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I've been enjoying my custom ST35 but find myself in the tweaking mood again and wonder about making monoblocks. Layout wise I was thinking of building sort of mini-Mk3's. I was wondering if simply splitting the ST35 in half would be doable. I have an extra PT.

Searching the forum has unfortunately not been helpful. I'm not the first to propose this but many seem to discourage the idea. It's a shame because I think it would be a cool project. Considering how simple these circuits are I would think it would be pretty easy.

If anyone has any helpful advice I would surely appreciate it.

I suppose that a couple of the Poseidon boards could be used as drivers, just re-calculate the plate and cathode resistors for operation with 325-365 VDC B+

An LTPI will play well with a PP pair of EL84s

/ed B

At first I was thinking I would simply split the St35 driver in half and retain the 12AX7 for voltage amp and 12AU7 for phase splitter but basically only using half of each tube.

Then I thought surely this could be accomplished with one tube. Could a 12AU7 or 6BQ7 work for both sections? I understand it has less gain but I have too much gain with my preamp anyway. I imagine changing the resistor values might be a good idea but I don't quite understand how to calculate this.

Also, since I still want to use the PA774 that means changing the power supply resistors as well. I read that those need to be doubled. I was also considering a choke after the first cap similar to the other Dynaco circuits. Would the C354 or Triad C24X still work in a mono configuration or do I need to find a bigger one as well?

LMAO, the "Original" Dynaco circuit used a 7247/12DW7 tube, which is 1/2 of each!!

cdmawolf wrote:LMAO, the "Original" Dynaco circuit used a 7247/12DW7 tube, which is 1/2 of each!!

I know, I know. I would prefer either a tube that's not as popular or a tube people currently make and not some long lost nos tube. It seems like an old practice to use a high gain tube for the voltage amp and a low gain tube for phase splitter. If I have to do it that way so be it but it seems rather unimaginative.

6AV6 and a 6C4?

That would work, 6AV6 is like half of a 12AX7, 6C4 is like half of a 12AU7. They also have the advantage of being relatively cheap tubes

/ed B

corndog71 wrote: I would prefer either a tube that's not as popular or a tube people currently make and not some long lost nos tube.

If I am not mistaken, JJ 12DW7 is a current production tube.

Also look for J/J ECC832, which is Euro-version of 7247 / 12DW7, as the number suggests, it is half ECC83 and half ECC82.

/ed B

corndog71 wrote:

cdmawolf wrote:LMAO, the "Original" Dynaco circuit used a 7247/12DW7 tube, which is 1/2 of each!!

I know, I know. I would prefer either a tube that's not as popular or a tube people currently make and not some long lost nos tube. It seems like an old practice to use a high gain tube for the voltage amp and a low gain tube for phase splitter. If I have to do it that way so be it but it seems rather unimaginative.

Back in the day Ed Laurent published a 7199/7189 PP circuit in Radio Electronics. Since the 7189 is just a robust version of the EL 84 and the 7199 has been been replaced by the 6GH8A in the ST70 type II it seems like a slam dunk option for your monos to use the 6GH8A which you can find for $5 or less. I got a ton of them in sleeves for $2 a couple of years ago.

A 7199/7189(EL84) circuit would be a Dynaco SCA-35, the Integrated amp version of the ST-35.

Eico ST-70 used the 7247/12DW7, Dynaco ST-70 the 7199.

Each have their demons and charms

Yeah, I may just suck it up and go with the JJ 12DW7. At least they're cheap. I'm looking at using a 10x6 box.

I am wondering if it is possible to simply use a 12AX7 instead of a 12DW7/7247/ECC832.
Does driving 6BQ7s require so much power that a 12AX7 cannot supply enough current in the Concertina phase splitter?

I know a few designs use a 12AX7 in the para-phase or the floating-para-phase splitter to drive 6BQ7s.

How about the Concertina set up with a 12AX7?

Just curious.

Plugging a 12AX7 into a 12DW7/7247 will work (the amplifier will produce sound), but not very well. The phase splitter (the *au7 half) is setup to draw 4.5 ma, and the 12AX7 is designed to operate at less than 25% of that. If you feel the need to substitute another tube, the 12AT7 is a better choice.

The circuit could be re-designed to use the 12AX7 for the phase splitter, but since the circuit uses both direct coupling, and positive feedback, it’s a complex job, and it’s questionable if any improvement would result.
However replacing the 12AU7, where possible, (the DIY circuit) with a 12BH7 or 12FQ7 (requires a slight heater wire change), will make a measurable improvement, particularly if the B+ to the VA/PI is raised from 320 VDC to 365 VDC.

BTW; the 12FQ7 is electrically identical (other than heater voltage) to the 6SN7, which is two 6J5’s in one bottle. Sorry, they are both “octals”!