Replacement driver board for MK III

GlacierJohn wrote:Ok, I'm bumping this thread from last year. I'm too busy with outdoors stuff to do anything but listen to my stereo from May-December. Now in late winter I'm ready to play with my MK IIIs again, been listening to my wonderful MC225 since last spring.

I installed the SDS power supply mod last winter and with your help got them up and running. Now I'm ready to order the tube 4 hifi driver boards and try Snav's idea for lowering my B+ voltage. My line voltage out the wall is 123 volts and my B+ is plus and minus 564 volts. Will Snav's suggestion drop that B+ voltage enough? I'm about 90 volts higher than spec right now. I biased the amp at an even 1.50 volts. My amp runs pretty hot, not too hot to touch but way hotter than my 225. Any other ideas? What about installing that mod for switching to triode mode? Or the separate bias pots?

I have three matched sets of 6550s on hand, and the amp sounds good right now, though not as good as my restored Mc225, but what is. The MK IIIs have very strong tight bass and extended airy highs, I guess my point is I'm happy with the sound so any improvement is bonus, while running cooler and driving the 6550s a little easier would be good.

Bob Latino wrote:

GlacierJohn wrote:Ok, I'm bumping this thread from last year. I'm too busy with outdoors stuff to do anything but listen to my stereo from May-December. Now in late winter I'm ready to play with my MK IIIs again, been listening to my wonderful MC225 since last spring.

I installed the SDS power supply mod last winter and with your help got them up and running. Now I'm ready to order the tube 4 hifi driver boards and try Snav's idea for lowering my B+ voltage. My line voltage out the wall is 123 volts and my B+ is plus and minus 564 volts. Will Snav's suggestion drop that B+ voltage enough? I'm about 90 volts higher than spec right now. I biased the amp at an even 1.50 volts. My amp runs pretty hot, not too hot to touch but way hotter than my 225. Any other ideas? What about installing that mod for switching to triode mode? Or the separate bias pots?

I have three matched sets of 6550s on hand, and the amp sounds good right now, though not as good as my restored Mc225, but what is. The MK IIIs have very strong tight bass and extended airy highs, I guess my point is I'm happy with the sound so any improvement is bonus, while running cooler and driving the 6550s a little easier would be good.

If you run an older Mark III that was supposed to run on 1957 voltage (115 - 117 VAC USA line voltage) and then run that at 123 volts, the B+ voltage will be sky high as you have found out. 563 volts DC will shorten tube life. That voltage should be about 490 VDC. I bet if you measure across pins 2 and 7 of each output tube, your filament voltage will be much higher than 6.3 VAC. Probably up in the 7+ volt AC range. If you continue to run the amp(s) like that, tube life will be noticeably shortened.

Suggestions ..
1. If you are using a 5AR4 tube rectifier switch to a 5U4. This will drop all the DC voltages in the amp but will not the AC voltages. If you are using the SDS's power supply ALSO for solid state rectification > switch back to tube rectification. The worst thing you can to to an OLDER ORIGINAL Mark III or ST-70 is to use solid state rectification. Using solid state rectification causes even a higher B+ than a GZ34/5AR4 and your much higher than normal 123 VAC line voltage is making things even worse.

2. Get a VARIAC and set it to give an output of about 117 - 118 VAC. Now use tube rectification and your voltages should be pretty much normal again ...

Bob

zx wrote:just concentrate on the power supply which I did......
You put new caps in right.??..even if you go with a other frontend ......whats your B+???...If its over 500-550V like my 3pr are.....i have 125AC comeing from the wal here in Fl... you well wont to drop  it to about 480V thats what the amps were made to run at....for the best sound out these amps....
Well sounds best to me.....
Otheres can say on the sound of the new boards....the 6SN7 one look good to me....an a vary fair $$..if you dont think it sounds better than stock not out a lot .....i like the stock 6AN8-7199...one tube makes less noise.. so my ears say.... others ears may say diff....an thay can speaker for there self......hehe ....have fun with tubes.........

Thanks for the site Bob......

Tiziano73 wrote:

zx wrote:just concentrate on the power supply which I did......
You put new caps in right.??..even if you go with a other frontend ......whats your B+???...If its over 500-550V like my 3pr are.....i have 125AC comeing from the wal here in Fl... you well wont to drop  it to about 480V thats what the amps were made to run at....for the best sound out these amps....
Well sounds best to me.....
Otheres can say on the sound of the new boards....the 6SN7 one look good to me....an a vary fair $$..if you dont think it sounds better than stock not out a lot .....i like the stock 6AN8-7199...one tube makes less noise.. so my ears say.... others ears may say diff....an thay can speaker for there self......hehe ....have fun with tubes.........

Thanks for the site Bob......

I am in agreement, i prefer stock boards with good 6AN8 matching (same triode current and same Gm on pentode). The best thing to do it's control the power supply.

Tiziano.

MontanaWay wrote:

Tiziano73 wrote:

zx wrote:just concentrate on the power supply which I did......
You put new caps in right.??..even if you go with a other frontend ......whats your B+???...If its over 500-550V like my 3pr are.....i have 125AC comeing from the wal here in Fl... you well wont to drop  it to about 480V thats what the amps were made to run at....for the best sound out these amps....
Well sounds best to me.....
Otheres can say on the sound of the new boards....the 6SN7 one look good to me....an a vary fair $$..if you dont think it sounds better than stock not out a lot .....i like the stock 6AN8-7199...one tube makes less noise.. so my ears say.... others ears may say diff....an thay can speaker for there self......hehe ....have fun with tubes.........

Thanks for the site Bob......

I am in agreement, i prefer stock boards with good 6AN8 matching (same triode current and same Gm on pentode). The best thing to do it's control the power supply.

Tiziano.

It is important to have a good, clean and steady power supply, no argument there. But that is, I feel, only about half way there. Updating the driver board, in my opinion, will make the biggest change in sound.
To me, there is no comparison between the old driver circuit design to the updated Tubes4Hifi design, the latter being head and shoulder above the original. Some people prefer to keep a circuit as original as possible, and thats OK, but I feel that by doing that, you would not get the best sound out of your system. And don't forget, sound is extremely subjective as well!
Like a lot of original/early Dynaco designs, it was a compromise, one of the main aims being saving $.
For us, paying a little extra for two tubes and additional resistors etc is no big deal, but when you start talking about 100's even 1000's of kits, it makes a huge difference to the bottom line.

So, if you have the $ to upgrade the driver board, do it.

GlacierJohn wrote:

MontanaWay wrote:

Tiziano73 wrote:

zx wrote:just concentrate on the power supply which I did......
You put new caps in right.??..even if you go with a other frontend ......whats your B+???...If its over 500-550V like my 3pr are.....i have 125AC comeing from the wal here in Fl... you well wont to drop  it to about 480V thats what the amps were made to run at....for the best sound out these amps....
Well sounds best to me.....
Otheres can say on the sound of the new boards....the 6SN7 one look good to me....an a vary fair $$..if you dont think it sounds better than stock not out a lot .....i like the stock 6AN8-7199...one tube makes less noise.. so my ears say.... others ears may say diff....an thay can speaker for there self......hehe ....have fun with tubes.........

Thanks for the site Bob......

I am in agreement, i prefer stock boards with good 6AN8 matching (same triode current and same Gm on pentode). The best thing to do it's control the power supply.

Tiziano.

It is important to have a good, clean and steady power supply, no argument there. But that is, I feel, only about half way there. Updating the driver board, in my opinion, will make the biggest change in sound.
To me, there is no comparison between the old driver circuit design to the updated Tubes4Hifi design, the latter being head and shoulder above the original. Some people prefer to keep a circuit as original as possible, and thats OK, but I feel that by doing that, you would not get the best sound out of your system. And don't forget, sound is extremely subjective as well!
Like a lot of original/early Dynaco designs, it was a compromise, one of the main aims being saving $.
For us, paying a little extra for two tubes and additional resistors etc is no big deal, but when you start talking about 100's even 1000's of kits, it makes a huge difference to the bottom line.

So, if you have the $ to upgrade the driver board, do it.

