• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

6HJ5 SET

I just wanted to share my successful build using the 6HJ5 wired as a triode. These tubes are cheap and rugged, and oh yeah they sound great. The driver is a 6N7 with both triodes in parallel.

I have a SS Slewmonster and Mullard 5-20 monoblocks so this amp is my "colored" sound amp. It has that warmth of a 300b amp. I highly recommend people to go ahead and not be afraid to use this tube for audio. I know George has used them in Pete's big red board but I wanted to share the success of it wired as a triode.

-bird
 

Attachments

  • 20160322_190105.jpg
    20160322_190105.jpg
    282.6 KB · Views: 1,985
For anyone still interested here are some more details.

Pout= 5 watts

Power supply is a CLC and then I branch off separately to each channel via RC networks. The supply is tube rectified via 5U4GB. The choke is Triad C-14X, 6H 150R. The 50uF electrolytic cap value is for two 100uF 350v in series with 470k balance resistors. The reason is for using the 5U4GB and the B+ jumping up to 460v during turn on. I used the 5V winding center tap to feed B+ to the power supply filters. The power transformer is a Hammond 272HX. Current source for the 6N7's is a 10M45S module with a 1k CC resistor up very close to pin "G" and a 380R current set resistor.

Actual Measured data:

C1 (50uF) V=356
C2 (50uF) V=326
C3 & C4 (50uF Electrolytic | | 45uF Polypropylene motor run) V=318
C5 & C6 (22uF) V=297

6N7

Va=170

Vk=2



6HJ5

Va=295

Vk=50 Rk=620

Ig2=4mA

g3 is tied to cathode


With my ear directly up against my Klipsch La Scala's there is zero audible noise or hum, dead silence.
 

Attachments

  • Simcoe.png
    Simcoe.png
    35.3 KB · Views: 1,858
  • P1010133.jpg
    P1010133.jpg
    576.8 KB · Views: 1,795
Last edited:
Thank you MrCurwen! The metal can tubes get no love because you can't see the filament. That's fine by me, that makes them cheap and easy to find :)

I was worried about using a sweep tube but they seem very stable. I figured it was time to reveal it knowing there hasn't been any issues. I used CC resistors right up against the sockets to prevent oscillation. No runaway tubes so far. The 6HJ5 g2 is rated for 6 watts! That's almost 3 more watts than the more popular 6CB5a. It is also rated for slightly higher voltage. I started out pushing these tubes and I didn't have any issues but I settled on a pretty relaxed operating point. I don't require tons of power with this amp paired to Klipsch La Scala's which I believe are 104db efficiency. At the price of these tubes ($4) I am surprised they aren't more popular.
 
A couple of 6HJ5 tubes in triode. Usually very consistent. Raytheon tubes. (Argh, just found the zero was off by 2V on the left picture, left most curve is -2V then 7 V steps down from that.)
50 mA/div Vert., 50V/div Horiz., 7 V steps on grid 1
 

Attachments

  • rsz_6hj5_t_7vs_50ma_50v_ray.jpg
    rsz_6hj5_t_7vs_50ma_50v_ray.jpg
    71.5 KB · Views: 1,646
  • rsz_6hj5_t.jpg
    rsz_6hj5_t.jpg
    36.4 KB · Views: 1,521
Last edited:
Most hobbyists don't really do much design, they build schematics they find online. I guess there's not that many sweep tube schematics around, other than maybe nowadays on this forum.

Regarding stability: You can always decrease gm by putting an unbypassed resistor (22, 33 or maybe 47 ohm) under the cathode. Also a bigger g2 to plate triode strapping resistor helps; I didn't get my 6E5P balanced amp stable until the triode strapping resistor was 8.2k. I used 10k straight away in my SE build. These tubes have a lot of gm so in my opinion doesn't hurt at all to lose some, especially if power is not a priority.
 
A couple of 6HJ5 tubes in triode. Usually very consistent. Raytheon tubes.

I bought 5 sleeves of Raytheon tubes from the same production batch. Of the dozen I put in the amp they all biased up relatively similar so consistency is good with the Raytheon tubes. I think RCA made some too but I don't have any of them.


Most hobbyists don't really do much design, they build schematics they find online. I guess there's not that many sweep tube schematics around, other than maybe nowadays on this forum.

Well I admit I am only a hobbyist but I like to try and design my own stuff, with the help of you guys of course :) I felt like if I posted my build it might get some diy'ers to start using more sweep tubes. I really like the 6HJ5 a lot.

Regarding stability: You can always decrease gm by putting an unbypassed resistor (22, 33 or maybe 47 ohm) under the cathode. Also a bigger g2 to plate triode strapping resistor helps; I didn't get my 6E5P balanced amp stable until the triode strapping resistor was 8.2k. I used 10k straight away in my SE build. These tubes have a lot of gm so in my opinion doesn't hurt at all to lose some, especially if power is not a priority.

I was a little worried at first, maybe I still am a bit but at the operating point I am using the gm is probably down from 10mA/V to 7-8mA/V, the 6E5P is beast at 30mA/V!!! I can see having issues with stability with that tube. I am using 180 ohms for the triode strapping resistor. If I saw instability I was going to keep increasing it but everything seems stable.
 
21LG6A is a very similar tube to the 6HJ5, except for base pinout and a plate cap and heater V. It's also $4. Then there is 26DQ5 for $3, but its Octal base.

Might want to give it a try sometime, but the wiring to some pins have to be changed to try it.

Have you tried the 6CB5 before? Would be interesting to compare the two.
 
Last edited:
Have you tried the 6CB5 before? Would be interesting to compare the two.


Nope but they look very similar. Many forum members have built VinylSavor's 6CB5a amp with great success. Forum member Wdecho is a big advocate of that tube and amp. In fact he is the one whom brought to my attention the 6N7 while I was trying different drivers out. I ended up staying with the 6N7, it's a great tube IMHO.

I wanted to use a tube nobody uses for "the cause". I felt if the 6CB5a is a great tube triode strapped then the 6HJ5 would be too, it seems beefier.
 
Nice to see this design. This tube 6HJ5 was used in the famous H3 Futterman OTL amp.


Yes you are correct in that the 6HJ5 was used in the Futterman H3 OTL amp. I have never had the chance to hear one of those amps. Thanks for the interest shown in my build. I couldn't have done it without all the help from the sweep tube gurus lurking around these parts:)

I should have a full schematic ready to post in the next couple days.
 
Hey Ron, it sounds fantastic! Nice tight bass and the midrange is just heavenly. Overall it has a very relaxed nature to it in that it's so pleasing to listen too, I tend to get listening fatigue from some amps but this amp is the opposite, it has an addictive nature to it! It's 1:30am and I am up late listening to it :)

The power transformer you have is very similar to the one I used and you are aiming for the same plate voltage. Your 2A3 needs the same drive requirements as the 6HJ5 in my amp, in fact there are lots of parallels. I look forward in hopefully lending a hand in getting your amp build a dream come true. Soon you will be in SET audio nirvana!
 
When looking at datasheet g2 max voltage specifications I figure this is for operating conditions. I built my power supply for worst case scenario situation (no tubes installed and power on) so it's rated for 700v. Because of the 5U4GB conducting so quickly the g2 on the 6HJ5 is over 400v. I have had zero tube failures or any sort of internal arcing on start up. I have tried several of my specimens.

So should I be worried about the brief high voltage on g2?