[nnck]

Just like the topic says. I've been seeing a lot of hype online about the new(er) KT150 tubes available which can sometimes be used in place of KT88 or KT120 tubes. Has anyone tried using these these tubes in their RM200II amp? Results?

http://www.tungsol.com/tungsol/html/kt150-tung-sol.html

[Roger A. Modjeski]

Haven't tried it but the specs are not encouraging for use in my or any other KT-88 or KT-120. Note the bias is only -14 where a KT-88 is about 40V. The transconductance is about twice the KT-88 which may cause amps to oscillate.

[nnck]

Haven't tried it but the specs are not encouraging for use in my or any other KT-88 or KT-120. Note the bias is only -14 where a KT-88 is about 40V. The transconductance is about twice the KT-88 which may cause amps to oscillate.

Thanks for the info Roger. If you determine they are ok, let us know. But if it doesn't look good....well, I wouldn't want to try anything if you havent first tested it.

On a related note, I did see you mention that KT120s were okay to use in the RM200II. Can you tell us what the bias should be set at for these tubes if we decide to switch from KT88 to KT120?

And finally, are EL34s okay to use in an RM200II? If so, what should the bias be set at for those, and what sort of difference in sound might we expect compared to KT88 or KT120.

Thanks

[Roger A. Modjeski]

you can set the bias (idle current) for 30 MV (30MA) for EL34 and 50 for the KT-120

[nnck]

you can set the bias (idle current) for 30 MV (30MA) for EL34 and 50 for the KT-120

Thanks Roger. Can you guide us on what sort of subtle differences we might expect for KT88 vs KT120 vs EL34 tubes?

Also, there was some discussion on the forums that suggested you didnt exactly recommend the EL34 for the RM200II- Is that an inaccurate statement? I'm wondering why at least one other person mentioned that to me, and yet your comment would suggest that they are fine to use.

[Roger A. Modjeski]

Thanks Roger. Can you guide us on what sort of subtle differences we might expect for KT88 vs KT120 vs EL34 tubes?

Also, there was some discussion on the forums that suggested you didnt exactly recommend the EL34 for the RM200II- Is that an inaccurate statement? I'm wondering why at least one other person mentioned that to me, and yet your comment would suggest that they are fine to use.

I generally don't comment on tube differences as those differ with associated equipment and the listener.  There are so many variables, however I will say that the EL-34 has to run at lower idle current (30 mA per tube) due to its lower dissipation limit and it has less available current than the other tubes. Since I want the RM-200 to be a 100 watt per channel amplifier rather than an 80 watt amplifier I tend not to recommend it. However you may like it and there is nothing wrong if you do. Perhaps you will write about it and inspire others to try it. I do suggest you get the Siemens EL-34 from us as it is the best sounding and most reliable EL-34 available. We also know how to match it for the RM-200 which has very different parameters from the typical EL-34 ultralinear operation in which it usually finds itself applied.

As to the differences, Yes I expect there will be some.

[rbwalt]

yes get the EL34 Siemens from roger. i tried various tubes in my RM9SE( came with the Siemens) such as Valve Art and JJ e34l's. The Valve Arts were very warm sounding with a muddy bottom end. The JJ's were quite good but they tended to self destruct after only about 150 hrs. So after all of this i went back to the Siemens. Roger is correct they are a fantastic tube. very smooth and continuous from top to bottom. a great midrange, airy top end with a very good bottom end. all in all a dam fine well balanced tube.  they are not cheap but well worth it. roger sent me 8 very well matched tubes and they are fine. they take about 50 hrs before they start to sound really good but right out of the box they are good.

rob w.

[nnck]

yes get the EL34 Siemens from roger. i tried various tubes in my RM9SE( came with the Siemens) such as Valve Art and JJ e34l's. The Valve Arts were very warm sounding with a muddy bottom end. The JJ's were quite good but they tended to self destruct after only about 150 hrs. So after all of this i went back to the Siemens. Roger is correct they are a fantastic tube. very smooth and continuous from top to bottom. a great midrange, airy top end with a very good bottom end. all in all a dam fine well balanced tube.  they are not cheap but well worth it. roger sent me 8 very well matched tubes and they are fine. they take about 50 hrs before they start to sound really good but right out of the box they are good.

rob w.

Thanks for the info Rob. As I said a few times, I probably will try another type of tube to compare to the KT88 eventually. For now, I have only had the amp for a few weeks, so I'd like to evaluate it some months before changing. I have been playing  bit with the speaker taps currently and that is keeping me busy as well.

When I do try another type of tube eventually, I'd like to try something that is the most 'different' than the KT88. I think that would be the EL34s as opposed to KT120s (KT150s still not cleared to use, so I wont consider them until then). I dont know that for a fact though, but I often hear people describing the sound of KT120 to be somewhat more similar to KT88 and EL34 as sounding more 'tube-like'. But I'm just going to keep reading and researching for a while.

[rbwalt]

if the amp is brand new put allot of hrs on it before you try different tubes. let things settle in/ break in. your tube choice will be determined somewhat by your speakers.

let us know.

ps. get your tubes from roger as he will match  them much better.

[ttan98]

Roger,

I hope you can answer this query I have in mind, my KT88 runs a B+ of 450Vdc, I can only get about 60 Watts in Class AB mode. The RM200 gives about 100W per channel, what B+ are you running at to give you that wattage, and it still runs in Class AB mode at 50mA per tube for KT88?

I would appreciate a response please.

[Roger A. Modjeski]

The RM-200 is rather unusual in that the B+ is 700 V and the screen is 320V.  The answers to watts, bias and loads and etc is not a simple one but one I do cover in my classes. As I have often stated,  bias has nothing to do with output power. I get 100 watts per pair because of the chosen operating parameters mentioned above.

[ttan98]

The RM-200 is rather unusual in that the B+ is 700 V and the screen is 320V.  The answers to watts, bias and loads and etc is not a simple one but one I do cover in my classes. As I have often stated,  bias has nothing to do with output power. I get 100 watts per pair because of the chosen operating parameters mentioned above.

Thanks Roger looks like you operate the tube in Pentode mode rather than Ultra linear, my experience with older Chinese KT88 is that its the screen voltage cannot handle more than 450V hence ultralinear is not recommended for B+ greater than 450Vdc. Pentode is better because the screen voltage is about half the anode hence you get better reliability out of the tube. Having said that Pentode mode KT88 has much higher output impedance how do you reduce it by applying more/greater feedback?