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EL34s with A430 vs. A470?

kward

kward

AK Subscriber
Subscriber
Greetings.

I know EL34's are normally paired with A470s, and KT88s/6550s are normally paired with A430s. But in my situation I've got a donor chassis pre-drilled for A430s, and yet I want to build an EL34 PP AB1 amp. The output stage will have approximately 440V quiescent plate voltage, will be fixed biased, will be UL connected, and will deliver approximately 35 watts output.

Manufacturer on both OPT models in question is Dynakit Parts.

So basic question is about electrical suitability of mating an EL34 output stage to A430s. I am aware that both A430s and A470s have have the same primary impedance (4.3KΩ) and same UL tap points (33%), but that the A430s support 60 watts and the A470s support 35 watts. Are there any issues or concerns with the "smaller" output stage driving the "larger" OPTs?

Regards,
Kevin
 
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If both opt are in the 4K area and you are using fixed bias there should be no issue at all , just bigger iron. Not a bad thing . I also would think you will get more then 35w across the frequency band with the larger opt.
Size the power trans big enough so you can try El34 and 6550.
I’ve done some super st70 jobs using kt88 with the 35w opts. Definitely get more then 35w out of them until they saturate with low frequencies.
 
Do you need KT88 level current to drive A430s well? They are bigger than A470s so they might need more oomph to drive them well.

Wasn’t the Mark II a stepping stone to the Mark III. Meaning although they did make the Mark II, the Mark III replaced it as the better design. Those were all questions even if not written with question marks.
 
kward said:
Do you need KT88 level current to drive A430s well? They are bigger than A470s so they might need more oomph to drive them well.

Wasn’t the Mark II a stepping stone to the Mark III. Meaning although they did make the Mark II, the Mark III replaced it as the better design. Those were all questions even if not written with question marks.
Click to expand...
MkII was close to identical with MkIII, a resistor instead of a choke and EL34 instead of 6550 as far as i remember.
/PS
checking the assembly manual a few more details differ :
5U4 instead of 5AR4
12ohm cathode resistor instead of 11.2
/DS
 
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K-- No.No.Yes. Not a better design, but an improved design (for example; added a choke, heater bias and improved HF Stability) -- and -- one that took advantage of the KT88 that had just been made available. Those were all answers to your questions.

Now, because I know you've been down this road before, here's one little additional fact that might play into your decision making: The A-470 is a much better transformer from the standpoint of HF stability. The A-430 and A-431 transformers -- whether in the Dynaco designs or others employing 20 db of NFB -- generally cannot tolerate more than .0075 uF across an unloaded output (although a Zobel will improve this somewhat). In return however, they also typically will have a supersonic response that goes out to about 60 kHz, being typically down no more than 1 db at that frequency. On the other hand, the A-430 has a response that is only good to about 40 kHz at best -- but -- still produces a beautiful 10 kHz square wave, and displays absolute stability at each output tap. You simply cannot make the things squeal. To be fair, the A-430 and A-431 deliver superb performance, and in practice, the level of HF stability they deliver has never been made issue of. But these differences are there and very real between the two transformers, so plug that in as you preference so dictates.

I hope this helps!

Dave
 
Alrighty. Thanks for the quick replies! I have the info I need to decide which to use now. And with the added tidbit of the better HF response on the A470 does not make it an easier decision!

I’ve never designed around the A470 so I’m kinda chomping at the bit for that. I might go with a separate chassis and save the current chassis for a KT88 build.

Stay safe everyone.
 
gadget73 said:
The Mark IV is what you're after in mono, EL34, and A470 transformer. Its half an ST-70, and good for a few more watts.
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I would love to have a couple Mark IV's over the ST-70 I have. If I had to do it all over again I would have bought a couple Mark IV mononblock kits from Dynakit and use the VTA driver boards in them. Either that or a couple used Dynaco Mark lV's.
The 5AR4 should he a lot happier too in the lV's I would think.
 
Well I’ll be into these for short change and can flip them for some IVs if they don’t wow me. I believe the IVs go for more money and are less common but we’ll just see how these turn out first.
 
Blackdog61 said:
Well I’ll be into these for short change and can flip them for some IVs if they don’t wow me. I believe the IVs go for more money and are less common but we’ll just see how these turn out first.
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At any rate, they should be a fun project with plenty of options to explore for the the amp build. And if you don't like how they sound they should be pretty easy to flip. But if you like the ST-70, thwn you should like the Mark lV's.
 
I've turned a pair of ST70s into something very similar to a set of Mark IVs. Added the VTA boards, Roy Mottram's solid state rectifier, and some Tung Sol 6550s and I'm a happy camper.

dec2025C.jpg
 
knotscott said:
I've turned a pair of ST70s into something very similar to a set of Mark IVs. Added the VTA boards, Roy Mottram's solid state rectifier, and some Tung Sol 6550s and I'm a happy camper.

View attachment 3846348
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I tried KT-88's in my VTA ST-70 and while the bass was better, the midrange didn't sound right to me. The best way I can describe it is that the midrange sounded kind of recessed, or quieter? Those same KT-88's sound great in the mono blocks I have that were designed for KT-88's and 6550's. Maybe I should try some 6550's in the ST-70. I have yet to try 6550's in any of my amps.
I have heard 6550's in a pair of mono blocks owned by another AK'r that sounded really nice though.
 
Lavane said:
I would love to have a couple Mark IV's over the ST-70 I have. If I had to do it all over again I would have bought a couple Mark IV mononblock kits from Dynakit and use the VTA driver boards in them. Either that or a couple used Dynaco Mark lV's.
The 5AR4 should he a lot happier too in the lV's I would think.
Click to expand...
Sweet pair of IVs in Barter Town. No affiliation but if I had the bread……
 
I've seen a recommendation in the past to use two ST70's, remove an OPT from each and install a nice choke where the second OPT was removed thus turning them into monoblocks and allowing the original power transformer to not be so heavily loaded. Then you have a spare pair of outputs to either sell or make into another project. I think it was @PakProtector who suggested this.
 
visually that would bother me entirely too much. I don't mind the notion electrically, it would give a heck of a stout power supply, but I don't think I could stand to look at that. Possibly could allow it with some physical re-arranging though. One output tube on each side, a fill plate where the original driver is, built point to point for a single channel drive circuit thats physically centered, blank over the extra sockets. Also the choke would have to be basically the same dimensions as the stock output transformer so the whole thing ends up at least symmetrical.
 
Most monoblocks aren't very symmetrical. The EL34 sockets on the choke side could be damper diodes. Can use the filament winding for that side to heat them. The choke could likely be about the same size as the OPT. I agree that PtoP for the driver stages on an aluminum plate would be great. If I had a second ST-70 I would probably try this. Come to think of it, a pair of custom chassis and a pair of the original pwr transformers, which I have, could work just fine. Might be ok to use EFB as well. Likely could get something back for the original chassis and driver board. Rats, I don't need another project, but this sounds fun. Would effectively but custom MkIV's.
 
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