Dynaco Mark III

Ty_Bower · 23rd August 2007, 02:23 AM

In the March 2001 issue of Audio Xpress there was an article titled "The Cure for the Common Dynaco MKIII". Kara Chaffee described a modification for the Mark III which replaced the 6AN8 driver. It was supposed to be a relatively inexpensive mod which improved the sound.

Does anyone have any specific details about this article? I'm curious to know exactly what the mod involved.

ArtG · 24th August 2007, 09:10 PM

The modification involves the front end of the amp and a slight mod to the feedback circuit. The input 6AN8 is replaced by one half of a 12AT7 driving a 6CG7/6FQ7 Schmitt phase splitter. The feedback resistor is changed to a pot to allow adjustment. ( I see this technique shown on a number of mod circuits to tube amps. I wonder how many folks realize that changing the "feedback" in this fashion, also varies the bias on the input tube.) Additionally the author adds another filter stage to the power supply. These are probably all worthwhile changes to improve the MkIII.

SY · 24th August 2007, 10:23 PM

There are at least a hundred Mark III mods out there which replace the input stage. Probably worthwhile to do, but I'd put it behind power supply upgrades in effectiveness.

Bandersnatch · 25th August 2007, 01:32 AM

hey-Hey!!!,
The amp is a complete package. If you improve a piece of it, the shortfalls of the rest become more obvious. If you insist on using the Iron, put it on a chassis that has room to work in. Once that is done, there is far less limit to the sorts of mods available to you.
cheers,
Douglas

Cobra2 · 25th August 2007, 08:31 AM

Quote:

Originally posted by SY
There are at least a hundred Mark III mods out there which replace the input stage. Probably worthwhile to do, but I'd put it behind power supply upgrades in effectiveness.

I have a pair MkIII, and need new power-trafos...for 230V.
Can you please suggest were I can find a suitable upgrade?
I will probably use a SS rectifier with relay/delayed HV.
and new 550V JJ caps...or a cap-upgrade board, and have already finished a pair "diytube.com" "Poseidon" driver boards.

I have been tempted to run the two 115V trafos in series...but afraid that one will "over-power" if the loads are different?

Arne K

SY · 25th August 2007, 12:47 PM

You probably don't want to put the primaries of two different transformers in series.

Two major PS upgrades are desirable: first, larger reservoir capacitance. That will be the biggest bang-for-buck. Increasing the decoupling capacitance for the driver stage is also very worthwhile. Second, regulation of the B+ and the bias supply. Bias supply regulation is easy, needing nothing fancier than a Zener diode. B+ regulation is a bit more complex, but still not overwhelming- check out Joe Curcio's reg for the Stereo 70 published some years ago in Glass Audio.

If it were me, I'd use something a little simpler like a Maida reg or a simple series regulator with a one transistor error amp. At minimum, regulate the driver stage supplies, but best practice is to regulate the output stage supply as well.

One thing important to note- output and bias supplies should both be regulated or neither should be. Don't just do one.

j beede · 28th March 2011, 03:29 AM

You'll want to excise that 525V quad cap before replacing the 5AR4 with a bridge. The higher B+ also means KT-88s not 6550 for outputs. I was able to fit ten 47µF, 450V radial caps on 2.5" x 4" board that fits nicely under the PT of the Mark III to give me 50-25-25-25 µF at ~900WVDC. I left the liberated quad cap in place as a reminder of what a flawed component it is. With 122VAC coming out of the wall I see pin #8 of my 5AR4 at 525V all day long.

Cobra2 · 28th March 2011, 08:01 PM

I just finished the cleanup / upgrade of my first pair of MkIII, and WOW !!! -this is a totally different "beast" than a ST-70 !
New power-trafos for 230V, "Poseidon" pcb,( diytube.com :: View Forum - poseidon ) and PSU-cap-boards.
If I had known the difference (from ST-70) was that big, I would have finished them long ago!

Arne K

j beede · 30th March 2011, 04:05 AM

Quote:

Originally Posted by Cobra2

I just finished the cleanup / upgrade of my first pair of MkIII, and WOW !!! -this is a totally different "beast" than a ST-70 !
New power-trafos for 230V, "Poseidon" pcb,( diytube.com :: View Forum - poseidon ) and PSU-cap-boards.
If I had known the difference (from ST-70) was that big, I would have finished them long ago!

