modify parallel push-pull EL84 to single push-pull - 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 28th October 2005, 08:10 AM   #1
diyAudio Member
 
Join Date: Mar 2005
Location: HK
Default modify parallel push-pull EL84 to single push-pull

Recently I bought a integrated amp. The power output stage is using parallel push-pull EL84. Total 8 tubes.

The output transformer is 4K pri. impedance.

I don't need that much power and considering using only 4 tubes. That is changing from parallel push-pull to single push-pull.

What modifications do I need to do without changing the output transformer? Bigger cathode resistor? Or no need to modify anything?

The B+ is kind of high, 450 VDC.

Thanks for any help.
  Reply With Quote
Old 28th October 2005, 09:22 AM   #2
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
You will need a higher load impedance. I doubt if you can get away without changing the OP tranny, unless you can use different secondary taps for the speakers, e.g. connect 8 ohm speakers to the 4 ohm tap.
  Reply With Quote
Old 28th October 2005, 09:35 AM   #3
diyAudio Member
 
Join Date: Mar 2005
Location: HK
I have forgotten to mention that the output transformer is ultralinear.

Tks.
  Reply With Quote
Old 28th October 2005, 09:37 AM   #4
diyAudio Member
 
Join Date: Mar 2005
Location: HK
It does has 4 ohm taps.

One thing I dont understand is that even the ratio is correct, the load impedance still doesn't match.
  Reply With Quote
Old 28th October 2005, 02:17 PM   #5
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
Quote:
. . . even the ratio is correct, the load impedance still doesn't match
How do you know it doesn't match? The normal p-p load I've seen for a pair of EL84 in UL PP is 8k. Connecting 8 ohm spkr to 4 ohm tap on a 4k p-p OPT should give you 8k. Just be careful if the paralleled EL84s share a cathode resistor. You will need to double the value to get the same bias voltage, since you will be halving the current.
  Reply With Quote
Old 28th October 2005, 03:03 PM   #6
rdf is offline rdf  Canada
diyAudio Member
 
rdf's Avatar
 
Join Date: Jun 2004
Location: big smoke
If you don't need that much power why not try strapping them triode? Assuming the driver cleanly handles the increased grid capacitance.
  Reply With Quote
Old 31st October 2005, 01:40 AM   #7
diyAudio Member
 
Join Date: Mar 2005
Location: HK
Thanks a lot! I will try that.

It is using parallel PP and I thought that make it single PP may have a positive effect on the sonic quality.

The cathode resistor is 10R for each tube. So I will double it for the 4 tubes.

What is good for cathode resistor for output stage? WW or carbon composite?

Thanks for your help.
  Reply With Quote
Old 31st October 2005, 04:14 AM   #8
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
Quote:
The cathode resistor is 10R for each tube. So I will double it for the 4 tubes.
Hold on! I don't follow your logic. If there is really a separate 10R resistor at each EL84 cathode, then there is no need to double anything. Also, with only 10R present, it suggests that the amp is using grid bias, not cathode bias, which would probably require no adjustment to change from parallel to single PP.

Also, I don't believe changing from parallell to single PP will make any audible improvement to the sonics. Indeed, the opposite is more likely to be true. If you want better sonics, then it might be better to try rdf's suggestion.
  Reply With Quote
Old 31st October 2005, 04:35 AM   #9
diyAudio Member
 
Join Date: Mar 2005
Location: HK
Here is the schematic of the output.
  Reply With Quote
Old 31st October 2005, 06:02 AM   #10
diyAudio Member
 
analog_sa's Avatar
 
Join Date: Aug 2002
Location: Sofia
Quote:
Also, I don't believe changing from parallell to single PP will make any audible improvement to the sonics.

It very probably will. I have never heard parallel tubes sound better. For reasons i don't understand parallel EL84 sound particularly bad.
  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
EL84 push pull slideman82 Tubes / Valves 4 19th June 2009 03:40 PM
Will this work for an EL84 push pull? zobsky Tubes / Valves 5 10th October 2006 07:42 PM
EL84 Push Pull amp. hacknet Tubes / Valves 8 23rd September 2004 05:53 AM
Parallel single ended or push-pull? Jeffy74 Tubes / Valves 6 12th February 2004 06:04 AM
Double push-pull versus single push-pull jraraujo Tubes / Valves 19 19th October 2002 12:23 AM


New To Site? Need Help?

All times are GMT. The time now is 01:33 AM.


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