Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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 16th July 2018, 12:55 PM   #11
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Quote:
Originally Posted by Sorento View Post
Concerning the original Schadeode schematic, why is the cathode bypass cap connected to B+ rather than gnd? Makes PSR worse and requires a high voltage cap ... Cannot see why it should be beneficial .
That's the 'ultrapath' connection of cathode bypass. It's controversial.

In this particular amp, since the MOSFET drain is DC-coupled to the 6L6 grid, the 6L6 cathode has to sit at >70V, so a high voltage capacitor would be required for either connection.

Ultrapath connection would need 500V - 70V = 430V or higher rated. Normal cathode bypass to ground would need 70V or higher rated.

In my proposed push-pull variation, the B+ would be at about 450V, so a 450V rated cap would work (450V - 70V = 380V). Those are easy to find.

No comments on whether a push-pull version would be do-able? I'm wondering how difficult it would be to select a good pair of DN2540 close enough in both current draw and gain/transconductance to work as a push-pull pair.

Quote:
Originally Posted by Wavebourn View Post
Yes, if feedback divider's ratio is stable. But if is non-linear, the result is non-linear. Thst's why I always use the best resistors in feedback voltage divider.
So, when one of the resistors is replaced by the plate resistance of the driver tube (or drain-source resistance of a MOSFET), does that mean you now have a lower quality resistor in the feedback divider?

How variable is the Rd-s of a DN2540 MOSFET with only about 4mA Id-s?
  Reply With Quote
Old 16th July 2018, 03:07 PM   #12
indra1 is offline indra1  Indonesia
diyAudio Member
 
Join Date: Nov 2010
Location: Bogor
Quote:
Originally Posted by rongon View Post
So, when one of the resistors is replaced by the plate resistance of the driver tube (or drain-source resistance of a MOSFET), does that mean you now have a lower quality resistor in the feedback divider?
Mosfet output impedance in schadeode is assumed to be high and not a concern in Wavebourne's comment. Non linear effective feedback resistance (R1 / gain of V3) due to non linear gain of V3 is. I think he prefers using a different topology, a low output impedance stage driving a voltage divider or something similar.
Quote:
How variable is the Rd-s of a DN2540 MOSFET with only about 4mA Id-s?
I would use Id > 10ma if I want to use mosfet as input, member TheGimp discuss FET Source Follower Distortion thread at low Id.
  Reply With Quote
Old 16th July 2018, 04:19 PM   #13
smoking-amp is offline smoking-amp  United States
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
I think the usual complaint about leaving out the driver load resistor is that it is hard to get enough operating current thru a driver pentode that way (large voltage drop thru Rfdbk). The Mosfet should be fine on that account. A driver triode would certainly call for some load resistor to control the Rfdbk ratio.

Pentode or Mosfet driver output Z should be high enough to ignore in the Rfdbk divider. However, both pentode and Mosfet have an Iout = k Vin ^2 transfer (square law if current is up enough) so a driver cathode or source resistor is called for to linearize the driver V to I in the operating range.

While the "Schade" shunt Fdbk works well to turn the output device into a linear triode, one issue to consider is the OT leakage L. The plate is not the best actual Vout sampling point. Typical OTs with UL taps should have better coupling to the secondary at the UL taps (for UL use-age), so could make a better place to take the "Schade" Fdbk from. -If- it remains stable with the OT leakage L now (more so) in the N Fdbk loop. And the two P-P sides usually have wildly different leakage L for typical single bobbin OTs. But since these are near constants, they could be compensated for in the driver loops.

Last edited by smoking-amp; 16th July 2018 at 04:25 PM.
  Reply With Quote
Old 16th July 2018, 04:48 PM   #14
Bigun is offline Bigun  Canada
diyAudio Member
 
Bigun's Avatar
 
Join Date: Jan 2009
Location: Waterloo, ON or Herefordshire UK
Exactly, that's confirming how I usually see it - the plate of the driver shunts the high impedance feedback signal coming from the plate of the output tube. If the impedance of the driver plate is too low, it shunts the feedback signal too heavily. Since a Pentode has a high impedance at it's plate, it is far better suited to this style of feedback. If you have to use a triode, increase it's effective plate resistance by using a high-mu triode with an un-bypassed cathode resistor.
__________________
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.
  Reply With Quote
Old 16th July 2018, 05:37 PM   #15
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Quote:
Originally Posted by smoking-amp View Post
Pentode or Mosfet driver output Z should be high enough to ignore in the Rfdbk divider. However, both pentode and Mosfet have an Iout = k Vin ^2 transfer (square law if current is up enough) so a driver cathode or source resistor is called for to linearize the driver V to I in the operating range.
Yes, I found that to be necessary in the MOSFET LTP. In my schematic, both DN2540s have 220 ohm source load resistors, 'above' the ccs in the tail. In spice simulation, without those source load resistors gain goes wildly high and I'm sure in real life balance would go right out the window.

Quote:
Originally Posted by smoking-amp View Post
While the "Schade" shunt Fdbk works well to turn the output device into a linear triode, one issue to consider is the OT leakage L. The plate is not the best actual Vout sampling point. Typical OTs with UL taps should have better coupling to the secondary at the UL taps (for UL use-age), so could make a better place to take the "Schade" Fdbk from. -If- it remains stable with the OT leakage L now (more so) in the N Fdbk loop. And the two P-P sides usually have wildly different leakage L for typical single bobbin OTs. But since these are near constants, they could be compensated for in the driver loops.
Is that the argument for 'E-Linear' connection? OK, I'll try a simulation using the screen taps to feed the drain load/Rnfb resistors to the MOSFET LTP. The resulting amp would have less gain that way, certainly.

I have a beater Dynaco ST70 chassis that's been stripped down to its sockets and transformers several times over its lifetime. I was thinking of building this up in that chassis. It will be tight, though. The 1.5k cathode resistors for the 6L6s will need breathing room, as they will be dissipating over 6W each. I'll need to use 15W rated resistors minimum. 25 watters if I can find them.
--
  Reply With Quote
Old 16th July 2018, 06:09 PM   #16
Ketje is offline Ketje  Belgium
diyAudio Member
 
Ketje's Avatar
 
Join Date: Dec 2012
Location: Flanders
Quote:
Originally Posted by Wavebourn View Post
Yes, if feedback divider's ratio is stable. But if is non-linear, the result is non-linear. Thst's why I always use the best resistors in feedback voltage divider.
If the driver is a triode you are right,the Ri changes giving a non-linear devider.
But here the driver is a mosfet, allmost a constant current source, no real divider there.It has to produce a current that gives Va-Vg1 on 100k resistance.
With no feedback (100k to +B) the driver current has to give only Vg1 on the resistance.Less distortion at the cost of more input, as usual with fb.
Mona
  Reply With Quote
Old 16th July 2018, 07:46 PM   #17
Wavebourn is offline Wavebourn  United States
Designer & Technologist
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Quote:
Originally Posted by Ketje View Post
If the driver is a triode you are right,the Ri changes giving a non-linear devider.
But here the driver is a mosfet, allmost a constant current source, no real divider there.It has to produce a current that gives Va-Vg1 on 100k resistance.
With no feedback (100k to +B) the driver current has to give only Vg1 on the resistance.Less distortion at the cost of more input, as usual with fb.
Mona
The same way we can assume that an output tube has similarly almost linear transfer curve (well, straight line), so why feedback?

But if the tube is not linear, driver's current would drive non-linear impedance of non-linear device that is the feedback resistor's resistance divided by real output stage's momentary gain. What would we get then?
__________________
Nothing in the universe is perfect. The ideal things are the ones that are most optimal. Optimization criteria, what matters. When I hear "No Compromise Design", I want to take a sledgehammer and test how impact-proof it is.
  Reply With Quote
Old 16th July 2018, 08:09 PM   #18
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
I combined a bunch of stuff in this idea.

1) M. Koster 'Schadeode' but with push-pull and ultralinear OPT.
2) Doug "Bandersnatch" 'E-Linear' screen grid to control grid feedback.
3) RCA-style negative feedback from output tube plate to driver source.

Combining them all, I get what looks like a well-performing circuit. LTspice predicts THD of around 0.05% at 1W out into 4 ohms, about 1% THD at 20W out. H2 and H3 equal (about -75dB) at 1W out.

However, the straight push-pull 'Schadeode' with plate-to-grid feedback and ultralinear OPT connection looks 'too good to be true' in LTspice. Would you believe 0.005% THD at 1W out into 4 ohms? Nah, I don't either.

