Output Tube Type Comparison. Perfect Amp format. Twelve + years of use.

[arlum]

I own a Cornford Carrera. IMO .... one of the best Class A amps ever made. It's also possibly the best amp for doing head to head tube comparisons. I say that because it works with virtually any type of tube, uses a solid state rectifier keeping tube rectification out of the mix so it fits better with modern amp build types and uses only on output tube. The tube in play is the tube being compared. I'll include a link here for those interested in more on the amp itself.

Cornford Amplification Carrera review

It was back in 1999 that UK maker Cornford Amplification launched its first Harlequin model. It was a radical move: a full-sized 1 x 12 combo that kicked out just six watts.Since then, many other manu

www.musicradar.com

Over the years I've put many hours on the Cornford Carrera using any tube type I could lay my hands on. For most tube types I've used multiple brands. They do differ but not so much about type. The Carrera has two tube slots. An 8 pin and a 9 pin with a switch on the top plate to select between the two. Here again I scored with this purchase because my all time favorite output tube is an EL-84 and it's always onboard. It allow me to campare my "cream" to the best of all other tube offerings. At this point please note ..... my EL-84s are not the best for all applications. With this amp, whenever the EL-84s just aren't delivering to my expectations, I can compare them head to head with all the other tube types and find the one that is most perfect for the particular recording I'm currently working on. The second output tube slot will handle most all of the other output tube types other than some of the recent extreme low output tunes, (1 watt / 2 watt), that weren't considered when the Cornford was designed. Tubes I've used in the second position include 6V6, 6L6GC / 5881, EL34, KT66, KT77, KT88, and 6550. I've also tried a few others but they didn't earn enough respect to mention them.

I love clean tones and like the American clean tone the best. Because of that the 6L6GC is my first go to for those tones when my EL-84 doesn't deliver.

While I dislike British crunch I do very much like the milder tweed crunch and have always preferred 6V6 tubes over EL-84s for this application.

KT66 tubes don't get used much with the Carrera because the amp itself tends to be a bit on the dark side. I love KT66's for the way they add body to otherwise bright amps but not so much with the Carrera. That said ..... I always have one in stock and will use it during times I want the Carrera to have a very smooth thick tone. Some of Santana's instrumentals sound their best using this tube.

If I had to pick a second favorite tube after the EL-84 it would be the KT-77. As I've already said I'm not a fan of British crunch but I'm a huge fan of American clean. The KT-77 is like the perfect blend of an EL-34, (British crunch), and a 6L6GC, (American tone). It adds all the good tonal parts of an EL-34 to the tone of a 6L6GC. More mids. More human voiced. Minus the crunchy British breakup.

Tubes labelled 5881 are supposed to be the same as 6L6GCs but they always sound a little brighter and don't, (to my ears), break up in the same way. They seem too controlled. The breakup is like comparing an audio vs. linear tone pot. There's no magic spot.

KT88s are just to much for my uses. They seem more focused on output volume than anything else. I've tried two different recommended brands but they just didn't work for me.

6550s don't work for me. They just don't have the musical quality of the other tubes and I no longer keep them in stock.

 

[CoolBlueGlow]

"Tubes labelled 5881 are supposed to be the same as 6L6GCs but they always sound a little brighter and don't, (to my ears), break up in the same way. They seem too controlled. The breakup is like comparing an audio vs. linear tone pot. There's no magic spot."

I don't delve into subjective judgments about tube tones, but I certainly appreciate your work in trying different types.

However, from a design and engineering perspective the 5881 is definitely not a 6L6GC. The nearest JAN numbered beam tetrode to the 6L6GC would be the 7581/7581A, which is electrically identical, but with the low loss mica base borrowed from the 5881, a step which kept the JAN folks happy. But the 5881 and 6L6GC are definitely different tubes in several critical features, even though we guitar types swap them with some impunity.

Historically, the 5881 was originally developed in the early 1950's as an uprated 6L6G under the auspices of military guidance through the JAN program, to meet the requirement for a ruggedized and physically smaller enveloped 6L6G for certain military applications (i.e. electronic servo control amplifiers in the then new B-47 and YB-52A, etc. see U.S. Patent 2524053A for an example of the circuit) The 5881 had features like extra strong spot welds, larger getters, low loss bases, larger diameter support rods and a host of other refinements to make it more rugged. The plate dissipation rating of the 5881 reflects its similarity to the 6L6GB, with which it shares some features. (To give you some idea of the demand, a B-52 used nearly a thousand vacuum tubes in just the rear gun laying control and the bombing radar!)

