Proposed screen-drive amp design... comments, please? - 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 14th March 2008, 01:58 PM   #1
GordonW is offline GordonW  United States
diyAudio Member
 
GordonW's Avatar
 
Join Date: Mar 2005
Location: Marietta/Moultrie GA
Default Proposed screen-drive amp design... comments, please?

I've been thinking, lately, about trying to build a screen-drive amp, after reading some of the results that tubelab and others have gotten, from stuff like 6AV5s delivering up to 80w per push-pull pair...

I went looking around in my stash, and I actually had 5 12AV5s... but while searching, I also discovered a stash of NINETEEN NOS 6JB6s. Hmmm...

So, off I went, trying to figure out how to use them. My first stop, was looking at the schematic for the Berning EA230... kind of a standard starting-point for many screen-drive designs. My thought was, though- instead of a Williamson front-end (voltage amp, cathodyne phase splitter, intermediate gain amp, feeding a cathode follower which drives the output tubes), why not try to incorporate a Mulllard-style (input gain amp, LTP, cathode follower, output)? The potential ability to remove not one, but TWO sets of coupling caps between stages, was pretty appealing.

Through the help of folks in a discussion about this over at AudioKarma, I've gone through several design iterations... getting rid of bone-head errors. , working to improve the phase splitter symmetry, and trying to get enough output tube drive, without going to solid-state components (call me a hopeless traditionalists, but I don't want any silicon in the "direct drive path" of the amp, unless it absolutely can't be dispensed with).

I originally thought of using a pentode front end (and I may still, if it looks to be significantly advantageous), but as of now, I'm thinking of using a 12AT7 in the front end... just much easier to tune (no confusion trying to work with screen grids, that kind of thing).

I've been looking for parts for this, that would be feasible for construction- so far, I've found an Edcor output transformer ( the CXPP100-MS-5K) that should work- I'd think I could at least get 60w output from the amp, without problems, and maybe as much as 80w, with this iron. For main B+, I'm thinking Hammond 278CX (465ma/400-0-400v), one per monoblock. I've also located some cheap small power transformers that would work for the lower-voltage B+ and B- rails (+ and - 250V), and some chokes... it's going to be a choke 'n' transformer farm, but fortunately, none of them are that expensive.

My main questions now are:
-How would I go about calculating the proper values of the resistors and caps in both the inner and outer feedback loops?

-What would be reasonable tunings for the bandwidths of the first two stages (input amp/LTP) for maximum stability (now, the input stage has about a 600Khz bandwidth feeding from a 10K output-impedance preamp driving the input, and even more bandwidth feeding from a lower impedance preamp- it drops to 200K or so driven by a 30K impedance- is that enough, for one example?)?

I would also like to hear from anyone, about any other potential problems I might be inadvertently setting myself up for here, or any ways I could simplify the circuit while improving it... any comments welcome!

BTW: All resistor values in the main circuit are "good estimates", gotten through modeling each stage using TubeCad, to set bias and operating points. I realize that many of these values will change, somewhat, in practice... I would also like comments from anyone, if they think any of these operating points could be better-set, of course!

Thanks in advance!

Regards,
Gordon.
Attached Images
File Type: jpg screen amp 3-10-08.jpg (32.9 KB, 1112 views)
__________________
Speaker Design, Restoration and Repair- since 1985.
  Reply With Quote
Old 15th March 2008, 05:22 PM   #2
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
The 6JB6 is a good candidate. What you'll need attention to is gain and linearity in the input and driver stage- as it's set up now, I would guess that both are inadequate; figure you'll need an open loop gain of at least 400 (52dB) out of the driver. You're running the 12AT7s with a very low plate load which means low gain, high distortion.
__________________
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 15th March 2008, 06:41 PM   #3
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
Gordon,

For the 12AT7 LTP, 200-200 V. on the plate and IB = 3 mA. works WELL. 570 V. of B+ would allow the use of 120 KOhm load resistors in the LTP.

A 6GK5 as the voltage gain triode will provide more oomph and less distortion than a 'T7 section. Both mu and gm are higher in the 6GK5 than in the 12AT7, while RP is lower.

IMO, "sand" can be a very good "friend" in this project. Cascoded DN2540s in the LTP tail CCS allow the use of a much "shorter" negative rail, while enhancing performance. IRFBC20s in the DC coupled driver positions along the lines provided in MOSFET Follies will do very nicely too.
__________________
Eli D.
  Reply With Quote
Old 15th March 2008, 08:55 PM   #4
cerrem is offline cerrem  United States
diyAudio Member
 
Join Date: Jun 2005
Location: San Diego, CA
I fugure a PENTODE such as EF86 ..ect... Would be beneficial as the CCS in the tail....
You need a high impedance in the tail for better coupling, also lower capacitance for better coupling at high frequency....