Obviously a matter of opinion, but having googled the topic and read every related thread on various forums going back 15 years, your opinion seems to predominate. As stated I have already upgraded the power supply with the SDS mod, now I am committed to going whole hog with new tube sockets, speaker jacks, input plug, triode/pentode switch and driver board. The question was which driver board was the best, there is no definitive answer to that, whether it is the Poseidon, Tunes4Hifi, Octal whatever, everyone seems to think they are all an upgrade over the original. As a bonus, with the new driver board I will get two bias pots and that replacement terminal to bias each tube separately.

BTW, I just re-measured my pin voltages, my heater voltage is still too high 6.7 volts, but the B+ voltages now measure around 490 volts, much closer to spec.

zx wrote:I... get better sound with one pot for the outputs........... I have tube amps setup bouth ways...
The sound of the amps with a pot for each tube .....to me is like... if you were trying to dance with one foot nailed to the ground....the outputs cant talk to each other...an it sound like you Dont get full output.....
when the outputs cathoides are tied to gather then grounded... thay can talk to each other.... the sound is sweeter an fuller....to me!
The B+ being to high... like  over 500v....kills the sound!
480v is a sweet spot..........
With any drive board.... that has pot for ea output... can ezzly be setup for one pot for the outputs....
40 years I have had a lot of tube amps old an new......it all about the sound to me...
An as for the sound of the 6AN8/7199 frount end.....one tube has less noise!
but I have 6sn7 in preamps....... front ends of amps .....big tone..great tube...some say the best driver.
The only way I have found the sound I have an like......is by trying ever thing I could...
so this is just one mans o-pine.....if you don't go you don't know.....hehe have fun with tubes
An thanks for any an all info on getting better sound out of tubes


thanks for the site Bob...................

GlacierJohn wrote:

zx wrote:I... get better sound with one pot for the outputs........... I have tube amps setup bouth ways...
The sound of the amps with a pot for each tube .....to me is like... if you were trying to dance with one foot nailed to the ground....the outputs cant talk to each other...an it sound like you Dont get full output.....
when the outputs cathoides are tied to gather then grounded... thay can talk to each other.... the sound is sweeter an fuller....to me!
The B+ being to high... like  over 500v....kills the sound!
480v is a sweet spot..........
With any drive board.... that has pot for ea output... can ezzly be setup for one pot for the outputs....
40 years I have had a lot of tube amps old an new......it all about the sound to me...
An as for the sound of the 6AN8/7199 frount end.....one tube has less noise!
but I have 6sn7 in preamps....... front ends of amps .....big tone..great tube...some say the best driver.
The only way I have found the sound I have an like......is by trying ever thing I could...
so this is just one mans o-pine.....if you don't go you don't know.....hehe have fun with tubes
An thanks for any an all info on getting better sound out of tubes


thanks for the site Bob...................

Now my heads starting to spin. So individual bias pots is a bad thing? I'm not worried about voltage now because I will install a Variac and run the system at 117 volts. Currently my tubes are matched, so maybe the bias isn't an issue, but it seemed like a harmless option to keep open. The one tube driver vs two tube drive. Sound of the octal 6SN7/6SL7 vs 12AU7/12BH7. I don't know if it means anything, but I love the sound of my Mcintosh MC-225 which has 12BH7 driver tubes. I was just getting to order everything from Roy. I know it's not an end of the world decision, I'm sure they all sound good, but anybody else have an opinion on this?

MontanaWay wrote:

GlacierJohn wrote:

zx wrote:I... get better sound with one pot for the outputs........... I have tube amps setup bouth ways...
The sound of the amps with a pot for each tube .....to me is like... if you were trying to dance with one foot nailed to the ground....the outputs cant talk to each other...an it sound like you Dont get full output.....
when the outputs cathoides are tied to gather then grounded... thay can talk to each other.... the sound is sweeter an fuller....to me!
The B+ being to high... like  over 500v....kills the sound!
480v is a sweet spot..........
With any drive board.... that has pot for ea output... can ezzly be setup for one pot for the outputs....
40 years I have had a lot of tube amps old an new......it all about the sound to me...
An as for the sound of the 6AN8/7199 frount end.....one tube has less noise!
but I have 6sn7 in preamps....... front ends of amps .....big tone..great tube...some say the best driver.
The only way I have found the sound I have an like......is by trying ever thing I could...
so this is just one mans o-pine.....if you don't go you don't know.....hehe have fun with tubes
An thanks for any an all info on getting better sound out of tubes


thanks for the site Bob...................

