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Old 28th May 2007, 05:03 PM   #1
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Default Advice for retubing VTL Ichiban`s

Hi all of you.

I recently bought a pair of VTl Ichiban mono amps. These are 200W triode amps using 12! EL34 / KT90 /6550C pr. channel. I bought them as projects. They have not been used for several years and needs going over with new tubes, sockets ect. Bying 24 output tubes is quite an investment so I would like to get it right the first time. These amps are only operated in tiode mode and the current in each tube is quite modest. (about 25mA)
I am considering The EH KT90, the amps were sold with EI KT90 when new. What difference in sound should i expect from these compard to svetlana 6550C in the same circuit.
I neet a base that is as tight as posible as i am driving a pair of Quadral Titan MK III transmisionline speakers. For the rest of the fq. band i like "Telefunken " sound a lot. Anny input is welcome.
Thanks - Peter Jensen, Denmark.
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Old 28th May 2007, 10:57 PM   #2
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Peter,

The EH KT90 is not a particularly good tube. The sound stage is flat. See Jim McShane's report.

The good tubes available are the EH KT88 and the SED 6550C. The EH is a bit more energetic up top. You want solid bass performance; so, my guess is the 6550C is the better choice for you.

The Saratov, Russia made "reissue" GEC KT88 just might be "perfect". Sourcing them in the quantity you need will be both difficult and expensive. Production volume for this recent introduction is small. Hopefully, New Sensor will ramp things up.
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Old 29th May 2007, 07:17 PM   #3
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Imho, VTL pushes their tubes too much so that they can get "impressive" power output specs.

No matter if you run it with the screens strapped for triode (switches?) or in pentode/UL mode, my advice is to get in there and both check the peak screen voltage and current under load at full power, and if it looks like I think it will change the screen voltage and/or the screen current limiting resistor so that a reasonable amount of screen current is drawn.

You probably can not reduce the B+, so that's a fixed value that you have to work with.

The result will probably be a ~20% reduction in max power output, but likely better sound and lower distortion at peak levels.

In my experience that's what eats up tubes in the "california" style commercial amp brands.

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Old 4th June 2007, 06:35 PM   #4
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Hi again

Thanks for your inputs. The Ichiban amps can only be operated in triode mode. I did not have the impression that the tubes were overstressed in this amp, but as I understand it the amp is using voltages close to the limit of the tubes. When I start working on the amps I will post some pictures of the inside of the amp. I don`t have the schematics fot it but it uses ECC81 for the imput and 2 x 12BH7A as drivers. I have found a source for SED 6550C tubes so I wioll go for these as output tubes.

Regards, Peter Jensen
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Old 4th June 2007, 08:48 PM   #5
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Hi Peter,
I agree with Bear on this one. Look at the screen grid rating very carefully. This is what will kill the tube if you pick the wrong one. A beam power tube might have a better survival rate since the screens are in the "electronic shadow" of the control grid. That should cut down on screen grid current.

Hi Eli,
I've found that EH tend to be good on the top and the bottom. Possibly because the tube has more top and mids, it only appears weaker in the bass. I've found they are a little more solid on the bottom end along with cleaner mids and highs. I tend to disagree with some reviews as my experience doesn't agree with most guitar amp based reviews. The output transformer may be a key difference there.

-Chris
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