The original 6BG6 and 6BG6GA were 6L6GA's with a plate cap and a different pinout. 807's were also 6L6GA's with a plate cap and a different base. The limiting factor in the plate voltage on a 6L6 type tube is the spacing between pin 2 and pin 3. The plate cap removes that limitation and allows a much higher plate voltage. The screen rating is 350 to 375 volts depending on whose specs you read, but this one can be pushed a bit especially in triode.
As the 6L6GA was discontinued and replaced with the 6L6GB and 6L6GC there was still some demand for 807's and 6BG6GA's so there are a few of them around with 6L6GB and 6L6GC guts inside them. There were also some 6BG6GA's made by Sylvania and sold under the Sylvania brand or the Phillips ECG brand that contained 7027A guts. These are the ones to use if you are pushing the ratings a bit.
I got some of these when they were $6:
6BG6 To 6L6 Vacuum Tube List
A true TV horizontal sweep (line output) tube is not intended to be used as a linear amplifier, although most do just fine. It is used as a constant current switch in TV service. The control grid is the switch element and the screen voltage sets the current. This is why you find screen curves with G1 = 0 volts in the data sheets. The 6BG6 was not designed as a sweep tube, it just works good in that service.
Note, the vertical sweep tube (frame output) IS intended to run as a linear amp, and it operates as a single frequency SE audio amp (50 or 60 Hz) making 2 to 8 watts depending on the size of the TV. As a teenager I would rip the entire vertical sweep circuit from a discarded TV set to make guitar amps. The vertical output transformer will drive a speaker just as well as a deflection yoke.
How Does 6CB5A Compare To 6BG6G?
I also have a 6CB5A project breadboarded on the bench. Truth is it's been in residence there and dormant for quite a while but I'm trying to revive the project now that things at work have settled a bit. Mine is PP and was originally set up in pentode but I'll now be trying triode with a higher voltage PS. I've been thinking about trying some 6BG6G also. How do the two tubes sound in comparison? Since you built your amp with the 6CB5A, what did you like better about that tube?
I'm planning on trying a few different front end configurations. Current one uses 6GK5s in paraphase.
Thanks . . . Charlie
I also have a 6CB5A project breadboarded on the bench. Truth is it's been in residence there and dormant for quite a while but I'm trying to revive the project now that things at work have settled a bit. Mine is PP and was originally set up in pentode but I'll now be trying triode with a higher voltage PS. I've been thinking about trying some 6BG6G also. How do the two tubes sound in comparison? Since you built your amp with the 6CB5A, what did you like better about that tube?
I'm planning on trying a few different front end configurations. Current one uses 6GK5s in paraphase.
Thanks . . . Charlie
It depends on a lot of variables, I can not tell you how they will sound in another design. The 6BG6 in many different trials always sounded on the bright side to me, especially in pentode I ran some 6BG6'S with 700V plates and 320v on screens in quads push pull, there was aprox 80 watts output but it was very snappy, great sound for guitar but not for music listening. That is just my opinion, some people like the brighter sound but not me. The 6CB5'S are the first generation type st shape that I'm listening to right now and they just sound sonically correct to me a warm lush sound covering everything and nothing offensive but this is in Thomas Mayer's design it sounds great! There are at least 8 discussions and complete details on the build on his site at vinylsavor.blogspot.com
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I've tried the "super" 6BG6GA in my PP Jolida 302b. They had incredible bass, good mids, but a little rolled off in the treble. I'm no longer using them, as I had one fail, so had considered them for SE duty. I didn't try the 6CB5A's in my amp because of the low screen rating and the high heater current requirement. Maybe I'll snag a few to try in an SE amp, they'll work in my socket adapters.
I'll be watching with interest.
jeff
Hi!
You cannot transfer sound results you got from a particular tube in one amp to another. Especially not from SE to PP. There as so much more which has an impact on the overall sound than just the tube.
Yes the 2,5A heater current are a drawback of the 6CB5A, but probably this also gives some hint to the quality of the tube. Not sure if the heater current says anything about the technical properties of the cathode. But it could be an indication of a very sturdy cathode ?
Best regards
Thomas
You cannot transfer sound results you got from a particular tube in one amp to another. Especially not from SE to PP. There as so much more which has an impact on the overall sound than just the tube.
Yes the 2,5A heater current are a drawback of the 6CB5A, but probably this also gives some hint to the quality of the tube. Not sure if the heater current says anything about the technical properties of the cathode. But it could be an indication of a very sturdy cathode ?
Best regards
Thomas
PP 6CB5A Project
Finally found some time to work on my PP 6CB5A project. This is a very low budget design using only parts I have on hand and scrounged transformers. It's just breadboarded at the moment. 6CB5As are in triode and the front end is 6GK5s in paraphase. The power transformer was found cheap on ebay (~$40 I think) and the outputs are from a Pilot console that ran PP 7591s. I think I paid ~$25 for the console. I'll try to attach the current schematics.
. . . Charlie
Finally found some time to work on my PP 6CB5A project. This is a very low budget design using only parts I have on hand and scrounged transformers. It's just breadboarded at the moment. 6CB5As are in triode and the front end is 6GK5s in paraphase. The power transformer was found cheap on ebay (~$40 I think) and the outputs are from a Pilot console that ran PP 7591s. I think I paid ~$25 for the console. I'll try to attach the current schematics.
