MKII vs. MKIII

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Could anyone tell me the difference bewteen the two? Also, what's the difference between MKIII and MKIV? Do they all use the same irons? Thanks!

[Shannon Parks]

The Mark IV is basically half of an ST70 - kinda.

http://www.geocities.com/vintageaudio/

[Ty_Bower]

Odd... I thought the Mark IV was half a ST70. It used the A470 output transformer, same as the ST70.

The Mark II had A430 outputs, and the Mark III had A431 outputs. Both pieces of iron appear to be essentially the same, except the A431 has taps for 4, 8, and 16 ohms. The A430 only supported 8 and 16.

http://home.indy.net/~gregdunn/dynaco/catalog.html

[Shannon Parks]

Odd... I thought the Mark IV was half a ST70. It used the A470 output transformer, same as the ST70.

What was I thinking? You are correct. I admit to failing Roman Numerology in high school.

The Mark II had A430 outputs, and the Mark III had A431 outputs. Both pieces of iron appear to be essentially the same, except the A431 has taps for 4, 8, and 16 ohms. The A430 only supported 8 and 16.

http://home.indy.net/~gregdunn/dynaco/catalog.html

In addition to that output transformer change, the Mark II used a 5U4G rectifier and EL34s. So basically it is a stripped down (ie cheaper) Mark III. Makes sense.

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Ok, do you think $300 for a pair of MKIIs is a good deal? One of them is missing some parts like resistors and a can cap etc. The irons are still good as are the original driver boards. So Shannon, with poseidon boards you can mod MKIIs and drive them with different tubes? Also, is it possible to use other tubes with the original desgin? If not, what mods can be made? Thanks.

[Ty_Bower]

Ok, do you think $300 for a pair of MKIIs is a good deal? One of them is missing some parts like resistors and a can cap etc. The irons are still good...

Go for it. The transformers alone are probably worth it.

[Shannon Parks]

I think you can just drop the MKIII build directly in. I didn't see any circuit differences in a cursory viewing of the schematic.

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Didn't MKIIs come with filter chokes? My MKIIs don't seem to have any. The schematic calls for a 50 ohms 10W resistor instead. Any recommendation for sourcing can caps for MKIIs? I see some on ebay for 35 bucks. Thanks!

[Ty_Bower]

If you check out the link in the second post of this thread, Shannon points out a page which has the schematic for the Mark II. Apparently, it does not have a choke. Many DIY ST35 owners have subbed the C354 (or equivalent) for the 50 ohm resistor in the power supply. It makes sense that Dynaco did not provide the choke on the Mark II. It would have been a little cheaper, and probably doesn't hurt performance all that much.

I'd consider the usual suspects for the can cap... Triode Electronics, etc. They've got a can which says it's good for the Mark II, but not the Mark III. Look here. Not being intimately familiar with the Mark II/III, that doesn't make much sense to me. If it's not recommended for the III, why is it good enough for the II? Anyhoo, they suggest looking at the SDS cap board. That might be a good solution for you, although it won't be as aesthetically pleasing as a real can.
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[Ty_Bower]

Aha! I see the difference. The Mark II schematic has a 5U4. The Mark III shows a 5AR4.

I'd seek out a higher rated set of power supply caps for both blocks, and use a 5AR4 in each instead.

[Shannon Parks]

Any recommendation for sourcing can caps for MKIIs? I see some on ebay for 35 bucks. Thanks!

Looks like the one at Antique Electronics is also around $35 now. Search for C-EC80-40-30-20 .

I imagine Triode Electronics is still selling the choke for $10.

[kheper]

Both the MK II and MK III used the same
power supply, the P-782. In the original
schematics, the advertized B+ for the MKII
is 465V. For the MKIII, 480V.

The first filter cap on the MKII is rated
at 500V. This seems way low, because
there'll be a turn-on surge greater than 500V,
because today's line voltage is higher than in
the past. The 5U4GB may drop the voltage
15V more than the 5AR4, but a cap rated
for 500V is out. A cap rated for 525V may
be serviceable, but it, too, is pushing it limits.
You may want to look for a cap-board or
use 550V rated electrolytics. F&T make
such caps.

A NOS 5AR4, a cap-board, and a choke will
provide the best regulation. The EL-34s
will be able to handle the 15 extra volts, as
well. Some people use KT-88s in the MK II.
But you may have to alter the voltage divider
resistors in the bias supply for the proper
voltage.

The MK IV used A-470 outputs. The MK III
used A-431 outputs. I'd kill for a cheap pair
of MKIIs.