• 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.

Tube Virgin

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Hi Guys

Over the weekend, a friend gave me two boxes of tubes. If I may list some of them, are any good for an audio project or two?

Telefunken EBF 80
Telefunken 6BA6
Haltron EF93/6BA6
Mullard EF80
Mullard EF95
Mullard EC92
Torvac 6AK5
Sylvania 12BA6
Brimar 6AL5
Brimar EF91
Haltron 6C4
Brimar 6C4
Brimar EF91
Philco 7C5 (2 of)
Philips UCH21

And then there's a second box with

Mullard UCH21

Philco 787
Purotron? 7A7
? 7B5
Philco 7C5
Philips UCH21
K-R 7C5
Zenith 7C6

Much obliged

OK let's see here...

EBF80 is a 6.3V heater dual diode + remote cut-off pentode combo, which is mostly used for IF and detector work in receivers. There is mention of audio amplification in it's datasheet at http://www.mif.pg.gda.pl/homepages/frank/sheets/030/e/EBF80.pdf

6BA6 a.k.a. EF93 is another remote cut-off pentode used for IF work in receivers, doubtfuly of use for audio. Datasheet at http://www.mif.pg.gda.pl/homepages/frank/sheets/020/6/6BA6.pdf in french.

EF80 is a very common pentode that can be used in audio, assuming you get a particular piece that is not microphonic. It's original use was for IF in receivers, so low microphonics were never a design goal, that being said, there are good ones out there. It makes a great driver tube or general amplifier, also great in triode mode.
Datasheet here: http://www.mif.pg.gda.pl/homepages/frank/sheets/030/e/EF80.pdf
Triode mode curves here: http://www.tubes.mynetcologne.de/roehren/daten/ef80pentode_as_triode.pdf

EF95 is a pentode used for HF amplification. The datasheet is very short and does not mention if it is a remote or sharp cut-off type, nor does it give any curves, it is at http://www.mif.pg.gda.pl/homepages/frank/sheets/030/e/EF95.pdf

EC92 is a single miniature triode which is actually a single section of the common ECC81/12AT7 double triode, commonly used for audio in various roles, datasheet here: http://www.mif.pg.gda.pl/homepages/frank/sheets/010/e/EC92.pdf

6AK5(W) also known as 5654 or 6096, is a HF amplification pentode, that can be used for small signal linear amplification in pentode mode, or as a very nice triode, with mu around 30 and gm about 5mA/V. This datasheet also gives triode mode curves: http://www.mif.pg.gda.pl/homepages/frank/sheets/138/5/5654.pdf

12BA6 is a remote cut-off pentode intended for HF work, audio use is dubious. Datasheet: http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6BA6.pdf

6AL5 is a miniature double diode, often used in radio equipment, and likely holds the dubious distinction of being the least usable tube for audio ever :)
Datasheet: http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6AL5.pdf

EF91 is also known as 6AM6, and it is a nice RF pentode also usable for audio - and it also makes a very nice liner triode, with a mu of about 70 and gm at about 7mA/V, which makes it eminently usable for a variety of applications in audio.
Datasheet here: http://www.mif.pg.gda.pl/homepages/frank/sheets/154/e/EF91.pdf
Triode curves here: http://www.tubes.mynetcologne.de/roehren/daten/ef91pentode_as_triode.pdf

7C5 is a loctal socket version of the popular 6V6GT beam power pentode, which is a popular medium power audio output tube, which can also be used as a low mu triode for line stages.
Datasheet: http://www.mif.pg.gda.pl/homepages/frank/sheets/084/7/7C5.pdf
See also type 6V6GT

UCH21 is a triode-heptode with a loctal base, the usability of which is unfortunately limited by it having a common cathode. Still, the Philips darasheet shows audio amplifier and phase splitter applications, here:
It can be used to build a push-pull ampifier front end, which works very well with a pair of UBL21 tubes for output. The slight drawback is the 100mA heater. which is intended for series connection in mains powered radio receivers, which makes for a 20V approximate heater voltage rating.

