Hmm! OP did not say he was a beginner. See post #13. Each of the links I posted have schematics if you care to look. They will need feedback modification, of course, if they are built with different output transformers.
I misunderstood your comment a bit. I meant to say that he has some output transformers in mind which will determine what amp he builds. If he can get a pair of Dynaco A470s, I feel confident he can build an ST70 without much trouble.
Certainly if he can find another tested circuit that is designed for a specific transformer that he is able to source, then that would be good as well.
Dave Gillespie's Eico mods are excellent, but substituting a different output transformer would be extremely difficult, IMO.
I agree that Nick Soudas' "Edcor" Eico HF87 looks very well done, and Edcor says they ship overseas, so that could be a good project for the OP. The price for the CXPP50-6.6 is very reasonable, but could be much higher with shipping and VAT. And it's only 35wpc, so an ST-70 clone would offer the same power.
I did suggest he could try the the Mark II with the Hammond 1650NA, but I for one would be worried that it would be a big struggle to get the amp to perform properly, even with some experience and a scope.
"Btw, As you might see, I'm new in tubes."
I do not recommend the Dyna circuits unless Dyna transformers are used.
I do not recommend the Dyna circuits unless Dyna transformers are used.
A commercial tube amplifier is designed around the output transformer. Once the design is completed and tested,
the circuit should not be "copied" unless the same output transformer is used.
the circuit should not be "copied" unless the same output transformer is used.
Well, this Eico with Edcor transformer would be good, but still costs of shipping, shipping time and VAT are little bit too big. But there's still only 32W @ 2% THD. Even Dynaco MK4 offers 40W @ 1% THD, and I can buy A470s in UE.
In case of Hammond, it would be good too, but I can't find any amplifier project that uses 1650NA transformers.
I think I'll stay with MK4. But can I replace a 7199 tube with something? I can't find it anywhere locally. Maybe 6AN8?
And what do you think about removing the bias adjustment and place there a fixed resistor?
In case of Hammond, it would be good too, but I can't find any amplifier project that uses 1650NA transformers.
I think I'll stay with MK4. But can I replace a 7199 tube with something? I can't find it anywhere locally. Maybe 6AN8?
And what do you think about removing the bias adjustment and place there a fixed resistor?
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ECF80 ECF82 and 6AN8 would all be good replacements for 7199
What do you mean with "removing the bias adjustment and .." ?
Do I need to change something in this project or just match the connectors of tube?
I mean to do it like here: https://www.diyaudio.com/community/...stereo-amp-clone_diyaudio_092922-jpg.1095384/
This resistor 235R/20W created dropdown voltage to polarize EL34 grid? In this case I don't need to care about periodic bias adjustment.
As for the tubes, in a ST70-like configuration the screen resistor ( 1.5M) might need to be adjusted
to find the plate on 1/3 of B+ This is to have the cathodyn pase splitter work in the middle of it's range.
Ah, that is a cathode bias. That's right the tubes will automatically find their bias, provided that they are matched.
Yes, they will be paired. So, should I use a 294R/10W cathode resistor? It should give about 34V dropdown voltage @ 115mA current. Do I need to change something else in amp design? (of course except removing the transformer bias tap and its rectifier circuit). I mean, will it affect EL34 working point?
The screen resistor I think the simplest will be to decrease it to 1.0M and give it in series 1.0M pot?
The screen resistor I think the simplest will be to decrease it to 1.0M and give it in series 1.0M pot?
If you choose to buy Dynaco A470 clone (or real) output transformers then build a Dynaco Mk 4 with adjustable grid bias a per schematic. Be aware that those A470 clones are in short supply at the moment and expensive if you find them.
Cathode bias is slightly simpler build and operate, but a compromise that provides less power and higher distortion than fixed bias in the Dynaco design. Additionally, with cathode bias you will have to hope you always have matched power tubes THAT AGE PERFECTLY in pairs.
Read Dave Gillespie’s analysis and measurements of the performance of the stock Dynaco ST70. If you want to ”improve” on the Dynaco ST70/Mk4 design the Gilliespie EFB(tm) bias arrangement is a good approach albeit marginally so in case of the ST70. http://www.audioregenesis.com/documents/ST-70 Base Line Testing.pdf
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Regarding "screen resistor", i was reffering to the screen resistor in the dynaco st-70 7199 .
294 ohm ? make it some "easy value", such as 2 150ohm in series to make purchasing easier. 2 resistors
will also mean that the heat is spread among a larger area.
Well, @Francois G convinced me to leave the bias as it is.
I think I could implement this EFB circuit into MK4s. I'll read this whole pdf later, but just as a shortcut, what EFB can give me?
I think I could implement this EFB circuit into MK4s. I'll read this whole pdf later, but just as a shortcut, what EFB can give me?
Dave Gillespie’s EFB (tm) stands for ”Enhanced Flexible Bias”. It is in short a way to modulate the bias to remain in a fixed ratio with the B+ supply, as it sags under heavy loads. If you read Dave’s article he concludes that in case of the Dyna St70 that the original was well designed and doesn’t benefit much from EFB, unlike other amplifiers that he modified, especially the fixed bias Fishers and Scotts with pentode output tube configuration.
Build your Mk4s (if you can acquire A470 transformers) as originally designed, listen to it for a while and tweek/modify later if there still is an itch. Replace the 7199 of one of the compatibles already mentioned. In absence of A470 transformers you could rethink a different amp with good, inexpensive Toroidy output transformers.
Peter, you mean ECF82? 😉
For the record, 6U8a, 6ae8, 6gh8a, etc are not the same tube as a 7199 with different pinout. Check and compare the data sheets. Dave Gillespie favors the 6gh8a as replacement for 7199 in his updated Dynaco SCA35, and there are small differences in the values of optimized components between the two. See: http://www.tronola.com/PC-10A_Assembly.pdf and http://www.tronola.com/A_New_Look_At_An_Old_Friend.pdf
Of an earlier mod for the AVA Super 70 he wrote:”the best news of all is that the 6GH8A makes the Super Seventy work much better than ever (i.e. with 7199). You could read about it here: http://www.tronola.com/6gh8a_tubes.pdf
For the record, 6U8a, 6ae8, 6gh8a, etc are not the same tube as a 7199 with different pinout. Check and compare the data sheets. Dave Gillespie favors the 6gh8a as replacement for 7199 in his updated Dynaco SCA35, and there are small differences in the values of optimized components between the two. See: http://www.tronola.com/PC-10A_Assembly.pdf and http://www.tronola.com/A_New_Look_At_An_Old_Friend.pdf
Of an earlier mod for the AVA Super 70 he wrote:”the best news of all is that the 6GH8A makes the Super Seventy work much better than ever (i.e. with 7199). You could read about it here: http://www.tronola.com/6gh8a_tubes.pdf
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Yes of course. 6U8 == ECF82 My mistake to write something else.
Myself did a comparisment with a small number of tubes :Alternative to 7199
All done in a dynaco sca35 with a adapter that does the pin swaps.