I was going through the closet, looking at what's sitting there on the shelf. It seems I squirreled away the iron from an old Dyna ST35 amplifier. (Its PCBs burned up back in the days before JLPCB made it so cheap 'n easy to have your own made.) I was recently given a nice aluminum chassis that's pre-drilled for a quartet of 9-pin output tubes and a couple of 9-pin driver tubes, with room for transformers, etc. Obviously, this has to be a project.
The speakers I want to use are all 4 ohms. I have a pair of Snell E/III that I like, and are supposed to have 91dB sensitivity (but at 1W? 2.83V? I don't know).
The Z565 OPTs have 8 and 16 ohm secondary taps. I think their primary impedance is 8k plate-plate.
What would happen if I used 6P43P (EL86) tubes instead of the EL84 (6P14P) tubes the OPT was designed for?
Thanks for any help with this.
The speakers I want to use are all 4 ohms. I have a pair of Snell E/III that I like, and are supposed to have 91dB sensitivity (but at 1W? 2.83V? I don't know).
The Z565 OPTs have 8 and 16 ohm secondary taps. I think their primary impedance is 8k plate-plate.
What would happen if I used 6P43P (EL86) tubes instead of the EL84 (6P14P) tubes the OPT was designed for?
- Running the 8k:8 rated OPT into a 4 ohm load on secondary would make it 4k:4, would it not?
- 6P43P has half the rp of EL84, so should be happy into a 4k p-p primary load.
- Z565 is supposed to present 100H primary inductance. Does that change if the OPT secondary is underloaded? Or is the primary L relatively constant?
Thanks for any help with this.
@rongon, sounds like an interesting project. Please keep us up to date with your progress.
I second @baudouin0, regarding the "go for it". I believe you know that EL86 runs lower Va, Vg1 and higher current than EL84 and has higher drive requirements. You will account for them in your circuit design? Share your circuit if you please.
I second @baudouin0, regarding the "go for it". I believe you know that EL86 runs lower Va, Vg1 and higher current than EL84 and has higher drive requirements. You will account for them in your circuit design? Share your circuit if you please.
I'm thinking of basically copying one of kodabmx's PP amp designs using 6F12P, even though he uses 220VAC transformers as OPTs. The B+ is likely going to be down around 300V, so I will need a driver stage that can swing +/-30V from 1V input signal with a low plate voltage. Perhaps a 12AT7 LTP can pull that off, or maybe I'll need to look at using a pentode input stage into a cathodyne phase splitter. Or... A paralleled 12AX7 as voltage amp into a MOSFET 'source-o-dyne'?
Another possibility could be making a PP EL84 (or 6P15P) pentode amp with plate-grid feedback, or Bandersnatch-style g2-g1 feedback, using 7 pin pentodes for an LTP driver stage. 6AU6A, 6CF6, 6CB6, etc.
@je245 is your ST35 stock, using EL84s?
Another possibility could be making a PP EL84 (or 6P15P) pentode amp with plate-grid feedback, or Bandersnatch-style g2-g1 feedback, using 7 pin pentodes for an LTP driver stage. 6AU6A, 6CF6, 6CB6, etc.
@je245 is your ST35 stock, using EL84s?
Last edited:
6F12P will work. This schematic will do the trick - just run it from 300V instead and perhaps reduce 10k to 2k (3 watt)
Here's my implementation of 6F12P and 6P43P as in the Little Miracle amp.
The 6P43P cathodes are around 30V and I used 1uF coupling caps instead of 0.22u from 6F12P to 6P43P.
Here's my implementation of 6F12P and 6P43P as in the Little Miracle amp.
The 6P43P cathodes are around 30V and I used 1uF coupling caps instead of 0.22u from 6F12P to 6P43P.
Last edited:
Yes, stock circuit but I removed the input cap and added a filter choke in the PS.
Thanks Koda, that's the one I was thinking of. I even have a few TL783 in my parts stash.
Interesting that the stock ST35 can drive those 4 ohm speakers. Perhaps the NFB makes that possible. It looks like the choke in the PSU helps.
Last edited:
FWIW, when I visited Joe Roberts a couple of years ago, he was using a stock SCA35 to drive WE728Bs because he was too lazy to rewire his Silbatone 300B amp to 120V.
Whichever way you decide, happy listening!
Koda, have you measured the current through the pair of 6P43Ps in you circuit? I wondered if the primaries on rongon’s Z565s can handle that much current. I tried to find a specification for the current capability of a Z565, but have come up empty so far.
Well, it certainly would be easier to stick with EL84s. They require so little drive voltage swing, and I could goose the B+ to 310V and drop about 12V or so across the cathode resistor(s). It is really difficult to screw up the basic cathodyne > PP UL EL84 recipe.
