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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Blue glowing tube

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I've not been able to find it, but not so long ago, I saw on this forum a tube amplifier with blue glowing tubes. What kind of tubes are they? I've read somewhere it has to do with mercury gas but I don't really know...

Anyway, the point is that I want to build a tube headphone amplifier and I believe it would be very cool if it could use that kind of tubes. Is there good designs using these (diy-ready)?

thanks a lot,
They are no good for this kind of stuff.. overkill, plus a PITA to implement.

You need to warm them up before you turn on the HV, or they croak.

Plus a headphone amp just doesnt need that much power..

I've got some 866's, they're good for 250mA @ 10Kv or something crazy like that, IIRC.

The UV might be bad for your eyes too, or does the glass stop it?

I had a better response typed up, but IE screwed up :(

heres a pic of a 866 (my poor camera doesnt do it justice.. looks much better in real life).


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diyAudio Senior Member
Joined 2002


Those huge valves aren't the only ones.
Way back then,in the early days of radio you had quite a few of these cute blue tubes.
Well,in Europe at least.
With the most lovely shaped bulbs to boot.
IMO,but check it out with the HAM folks, it served the purpose of tempering the influence of incoming light.
Say much the same way you will find some graphite coating on the inside of some older coke bottle shape 6L6's,6Y6 to name but a few.
And I agree it looks cool.:cool:
I recall former Tesla EL34's whitch we're done with the same kind of blue glass.
The Japanese audio scene was the major buyer of these.

I have never seen one myself, but I understand that mercury
rectifiers once was the only way to rectify high currents/voltages.
I have heard people referring to the strong blue glow from the
inner of certain electric railway engines, eg. the sewdish Ra
and Rb series back in the late 50s/early 60s, before high
power thyristors were available. In this case we are not
talking W or kW, but MW! These were not small valves/tubes
but tall glass columns, I am told.

Sorry for posting off-topic for the forum, but it seemed
a bit on-topic for the thread.

There is very simple way to make every tube glowing blue.
There is no any sound difference however.
It just looks very unusuall and cool.

You simply install high-efficiency Blue LED below tube !:)
I tried, and it looks very different than the rest normal glowing

Best regards,
Kristijan Kljucaric


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Don't know what you saw before

but I've got a couple of pairs of 872's that are mercury vapor types that burn blue. They will be used for rectification on a 304TL amp - you're looking at about 1500 volts here -

The 304tl amp will be SET but due to the higher voltage will put out in the neighborhood of 45 watts.

The most widely known of the 304tl's is probably from Moth Audio.


I've been assembling parts for a Jim Dowdy design for almost a year - interstage transformer due in soon which only leaves the Plitron output and I'll have all the main parts


Ken L
Nicely done

Good looking preamp.

Is it sounding like you want?

I've recently finished (?) a Dowdy designed Preamp - debugging it - having problems getting the AC hum out -

Real Life intrudes in the form of work (some), and home renovation (major), I left it with Jim a few weeks ago -because we're moving everything out of the house we can due to the renovation.

What part of Va are you located in?

Did I meet you in Knoxville a couple of weeks ago at Phil's??

Ken L
Thanks Ken!
Here is a pic of the actual preamp I am using with that power supply..

An externally hosted image should be here but it was not working when we last tested it.

I made it so it is like a breadboard so I can try out many types of tubes. The little switches next to some of the sockets are for changing filament voltage. The group of 6 sockets on the left is the phono premp. The middle 2 sockets in that group can take (2) 6SU7, (2) 6SL7 or (2) 12SL7's. The outer 4 sockets are for 6K5G's single triodes that can be used instead of the dual triode octals. The phono preamp uses passive eq and no neg feedback. The tube in the middle is a 6SN7 cathode follower for tape loop isolation. There are 5 sockets on the right for the line section. The 5 pin sockets are for 27, 56 and 76 used as a gain stage and there is a switch to change the filament voltage as needed. The middle socket is for a dual troide such as a 6SN7 or 12SN7 cathode follower. The 2 back sockets are for single triode cathode follower and there are 6P5G's there now but manu others can be used such as 7193, 6J5g etc. This way I can compare the sound between various types of tubes. The follower sockets are wired so I can either use 1 dual triode or 2 single triodes but not both at the same time. I figure once I get the sound I like best I may build a more compact better looking version... The power supply in my previous post uses (2) 816 Mercury vapor 1/2 wave rectifiers and a Amperite time delay relay to give them 90 secs to warm up before B+ is applied. Each channel has its own LC network leg.
I live near Richmond, VA and I do not know Phil and wasn't in Knoxville. I am also in the process of renovation/restoration and have been for the last 2 years so the audio projects get a backseat from time to time, usually when I am burnt out from working on the 1764 house.

that is one exotic preamp

Always interested in seeing something well executed. This is not only well executed but original thinking also.

The Dixie Bottleheads (essentially exotic tube types) have been meeting in Knoxville, and I met a guy from VA, that I was thinking was named Dave - the reason for that question - thinking it might have been you-

It'll be interesting to see if any other users respond to fdegrove's post.

If your restoration is the same quality as that hardware you built (and I'm sure it is) you're going to have something really nice.

Ken L
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