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-   -   Bewitch KT88 (http://www.diyaudio.com/forums/tubes-valves/226866-bewitch-kt88.html)

pkrussel 2nd January 2013 06:49 AM

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

HpW 4th January 2013 06:42 AM

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:

http://www.diyaudio.com/forums/parts...apacitors.html

DF96 4th January 2013 09:41 AM

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.

roline 4th January 2013 11:24 AM

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.

pkrussel 5th January 2013 10:01 PM

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.

JonSnell Electronic 5th January 2013 10:39 PM

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. :sax:

pkrussel 6th January 2013 08:58 AM

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.

Kay Pirinha 6th January 2013 09:42 AM

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!

DF96 6th January 2013 02:16 PM

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!

Kay Pirinha 6th January 2013 05:03 PM

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!


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