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engels 1st August 2007 05:01 PM

Got oddball tubes!
Just received a package with some very odd NOS tubes - I won 3 lots of 10 tubes for $3.50 each. RCA, tungsram, tungsol, raytheon etc. The majority of the odd tubes are compactrons - I wonder if they're good for anything like audio output.
Here's the list of the tubes, if you have any idea where to stick them, give me a hint!!!!

6CA4 (no questions about this one)
6X5GT (okay, okay...)
RCA 4422 (very odd!!!)
6080 (no questions)
6V6 (I wonder what this tube is used for)
12AT7/Ecc81 (not again!)
PL84 (~EL84 wih different heater V)
UBL1 (only one unfortunately)

many of them came in pairs so PP's are possible.

A pair of 33GY7A: any chance for PP??

RCA's -
upper row: 6080, 38HE7, 6AX5GT, 6V6


ilimzn 1st August 2007 06:18 PM

Oh, come on, some of these are realy standard (or should be) :)

PCL84 is a 300mA version of the ECL84, a decent triode with a mu of 70 and mid gm, better high swing linearity than a 12AT7, and a nice video output pentode which strapped as a triode gives you something very similar to an ECC88/6DJ8 section on steroids, see here:

12FQ7 is a 12V version of the 6FQ7 (there is also an 8FQ7), which is essentially a noval version of the 12SN7, and that is a 12V version of the well known 6SN7 :)

EF184 is a high gm video pentode that makes a great triode when triode strapped. Agan, see

PL84 is not a different heater EL84 but a different heater EL86 - this is a common mistake. PL84 is a 300mA heater version of the EL86.

PL81 is a BPT designed for horizontal sweep in small BW TV sets. It's datasheet also gives class (A)B operating point for a 20W PP amplifier.

This is just off the top of my head, will give it a better look later on...

engels 1st August 2007 06:27 PM

first candidate-
heater = 33.6V, 0.45A
average: plate = 130V, 48mA
screen = 130V, 2.9mA
plate dissipation = 9W, screen = 3W

second candidate:


SY 1st August 2007 06:53 PM

They'd work pretty well in class B push-pull screen drive. The 38HE7 looks like it would be fairly linear, and you could probably squeeze 20-25 watts out of a pair.

engels 2nd August 2007 07:53 AM

A question about UBL1 :

First of all, I have to say this old tube looks awesome and as I've got the socket and the cap I may already make a nice-looking SE amp with it...


So I've got a question: The filament of UBL1 is 55V@0.1A, and the plate + screen voltages are low, under 200V.
I'm thinking about using one 230V:115V power transformer for everything: First, getting the B+ of about 145V with a bridge (although it seems a bit low) and then putting a VR90 tube (I've got one nice ST) and wiring the heater of UBL1 in series with some preamp tubes to get 90V total. There are several choises for 0.1A heaters tubes, but the cheap tubes are mostly noval or 7-pin type which looks not good together with UBL1. There's a UF9 pentode that looks proper, but it's a bit expensive ($13.90 at Antique Electronics) and the heater is 12.6V so I will need three of them. I may wire them as triodes and it'll be fine but still a bit too Xpensive.

Is there a problem with this setup - first B+, then 0.1A heaters from the same secondary?

And if that's okay, what other tubes may fit the preamp

Geek 2nd August 2007 08:19 AM

Not a bad score! :)

The EF184 is a real nice one in triode. Make a mu-follower with it and you have an extremely linear, large voltage swing driver for those PITA to drive tubes (sans interstage iron), like 6080, 2A3, 300B or trioded sweep tubes.


aletheian 2nd August 2007 08:52 AM

Well, I will be the odd man out and say that I like 6BR8... but I have only used them in guitar amps. They are heinously non-linear and they sound pretty cool if they are not microphonic.

and the 3v4... OK I'll admit that I have built up 3v4 monoblocs for kicks and giggles. Aside from te low output , they actually didn't sound as bad as I thought thty would, so that could make for a fun afternoon.

Tubes4e4 2nd August 2007 01:00 PM

Re: Got oddball tubes!


