I’m planning my first PP build and the ST-70 topology is of interest.
It didn’t take me long to realize the main issue with this one is the scarce/expensive 7199 input/driver tube. Which some have swapped with things like 6GH8s and similar. I’ve seen some kits, articles, posts about using other tubes but can’t really figure out which one would be the best.
So as of right now in 2022, i’m wondering if there is a “generally accepted” alternative driver section for the ST-70 which uses separate triode and pentode tubes or just triodes?
It didn’t take me long to realize the main issue with this one is the scarce/expensive 7199 input/driver tube. Which some have swapped with things like 6GH8s and similar. I’ve seen some kits, articles, posts about using other tubes but can’t really figure out which one would be the best.
So as of right now in 2022, i’m wondering if there is a “generally accepted” alternative driver section for the ST-70 which uses separate triode and pentode tubes or just triodes?
Correct tube is the price of admission. If a problem... look to a design which uses easier to afford tubes?
I feared this might have been the case where it’s a design that isn’t very tolerant to substitution. I guess I’m more concerned about availability rather than price, like something thats available as current production such as 12AU7, 6SN7, etc… Those aren’t exactly cheap tubes these days but definitley available.
I would think that the OPT is much more critical than the 7199. So if the OPT's you're going the use are not identical to the original ones there's not much to say about which alternative for the 7199 would be best.
Excellent point I have not considered… The transformers I have on hand for this project are the Hammond 1650N’s
There are numerous alternate triode based ST-70 drivers, but they will involve at minimum a new driver PCB.
Maybe consider a Williamson circuit?
Maybe consider a Williamson circuit?
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I’m definitely open to other designs to consider. Given that this is my first PP project - something that’s a bit forgiving of non-exact parts (even if it means it requires more parts)
Do you have a pair of Hammond 1650N transformers and a blank slate for DIY, or do you have a chassis that limits you to the Dyna ST70 topology and must you use EL34s?
If a blank slate I suggest you take a look at the Eico ST87, or the HK Citation V schematics. Great amps had been built using the 1650N and either schematics could be used with minor adaptations. However, at 60w those Hammond 1650Ns are more like Dyna MkII thank ST70. If I had a pair of those outputs now I would build this ETF bias design of Kevin Ward, described here:
https://audiokarma.org/forums/index.php?threads/kt120-amp-mk-3.828722/
If it must be closer to the original Dyna ST, I can recommend http://www.tubes4hifi.com/ST70.htm
I am not sure if 6F12P is "“generally accepted” to be a substitute for 7199, but for sure the performance of 6F12P is much better and price relatively low.
I have used 6F12P as a voltage amplifier/cathodyne a lot. Attached the schematic of well tested circuit with 6F12P.
And also a version with EL34 UL PP, quite similar to Dynaco ST-70.
Attachments
These trannies and a blank slate for DIY 🙂 the Eico HF87 has defninjtely piqued my interest. Too bad I just gave away all my 12BY7s to a ham radio friend otherwise I would have considered the Citation V too!
This was a catalyst for me choosing the 6SN7 driver board from Tubes for Hifi many years ago. A the time Andy at Vintage Tube Services had old stock tested chrome dome 6SN7 tubes for about $18. The price for these tubes has doubled / tripled in the past 15 years. Maybe the 12AU7 / 12AT7 tubes are on par now from a cost perspective, but I love the sound of the 6SN7 driver board. Also, Roy Mottram was extremely helpful resolving an assembly error I made. I think that either Bob Latino or Roy Mottram have departed Tubes4hifi, but this board would easily be my choice. // I had another popular driver board in my ST70 that was inferior. This board has been updated since my experience, and I am not savvy with the current model.
6Gh8/6BL8/6U8 are all good alternatives ( or ECF80/ECF82) . I have seen best result with ECF80 and these
tubes are i good supply.
dynakitparts.com have readymade boards for these tubes. They also have transformers identical
to the originals.
artosalo,
Regarding your post # 9:
Thanks for the idea!
The major changes versus the original Dynaco ST70 that I see are:
1. AC coupling from the input pentode to the concertina splitter.
That makes the amplifier more like a Williamson, correct?
2. No dominant pole series RC network from the pentode plate to ground.
Where is the dominant pole?
