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Old 2nd May 2009, 07:29 AM   #1
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Default Beam pentodes and suppressor-grid pentodes of same type

I just acquired a small lot of RCA 6BA8A for a vintage amp of mine... but, here's the kicker - half of them have suppressor grids, and the other have beam-forming plates. I did notice a significant difference in sound (before realizing that they were different tubes inside) - the pentodes being "drier" and more odd-order-harmonic-sounding than the beamers, which have a clear but more "colorful" sound (at least in this amp). I have no real preference, but it was a shock to see!

So - this raises an interesting consideration - even RCA switched things up a bit in their tube production. These aren't super-late-production tubes either, although the beam versions do seem to be a few years later than the pentodes. Maybe this is why some pentode data sheet curves look suspiciously like beam tube curves - of course, the 6BA8 curves clearly show that they were originally designed as pentodes
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Old 2nd May 2009, 07:10 PM   #2
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Quote:
even RCA switched things up a bit in their tube production
Possible, but it is likely that some of the tubes were made by RCA, and the othere were made by someone else and relabled by RCA. This was quite common. I have seen RCA branded tubes made by several different vendors.

Look at the type number. True RCA tubes have the "RCA stop sign" shape surrounding the number. GE made tubes have the GE dots below the number, and may have their EIA code number 188 somewhere on the tube. Sylvania made tubes will have the type number in block text with USA underneath them. There are some Japanese made tubes that have an elongated stop sign logo. RCA sold these too.

I am looking at a GE brand tube with the 188-5 code on it. There is the block text 6LR8 with USA under it indicating that Sylvania actually made this tube.
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Old 2nd May 2009, 11:38 PM   #3
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Default Re: Beam pentodes and suppressor-grid pentodes of same type

Quote:
Originally posted by jon_010101
So - this raises an interesting consideration - even RCA switched things up a bit in their tube production. These aren't super-late-production tubes either, although the beam versions do seem to be a few years later than the pentodes. Maybe this is why some pentode data sheet curves look suspiciously like beam tube curves - of course, the 6BA8 curves clearly show that they were originally designed as pentodes
All they had to do was meet the published specs for the type. Didn't matter how they did that. You see the same deal with 6BQ5s: some of these are pents with the suppressor grids, and others are beam formers.
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Old 3rd May 2009, 12:18 AM   #4
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Quote:
Originally posted by tubelab.com

Possible, but it is likely that some of the tubes were made by RCA, and the othere were made by someone else and relabled by RCA. This was quite common. I have seen RCA branded tubes made by several different vendors.

Look at the type number. True RCA tubes have the "RCA stop sign" shape surrounding the number.
Believe it or not, both are "real" RCA USA with the stop sign - the marking is identical for the beamers and the pentodes... both have the same halo getters, and the triode sections look identical. Actually, even the pentode plates are the same, until you notice the beam formers inside, and the different connections at the mica. Maybe they combined two tube types, or found that this configuration worked better. Unfortunately my camera doesn't do good macro, or I'd post some pics.

Quote:
Originally posted by Miles Prower
All they had to do was meet the published specs for the type. Didn't matter how they did that. You see the same deal with 6BQ5s: some of these are pents with the suppressor grids, and others are beam formers.
Well that's interesting! Even if the performance is the same at small signals, with similar specs, the behavior at the knee, and tolerance of mis-matched loads should be different. I'd be curious to see curve trace results for each.
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