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 13th August 2012, 05:57 PM   #1
lechuck is offline lechuck  Netherlands
diyAudio Member
 
Join Date: Mar 2011
Location: Amsterdam
Question Found a new interstage coupling method (?)

Hi Guys,

In my personal search for removing the interstage coupling cap I have found a different way to couple between valve amplifier stages, a way that I have not seen anywhere before. Because I am not sure if this is new or not I would like to ask you if you have seen anything similar to what I will describe in this topic.

Because I am careful not to disclose my design yet I will only describe it as good as I can. Unfortunately the topology is so extremely simple that I cannot even say that much about it, but let's give it a try.

I have noticed that it is possible to shift the DC voltage between two stages without using a cap or transformer, while retaining the original AC signal. The topology that I have found uses medium-mu valves to couple between two amplifier stages.

I have found the topology by using SPICE and I have build a Single Ended stereo amplifier to try out the topology in real-life. In my test amplifier I am using a simple grounded grid voltage amplifier with a ECC83, the coupling valve is a 6CG7 and the output valve is a 6550. The DC input voltage for the coupling stage within this amplifier is +160 volts, the output DC voltage is -43 volts.

The coupling stage is capable to drive the grid of the 6550 positive. In my simulations I have noticed that it will go positive for as long as the grid current of the output tube is not too much. For real subclass 2 operation an extra buffer stage should be implemented to allow for true subclass 2 operation.

The coupling stage does not show changes in resistance with varying frequencies and also shows a zero phase shift.

Because the coupling valve is also responsible for setting the idle current of the output valve I have been looking for a method to use a DC servo. I am now in the middle of developing a push pull version in which I will make use of a DC servo, connected to the coupling stage, to set the idle current of the output valves.

The topology described here is not the same as the topology as described by Darius-Karim Mottaghian-Milani (url: Coupling Triodes). I have sim'd his topology, didn't like the results and dropped his idea.

Please forgive me if I have overlooked certain properties. I am not technically educated, so it could be possible that I am not aware of certain key properties that require explanation.

I would be very interested to know if any of you forum members recognize what I am doing, and perhaps could provide me with examples of similar topologies. Also any other feedback is more than welcome.

Best regards,
Anthonie

PS.
I have attached a few pictures of my test amplifier. I do realize that there is a cap on the circuit boards that might look like a interstage coupling cap. This is a 10u cap that is used for de-coupling the high voltage for the first two amplifier stages.

Click the image to open in full size.
Click the image to open in full size.
  Reply With Quote
Old 13th August 2012, 06:16 PM   #2
diyAudio Member
 
Michael Koster's Avatar
 
Join Date: Jan 2008
Location: Eureka, CA
Because I am careful not to disclose my design yet
  Reply With Quote
Old 13th August 2012, 06:22 PM   #3
lechuck is offline lechuck  Netherlands
diyAudio Member
 
Join Date: Mar 2011
Location: Amsterdam
Quote:
Originally Posted by Michael Koster View Post
Because I am careful not to disclose my design yet
With that I am referring to the electrical circuit itself. I don't know if this is a new invention, or that it already exists, that is the reason for my precautions.
  Reply With Quote
Old 13th August 2012, 07:26 PM   #4
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
I was going to guess the inverted triode scheme (long known but obscure, ie the last tube in the Darius et al scheme using a low Mu tube), but since you say yours is different ... I can only guess that you have reversed the plate and grid in the Darius' scheme and put negative V on the plate to reduce the grid current. Steve Bench played around with some flipped tube configurations, but I think they were oriented toward still getting some power gain from the stage. Might want to check out his stuff.

Or, there is a scheme around that just uses the tube as a diode to drop voltage, used for cathode bias sometimes, usually using damper diodes.

A current source through a resistor also works fine for dropping voltage. Needs a SS CCS to get to useful low enough voltages for grid coupling though, unless you were to run a pentode or high Mu triode CCS from a negative cathode level.

Probably could also just put a resistor voltage divider across a triode: plate to cathode divider with the middle tap to the grid. Sort of a shunt V regulator. A top zener in the divider would stiffen it up more.
__________________
I want a Huvr-Board!!

Last edited by smoking-amp; 13th August 2012 at 07:44 PM.
  Reply With Quote
Old 13th August 2012, 07:44 PM   #5
ChrisA is offline ChrisA  United States
diyAudio Member
 
Join Date: Jan 2008
Quote:
Originally Posted by lechuck View Post
Hi Guys,

In my personal search for removing the interstage coupling cap I have found a different way to couple between valve amplifier stages, a way that I have not seen anywhere before. ...
I think the term is "direct coupled amp" or sometimes they call it a DC coupled amp. I think you are about 50 years to late to invent a new valve amp topology.

There are many variations of it. but in every case the trick is to bias the second stage so the output of the preceding stage can be directly connected to the grid of the second stage.

Williamson might have beed first to use this? Actually I doubt that but Williamson is the fist really well known amp to use it

The Williamson Amp, part one | Preservation Sound

Another common example of this is seen in the Dynaco ST35 there the gain stage triode is directly coupled to the grid of the phase splitter. But the second tube could be a gain stage too.

Dynaco ST35 Tube Amplifier Schematic and Manual


Using only two triodes there are only a few ways we can connect wires to six terminals. I think it comes to something like only 50 possible topologies. Many of them already have names.
  Reply With Quote
Old 13th August 2012, 08:06 PM   #6
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
If I read the op post correctly, he has eliminated the coupling cap to the output tube(s) as well.

So it is a fully direct coupled amp.

The second tube (6CG7) is acting as a DC Level Shifter.
  Reply With Quote
Old 13th August 2012, 08:48 PM   #7
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
If it works it was probably invented first either by WW2 radar designers or Tektronix 'scope designers!
  Reply With Quote
Old 13th August 2012, 08:54 PM   #8
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
Quote:
The second tube (6CG7) is acting as a DC Level Shifter.
A floating shunt V regulator would be a good fit. (resistive V divider across the tube, plate to grid to cathode)

On the other hand, I see a bunch of heatsinks on the board which could be CCS's. So just a triode configured as a resistor would work to drop a fixed voltage using a CCS source.

Have to agree on the Tektronix scope designer or radar designers comment. If it is practical and works well, DC coupling would have been their forte.
__________________
I want a Huvr-Board!!

Last edited by smoking-amp; 13th August 2012 at 09:00 PM.
  Reply With Quote
Old 13th August 2012, 09:03 PM   #9
diyAudio Member
 
smoking-amp's Avatar
 
Join Date: Dec 2001
Location: Hickory, NC
Since there are so many DC coupling work-arounds, including just a resistor with a cap across it, I don't think a patent here is going to collect much royalty. Ego trip and a big hole in the wallet maybe.

Hmmm, post #9 (that I was responding to) seems to have vanished?
__________________
I want a Huvr-Board!!
  Reply With Quote
Old 13th August 2012, 10:01 PM   #10
7N7 is offline 7N7  United Kingdom
diyAudio Member
 
7N7's Avatar
 
Join Date: Jan 2003
Location: England
Quote:
Originally Posted by DF96 View Post
If it works it was probably invented first either by WW2 radar designers or Tektronix 'scope designers!
Alan Dower Blumlein?

7N7
__________________
Plug them in and light them up
  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
Interstage coupling feedback Ronken Tubes / Valves 2 17th January 2012 07:36 PM
TSE interstage coupling cap jrenkin Tubelab 4 31st December 2011 01:04 AM
interstage coupling Raj1 Tubes / Valves 18 19th December 2003 05:28 PM
Interstage coupling transformers Circlotron Everything Else 2 15th June 2002 11:36 PM


New To Site? Need Help?

All times are GMT. The time now is 02:51 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