Restore Three Mark II Amps

I posted photos, in the photo section, of three Mark IIs that a friend wants me to restore:

Three Mark II Amps

I've done a little research, in this forum and elsewhere, so I am aware that many recommend upgrading to the Mark III, and some recommend getting new circuit boards populated with metal film resistors. At this point, my main goal is to simply get each amp running, and have them produce sound that is equivalent to how they worked when new.

I have another friend who has a Hickok tube tester and a Variac. I have an isolation transformer. If he doesn't come through with the tester, I may be dead in the water, since even in Silicon Valley (100 miles to my north), there are no public tube testers anymore.

At this point my plan is to:

1. Test the tubes and replace both the missing tubes and any which test bad.

2. "Re-cap" each amp, although I need to read more to figure out whether I can just use modern caps from Mouser or Digikey, or whether I need to find something more exotic. I don't know yet where to get a replacement for the big can capacitor.

3. Power up, adjust the bias, and go home.

There appears to be a small selenium rectifier. I've replaced those before (in my old Wollensak T-1500 tape recorder), but figuring out the right value for the series resistor is a little hit or miss, and I'm not sure the work was necessary for my Wollensak, and it may not be for this either. Thus, I'm not planning to do anything with that at the moment.

I'll post back again as the work progresses.

Hi John,

I have a pair of MkIIIs that I redid last year.  I replaced the quad cap and built/installed Roy's driver boards.  I had also previously removed the selenium rectifier -- replaced with diode.  In essence, the only thing left from original was the chassis, transformers, and choke.

The key point is that Roy's driver circuit make the amp sound amazing.  I did the octal version, but you might want to go with the 12AU7 version due to the added current load from the octal 6SN7 tubes.  Ask Roy or Bob.  They can also advise you on any differences vis-a-vis MkII vs. MkIII.

Here's the MkIII page on Tubes 4 Hifi -- Bob and Roy's site:

http://www.tubes4hifi.com/MK3.htm

dave

Many thanks. It is tempting to go for the upgrade, but at this point I will be happy if I can get them working again, producing sound that is reasonable. I'm 69 and my client is 83, so neither of us have quite the hearing we used to.

The conversion doesn't look that daunting, but I want to take stock of what problems I have before I go driving down the road of refurbishment and/or upgrade.

In the few hours since I started this I was able to borrow an operational Hickok 539C tester (see photo below). My friend had a bunch of EL34 tubes, so we performed a test on one of those, just to make sure it is still working (it is).

He also loaned me his Variac which he had modified so you can easily put an ammeter in series while powering up, to watch for current spikes as the old capacitors reform (or not!). That was a bonus I hadn't expected.

So here's the current plan:

• I'll test the tubes;
  
• I'll do a quick in-circuit test of the caps with a capacitance meter;
  
• I'll check solder joints, etc., fix the broken fuse holder, etc.,
  ... and then, after putting a dummy 8 ohm load on the output;
  
• I'll see if any of these three amps will power up, slowly, using the Variac.
  

Hopefully I'll have enough operational tubes from the three amps so I can populate each amp, one at a time, with good tubes while I do the power-up tests.

Depending on the results of that effort, I'll decide whether to upgrade. It looks like a populated Mark III upgrade board is $90/amp. If I do the upgrade, I'll need different power tubes (I may need new tubes anyway, even for the Mark II, depending on what my tester shows). I'll also probably need to do something with the quad cap. It looks like most people are just using four (or eight, depending on voltage) modern caps to replace that, hiding them under the chassis, and then leaving the can in place for looks.

If that's not what I should do, let me know.

More to come ...

Well, I'm 75 and I can hear how good my MkIIIs sound -- with my turntable and VTA tube preamp stages.

If you want bad sound, just get a cheap transistor amp and avoid the trouble, eh?

solderblob wrote:Well, I'm 75 and I can hear how good my MkIIIs sound -- with my turntable and VTA tube preamp stages.

I can also still hear pretty well (although the eyesight isn't so good). My Altec 604 "Sound of the Theater" speakers still sound amazing (see pics below).

I am not questioning whether the Mark III upgrade will sound better, but whether I want to do that before I determine what other problems I might have. I've now read dozens of posts in this forum and elsewhere and I note a lot of people have problems because they rebuild before they understand the circuit and wiring that they already have.

johnmeyer wrote:I posted photos, in the photo section, of three Mark IIs that a friend wants me to restore:

Three Mark II Amps

I've done a little research, in this forum and elsewhere, so I am aware that many recommend upgrading to the Mark III, and some recommend getting new circuit boards populated with metal film resistors. At this point, my main goal is to simply get each amp running, and have them produce sound that is equivalent to how they worked when new.

