Looking at the ST 70 Driver board but want to know if/what mods will conflict

Hello, I picked up an obviously modified ST70 but I don't know how to identify what mods are installed. I can tell rca's are in the front and left and right set has individual biasing.

I was looking to get the driver board upgrade and wondering if anything would conflict this from being an easy install.

If you did not get a schematic of the modified st70, then disassembling and rebuild as per the dynaco
manual might be a good solution.

It's always a pain in the *** to try to disasseble someone else's build. It will remain a pain until
the schematic is in agreement with the amp.

Hi --- Welcome to the Forum.

Mods that I see on your ST70 are:
(1) --- New Electrolytic Caps on the "Bias Tag Strip" and new coupling Caps on the Driver PCB
(2) --- A new Modern Quad Cap
(3) --- 2 Red Biasing Jacks that are Mounted between each pair of output tubes
(4) --- Removal of the old L/R RCAs and the Stereo/Mono Switch and wiring.
(5) --- Removal of the two Front Panel Tube Sockets and wiring to the Driver PCB.
(6) --- Addition of new Modern RCAs in the Front Panel space that was occupied by the Tube Sockets and necessary wiring to the driver PCB.

=====>>>>> I am assuming that you want to replace the original Driver PCB with Roy's Tubes4Hifi ST70 PCB.

Item (6) wires from New RCAs will go to Roy's PCB
Item (2) Quad Cap will be rewired for Roy's PCB

Item (3) Biasing Jacks and wires will be deleted, since Roy's PCB has 4 Biasing Points, one for each output tube.
Item (1) new electrolytic caps and tag strip will be deleted since Roy's PCB has Biasing Capacitors added.   <<<==== see the colorized schematic below.

If you go to http://www.tubes4hifi.com/ST70.htm - there is a link for reworking the ST70 for Roy's PCB and a colorized schematic diagram of the VTA 70.  
If link disabled, message Roy and request Link Activation.

Any questions, just ask.   Also compare your unit with pictures of original ST70s and see if there are any other differences.

Stew

Looks like you have a 5V3 or 5V3A rectifier tube. Hope it's a 5V3A as it has a signficantly lower filament current than a 5V3, 3.0 vs. 3.8 amps. I don't know how much filament current the early Dynaco power transformers can handle.
i've been running the same 5V3A in my ST 120 for a few years now with excellent results.

stewdan wrote:Hi --- Welcome to the Forum.

Mods that I see on your ST70 are:
(1) --- New Electrolytic Caps on the "Bias Tag Strip" and new coupling Caps on the Driver PCB
(2) --- A new Modern Quad Cap
(3) --- 2 Red Biasing Jacks that are Mounted between each pair of output tubes
(4) --- Removal of the old L/R RCAs and the Stereo/Mono Switch and wiring.
(5) --- Removal of the two Front Panel Tube Sockets and wiring to the Driver PCB.
(6) --- Addition of new Modern RCAs in the Front Panel space that was occupied by the Tube Sockets and necessary wiring to the driver PCB.

=====>>>>> I am assuming that you want to replace the original Driver PCB with Roy's Tubes4Hifi ST70 PCB.

Item (6) wires from New RCAs will go to Roy's PCB
Item (2) Quad Cap will be rewired for Roy's PCB

Item (3) Biasing Jacks and wires will be deleted, since Roy's PCB has 4 Biasing Points, one for each output tube.


Any questions, just ask.   Also compare your unit with pictures of original ST70s and see if there are any other differences.

