[spritzer]

A small update is in order. I've been trying the amp with a quad of 7f7's (the loktal version of the 6SL7) as drivers and it is certainly a different sound, leaner and a bit dryer with less bass bloom. A small issue has crept up though as the amp can't be properly biased with these tubes. It's only off by about 15VDC so it isn't a big deal but should be noted none the less.

Speaking of biasing the amp, I've been asked on a couple of occasions how this is done now so I figured I'd post it here for posterity. As can be seen on the picture below each channel has 5 test points and 4 pots to adjust the bias.



The amp is mostly DC coupled (as in any change in one stage is carried over to the next stage) but the output stage is AC coupled to the driver stages so effectively isolated.

Looking at the schematic on the first page there are four points marked TP1 and through TP4 and those are indeed the test points. The fifth is ground and connected to the common ground inside the amp. On this amp then black and red are the two test points per stage with white being ground.

For the driver stages we have two pots to adjust, a balance pot that balances the sections (which was there on the original Stax design) and we also have a pot connected to the B+ which acts as a voltage divider to decrease the distortion in the amp. This was a later addition to the design by Dr. Gilmore and really works a treat. Now to bias this stage you place a voltmeter between the two test points and adjust the balance pot for 0VDC. Next step is to adjust the B+ and you do that by placing the red probe on either test point and the other to ground and adjust the second pot for half of the amps B+ voltage. My amp is running at +430VDC so +215VDC would be the target here. One has to go back and check the balance after adjusting to get the correct values so using two voltmeters at the same time would be a good idea.

Now any changes made here will not affect the output stage one bit so it's easy work to roll in some new driver tubes. As I said above, that doesn't mean you can use just about any tube with this amp like Mikhail led his customers to believe. The changes made to the circuit were done for the 6SN7/7N7 in mind so with some tubes you might not be able to bias it properly. There is also the issue of distortion which will increase dramatically with some tubes. For lowest distortion then the 12AT7/ECC81 is the tube of choice but the 6SN7 is a good compromise since it would have been a pain to retrofit noval sockets to this chassis.

Now to adjust the output stage. This is rather simpler with just two test points and two pots which ideally should be adjusted at the same time with two meters running. Both are to be adjusted for 0VDC (something which the stock amp could never do, always stuck at -100VDC) with one probe connected to a test point and the other connected to ground. This adjustment is a bit fiddly as you need to balance the two pots per channel and they work against each other at times but is shouldn't take long to get it down to 0V or there about. Now once the amp has been adjusted it drifts very little, a couple of volts max.

Now one last thing, while I do love the cheap thrill of the loktal tubes I must say I've been having some slight trouble with them. They are known to be a bit noisy due to the pins not having the contact area of their octal cousins and I've been experiencing this from time to time. Could also be due to the quality of these cheap Chinese sockets so I'll probably look into replacing them with some teflon units soon.

 

[Vandal]

Your work is nothing short of amazing, especially for someone like me who knows nothing about soldering. And I like the work on the casing, and the cleaner rewiring. But speaking about the stuff the amp was "supposed" to have, I must say one thing "Mikhail is a thief", and that too, of the lowest order, and belongs in a cell. And this coming from someone who has never even seen one of his amps.
 
BTW nice pics too.
 

 

[spritzer]

Thanks for the kind words. 

  Sadly not many ES-1/2's have had a full restoration like this to date, only simple remedies to fix the worst issues and make the amps safe.  Hardly surprising though since something like this costs a lot of money to do, even as a DIY project.  Add labor costs to the mix and it is a small fortune.  I must say it is worth it though since this is about as good as it gets for a vintage design and really holds is own, even with a BHSE sitting next to it.  Speaking of which, here they are together:
 

 

[neilvg]

I've been out of the game for a while but I recently got 3 offers for my HE90's which has had the effect of rekindling my interest in my sad and unused ES2. I hope to one day find resolution to this sad story for myself -- and as always Spritzer and Kevin have provided rays of hope

And speaking of hope, I one day hope to hear this thing which looks so beautiful and probably sounds the way it always should have.

