DIY Audio Projects Forum

New 6JT8 build working
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Author:  ferronier [ 02 May 2011, 21:43 ]
Post subject:  New 6JT8 build working

I went to my first hamfest where a guy sells NOS tubes in bulk, I got 30 for $20, all triode, pentode, audio stuff. I could never find schematics for the tubes I have, lots of forum questions if anyone had tried these tubes, but never any follow up . So, I finally took the plunge and started building from scratch. Chose the 6JT8 because it looked promising as a one tube per channel mini amp. I got one sounding good, so I finished the other channel and was rewarded with a nice little stereo amp. I learned more building this guy from scratch with no plans than any other project I've done. Now I know how to bias, now I know how to adjust input stability, now I know how to apply negative feedback, it was a real learning breakthrough for me, so I'm a little excited.
Turns out, the 6KT8 tubes I grabbed work in the same circuit, but I prefer the 6JT8, especially the fat Sylvania ones seem to be good. Here's a pic of the results:

Author:  Geek [ 03 May 2011, 02:27 ]
Post subject:  Re: New 6JT8 build working

I like those! :D

Put out about 2W SE into 2.5K load. The fatbottle with minature bases look cool, too.

The only thing to watch for is they loooove to oscillate :?


Author:  ferronier [ 05 May 2011, 12:25 ]
Post subject:  Re: New 6JT8 build working

Ah, did not know they had 2.5K load; been using cheapo 5K SE OT.... so I could run them in a PP with 5K primary PP OT?

Author:  Geek [ 05 May 2011, 15:35 ]
Post subject:  Re: New 6JT8 build working


They aren't that fussy I don't think. 2.5K was where the SE loadline put it.

I haven't done a PP loadline (I'm really bad at those anyway), so your guess is as good as mine.


Author:  CrazzyAbtTubes [ 28 May 2011, 17:22 ]
Post subject:  Re: New 6JT8 build working

That is a great little project, I am glad you learned something from it too, it must have felt really good. :D

Do you have a schematic for this circuit? That tube looks more like the type that would be used in a tube TV rather than an audio circuit, maybe the audio circuit of a tube TV. Of course it has the standard 9 pin socket unlike the strange three section compactron TV tubes I have laying around.

Author:  ferronier [ 31 May 2011, 14:43 ]
Post subject:  Re: New 6JT8 build working

It was made as a TV tube. It contains a hi-mu triode and sharp cuttoff pentode so it's ideal for audio use as a one tube amp (like the 6T9, but in a 9pin package). Here's my working schematic.
I have tested it with 6JT8, 6KT8 and 6LY8 tubes. You will get different results depending on brand too, I've tried RCA, Sylvania, GE, Magnavox and Westinghouse. All resistors 1/2W unless noted. Input resistor 4.7K works for CD input, leave out for ipod input. There is room to experiment on this resistor according to your input source...also the .0033 cap from input to ground is optional; I use it on one channel. I found the circuit to be unstable if both channels are identical. After reading a lot about this I found stability by adding the cap on one side. Note this is setup for 100K pot, if you change that you will have to modify input section; it's pretty sensitive.
B+ can be from 200-250v, I shoot for something in the middle; each tube type and brand will pull a different voltage and current but the three I mentioned all work in this circuit.

Author:  billyboy [ 02 Jun 2011, 12:09 ]
Post subject:  Re: New 6JT8 build working

Boy, I'm glad to hear from you guys. There are bundles of tubes that were originally intended for RF, that will work very well, thank you, for audio purposes. The lowly 6GH8A. It is a wonderful triode/pentode, that was produced by the millions. Some RCA TV's had ten of the buggers each, in the color circuitry. Repairmen would replace all of them at a time, and usually cure any color problems.They should still be available and cheep, nowadays. I know I have scores of them, still. They would make a wonderful low powered amp, or as the drivers for a more powerful brute.

Author:  ferronier [ 02 Jun 2011, 14:01 ]
Post subject:  Re: New 6JT8 build working

According to the NJ7P database, these tubes should also work:


Just keep an eye on the current being pulled by the plate and adjust voltage accordingly.

They are a little low on power so I'd like to try in PP next....

Author:  CrazzyAbtTubes [ 02 Jun 2011, 17:33 ]
Post subject:  Re: New 6JT8 build working

This is all Great information ferronier, because I am sure there are many boxes of TV tubes laying around out there that are other wise useless without the TV, so this kind of project would be quite cheap and easy to gather parts for in the case of a beginner who want to build a tube amp. :D

Author:  Gio [ 02 Jun 2011, 20:38 ]
Post subject:  Re: New 6JT8 build working

Nice work ferronier. :up:

billyboy wrote:
6GH8A. It is a wonderful triode/pentode, that was produced by the millions. ... They should still be available and cheep, nowadays.

The 6GH8A is a common substitute tube for the 7199. I am seeing them being used more and more often in Dynaco ST-70 amps. Socket converters make tube substitution ultra simple so unfortunately, they are not that cheap anymore.


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