets have the same curves as a pentode tube. The reason they sound similar.
I asked the same question on the Dynaco forum and got a quick response from Bob Latino the original designer and previous owner of tube4hifi amplifier. Dan has changed the design from what Bob designed. Dan Willoughby is the guys name that bought out tube4hifi. There are some similarities between the Dynaco ST-70 and the VTA clone but there is so much that is different I would not even call it a Dynaco ST-70. The true Dynaco St-70 is being sold by Dynakit. I did contact them about purchasing a kit and was told at that time it was a 2 month waiting period. I just had another heart attack in March, ambulance, 2 stents implanted and did not want to wait that long. Who knows how long old people have. That and the Antek kit came up for sale.
The VTA amplifier has the appearance of a Dynaco but that is about it as far as I am concerned. More like an improved Dyanco design, in some areas. Some may even say the original Dynaco St-70 has it's roots in the Williamson amplifier.
The reasons I wanted to build a PP EL-34 amplifier is because before this one the only other PP tube amplifier I have is a class A EL84 among my many SE tube amplifiers. That and I love the sound of my SE EL-34 amplifier. I have a SE KT-88, a SE KT77 and a SE 6L6 amplifier and prefer the EL-34 tube better.
I have also built most of Nelson Pass' clones he has published at diyaudio.com. This Antek ST-70 amplifier sounds very similar to one of his PP class A mosfet amplifiers. Nelson has made a career getting a SS amplifier to sound like a tube amplifier. Mosfets have the same curves as a pentode tube. The reason they sound similar.
whoaru99
Epic Member
I'm talking very specifically about appearance of the driver boards.
A "Dynaco style" driver board only has two tubes. VTA/tubes4hifi driver board has three tubes.
The images in the first two links represent with two tubes on the driver board, so where do the tubes4hifi boards come into play re. the Antek kits?
Were they (VTA/tubes4hifi board) an option from Antek, are the images not matching actual product, or are they obtained through completely separate, non-Antek related transactions?
A "Dynaco style" driver board only has two tubes. VTA/tubes4hifi driver board has three tubes.
The images in the first two links represent with two tubes on the driver board, so where do the tubes4hifi boards come into play re. the Antek kits?
Were they (VTA/tubes4hifi board) an option from Antek, are the images not matching actual product, or are they obtained through completely separate, non-Antek related transactions?
No, the board that comes with the Antek kit is worthless as far as I am concerned. I could not get it to run. Tried for weeks to make it work. Found myself trying to be a designer which I am not. At that time though I was trying to get the driver board with the transformer that came with the kit to work. The voltages were way to low to run the supplied board under load. The supplied transformer is a 300V one and what is needed is a 360V one. Even with the larger transformer I had terrible hum and at times oscillation with the supplied board. In my opinion poor ground layout in the supplied board. There is no way whoever designed that board and put the kit together ever built that amplifier and made it work, in my opinion. The supplied schematic had no voltage measurements to go by. I have a basic understanding of the voltages needed for input and output tubes to work and started changing parts trying to make it work. After a few weeks I gave up on both of the supplied boards. Nothing inherently wrong with the PS board that I found but did not use it because the supplied PS board has the negative bias circuit on it and the VTA driver board has it on it as well. That and the PS capacitors for the input are on the VTA board as well. I hope this makes some sense. I have a tendency to ramble.
(Were they (VTA/tubes4hifi board) an option from Antek, are the images not matching actual product, or are they obtained through completely separate, non-Antek related transactions.)
No option. Completely separate. The only similarity is using the 6GH8A tube like Dynakit sells being the 7199 tube is expensive and hard to find. Completely different circuit and layout, I assume. Never have seen the Dynakit 6GH8A board in person. Others in the past have designed and built a replacement 6GH8A driver board for the Dynaco.
NOS SOUND VALVES / DYNACO Bare/Blank 6GH8A Driver Board (STEREO 70, ST70, ST-70) for one. Being sold right now on Ebay. I bought one but like the advantages of individual bias of each output tube and the 12AU7s tube on the VTA driver board. Both are proven designs many have built and worked fine. Cannot say that about the Antek board offering. Take someone smarter than me to get that kit to work as is being sold.
Mine came with a tubes4hifi.com board and the loose parts to build the board. The board has the name right on it and is the same as those sold on tubes4hifil.com website. They have three tubes and is the exact board that is on their webpage when you go to their homesite. This board doesn't need to be redesigned as it is suppose to sound great just the way it is. You can upgrade it if you want but it works just the way it is.
I have not finished soldering everything yet, so some parts are not included in the pic.
I have not finished soldering everything yet, so some parts are not included in the pic.
Which kit are you talking about? The VTA kit or have you bought the Antek kit with the toroidal transformers like myself?
Looking good stuffing board.
My advice, start with 15K resistors for R37 and R38.
I purchased the one with the toroidal transformers, and it came with the black plate to cover where the board is with two holes for driver tubes but the board I received is the one in the picture above, it is a genuine tubes4hifi board.
This is not really a kit to build, it was really just some parts to get started. But board is a perfectly good board but it would seem to me if your selling boards to upgrade an amp with you should provide drawings.
This is not really a kit to build, it was really just some parts to get started. But board is a perfectly good board but it would seem to me if your selling boards to upgrade an amp with you should provide drawings.
whoaru99
Epic Member
I was thinking about popping for a couple of the Antek KT88 monoblock kits. Had them in my cart even, but looking at the BOMs and schematics started having questions.
Besides, I have all the parts to make a pair of KT88 PSE UL monoblock amps from Kegger's schematics using Edcor OPTs and Antek PTs.
Besides, I have all the parts to make a pair of KT88 PSE UL monoblock amps from Kegger's schematics using Edcor OPTs and Antek PTs.
That is good news. I bought mine a month or more ago and it did not come with the VTA board.