Low voltage power transformer

Hi everyone,
This is my first post. I have found the more I learn about the ST 70 the more questions I have. My ST 70 is mostly built from old stock I purchased from Stereo Cost Cutters in the mid 80's. They didn't have any power transformers so I bought one from a highly regarded name in tube electronics (he is still in business). This replacement transformer doesn't have a 5 volt tap for a 5AR4 which he claimed was better because this tube was getting rare and expensive.
So for the next few years I tried a couple of different driver boards and even splurged on a SDS Labs cap board but the amp never sounded quite right so I put it in storage. A couple of weeks ago I pulled it out of storage and installed a Diytube driver board (the one that uses two 12au7's and one 12ax7.) The manual can be found at tubezone. (I have the link but I am too new to post links.) The documentation that came with this board has a voltage chart and that's when I discovered the real problem. The power transformer only puts out 630vct instead of the 720vct for a
PA060. So now I have many questions. On the driver board J19 should be 300v and J20 should be 350v. Both were 50v lower so I changed the 6.8K power supply resistor to 3K and that brought the voltage back up to where it should be. There is only 390v on pin 3 of the EL34's and I'm using the 15.6 ohm resistors. My EL34's are original Dynaco and I would like to keep the bias current on the safe side. How do I figure this out? What value would a safe current be? 35ma or should it be lower? The two pots on the driver board (R29 and R30) are supposed to be adjusted for zero volts but the best I can get is minus .9 volts. How important is this setting? Can I change values on the driver board to get it closer to zero? I would be grateful for any advice. Maybe I need to buy a new power transformer? I wouldn't mind lower wattage if it can be done properly.
Thanks,
Gary

gwvenus56 wrote:Hi everyone,
This is my first post. I have found the more I learn about the ST 70 the more questions I have. My ST 70 is mostly built from old stock I purchased from Stereo Cost Cutters in the mid 80's. They didn't have any power transformers so I bought one from a highly regarded name in tube electronics (he is still in business). This replacement transformer doesn't have a 5 volt tap for a 5AR4 which he claimed was better because this tube was getting rare and expensive.
So for the next few years I tried a couple of different driver boards and even splurged on a SDS Labs cap board but the amp never sounded quite right so I put it in storage. A couple of weeks ago I pulled it out of storage and installed a Diytube driver board (the one that uses two 12au7's and one 12ax7.) The manual can be found at tubezone. (I have the link but I am too new to post links.) The documentation that came with this board has a voltage chart and that's when I discovered the real problem. The power transformer only puts out 630vct instead of the 720vct for a
PA060. So now I have many questions. On the driver board J19 should be 300v and J20 should be 350v. Both were 50v lower so I changed the 6.8K power supply resistor to 3K and that brought the voltage back up to where it should be. There is only 390v on pin 3 of the EL34's and I'm using the 15.6 ohm resistors. My EL34's are original Dynaco and I would like to keep the bias current on the safe side. How do I figure this out? What value would a safe current be? 35ma or should it be lower? The two pots on the driver board (R29 and R30) are supposed to be adjusted for zero volts but the best I can get is minus .9 volts. How important is this setting? Can I change values on the driver board to get it closer to zero? I would be grateful for any advice. Maybe I need to buy a new power transformer? I wouldn't mind lower wattage if it can be done properly.
Thanks,
Gary

The power transformer seems to be common to most of your problems, maybe you
should start with installing a genuin PA60 or one of the equivalents ?

Hi,
That would be the ideal solution but I live in Canada and would have to order one from the U.S. After rate of exchange and customs it would be over $300! The transformer I have now would end up in a landfill. I'm hoping with a few modifications it can still be used.

gwvenus56 wrote:Hi,
That would be the ideal solution but I live in Canada and would have to order one from the U.S. After rate of exchange and customs it would be over $300! The transformer I have now would end up in a landfill. I'm hoping with a few modifications it can still be used.

That's one view. Another is that you might have payed more then $300 in your
efforts to get the amp working, and still no success.

I'd start with a dynaco transformer. The existing might be sold at ebay ( where you also
might find the correct transformer)