Stew

Thanks

Looks like the picafarads were removed as well. To be honest with you I would replace the current board with a new one. You can replace it with the VTA board if you like but I have always preferred the original board. I have assembled more of these than I can remember. Please don't get me wrong, Bob has offered many products and designs that have helped the Dynaco line without a doubt. He gave us an option to replace the 7199 tube that has helped many. I have modified the original circuit in 3 ways. I use the VanAlstine bandwidth filter at the inputs. This has worked great at eliminating any mud in the base response. 2. I then replace the .1uF coupling capacitors with a Janzen Z series 1.5 uF. 3. I replace the 7199 tube with a Japan manufactured 6GH8A and use an adapter that I made and sold for about 3 years that makes this amp sound brilliant! I purchased 500 of these tubes and have a couple of hundred left. I replace the picafarad caps with polystyrenes. I have a couple of boards that were made to use the 6GH8A tube but I have not built them yet. You can get the schematics for this amp in many places on the web. I think the one that helped us the most providing the best tube gear was David Hafler himself. I use the Hafler DH 110 preamp on my system. This amp by the way sounds like crap until you modify and replace certain caps and remove the muting circuit. FYI, you can purchase a 6U8 to 7199 adapter on one of the websites out there. I can not compete with their price for the adapter. It has the same pinout as the 6GH8A. Good luck with this project. Regards, Jamey.

These are my before and after pics.  The previous owner had done alot of messy mods.  I undid some of those then did some more of my own.

Finally last year I pretty much gutted it down to the chassis and transformers.  Also kept the rca's and speaker binding posts.  Then I did Roy's VTA board mod, diode mod, new quad cap, and triode/pentode switches.

Works reliably and sounds great.

Good work!
I noticed that you isolated the 4 "russian caps" , i did not Some other day i will isolate them, but as
long as no other trouble comes with the amp i guess it will remain without isolation)

Jbacik wrote:Looks like the picafarads were removed as well. To be honest with you I would replace the current board with a new one. You can replace it with the VTA board if you like but I have always preferred the original board. I have assembled more of these than I can remember. Please don't get me wrong, Bob has offered many products and designs that have helped the Dynaco line without a doubt. He gave us an option to replace the 7199 tube that has helped many. I have modified the original circuit in 3 ways. I use the VanAlstine bandwidth filter at the inputs. This has worked great at eliminating any mud in the base response. 2. I then replace the .1uF coupling capacitors with a Janzen Z series 1.5 uF. 3. I replace the 7199 tube with a Japan manufactured 6GH8A and use an adapter that I made and sold for about 3 years that makes this amp sound brilliant! I purchased 500 of these tubes and have a couple of hundred left. I replace the picafarad caps with polystyrenes. I have a couple of boards that were made to use the 6GH8A tube but I have not built them yet. You can get the schematics for this amp in many places on the web. I think the one that helped us the most providing the best tube gear was David Hafler himself. I use the Hafler DH 110 preamp on my system. This amp by the way sounds like crap until you modify and replace certain caps and remove the muting circuit. FYI, you can purchase a 6U8 to 7199 adapter on one of the websites out there. I can not compete with their price for the adapter. It has the same pinout as the 6GH8A. Good luck with this project. Regards, Jamey.

I find the  "VanAlstine bandwidth filter" an intriguing idea (is it bandpass, not bandwidth?).  I wonder if it would improve the bass on Bob's ST-120?  I have that with the auto bias board and improved caps.  I have not found but one possible schematic of the filter.  I use a ST-9 preamp which really helps as to the eq issues I have found with many LPs (not to mention that I don't have the best speakers but have worked a lot on them).  I have made many adjustments to get good clean rich music and probably could do more with my speakers.  I was wondering about trying the filter to see if there are any significant improvements.

It is called a bandwidth filter by VanAlstine. I took the calculation and put it into a spread sheet so I could dial the values of the resistors to match the cut in and cutout frequencies. I could send you a copy if you like. It is fairly simple. I used some small boards the Radio jap used to cary. Not sure if you can still get them. They are 2x2 inches I believe. I can send the spread sheet with the values that I use already in the cells. It will spit out the range as you change the values. It will show the low and high frequencies. Just send me a PM if you would like. I think my e-mail is in there. Regards, Jamey.