Anyway, here's the mish-mash version, attached. Please feel free to criticize it for all it's worth.
--
Attached Images
File Type: png _PP-UL_Schadeode_ELinear_plate-to-cathode-FB_idea.png (43.2 KB, 158 views)

Last edited by rongon; 16th July 2018 at 08:24 PM.
  Reply With Quote
Old 16th July 2018, 09:00 PM   #19
smoking-amp is offline smoking-amp  United States
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
RE: Wavebourn

The output tube impedance variation and near square law Vgrid to Iplate are distortion causing factors for sure. But the voltage on the OT/plate is the real deal output, what we are measuring, so getting the N Fdbk to linearize that is what we want.

----------------------------------------------------

Gee, I was just about to mention the RCA 50 Watt'r schematic with "Schade" N Fdbk -and- N Fdbk to the driver cathodes, driver CFB (Mosfet sources here). (schematic below) The N Fdbk to the driver cathode/source linearizes the V to I conversion in the driver stage nicely. The low Z plate load for the driver, however, mostly winds down the overall driver CFB gain.

0.005% dist. Huh! Probably some coincidence from the device models, but still, very interesting, can't be much wrong with it. Would leave Citation II and Mac 275 way behind. Schematic? (just delete C3 and C4?)

You might try C3 and C4 connected to the SG1 and SG2 OT taps instead of the plates.

And then maybe try changing the R4,R13 and R5,R14 Schade Fdbks into just loads to B+, so making it totally driver CFB based. Some optimizing R values may be needed for 6L6 grid bias level. Then we have some comparison between driver CFB and shunt "Schade" effectiveness.
Attached Images
File Type: jpg RCA_50Watt_Amp.JPG (534.0 KB, 150 views)

Last edited by smoking-amp; 16th July 2018 at 09:21 PM.
  Reply With Quote
Old 16th July 2018, 09:47 PM   #20
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Quote:
Originally Posted by smoking-amp View Post
0.005% dist. Huh! Probably some coincidence from the device models, but still, very interesting, can't be much wrong with it. Would leave Citation II and Mac 275 way behind. Schematic? (just delete C3 and C4?)
The problem with removing C3 and C4, allowing DC current to flow from output tube plates to driver MOSFET sources, is that it seriously messes with the driver stage biasing. I could RC-couple the MOSFET drains to the output tubes' grids, but then I'll be increasing the parts count even more. I was going to attempt to cram all of this into a Dyna ST70 chassis, and it's tight in there.

That 0.005% THD at 1W into 4 ohms figure was attained by really goosing the current through the 6L6s, beyond what the ST70 power transformer can do. But even with it cooled down to within the ST70 power transformer's current budget, LTspice is saying 0.01% THD at 1W. The schematic is attached to this post.

To get it to that 0.005% THD @ 1W/4R figure, change R5 and R20 to 39k, leaveing R4 and R19 at 47k. That will raise the voltage at the DN2540 drains, reducing the grid bias on the 6L6s so that they draw 68mA each instead of 55mA each. That will draw a total of 150mA from the B+ supply. The extra current reduces 3rd harmonic distortion a bit.

In simulation, THD is lower if you take the feedback from the output tubes' plates to the driver sources. I think there's just more gain there, so more feedback. There's less gain from the screen grids, so there's less gain for feedback available from there.

Quote:
Originally Posted by smoking-amp
You might try C3 and C4 connected to the SG1 and SG2 OT taps instead of the plates.
Do you mean don't tap feedback off the plates at all? Tap two feedback loops from the screens? (Output tube screens to MOSFET drains, and also output tube screens to MOSFET sources?)
Attached Images
File Type: png _PP-Schadeode_EXPERIMENTAL_01.png (44.9 KB, 154 views)

Last edited by rongon; 16th July 2018 at 10:01 PM.
  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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
"Iron Garter" Push-Pull Amplifier wrenchone Tubes / Valves 54 16th October 2017 07:32 AM
2 Push-Pull "universal" driver PCBs ta240 Swap Meet 1 10th October 2017 06:03 AM
"Killer" Screen Driven Push-Pull Amp Initial Queries wrenchone Tubes / Valves 55 7th September 2014 11:39 PM
"Push-Pull" for push und "Push-Pull" for pull (like Gamnut) - whats the right name? tiefbassuebertr Solid State 70 15th August 2012 08:08 PM
829B/3E29 "Triode" Push Pull Paralleled Sections coldcathode Tubes / Valves 26 28th October 2010 06:39 PM


New To Site? Need Help?

All times are GMT. The time now is 02:26 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki