6L6GB versus 6L6GC?

[Musekatcher]

Just got a RK Twin, and it has four Electro-Harmonix 6L6GBs in it. Schem sez it should have 6L6GCs in it. Does anyone have experience with how much clean and power are gained/lost when subbing 6L6GBs for 6L6GC's? And, is there any risk to damaging circuits or transformers?

 

[Johnny Cache]

There is a wide range of 6L6 based tubes, not all are equal. Without my RCA tube reference book in front of me I'll have to say from memory that the 6L6GB (at least the old ones) had a lower PV rating that the 6L6GC. I'm not sure that will be a concern with your amp, just a note of caution. It could be they are 5881's. Today's tubes are all over the place with quality and ratings, and I've never been a fan of EH tubes. I've had more trouble with them than any other brand.

 

[uriah1]

Yea. The b version is pretty low. Close to 5881. IMHO

 

[vanr]

Unless those are really that Sovtek 5881/6L6WGB tube thing with EH paint on it. If so then you can't kill it.

 

[6942]

6L6....................19w dissipation
6L6G...................19w "
6L6GB..................19w "
6L6WGB/5881........23w "
6L6GC..................30w "

 

[Wyatt]

The risk would be to the tubes.

First a valve theory, using real North American 6L6 tube specs...

• 6L6GB/5881 has a published maximum plate voltage of 360VDC for a maximum dissipation of 19/23 watts
• 6L6WGB/5881 has a published maximum plate voltage of 400VDC for a maximum dissipation of 26 watts (these were later models than the ones above)
• 6L6GC has a published maximum plate voltage of 500VDC for a maximum dissipation of 30 watts

But all of these tubes put out the same output wattage at the same plate voltages (they all put out ~23 watts at 360 VDC on the plates with ~4.2K ohms of resistance). The reason the GC can put out more watts overall is because it can handle more plate voltage. Energy out requires energy in...tubes use the DC voltage being applied to their plates (anode) to create the voltage swing and amplify the AC signal voltage.

Valve theory over.

Now, all this goes out the door with modern tubes...especially modern tubes from New Sensor/Reflektor. The names/designations of these tubes has more to do with marketing and very little to do with historic tube data. When they do publish tube data, usually it's nothing more than a scan of 50-eyear old charts with their brand name added to the top...they aren't the same tubes those charts were design for. So, it's nearly impossible to match up tube specs and requirements for application. And many of their tubes are the exact same tube with a different name and designation and a different "published" tube data to match the name. It's a hassle sorting it all out and knowing which tubes to trust.

For instance...I can't find any web listing or data for a Electro Harmonix 6L6GB offered....only their 6L6EH, which was supposed to be a GC.

With a plate voltage of 469VDC in "High Power" mode (according to the schematic), the amp really needs a true 6L6GC installed that can handle that plate voltage. That's not to say the EH 6L6GB isn't robust enough for the application...but it's impossible to know without true, accurate tube data. If the EH really is a lower output tube, you'll probably notice a little more clean headroom by going to a GC.

 

[jefrs]

If it's a modern manufacture then in practice no difference.

 

[CoolBlueGlow]

6L6GB vs. GC

The terms B and C are now muddied up by poor behaviors of current mfg.
As usual, there is a back story. Here is the highly compressed version.

When discussing the original specs which warranted a tube variant designation, during the golden age of vacuum tube manufacturing RETMA (Radio Electronic Television Manufacturers Association) controlled the issuance of tube variant letters and numbers by common industry agreement amongst the major manufacturers of the time. RETMA granted the "C" designation in November 1958. Period RETMA literature does not indicate to whom they granted it, but "6L6GC" almost certainly occurred after General Electric demonstrated this new uprated 6L6 variant which they announced to the public in early 1960.

It was at the time a major success story for GE over rivals RCA and Sylvania.

Here's why.

At the time (circa 1958-59) The General Electric 6L6B and C were dimensionally and mechanically identical, being made on the same tooling. why does that matter? Simple - TOOLING was and is the name of the game in tube manufacture...it is expensive, requires tons of research, design, and development. Then one must build the line, staff and train, troublehsoot and publicize the new higher power tube, convince manufacturers to design and launch new devices which use it, etc. bottom line: A new high power tube line was major bucks.