I have another friend who has a Hitchcock tube tester and a Variac. I have an isolation transformer. If he doesn't come through with the tester, I may be dead in the water, since even in Silicon Valley (100 miles to my north), there are no public tube testers anymore.

At this point my plan is to:

1. Test the tubes and replace both the missing tubes and any which test bad.

2. "Re-cap" each amp, although I need to read more to figure out whether I can just use modern caps from Mouser or Digikey, or whether I need to find something more exotic. I don't know yet where to get a replacement for the big can capacitor.

3. Power up, adjust the bias, and go home.

There appears to be a small selenium rectifier. I've replaced those before (in my old Wollensak T-1500 tape recorder), but figuring out the right value for the series resistor is a little hit or miss, and I'm not sure the work was necessary for my Wollensak, and it may not be for this either. Thus, I'm not planning to do anything with that at the moment.

I'll post back again as the work progresses.

In addition to the EL34 tubes there is 2 items that needs your attention :

-The can cap. It might be leaky, it might short. New ones is available from dynakitparts.com
- the Se rectfier. This is a reliable item but if it breaks you will loose all bias, which could be damaging the tubes and or transformer. Get a replacement kit from dynakitparts.com

Apart from that you should most likely have a working amp.

peterh wrote:In addition to the EL34 tubes there is 2 items that needs your attention :

-The can cap. It might be leaky, it might short.  New ones is available from dynakitparts.com
- the Se rectfier. This is a reliable item but if it breaks you will loose all bias, which could be damaging the tubes and or transformer.  Get a replacement kit from dynakitparts.com

Apart from that you should most likely have a working amp.

I did some testing last night, in preparation for more work today. I tested the can cap on the two stock Mark II amps and the four caps (used instead of the quad) on the modified Mark II built on the big chassis. Some sections test OK, and others do not. The other electrolytic, the 100 uF 50V cap, tests OK on two of the amps and is as dead as a door nail on the third one.

(It's nice to have three almost-identical units to work on, so I can compare readings.)

I used my bench power supply, set to limit current to a a few milliamps, to partially reform each capacitor. I watch the current reading on the power supply display as I increase the voltage, and it momentarily increases each time I increase the voltage and then it stops. On a good cap it should go up as the voltage is increased, and then settle at zero once I stop changing the voltage, but only if there is no resistance in parallel with the cap. If there is resistance in parallel with the capacitor, it will settle to the reading for the parallel resistance.

I did this for each section of the cans. I then went back and used the capacitance meter again. This reforming helped a little on one of the cans, but was otherwise ineffective. My bench supply only goes to 30VDC, so I wasn't expecting miracles. It is still a useful first step.

Based on this work, one of the sections of one quad can is defective. That can must be replaced. The dead 100uF cap must be replaced, and since it is trivial to replace, and costs two bits, I'll replace that on all three amps.

I was going to replace the cans with individual caps. However, I found that 20 uF 525V caps are $25-40 each at Mouser and Digikey, so four of them could be close to $140. That's more expensive than replacing the can with another quad can. However, if I instead order 40 uF caps at 275V (or higher), they are only $1.69 each.

https://www.digikey.com/en/products/detail/rubycon/350BXW47MEFR12-5X25/6047826

With two in series that's $3.38/cap section on the quad capacitor, or about $14 total (a little more since one needs to be 30uF total). The replacement quad can from Dynaparts is $70:

https://www.digikey.com/en/products/detail/rubycon/350BXW47MEFR12-5X25/6047826

However, replacing the can with another can is dirt simple, and since I'm doing this for a friend for no pay, I don't want to spend hours and days on this.

So my question is: can anyone point me towards a post which shows a clever way to mount eight capacitors under the chassis that is secure?

I would keep them as MkII’s for the simple fact that EL34 tubes are cheaper, sound just as good, and still provide a decent amount of power.

I would also:
- replace the can caps with new ones
- install the VTA 12AU7 driver boards
- replace the octal sockets with new ones.
- replace all power cords with a good 14awg cable.
- Get a proper chassis or at least a smaller one for that first amp. If they were mine I would replace all of the boxes.

Have fun!

corndog71 wrote:I would keep them as MkII’s for the simple fact that EL34 tubes are cheaper, sound just as good, and still provide a decent amount of power.

