Dynaco Mark III?

[gadget73]

I wouldn't do a Mark II, you want the Mark III with the choke. Better power supply filtering.

If you're truly married to the ST70, you want a Mark IV. Its literally half an ST70, same output transformers, same circuit. Does an insignificant amount more power purely because the power transformer isn't as stressed.

 

[ConradH]

FWIW, I upgraded my MkIIIs with RN70D resistors and new caps. In hindsight, polypropylene would have been better, but this was long before I had a clue.

 

[nerdorama]

I wouldn't do a Mark II, you want the Mark III with the choke. Better power supply filtering.

I added the chokes to my Mk2’s. Not difficult or expensive. I used a 1H Triad with a higher current rating.

 

[peterh]

Seems like the thing to do is to confirm the transformers are right and build new Mark IIs then.

No, MkIII is what you want !

 

[joeriz]

At that price you cannot resist.
Contrary to others in this thread : MkIII in original shape does what the ST-70 is good at with more power and at better reliability.

I agree with all of this but not sure about better reliability. I have 2 ST-70s and a pair of MKIIIs. They are all equally reliable if rebuilt correctly and, within their power limitations, sound quality is very, very similar. MKIIIs perhaps get the nod on bass.

Also, you can use EL34s in a MKIII with a 5U4 rectifier and rebiasing.

That is a great price. The questions are: Would you enjoy rebuilding them? And would you use them?

 

[Lavane]

But not the slow start. Doesn’t a 5U4 hit hard o when powered on?

There are still old console amps that work, although should be recapped etc, that use 5U4's. I don't worry too much about the harder start of rectifiers like the 5U4. Yes, the softer start of the GZ34, GZ37, etc is nice, its not like using a 5U4 is going to blow up your amp though. Just make sure you use higher voltage caps.
Instead of 450v caps in my ST-70 it has 500V and 550V electrolytic caps.
Most of my amps have SS rectifiers. They've been running like that for some years now. One is an old Magnavox 9303 EL84 amp. I built a whole new power supply using a couple diodes instead of a tube rectifier. Its been working fine like that for around 14 years. The amp is only on its second set of tubes since its manufacture in the 60's. I used the original set up until maybe two years ago when one of the original EL84's finely lost vacume.
I've been using a Coppertop WZ-34 SS rectifiers in my ST-70, and WZ-68's in my mono blocks. I've had zero issues with them. The GZ68's in my mono blocks mimic dual 5AR4's with resistors and a thermistor inside. They've been in those amps since I built them in 2021. So I finely put a WZ34 that mimics the 17V drop of a 5AR4 and has a thermistor for a bit of a softer start.
The ST-70 is kind of hard on the 5AR4 to begin with. A 5U4 does have more voltage drop, but I dought you could hear a difference in the few watts less the amp would produce. Plus the 5U4 isn't going to be stressed near as much as the 5AR4 at higher volumes.
I see Tubes4hifi is selling 5U4's now instead of a 5AR4's for his ST-70 kits, and has a dual 5U4 rectifier board option for there ST-120's.

 

[peterh]

I agree with all of this but not sure about better reliability. I have 2 ST-70s and a pair of MKIIIs. They are all equally reliable if rebuilt correctly and, within their power limitations, sound quality is very, very similar. MKIIIs perhaps get the nod on bass.

Also, you can use EL34s in a MKIII with a 5U4 rectifier and rebiasing.

That is a great price. The questions are: Would you enjoy rebuilding them? And would you use them?

The singel issue with reliability is the single 5ar4 that is very loaded in the ST-70. The MkIII also uses 5AR4 but here it is well inside spec.

 

[Blackdog61]

Thread 'EL34s with A430 vs. A470?'

Apr 17, 2020

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...

• kward
• Replies: 37
• Forum: Tube Audio

What happens if I build them as Mark 3s, drop in 5u4s, EL34s and bias accordingly?

I’m inclined to build them due to the price of admission.

 

[gadget73]

honestly I wouldn't even get concerned about the 5U4 if you have good GZ34 rectifiers. Its about 15 more volts, well within the capabilities of an EL34.

If you don't have GZ34's, then by all means. 5U4GB's are fairly cheap and plenty robust. Just go with the Mark III's higher voltage rated caps if possible, the turn-on surge voltage is likely to be higher than with a GZ34 even if the running voltage is lower.

 

[Blackdog61]

OK. Build them as IIIs with 5ar4 rectifiers and bias for 34s.

 

[nerdorama]

Sounds ok to me. I built my 2’a with SS diodes and used the location vacated by the can cap for a damper diode for a slow start and a small voltage drop.

 

[Blackdog61]

Sounds ok to me. I built my 2’a with SS diodes and used the location vacated by the can cap for a damper diode for a slow start and a small voltage drop.

Tell me

 

[nerdorama]

Slightly wrong. I put a nylon vent plug in place of the can cap. Damper went where the original rectifier was located. I used a cap board designed by Sheldon Stokes so didn’t need a can cap. The damper diode is a 6CG3 which runs ok on the 5V winding at the current of this amp which is a fair bit below the diodes rated current. I also use an auto bias board d and a 6SN7 driver board.

 

[Blackdog61]

Looks beautiful, expensive and possibly beyond my ability.

 

[nerdorama]

You don't have to do it like mine. You can use EL34's with SS rectifiers or a tube rectifier. I used an inexpensive damper diode as a slow start instead of a 5AR4. My main point was that you can do many things with a Mk2.

The outputs may be a little different but not much. John Atwood of One Electron published measurements of many outputs some years ago. The first page includes the Dynaco outputs.

 

[Blackdog61]

What’s the best option for boards? Various 6AN8 boards seem to be the cheapest option but I just don’t know. I prefer to stay close to the original sound.

 

[gadget73]

6AN8 is the stock driver circuit. If they are all built to the stock schematic, not sure it matters a ton which. The passive bits on the board would matter more if its populated. Don't get one with garbage parts on it.

 

[Blackdog61]

I’ve got other things going on but I really appreciate the help in deciding whether to take this on and how to do it. First step will be stripping them down to paint the chassis’ (probably grey hammer tone). Might not get these done this year.

 

[TomD62]

Will be following your progress as I have picked up a pair of Mark III's as well. Already purchased a quad of NOS GE JAN 6550A tubes (ouch!), a pair of Matsushita Japan Realistic Lifetime Gold Clad 5AR4 / GZ34 rectifier tubes to replaced the installed JJ's, and the tubesforhifi 6SN7 boards. These will be Project #2 this Winter.

 

[BNR_1]

I have used a pair of TV damper diodes, type 19AU4 to power a stereo 50W/ch amplifier. Damper diodes are excellent rectifiers, and are probably one of the last good deals left in NOS tubes, but they're a little awkward to implement, because you need a pair, and the heater current requirement is fairly high.

Will be following your progress as I have picked up a pair of Mark III's as well. Already purchased a quad of NOS GE JAN 6550A tubes (ouch!), a pair of Matsushita Japan Realistic Lifetime Gold Clad 5AR4 / GZ34 rectifier tubes to replaced the installed JJ's, and the tubesforhifi 6SN7 boards. These will be Project #2 this Winter.

Nice. I always wondered why this mono amp did not get greater recognition given the general market price, performance, and shared knowledge. So much information about this amp and mods are available. This amp does not appear dificult to maintain. Replacement parts are available. It uses 6550/KT88 plus 5AR4 tubes, which I dig. It falls within the 60W window. Plus it is a mono....easier on the back.