PL84 (~EL84 wih different heater V)
Sorry, but this is a common misconception (and one of many nonlinearities in the European numbering scheme), since PL84 is the 300mA series heater variant of EL86 / 6CW5, which is quite different from EL84.



ilimzn 2nd August 2007 01:08 PM

OK, I am siding with SY here :) regarding the various sweep tubes.
Your first choices should be the 21GY5 and the 21JZ6. These are 18W plate dissipation horizontal sweep BPTs. The 33GY7 and 38HE7 are 9 and 10W respectively. Most of what I am going to write here applies to all of them, but the latter two require some additional consideration.
You can use them, basically, in two different ways:

1) As 'pentode' connected with low G2 voltage (other refinements as plate-to-plate and supertriode techniques could also be applied). They appear sufficiently linear as such, with two caveats, which should be obvios from the plate curves:
- The plate currents go VERY high even with a very low Vg2 of 130V, way above the maximum dissipation curve it heps if you draw it into the curve plot, connecting points where Ip * Up = plate dissipation max). This means that for them to be useful in linear mode, a comparatively very low G2 voltage will have to be used, but because of this, you have a problem - you wuld need to plot the plate curves for this lower voltage yourself and see how linear they end up looking.
- The G2 current fluctuates wildly with Vp change (see the dashed lines). Although a lower Vg2 is going to make this easyer to manage, this means that Cg2 must come from a very good and stiff voltage regulator.

2) Screen (G2) drive, AKA enhanced triode. The curves look very linear and again the very high plate current sensitivity to Vg2 can be seen in the graphs in the datasheet. Again, you will probably be using very low voltages (50V or below) for Vg2 in this mode, but look at Ig2 - it can be considerable, and your driver circuitry has to source them. You are most likely looking at driving G2's with something like a MOSFET follower.

The great distinction between these two modes of operation will be the output power. Screen grid drive normally requires very low standing current, which is considered class B operation. Unlike regular pentode mode and class B, screen drive can be amazingly low distortion in class B. Because the idle dissipation in class B is very low compared to the usual thing one finds with class AB bias, almost all of the plate dissipation capability of the tube is left to be used by the actual signal swing - and being class B, the tube only dissipates heat during half of the output signal cycle. You can get signifficantly higher power, in theory approaching twice that of high bias class AB, which would have to be used in pentode mode.

Triode operation for either of these tubes is dubious, because of the very low maximum Vg2 - 150V - unless you want to go seriously over spec. Mind you, it IS possible, but at low plate voltages. Couple that with less efficiency from a triode, and you would get a very unfavorable ratio of power supplied, to power delivered to the speaker.

38HE7 and 33GY7 also contain a damper diode, which in tjis case may prove to be a serious hindrance, because it takes up heater power and limits power dissipation of the 'business' part, that being the BPT section. However, if you look at the heater to cathode voltage spec, you will see that it provides up to 500VDC, with heater negative WRT cathode. Since it is the cathode that is normally the source of B+ in a tube rectifier, this means the damper diode could be used as one of the two rectifier diodes to power the rest of the amp. Damper diodes are very low voltage drop as tube rectifiers go, but require serious heaters - in your case, not a great problem as they are inside the bulb already, so the proposition looks interesting. But there is a caveat - even though two tubes for a PP amp give you two diodes to make up a rectifier, in case a tube is disconnected (removed...), you get half wave rectification and soon a very fried power transformer. Also, in a stereo amp, you get 4 diodes, with two being relatively superfluous. Maybe use them as cathode bias 'resitors' in pentode mode?

ilimzn 2nd August 2007 02:27 PM


PABC80 is a triode duodiode diode combo for 300mA series heater chains, with approxcimately 7V heater voltage. The dual diode section is capable of fairly high currents and could be used for bias, or, simply left unconnected along with the remaining diode plate, and only the triode used. It has a nice triode section vaguely reminiscent of a 6SL7 triode section but with limited plate dissipation and makimum plate voltage limited to 250V.

6AX5 is a nice tube rectifier, use it in power supplies.

6EA8 is a triode-pentode combo tube. The triode seems to have a variable mu so is of limited use, but the pentode section may be interesting triode strapped.

1X2B is a high voltage rectifier for TV CRT tubes. To be quite honest, you can probably chuck them in the bin, they are of little use but for the purpose they were designed.

3V4 is a small direct heated output pentode designed for battery powered equipment. This means that the output power is very low in this role, however, it could make a pseudo DHT for line preamp use, with some creativity.

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