Chris
  Reply With Quote
Old 15th March 2008, 09:32 PM   #5
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
Quote:
You need a high impedance in the tail for better coupling, also lower capacitance for better coupling at high frequency....
I agree a pentode would be an improvement but you could get a much higher impedance in the tail from a transistor cascode CCS than from any tube. Even if it made no audible difference, the practical advantages alone of an SS CCS would be worth it.

As Eli said, you could use a better tube that a 12AT7 for the first stage. A pentode would give you a useful open-loop gain - maybe EF86 or 6AU6. You might find it necessary, also, with that global NFB loop, to control HF stability by adding a zobel network across the plate load of the first stage.
  Reply With Quote
Old 17th March 2008, 04:57 PM   #6
GordonW is offline GordonW  United States
diyAudio Member
 
GordonW's Avatar
 
Join Date: Mar 2005
Location: Marietta/Moultrie GA
Quote:
Originally posted by SY
The 6JB6 is a good candidate. What you'll need attention to is gain and linearity in the input and driver stage- as it's set up now, I would guess that both are inadequate; figure you'll need an open loop gain of at least 400 (52dB) out of the driver. You're running the 12AT7s with a very low plate load which means low gain, high distortion.
I went and compared my CURRENT gain structure to that of the Berning EA230... and it's actually 6 dB MORE gain than the original amp... I actually went and reduced the gain a bit on the first stage, as it was MUCH higher than this (which was compromising open-input bandwidth, of high-impedance source). The input 12AT7 now has about a 13K anode resistor and a 600 ohm cathode resistor, in round terms...

The input stage of the original Berning design (which also used 12AT7s, just under lower current- 1.2ma as opposed to about 4ma for mine) had a gain of about 11, while mine is about 11.9... about .75 dB more gain. I originally started out with a gain of nearly 30... which turned out to be FAR higher than the original (29 dB as opposed to about 21 dB for the original). I couldn't get an open-input bandwidth matching the original (about 90KHz, with open input, the only input loading being a 100K resistor to ground paralleled with the input jack), until I reduced the gain to that point.

The phase splitter/voltage amp of the Berning had a gain of about 24 (about 27.6 dB, all in the voltage amp between the concertina and the cathode follower- the concerina has no gain of its own), while my LTP with CCS has a gain of nearly 45 (33dB)... about a 5.4 dB increase.

Comparing the cathode follower sections... Berning's 6SN7 and my 6BL7 both have about -.75dB gain... about 3/4 dB below unity. No real difference there, other than the ability of mine to handle MUCH more current supply to the screens of the output tubes (just over 1/3 the output impedance of the original cathode followers).

So, for my input stages, if you add the gains (21.5 dB input stage, 33 dB phase splitter, -.75dB cathode folllower), you get a gain of about 53 dB, at the grid of the output tube, open loop, referenced to the input.

As for low gain, high distortion... I'm a bit confused there. I have been using TubeCad to set the DC bias points of these tubes- and it keeps insisting that I'm actually using TOO LITTLE plate current... and that it wants the plate resistors to be even SMALLER numeric value (and even LESS gain). I'm already running the tubes at an average of THREE TIMES the plate current of the originals... but I'm still well below the max current for the 12AT7 (6 ma for the input stage, about 4ma for the LTP main tubes and 8ma for the CCS- both below the max normal rating of the 12AT7, which is either 10ma or 15ma, depending on which data sheet you look at...

AFAIK, the 6JB6 and 6JN6 have about the same screen gain... they both have the same maximum plate voltage and screen voltage ratings. and the graphs look almost identical for the plate curves.

So... even considering that I'm trying to squeeze about 3 dB more output out of the thing, that leaves me with 3 dB MORE gain to use in the feedback circuit, than the original design. I don't know if people thought that the Berning had inadequate feedback originally, but at least my design has a LITTLE more...

I appreciate the comments so far... please keep 'em coming. I think I'm finally STARTING to wrap my head around some of the more esoteric aspects of the design (as in how to predict current draw and loading of the PARALLELED cathode loads of the cathode resistor and the grid of the 6JB6 output tube, on the 6BL7- how to roughly draw a load-line for a "stepped load" depending on voltage, for one thing). But, there's still several things that I'll probably be asking about, in the next couple weeks!!

Regards,
Gordon.
__________________
Speaker Design, Restoration and Repair- since 1985.
  Reply With Quote
Old 17th March 2008, 05:28 PM   #7
GordonW is offline GordonW  United States
diyAudio Member
 
GordonW's Avatar
 
Join Date: Mar 2005
Location: Marietta/Moultrie GA
Quote:
Originally posted by Eli Duttman
[B]Gordon,

For the 12AT7 LTP, 200-200 V. on the plate and IB = 3 mA. works WELL. 570 V. of B+ would allow the use of 120 KOhm load resistors in the LTP.

A 6GK5 as the voltage gain triode will provide more oomph and less distortion than a 'T7 section. Both mu and gm are higher in the 6GK5 than in the 12AT7, while RP is lower.
I tried using larger plate resistors on the LTP... but kept running into bandwidth problems. On the original Berning design, they had about 350K bandwidth on the phase splitter/driver amp stage (concertina plus grounded-cathode driver amps); I managed to get up to around 360KHz out of the setup I have now.

As for the current through the 12AT7s... it's currently at about 3.8ma on each of the LTP main sections, and double that (7.5ma) through the CCS...

As it is now, this is the ONLY alignment between the input amp and LTP phase splitter that I could reasonably find, that gave gain similar to the original (6 dB more, in fact), sufficiently higher voltage swing (about 40% more than the original, commensurate with me wanting 60w output as opposed to 30w for the original Berning design) at the output of the LTP, and similar bandwidths to the original stages as well, while at the same time giving the ability to "voltage-match" the DC plate voltage of the input amp to the needed grid (bias) voltage of the LTP top tube (in order to run it in direct-coupled "Mullard-style")...

I would still like to hear any comments anyone might have, about methodologies of calculating good ballpark ranges for the feedback loop resistors and caps. I would also like to hear anyone's thoughts on potential values for a input-amp Zobel around the plate resistor as well... I had thought of that kind of thing, but didn't know exactly which way I wanted to go...

Thanks!!

Regards,
Gordon.
__________________
Speaker Design, Restoration and Repair- since 1985.
  Reply With Quote
Old 17th March 2008, 06:16 PM   #8
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Gordon, thanks for all the pertinent comments. I've never cloned an EA230, so I watch your project with great interest.

As for linearity questions, there are flashing lights on your screen which read, "Be suspicious of this sim!" Take a real 12AT7 and set it up as a voltage amp. Take into account the effect of the feedback look on the local feedback in that stage (usually, it overwhelms the local feedback) when choosing biasing. Measure the output spectra with sine wave excitation. You'll find pretty quickly that you need to run them with much higher plate loads to maximize linearity.

If you find something different than that, please post!

Finally, do not be afraid of using MOSFETs to drive the screens. They work significantly better than cathode followers in that position (source impedance). That's what I've done for my last two amps and I'll never look back.
__________________
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 17th March 2008, 07:22 PM   #9
GordonW is offline GordonW  United States
diyAudio Member
 
GordonW's Avatar
 
Join Date: Mar 2005
Location: Marietta/Moultrie GA
I've taken some time to re-adjust some of these values... some before reading the comments here, some afterwards. I will definitely keep Sy's admonition in mind as far as loading is concerned... but, it could wind up being a PITA to increase the plate resistors, while keeping the voltage arrangement between the first stage plate and the LTP grid in the right range, to bias the LTP correctly...

However, those problems CAN be solved... it's just a matter of learning to artfully juggle...

Here's my current take on the schematic... I'd appreciate any advice on how to set a preliminary Zobel value for the plate of the input tube, as well as any suggestions for starting values for feedback loop resistors and caps...

Thanks!

Regards,
Gordon.
Attached Images
File Type: jpg screen amp 3-17-08.jpg (49.2 KB, 819 views)
__________________
Speaker Design, Restoration and Repair- since 1985.
  Reply With Quote
Old 19th March 2008, 09:10 PM   #10
GordonW is offline GordonW  United States
diyAudio Member
 
GordonW's Avatar
 
Join Date: Mar 2005
Location: Marietta/Moultrie GA
Default Keepin' on pluggin'...

I decided to make life easier on everyone... I found a set of tube schematic stencils, and made the schematic MUCH neater (see below!).

I also have changed the input stage, for higher-value anode and cathode resistors... Sy, is this more along the lines of what you were thinking?

I managed to preserve my voltage ratios, while reducing the loading... the input 12AT7 stage is now idling at about 2.1ma (up from 1.2 in the original), while the LTP is about 3.8ma per section. Both the input stage and the LTP have about 205-210 volts across the tube at idle...

Thanks!

Regards,
Gordon.
Attached Images
File Type: jpg screen amp 3-19-08-redraw.jpg (78.2 KB, 820 views)
__________________
Speaker Design, Restoration and Repair- since 1985.
  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
How do I design "SCREEN GRID DRIVE" ANALOG GUY Tubes / Valves 27 26th December 2013 02:35 AM
Opinions on proposed design? leadbelly Multi-Way 13 18th September 2006 09:16 AM
Need comments on a proposed sub design cdwitmer Multi-Way 16 23rd August 2004 09:56 AM
proposed 833 design James D. Tubes / Valves 0 25th March 2004 12:42 PM
Comments invited: proposed remote volume control tcpip Parts 2 2nd February 2004 07:07 AM


New To Site? Need Help?

All times are GMT. The time now is 11:48 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