UBL21 as mentioned in the above paragraph is an loctal base output pentode + dual diode. This fairly rare tube holds an excellent output pentode of very high sensitivity, needing a very low input signal. Because of this it must be used with automatic bias only. The datasheet unfortunately gives only data for fairly low power rail voltages, because of the intended use of the tube, in mains powered radio sets. This also implies a 100mA relatively high voltage heater (55V). A push-pull pair gives about 13W from a 200V power supply at 4% distortion. That being said, some characteristics are remarkably similar to the very popular EL84 output pentode, so it is possible that this tube can be used under similar circumstances, with corrections to the biassing. The diodes in the tube were intended for demodulation in radio sets, and for audio applications are not used. Datasheet: http://www.mif.pg.gda.pl/homepages/frank/sheets/030/u/UBL21.pdf

7A7 is the loctal socket equivalent of the octal 6SK7GT remote cut-off pentode. Because of the remote cut-off, it is of little use in audio. Datasheet: http://www.mif.pg.gda.pl/homepages/frank/sheets/084/7/7A7.pdf

7B5 is a loctal socket equivalent of the 6K6 and 41 tubes, so it's a power pentode (unlike the 6V6 which is a beam power tube). It is intended for audio output stages, datasheet here:

7A8 is a loctal socket octode converter, octodes in general are very rare, and intended as mixer circuits in radio receivers. Although it is possible to connect it as a pentode or triode, such use for audio would probably be either as an experiment, an attempt to be different as a means unto itself, or as an act of desperation when it's the only tube left :)
Datasheet: http://www.mif.pg.gda.pl/homepages/frank/sheets/127/7/7A8.pdf

7C6 has a high mu triode similar to type 75 along with two small signal diodes in the same envelope. The triode is suitable for grid leak bias operation and has a mu of about 100, making it similar to a section of a 12AX7 or a higher mu 6SL7. Datasheet: http://www.mif.pg.gda.pl/homepages/frank/sheets/084/7/7C6.pdf

I called myself a "Tube Virgin" but it's not quite that bad;) Here's some of my Leak things, set up for a "do" last weekend when my engineering society had an "open day" for the general public.



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Hello guys

As the recipient of ilimzn's generosity of spirit and knowledge
I can only but agree that The MAN from Zagreb is an example to me and to all of us.

Although I have some general audio knowledge, tubes (or valves in South African-speak) are indeed virgin territory for me. Many years ago - about 45 to be exact - I did build a small valve amp while still at school. A steep learning curve awaits again...

6AL5 is a miniature double diode, often used in radio equipment, and likely holds the dubious distinction of being the least usable tube for audio ever

Now, now, be fair. We made excellent use of the 'AL5 in the B- supply of "El Cheapo". :D The type works quite well in low current hybrid bridge rectified supplies. 18 mA. should be enough for a buffered line stage. Double up tubes for more current.

Thanks for the info. I had not seen the triode curves for the 6AK5 before since they are not on the data sheet that I have. I have a few zillion of these tubes and will be doing some more testing in the near future. Ditto the 6AM6.

6AL5 is a miniature double diode, often used in radio equipment, and likely holds the dubious distinction of being the least usable tube for audio ever

I tend to agree here. I don't use tubes for bias rectifiers since I want to be sure that the B- is available before the B+ is. A 6AL5 is slower than a 5Y3, 5U4, and some 5AR4's. The second most useless audio tube has got to be the 2D21. Too bad that I have about 1000 of each!
Over the weekend, a friend gave me two boxes of tubes. If I may list some of them, are any good for an audio project or two?


As previously indicated, the 7C5s are Locktal based members of the 6V6 family. It's very likely the tubes you possess were made by Sylvania, regardless of the label. I'm disinclined to suggest you use the 4 tubes in a PP circuit, as they rate to be poorly matched. However, you could build a singled ended amp around 2X 7C5s and have a spare set of O/P tubes. I didn't notice a suitable driver tube in the collection. Can you acquire a 5965?
I agree that a bias supply (C-) should be instant on. OTOH, the soft start is fine in the negative rail under a LTP (B-). FWIW, the "finals" in "El Cheapo" are cathode biased.

You are indeed correct. I looked at the schematic of the El Cheapo when you first posted it, and have forgotten the details since then. Many of my designs are fixed bias and often use a negative voltage source for other functions. Since B- and C- are often the same source, I have gotten used to calling the supply B-. I prefer to have the negative voltage arrive first since it often is used for bias. I might consider a tube rectifier for the negative voltage if it was not being used for bias.
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