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Help me understand - 7061 vs. 12ab5 (and the rest of the 6V6 family)

ericj

Member
2008-12-08 10:24 pm
I know the 6v6 family is a big family.

Recently I came across some 7061 tubes. There doesn't seem to be a lot of information about them out there, and it's muddied terribly by the existence of a furnace part with the same designation. Sometimes google can suck.

I've only found the one RCA sheet for the 7061:

http://www.mif.pg.gda.pl/homepages/frank/sheets/049/7/7061.pdf

It's said to have been 'derived from' the 12ab5. Indeed, the spec sheets are very similar.

inter-electrode capacitances are the same.

Heater specs have subtle differences. Both are specified for use in systems powered by 6-cell storage batteries, but the 12ab5 sheets give 12.6v as the nominal heater voltage and the 7061 sheets give 13.5v as the nominal heater voltage -- perhaps just a realization that in a car you're going to have a charging system increasing the voltage?

I've seen people assert that the 12ab5 heater is DC only, but I've seen at least one version of the spec sheet that clearly states AC or DC.

What sticks out to me is the maximum ratings in class A1.

12ab5: 315v plate, 285v screen, peak heater to cathode 90v, max plate dissipation 12w.

7061: 345v plate, 310v screen, peak heater to cathode 120v, max plate dissipation 9w?

Why three less watts?

The spec sheets go on mostly the same, except the examples are given at different voltages.
 
Well they both seem to have the same size plate and the same g2 dissipation spec. and similar heater power. 7061 was registered by RCA in 1956, and 12AB5 was registered by TungSol in 1955. Maybe it's just a reliability issue, due to the tube being hard to get to in a dashboard radio. The 12AB5 datasheet does say it's got an oversized plate, but it looks the same. Looks like a small grid radiator fin above the top mica in the 12AB5 (right pic), can't tell for the 7061 (left).
 

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One difference I see now.
In the operational data. The 12AB5 is drawing 10% of plate current for screen current (4.5 mA per 45 mA) (6V6 is also 4.5 mA per 45 mA). The 7061 is drawing 25% of plate current for screen current (9 mA per 35.5 mA). You would think that similar designs (gm, Rp) would have similar screen current ratios? The higher screen current would limit the power that could be used. 12AB5 are only $3 at ESRC. 7061 looks like you will NEVER find any replacements.

Another 6V6 copycat is the 6JC5/6JB5/6HE5/6EZ5 series. These tubes have squarer plate curve knees than 6V6, (normally preferred for efficiency, couldn't say how it sounds compared to 6V6 though) and even lower screen current. 3.5mA per 43 mA. The 6JC5 is rated 19 Watts and is 12 pin. The 12 Watt 6EZ5 fits the same octal socket as the 6V6.

6JC5 is $5. 6EY5/6EZ5 is $3 at Vacuumtubes.net. (6EY5/6EZ5 was on the $1 list at Vacuumtubes.net several years back)
 
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It's odd that RCA would come out with such a tube as the 7061 a year after the 12AB5 was released. One would expect something to be "improved" about it, not worse. Would seem like efficiency would be important for battery powered stuff, but maybe not in a car. Maybe they just saw it as a mass produced tube for the auto market and it was kept "cheap" by not aligning the screen grid well. (maybe just a loose spec, ie low quality control on alignment provided) If you have some 7061 and they have some kind of plate "window" to observe grid wire alignment, could check to see if they are aligned and/or sort out ones that are well aligned. Those might meet the 12AB5 spec then.

There was a 6JC5 (DIY) amplifier that got mentioned here a few years back in a thread about the 6V6. They claimed it was an improved 6V6. No measurements were offered. I assume it sounds OK then. I recall George (Tubelab) commenting on the 6EZ5 once too. Seeing how much cheaper the 6JC5 or 6EZ5 is than a 6V6, certainly some incentive to try it.
 
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I like tubes that offer more current capability in general, since the OT primary Z can be lower for better OT performance. The 6JC5 offers a 75 mA DC spec versus the 40 mA DC spec of the 6V6. If it sounds the same as the long respected 6V6, that would make it a winner. Only issue would be that one would also like to see increased gm to go with that increased current level, so that no more drive voltage would be needed.

TV Horiz. sweeps of similar wattage (like 18 Watt for 21HB5) offer around a 230 mA DC spec. But one pays for it with more heater power. They do offer more gm to go with that current at least.
 

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6EY5/6EZ5 was on the $1 list at Vacuumtubes.net several years back......I recall George (Tubelab) commenting on the 6EZ5 once too.

It was on ESRC's dollar list too, and Stan had thousands in stock. I mentioned it, and POOF, they all disappeared. An "Asian buyer" grabbed ALL of them. All the 6EZ5's that I have (about 35) appear to be identical GE made regardless of brand. They worked very well in a low voltage (325 volt) cathode biased SSE which I built for 6V6GT's.

Ditto the 6HB6. We talked about then, and 7,000 bulk packed green label GE's vanished, another "Asian buyer." The jury is still out on these tubes since there seems to be at least 3 different versions, one of which is very different from the other two. I do know that the green label GE's worked good in a push pull amp making 25 watts without issue, but all were gone by the time I figured this out.

It's odd that RCA would come out with such a tube as the 7061 a year after the 12AB5 was released.

Note the specific testing listed at the end of the original RCA data sheet. Cycled testing with 17 volts on the heater, and 2.5G's vibration testing at 25 Hz. This tells me that RCA had a contract to fulfill, probably for a military Jeep, that would allow them to put a new number on the tube, test them to this spec, and charge the government a lot of money for it.
 

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12AB5 arrived for car radios with the change from 6V to 12V batteries, but power transistors and the space charge tubes appeared not much later, so had limited use. Webcor and Wollensak used them in tape recorders and console amps. The 7061 was used in mobile radio, maybe even CB radios, and may have been specified mainly for class C use. They were typically in the shorter 6CG7 size, 12AB5 were usually 6BQ5 size, though I have shorter ones too.