Tubelab using Pete's big red board, with 6HJ5 in P-P, got 75 Watts readily.
6HJ5, 26/6DQ5, 21LG6, 6CB5 are all fairly similar tubes and cheap ($3 to $5). (ESRC, Vacuumtubes.net) 6HJ5 is the only one without a plate cap.
For the ultimate in linearity and efficiency take a look at this:
http://www.diyaudio.com/forums/tube...nificent-television-tubes-67.html#post4675483
The same scheme could be used for a SE 6HJ5 amp too. Would need some Schade or local N Fdbk to lower the output Z.
6HJ5 plate curves using "Crazy Drive" or "Twin Drive":
6HJ5, 26/6DQ5, 21LG6, 6CB5 are all fairly similar tubes and cheap ($3 to $5). (ESRC, Vacuumtubes.net) 6HJ5 is the only one without a plate cap.
For the ultimate in linearity and efficiency take a look at this:
http://www.diyaudio.com/forums/tube...nificent-television-tubes-67.html#post4675483
The same scheme could be used for a SE 6HJ5 amp too. Would need some Schade or local N Fdbk to lower the output Z.
6HJ5 plate curves using "Crazy Drive" or "Twin Drive":
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Hey guys fall is here and I am getting back into more amp building projects for the winter, crazy drive w/ a 6HJ5 is high on the list but I am trying to finish a Tubelab SE board w/ 45's, it should be done in the next week or so.
For anyone still interested in my 6HJ5 amp I finally got got around to look at the distortion spectrum on an FFT plot.
I have very efficient horns and probably never go over 1 watt, the amp sounds very nice!
Here is the FFT spectrum at 1 watt and 2 watts.
For anyone still interested in my 6HJ5 amp I finally got got around to look at the distortion spectrum on an FFT plot.
I have very efficient horns and probably never go over 1 watt, the amp sounds very nice!
Here is the FFT spectrum at 1 watt and 2 watts.
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It never ceases to astonish me that fantastic compactrons like this were and are completely ignored by high end audio manufacturers. The unit to unit consistency is so high and they were so cheap in big quantities back in the 80s and 90s. And all those guys used were 12ax7s, 6dj8s, 6bq5s and junk 6550s. No imagination whatsoever.
Yes indeed, I really look forward to hearing George's circuit and comparing a real DHT to a triode strapped pentode.
I couldn't agree more but I am kind of happy that they ignore these tubes, they are much cheaper for me this way
I have always enjoyed doing things differently and since this is diy I wanted to take the path that has been walked the least. I bought 5 sleeves of the 6HJ5 and you are correct in that the consistency is very good, these are well built rugged tubes. I am glad I settled on using them. The FFT proves what I am hearing, that these tubes are well suited for audio use.
I await four capacitors and then I can hear George's SE amp!! I may or may not start another thread about that amp.
The thing that is killing me is if I didn't forget these caps on the last parts order I could be enjoying it right now
Here is a popular design from the far east.
SRPP front end into a 6L6 wired as a pentode with global feedback, good for 5 watts out.
It's a good sounding amp but not as pleasant as the 6HJ5.
Compared at 2 watts out into an 8 ohm load. You can see that the 2H is much lower but the odds are higher.
SRPP front end into a 6L6 wired as a pentode with global feedback, good for 5 watts out.
It's a good sounding amp but not as pleasant as the 6HJ5.
Compared at 2 watts out into an 8 ohm load. You can see that the 2H is much lower but the odds are higher.
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Not to intrude, but I am starting to plan my PP 6hj5 amp and have a question about screen voltage and its effects on total output power. With a limit of 220V on the screen, it is not likely that we can get more than around 12W in triode if we push the upper limits and use a B+ of 200v and 6500k transformer in PP. I would like to get a bit more than that but want to retain the triode character. Schade/shunt feedback requires setting the screen voltage at a fixed voltage, independent of the plate voltage. Does this allow us to cheat the power limitation due to screen voltage or does it still ultimitaelt affect output power? In other words, could you fix the screen at 200v and run the plate at 300V? Sorry about the troll, but not many threads talk on this particular tube and I am new to the subject.
I think it was discussed earlier in the thread but the screen rating in the datasheet is most likely for Pentode operation. In triode operation I was mainly concerned with screen dissipation but it's a beefy tube and the screen rating is 6 watts continuous and 12 watts warm up and surge. 6 watts at 325v is 18mA, I didn't encounter any issues when running over 300v in triode mode. I did end up using cathode bias which gives me Va-k=250 but I run more current through them @ 80mA. I think these tubes like current and I didn't need power so I was happy with the operating point of Va-k=250 and Ia=80mA for SET operation.
