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Old 08-13-2011, 07:59 PM
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David Roper David Roper is offline
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Well-kept secret?

I refer to the apparent early obsolescence of a common tube.

I've noted the use of some odd and/or anachronistic tubes in postwar TV sets, from 1-V to 117N7. But one tube I have never seen used in any postwar set is the seemingly very common 6F6. Every other post-fat-pin output tube imaginable can be found in 40s and 50s sets, including a pseudo-12 volt version of 6F6, namely the 12A6. I thought I'd found a rare exception when a 6F6 turned up in a 16" Motorola set, but it turned out to be nothing more than a doofus mindlessly filling an empty 6H6 socket with what was handy.

RCA tube manuals continued to highlight the 6F6 in their main sections for many years after it ceased being incorporated into new equipment. Is there a postwar set using 6F6 which has escaped my notice?
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Old 08-14-2011, 08:46 PM
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Those RCA post war projection sets used a pair of them in push-pull, on the separate amplifier chassis.
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Old 08-14-2011, 10:21 PM
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There we go, that counts. If we go back to prewar sets, then RCA TRK-9 and -12 also had push-pull 6F6s for the sound, while GE prewar sets used one as the video amp.
Meissner kit literature also shows 6F6 for video amp but the few surviving sets I'm aware of use a 6AC7.
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Old 08-14-2011, 11:09 PM
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I think the 6V6 made the 6F6 obsolete in most applications by the late 30s.
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Old 08-14-2011, 11:54 PM
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After the war TVs were still expensive, and many tubes were in short supply so if it was avaliable, cheap, and could do the job in it went.
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Old 08-15-2011, 12:29 AM
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Quote:
Originally Posted by maxhifi View Post
I think the 6V6 made the 6F6 obsolete in most applications by the late 30s.
Maybe, but it seems odd that the 6F6 was entirely superseded while the 6K6 continued to thrive. 6K6 is nothing but a derated 6F6 yet the RCA 630 uses what--five of them (and no 6V6s)?
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Old 08-15-2011, 12:37 AM
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Originally Posted by Electronic M View Post
After the war TVs were still expensive, and many tubes were in short supply so if it was avaliable, cheap, and could do the job in it went.
There's more to it than that though. There are some eccentric designs lurking in old service literature, such as sets for use on 110 volts DC. Sometimes it's fun just to browse old Rider volumes like one would a catalog just to see how creative the engineers could be.
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Old 08-15-2011, 12:57 AM
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Quote:
Originally Posted by David Roper View Post
Maybe, but it seems odd that the 6F6 was entirely superseded while the 6K6 continued to thrive. 6K6 is nothing but a derated 6F6 yet the RCA 630 uses what--five of them (and no 6V6s)?
Hmm you raise an interesting point. I was just thinking how sharply the transition was made in radios.. after the 6L6 and 6V6 came out in 1937, the 6V6 very quickly became the go-to medium sized power tube. I can't think of any post-war radios with a 6F6 output stage either, although someone could probably prove me wrong. It would be the exception though, and I also don't know of any hi-fi amps with 6F6 output stages, so the 6V6 must have had some clear advantages.

As for the 630 using a quantity of 6K6's, well, it's anyone's guess. I am sure some very strong opinions existed within RCA at the time, and chances are most of them are lost to time. It's fair to say though that not long after the 630, specialized TV types took over future designs, rather than general purpose radio tubes used in the early sets.

The 6K6 did not endure the same way the 6V6 has - the 6V6 survived in TVs well into the 60s as the 6AQ5, and even lives on today. I was in a guitar store a couple months ago and saw a brand new Made in China Fender Champ with a 6V6 and a 12AX7 in it, for less than $200. That's some amazing longevity, I wonder if someone will still be making 2N3055's in 2043!

Last edited by maxhifi; 08-15-2011 at 01:03 AM.
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Old 08-15-2011, 08:32 PM
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I think the Hallicrafters S-40A communications receiver still used a 6F6 output and 80 rectifier up through the late 40's/early 50's...some of the later models may have replaced the 80 with a 5Y3 but I think they all used the 6F6.
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Old 08-15-2011, 09:20 PM
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i have an s-40, just went to look at it - i thought it had a 6VGG, and yes, mine does, but 6F6 is clearly marked beside the tube socket! That is a very solid radio, i think i will fire it up tonight, i haven't in a coule years
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Old 08-16-2011, 06:28 PM
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6F6 output

My RCA model 19K uses two 6F6s by design as the push pull output .
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Old 08-23-2011, 03:11 PM
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Count on Stromberg-Carlson to use parts well past their prime (just like their design style...). AR-37 amp used push-pull 6F6s (1952 Sams folder, possibly earlier). Also the AR-410 amp (1953). And the SR-405 mono receiver used 6F6s (1954). Sams folders and other information are on my Stromberg page: http://www.audiophool.cjb.net/MadeInRoch.html
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Old 11-05-2018, 04:32 PM
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I hate to drag up an ancient thread, but I've had the opportunity to restore the Stromberg-Carlson AR-410 amplifier (albeit for someone else) that Tom Bavis mentioned. Mein Gott! Talk about a very nice sounding amplifier. And it measures much better than the typical 6V6 design from the same era. I was getting a solid, actual 10 watts out of it, cathode bias of course, with only 0.5% THD and negligible IM once recapped and cleaned up. All this with a rather shoddily designed 12AT7 input and phase splitter stage to boot! It does bust the plate and screen voltage specs, but not terribly. I would still expect acceptable life from the 6F6 outputs.

