GM-70 cathode feed SE amplifier - 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 3rd April 2009, 07:56 PM   #1
diyAudio Member
 
kavermei's Avatar
 
Join Date: Jan 2005
Location: Lokeren, Belgium
Send a message via MSN to kavermei
Default GM-70 cathode feed SE amplifier

Hi everybody,

I'd like to throw the circuit below out in the open and get some comments on it.
I'm not at all experienced in designing my own circuits. The tube choices are based mainly on what I have lying around, it's the overall topology I'm not so sure about. Will it work well?

Credit where it's due: the voltage stage (around the 6SN7) was taken from the 'Lyrebird' amp.

Please comment,
nukkel
Attached Images
File Type: png nukkel_gm70.png (88.1 KB, 872 views)
  Reply With Quote
Old 3rd April 2009, 08:56 PM   #2
diyAudio Member
 
korneluk's Avatar
 
Join Date: Jan 2005
Location: South Florida
I have never worked with the GM70, so I'll let others comment on the circuit and specific op-points.

However, the EL84 is running at 93% of max power dissipation. I recommend you replace it with another power pentode with larger plate dissipation - i.e. another 807 or an EL34.

Enjoy,
  Reply With Quote
Old 4th April 2009, 10:24 PM   #3
R.I.P.
 
Join Date: Dec 2001
Location: Schaffhausen Switzerland
Kavamel,
Running the output tube as a grounded grid and no NFB will give you a VERY high output impedance - if that's what you want then go for it.

But if it's not what you or your speakers want, then you will have every odd sounding amp.

Your driver staes could be much simplier if you use conventional grid drive, and the OPZ will be down into a "normal" range.

Regards, Allen (Vacuum State)
  Reply With Quote
Old 5th April 2009, 12:14 AM   #4
diyAudio Member
 
Join Date: Jul 2003
Location: Ann Arbor, MI
Quote:
Originally posted by Allen Wright
Kavamel,
Running the output tube as a grounded grid and no NFB will give you a VERY high output impedance

Regards, Allen (Vacuum State)
How do you figure that Allen? The cathode impedance does change the plate characteristic. Roughly by factor of mu times the output Z of the driving stage.

The bigger issue I see here is cap coupling. The cathode impedance( load to be driven ) is ~200 Ohms. The 211 is much more responsive to this sort of thing; its A1 grid drive capability is small compared to what is possible with grid current. Driving the cathode leaves no step change in load when grid current starts to flow, as well as using the grid as a screen to avoid Miller. Zero bias with B+ of ~600V can yeild nearly 100W in PP without resorting to AB. Since we're taking SE, the drive is slightly more difficult...still need a power amp in front of it.
cheers,
Douglas
__________________
the Tnuctipun will return
  Reply With Quote
Old 5th April 2009, 12:35 AM   #5
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
The bigger issue here is the driver has to deliver the same current the output transformer will see, and a voltage swing roughly twice of an idle grid-cathode bias. Think of such power requirements, and add 100 Ohm load (2 x 200 Ohm in parallel) and guess, do you need an output stage if your EL84 delivers more power than you get on the speaker...

__________________
The Devil is not so terrible as his math model is!
  Reply With Quote
Old 5th April 2009, 02:01 AM   #6
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
I played with cathode drive several years ago when I was designing my 845SE. I never could get it to sound right, so I kept experimenting and wound up with what I now call PowerDrive.

There are several ways to do cathode drive that are far simpler than this. Some of these ideas work with the 845 because of its large bias and drive requirements, they may be harder to implement with less voltage.

1) Use a 6AS7 for your output tubes cathode resistor and drive its grid. The grid voltage on the 6AS7 sets the current through the output tube stack, and you get extra gain (not much).

2) Make an LTP using the output tube for one half and a decent sized triode (or triode wired pentode) of similar Mu for the other half (I used a KT88). Hang a CCS chip in the tail. The drive goes to the smaller tubes grid and its plate goes to a medium voltage supply.

3) Use a P channel mosfet in the cathode, Source to the cathode , drain is grounded, bias pot and drive to the gate. This works the best.

I never really thought about the output impedance, but I believe Allen is right. The cathode to grid drive voltage is the same whether the grid is grounded and the cathode is driven, or the cathode is grounded and the grid driven. The cathode to plate voltage is not the same. There is a positive feedback type action going on since the plate to cathode voltage is modulated by the drive voltage. This will raise the output impedance but may allow for more power output from a given tube. I was trying to exploit the "more power" idea, but it sounded poor.

Ham radio guys realized long ago that the drive power is added to the output power allowing for a bit more power output from a given amount of DC power input to the final stage. This is why grounded grid RF amplifiers were popular when the FCC rules specified a maximum DC input to the final stage. Those rules were changed a long time ago (I don't remember exactly when).
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 5th April 2009, 02:47 AM   #7
Jeb-D. is offline Jeb-D.  United States
diyAudio Member
 
Jeb-D.'s Avatar
 
Join Date: Nov 2005
Location: SoCal
Quote:
I never really thought about the output impedance, but I believe Allen is right.
I think he is too. The driver and output stage together are working kind of like an AC coupled cascode variant. Good for a pre-amp, but probably the worst candidate for an audio amplifiers output stage.

Quote:
The bigger issue I see here is cap coupling.
That would defiantly be another major obstacle.


I wouldn't pursue that output stage, if I were you. Sure you will probably get sound out of it, but I can't imagine the results being better than a conventional grid driven config.
  Reply With Quote
Old 6th April 2009, 07:14 AM   #8
diyAudio Member
 
kavermei's Avatar
 
Join Date: Jan 2005
Location: Lokeren, Belgium
Send a message via MSN to kavermei
Thanks everyone for your feedback. Based on it I guess the circuit as it is isn't worth building. At the moment I see two best options:

A- Stick with conventional grid drive and keep the output tube in A1 as much as possible (to avoid the sudden load change to the driver)
B- Try a better way of implementing cathode drive (Tubelab, thanks for your suggestions! I'll be looking into them)

I'm leaning towards (A), but I'm also curious how those on-paper benefits of a grounded grid stage would translate into sound...

All the best
kavermei
  Reply With Quote
Old 6th April 2009, 12:03 PM   #9
R.I.P.
 
Join Date: Dec 2001
Location: Schaffhausen Switzerland
I'm curious. What "on paper"benefits do you expect from ground grid operation?

I'm not aware of any...

Regards, Allen (Vacuum State)
  Reply With Quote
Old 6th April 2009, 12:23 PM   #10
jane is offline jane  Norway
diyAudio Member
 
jane's Avatar
 
Join Date: Jul 2004
Location: Norway
Quote:
Originally posted by Allen Wright
I'm curious. What "on paper"benefits do you expect from ground grid operation?

I'm not aware of any...

Regards, Allen (Vacuum State)
Maybe that the driver requirements are about the same in class A1 and A2 and that the transition from A1->A2 not will affect the driver very much?

Jan E Veiset
__________________
Life is hard - Then you die.
  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
20W cathode follower amplifier yan24 Tubes / Valves 125 8th October 2014 08:28 PM
UL to triode mode conversion on cathode bias amp-different cathode resistors needed? bigwill Tubes / Valves 6 30th December 2007 01:37 AM
TCJ Cathode-Coupled Amplifier seventenths Tubes / Valves 3 18th July 2007 06:05 PM
hybrid common cathode amplifier? mwmitchell Tubes / Valves 19 16th January 2003 02:51 PM


New To Site? Need Help?

All times are GMT. The time now is 10:00 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