Dynaco Mark III - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 23rd August 2007, 02:23 AM   #1
diyAudio Member
 
Ty_Bower's Avatar
 
Join Date: Mar 2007
Location: Newark, DE
Default 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.
  Reply With Quote
Old 24th August 2007, 09:10 PM   #2
ArtG is offline ArtG  United States
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.
  Reply With Quote
Old 24th August 2007, 10:23 PM   #3
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
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.
__________________
The more you pay for it, the less inclined you are to doubt it.- George Smiley
  Reply With Quote
Old 25th August 2007, 01:32 AM   #4
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
  Reply With Quote
Old 25th August 2007, 08:31 AM   #5
Cobra2 is offline Cobra2  Norway
DIY !
diyAudio Member
 
Cobra2's Avatar
 
Join Date: Apr 2002
Location: Stavanger, Norway
Send a message via Skype™ to Cobra2
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
__________________
Don't believe everything you think...
  Reply With Quote
Old 25th August 2007, 12:47 PM   #6
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
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.
__________________
The more you pay for it, the less inclined you are to doubt it.- George Smiley
  Reply With Quote
Old 28th March 2011, 03:29 AM   #7
j beede is offline j beede  United States
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 47F, 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 F 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.

Click the image to open in full size.
Click the image to open in full size.
  Reply With Quote
Old 28th March 2011, 08:01 PM   #8
Cobra2 is offline Cobra2  Norway
DIY !
diyAudio Member
 
Cobra2's Avatar
 
Join Date: Apr 2002
Location: Stavanger, Norway
Send a message via Skype™ to 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
__________________
Don't believe everything you think...
  Reply With Quote
Old 30th March 2011, 04:05 AM   #9
j beede is offline j beede  United States
diyAudio Member
 
Join Date: Dec 2007
Quote:
Originally Posted by Cobra2 View Post
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

Click the image to open in full size.
  Reply With Quote
Old 17th September 2011, 06:39 PM   #10
Cobra2 is offline Cobra2  Norway
DIY !
diyAudio Member
 
Cobra2's Avatar
 
Join Date: Apr 2002
Location: Stavanger, Norway
Send a message via Skype™ to Cobra2
Default More Mk III

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
Attached Images
File Type: jpg Dyna MkIII (Medium).JPG (142.8 KB, 373 views)
__________________
Don't believe everything you think...
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Dynaco Mark VI Alex007 Tubes / Valves 39 13th September 2013 12:14 PM
Dynaco Mark III Kilowatt Tubes / Valves 17 1st February 2013 07:56 PM
Trade 2-Dynaco Mark IV's for Coldamps Wa Swap Meet 1 23rd January 2008 12:39 AM
Dynaco Information MArk II Required jbldriven Tubes / Valves 0 25th February 2007 11:24 PM


New To Site? Need Help?

All times are GMT. The time now is 03:09 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2