In contrast, the 6L6GC which happened in the very late 1950's was a different refinement of the 6L6GB. It was a consumer tube through and through and started life as a response to the demand for higher power tetrodes in the then rapidly growing Hi-Fi market of that era. (Tung-Sol's 6550 and the English made KT-88 were kings of the hill at that time.) I have written extensively on this elsewhere, so I'm not going to repeat it at length, but the main secret sauce of the 6L6CG was the explosion-forged five layer plate material (Iron/copper/aluminum). THIS is the reason a 6L6GC has the "C" designation and it is the sole reason it can dissipate approximately 25% more power than the B version (or the 5881) Other than that, a 6L6GC is mechanically identical to a 6L6GB, literally coming off the same tooling (excepting the anode material)

The 6L6GC, the 7581A, the 7189A, and eventually the 6550A all benefitted from the direct application of this innovative materials work. Most obviously, no new tooling was required for these types - only a change in plate materials - to make an immediate 26% increase in available plate dissipation against the same exact types made without five layer plates. As you might imagine, this put GE at considerable market advantage against RCA, Tung-Sol, Mullard, and others. It was such a huge advantage that other manufacturers immediately licensed it (or did as soon as GE would let them

They did this because the cost and time to develop and market an all new tetrode (like RCA's 8417 for example) was quite high. With the advent of explosion forged plate material, I would suggest that the 6L6GC and the 6550A essentially spelled the end of the line for the audio tetrode arms race. (not mentioning exotics like the Bendix 6384, of course). Within ten years, transistors would dominate the market, and there would simply be no market impetus to go further with power tetrode or pentode development. Big power tubes were a dying species (or so the manufacturers all thought.)

It is therefore not surprising that 5881 types perform somewhat differently than the 6L6GC in your application. But the engineering facts demand that they will produce less power, and therefore reach their 2% and 10% distortion characteristics at a lower volume in the same circuit - as compared to the 6L6GC - for the reasons I've outlined above.

Cheers!
CBG

 

[BB]

POOOOOOOOOOOF!!!!! The sound of my brain exploding! Cornfords are very cool amps. I've not had the pleasure to try one for myself, but I remember the glowing reviews stating it's tonal and dynamic response were pretty darn amazing.

I love dynamic, touch sensitive amps that seem to connect you with your guitar....... congratulations on a super cool amp.

 

[uriah1]

I own a Cornford Carrera. IMO .... one of the best Class A amps ever made. It's also possibly the best amp for doing head to head tube comparisons. I say that because it works with virtually any type of tube, uses a solid state rectifier keeping tube rectification out of the mix so it fits better with modern amp build types and uses only on output tube. The tube in play is the tube being compared. I'll include a link here for those interested in more on the amp itself.

Cornford Amplification Carrera review

It was back in 1999 that UK maker Cornford Amplification launched its first Harlequin model. It was a radical move: a full-sized 1 x 12 combo that kicked out just six watts.Since then, many other manu

www.musicradar.com

Over the years I've put many hours on the Cornford Carrera using any tube type I could lay my hands on. For most tube types I've used multiple brands. They do differ but not so much about type. The Carrera has two tube slots. An 8 pin and a 9 pin with a switch on the top plate to select between the two. Here again I scored with this purchase because my all time favorite output tube is an EL-84 and it's always onboard. It allow me to campare my "cream" to the best of all other tube offerings. At this point please note ..... my EL-84s are not the best for all applications. With this amp, whenever the EL-84s just aren't delivering to my expectations, I can compare them head to head with all the other tube types and find the one that is most perfect for the particular recording I'm currently working on. The second output tube slot will handle most all of the other output tube types other than some of the recent extreme low output tunes, (1 watt / 2 watt), that weren't considered when the Cornford was designed. Tubes I've used in the second position include 6V6, 6L6GC / 5881, EL34, KT66, KT77, KT88, and 6550. I've also tried a few others but they didn't earn enough respect to mention them.

I love clean tones and like the American clean tone the best. Because of that the 6L6GC is my first go to for those tones when my EL-84 doesn't deliver.

While I dislike British crunch I do very much like the milder tweed crunch and have always preferred 6V6 tubes over EL-84s for this application.

KT66 tubes don't get used much with the Carrera because the amp itself tends to be a bit on the dark side. I love KT66's for the way they add body to otherwise bright amps but not so much with the Carrera. That said ..... I always have one in stock and will use it during times I want the Carrera to have a very smooth thick tone. Some of Santana's instrumentals sound their best using this tube.