I hear Blubliss has had some luck or at least made some progress in getting his brother ES2 (to mine) fixed up.

Neil

 

[purk]

Quote:

I've been out of the game for a while but I recently got 3 offers for my HE90's which has had the effect of rekindling my interest in my sad and unused ES2. I hope to one day find resolution to this sad story for myself -- and as always Spritzer and Kevin have provided rays of hope

And speaking of hope, I one day hope to hear this thing which looks so beautiful and probably sounds the way it always should have.

I hear Blubliss has had some luck or at least made some progress in getting his brother ES2 (to mine) fixed up.

Neil

 
Neil,
 
I hope you will one day get a chance to enjoy the combo again.  I feel very fortunate for not keeping the ES-1 or purchasing the SP Concerto.  Can you still enjoy the HE90 with your HEV90 though?

 

 

[spritzer]

Andy has indeed made some real improvements to his ES-2.  KG modernized the design even further by scrapping the plate resistors for the EL34 and replaced them with a constant current source (similar to the one fitted to the BHSE).  The thick chassis of the ES-2 helps here since you can use it as a heatsink.  Andy has been pretty quiet about the amp lately since he's been drooling over his newly built T2DIY...

 
I have made some improvements to my amp as well over the last few months.  The biggest one was to replace all the driver tubes with 7F7's (6SL7's in a loktal base) and to modify one resistor per channel.  If you look at the schematic one the first page of this thread (chapter 2.1) there is a 1k5 common cathode resistor for the EL34´s.  This one sets the gain of the output stage which I was never quite happy with when running the amp single ended.  Now simply putting in a lower value here will increase the gain but then the issue of managing the DC offset on the output crops up.  That is governed by the C- supply so if there is some headroom left there, the gain can be increased quite a bit. 
 
As a card carrying member of team overkill I went completely over the top with the C- supply in my amp so I could put in a 470R cathode resistor and still have plenty of headroom left.  This means I no longer have any channel imbalance when using the RCA inputs and more gain is always a nice thing to have.  I'm not sure if I'll add the output stage CCS to my amp since I would have to add heatsinks and I do have plenty of other amps with this but just a handful of pure tube amps. 

 

[blubliss]

My ES-2, while operational, sounds nothing like it used to or even close to the realm of the T2.  It definitely needs more work.  I know Spritzers has been built correctly but mine is still missing some vital tweaks.  At least KG helped me get it much safer and operational and I thank him for that!
 
It is currently lent out to a friend, but we may upgrade it further when we get the itch.
 
Both my SP amps are on loan now, and I must say it is nice to have them out of my sight.

 

[purk]

Quote:

My ES-2, while operational, sounds nothing like it used to or even close to the realm of the T2.  It definitely needs more work.  I know Spritzers has been built correctly but mine is still missing some vital tweaks.  At least KG helped me get it much safer and operational and I thank him for that!
 
It is currently lent out to a friend, but we may upgrade it further when we get the itch.
 
Both my SP amps are on loan now, and I must say it is nice to have them out of my sight.

Howdy Andy?  Glad you are getting your SP problems sort out finally.

 

[blubliss]

So, time for an update on my ES-2 saga which now seems to be sorted finally.  My friend Steve (stv1756) who also helped me build the T2, has repaired the ES-2 following the close instruction of Kevin Gilmore and the great documentation by spritzer in this thread.  Since I have the T2, the ES-2 is in the hands of Steve and I have not heard it yet.  He has heard it at all stages so I will post his impressions, thoughts here:
 
"Am listening to the ES-2 as I’m writing this post and am truly amazed at how far we have come in terms of SQ and stability. In the beginning of the restoration project (or salvage, perhaps) I was a little concerned that this circuit was not only dated but also fundamentally flawed. The amp never really performed that well (considering its cost). And there was plenty of smoke (and tears)! Well I’ve become a believer and can say that with this circuit (properly engineered) you can really hear the tubes. So if you have great tubes like my Ampex Bugle Boys driven by some crisp, clean NOS 6SL7s, you have one noteworthy, albeit kick-ass amp! Needless to say I’m deeply satisfied with the outcome. A special thanks to Spritzer for putting together this extremely helpful thread and to KG for his encouragement and helpful suggestions.
 
