SPICE Simulator Giving Negative Gain - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs 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
Thread Tools Search this Thread
Old 1st October 2014, 02:14 PM   #1
diyAudio Member
Join Date: Mar 2013
Exclamation SPICE Simulator Giving Negative Gain

2A3 Question New Post At Bottom ****

Hey guys, so for my STEM project, I am learning the basics of designing tube amplifiers. My first project is to design a simple headphone amp, and I have chosen the 6SN7 and the 2A3. Just to get a basic design going, I set up the 6SN7 as the input with a 2A3 output and an output transformer. This is pretty basic and really doesn't qualify for my project, but I ran some simulations...

When making the transformer ratio 2.5, and using a 600R load (eg. Beyer T1) I got about 7 dB gain. That was fine for a TC amp with high impedence.

The problem was when I changed my load to anything less, and adjusted the tranny turns, I got negative gain. I have tried every possible turns ratio and many different impedences from 32-100R and I can't get the thing to give me a possitive gain.

Here is a screenshot. Anything wrong with my design? I can't seem to figure this one out.
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

Just to reitterate, this is NOT my final design, it is simply the basis for my design, which won't be that much more complex, but still won't be quite this basic.

Last edited by dutchgfx; 4th October 2014 at 04:32 PM.
  Reply With Quote
Old 1st October 2014, 02:22 PM   #2
diyAudio Member
Join Date: Feb 2010
Location: Helsinki
A step-down transformer by definition gives negative voltage gain. This may or may not make the total gain of the circuit negative. The OTs job is to make it possible for a high impedance source (such as a vacuum tube) to be able to drive low impedance devices, such as headphones and speakers.
  Reply With Quote
Old 1st October 2014, 03:08 PM   #3
Merlinb is offline Merlinb  United Kingdom
diyAudio Member
Merlinb's Avatar
Join Date: May 2006
Location: Lancashire
You appear to be showing a step UP transformer in your sims...

If you change the turns ratio to maintain the same primary impedance then you will get the same POWER into the load, but this implies the VOLTAGE gain of the whole circuit will be divided by sqrt(Load 1 / Load 2). The voltage gain to the anode of the 2A2, however, should remain the same.

Last edited by Merlinb; 1st October 2014 at 03:17 PM.
  Reply With Quote
Old 1st October 2014, 03:21 PM   #4
kevinkr is offline kevinkr  United States
diyAudio Moderator
kevinkr's Avatar
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
First big problem I can see is in the driver stage, your plate load resistor is too low, needlessly reducing the available voltage gain.

I would recommend a plate resistor of minimum 3X the rp of the 6SN7..

Supply voltage should probably be boosted to the vicinity of 300V which would give you the headroom required for reasonable drive out of the 6SN7.

To drive 32 ohm headphones a step down ratio of 9:1 to 10:1 would be close to ideal with a 2A3. If you want to drive a speaker or 8 ohm headphones 18:1 - 20:1 would be about right.

Note that the cathode resistor on the 2A3 must be bypassed in order to achieve gain approaching mu, and to maintain an rp of around 600 ohms, unbypassed your rp will be much higher, substantially greater than 3K.

You should be able to achieve at least 10dB of gain from input to transformer secondary if you have done things correctly.
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
  Reply With Quote
Old 4th October 2014, 04:30 PM   #5
diyAudio Member
Join Date: Mar 2013
Related question, I can't seem to figure out how to wire the 2A3 filament. If I design a DC Supply for the filaments, how do I physically wire it. Since the filament supply would affect the Cathode voltage , how do I keep my bias at whatever voltage I have it set as with a resitor?

I understand I solder one end of the supply, the +, to the filament, and then the other part, the ground, to the other filament terminal. Where do I put my bias resistor and capacitor to ground? A diagram would help if possible, thanks!
  Reply With Quote
Old 5th October 2014, 10:28 AM   #6
diyAudio Member
Join Date: Feb 2010
Location: Helsinki
Float the filament supply. Use positive side of filament supply as cathode. The filament supply has no 'ground', only negative and positive.
  Reply With Quote


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
Spice simulator engines rtarbell The Lounge 1 26th July 2007 11:17 AM
Free Spice Or Cheap Spice Simulator-Where To Start? kelticwizard Everything Else 29 15th February 2007 01:38 AM
Anode loaded 6080 giving less than unity gain, Why?? Shoog Tubes / Valves 28 25th May 2006 03:54 AM
Spice/simulator lumanauw Everything Else 5 24th January 2004 08:53 AM
Free SPICE simulator.. tschrama Everything Else 25 1st July 2002 10:14 PM

New To Site? Need Help?

All times are GMT. The time now is 07:44 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2