If I had to pick a second favorite tube after the EL-84 it would be the KT-77. As I've already said I'm not a fan of British crunch but I'm a huge fan of American clean. The KT-77 is like the perfect blend of an EL-34, (British crunch), and a 6L6GC, (American tone). It adds all the good tonal parts of an EL-34 to the tone of a 6L6GC. More mids. More human voiced. Minus the crunchy British breakup.

Tubes labelled 5881 are supposed to be the same as 6L6GCs but they always sound a little brighter and don't, (to my ears), break up in the same way. They seem too controlled. The breakup is like comparing an audio vs. linear tone pot. There's no magic spot.

KT88s are just to much for my uses. They seem more focused on output volume than anything else. I've tried two different recommended brands but they just didn't work for me.

6550s don't work for me. They just don't have the musical quality of the other tubes and I no longer keep them in stock.

I think the 5881 were more akin to a 6L6GB.. not the GC

 

[dukewellington]

"Tubes labelled 5881 are supposed to be the same as 6L6GCs but they always sound a little brighter and don't, (to my ears), break up in the same way. They seem too controlled. The breakup is like comparing an audio vs. linear tone pot. There's no magic spot."

I don't delve into subjective judgments about tube tones, but I certainly appreciate your work in trying different types.

However, from a design and engineering perspective the 5881 is definitely not a 6L6GC. The nearest JAN numbered beam tetrode to the 6L6GC would be the 7581/7581A, which is electrically identical, but with the low loss mica base borrowed from the 5881, a step which kept the JAN folks happy. But the 5881 and 6L6GC are definitely different tubes in several critical features, even though we guitar types swap them with some impunity.

Historically, the 5881 was originally developed in the early 1950's as an uprated 6L6G under the auspices of military guidance through the JAN program, to meet the requirement for a ruggedized and physically smaller enveloped 6L6G for certain military applications (i.e. electronic servo control amplifiers in the then new B-47 and YB-52A, etc. see U.S. Patent 2524053A for an example of the circuit) The 5881 had features like extra strong spot welds, larger getters, low loss bases, larger diameter support rods and a host of other refinements to make it more rugged. The plate dissipation rating of the 5881 reflects its similarity to the 6L6GB, with which it shares some features. (To give you some idea of the demand, a B-52 used nearly a thousand vacuum tubes in just the rear gun laying control and the bombing radar!)

In contrast, the 6L6GC which happened in the very late 1950's was a different refinement of the 6L6GB. It was a consumer tube through and through and started life as a response to the demand for higher power tetrodes in the then rapidly growing Hi-Fi market of that era. (Tung-Sol's 6550 and the English made KT-88 were kings of the hill at that time.) I have written extensively on this elsewhere, so I'm not going to repeat it at length, but the main secret sauce of the 6L6CG was the explosion-forged five layer plate material (Iron/copper/aluminum). THIS is the reason a 6L6GC has the "C" designation and it is the sole reason it can dissipate approximately 25% more power than the B version (or the 5881) Other than that, a 6L6GC is mechanically identical to a 6L6GB, literally coming off the same tooling (excepting the anode material)

The 6L6GC, the 7581A, the 7189A, and eventually the 6550A all benefitted from the direct application of this innovative materials work. Most obviously, no new tooling was required for these types - only a change in plate materials - to make an immediate 26% increase in available plate dissipation against the same exact types made without five layer plates. As you might imagine, this put GE at considerable market advantage against RCA, Tung-Sol, Mullard, and others. It was such a huge advantage that other manufacturers immediately licensed it (or did as soon as GE would let them

They did this because the cost and time to develop and market an all new tetrode (like RCA's 8417 for example) was quite high. With the advent of explosion forged plate material, I would suggest that the 6L6GC and the 6550A essentially spelled the end of the line for the audio tetrode arms race. (not mentioning exotics like the Bendix 6384, of course). Within ten years, transistors would dominate the market, and there would simply be no market impetus to go further with power tetrode or pentode development. Big power tubes were a dying species (or so the manufacturers all thought.)

It is therefore not surprising that 5881 types perform somewhat differently than the 6L6GC in your application. But the engineering facts demand that they will produce less power, and therefore reach their 2% and 10% distortion characteristics at a lower volume in the same circuit - as compared to the 6L6GC - for the reasons I've outlined above.

Cheers!
CBG

Good infos… I know it’s somewhat off the topic of the OP but I wanted to point out that what you said about licensing vs. new tech development with regard to other tube companies, all in competition. There is a lot of confusion about who made which tube, and when, in the guitar and music communities at least, and this is often misunderstood or overlooked. GE was absolutely a huge player (especially with the Ken-Rad plant) and their licensing of technology and batch manufacturing to all the other companies making tubes… no doubt with variation as we see in the performance of all these different tubes.