I would also:
- replace the can caps with new ones
- install the VTA 12AU7 driver boards
- replace the octal sockets with new ones.  
- replace all power cords with a good 14awg cable.
- Get a proper chassis or at least a smaller one for that first amp. If they were mine I would replace all of the boxes.

Have fun!

OK, your thinking and mine align. When you say "replace the can caps," do you mean replace with cans, or should I replace with an 8-capacitor pack stuffed under the chassis? I keep going back and forth on this. The capacitor pack can have better specs, and will be a LOT cheaper ($16 vs. $75 for each amp, per my earlier post), but the can will be a lot simpler to install.

I hadn't planned to re-do the driver boards at this point. The caps on those boards are not electrolytic (and the capacitance test is perfect on all of them), and the carbon resistors will last until the next ice age. The one advantage I see is in getting a new board is to upgrade to metal film resistors, but I may be missing something.

The first thing I check on all old equipment is the power cord. Much to my surprise, all three show no signs of cracking. Of course they are cosmetically awful, but this is a Mark II, with no chrome and no visual appeal, so I don't want to try to make a silk amplifier out of a cow's behind of a chassis (if I can mix several metaphors).

Regardless of your answer, if I do end up doing that board upgrade, it will be after I get it running. I'm just about to test all the tubes, and I've found some caps in my drawer that I can use to replace the 100uF 50V capacitor that is dead on the big chassis build. (Good idea, BTW, about rebuilding that on a more appropriate sized chassis, but that is for another time. I'm doing this as a favor to a friend, and even for a friend, there is a limit to what I'll do for free  .)

Have a look at the MKII/MKIII capacitor board at Triode Electronics.  It’s $45 for the blank board and $100 for a stuffed board.  The blank board would be a better option since you’re confident at sourcing parts, and also because you can increase the capacitance over what Triode supplies and what Dynaco designed.  Triode uses two 100uF in series for all four “sections”.  I used two Nichicon part number LKX2G121MESA25 in series for the first section.  It’s a 120uF 400V snap in cap so it’s a 60uF 800V equivalent, where 60uF is the maximum recommended capacitance for a GZ34/5AR4.  For the subsequent three sections, I used Nichicon LGL2G221MELA25, which is a 220uF 400V cap.

Note that Triode specifies a maximum capacitor height and diameter of 25mm in order to fit in the MKII/MKIII and on their board.  You’ll also need eight balance/bleeder resistors and four film bypass caps, and two 0.375 inch 8-32 standoffs with nuts or screws to mount the board to the mounting bolts of the output transformer.

http://www.triodeelectronics.com/sdslabmk3cap1.html

Having bought and populated one of these boards recently, and needing to upgrade four more MKIII amps, I’ll be building my own boards out of perfboard and point-to-point wiring them, since these are simple boards without complicated trace layouts.  The parts mentioned above, not including perfboard or the blank board, are about $54 at Mouser.  If I build my own board, I can increase the capacitance after the rectifier even more if I like (there are some nice 25mm high 330uF 400V snap in electrolytics on Mouser that are 30mm diameter).

danielrhall wrote:Have a look at the MKII/MKIII capacitor board at Triode Electronics.  It’s $45 for the blank board and $100 for a stuffed board.  

Thanks for that link. That's similar to what I was thinking of doing, although I'm now shying away from it, because there are a few non-standard ways each unit was built, and I'm not sure I'll have the space for that board the way it is configured. I was going to use caps with a totally different form factor, and I think it would be easier to fit them into the space (I ws going to use long and skinny caps that sit on their sides).

Update

I've tested all the tubes on my loaner Hickok 539C tester. Two of the EL34s are perfectly matched. One looked dead, and was in fact dead. Each of the two 6AN8 tubes tested fine on one half (triode and pentode in the same enclosure) but not fine on the other, but each was bad in the opposite section from the other. Since the results were opposite, I'm confident the tester is working, and that I had the setting correct (the 539C is a complicated little devil).

Both 6AN8 tubes need to be replaced.

I'm only going to buy enough tubes to get the two stock chassis Mark IIs working. I'll recap the big chassis as needed, and make sure it works by using the EL34 and 6AN8 from the other amps, but I'll leave it up to my friend to decide if he wants to spend the money on the additional tubes.

Again, as I've progressed through this project over the past two days, I have focused on getting the units to work, with a minimal investment ($180 is my current estimate), and then after he auditions the results, we can decide on better tubes, Mark III upgrade, etc.

For these I would say replace the can caps with new ones.
https://www.dynakitparts.com/shop/multi-section-capacitor-1-2/


You could build the power supply using modern caps if you like. You don’t need a fancy pcb. You could use terminal strips or a bread board. I would recommend the first cap be 47uF or less which I think limits inrush current.

I don’t trust old driver boards. The VTA boards sound excellent and are very reasonably priced.

corndog71 wrote:For these I would say replace the can caps with new ones.  
https://www.dynakitparts.com/shop/multi-section-capacitor-1-2/

Yes, that is what I've decided to do. As soon as my friend OKs the price tag, I'll get the tubes, caps, and misc. parts and hopefully have sound within a week or so.

As with all these antique amps, changing them "modify" will decrease their value and in most
cases will not add anything to the sound quality.

Replacement of the Se rectifier with associated cap is one exception.

As for driver board, there is no shortage of 6AN8 tubes thus no pressing need to change design.

peterh wrote:As with all these antique amps, changing them "modify"  will decrease their value and in most
cases will not add anything to the sound quality.

Replacement of the Se rectifier with associated cap is one exception.

As for driver board, there is no shortage of 6AN8 tubes thus no pressing need to change design.

Yup, I'm trying to first get it working before even contemplating upgrades.

As for the rectifier, I replaced the selenium rectifier in my old Wollensak T-1500 tape recorder, but have regretted doing that. I did it eight years ago because of recommendations in a similar forum to this one, with the reason given that when they fail, the cause a massive stink. However, I've since read many articles and posts that, unlike capacitors which degrade or have already failed (like some in my current project), the SE rectifiers still work just fine, and don't fail that often.

So, at present, I'm holding off on replacing the SE rectifier.

BTW, something I didn't mention is that one of the "stock" Mark II (on the regular-sized chassis) does have a modification: a very large transformer, which I think is being used as a choke, that is wired between the rectifier and one leg of the quad cap (the leg which goes to the bias pot). I've certainly seen chokes used as part of line filter circuits (especially in switch mode supplies where they are more effective because of the high frequencies) but this is bigger than anything I'd seen before. The wiring looks otherwise identical to the other stock-chassis Mark II, so it will be interesting to compare hum and noise when I get both of them working. I don't have a notch filter to do hum and distortion measurements, so I'll just have to do it "by ear."

corndog71 wrote:For these I would say replace the can caps with new ones.  
https://www.dynakitparts.com/shop/multi-section-capacitor-1-2/

If I were going to replace a can cap in a MK-II or MK-III, I'd choose this one instead https://www.dynakitparts.com/shop/multi-section-capacitor-1-2-3/, for a couple of reasons. Both amps use the same P-782 power transformer and the surge voltage at power up can be quite high. Granted, the MK-II sees less voltage on its first section of the quad cap than the MK-III (470 vs 490 vdc) but with either amp, a cap with a higher surge rating should make the cap last longer, especially with today's higher AC voltages.

A second reason for that can cap choice is because it's rated at 70 ºC, while the lower voltage one is only rated at 55 ºC. And as you can see from Bob's great ST-70 temperature photo, https://dynacotubeaudio.forumotion.com/t828-tube-amps-get-pretty-warm-especially-in-the-summer-photo?highlight=temp a 55 ºC rated cap would be running right at the limit of Bob's reading of 129 ºF (53.89 ºC) taken at the bottom of the can.

For anyone considering the stuffed SDS board from Triode, note that they use 50 uF in all sections, including the first section.  I like to keep the value of the first cap lower, so it doesn't stress the rectifier tube as much. I bought the unstuffed boards, and used two Nichicon KX, 56 uF as the first two caps in series, so the first cap is 28 uF @ 800 VDC. The three remaining sections are 41 uF each (82 uf in series) The Nichicon caps are rated at 105 ºC, which is nice. If you opt for the blank board, note that mine was stenciled incorrectly. That's not an issue if you're going to use all the same value caps, but since I chose less capacitance for the first section, Triode's stenciling indicates the first two caps in series are marked #1 and #5, instead of #1 and #2. If someone buys the board, follow the layout of the traces and not the stenciling.

Good luck with your restoration.

1973shovel wrote:If I were going to replace a can cap in a MK-II or MK-III, I'd choose this one instead https://www.dynakitparts.com/shop/multi-section-capacitor-1-2-3/, for a couple of reasons. Both amps use the same P-782 power transformer and the surge voltage at power up can be quite high. Granted, the MK-II sees less voltage on its first section of the quad cap than the MK-III (470 vs 490 vdc) but with either amp, a cap with a higher surge rating should make the cap last longer, especially with today's higher AC voltages.

I agree. I didn't use that vendor, but those are the specs of the cap I bought and which is on its way: 525 WVDC as opposed to the 500 WVDC of the 1950 cap I'm replacing.

I'm a little late to reply as I don't check into the forum very often, most of my customers contact me directly via email.
That said, first, restore the amps to original working condition. Definitely new high-voltage quad caps, you can get them from me or from DynaKitParts,
same price, $75 each.
2nd, obviously PeterH has never heard a VTA amp, a very simple mod that about 6000 other people will tell you that makes the amp sound like
something you just paid $10,000 for, instead of a 60 year old restored junker. Once you have the amps working, the upgrade is $100 the pair.
Even better with the 6SN7 octal version, the best audiophile tube made, rather than a 60 year old obsolete 6AN8 tube.
The sound is 10x better at least. They still look stock, and it will not lower the value, it will raise the value because so many people around
the world know the VTA board is THE BEST board in the world for a Dynaco amp. Plus you can bias each tube individually, and not worry
about matched tubes.
Also, if you ever decide to sell those speakers, let me know.

Roy
www.tubes4hifi.com/MK3.htm

tubes4hifi wrote:... never heard a VTA amp, a very simple mod that about 6000 other people will tell you that makes the amp sound like something you just paid $10,000 for, instead of a 60 year old restored junker.  Once you have the amps working, the upgrade is $100 the pair.

Yours was one of the first sites I went to when I started this project. However, I just looked at it again and it appears to be oriented towards the ST-70 and I saw no direct mention, pictures, etc. of my Mark II amps.

Just to be sure I wasn't missing some hard-to-find page I had Google search your site for "Mark II" using this search string:

"mark II" site:tubes4hifi.com

All I got were PDFs of the manual. No mention of using the VTA for upgrading a Mark II. However, I now searched beyond your site (and also searched this site) and I do see posts about using the VTA board with Mark II & III amps.

So, I'll wait to see how far my friend wants to go with these old amps that he has owned since the 1950s. The tubes and caps I ordered are due here tomorrow and it shouldn't take too long to get the amps ready to test.

tubes4hifi wrote:Also, if you ever decide to sell those speakers, let me know.

You'll have to talk to my heirs, when the time comes.  

Those old "sound of the theater" speakers sure do sound good. Same with some of my other old equipment. I'm transferring some spoken word tapes for a friend, using my Wollensak T-1500 that we got back in 1960. I re-capped that about six years ago and replaced the SE rectifier with a silicon diode and resistor. Sounds great, given that it is not an audiophile deck. It is perfect for digitizing voice recordings.

John,
look at my MK3 page, the MK3 is the same as the MK2 except for the output tubes, although early versions of the MK2 used a SQUARE board,
later versions used the same board as the MK3 (rectangular) and the early versions also had slightly smaller transformers.
But if you have a later version it's virtually identical to a MK3 other than the output tubes.
I also have both the MK2 and MK3 manuals on my download page
http://www.tubes4hifi.com/downloads.htm
lots of photos and illustrations on my MK3 page
http://www.tubes4hifi.com/MK3.htm

Roy info@tubes4hifi.com

also, the pictorial diagram on the last page of the MK3 is identical to later version of the MK2, so it makes wiring the amp up correctly super EZ without reading dozens of pages

tubes4hifi wrote:... look at my MK3 page, the MK3 is the same as the MK2 except for the output tubes, although early versions of the MK2 used a SQUARE board,later versions used the same board as the MK3 (rectangular) and the early versions also had slightly smaller transformers.

I posted, over in the pictures section, photos of the three amps I'm working on:

Three Mark II Amps

The two "stock" Mark IIs on the right have rectangular boards. The large-chassis Mark II has a square board. I have no plans to do anything with that odd, large Mark II other than getting it to work.

It would be fun to get the two Mark II amps working the way you describe. If I do that, I should also probably pay a little attention to cosmetics. These old steel chassis are dull and corroded. Has anyone attempted to clean these? The only part that really matters are the two exposed ends. I was thinking of covering the electronics and then using steel wool to clean it up. I'm sure that people who have done this before will have better ideas.

However, I am not going to completely re-build on a new chassis.

steel wool will clean it up but it will leave thousands of scratches. After that, you can use some metal polish.
Of course most people who attempt this will COMPLETELY dismantle the entire amplifier and start from scratch.
If you do that it's a pretty simple matter of either completely polishing the metal, or better, powder coated paint.
Or just primer and regular Rustoleum paint. But that's a ton of work. Been there, done that !