I would suggest you solve your problems one at a time.   In dealing with the tube bias question, you can pretty easily 'calculate' what your bias current draw is.  With 390V on the plate (Pin 3), you need to also check the screen voltage at pin 4.   It will be 'slightly' higher than that of the plate voltage, simply because the screen taps on the output transformer are of less overall internal resistance.   The 'complete' primary winding with it's full resistance is what connects the B+ voltage to the plate.  The screen taps have less winding resistance, so a slightly higher voltage.  This will be only a few volts.  But, to the point, at 390V, you're in the basic ball park for what should be a sonic performance that is basically on par with most any other amp.  In other words, while some of it's peak performance levels may be compromised, at general listening levels, I sincerely doubt that you or most anyone else could ever hear a difference.   Most likely even at the peak levels this would be more of something that would require measurement tools and equipment......not the typical human ear!   As to bias level, how are you measuring this?   You have indicated that you still have the 15.6 ohm current sensing resistors in your amp.   If this is true, and if they're wired as per a 'stock' amp, then when you measure the voltage across that resistor it will give you a reading that represents the status of both tubes in that operating pair.   So, if you have the bias adjustment set to read the 'factory' level of 1.56 volts at the front test point on the octal socket, that means that the 'current' can be calculate across the 15.6 ohm resistor as being 100ma, or 50ma per tube.   As that's the Dynaco 'spec' for that amp, and you indicate that you're running at 35ma, perhaps you're just too low and maybe that's why the amp didn't sound so good?  So, maybe start by checking the voltage on pins 3 and 4 of each output tube, make sure you've got adequate plate and screen voltage, then set the bias up closer to the 'factory' recommended level, then give the amp a listen.   Oh, I'd also check your 15.6 ohm resistors to make sure they're 'on' spec.   If they've drifted too much in either direction, you should consider replacements, but at least compensate when you make your adjustments.   Finally, I'd go no lower than 45ma to start, so 1.45 Volts at the bias test point.   If after you do all that it still sounds crappy..... then go on to possible issues with the driver board.

Thank you very much! I've never set the bias higher then 1 volt. I'll be trying this today.

You've probably never checked the value of the 15.6 ohm sensing resistors either. If they've drifted 'up' in value, like to 16 or even 17 ohms, and if you're using 1 volt at the test point, then you're at about 30ma or less on the tube bias....which in reality is 40% lower than spec (or more). So, this alone could account for some reduced sonic performance. If you get the bias up closer to where it's designed to be you may be surprised to find that a lot of things will fall in line and the amp will sound a lot better.

You are right. I'll check that first.

Do me a favor and report back after you do 'just this'......and advise if this resolved a good bit of your issues. As Peter mentioned, having a 'proper' power transformer is probably a good idea, but SO many 'Dynaclone' amps have been built using 'other than' OE type transformers that you're probably not as bad off as it may appear. Perhaps even the statement made about not having to use a 5AR4 might have some value. Personally, I like the use of solid state rectification in the ST-70, and the SDS boards are a great way to achieve that, as well as some added bolstering to the power supply for transient performance enhancement. If you're getting 390V at the plates and probably about 395 or so at the screens, then with the bias set at about 45 to 48ma, assuming all is 'correct' with your driver board and in the rest of your system, your amp should sound pretty good!

Hi,
The amp sounds much better. My bias bottoms out at 1.2v so I'm going to have to lower a resistor value in the bias circuit.
I bought the power transformer custom made by Frank Van Alstine in the early 90's but this amp has been in storage for close to thirty years so it looks like new. I brought it up slowly on my variac but I kept the bias low because it still has the original Dynaco EL34's with very low hours on them. Even at 1.2v I can now lower the AC balance from minus .9v to minus .6v
Thanks again for your expertise, it's very much appreciated. I built a Van Alstine Super Pas 3 just last week so things are looking good!

Yea, I think if you can get that bias up a bit more you'll be OK. FVA's stuff is generally quite good, and he's also quite reputable about what he sells and how he goes about his developments. I did one of his early driver boards some 20+ years ago....... and I found it quite nice. In the past 20 years I've rather migrated away from all the other 'super' boards and just stuck with the OE design. Nothing against all the high-tech improvements made, but I just find the complexity of what all that's been added and re-designed hasn't really improved the overall sonics of the amp from my perspective. About he only 'mod' I do to the driver section is to re-wire the 'stock' driver board to accept a 6GH8A tube vs the 7199. Oddly, you mentioning Frank Van Alstine, it was Frank who did some extensive research some 20+ years ago to compare the characteristics of the 6GH8A with the 7199 in the 'stock' Dynaco driver circuit. His published results are still out there somewhere, but in short, he found the 6GH8A actually out-performed the 7199 in some areas, and was essentially 'equal' to it in others......making it a 'better' choice in some ways if not for cost alone. I think he recommended 2 minor changes, neither of which are really all that critical. Many people simply use the 7199 to 6GH8A converters and just do a 'drop in' replacement for the 7199 and never skip a beat.

Well your advice was spot on. I ended up changing the value of two resistors and setting the bias to 1.45v as per your post. It's rock solid and sounds really good. I did get Franks Super Seventy driver board (his latest design) He uses a lot of zener diodes to control the tube parameters but he does not supply a schematic or a voltage chart so one is really in the dark when it comes to troubleshooting. The Diytube
driver board works like a charm and sounds great. Thanks again for your help.
Cheers,
Gary

Thats another reason why I try to stick with the 'stock' driver board. Its very well documented, easy to work on or repair, and when operating as it should with good tubes its as accurate as anything else you can spend your money on. Glad you got it working without dropping that extra $300!! Tom D