Arne K

I know what you mean. I had never heard Mark IIIs before I finished reworking the set that I rescued from an outdoor shed a few weeks back. They sound nothing like any ST-70 I have heard

Cobra2 · 17th September 2011, 06:39 PM

Another pair finished, this time with all new parts.
-as good as the old ones I upgraded... maybe even better!
(Stainless steel chassi, SDS-cap-boards, Poseidon driver,...)

Arne K

23rd August 2007, 02:23 AM	#1
Ty_Bower diyAudio Member Join Date: Mar 2007 Location: Newark, DE	Dynaco Mark III In the March 2001 issue of Audio Xpress there was an article titled "The Cure for the Common Dynaco MKIII". Kara Chaffee described a modification for the Mark III which replaced the 6AN8 driver. It was supposed to be a relatively inexpensive mod which improved the sound. Does anyone have any specific details about this article? I'm curious to know exactly what the mod involved.

24th August 2007, 10:23 PM	#3
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	There are at least a hundred Mark III mods out there which replace the input stage. Probably worthwhile to do, but I'd put it behind power supply upgrades in effectiveness. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

25th August 2007, 01:32 AM	#4
Bandersnatch diyAudio Member Join Date: Jul 2003 Location: Ann Arbor, MI	hey-Hey!!!, The amp is a complete package. If you improve a piece of it, the shortfalls of the rest become more obvious. If you insist on using the Iron, put it on a chassis that has room to work in. Once that is done, there is far less limit to the sorts of mods available to you. cheers, Douglas __________________ the Tnuctipun will return

25th August 2007, 12:47 PM	#6
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	You probably don't want to put the primaries of two different transformers in series. Two major PS upgrades are desirable: first, larger reservoir capacitance. That will be the biggest bang-for-buck. Increasing the decoupling capacitance for the driver stage is also very worthwhile. Second, regulation of the B+ and the bias supply. Bias supply regulation is easy, needing nothing fancier than a Zener diode. B+ regulation is a bit more complex, but still not overwhelming- check out Joe Curcio's reg for the Stereo 70 published some years ago in Glass Audio. If it were me, I'd use something a little simpler like a Maida reg or a simple series regulator with a one transistor error amp. At minimum, regulate the driver stage supplies, but best practice is to regulate the output stage supply as well. One thing important to note- output and bias supplies should both be regulated or neither should be. Don't just do one. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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24th August 2007, 09:10 PM	#2
ArtG diyAudio Member Join Date: Nov 2006 Location: Central PA	The modification involves the front end of the amp and a slight mod to the feedback circuit. The input 6AN8 is replaced by one half of a 12AT7 driving a 6CG7/6FQ7 Schmitt phase splitter. The feedback resistor is changed to a pot to allow adjustment. ( I see this technique shown on a number of mod circuits to tube amps. I wonder how many folks realize that changing the "feedback" in this fashion, also varies the bias on the input tube.) Additionally the author adds another filter stage to the power supply. These are probably all worthwhile changes to improve the MkIII.

28th March 2011, 03:29 AM	#7
j beede diyAudio Member Join Date: Dec 2007	You'll want to excise that 525V quad cap before replacing the 5AR4 with a bridge. The higher B+ also means KT-88s not 6550 for outputs. I was able to fit ten 47µF, 450V radial caps on 2.5" x 4" board that fits nicely under the PT of the Mark III to give me 50-25-25-25 µF at ~900WVDC. I left the liberated quad cap in place as a reminder of what a flawed component it is. With 122VAC coming out of the wall I see pin #8 of my 5AR4 at 525V all day long.

28th March 2011, 08:01 PM	#8
Cobra2 DIY ! diyAudio Member Join Date: Apr 2002 Location: Stavanger, Norway	I just finished the cleanup / upgrade of my first pair of MkIII, and WOW !!! -this is a totally different "beast" than a ST-70 ! New power-trafos for 230V, "Poseidon" pcb,( diytube.com :: View Forum - poseidon ) and PSU-cap-boards. If I had known the difference (from ST-70) was that big, I would have finished them long ago! Arne K __________________ Ars longa, vita brevis

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