Most cathode bias 6V6 push pull amps produce 8 to 10 actual watts of audio power on the secondary, at 1% or more THD with ear-ripping IM.

I am seriously considering doing a partial clone of the Stromberg 6F6 amplifier. The input stage is less than ideal; a 12AT7 makes a pretty terrible voltage amplifier and a marginal cathodyne if you're after low THD. A much better front end would consist of say a 6SF5 or 6F5 DC coupled to a 6J5 or 6C5 running as the phase splitter. The 6SF5 is much happier with ~90 volts on the plate than a 12AT7 would be. You could probably get down around 0.1% THD to 0.2% THD at clipping with a similar, moderate amount of feedback and the front end I described. A 12DW7 would also work admirably in such an arrangement and would save a hole in the chassis and 0.3 amps of heater current.

The 6F6 should have lived on longer as it is an exceptionally linear pentode... once you bust the plate and screen voltage specs a bit.
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Old 11-06-2018, 10:15 AM
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Quote:
Originally Posted by benman94 View Post
I hate to drag up an ancient thread, but I've had the opportunity to restore the Stromberg-Carlson AR-410 amplifier (albeit for someone else) that Tom Bavis mentioned. Mein Gott! Talk about a very nice sounding amplifier. And it measures much better than the typical 6V6 design from the same era. I was getting a solid, actual 10 watts out of it, cathode bias of course, with only 0.5% THD and negligible IM once recapped and cleaned up. All this with a rather shoddily designed 12AT7 input and phase splitter stage to boot! It does bust the plate and screen voltage specs, but not terribly. I would still expect acceptable life from the 6F6 outputs.

Most cathode bias 6V6 push pull amps produce 8 to 10 actual watts of audio power on the secondary, at 1% or more THD with ear-ripping IM.

I am seriously considering doing a partial clone of the Stromberg 6F6 amplifier. The input stage is less than ideal; a 12AT7 makes a pretty terrible voltage amplifier and a marginal cathodyne if you're after low THD. A much better front end would consist of say a 6SF5 or 6F5 DC coupled to a 6J5 or 6C5 running as the phase splitter. The 6SF5 is much happier with ~90 volts on the plate than a 12AT7 would be. You could probably get down around 0.1% THD to 0.2% THD at clipping with a similar, moderate amount of feedback and the front end I described. A 12DW7 would also work admirably in such an arrangement and would save a hole in the chassis and 0.3 amps of heater current.

The 6F6 should have lived on longer as it is an exceptionally linear pentode... once you bust the plate and screen voltage specs a bit.
A poor output transformer design is probably responsible for much of the distortion in many designs.
I know an amp designer that uses 12AT7's and even 6BK7's in the voltage amp and invertor stages.
I'll have to see if I have a Sams on the various S-C amplifiers.
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Old 11-07-2018, 05:27 PM
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Quote:
Originally Posted by dieseljeep View Post
A poor output transformer design is probably responsible for much of the distortion in many designs.
I know an amp designer that uses 12AT7's and even 6BK7's in the voltage amp and invertor stages.
I'll have to see if I have a Sams on the various S-C amplifiers.
I would agree up to a point; at the extremes of frequency the OPT must be the limiting factor. Mid-band however the output stage itself is probably the dominant term in the total distortion of the amplifier, followed by the input stage. A cathodyne operates ~ unity gain with a tremendous amount of feedback, so it's term can be dropped as negligible.

Now, that said, look at the plate curves of a 12AT7 and compare them to a 6F5, 6SL7, 5751, or 12AX7. They are all substantially more linear triodes than the 12AT7.

A 12AT7 would not be my first choice in a voltage amp. I think it is popular only because a) 12DW7s aren't cheap and b) it had better drive capabilities than a 12AX7, 6SL7, or 5751 when the second section is used as a cathodyne.

Another, superior option would be a 6AV6 with the diodes tied to ground and a 6C4 running as the cathodyne. Lots of ways to set up the voltage amp up front that would work.

My absolute favorite circuit for an input stage is the Van Scoyoc cross-coupled inverter. Gain is high, easy to wrap a feedback loop around it without concerns for stability at the high and low end, it has pretty decent drive ability all things considered, and the balance is only bested by a long tailed pair with a CCS in the tail, or a cathodyne. Balance is typically held to a factor of 1/mu+1 for the final, high gain triodes. For a 6SL7, that's about 1.4 percent, for a 12AX7 about 0.99 percent.
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