If 6P43P really cannot withstand more than 200V on its screen grids, then I'd best triode-wire them and run them low and hot, maybe 250V on the plates, 50mA Ip each. Maybe I should use a different pair of OPTs for that. The idea here is to use these very nice vintage Z565s, since I have them.
I get so confused every time I embark on a project... So many choices to make.
If 6P43P really cannot withstand more than 200V on its screen grids, then I'd best triode-wire them and run them low and hot, maybe 250V on the plates, 50mA Ip each. Maybe I should use a different pair of OPTs for that. The idea here is to use these very nice vintage Z565s, since I have them.
I get so confused every time I embark on a project... So many choices to make.
If you decide to go that route (sticking with EL84) consider a “real Dyna ST35” as Dave Gillespie enhanced it with EFB to maximize potential resale value. He has the EFB power supply PCB and very nice FR4 PCB replacements following the ST35 circuit precisely in his store: http://www.tronola.com/html/daves_store.html
In case you haven’t read it recently, Dave’s new look at “old friend SCA3” is always informative: http://www.tronola.com/html/a_new_look.html
In case you haven’t read it recently, Dave’s new look at “old friend SCA3” is always informative: http://www.tronola.com/html/a_new_look.html
In a recent post at the other website Dave said: “For reference, with all else being equal, when adding the EFB modification to an ST-35 (or SCA-35), the impact of the reduced heat from the plate and screen elements of the output tubes versus their operation in the original design (-20.95 watts), and the reduced heat from the EFB™ regulator versus that produced by the original cathode bias resistor (-0.86 watts), together produces a total reduction of 21.8 watts from the amplifier, or a net reduction of 35% of the heat originally produced by these elements. The reduction in heat is made even greater if the reduced heat produced by the power transformer is considered as well. The amplifier still produces significant heat like any power amplifier does. But it is well off the high water mark of the original design, not to mention the added benefits of increased power output and lower distortion produced from the modification.”
https://audiokarma.org/forums/index...lot-232-power-amp.509579/page-2#post-15943969
https://audiokarma.org/forums/index...lot-232-power-amp.509579/page-2#post-15943969
I no longer have the ST35 chassis, but I have the iron. So I don't need to follow the physical layout of the original ST35 amp. I consider that a good thing, as the old ST35 was a crowded little chassis. Mine eventually cooked its audio PCBs so badly the amp failed. This was maybe 25 years ago. I eventually sold the chassis thinking I'd never use it. So I have the iron, nothing more.
I figure the iron is good, especially the Z565 OPTs.
Perhaps this is my chance to finally try that 'Schade' feedback everyone was so excited about 10 years ago. I was thinking of something like this:
I've been curious to hear what this kind of circuit sounds like. It should get a reasonably clean 10W per channel, which would be good enough.
Can that Dynaco power transformer deliver 200mA? I'm not so sure. I have other transformers I could throw at this...
It might be worthwhile changing the output stage to fixed bias, or perhaps the EFB boards can be shoehorned into this circuit.
I figure the iron is good, especially the Z565 OPTs.
Perhaps this is my chance to finally try that 'Schade' feedback everyone was so excited about 10 years ago. I was thinking of something like this:
I've been curious to hear what this kind of circuit sounds like. It should get a reasonably clean 10W per channel, which would be good enough.
Can that Dynaco power transformer deliver 200mA? I'm not so sure. I have other transformers I could throw at this...
It might be worthwhile changing the output stage to fixed bias, or perhaps the EFB boards can be shoehorned into this circuit.
I'm not worried about really close impedance matching, just the very general idea that one gets lower THD and better damping if the load is lighter on the amplifying device(s). With the OPT in the mix, if I put a 4 ohm speaker on the 8 ohm secondary tap, and the nominal impedance ratio of the OPT is 8k:8, then I've changed the transformer's nominal impedance ratio to 4k:4.
On paper, 4k:4 doesn't look great for PP EL84s. but it's nearly ideal for PP EL86s.
That's as far as I went in my thinking about it. Am I overthinking it even on that level?
One reason I was thinking of going the 'Schade' feedback route was that the OPT is not in the feedback loop, and the NFB reduces the plate resistance of the output tubes by quite a bit. So the impedance ratio of primary:secondary becomes even less of an issue. That, and I'm really curious to hear what this topology sounds like....
On paper, 4k:4 doesn't look great for PP EL84s. but it's nearly ideal for PP EL86s.
That's as far as I went in my thinking about it. Am I overthinking it even on that level?
One reason I was thinking of going the 'Schade' feedback route was that the OPT is not in the feedback loop, and the NFB reduces the plate resistance of the output tubes by quite a bit. So the impedance ratio of primary:secondary becomes even less of an issue. That, and I'm really curious to hear what this topology sounds like....