Replacement driver board for MK III

just a quick blurb on so-called "tube matching". Hopefully you guys know they test the tubes at one specific voltage and one specific bias point,
and then check the current reading. So who cares if the tubes are "matched" at 250v or 350v, or if they are matched at 30ma.
What really MATTERS is that they are matched in YOUR specific amplifier running with YOUR specific characteristics.
The only way you can do that is to match them yourselves, with your own individual bias pot adjustment on each tube.
I have bought hundreds and hundreds of both "matched" tube and unmatched tubes in the past 20+ years. Matched tubes rarely match
unless you redesign the amplifier to match the matched tubes (by running it at a lower voltage and at whatever specific bias point they were tested at.
That not the optimum operating point for what you want, believe me!!
Call up any place that offers "tube matching" and ask them to give you the details of how they match. A few will be honest.
Most will tell you it's all computerized, and a lot of them will be clueless.

Another thing, for anyone (that's at least 80% of you reading this) that has a VTA driver board in their amplifier, take a look at the position of the bias pots
in your amp. If they are all set at about the same position (let's say 2 PM on a clock scale) then your tubes are matched!! If one or two tubes has the bias pots
cranked to some other position more than an hour away, they are definitely not matched.
This also becomes very obvious if you run your amp with a variac. Try running your amp on 100v instead of 120v. Check your bias readings.
Are they still matched?

tubes4hifi wrote:just a quick blurb on so-called "tube matching".   Hopefully you guys know they test the tubes at one specific voltage and one specific bias point,
and then check the current reading.   So who cares if the tubes are "matched" at 250v or 350v, or if they are matched at 30ma.
What really MATTERS is that they are matched in YOUR specific amplifier running with YOUR specific characteristics.
The only way you can do that is to match them yourselves, with your own individual bias pot adjustment on each tube.
I have bought hundreds and hundreds of both "matched" tube and unmatched tubes in the past 20+ years.   Matched tubes rarely match
unless you redesign the amplifier to match the matched tubes (by running it at a lower voltage and at whatever specific bias point they were tested at.
That not the optimum operating point for what you want, believe me!!
Call up any place that offers "tube matching" and ask them to give you the details of how they match.   A few will be honest.
Most will tell you it's all computerized, and a lot of them will be clueless.

Another thing, for anyone (that's at least 80% of you reading this) that has a VTA driver board in their amplifier, take a look at the position of the bias pots
in your amp.   If they are all set at about the same position (let's say 2 PM on a clock scale) then your tubes are matched!!   If one or two tubes has the bias pots
cranked to some other position more than an hour away, they are definitely not matched.
This also becomes very obvious if you run your amp with a variac.    Try running your amp on 100v instead of 120v.    Check your bias readings.
Are they still matched?

This is a very true.

Stating HOW the matching is done ( Va , Ia etc) or with what ( maximatcher ) is importent.

Another fact is that tubes does differ greatly, buying "unmatched" might give you very unbalanced tubesets, matching with any criteria will increase the chance that the tubes will be similar.
This also applies for old NOS tubes , they also differ wastly, and in addition is the likehood that
the ones sold today is the tubes that was rejected 30 years ago, but never thrown as waste.

There exists vendors that does a good job with matching, but finding which one is a matter of
trying them ( they all claim to make good matches, experience will show which ones really do)

Places like this is an excellent place to mention whenever a vendor does a good ( or bad ) job. Please
use your right as consumers to tell others your experience. It also is considered "good habits" to
give a criticized vendor a chance to respond.

tubes4hifi wrote:just a quick blurb on so-called "tube matching".   Hopefully you guys know they test the tubes at one specific voltage and one specific bias point,
and then check the current reading.   So who cares if the tubes are "matched" at 250v or 350v, or if they are matched at 30ma.
What really MATTERS is that they are matched in YOUR specific amplifier running with YOUR specific characteristics.
The only way you can do that is to match them yourselves, with your own individual bias pot adjustment on each tube.
I have bought hundreds and hundreds of both "matched" tube and unmatched tubes in the past 20+ years.   Matched tubes rarely match
unless you redesign the amplifier to match the matched tubes (by running it at a lower voltage and at whatever specific bias point they were tested at.
That not the optimum operating point for what you want, believe me!!
Call up any place that offers "tube matching" and ask them to give you the details of how they match.   A few will be honest.
Most will tell you it's all computerized, and a lot of them will be clueless.

Another thing, for anyone (that's at least 80% of you reading this) that has a VTA driver board in their amplifier, take a look at the position of the bias pots
in your amp.   If they are all set at about the same position (let's say 2 PM on a clock scale) then your tubes are matched!!   If one or two tubes has the bias pots
cranked to some other position more than an hour away, they are definitely not matched.
This also becomes very obvious if you run your amp with a variac.    Try running your amp on 100v instead of 120v.    Check your bias readings.
Are they still matched?

The Stock Mark III uses a very simple circuit, and it has simple regulation for bias on power tubes. It employs only three valves in the active stages plus one in power supply. Talking about the stock driver boards, in the entrance of 6AN8, a triode-pentode in which the pentode is used in input and provides virtually all of the voltage gain of the amplifier while the triode section is coupled directly to the previous phase inverter distributed load and goes to directly drive the finals. With this characteristic driver circuit it’s important to have matched driver tubes because if a triode draw 11mA and the other 21 mA it’s not the same at all, and also we can say something for the voltage amplification of pentode that give all of the voltage gain of the amplifier. The pentode section should be matched. I don’t know the VTA circuit because i never see the schematic but sure it’s not important to have matched tubes with that boards. About the sound i think it’s a taste choice but this it’s only my opinion.

This has been a helpful thread but like everything on the internet and in life, there are differing opinions. Also people who are so intensely focused on any detailed subject can make small differences seem big in discussion. So I'm just going to have to make an educated decision and go with the stock VTA driverboard and try to duplicate the Tubes4Hifi Mk III kit as much as possible. That kit has gotten rave reviews from those that have listened to them. My goal is a cooler running, more dependable MK III. I like the sound of my MK IIIs with their new 6550s and SDS cap board, but they run hot, have a high driver voltage and are slightly less tube sounding than my MC-225.

A check that I was waiting for just arrived, so I plan on ordering my stuff from Roy tomorrow, but a conflict between the SDS cap board I have already installed and the new VTA diver board was pointed out on another thread, so once again, I'm not sure which driver board to order.

Here's a pic of the underside of my amp...


From the pics, it looks like there are a couple caps on the edge of the VTA board that would hit the SDS board. Is there a way around this other than going with another board? While I have the pic posted a few other questions or concerns:

1) The SDS board also crowds the new triple speaker terminals I want to install, but I think I can bend the tabs up and out of the way.
2) You can see two caps; one between the small terminal strip just above the fuse holder and the other terminal strip with the diode. The other cap is between the middle tab on the bias pot and ground. The original manual calls for 50mfd caps, but on Chad at Triode's recommendation I up graded them to 100mfd. I don't see these caps on the VTA MK III's, are they replaced by the caps on the new driver board?
3) I like the idea of the Triode/Pentode switch that goes in the hole where the original bias pot was installed. That works because the new driver board has two tiny bias pots installed there. If I have to go with a different, say Poseidon board or other, will they also have an integral bias pot?
4) Any other comments or observations?

I realize I could throw the SDS board away and just install a new quad cap, but I hate wasting the time and money I just spent to build and install them. That plus it seemed like a worthwhile upgrade, I do hear a difference, so I would like to keep the SDS mod intact.

Thanks,

John

well its my way of dealing with the MK3s.....
if I ever need to go from Triode/UL...
Pentode....is a diff setup....................the Mk3 is so ezey to work on....you should just try the triode setup.....I like it.... but I have never live with triode.....even with K-horns.....just not a nef power...
I like the new b+ board.....................front end board....
I like the 6AN8....but the 6SN7 board would be for me......or one pr of ea. type... never know if you don't go.............have fun



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

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. 564 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

Bump for my friend John Mitchell