 

[northernguitar]

FWIW, I think the best EL84 is the Soviet 6P14P-EV. I've been using these for over a decade and no other modern tube comes close to its rugged reliability. I have no gauge, but I believe I got over 2000+ hours out of a pair before one crapped out. Impressive. My thoughts on how much output tubes affect tone are they don't. But these sound as good as any other and I run my amps full-tilt. I just picked up a pair off of a Ukraine vendor on eBay. I didn't even need them, I just wanted to shoot some business to someone in Kiev. I got them in less than three weeks for less than a pair of new JJ's.

 

[VintageSG]

FWIW, I think the best EL84 is the Soviet 6P14P-EV.

+1 on the above.
They sound great, they last ( seemingly ) forever, and they're not too expensive. Bit of a bargain really.

 

[CoolBlueGlow]

Good infos… I know it’s somewhat off the topic of the OP but I wanted to point out that what you said about licensing vs. new tech development with regard to other tube companies, all in competition. There is a lot of confusion about who made which tube, and when, in the guitar and music communities at least, and this is often misunderstood or overlooked. GE was absolutely a huge player (especially with the Ken-Rad plant) and their licensing of technology and batch manufacturing to all the other companies making tubes… no doubt with variation as we see in the performance of all these different tubes.

Yes, indeed - and the amount of cross-branding also depends on the era in which the tube was manufactured. During the competitive era (1940's through mid 1960ish) There was relatively little intra-corporate licensing, and a lot of downright anti-competitive behavior. However, as the management of the various larger players observed the approaching end of the tube era, their song changed. They held various conflabs to figure out how to share the costs and risks of manufacturing during the "transitional era". The motive was almost exclusively to protect each company from warranty service issues on existing unsold inventory, and units in the field but still under warranty, and to reduce manufacturing duplication in order to improve profits without wasting investment on new infrastructure for what they saw as an obsolete product. To that end, the larger manufacturers agreed to share inventory and manufacturing responsibilties for specific lines of tubes and for a specific number of years. (Many television tubes were cross-branded this way, but also many audio and industrial tubes, for the same reason). Dr. Kathryn Harrigan, a professor at the Columbia Business School, wrote extensively on this topic in her doctoral thesis in the 1980's, "Strategies for Declining Business" At that time she interviewed a number of still living corporate leaders who were or had recently been representatives of major tube manufacturers. She gathered many insights directly from the people who did the work and made the decisions. They are of course now long deceased. I have corresponded with Dr. Harrigan and found her doctoral work to be most helpful in understanding the times. Her doctoral thesis was eventually published as a book by the same title. The tube industry was considered in Chapter 4. Well worth the read, if you can find the book. It is fairly obscure, but Dr. Harrigan is a delightful person and a wealth of information about the final years of the tube manufacturing industry.

 

[dukewellington]

Yes, indeed - and the amount of cross-branding also depends on the era in which the tube was manufactured. During the competitive era (1940's through mid 1960ish) There was relatively little intra-corporate licensing, and a lot of downright anti-competitive behavior. However, as the management of the various larger players observed the approaching end of the tube era, their song changed. They held various conflabs to figure out how to share the costs and risks of manufacturing during the "transitional era". The motive was almost exclusively to protect each company from warranty service issues on existing unsold inventory, and units in the field but still under warranty, and to reduce manufacturing duplication in order to improve profits without wasting investment on new infrastructure for what they saw as an obsolete product. To that end, the larger manufacturers agreed to share inventory and manufacturing responsibilties for specific lines of tubes and for a specific number of years. (Many television tubes were cross-branded this way, but also many audio and industrial tubes, for the same reason). Dr. Kathryn Harrigan, a professor at the Columbia Business School, wrote extensively on this topic in her doctoral thesis in the 1980's, "Strategies for Declining Business" At that time she interviewed a number of still living corporate leaders who were or had recently been representatives of major tube manufacturers. She gathered many insights directly from the people who did the work and made the decisions. They are of course now long deceased. I have corresponded with Dr. Harrigan and found her doctoral work to be most helpful in understanding the times. Her doctoral thesis was eventually published as a book by the same title. The tube industry was considered in Chapter 4. Well worth the read, if you can find the book. It is fairly obscure, but Dr. Harrigan is a delightful person and a wealth of information about the final years of the tube manufacturing industry.

So they circled the wagons to protect each other against warranty service. Hohner did something very similar with other accordion and harmonica manufacturers in the first decades of the 2th century, and ended up absorbing its competitors.

I looked at the abstract for Harrigan’s paper. Love how she went to the sources instead of speculating. I’m sending her an email to see if she’ll send it to me, as I am behind a paywall.