Now my heads starting to spin. So individual bias pots is a bad thing? I'm not worried about voltage now because I will install a Variac and run the system at 117 volts. Currently my tubes are matched, so maybe the bias isn't an issue, but it seemed like a harmless option to keep open. The one tube driver vs two tube drive. Sound of the octal 6SN7/6SL7 vs 12AU7/12BH7. I don't know if it means anything, but I love the sound of my Mcintosh MC-225 which has 12BH7 driver tubes. I was just getting to order everything from Roy. I know it's not an end of the world decision, I'm sure they all sound good, but anybody else have an opinion on this?

individual biasing of the output tubes is preferred....absolutely. This way you can 'match' the output tubes to the best.
As has been said many times here, sound is extremely subjective. What I know though is that for many people, who have heard the 12BH7 then the 6XX7 series in a similar amp, mostly prefer the 6XX7's.

GlacierJohn wrote:

MontanaWay wrote:

GlacierJohn wrote:

zx wrote:I... get better sound with one pot for the outputs........... I have tube amps setup bouth ways...
The sound of the amps with a pot for each tube .....to me is like... if you were trying to dance with one foot nailed to the ground....the outputs cant talk to each other...an it sound like you Dont get full output.....
when the outputs cathoides are tied to gather then grounded... thay can talk to each other.... the sound is sweeter an fuller....to me!
The B+ being to high... like  over 500v....kills the sound!
480v is a sweet spot..........
With any drive board.... that has pot for ea output... can ezzly be setup for one pot for the outputs....
40 years I have had a lot of tube amps old an new......it all about the sound to me...
An as for the sound of the 6AN8/7199 frount end.....one tube has less noise!
but I have 6sn7 in preamps....... front ends of amps .....big tone..great tube...some say the best driver.
The only way I have found the sound I have an like......is by trying ever thing I could...
so this is just one mans o-pine.....if you don't go you don't know.....hehe have fun with tubes
An thanks for any an all info on getting better sound out of tubes


thanks for the site Bob...................

Now my heads starting to spin. So individual bias pots is a bad thing? I'm not worried about voltage now because I will install a Variac and run the system at 117 volts. Currently my tubes are matched, so maybe the bias isn't an issue, but it seemed like a harmless option to keep open. The one tube driver vs two tube drive. Sound of the octal 6SN7/6SL7 vs 12AU7/12BH7. I don't know if it means anything, but I love the sound of my Mcintosh MC-225 which has 12BH7 driver tubes. I was just getting to order everything from Roy. I know it's not an end of the world decision, I'm sure they all sound good, but anybody else have an opinion on this?

individual biasing of the output tubes is preferred....absolutely. This way you can 'match' the output tubes to the best.
As has been said many times here, sound is extremely subjective. What I know though is that for many people, who have heard the 12BH7 then the 6XX7 series in a similar amp, mostly prefer the 6XX7's.

Thank you, that is helpful. Is it snowing down your way? Just started up here in Bigfork.

zx wrote:With Bias...if the tubes are not matched.....as most are an vary close to day...with the pot for each output tube.....when the tubes is pushed... one well pull more bias V....from where?? the other output tube!.....
This is not too tell anyone what thay hear.....or need!
I like the Sound of the Stock MK3s I have.....I have hard to drive speakers like Apogee an Acoustats ESL...An Magnepans........So getting all the output I can out of the Amps I drive them with.. is a must.....so having the Cathoides ran togather befor the one res ... gives me that...
But like I said....try it all....are you may never know......or find the sound that's for you....that's all that matters in the end....by the way a lot of people think the MC225 is the best sounding tube peace MC ever made....even me....but not a nef power for my speakers....but if your into horns....sweet sound ...I have never got out of a Dynaco
An as for the 6AN8/7199 Dynaco stock driver tube.....costing less....so that was why thay were used...Hehe...
I don't buy it!.....in 1959 the 12xx7 an even the 6XX7 cost 10-20 cent ea...at the drug store....I used to buy tubes for my grandfather TV....you could say ....if you need a lot of tubes back then......thay were a dime a dozen.....hehe.....have fun with tubes



thanks for the site Bob..............

I not saleing any thing...........good luck

GlacierJohn wrote:

zx wrote:With Bias...if the tubes are not matched.....as most are an vary close to day...with the pot for each output tube.....when the tubes is pushed... one well pull more bias V....from where?? the other output tube!.....
This is not too tell anyone what thay hear.....or need!
I like the Sound of the Stock MK3s I have.....I have hard to drive speakers like Apogee an Acoustats ESL...An Magnepans........So getting all the output I can out of the Amps I drive them with.. is a must.....so having the Cathoides ran togather befor the one res ... gives me that...
But like I said....try it all....are you may never know......or find the sound that's for you....that's all that matters in the end....by the way a lot of people think the MC225 is the best sounding tube peace MC ever made....even me....but not a nef power for my speakers....but if your into horns....sweet sound ...I have never got out of a Dynaco
An as for the 6AN8/7199 Dynaco stock driver tube.....costing less....so that was why thay were used...Hehe...
I don't buy it!.....in 1959 the 12xx7 an even the 6XX7 cost 10-20 cent ea...at the drug store....I used to buy tubes for my grandfather TV....you could say ....if you need a lot of tubes back then......thay were a dime a dozen.....hehe.....have fun with tubes



thanks for the site Bob..............

I not saleing any thing...........good luck

Thank you. Just to be clear I listened to them in stock form from 1988 to 1995 when I got away from two channel for a bit, then again this past year since I upgraded the power supply. I actually traded a very nice Conrad Johnson straight across for the MK IIIs, probably not smartest monetary decision, but I liked the sound of the old Dynaco's better. At the time I was running three pair of speakers from four different power amps. My 1975 JBL L-65s, Martin Logan CLS's and a pair of B&W DM 2000s. Driving them I switched around between Conrad Johnson, Dynaco Pat 5 and Van Alstine Super Pas III pre amps and the MV 75 - Mark IIIs, Dynaco ST 400, Hafler DM 2000 that was modified by Musical Concepts and my favorite of all time McIntosh MC225. Slowly over the years my tubes burned out and living where I do I would just set each amp up on a shelf until there was only my Yamaha CR2020 receiver.

Point is I am very familiar with the stock MK III sound as well as others and would like to try something different. I would like a little more sweetness but with medium power. I don't need a lot since I'm set up in a small 10'x12' room, just a change of pace amp from the sweet little Mc225.

MontanaWay wrote:

Tiziano73 wrote:

zx wrote:just concentrate on the power supply which I did......
You put new caps in right.??..even if you go with a other frontend ......whats your B+???...If its over 500-550V like my 3pr are.....i have 125AC comeing from the wal here in Fl... you well wont to drop  it to about 480V thats what the amps were made to run at....for the best sound out these amps....
Well sounds best to me.....
Otheres can say on the sound of the new boards....the 6SN7 one look good to me....an a vary fair $$..if you dont think it sounds better than stock not out a lot .....i like the stock 6AN8-7199...one tube makes less noise.. so my ears say.... others ears may say diff....an thay can speaker for there self......hehe ....have fun with tubes.........

Thanks for the site Bob......

I am in agreement, i prefer stock boards with good 6AN8 matching (same triode current and same Gm on pentode). The best thing to do it's control the power supply.

Tiziano.

It is important to have a good, clean and steady power supply, no argument there. But that is, I feel, only about half way there. Updating the driver board, in my opinion, will make the biggest change in sound.
To me, there is no comparison between the old driver circuit design to the updated Tubes4Hifi design, the latter being head and shoulder above the original. Some people prefer to keep a circuit as original as possible, and thats OK, but I feel that by doing that, you would not get the best sound out of your system. And don't forget, sound is extremely subjective as well!
Like a lot of original/early Dynaco designs, it was a compromise, one of the main aims being saving $.
For us, paying a little extra for two tubes and additional resistors etc is no big deal, but when you start talking about 100's even 1000's of kits, it makes a huge difference to the bottom line.

So, if you have the $ to upgrade the driver board, do it.