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

6L6GC PP amplifier with SS phase inverter
6L6GC PP amplifier with SS phase inverter
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 7th October 2018, 12:19 AM   #1
H713 is offline H713  United States
diyAudio Member
 
Join Date: Nov 2017
Location: Wisconsin
Default 6L6GC PP amplifier with SS phase inverter

I am fully aware that this is considered to be sacrilege by many, but I myself am not afraid to utilize solid-state devices with tubes. In my eyes, a lot of the best solutions utilize the best of both worlds.

For me, it's just an experiment.

So here's the design. I only used the AD713 because 1) I have them, and 2) LTSpice has this particular Analog Devices chip in its default library. I may switch this out for an OPA2604.

The Balanced input is not necessary and somewhat redundant, but I like to use XLR connectors for my amplifiers because it's a great connector.

The challenging limitation here is the fact that I am limited to a 6-pin Harting connector connecting the PSU chassis to the amp chassis. It also has a dedicated ground pin. This will likely be tested in my PP 1625 amp, as there's a fair bit of space in the chassis for a test board. Due to the pin limitation on the connector, I'm hoping I can run the Op-Amps off of the bias supply. The transformer is more than large enough, so no worries on that, but obviously I don't have enough pins to run a dual rail supply. So this is what I came up with for a rail splitting device, not sure if it's the best way.

The negative feedback circuit has not been worked out yet, if anyone has suggestions on where to connect it, please share. My experience with op-amp design is pretty minimal, so there may be some glaring issues. That said, the Spice model looks good, but obviously LTspice != Reality. However, I do believe that this could be very easily adapted to work with a huge variety of tubes from 6V6s (or smaller) all the way up to big sweep tubes and KT88s.

Also, the schematic does not show all the correct resistor values for the bias supply.
Attached Images
File Type: png H713 SS Inverter.PNG (128.3 KB, 898 views)
  Reply With Quote
Old 7th October 2018, 01:58 AM   #2
Tubelab_com is offline Tubelab_com  United States
diyAudio Member
 
Tubelab_com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
I believe that some DC path for the first opamp's inputs needs to be in place. Any DC on pin 6 will affect the phase inverter chips. That will vary with the volume control setting.

There definitely needs to be a DC path to a fixed voltage source for the 6SN7 grids. If you have a negative voltage source tied to the CCS on the cathode instead of ground, then the grids can be grounded through a high valued resistor, 220 K to 1 meg. The -15 volt supply for the opamps is suffient for most CCS chips. The -68 volt supply will work for the 10M45. If the CCS is indeed grounded, there should be a small positive voltage on the grids for the CCS to have sufficient headroom to function properly under large signal conditions.

The standard 7918 chips can't eat 68 volts, most are 30 to 35 volts maximum. A positive regulator chip like the 7818 will not work with a negative voltage on it's input. Smoke or dead parts happen.
__________________
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
  Reply With Quote
Old 7th October 2018, 02:53 AM   #3
H713 is offline H713  United States
diyAudio Member
 
Join Date: Nov 2017
Location: Wisconsin
Yeah, looking back on the schematic I see a few of the issues you mentioned. In theory, any issues now are design related, not drawing related. The power rail splitting thing I was trying to show before is now drawn the way I mean to draw it. Blocking capacitor between the first op-amp and the second is now in place (forgot about that... probably a smart thing to do). 10M45 is now tied to -68V, and the 6SN7 grids are grounded through a 1 Meg resistor. The regulators are still dropping a little more voltage than I'd really like, so I may insert a dropping resistor in there beforehand.
Attached Images
File Type: png V3 6L6 SS Inverter PNG.PNG (131.9 KB, 887 views)
  Reply With Quote
Old 7th October 2018, 05:51 AM   #4
leadbelly is offline leadbelly  Canada
diyAudio Member
 
leadbelly's Avatar
 
Join Date: Dec 2002
Location: Calgary, Alberta
Don't understand the point of your design. You left the driver as a tube. That's the most important place where SS wins over tubes. Even in an AB1 amp, you will still get better overload recovery if you go with an SS driver.
__________________
Wise men speak because they have something to say; Fools because they have to say something. Plato
  Reply With Quote
Old 7th October 2018, 01:54 PM   #5
Tubelab_com is offline Tubelab_com  United States
diyAudio Member
 
Tubelab_com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
Quote:
You left the driver as a tube. That's the most important place where SS wins over tubes.
I would also tend to agree. you need some voltage gain between the opamp and the output tube, and a 6SN7 is a good choice, but the best parts to directly feed the grids of the output tubes are mosfets.

The 6CG7 is a shrunken 6SN7, so look at the second stage and mosfet drivers in this design. Copy whatever you want from it.

Tubelab Universal Driver Board, 2015 version
__________________
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
  Reply With Quote
Old 7th October 2018, 03:46 PM   #6
H713 is offline H713  United States
diyAudio Member
 
Join Date: Nov 2017
Location: Wisconsin
Hopefully this week I'll have time to play around with models for a source-follower for the driver portion just to see what it looks like.

Am I correct that the reason a FET is a better driver than a 6SN7 is because they can supply current if the grid of the output tube is driven positive? That was on my mind as I was drawing this, but I ultimately concluded that it's still probably about as capable in this regard as a tube-based LTP driving the outputs, just with a (theoretically) more closely balanced phase inverter.

If I have time (and if it ever stops raining long enough for the basement to dry up) I may try breadboarding up this current design, just to see how it performs. I can also try some source followers at that point.
  Reply With Quote
Old 7th October 2018, 05:21 PM   #7
leadbelly is offline leadbelly  Canada
diyAudio Member
 
leadbelly's Avatar
 
Join Date: Dec 2002
Location: Calgary, Alberta
Why do you think you have a better balanced PI with that design?

Your input stage doesn't make sense by the way. The AD713 is a regular opamp, so the non-inverting and inverting inputs have very different input impedances. It's not the way to implement a balanced input with opamps.
__________________
Wise men speak because they have something to say; Fools because they have to say something. Plato
  Reply With Quote
Old 7th October 2018, 06:56 PM   #8
waltube is offline waltube  Italy
diyAudio Member
 
waltube's Avatar
 
Join Date: Feb 2005
Location: roma
Oh my God!!!!


Walter
  Reply With Quote
Old 7th October 2018, 09:44 PM   #9
SpreadSpectrum is offline SpreadSpectrum  United States
diyAudio Member
 
SpreadSpectrum's Avatar
 
Join Date: Nov 2007
I built a signal generator for bench testing tube stages using an ADA4700 and two cheap 48V switching supplies to provide the rails. The distortion it produces is so much less than a 6SN7 at comparable levels it is ridiculous, and it is low-order as well. I can't detect anything over 3rd harmonic at any level all the way up to 30Vrms. There is nothing wrong with using op amps if done right.

I wanted to kill the ground loop sounds in my KT88 Unity-Coupled amp permanently and was looking at adding input transformers. The only trouble was the space constraints wouldn't allow it, so I replaced the 6SN7 input stage with input transformers and an ADA4700 input stage (in the configuration of the input stage of an instrumentation amp). Transformers sit where the tubes used to and the amp is dead quiet.

You could probably choose a much better op amp for audio and you definitely need to add DC references at various points like has been mentioned. I'd use LM4562 wherever levels are low and ADA4700 where you need to swing some volts. If you really want some good ideas, get Douglas Self's Small Signal Audio Design book for best circuits for all of the low-level stuff.
__________________
My Blog: http://tubeswithatwist.blogspot.com/
  Reply With Quote
Old 7th October 2018, 09:46 PM   #10
azazello is offline azazello  Bulgaria
diyAudio Member
 
azazello's Avatar
 
Join Date: Aug 2008
Location: USA
SS ph. inv. will cause SS sound. How do You like?
__________________
Projects: SE GK71, SE 12BQ6, PP KT88, OTL-OCL 6AS7 Gen. Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, E84L Siemens, E236L Telefunken, headphone ampl. OTL Loftin White 6AS7 RCA,...
  Reply With Quote

Reply


6L6GC PP amplifier with SS phase inverterHide 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
6N7's in phase inverter service rsumperl Tubes / Valves 9 24th September 2016 08:38 PM
RCA phase inverter? KTMrida4life Car Audio 5 19th September 2015 10:33 PM
Need help identifying a phase inverter LATUBEGEEK Instruments and Amps 6 28th September 2013 12:59 AM
Phase inverter ThSpeakerDude88 Tubes / Valves 37 29th August 2007 04:15 PM
6C8G phase inverter engels Tubes / Valves 4 4th February 2007 07:01 PM


New To Site? Need Help?

All times are GMT. The time now is 06:51 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki