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GordonW 11th June 2008 12:02 PM

Small-signal (or small power) pentode that can handle 400V (or more)?
Been looking around for phase splitter (LTP) pentodes, for possibly building a Hegeman-style (all-pentode) front end for a screen-drive amp.

The issue here, is the need to be able to swing something in the order of plus and minus 200 to 250 volts, to drive the screen grid (through a cathode follower, but naturally, those don't have any additional gain, so effectively the same voltage is on the screen of the output tube), and have enough headroom to avoid distortion (near-clipping).

Anyone know of a high-gain pentode that can handle 400 or so volts on the plate? I can find plenty of triodes (6SN7, etc), but the highest voltage pentodes I've found are about 330v (12BY7, etc). It will be seeing an extremely high load impedance on its output (driving a cathode follower), and will be run from a pentode voltage amp before it (probably either an EF86/7C7-type or a 12BY7/6CL6 type).



DF96 11th June 2008 04:10 PM

I used Duncan Amps TDSL tool. There are of course lots of big power pentodes which will do the job (use a high power driver and scrap the cathode follower?).

The only smaller pentode I could find was 7189A - a souped-up version of the 6BQ5 (EL84 to us Europeans). The 7189A will go to 400V.

Alternatively, take a chance with a smaller pentode. These can under dynamic conditions take high voltages - a SE output can drive the anode/plate voltage up well above the HT supply on signal peaks. Does the valve know the difference between driving a transformer from a 300V line, or driving a resistor from a 400V line? You might need to make some arrangement to limit the supply voltage during switch on, to avoid arcing. A low gain valve might be more robust - 6BW6 (6V6 in a small bottle)?

Yvesm 11th June 2008 04:43 PM

Remember that in a classic output stage, the plate voltage swings from (ideally) zero volt to the double of the supply.

That means that a 6BQ5 conservativly powered at 250 Vots sees it's plate rising near 500 Volts.

Have a look at very interesting penthodes primarly intended for video stage in B&W and Color TV, they are gems !

Browse Pete Millet site ;)

Here, a single one is able to deliver 300V pp:

Of course, you'll need some addotional current buffering if you intend to drive G2's !


pmillett 11th June 2008 05:06 PM

I've used 12BY7 as an error amp in an adjustable bench supply that has up to 500-600V DC plate-to-cathode. No problems.

I picked the 12BY7 after looking at the physical structure - the distance between the plate and other elements is very large.

The design-center rating is 300V, so as Yves noted, it should be OK up to 600V...


Rod Coleman 11th June 2008 05:31 PM

How about the C3m?

Cold anode spec (Vao) is 550V, and although the dc operating Va max is 300V, I am confident you would get away with:

B+=550V; Vg2=220V by divider, Rk=5,1K, Ra=100K; Ia would be 2mA and S would be around 3mA/V for a gain of ~300. It should be able to swing nearly 400V, and a small liberty with the Supply voltage should take it above that.

Your anode dissipation will be below 1W in a 4W valve, so getting quite near the Va0 value is much less likely to burn up the part too soon. And it's a 10000 hour rated SQ item in the first place!

Tubes4e4 12th June 2008 08:34 AM

When doing my tube characteristics plots/traces, I often run even tiny triodes or pentodes quasi-statically (for some minutes) at anode voltages of up to, say, 550-600V. This should not be an inivitation to exceed manufacturer specs regularly and thoughtlessly, though.

Besides (external and internal) isolation considerations, in my opinion one general but major issue with tubes running on massively exceeded (statical) anode voltages is the danger of cathode material spalling during heat-up time with the anode voltage already being present. So, first let the heater come up to regular operation temperature, then raise the anode voltage to the desired value slowly, but not abruptly.


Tom Schlangen

Rod Coleman 12th June 2008 10:16 AM


So, first let the heater come up to regular operation temperature, then raise the anode voltage to the desired value slowly, but not abruptly.
Tom, sounds like you're recommending some TV Boost diodes (damper diodes with nice slow warm-up) for mains rectification duty, a very good idea!


bedeutet dieses Wort - "verderbend, (spoiling)"?

Tweeker 12th June 2008 10:28 AM

Wikipedia-Spall are flakes of a material that are broken off a larger solid body and can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure (as in a ball bearing). Spalling and spallation both describe the process of surface failure in which spall is shed.

Seen often with concrete.

oldeurope 12th June 2008 10:32 AM

Hi Gordon,
E(C)L80 :)

Tubes4e4 12th June 2008 03:59 PM

Rod, Tweeker,

well, translating technical terms by picking from several alternatives from a dictionary sometimes gives funny results :) but I think "spalling" is quite to the point - at least better suited than "blistering", I think.

The cathode paste surface can get micro-cracks under such conditions, with little "chips" or "spalls" literally being ripped off the surface by the high tension.

This high tension problem is a general one not only on heat-up but also continuous service with indirectly heated cathodes and even with dull emitters (coated and directly heated cathodes).

That is why bright emitters (uncoated directly heated pure tungsten cathodes) are preferred for high tension and high power duty, like with transmitting tubes running typically at anode voltages starting with, say 1kV and above.



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