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Replacing KT88 with KT90 - Beginner's questions

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#21
Jim#
Once I had a Citation 2 but in a knee-jerk reaction gotton rid of it as I found was borderline unstable with high order LS crossovers and flighty with electrostatic speakers. That was around 1965 or so.
On hindsight now 40 years later I should have re-examined it as there is alot of merit with a two type tube design.. The driver stage inverse feedback makes good use of the 12BY7 gain. In my opinion an excellent video tube and has decent gain in triode mode. I use these tubes as balanced voltage drivers with split cathode grid feedback in my amps and this reduces IM thd. I run these tubes as drivers at roughly 15mA each and can easily take on parallel output stages.
The problem with the KT88 upper timbre I shall look in to...but the o/p tranny on the Citation 2 is a tapped secondary (>right ?) and I’m wondering if it is this causing top end timbre troubles.
I’m using a custom wound (Majestic) with 8 sep secondary windings (series parallel arrangement) to reduce leakage inductance and "up" the >HF response, and cannot detect a "squashed top end" response.
I’m not sure if the original Citation 2 design will accept o/p trannies from other vendors without re-tweaking those pF's
Have you tried it ?

richj
 
#22
Tweeker said:
I think Morgan Jones is just gettering them without the strain they would experience of being on while gassy, if gassy. Beware of Bakelite if baking tubes.
Click to expand...

Quite Right. I've never had problems with the bakelite bases, but even my 'old' tubes aren't really that old... mostly from gittar amps that 'need retubing'.. 😀

I bring them up really slow - step wise over a couple of hours to about 200 degrees celcius. I leave them there for only two hours though. Then REALLY IMPORTANT - Turn off and allow to cool completely IN THE OVEN before moving them.

I feel this works marginally better than 150 degrees for 4 hours. Only because I repeated the process with a few I had done earlier to 150 degrees, but were still a bit gassy. After the 200 degrees cycle they were much better - and to this day are no longer gassy...
 
#23
richwalters said:
Jim#
Once I had a Citation 2 but in a knee-jerk reaction gotton rid of it as I found was borderline unstable with high order LS crossovers and flighty with electrostatic speakers. That was around 1965 or so.
On hindsight now 40 years later I should have re-examined it as there is alot of merit with a two type tube design.. The driver stage inverse feedback makes good use of the 12BY7 gain. In my opinion an excellent video tube and has decent gain in triode mode. I use these tubes as balanced voltage drivers with split cathode grid feedback in my amps and this reduces IM thd. I run these tubes as drivers at roughly 15mA each and can easily take on parallel output stages.
The problem with the KT88 upper timbre I shall look in to...but the o/p tranny on the Citation 2 is a tapped secondary (>right ?) and I’m wondering if it is this causing top end timbre troubles.
I’m using a custom wound (Majestic) with 8 sep secondary windings (series parallel arrangement) to reduce leakage inductance and "up" the >HF response, and cannot detect a "squashed top end" response.
I’m not sure if the original Citation 2 design will accept o/p trannies from other vendors without re-tweaking those pF's
Have you tried it ?

richj
Click to expand...

Rich,

I have never even CONSIDERED another OPT for a Cit II. Many consider them the best ever wound, and even if not the best, always in the top 2 or 3. The high frequency performance of the Citation OPTS is remarkable. A resonant frequency of 450KHz (!), and a full power bandwidth (+0, - 1 db) of 18Hz-40KHz at 60 watts (2Hz-80KHz at 1 watt). I am quite certain the top end issues you had weren't due to the OPTs, tapped secondary or not.

Sadly, many a Cit II was stripped for those incredible transformers.

Honestly, I'd have to have my head examined to consider a different OPT!
 
#24
Jim#

I'm thinking of a possible build but I'm at log-aheads with the orig Citation 2 spec. It claims Harmonic distortion: Less than 0.5% @ 60W; less than 0.2% @ 20W .
I presume this is at 1KHz ?
This seems quite high considering 32dB of feedback used and
wondering where this orignates from ? (the output stage).

With other type amps I'm engineering under 0.05% at 50W with only 20dB nfb.

Comments`?

richj
 
#25
richwalters said:
Jim#

I'm thinking of a possible build but I'm at log-aheads with the orig Citation 2 spec. It claims Harmonic distortion: Less than 0.5% @ 60W; less than 0.2% @ 20W . I presume this is at 1KHz ? This seems quite high considering 32dB of feedback used and wondering where this orignates from ? (the output stage).

With other type amps I'm engineering under 0.05% at 50W with only 20dB nfb.

Comments`?

richj
Click to expand...

Yes...

1. The THD spec given is at 60 watts from 20Hz to 20KHz - NOT at 1KHz.

2. The spec is VERY conservative. I see less than .2% typically in the 20-20K bandwidth. At 1K it's better yet - there is some variance form amp to amp, but most all are under .1%.

3. At 1 watt the orginal spec sheet says - "unmeasurable".
 
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