Using a spdt switch to select between triode mode and pentode mode on a guitar amp. - 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 8th March 2006, 10:01 AM   #1
G is offline G  United States
diyAudio Member
 
G's Avatar
 
Join Date: Apr 2002
Location: Belleville, IL.
Default Using a spdt switch to select between triode mode and pentode mode on a guitar amp.

Hi all. I'm building a little guitar amp and would like to incorporate a switch to select either triode or pentode operation of the output tube (EL34). Has anyone done this personally? I'm wondering about the consequences of switching modes while powered up and playing.
__________________
Gavin
  Reply With Quote
Old 8th March 2006, 10:39 AM   #2
SHiFTY is offline SHiFTY  New Zealand
diyAudio Member
 
SHiFTY's Avatar
 
Join Date: Jul 2002
Location: New Zealand
I did this with 807s. As long as I turned down the input to zero, and the switchover was fast, there was only a small pop from the speaker. However try it with an old speaker first, YMMV...
  Reply With Quote
Old 8th March 2006, 12:13 PM   #3
G is offline G  United States
diyAudio Member
 
G's Avatar
 
Join Date: Apr 2002
Location: Belleville, IL.
Thank you sir.
__________________
Gavin
  Reply With Quote
Old 8th March 2006, 01:04 PM   #4
Sherman is offline Sherman  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Chicago area
I have also done this, both on a guitar amp and on a hifi amp. One thing to be aware of though- most switches are not rated for the voltage they will see in this application.

When switching with the power on there will be arcing across the switch. That arcing will cause the switch to fail sooner or later. You can avoid that by only switching with the power off. If there is no power and the PS caps are mostly discharged it won't arc and the switch should last indefinitely. Or you can just replace the switch occasionally when it fails.

(If your OPT has the taps for it you can also use a rotary switch to switch between triode, ultralinear and pentode. The same cautions about voltage and arcing apply.)
__________________
--Sherman
  Reply With Quote
Old 8th March 2006, 02:39 PM   #5
G is offline G  United States
diyAudio Member
 
G's Avatar
 
Join Date: Apr 2002
Location: Belleville, IL.
Quote:
Originally posted by Sherman
I have also done this, both on a guitar amp and on a hifi amp. One thing to be aware of though- most switches are not rated for the voltage they will see in this application.

When switching with the power on there will be arcing across the switch. That arcing will cause the switch to fail sooner or later. You can avoid that by only switching with the power off. If there is no power and the PS caps are mostly discharged it won't arc and the switch should last indefinitely. Or you can just replace the switch occasionally when it fails.

(If your OPT has the taps for it you can also use a rotary switch to switch between triode, ultralinear and pentode. The same cautions about voltage and arcing apply.)
A very solid point Sherman. I had just considered that conundrum while considering what switch to use. I will have to consider this problem for a while. Thank you for your reply.
__________________
Gavin
  Reply With Quote
Old 8th March 2006, 02:57 PM   #6
G is offline G  United States
diyAudio Member
 
G's Avatar
 
Join Date: Apr 2002
Location: Belleville, IL.
I just considered something. Since there is essentially no load connected to the contacts would the switch arc? Have you seen them arc? If you have then conversation over but it would seem to me that with almost no current flow there would be minimal arcing. Maybe I'm looking at it wrong.
__________________
Gavin
  Reply With Quote
Old 8th March 2006, 03:01 PM   #7
diyAudio Member
 
Join Date: Mar 2003
Location: Zürich
This has been asked recently. I remember SY's post. He stated that although the switching happens at high voltages, maybe in excess of the swith rating, the difference between the terminals of the switch are not of a high value. An example, to explain it better. with a swith you connect one side to the b+ before the tap, the other end to the b+ at the plate of the output tube. The middle connection goes to the grid. (logical). Now the difference between the B+ before the tap and the B+ at the anode isn't much (just the DCR ot the trafo*the quiescent current, some volts, no more). when the difference is not that high, there will not be arcing! Bu be aware that the swich is well isolated, because it can be there at a high B+, working fine. But when YOU put your fingers there, than you will form the path to ground...
  Reply With Quote
Old 8th March 2006, 03:26 PM   #8
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
I have used both a 2 position and 3 position switches for the "mode" switch on a single ended champ style guitar amp. The switches that I used were rated for 300 volts, and the B+ could go up to almost 400 volts (yet another switch). I personally saw a 6V6 sacrificed to the fire gods by flipping the screen grid switch while tha amp was operating at full clip trying to squeeze out far more power than it was made to do. The switch was toast too. This may sound exterme, but it is a common occurrence in the life of a guitar amp. The voltage at the plate is the B+ voltage PLUS the instanteous peak AC signal voltage. There can be up to 700 volts at the plate in this situation. I verified this voltage with a scope in this amp. This is only a 5 watt amp. More power = more voltage.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 8th March 2006, 03:32 PM   #9
G is offline G  United States
diyAudio Member
 
G's Avatar
 
Join Date: Apr 2002
Location: Belleville, IL.
Quote:
Originally posted by tubelab.com
I have used both a 2 position and 3 position switches for the "mode" switch on a single ended champ style guitar amp. The switches that I used were rated for 300 volts, and the B+ could go up to almost 400 volts (yet another switch). I personally saw a 6V6 sacrificed to the fire gods by flipping the screen grid switch while tha amp was operating at full clip trying to squeeze out far more power than it was made to do. The switch was toast too. This may sound exterme, but it is a common occurrence in the life of a guitar amp. The voltage at the plate is the B+ voltage PLUS the instanteous peak AC signal voltage. There can be up to 700 volts at the plate in this situation. I verified this voltage with a scope in this amp. This is only a 5 watt amp. More power = more voltage.

So I could use a "mute" switch to ground the signal at the input jack and safely switch from triode to pentode operation? I think that would work. I just plink around at home. No gigging for me. Too much electrical work to be done.
__________________
Gavin
  Reply With Quote
Old 8th March 2006, 03:36 PM   #10
G is offline G  United States
diyAudio Member
 
G's Avatar
 
Join Date: Apr 2002
Location: Belleville, IL.
Quote:
Originally posted by ErikdeBest
This has been asked recently. I remember SY's post. He stated that although the switching happens at high voltages, maybe in excess of the swith rating, the difference between the terminals of the switch are not of a high value. An example, to explain it better. with a swith you connect one side to the b+ before the tap, the other end to the b+ at the plate of the output tube. The middle connection goes to the grid. (logical). Now the difference between the B+ before the tap and the B+ at the anode isn't much (just the DCR ot the trafo*the quiescent current, some volts, no more). when the difference is not that high, there will not be arcing! Bu be aware that the swich is well isolated, because it can be there at a high B+, working fine. But when YOU put your fingers there, than you will form the path to ground...

That would make sense. No difference in potential (or very little) means no current flow (or very little). Hence no arcing. My view may be simplistic though.
__________________
Gavin
  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
pentode in triode mode SE vs a DHT SE like 2a3/300b jarthel Tubes / Valves 67 8th July 2014 02:51 AM
budget xformer for 829B pentode mode PP (class AB) ? vladn Tubes / Valves 42 12th October 2008 07:03 PM
E80L pentode mode as driver? pmillett Tubes / Valves 1 3rd April 2008 09:49 PM
7189 in pentode mode? G Tubes / Valves 30 12th March 2003 05:13 PM


New To Site? Need Help?

All times are GMT. The time now is 05:36 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