3. No capacitor feedback from the "pull" EL34 screen, back to the input tube cathode NFB node.
4. Addition of a 20k potentiometer to balance the DC quiescent current of the EL34 tubes.
5. Addition of a 20k potentiometer to balance the signal currents of the EL34 plates (Any unbalanced transconductance of the EL34 tubes).
A true concertina does not need balancing for itself, but may need to be have different gain, in order to make up for any following stage(s) that are not balanced.
(A Concertina with identical AC load resistors of the cathode and plate are Intrinsically and automatically perfectly balanced, as long as the loads are equal, including the Rk and Rp of the concertina, and also the Rg's of the two EL34 tubes).
The only exception is the condition of any grid current in the concertina.
And grid current of a good tube and a good circuit design that is not overdriven by large sitnals, are essentially zero.
And, if there is any grid current with large signals, the RC coupling from the pentode plate to the concertina grid will cause a bias shift, you do not want that.
6. The particular model of output transformers, original versus a substitute.
Other than that, it looks very much like a Dyna ST70.
Just my observations and opinions.
Regarding your post # 9:
Thanks for the idea!
The major changes versus the original Dynaco ST70 that I see are:
1. AC coupling from the input pentode to the concertina splitter.
That makes the amplifier more like a Williamson, correct?
2. No dominant pole series RC network from the pentode plate to ground.
Where is the dominant pole?
3. No capacitor feedback from the "pull" EL34 screen, back to the input tube cathode NFB node.
4. Addition of a 20k potentiometer to balance the DC quiescent current of the EL34 tubes.
5. Addition of a 20k potentiometer to balance the signal currents of the EL34 plates (Any unbalanced transconductance of the EL34 tubes).
A true concertina does not need balancing for itself, but may need to be have different gain, in order to make up for any following stage(s) that are not balanced.
(A Concertina with identical AC load resistors of the cathode and plate are Intrinsically and automatically perfectly balanced, as long as the loads are equal, including the Rk and Rp of the concertina, and also the Rg's of the two EL34 tubes).
The only exception is the condition of any grid current in the concertina.
And grid current of a good tube and a good circuit design that is not overdriven by large sitnals, are essentially zero.
And, if there is any grid current with large signals, the RC coupling from the pentode plate to the concertina grid will cause a bias shift, you do not want that.
6. The particular model of output transformers, original versus a substitute.
Other than that, it looks very much like a Dyna ST70.
Just my observations and opinions.
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I ended up using the K&K Audio driver board for my ST-70. While I like it better, I could have easily lived with the low-gain version of the VTA driver board that I was using prior and that is what I would recommend. Well, that, and a more robust power supply.
To first Q-
Rolled bunch of different tri/pent's in my first tube build... Chifi ebay chunk which used 6P2 up front- same topo as 7199
Tried 6GH8, 6U8, 6EA8... found the last to have the lowest noise floor...
Then decided to try dual triode front end like Dynaco SCA-35- Dissimilar ECC832- hum/hiss gone!! More so then any pent... Lost lil gain, but manageable.
Guessing your ST-70 is the EICO. If so, uses 7591 outputs. If so (again) then, I'm using same output PP config w/ 6GM5's, design borrowed from Pioneer SA-810. Fixed bias, big plates, capable of scaring off small rodents... lol
It's a hybrid, but it works- need to put some meters on it & balance it out.
Jim
ps. there's bunch of 'hot rod' circuit upgrades for the ST-70, including a quad
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My apologies if this turns out to be an off-topic distraction, but...
Another strategy could be a push-pull-parallel (PPP) output stage made of four EL84/6P14P per channel, which should work well with the Hammond 1650N plate-plate primary load of 4.3k:VC. The Russian 6P14P-E or 6P14P-EV are fine members of the EL84 family. The JJ EL84 is also supposed to be good, but I haven't tried it.
You could also do PPP 6V6GT (Russian 6P6S).
That 6F12P driver should drive the snot out of EL84s, but it would need to be run from a lower B+ voltage, perhaps +300V or something like that. You could even triode-wire the input stage pentode and have enough gain.
Another strategy could be a push-pull-parallel (PPP) output stage made of four EL84/6P14P per channel, which should work well with the Hammond 1650N plate-plate primary load of 4.3k:VC. The Russian 6P14P-E or 6P14P-EV are fine members of the EL84 family. The JJ EL84 is also supposed to be good, but I haven't tried it.
You could also do PPP 6V6GT (Russian 6P6S).
That 6F12P driver should drive the snot out of EL84s, but it would need to be run from a lower B+ voltage, perhaps +300V or something like that. You could even triode-wire the input stage pentode and have enough gain.
I think that DC-coupling between voltage amplifying stage and cathodyne phase splitter is a compromise. Operating point of the cathodyne in case of DC-coupling, is determined by the plate voltage of the amplifying stage. It is better to bias both stages independently. Then voltage amplifier can be biased to most linear operating point and phase splitter to largest headroom of output voltage.
I think that such RC network is only needed if very high GNFB is used (>20 dB) as is the case with Mullard, Williamson and Dynaco amplifiers.
I usually stay below 16...17 dB and have noticed no stability problems. Frequency response and transient response is adjusted by 2k7 and 100p RC-network at NFB loop.
I have not seen this necessary.
Yes, to compensate un match of output tubes.
I have used this circuit with many different type of tubes and output transformers and only minor modifications are needed, mainly the components of NFB loop. But this requires that the amount of GNFB is reasonable, not too high.
artosalo,
Good.
I am glad that different parts, etc. have worked for you in a number of amplifiers that use your circuit or similar.
But adjusting the negative feedback is not easy for some less experienced designers and builders.
I would like to point out that the high frequency stability of an amplifier, and the high frequency square wave response (look / shape) is dependent not only on the parts including the output transformers, but also on the wiring layout (wire crossings; parallel wires; wire lengths; all of those which cause parasitic L and C; as well as coupling from one circuit to another). This is an art and science.
Not all newbies have a lot of experience in the art and science of a low to moderate complexity 3 stage amplifier that uses global negative feedback.
I believe that the Dyna ST70 did not use high amounts of negative feedback; it was probably stable even without the EL34 screen to input stage cathode; but perhaps not stable without the dominant pole.
My impression that one of the most often listing of problems in Tubes / Valves is:
Low frequency stability of Williamson and similar circuits, with at least 3 stages that use RC coupling, that also use global negative feedback that includes all 3 of those RC networks in the feedback loop.
My impression is also that those postings often take a long time to solve, given the number of people who respond before the original poster's problem is solved.
Although I did try designing and building an input stage that was direct coupled to the concertina, even though it did work, I never liked that circuit.
Just my personal preference.
I can see why you prefer RC coupling between the input stage and the concertina.
One low frequency pole that is often overlooked / forgotton is the bypass cap that is across the input stage's self bias resistor.
Just one more phase shift.
Good.
I am glad that different parts, etc. have worked for you in a number of amplifiers that use your circuit or similar.
But adjusting the negative feedback is not easy for some less experienced designers and builders.
I would like to point out that the high frequency stability of an amplifier, and the high frequency square wave response (look / shape) is dependent not only on the parts including the output transformers, but also on the wiring layout (wire crossings; parallel wires; wire lengths; all of those which cause parasitic L and C; as well as coupling from one circuit to another). This is an art and science.
Not all newbies have a lot of experience in the art and science of a low to moderate complexity 3 stage amplifier that uses global negative feedback.
I believe that the Dyna ST70 did not use high amounts of negative feedback; it was probably stable even without the EL34 screen to input stage cathode; but perhaps not stable without the dominant pole.
My impression that one of the most often listing of problems in Tubes / Valves is:
Low frequency stability of Williamson and similar circuits, with at least 3 stages that use RC coupling, that also use global negative feedback that includes all 3 of those RC networks in the feedback loop.
My impression is also that those postings often take a long time to solve, given the number of people who respond before the original poster's problem is solved.
Although I did try designing and building an input stage that was direct coupled to the concertina, even though it did work, I never liked that circuit.
Just my personal preference.
I can see why you prefer RC coupling between the input stage and the concertina.
One low frequency pole that is often overlooked / forgotton is the bypass cap that is across the input stage's self bias resistor.
Just one more phase shift.
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