We have pretty much stayed true to Spritzer’s recommendations in this thread, installing a C- PS and floating the EL34 heaters with the help of a local technician. We then replaced each of the EL34 plate load resistors with an IXYS CCS, and reduced R19 to 1K. This past week we removed the original pots and replaced them with Bourns 3540s. In the front we used 4-5K pots (a voltage divider and a balancer for each channel) as the schematic indicates. We put 4-50k pots in the back (one for each of the EL34s) wired to the B- and C- and outputting through 500k to each of the EL34 grids. This works like a charm to get 0v at TP3 and TP4. The bonus surprise here with the installation of the Bourns pots was the elimination of an annoying hum.
 
With this arrangement we were able to get our output voltages correct, and were within 25v on the cathode/heater differential on U3/4. According to the data sheet for the 6SL7s the Vmax cathode/heater differential is 90v. We ran out of pot at 120v, but it doesn’t seem to be a problem. KG recommended the 6SL7s to help overcome a thinness and lack of definition at the top. I must admit that I am smitten by the 6SL7/EL34 combination. Really makes the MK2s come alive!
 
To summarize, the amp sounds incredible, handles difficult transients with ease, and gives a real naturalness to the sound of everything I’ve thrown at it.

"
 
 
 

 

[spritzer]

Glad to hear that troubled amp is now running as it should.  The circuit is indeed still great and it forms the basic backbone of the SRM-T2 so that accounts for something. 

 
As for the front end voltage divider, it was devised for the 6SN7's originally used in my amp so it needs to be tweaked a bit for the 6SL7's.  The difference is pretty small though and I'm using 7f7's which are dirt cheap so I don't really care enough to change it. 

 

[kevin gilmore]

complete restoration of my newly purchased unit in progress.
comments probably not necessary.
 
 

http://gilmore.chem.northwestern.edu/myesx1.jpg
http://gilmore.chem.northwestern.edu/myesx2.jpg
http://gilmore.chem.northwestern.edu/myesx3.jpg
http://gilmore.chem.northwestern.edu/myesx4.jpg
http://gilmore.chem.northwestern.edu/myesx5.jpg
http://gilmore.chem.northwestern.edu/myesx6.jpg
http://gilmore.chem.northwestern.edu/myesx7.jpg
http://gilmore.chem.northwestern.edu/myesx8.jpg
http://gilmore.chem.northwestern.edu/myesx9.jpg
http://gilmore.chem.northwestern.edu/myesx10.jpg
http://gilmore.chem.northwestern.edu/myesx11.jpg
http://gilmore.chem.northwestern.edu/myesx12.jpg
http://gilmore.chem.northwestern.edu/myesx13.jpg
http://gilmore.chem.northwestern.edu/myesx14.jpg
http://gilmore.chem.northwestern.edu/myesx15.jpg
http://gilmore.chem.northwestern.edu/myesx16.jpg
http://gilmore.chem.northwestern.edu/myesx17.jpg
http://gilmore.chem.northwestern.edu/myesx18.jpg
http://gilmore.chem.northwestern.edu/myesx19.jpg

 

[rosgr63]

How long will the restoration take?

 

[spritzer]

40-50 hours of stripping down and rebuilding shouldn't be too far off. 

 

[kevin gilmore]

14 hours so far to get the two chassis ready for rebuilding. Including cracking off the epoxied feet,
removing all the rusted and frozen screws, and machining the holes for the amphenol connectors
and one of my teflon stax sockets.
 
Now waiting for the rest of the parts to come in. Teflon tube sockets, teflon wire, teflon capacitors...
By the time i'm done, well over $1k in replacement parts.
 
40 to 50 hours total seems about right. Which is why these things are worthless. But they
sure do sound very nice when finished.

 

[purk]

Glad I returned my ES-1 when I had the chance.