KT88 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 20th January 2008, 09:11 PM   #1
diyAudio Member
 
denny9167's Avatar
 
Join Date: Nov 2007
Location: Graham, TX
Default KT88 Push-Pull

I have a schematic using a pair of 6SN7's to drive a pair of KT88's in a Push Pull config. it's rated at 30 watts, is it possible to add an additional pair in parallel to increase the power output?
  Reply With Quote
Old 20th January 2008, 10:06 PM   #2
diyAudio Member
 
tubewade's Avatar
 
Join Date: Jun 2007
It would be nice to have a look at your schematic to say for sure. Is it the classic GEC 30 W KT88 schematic? At any rate, you can probably add a second pair of valves for more power out. It will consume twice the current and require half the load impedance if you do that. There may be other things to consider such as bias, too, but it can probably be done.

Another thing to consider is that doing so will increase the cost tremendously, i.e. four valves, bigger output transformer, bigger power transformer, maybe more rectifiers, but will only increase your output by about 3 decibels. Is it worth the cost?

Wade
  Reply With Quote
Old 20th January 2008, 11:23 PM   #3
diyAudio Member
 
TubeHead Johnny's Avatar
 
Join Date: Jan 2005
Location: The emerald city, Seattle, WA.
IMHO, I think 4 KT88's are overkill, unless you're building a seriously loud bass amp, or P.A. With the right PT (and OT), you can get some incredible power with just a pair in push-pull, with fixed bias.
  Reply With Quote
Old 20th January 2008, 11:56 PM   #4
diyAudio Member
 
denny9167's Avatar
 
Join Date: Nov 2007
Location: Graham, TX
Quote:
Originally posted by tubewade
It would be nice to have a look at your schematic to say for sure. Is it the classic GEC 30 W KT88 schematic? At any rate, you can probably add a second pair of valves for more power out. It will consume twice the current and require half the load impedance if you do that. There may be other things to consider such as bias, too, but it can probably be done.

Another thing to consider is that doing so will increase the cost tremendously, i.e. four valves, bigger output transformer, bigger power transformer, maybe more rectifiers, but will only increase your output by about 3 decibels. Is it worth the cost?

Wade
You probably have a point,I thought what I might do is use the original design,and just bi-amp my system by using the tube amps on the mids and highs,and use the pair of SS monoblock amps I have for the low end.

Thanks for the advice though!
  Reply With Quote
Old 21st January 2008, 01:14 AM   #5
diyAudio Member
 
Join Date: May 2005
Location: Pretoria, South Africa
Denny9167,

It is not clear whether you already have some parts e.g. the output transformer, power transformer, etc. As Tubewade said you will have to reckon with cost.

But only 30W for a pair of KT88s? Is this triode connected? I don't know over there, but in RSA the KT88 is quite expensive, and it will pay to get as much as is safely possible from a pair. As pentodes they can give up to 100W, but I would prefer UL. There they can give an easy 60W with 500V h.t. and fixed bias.

I like 6L6s at half the heater current of the KT88. A twin pair (i.e. 4 in push-pull parallel) of 6L6GCs can give a comfortable 100W with fixed bias in UL. The list is endless, but just to position you within the scope of your question.
  Reply With Quote
Old 21st January 2008, 02:30 AM   #6
diyAudio Member
 
tubewade's Avatar
 
Join Date: Jun 2007
Denny,

If you use an active filtering setup, the P/P KT88s at 30 watts on the medium and high range drivers should certainly be more than enough power, even if the speakers are fairly inefficient.


Johan,

I believe the reason for the relative low power from this circuit is to promote extremely long life of the valves. Running them at the operating points required for 30 watts should make the valves last practically forever. I agree that it would also be very easy to get 50 or 60 watts from the same set of valves and still not be pushing them very hard.
  Reply With Quote
Old 21st January 2008, 03:46 AM   #7
diyAudio Member
 
TubeHead Johnny's Avatar
 
Join Date: Jan 2005
Location: The emerald city, Seattle, WA.
The 30 watt rating has to do with cathode biasing, no?
  Reply With Quote
Old 21st January 2008, 03:48 AM   #8
navin is offline navin  India
diyAudio Member
 
navin's Avatar
 
Join Date: Jan 2002
Location: Mumbai (Bombay), India
Send a message via MSN to navin Send a message via Yahoo to navin
Quote:
Originally posted by denny9167

You probably have a point,I thought what I might do is use the original design,and just bi-amp my system by using the tube amps on the mids and highs,and use the pair of SS monoblock amps I have for the low end.
This makes the most sense. There a circuit posted by Wayne on a thread called EL34 schematic confusion optimised for just such a configuration. It needs 1 6SL7, 2 6SN7 and 4 power tubes like the KT88. The original has been designed for the EL34 but I think the 6SN7 can drive KT88s as well.

Quote:
Originally posted by tubewade

If you use an active filtering setup, the P/P KT88s at 30 watts on the medium and high range drivers should certainly be more than enough power, even if the speakers are fairly inefficient..
if the speakers are 90db/1m/1m and 84-85db/1w/3m you will get 94-95db at 10W and 100db at 40W. More than enough for most applications.
__________________
...still looking for the holy grail.
  Reply With Quote
Old 21st January 2008, 04:27 AM   #9
diyAudio Member
 
tubewade's Avatar
 
Join Date: Jun 2007
Quote:
Originally posted by navin


if the speakers are 90db/1m/1m and 84-85db/1w/3m you will get 94-95db at 10W and 100db at 40W. More than enough for most applications.
This is true. Also consider that this is for only one channel. If there are two channels, there will an additional 3 decibels due to the additional electrical power, and an additional 3 decibels due to doubling the radiation area, for a total gain of +6dB. Considering that power distribution in music is about 50 percent below 350 cps and the remaining 50 percent above 350 cps, if filtering is done in this range or above the amplifier will sound MUCH larger than it is. Dispensing with power wasting passive filter and level matching networks will make things even louder. I agree with you that it will be more than enough.
  Reply With Quote
Old 21st January 2008, 08:54 PM   #10
diyAudio Member
 
richwalters's Avatar
 
Join Date: Sep 2003
Location: Alps:Tube amp designs over 150W, SMPS guru.
Default Re: KT88 Push-Pull

Quote:
Originally posted by denny9167
I have a schematic using a pair of 6SN7's to drive a pair of KT88's in a Push Pull config. it's rated at 30 watts, is it possible to add an additional pair in parallel to increase the power output?
So long one stays in class A (which you are implying) the load on the drivers is bearable and isn't suprising to find in some designs an ECC83 driving a KT88. The troubles really start when amp tips class B or specifically true power triodes with high Cag Miller cap.

Cribbing the literature for the GEC 400Wattt amp, it quotes << with four o/p valves the grid resistors could be increased to 100K and the cathode followers dispensed with.>>

This is a fixed bias set-up. With my sim designs I use separate grid coupling caps for each tube and bias presets, (a bit more complexity) but an advantage if unmatched tubes are used with differing neg bias volts ..


richj
  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
Suggest KT88 Push-Pull schematics, please. kacernator Tubes / Valves 9 17th October 2011 10:14 PM
KT88 and push-pull ultra-linear rossdt Tubes / Valves 4 25th July 2006 05:54 AM
Please advice on KT88 Push Pull Lucas_G Tubes / Valves 6 19th May 2006 05:37 PM
KT88 push pull Hojvaelde Tubes / Valves 5 10th November 2005 07:51 PM
KT88 push pull noob questions (sry) Baronty Tubes / Valves 3 20th October 2004 02:00 AM


New To Site? Need Help?

All times are GMT. The time now is 07:45 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright Đ1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2