While RCA and Sylvania were struggling with design/development/marketing of higher power and/or more compact NEW designs, (i.e. 8417) GE found a workaround for the faithful workhorse 6L6 and it was pure genius.

The technical difference which allowed the C to dissipate an additional 7 watts is strictly due to a newly developed plate material. That material consisted of five layers of dissimilar metal, specifically aluminum, steel, copper, steel, aluminum. These materials were composited by the then new and secret process of explosive forging. This composite material allowed the plate to evenly dissipate heat, resist deformation and thus dissipate an additional 7 watts. (See link below for "B" vs. "C" action photographs and an explanation by the original lead engineer at GE, Robert Moe. Mr. Moe was another extremely brilliant and all but forgotten contributor to the golden age. He didn't have an iPhone, computer, Twitter account or website either...

ANYWAY - The 6L6GC variant description was very likely approved by RETMA for General Electric after they demonstrated the uprated C variant in 1958.

Again, this went on while Sylvania and RCA were both struggling to launch higher power tubes NOT based on 6L6 tooling and specs. Meanwhile, GE quickly brought the 6L6GC to market. Fits existing designs...can be exploited for more power in new designs. Genius!

This quick turnaround was ONLY possible since no tooling changes were required. GE simultaneously applied the same explosion forged plate technology to other tubes, such as the 6BQ5, the 7189 and eventually the 6550 (which tells you what the "A" stands for in those tube variants!)

The Engineer in charge of this development was R.E. Moe, at GE Owensboro. Read his explanation of the story here - along with other tubes to which GE applied the technique. This is from April 1960...but the work was long done and the tubes were already in the marketplace by 1959.

http://n4trb.com/AmateurRadio/GE_HamNews/issues/GE Ham News Vol 15 No 1.pdf

The explosive forged anode material process was patented by Texas Instruments. Here's the Patent documentation. Applied for in late 1959 and granted in 1963.

http://www.google.com/patents/US3112185

Since TI (not GE) was sourcing the patented plate material, RCA and Sylvania quickly reverse engineered the idea...but GE was first.

TO the OP, THAT is the difference between the original B and C 6L6 variants.

 

[uriah1]

That could be..

The wimpier B version I referred to are recent 6l6GB out of Russia and China...which are akin to the lower 5881..not the early USA versions...

 

[screamin eagle]

I have an cathode biased amp that can take any variant of the 6l6 family. Sonic differences between the 6l6wbg (NOS Philips) and 6l6GC (NOS GE) (in this amp): wgb's seem to have more mids and compress a little--great bluesy tone. GC's seem to present a much stouter bass and treble to the amp--thumping presence and transparent highs.

Again, these sonic differences don't necessarily reflect the tone of these tubes in general (per say), as I can only relate what I hear in this particular amp with these tubes.

 

[CoolBlueGlow]

Hi Uriah!

Yes - agree and understand. Modern foreign production.

Bottom line, there is no mystery why the the current Chinese/Russian beam tetrodes cannot and do NOT dissipate as much power as golden era US versions. They don't use composite plates. That's why honest mfg. overseas can't really call their typical product a 6L6GC and why they won't RELIABLY dissipate as much power as a U.S. Made 6L6GC.

Very easy to verify the above. Break open the typical China / Rusky 6L6 tube of any brand and designator. Carefully immerse the plate structure in hydrochloric acid. In a few minutes, Nothing of the plate will remain. It is iron. (Maybe something has changed in the past few years with these newer foreign 6L6GC, but as recently as 2008 they were not copper core plates.)

In contrast, break open a General Electric 6L6GC date coded after 1/1960 and immerse the plates in the same. The aluminum and iron will dissolve away, leaving the copper plate core intact.

Voltage limits

With US made 5881/6L6GB/GC Max "design" voltage limits are not particularly a limiting factor (within reasonable limits). Leo was pushing 5881 in the high 400's in the early 60's without issue...within the current dissipation limits of the anode.

Voltage is most relevant when duty cycle is taken into account. One example would be the 6V6 (a cousin to the 6L6 and built using the same basic materials and construction features and tolerances)

You can see that confirmed in this tube data sheet...

