Dynaco ST35 B+ - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 26th September 2013, 01:28 AM   #1
diyAudio Member
 
mr2racer's Avatar
 
Join Date: Jul 2009
Default Dynaco ST35 B+

Hey Anyone? Does anyone know what the B+ for the preamp section of the Dynaco ST35 is? I have to revamp the power supply to get the B+ for the EL84's down to 300 volts for new JJ's. For some reason I have way too much voltage across the 12AX7 section of the 12DW7, over 400 volts.
  Reply With Quote
Old 27th September 2013, 12:25 PM   #2
diyAudio Member
 
mr2racer's Avatar
 
Join Date: Jul 2009
If anyone is listening I've done my best to figure this out for myself. Nearest I can figure is that the 12AX7 is drawing about 1.1 ma and the 12AU7 ABOUT 4.6 MA. From there I calculated the B+ for the preamp section is about 331 volts. Since the JJ EL84's are limited to 300 volts I'l build the supply for about 300 and see where that gets me.

The 12AX7 is a bit harder to calculate. I think its because of the shared voltage (DC coupled) from the 12AX7 plate to the 12AU7 grid.

Last edited by mr2racer; 27th September 2013 at 12:27 PM.
  Reply With Quote
Old 27th September 2013, 01:12 PM   #3
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
It's generally pretty high- 370-400V isn't unusual. Good for the 12AX7, not so much for weedy EL84. That said, I've pulled old 6BQ5 out of these units with 10-20 years of service and they're still going strong, despite the abuse of their maximum ratings.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 27th September 2013, 11:58 PM   #4
jerrys is offline jerrys  United States
diyAudio Member
 
Join Date: Mar 2011
Location: Spokane, Washington, USA
Quote:
Originally Posted by mr2racer View Post
If anyone is listening I've done my best to figure this out for myself. Nearest I can figure is that the 12AX7 is drawing about 1.1 ma and the 12AU7 ABOUT 4.6 MA. From there I calculated the B+ for the preamp section is about 331 volts. Since the JJ EL84's are limited to 300 volts I'l build the supply for about 300 and see where that gets me.

The 12AX7 is a bit harder to calculate. I think its because of the shared voltage (DC coupled) from the 12AX7 plate to the 12AU7 grid.
The voltage limit is for the TUBE, not the power supply voltage applied to the anode resistor. It's easy to get confused here because there is no anode resistor on the power tubes, so we think in terms of B+ being the limit as is the case with power tubes.
Different preamp tubes sound better with more voltages (llike the 6SN7) and some sound better with reduced voltage (like the 6n2p). Feel free to experiment a little bit and go with what your ears tell you. I don't use 12ax7 (they sound like crap), but I seem to recall from my early days of tube experimentation that they sounded better with a B+ of about 250. Obviously, the anode resistor is a factor here. Use Ohm's law and your cathode resistor to figure out current draw and make sure you have the current where you want it. Your cathode resistor is what is going to set the operating parameters for the tube. If the current draw isn't right, that's what you want to adjust to make it stay in the linear range.
  Reply With Quote
Old 28th September 2013, 12:07 AM   #5
AJT is offline AJT  Philippines
diyAudio Moderator
 
AJT's Avatar
 
Join Date: May 2003
Location: Palatiw, Pasig City
the dc resistance of the output transformer is in the 100ohms range so this also limits current and drops the voltage...

in many tube amp repairs i have done, i found that lowering the output stage B+ to be very close to the output tube plate rating does not materially affect the sound, probably clipping is earlier but under normal listening conditions, i did not hear any difference...
__________________
the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus....
  Reply With Quote
Old 28th September 2013, 03:35 AM   #6
jerrys is offline jerrys  United States
diyAudio Member
 
Join Date: Mar 2011
Location: Spokane, Washington, USA
Quote:
Originally Posted by AJT View Post
the dc resistance of the output transformer is in the 100ohms range so this also limits current and drops the voltage...

in many tube amp repairs i have done, i found that lowering the output stage B+ to be very close to the output tube plate rating does not materially affect the sound, probably clipping is earlier but under normal listening conditions, i did not hear any difference...
I would agree with you regarding the 12ax7 tube. Varying the voltage with them was not nearly as pronounced an effect as with the 6CG7, 6SN7, 6SL7, and also seemed to depend on the circuit topology. I had an amp with either 6CG7 or 6SN7 and the B+ was around 300. I bumped it up to closer to 400 and the sound quality was noticeably better. Enough of an improvement that I modified all of my amps (that had that design) in similar fashion. It may be dependent upon the circuit topology. And, again, the popular tubes being the 12xx7 lineup, I wouldn't expect to notice as big a diff with them, if at all. The guy that makes the Big Dawg guitar amps, he claims the 12au7 needs a pretty stiff voltage to sound decent. That's why I experiment a lot. I also use some weird designs of my own, so I have nothing to guide me and I'm forced to experiment a great deal.
  Reply With Quote
Old 28th September 2013, 05:03 AM   #7
jerrys is offline jerrys  United States
diyAudio Member
 
