Audio Asylum Thread Printer

Get a view of an entire thread on one page

FAQ / News / Events

Tube DIY Asylum

Looking for Dynaco MKIV owner's using the VTA boards set up for 12AU7 lo-gain

72.185.15.172

Michael Norona

I am currently using the VTA boards set up for the 12AT7 in my Dynaco MKIV's. There are a number of resistors that need to be changed when you configure the boards for the 12AU7. I have a few lists that I was sent by VTA, but there are differences in values between the lists. I would preface this by saying don't suggest going to Dynaco Tube Audio Forum, or VTA, That is not an option, and that's all I am saying. The resistors, and one capacitor in question are:

R11-12 68K changes to a 33K 2w or 36K 2w
R13-14 150K " " " 33K 2w or 36K 2w or 47K .5w
R15-16 7.5K " " " 6.8K .5w or 6.8K 2w
R17-18 12K .5w " " " 12K 1w or 12K 2w
C13 15 is it 680pF or 470pF

As far as the watts rating, going bigger can't hurt you, just cost a little more, and are a little bigger. The values I was told are flexible, but I discovered that is not true in all cases. I am interested in hearing what other MKIV owners, using the VTA/12AU7, are using in these resistor and capacitor locations. Thanks for your help.

Follow Ups:

RE: Looking for Dynaco MKIV owner's using the VTA boards set up for 12AU7 lo-gain - Neff 19:49:27 03/17/12 (0)
As reported the transformers can support the higher 12BH7 filament current. Being the case, drop-in Tungsram E80cc and enjoy a tube that plays as well if not better than the best manufactured vintage 6SN7.
I use the VTA low gain board in an ST-70, so there is no difference......... - viridian 10:03:57 03/17/12 (7)
R11-14 33K 2W
R15-16 6.8K 1W
R17-18 12K 2W
R19-22 30K 2W
For C-13-14 you can go as high as 120uF and for C15-16 as high as 47uF. I like the Pansonic parts here as they fit well, sound good and are sharply priced.
A bit of further advice, two of the .1uF caps are in the signal path, some people will give you BS that these have no effect on the sound. And that would be completely wrong. Use a high quality cap here just as you do in the coupling positions.
You might also like to try 12BH7s in place of the 12AU7s. The transformers will run a touch hotter, but with the independent power supplies it will not be an issue. They will yeild a sound that is darker and more centered, which may suit you......or not. Happy hunting.

Edits: 03/17/12
- RE: I use the VTA low gain board in an ST-70, so there is no difference......... - Michael Norona 10:18:12 03/17/12 (6)
  Many thanks!! I will check out the Panasonic caps, I have read good things about them. I have also read about using the 12BH7, is there any particular brand you like? I notice that they are not what you would call an inexpensive tube.
  - RE: I use the VTA low gain board in an ST-70, so there is no difference......... - Michael Samra 16:27:55 03/17/12 (5)
    I have been fighting off a respiratory infection so I haven't posted much here but I can tell you that you want to use a box of caps on that board..I would use either k40s for the ones from driver to output and use the panasonics for the .1s or even russian silver mica caps...You really need paper in oil caps on the VTA boards or they will be too analytical and dry sounding.Vird is correct in that the .1s make a huge difference. The panasonics are one of my favorites of the film caps at a reasonable price.Bob Latino sells the k40s as an upgrade kit on his amps that use the VTA board.
    Also,remove 47uf bypass cap on the cathode of the first stage..It is the R9 resistor and the cap is c9.
    
    When problems become increasingly difficult,we can always rely on our collective knowledge and seek relief.
    - "panasonics are one of my favorites of the film caps at a reasonable price".... - Cleantimestream 21:24:53 03/17/12 (0)
      Michael,
      
      Am aware of three types {Panasonic} one polyester, one metallized poly,
      
      and one is black of unknown composition.
      
      Which one are you referring to?
      The Mind has No Firewall~ U.S. Army War College.
    - Agree with my guru Mike on the K40Ys. And Mike I hope that you feel better......... - viridian 19:14:13 03/17/12 (3)
      ..for the .1s I like Auricaps, and they are not too spendy in these values. You can't use oilers for the .1s as you don't have the real estate available.
      
      GE is a nice, inexpensive 12BH7. What you might like is an RCA clear top for the input/phase splitter and GE 12BH7s for the drivers. Alternately, an RCA clear top for the input/phase splitter and some more RCA 12AU7s for the drivers. Pretty cost effective.
      
      For output tubes I have tried lots and, if you have the money, I think that the Tesla E34L with black base beat most of the vintage tubes that I own in this amp, Mullard XF2, Amperex brown base, GE 6L6 and 6CA7, etc.
      
      Of the currently available output tubes, I know that Mike likes the 6P3S-E and I have a set in there right now, juiced up to 42ma. For currently available tubes my favorite is the JJ E34L blue glass, but there are reported reliability problems. I like them so much that I really don't care. A distant second would be the 6P3S-e, followed by the older, big body Sovtek EL34 of the 1970s, followed by the much-maligned Sovtek EL34G then the Electo Harmonix 6CA7. But these are completely matters of taste and each of us will have our own favorites.
      - RE: Agree with my guru Mike on the K40Ys. And Mike I hope that you feel better......... - Michael Samra 02:09:16 03/18/12 (2)
        
        In Reply to: RE: Agree with my guru Mike on the K40Ys. And Mike I hope that you feel better......... posted by viridian on March 17, 2012 at 19:14:13
        
        You can use the .1@400v k40s just fine.On the vta 70 board you just hang them all underneath.I have used all k40s and it worked great..You can also put some on top and some below.This particular st70 has 6 k40s and 2 silver micas.The Silver Micas are tan ones mounted on each side of the chassis..You can see tho there was plenty of room for 2 more k40s.This amp is fantastic as it has a mostly all film cap power supply.
        
        When problems become increasingly difficult,we can always rely on our collective knowledge and seek relief.
        
        Edits: 03/18/12
        
        Nice build Mike! And you are right there is plenty of room. nt - viridian 08:47:01 03/18/12 (1)
        
        In Reply to: RE: Agree with my guru Mike on the K40Ys. And Mike I hope that you feel better......... posted by Michael Samra on March 18, 2012 at 02:09:16
        
        nt
        
        RE: Nice build Mike! And you are right there is plenty of room. nt - Michael Samra 13:21:02 03/18/12 (0)
        
        In Reply to: RE: Nice build Mike! And you are right there is plenty of room. nt posted by viridian on March 18, 2012 at 08:47:01
        
        Thanks
        
        When problems become increasingly difficult,we can always rely on our collective knowledge and seek relief.

Post a Followup:

FAQ

Post a Message!

Forgot Password?

Moniker (Username):
Password (Optional):
	Remember my Moniker & Password (What's this?) Eat Me
E-Mail (Optional):
Subject:

Message:	(Posts are subject to Content Rules)

Optional Link URL:
Optional Link Title:
Optional Image URL:
Upload Image:
E-mail Replies:	Automagically notify you when someone responds.

To view your new posting or follow-up, click on the RELOAD or REFRESH button on your browser.

This post is made possible by the generous support of people like you and our sponsors:

Amplified Parts