KT88 SE - 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 4th April 2015, 07:44 PM   #1
diyAudio Member
 
andrew_whitham's Avatar
 
Join Date: Oct 2005
Location: Great Britain
Default KT88 SE

Posting details here of my KT88 amp.

Project aims:
Simple Single ended design
Using transformers and Chassis I already had.
PCB's for supply and other modules to proof of board.

so with that in mind schematic is here: (I hope)
Attached Images
File Type: gif kt88 amp.gif (24.3 KB, 741 views)
  Reply With Quote
Old 4th April 2015, 07:46 PM   #2
diyAudio Member
 
andrew_whitham's Avatar
 
Join Date: Oct 2005
Location: Great Britain
I forgot - global feedback to be applied later (perhaps) I'm actually listening to this variation before going down that route again
  Reply With Quote
Old 4th April 2015, 08:08 PM   #3
diyAudio Member
 
andrew_whitham's Avatar
 
Join Date: Oct 2005
Location: Great Britain
finally in this splurge:

https://grabcad.com/library/kt88-amplifier-1
for all shiny 3d models and so on.

please like if you do it makes my inbox all funky :-)
  Reply With Quote
Old 5th April 2015, 06:47 AM   #4
diyAudio Member
 
soulmerchant's Avatar
 
Join Date: Sep 2006
Location: in the air
I like the KT88 triode strapped. Its also very nice in Pentode mode. What transformers did you use - or did you wind them yourself?
  Reply With Quote
Old 5th April 2015, 11:14 AM   #5
diyAudio Member
 
andrew_whitham's Avatar
 
Join Date: Oct 2005
Location: Great Britain
Soulmerchant. Transformers were 3.5k Chinese specials the type is not marked but were made by "full audio music". They are quite a lump but I've no baseline for comparison. I have some Sowter alternatives but they're reserved for a 300b (one day).
  Reply With Quote
Old 5th April 2015, 10:20 PM   #6
diyAudio Member
 
soulmerchant's Avatar
 
Join Date: Sep 2006
Location: in the air
Some of those chines specials can sound surprisingly good. Many use japanese Z11 laminations and Hitachi wire. Did you ever just hook up the sowter's to see what they sound like in comparison?
  Reply With Quote
Old 7th April 2015, 09:35 PM   #7
diyAudio Member
 
andrew_whitham's Avatar
 
Join Date: Oct 2005
Location: Great Britain
Default update

Hi Soulmerchant, didn't try the Sowter due to the way I cunningly hid the connections for the transformers under the switching board. .

I'd agree though, someone *admitted* to making them and I am quite pleased - they were not expensive. Sure I'd be disappointed in comparison though. The Sowters are SE08A, much larger and heavier - do need to do testing on the amp though to find response etc. I have new Sovtek tubes on their way so there is a round of optimisation to get through.

More generally I have a few numbers for now though! KT88's are biasing at -30v with a 325ohm cathode resistor so that's 80mA, which seems right according to the datasheets but I think I'd like to shove the operating point left. Need to dig out the dummy loads before getting any further.

preliminary prods with a scope show a nice sine wave at 1khz but, frankly, without those dummy loads it was all getting a bit loud and unpleasant.

Cheers

Andy
Attached Images
File Type: jpg ampinside.jpg (852.0 KB, 516 views)
  Reply With Quote
Old 8th April 2015, 08:13 PM   #8
diyAudio Member
 
andrew_whitham's Avatar
 
Join Date: Oct 2005
Location: Great Britain
ok so to keep posting:
One schematic with voltages. according to SEampCAD I'm not going to do much better with the O/P stage :-/ So here she is at present.

TO-DO
Change the OP caps! as there is a point in between the 6 and the 3
Reduce voltage to input stage with further RC filter say 300 V
Move op point for 6J5G to -4v rather than -6V
Add feedback from transformer OP to 6j5 cathode.

Notes:
At max volume there is a faint - faint hum with ear to speaker (Castle Harlech 2) deemed acceptable as I get far worse from commercially available amps. So that's NICE, passive supply works.

Those heading to Grabcad will find a 30sec delay timer as part of the switching board, not implemented but meant for switching of an SSR for HT
Attached Images
File Type: gif amp 08042015.gif (24.6 KB, 410 views)
  Reply With Quote
Old 10th April 2015, 05:43 PM   #9
diyAudio Member
 
andrew_whitham's Avatar
 
Join Date: Oct 2005
Location: Great Britain
edit - OP caps now 1000u 63v
much loudness has arrived :-)
  Reply With Quote
Old 10th April 2015, 09:03 PM   #10
diyAudio Member
 
soulmerchant's Avatar
 
Join Date: Sep 2006
Location: in the air
Hi Andrew, did you ever consider trying LED bias on the input tube? You might try a few green and a red in series to get something close to your preferred bias point. Bass response will be very firm after you try it...

Its not hard to do. If you want to try it, make a little rig with a 9V battery, and low ohm resistor in series with the LED, then match LED's forward drop voltages so you bias at exactly the same voltage channel to channel.

Just a thought. I have done it before. I always liked KT88 much more in triode mode.

Ian
  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
SE 6L6 to KT88 oldschooltube Tubes / Valves 5 9th July 2013 05:32 AM
SE KT88 Amp Cassiel Tubes / Valves 50 24th March 2011 08:33 PM
Kt88 Se overdrajv Tubes / Valves 0 16th December 2008 12:26 PM
Aikido and SE KT88 kacernator Tubes / Valves 10 1st April 2008 12:50 AM
Kt88 Se hacknet Tubes / Valves 9 10th September 2004 08:19 AM


New To Site? Need Help?

All times are GMT. The time now is 05:19 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright 1999-2015 diyAudio

Content Relevant URLs by vBSEO 3.3.2