Join Date: Mar 2011
Location: Spokane, Washington, USA
I think you are going to be fine at that level. If the 12au7 were a gain stage, I would say kick it up some, but since it's not, you should be fine, and 300 should be fine for the 12ax7 stage; I have seen that as the B+ for 12ax7 in many, many schematics. I suspect that is because many designers like to "push the envelope" with regard to B+. The 6BQ5 is a pretty sensitive tube and this arrangement should provide plenty of gain. The 12au7 is basically functioning like a cathode follower and should just pass the signal along with no gain and coloration of the tone.
  Reply With Quote
Old 28th September 2013, 05:37 AM   #8
AJT is offline AJT  Philippines
diyAudio Moderator
 
AJT's Avatar
 
Join Date: May 2003
Location: Palatiw, Pasig City
wrt 12au7 and 12at7 stages, i agree that more voltage is necessary and desireable...

it is the output stage and the power tubes that i lower the B+ and finds that it did not materially affect the sound...
__________________
the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus....
  Reply With Quote
Old 28th September 2013, 06:15 AM   #9
jerrys is offline jerrys  United States
diyAudio Member
 
Join Date: Mar 2011
Location: Spokane, Washington, USA
Quote:
Originally Posted by AJT View Post
wrt 12au7 and 12at7 stages, i agree that more voltage is necessary and desireable...

it is the output stage and the power tubes that i lower the B+ and finds that it did not materially affect the sound...
My experiences with power tubes would again force me to say, "depends." My first amp I experimented on was a 6V6 push pull pentode mode. I left the screen voltage alone, removed the NFB, and bumped up the plate voltage. Totally changed the sound. It was clearer, and not quite as warm. I left it that way as it was my midrange treble amp and I wanted the highs accentuated more. I was on a kick where I changed from tube to solid state rectifiers and, with the 6L6GB and GC tubes, it made them "punchier" more dynamic. Again, I was changing the spread between the anodes and screens. I assumed that changing the spread between anode and screen was responsible for the change in character. I think the 6V6 rep for "warmth" comes from the fact that the data sheets recommend a very small difference between screen voltage and plate voltage. Just guessing there.
Then, my latest build, the GU50 push-pull ultralinear. It has an anode limit of 1000 volts, but a screen limit of 300 and triode mode limit of 400 volts. Since I went with ultralinear, I used the 400 volt limit and put a pretty stiff resistor on all the screens. I also went fixed bias this time so that I would get that extra 25 or 30 volts across the tube because the GU50 definitely sounded better with more voltage across it. as the voltage fell from 400 volts, the sound quality really suffered. My next project is going to be GU50 screen drive where that screen voltage limitation won't be a factor.
And then again, I have had amps where changing the plate voltage didn't seem to have much effect at all. Tubes are mysterious, and I think that is why I like them. I try to not make assumptions and instead experiment a lot just to see what happens. Sometimes I get pleasantly surprised. My first unexpected surprise was when I was doing an experiment with a 6L6GB or GC amp and the power supply had a bad connection such that the voltage was seriously reduced (pentode mode amp). The sound was VERY sweet, like triode sweet. Just really pretty. Was it the power tubes being run at reduced voltage or the preamp tubes? ? I'm not sure. That was a long time ago and I keep meaning to get back to that concept and check it out, I just haven't done so yet.

I did notice that ultralinear mode on most amps (except the GU50) seemed a little more forgiving of B+ variations. I have concentrated on inexpensive but decent tubes so I can build more and experiment more within the confines of my budget, and I stay away from lower powered stuff anymore. They just don't sound as good (to me). I found that if the bass isn't solid, just as tight and distortion free as possible, it doesn't matter what u do with the rest of the system, it won't sound right. It may sound "okay," but it isn't going to knock anybody's socks off. Every instrument has it's base or resonant frequency solidly in the bass region (Bob Heil, "The Practical Guide To Concert Sound.")

Last edited by jerrys; 28th September 2013 at 06:23 AM.
  Reply With Quote
Old 28th September 2013, 06:30 AM   #10
AJT is offline AJT  Philippines
diyAudio Moderator
 
AJT's Avatar
 
Join Date: May 2003
Location: Palatiw, Pasig City
oh yes, "depends" is the right word, and i am not advocating that what i did is the right thing to do for others, it is just that i got those results and is happy with it....

here is another, a G2 regulated pentode can sound better than ultralinear pentode, again "it depends" is applicable here as well...
__________________
the best advertisement for a good audio design is the number of diy'ers wanting to build it after all the years....never the say so of so called gurus....
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Dynaco ST35 coolhandjjl Parts 2 26th July 2013 06:09 PM
Dynaco ST35 boards manonon Tubes / Valves 0 22nd February 2009 08:45 PM
Dynaco ST35 power? KJ2005 Tubes / Valves 15 15th August 2008 01:47 PM
Dynaco ST35 sds2000 Tubes / Valves 31 4th January 2007 04:06 AM
Wanted: Dynaco ST35 Family Swap Meet 0 28th October 2005 04:44 AM


New To Site? Need Help?

All times are GMT. The time now is 01:07 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2