Bewitch KT88 - 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 2nd January 2013, 07:49 AM   #1
diyAudio Member
 
Join Date: Jan 2013
Default Bewitch KT88

Hi,

I'm new to diy audio and have no experience in modifying amps. Few months ago I bought a Bewitch KT88 on ebay and so far I like it a lot, one of the reasons I bought the Bewitch was the positive review on the Lampizator website (Bewitch).
He also made a recommendation on some mods. I'm planning to follow his mods, but recently I heard from some people that the Lampizator mods might not be as good as he claims.
Can somebody give me some information about this mod?

http://lampizator.eu/AMPLIFIERS/CHIN...0/IMG_5577.jpg
  Reply With Quote
Old 4th January 2013, 07:42 AM   #2
HpW is offline HpW  Switzerland
diyAudio Member
 
Join Date: Mar 2002
Location: Switzerland (Bern)
My 2 cents about the modifications:

- Adding those Cap's should be simple

- Still the worst thing is the fixed bias using a Electrolyte Cap. The issue here is the absorption voltage given from the Electrolyte Cap. The get ride requires a negative voltage. I did this on my SLM-100 and the sound was more clear, no more echo's...

- An other simple tuning is to peel the Electrolyte Cap's as mentioned in old thread. This improved on my SLM-100 Cary a lot (some weeks ago I replaced the old cap's with new once, first without the peeling and the sound was very closed and after the peeling I went back again!) , but I do not know how this will improve on your amplifier.

see old thread from 2002:

Removing Plastic covers from Capacitors
__________________
www.hpw-works.com
  Reply With Quote
Old 4th January 2013, 10:41 AM   #3
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Removing the plastic cover from a cap might alter the sound, but not for the better. The most likely outcome is no change. The next most likely outcome is microphony or a poorly damped mechanical resonance (which the plastic was suppressing by being mechanically lossy).

Bypassing the electrolytics might improve things a little, or make things worse (by introducing a resonance), or make no audible change at all. Removing the global negative feedback will raise gain, distortion and output impedance while reducing bandwidth. It will certainly change the sound.

Mr. Lampizator's audio ideas are best regarded as being somewhat eccentric.
  Reply With Quote
Old 4th January 2013, 12:24 PM   #4
roline is offline roline  United States
diyAudio Member
 
roline's Avatar
 
Join Date: Sep 2010
Location: East Tennessee
Is there room to add a small transformer to get -90vdc for adjustable fixed bias? I just used a small 110 to 6vac iron connected backwards to the 6.3 filament output from the power transformer, diode and cap, resistive divider to a pot for each tube. if the pot fails, the voltage goes neg to turn off the tube. That should put an additional 40 volts across the output tubes, with 10ohm resistors to ground on their tails to monitor the bias current. Leave the GNFB, with the 180k there really is not a lot of it.
It would help to know the voltages for the bias points on the tubes.
__________________
SO many tubes, SO little time!!!
  Reply With Quote
Old 5th January 2013, 11:01 PM   #5
diyAudio Member
 
Join Date: Jan 2013
Thanx for all the feedback. I won't modify the Bewitch according to Lampzinator's scheme.
@roline. Im afraid adding an extra transformator is a bit out of my league.
  Reply With Quote
Old 5th January 2013, 11:39 PM   #6
diyAudio Member
 
Join Date: Dec 2012
Location: The Jurassic Coast, England. GB
Send a message via Skype™ to JonSnell Electronic
Please take a look at
http://frank.pocnet.net/sheets/164/k/KT88spec.pdf

The KT88 is a direct replacement for the 6550 except for an adjustment on the bias voltage and the heater current drawn!
Lovely valves. I have a pair working in my Fender Twin Reverb and they are so full of life compared with the original 6L6GC.
  Reply With Quote
Old 6th January 2013, 09:58 AM   #7
diyAudio Member
 
Join Date: Jan 2013
Hi Harleyjon,

My Bewitch is the KT88 version. It has EH KT88 tubes.
I like it. I replaced the Chinese tubes with US and Russian NOS tubes. Waiting for my Russian rectifier tubes to arrive.
I also replaced the 4 caps (C1..C4) with Obligato caps.
  Reply With Quote
Old 6th January 2013, 10:42 AM   #8
diyAudio Member
 
Join Date: Oct 2011
Location: Tauberbischofsheim, Germany
Btw, the overall design looks somewhat weird to me. Where does phase inversion occur? Within the first tube? With it's tiny common cathode resistor? So, within the second one? But why, in this instance, the quasi symmetric approach of the first stage?

Anyway, I agree with DF96 - removing the negative feedback loop is probably the worst thing anyone can do to an existing design.

Best regards!

Last edited by Kay Pirinha; 6th January 2013 at 10:43 AM. Reason: orthographics
  Reply With Quote
Old 6th January 2013, 03:16 PM   #9
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Yes, it looks like the first stage does a partial phase split which is then finished off by the second stage. Maybe the designer was a fan of circuits which look balanced, but actually are not? Feeding the NFB back into a partial phase splitter raises issues of common-mode distortion too, although the relatively low signal level might avoid too many problems.

Still, at least it doesn't have the apparently obligatory Chinese SRPP stage!
  Reply With Quote
Old 6th January 2013, 06:03 PM   #10
diyAudio Member
 
Join Date: Oct 2011
Location: Tauberbischofsheim, Germany
Other idea: The upper half of the first tube, which definitely gets near-to-zero drive out of the common cathode resistor R1, might only provide DC bias for the upper half of the PI tube?

Anyway, rather an odd design...

Best regards!
  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
6550C v. KT88 mr2racer Tubes / Valves 4 8th October 2012 04:07 AM
MA XDSE KT88 vs YAQIN MC-100B KT88 esangenisd Tubes / Valves 3 23rd May 2010 12:38 PM
Ebay KT88 agithegreat Tubes / Valves 12 5th January 2010 02:40 PM
Which KT88?? Andy G Tubes / Valves 81 5th February 2004 08:48 AM


New To Site? Need Help?

All times are GMT. The time now is 03:20 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