(link removed)

Here, a common garden variety 6V6 is rated at 1200VDC for a vertical deflection amplifier application (At low duty cycle, of course! that is ALSO carefully specified)

5881 max ratings.

As I have researched this issue of plate materials over the past two years, I have mused that it is at least plausible that U.S. Made 5881 produced after about 1960 MIGHT contain composite plate material. The 5881 was developed for the USAF Strategic bomber program in the early 50's. The additional cost of switching 5881 to composite plate material would have been only pennies a tube, and would have offered substantial peak rating gains for the military application. Imagine GE and others might have thought of this...if not acted on it.

It would therefore be interesting to break open a 5881 made after 1960 and see what the plates are made of, using the acid test.

Would certainly go a ways toward explain 5881 oddities, like conflicting inconsistent 5881 power handling stories here and there on the forum and the web. If it were so, it would substantially clarify and uprate the REAL power rating of the typical 60's mfg. date 5881,

AND probably drive up the price of 5881 on Fleabay.

Cheers!
CBG

 

[bparnell57]

The 6550A from GE was specifically made using these specially forged plates for the Fender 400PS amplifier. No other 6550's will deliver 135 watts of power per pair in that amp.

 

[FiddlinJim]

CBG, I must agree that GE engineers are the greatest! After all, I is one, albeit of the software variety...

 

[SamClemons]

All that being said.....I would probably buy a set of replacement 6L6'S, probably try them in the amp. If it did not make a huge difference, I would run the old tubes into the ground and save the new tubes for eventual replacements.

 

[Little Jay]

What an impressive amount of tube knowledge here! A pleasure to read it all! (Gotta love this forum!)

 

[Musekatcher]

Yes, thanks for all the great knowledge shared. I believe I read that Fender was using re-labeled Sovtek tubes when this amp was sold. So, it sounds like these EH's aren't far off from the OEM tubes, assuming not much has changed in the design of tubes in the last 30 years of "modern" russian tube production. I'll just run these while I look for vintage and modern tubes to experiment with.

 

[Bill Hicklin]

Unfortunately modern Russian/Chinese tubes, even Czech ones, play fast and loose with the old RETMA designations and you really need to check the mfr's spec sheets. For a while SovTek was even marketing 6G6Gs as KT-66s!

---------------------------

Comment on a couple of the above comments: the 5881 was not the 6L6GB, it was the beefier "ruggedized" 6L6WGB. The plain GB had the same specs as the (metal) 6L6, (big-bottle) 6L6G and (small-bottle) 6L6GA, but in a straight tube.

 

[Silverface]

If it's a modern manufacture then in practice no difference.

Yep. And ditto CoolBlueGlow.

Labeling of most modern tubes is a complete joke. JJ is about the only one I trust for proper nomenclature.

Things went sideways way back when Sovtek/EH first "saved us" by releasing the Sovtek wafer-base 5881. We FINALLY had a new 6L6 type!

Except they weren't 6L6's. or even 5881's. They are a Russian servo tube that with some creative re-basing (and switching a few pinouts) worked as an amp tube, somewhat like a 6L6.

And their first 6V6's - with the all-black glass - started as "6L6-types", slightly failed specs but passed 6V6 specs - and got new smaller black glass envelopes hiding the Russian tube construction.

More recently GT's "Made in U.S.A." 6L6-GE's had enough of a percentage of parts made here to qualify for a made in USA label - and shipped to China for assembly, glass, stenciling etc.

So if you are talking about NOS tubes - check the RCA tube manual. It's easy to find a used copy, and very handy.

But it has NOTHING to do with modern tube specs.

 

[reddart]

Any issues or concerns with running RCA 6L6 GBs in a silverface twin? They are a little cheaper than the GCs on the bay.

 

[schmee]

Fender heads like the Bandmaster and Bassman often came with 5881's in them. The brown base Tung Sols were common. One of the few things I remember from way back when. Those amps run 425 volts or higher. I have never considered worrying about using WGB's in 6L6GC amps and I often have used WGB's to tame things a bit. I think they run about 20-25% lower power rating. Maybe I've been lucky, but the old BF/SF Fender 6V6 amps run very high in voltage too.

I'm not very familiar with the GB's though.

Those old Sovtek/EH wafer-base 5881's were indestructible! A bit harsh sounding, but if you needing volume they did it!