After getting my SSE running again, I decided to spin-off my thread as I'm looking for commentary on running some of the bigger output tubes with the existing amp design. I have a bit more B+ due to the Edcor PT, and unless I need to beef up the cathode bias circuit R & C, should be set.
I've seen a few threads using the KT120 and KT150 and was curious if anyone has any longer term experience with these tubes. Did they last or did they suffer the same fate as some like the Russian TS 6550's where the getters fade too quickly and they go into runaway?
Also, any comparison between the two tube models themselves? They both should have more headroom and output capacity than the 6550's (and the KT88's I've had decent luck with). I'm just trying to decide whether they will be as good or better than the value priced EH KT88's, as so far they have been the best bang for the buck in this amp both in sound and lifespan.
Ensure your power transformer can supply the heater current required to run the KT100/20/50.
They draw considerably more current!
The Russian 6550s are wonderful but do not like being overloaded. It is not good to use a high bias current, they will get damaged due to overheating. As will the KT120.
They draw 1.8-2A heater current, so you would be pushing it with a 4A tap including your 12at7. Ideally you would want a 6A tap.
I have tried both the kt120 and kt150, I believe I was using an Rk of 680 ohms. Sold the amp to a friend as I preferred the tse.
Although I didn't play with operating points I did find the 120 and 150 had more grunt in the bottom but I preferred the 6L6 musically. My favorite sound with that amp was the 6v6, very close to the tse but you need efficient speakers.
I think your best option is to use a variable resistor for Rk and try different operating points.
Thanks for the input - I have a 5A tap for the heaters. Running 560 ohm presently with the 6550's. Would a larger Rk be recommended in this case? I am presently seeing about 46V across the resistor (82 mA current). What bias current is suggested?
82mA should be fine, I usually run 6550's and kt88's around 75mA.
Check the plate to cathode voltage and calculate your plate dissipation just to be sure you are under the max. rating for whatever tube you use.
Thanks so much for your input! In my research around the web, I found suggestions elsewhere that agree that 75 mA is a much better bias to run the TS 6550's. My new pair do not like 82 mA, as leaving them idle for 15 minutes or so produces a faint red glow on the plate seam.
I got a killer deal on some new KT120's so I will be giving them a spin in the amp. If anyone has an opinion on what bias current they should run it would be most welcome. Since some of the data sheet values are higher than the 6550, I suspect they won't be run as hard - but any real world experience is appreciated. If they actually aren't as different from the 6550/KT88 as they seem to claim, I suspect I'd want to change out some components for better lifespan.
I'm also curious if anyone has lowered their grid resistor to prevent runaway in this instance. The KT120 lists 240k max for self bias as opposed to the 250k for the 6550. I'm running the stock 220k resistors now.
Another update, the KT120's arrived and the bottles are big and imposing looking! I plugged a set of them into the amp and fired it up.
The first thing I've noticed is the transformer seems to run cooler than with the 6550's and potentially has less hum.
Conditions at startup 50V across 560 ohm cathode resistor, 477V plate to ground.
After 4 minutes, the voltage drops to 47V, where it remains stable 46V/47V across the channels and 460V on the plate at 10 minutes idle. The lowest I registered was after applying a signal to the amp and playing music for a while, 45V and 450V on the plate.
At 10 minute idle, I calculated a 84 mA cathode current (79 mA plate current) and 48% dissipation. How cold is too cold when running a set of tubes like this?
They sound good with a low input drive, but if I up the drive level too much things sound distorted. It may be perception, but I thought I could drive the 6550's with more input signal before they distorted. At lower drive levels the power output is increased and the sound is very clean, with sparkly highs, thick midrange, and tight bass. Vocals sound awesome. The volume control can be cranked up for high output at these drive levels.
So the big question here is what is the ideal range to run these, or any (especially newer production) tube. I see the most discussion about avoiding too hot of a bias point, and unlike the 6550's starting to glow, these tubes show no signs of being pushed too hard.
Has anyone rigged up a good system for easier bias adjustment on their SSE? My resistors presently sit against the top plate for heat sinking with some thermal paste, so I have to dismantle the amp and remove the PCB to change them and then carefully install the new ones so they make contact with the plate upon reassembly. I'd love a simpler alternative.
Any input is appreciated from those running KT120's on their bias point, especially if run in the SSE.
Leave your resistors where they are and add a resistor in parallel with it to increase the current. I used a switch to run one of 4 different resistors in parallel with the original 560 ohm. This of course will only increase the current, but my EH KT88's didn't show red even at 100 mA. 10 years later they still work. My B+ was around 450 with 70 mA and somewhat less at 100 since my undersized power transformer really didn't like this, it got too hot to touch. It never died, and I cranked it up to 100 mA for stuff that benefited from the high current like Pink Floyd quite often.
The added resistors will be in the 1K and up range for KT88's, you may need to go lower with those big tubes. Since the original 560 ohm will eat at least half of the power, the new resistors won't get as hot, and shouldn't need the heat sink treatment.
Thanks for the input - I'll get some resistors and try to see what I can do. I'm curious how to determine HV current headroom, though. When choosing a rating for the HV tap of the transformer, what all needs to be taken into account? The Edcor I'm using is rated at 200mA. I presume I shouldn't exceed this rating for long term use, but what is the source of the rating itself (the cathode currents of all tubes, or is it not that simple)? My 6.3V tap is 5A, so shouldn't be an issue there for the filament draw.
I'm trying to better understand the choice of an operating point a little better by using some calculators and it appears that the voltages should stay fairly stable (some variation) but the current will be the major item to change when the resistors are adjusted.
So what I'm left wondering is where to ideally operate these tubes, or any for that matter. If I read correctly from the bias calculator, somewhere from 70-80% of max, up to 90% if you want it hot. Here, your KT88's were pushing 100 mA which it says is around or above 100%. At 70 mA, closer to 70%. If the 6550's were indeed rated at the TS spec of 42W, I should have been OK, but those ran away unlike my former KT88's that seemed to be much happier.
Is there a different formula to use or something else I may be missing in trying to calculate reasonable bias values? This way I can determine what resistors to obtain and how to aim for decent tube longevity.
"A few things about ratings. Very few, if any, tube manufacturers stated the life of the tubes they made at full dissipation. When I visited the Sylvania factory I asked the question "how long will your tubes run at full dissipation" the chief engineer of 25 years said "Well you aren't supposed to run them at full dissipation because they will only last about 1000 hours. If you run them at half dissipation they will run 10,000 hours." Now Sylvania made some of the best American tubes and he also told me that their goal was to make a color TV run 10 years at 3 hours a day with their tubes and that they had pretty much achieved that if the designer of the set ran things properly. Unfortunately life vs dissipation is never given in any data book I have seen. I was lucky to have the Sylvania enginieer share that information early in my career so I didnt make the mistake some designers make and run the tubes full out. "
This author goes on to suggest running at 50% dissipation for long life, so I'm not far off from that (and this is in contrast to the other source which said 70% was "cold".