Don't be afraid to run 300v in triode operation, they are still relatively cheap so even 325 would probably be okay. I didn't try these tubes in any sort of push pull arrangement but I know people were getting 75w a pair in pentode operation. Triode AB push pull you can get 15w w/ fixed bias and 300 plate volts me thinks.
Don't be afraid to run 300v in triode operation, they are still relatively cheap so even 325 would probably be okay. I didn't try these tubes in any sort of push pull arrangement but I know people were getting 75w a pair in pentode operation. Triode AB push pull you can get 15w w/ fixed bias and 300 plate volts me thinks.
On another note I did get the Tubelab SE up and running with 45 tubes. What a beautiful sound indeed
Once I get it in the wooden cabinet I will upload some photos.
The load isn't quite optimal for the 45 @ 2.5k:8 but I am running them at Va=200. When I get some 2A3's I plan to up the plate volts to 250v.
I haven't done a frequency sweep yet but the Tubelab SE sounds like it has better top end extension but the low end isn't as clear as the 6HJ5, but the output iron is a huge factor here, I am currently using the 10 watt Edcor transformers so they are limiting the bass as opposed to the Transcendar in the 6HJ5. With the secondary open I measured 40H for the Transcendar primary inductance where the Edcor was 6H. Given the 6HJ5 rp is ~450 and the 45 is ~1.6k the high pass filter is the limiting factor, -3db @ 42Hz. 1600/(2*3.14*6)
BUT I have a subwoofer and so that doesn't really matter, my Klipsch La Scala's don't dig deep either. The midrange of the 45 amp is TO DIE FOR though Very articulate. The first CD I played was Jeff Beck album and the guitar was just in your face and alive. Good stuff
FFT @ 1watt vs 6HJ5 @ 1watt
Once I get it in the wooden cabinet I will upload some photos.
The load isn't quite optimal for the 45 @ 2.5k:8 but I am running them at Va=200. When I get some 2A3's I plan to up the plate volts to 250v.
I haven't done a frequency sweep yet but the Tubelab SE sounds like it has better top end extension but the low end isn't as clear as the 6HJ5, but the output iron is a huge factor here, I am currently using the 10 watt Edcor transformers so they are limiting the bass as opposed to the Transcendar in the 6HJ5. With the secondary open I measured 40H for the Transcendar primary inductance where the Edcor was 6H. Given the 6HJ5 rp is ~450 and the 45 is ~1.6k the high pass filter is the limiting factor, -3db @ 42Hz. 1600/(2*3.14*6)
BUT I have a subwoofer and so that doesn't really matter, my Klipsch La Scala's don't dig deep either. The midrange of the 45 amp is TO DIE FOR though
Very articulate. The first CD I played was Jeff Beck album and the guitar was just in your face and alive. Good stuffFFT @ 1watt vs 6HJ5 @ 1watt
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It's hard to say because in changing from fixed to cathode bias I too changed the operating point by lowering Va-k and increasing current through the valve. I would think where I am not running this amp anywhere near clipping the added sonic improvement was due to the increased current and shifting the bias to the left/hotter side of the curves. Increased current gives increased gm and lower rp, all good things for a zero feedback design. Well at least I think so.
I have heard this stated several times, and haven't really investigated the 6HJ5 in triode. I built a triode wired push pull sweep tube amp several years ago using 6LW6's which are the biggest of sweep tubes. After losing 4 tubes over about 2 years I rewired the amp to run 7403's, a Bendix built military tube rated to eat 850 volts in triode!
It is stated that it is OK to run the screen grid over spec in triode mode since the screen tracks the plate, and the plate eats most of the current. This is usually true. However some sweep tubes can be run way over the screen spec (6AV5GA) and some will runaway at 25 volts over spec (6CD6). The 6LW6 tubes did not fail while playing music, even while tortured by me playing the guitar through it at 20 dB into clipping.
They all failed while idling. The tube doesn't know triode from pentode from UL at idle. The screen and plate remain constant at nearly the same potential.
I started out running the amp at 400 volts against a 275 volt max screen spec. The last tube blew at about 340 volts. I had noticed that the idle current would remain constant after the first few days of a new tube settling. I didn't retouch the bias at all for months, then the slow upward creep set in. Every tube eventually went into runaway leading to a blown fuse. The tubes were not usable after this happened. Instant runaway within a minute or two. Why, I am not sure, but the same amp ran 500 volts with the 7403's and 430 with EH 6550s.
I bought my pair on EBay from Gery but it looks like he isn't in production at the moment.
Edcor I don't think makes a 3k:8 OPT but a 2.5k will get more power but at slightly higher distortion. EDCOR - GXSE10-2.5K
Here is my Tubelab 45SE build
