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Old 17th February 2014, 12:49 PM   #11
mike567 is offline mike567  United States
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Probably run out of juice.
twice the voltage = half the amperege.
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Old 17th February 2014, 03:36 PM   #12
tinitus is offline tinitus  Europe
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yes, it will be 175mA each channel

how much is needed ?
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Old 18th February 2014, 01:04 AM   #13
mike567 is offline mike567  United States
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Might work. but not much left for preamp stages or passives depending on how hard you want to run the kt150's
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Old 18th February 2014, 07:43 AM   #14
tinitus is offline tinitus  Europe
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would it help adding small trafo using negative bias ?
(I have a few small trafos with 2x22-24V, 1A)
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Old 18th February 2014, 03:55 PM   #15
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would 20-25watt be realistic with just SE ?...... Pretty sure the KT150 can be driven similar to a KT120 or KT88 for that matter and thinking just upping B+ voltage some......For output I'd think 10-12 watts tops.
I know that I will never spring for the $$$$ that these tubes go for, but I will offer these bits of insight.

10 to 12 watts is probably the limit in triode mode. I get 14 watts from my KT88 SSE in UL so I would GUESS that the KT150 could do 25 to 30 watts in UL under the right conditions, but what are the right conditions?

The plate structure is much bigger than that of the KT88. and the cathode appears to be longer. This would imply more peak current capability, and higher dissipation. To use all of this I would raise the B+ slightly, maybe 450 to 500 volts, and drop the load impedance.

I kave tested my SSE KT88 amp with B+ voltages to 525 volts and load impedances from 3000 to 6000 ohms. I currently use 3000 or 6000 ohms, 435 volts and 80 or 100 mA depending on music and speaker choices.

In most SE amps the power will increase, distortion will increase, and damping factor will decrease as the load impedance is reduced. There will be a point where the distortion increase starts to rise rather quickly. This is due to peak cathode emission limitations. Operating the amp in this region will shorten tube life.

Keep in mind that "8 ohm" speakers may be 8 ohms at one frequency.....maybe. In actual use the impedance will vary with frequency, AND dynamics. My 8 ohm Yamaha's have a published curve that runs from 7 ohms to 25 ohms over frequency, but these are static measurements. I have attempted to measure how they behave in transient conditions, and seen excursions down to two ohms, and possibly below. Admittedly my ability to accurately measure this is still crude, but the effects of back EMF are real and can cause higher than expected peak currents.

The power will increase, distortion will decrease, and the damping factor remain relatively constant as the B+ is increased. For many new production tubes there will be a point where the tube will tend toward bias instability and uncontrolled destructive runaway may occur. This can happen without warning and the point at which runaway occurs gets lower as the tubes age. This is due to to imperfect vacuum and sloppy construction. Observing the maximum grid circuit resistance spec and use of cathode bias is a good practice to avoid this!

The distortion will decrease, and damping factor will decrease, while output power remains relatively constant as the tubes bias current is increased. Obviously higher current causes higher dissipation which can reduce tube life. The conventional wisdom says to keep the tubes idle dissipation to less than 75 or 80% of rated maximum for optimum tube life.

Several SSE users have seen the getters fade in their expensive Russian Tung Sol 6550 reissues within a year at 75 mA. I still have the same pair of EH KT88's in my SSE after 5 years and they look new and work like new (although the blue glow has faded) and they have been runnung at 100 mA. These tubes are both made in the same factory.

As you can see, there are a lot of variables. Speaker and music choices will greatly affect them. I think the optimum point for these tubes will be somewhere around 3000 to 5000 ohms and 450 to 500 volts in UL with cathode feedback to improve the damping factor on bass heavy music. Switch to triode without feedback with more mellow music.

Speakers with published curves that trend toward impedances below the rated nominal, especially in the 100 Hz and down region may benefit from a higher impedance OPT (typically large woofered multi way), while speakers with curves that lie above the rated impedance may get by with a lower impedance OPT.

OPT's with multiple taps (4-8-16 ohms) can be purposely mismatched to allow for different music. I often run my 8 ohm speakers on the 4 ohm tap (with a 3000 ohm OPT) to give the KT88's a 6000 ohm load for simple music where low distortion is important.
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Old 18th February 2014, 04:39 PM   #16
tinitus is offline tinitus  Europe
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good info thanks George
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Old 19th February 2014, 05:11 PM   #17
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So with kt150s, the right transformers and tweeks my SSE could do 25-30 watts a channel?
I'm very interested!
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Old 19th February 2014, 07:40 PM   #18
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I finally found published data sheets for the KT120 and the KT150 on the Tubestore.com web site. Both tubes are specified to withstand over 600 volts on the screen grid. However, the typical operating conditions call for 225 volts on G2 and 400 volts on the plate, yielding 20 watts. Both tubes have exactly the same numbers.

This reminds me of the JJ EL509 that is sold as an audio tube, but is really a sweep tube with an octal base added. The G2 spec is 700 volts, but these tubes fail quickly if run in triode or UL because the screen grid can't eat 450 volts.

The KT120 data sheet at least calls for triode operation up to 650 volts, so it would seem that UL is possible at at least 500 volts. If this is indeed true, 20+ watts in UL should be possible.

It seems that there is little information out there on these tubes, and all I can find is reports on push pull amps.
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Old 21st February 2014, 01:15 AM   #19
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Originally Posted by the trooper View Post
So with kt150s, the right transformers and tweeks my SSE could do 25-30 watts a channel?
I'm very interested!
I need to know how to avoid burning up the IXYS10M45 regulator... is there another similar reg. that can handle more voltage? Would a single 5AR4 be up the the task for the SSE? Probably need a vintage 5AR4 and not new production, although the added diodes should help with the newer ones there.
Need to do something about electrolytic capacitors in the PSU....


Cheers,
Bob
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Old 21st February 2014, 04:04 AM   #20
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There is the 10M90. I bought some, but haven't tried them yet. I got them for a 750 volt 2 amp power supply I am working on. The 10M45 sees the B+ voltage minus the plate voltage on the 12AT7, minus the drop across the 10K resistor. Say 500 volts - 175 - 100 volts is 225 volts.....I don't think the 10M45 will fry.

I had an Antek 400-0-400 volt toroid hooked up to my SSE at one time. The 5AR4 didn't blow, but I am using a NOS (well it was NOS when I put it in 7 years ago) Sylvania. The B+ was 485 volts which is borderline for the 500 Volt Panasonics.

I then tried solid state rectification which resulted in 525 volts. Over the line for the Panasonics, but they survived for about an hour until I shut the amp down. Those caps are still alive as is the amp, but I wouldn't recommend this. Wire two 350 volt or higher caps in series, or use the polyprop motor run caps alone with no electrolytics in the board.

I had to shut the amp down because the EH KT88's tended toward runaway at this voltage The cathode current kept climbing slowly until one tube developed red plate syndrome. I let the amp cool, then swapped the tubes, and the problem followed the tube. This does not happen at more realistic voltages, and those tubes are still in the amp at 430 volts.

Another thing to watch out for when running high voltages with big, current hungry tubes, is the cathode voltage on the output tubes. The voltage my be above the 50 or 63 volt ratings of your caps, and the dissipation may be over 5 watts in the cathode resistor. I had a cathode resistor go open on me, which then overvoltaged the cap causing it to explode. I installed 100 volt caps and 10 watt resistors.
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