Jbacik wrote:It is called a bandwidth filter by VanAlstine. I took the calculation and put it into a spread sheet so I could dial the values of the resistors to match the cut in and cutout frequencies. I could send you a copy if you like. It is fairly simple. I used some small boards the Radio jap used to cary. Not sure if you can still get them. They are 2x2 inches I believe. I can send the spread sheet with the values that I use already in the cells. It will spit out the range as you change the values. It will show the low and high frequencies. Just send me a PM if you would like. I think my e-mail is in there. Regards, Jamey.

Please send the spread sheet and values you used. I have plenty of prototyping boards and supplies. You can the grid boards of all sizes at Amazon now day. All the caps and resistors too.

definitely helps with the 60 year old stock Dyna design, it limits bandwidth to about 15Hz to 30KHz, which is pushing it with a 7199 tube, which has marginal gain and bandwidth.
I measured all that 30 years ago, it needs ALOT of negative feedback to work above 5Khz (yes, that's correct) or below 100Hz.
The VTA70 driver with it's modern design can easily handle 5Hz to 100KHz flat response with full gain, but of course you can't hear anything below 20Hz or above 20KHz.
but it doesn't mess up anything else with limitations

After switching to the 6GH8a, this is when I could definitely could hear the muddyness in the bass on the ST70. The way I got the idea to use the 6gh8a was many years ago. I had an HH Scott LK48 that used the 7199. I then acquired a 222C which used the 6gh8a tube. So I knew that the 6gh8a would work. Again this was in the 90's. I researched this and found that only 3 pins needed to be re arranged. The gain was much greater than the 7199 and they were getting harder to find as Sovtec was not producing these anymore and needed a replacement. I made the adapter and went from there. It was only after the internet became available that I even knew that you had another solution. So this is when I looked at the VanAlstine filter and placed the calculations in a spreadsheet. I changed the values until I got to a value of 20hz to 20khz and this solved the muddy problem. I have used this on all my ST70 rebuilds since. They all sound like a hundred watts after all this when paired with the right speakers. I use the Cerwin-Vega D-3's. I also use an audio compressor which the tubes handle better than solid state if you ask me. Regards, Jamey.

I have an ST-120 with Technics SB-A53 speakers that I modified with compartment reinforcement, acoustic insulation and crossover that I calculated with parts from Parts Express (basic crossover board).

My wife has complained about the size of the these speakers but, the sound is phenomenal!  If I downsized to something like the Cerwin-Vega D-3, I am wondering how good the the sound quality.  
I found a pair locally for $100 (which is the same price I initially paid for the Technics).  Do you think it might be worth a try?

For a hundred bucks, get them while you can!!! These sound great!!! If you have a problem with the 10 inch surrounds, I may have some or I should be able to find them. I replaced my surround foam with the original orange color from Orange county speaker in CA. They were bought out by GLS. I am not sure we can get the surrounds from them but we should be able to find a pair of black ones. Regards, Jamey. P.S. Make sure the other components work as well before you buy.

Surrounds?  Are you talking about the rubber or foam gasket between the speaker and the case for mounting like these?
https://www.amazon.com/Cerwin-Vega-Speaker-Surround-Repair/dp/B00D4BAWDM

That is them. If you are not familiar with re edging a speaker, I used to cone speakers. It will need to be replaced very carefully. If you would like my assistance, let me know. This includes sending them to me if needed. No charge. I also hook them up to a signal generator to help center the cone once installed. Regards, Jamey.

Make sure they include the gaskets! Jamey.

Just now, those speakers were pulled. Any other suggestions from Cerwin-Vega or anyone else?

The woofers were removed?

No the speakers are not listed anymore. They probably sold them. Re-edging a speaker I have done many time. As far as cone, I did not think that that could be done. At that point I usually replace the whole thing.

I worked for a company here locally that re coned speakers. The coils, diafram, cone, gaskets, and the center covers. This was back in 1998. Jamey.

FYI, I just googled and came across https://reconekits.com/ No affiliation or experience with these guys

Pre assembled kits. Interesting.