. . . Charlie
That's very interesting,I hope that tranny can handle over 10amps current for the 6CB5 heaters? if your running four of them for stereo. I would build two mono blocks, it may cost a bit more,but you could use smaller power transformers that way. Anyway I think it's kind of cool you did it at such a low cost, I've done a lot of what I call dumpster diver projects from stuff just laying around.I'm also curious what the 6GK5 tube sounds like,I've tried lots of those little 7 pin triodes but not that one so far.Have you tried any feedback in that design?
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That's why I picked up this particular power tranny. It has two 6.3v windings - one rated at 12A and the other at 15A. I'm using the 15A section for the 6GK5s and two 6CB5A and the 12A section for the other pair of 6CB5A. The other secondaries are listed as 176v @ 700mA and 97v @ 550mA. No center taps on anything. The transformer is actually really small for those ratings (3" x 3.5" x 5") so I'm a bit skeptical but it barely feels warm after running for an hour or so. Looks like it's "potted" but with epoxy or something. Says it's made for Beckman by Electro Engineering Works. I couldn't find any info on it when I searched. It actually has 3 primary taps - 105v, 117v and 126v. I'm using the 126v primary.
I really like the sound - very balanced and extended with great dynamics. First time I've used the 6GK5 and since I like a lot of the vintage amps that use paraphase I thought I'd try it like that. I built a PP 6B4G a while back that uses paraphase with 2C22s.
Haven't tried feedback yet. I don't really know much about design - mostly I just cut and paste - so I need to learn how to figure out how much feedback (if any) to use. Any suggestions from the many gurus here are welcome, of course. Also haven't tried it with cathode bypass caps. I'll probably add some snubbers to the diodes at some point and try some other coupling caps - it's got Russian K40Y PIOs at the moment.
. . . Charlie
I really like the sound - very balanced and extended with great dynamics. First time I've used the 6GK5 and since I like a lot of the vintage amps that use paraphase I thought I'd try it like that. I built a PP 6B4G a while back that uses paraphase with 2C22s.
Haven't tried feedback yet. I don't really know much about design - mostly I just cut and paste - so I need to learn how to figure out how much feedback (if any) to use. Any suggestions from the many gurus here are welcome, of course. Also haven't tried it with cathode bypass caps. I'll probably add some snubbers to the diodes at some point and try some other coupling caps - it's got Russian K40Y PIOs at the moment.
. . . Charlie
Hi FlaCharlie,
Thanks very much for posting your schematics.
Your circuit looks very elegant. I have never used the 6GK5 and am wondering about the sensitivity of the circuit. Although it looks to me that it might have just enough gain to adequately drive the 6CB5A, I am wondering if you are using a preamp?
Ciao - M
Thanks very much for posting your schematics.
Your circuit looks very elegant. I have never used the 6GK5 and am wondering about the sensitivity of the circuit. Although it looks to me that it might have just enough gain to adequately drive the 6CB5A, I am wondering if you are using a preamp?
Ciao - M
As I mentioned, I'm not well versed in the technical/theory department so I don't really know if there's enough drive to push the 6CB5As to full output. Perhaps someone more knowledgeable will comment on the technical aspects.
I think I read somewhere that the 6CB5A was relatively easy to drive and I decided to try the 6GK5 after seeing that Poinz uses them in his Musical Machine PP 6V6 amp. I'm also planning on trying (cut and paste!) that front end too. It's a LTP with CCS. I want to listen to the paraphase for a while first though. Should be an interesting comparison.
That said, I'm not using a preamp and it seems to get pretty loud with my small, vintage, Genesis bookshelf speakers that I use on the bench. There's just the cheapy 100k stereo pot (made by Alps for Radio Shack) with a CD player as a source.
. . . Charlie
I think I read somewhere that the 6CB5A was relatively easy to drive and I decided to try the 6GK5 after seeing that Poinz uses them in his Musical Machine PP 6V6 amp. I'm also planning on trying (cut and paste!) that front end too. It's a LTP with CCS. I want to listen to the paraphase for a while first though. Should be an interesting comparison.
That said, I'm not using a preamp and it seems to get pretty loud with my small, vintage, Genesis bookshelf speakers that I use on the bench. There's just the cheapy 100k stereo pot (made by Alps for Radio Shack) with a CD player as a source.
. . . Charlie
I must be living under a mushroom!, I have been building tube amps for 20yrs now and I've never seen this paraphase configuration before, but since it's new to me I'll experiment with it a bit,maybe I have and didn't recognize the name? Anyway if you try bypass caps in the front end it will increase the gain a bit, sometimes a brighter tone will also result from it. If you would like to experiment with feedback without getting too technical, you can simply use a 50k pot from the output to the 6GK5 cathode, run a sine wave through the amp at 1kz, so you can hear it but not too loud then slowly decrease the pot until you hear the signal go down in volume just a bit that's it.If you turn it to far you will lose too much gain. There is another method if you want to optimize it, but to the experimenter it isn't useful unless your content with it and your never going to change anything again.The 6CB5 was not designed as an audio tube, so there is no information on what the maximum AF voltage is to grid one,I do know it only has a mu of 3.8 so it probably needs quite a kick if maximum output is desired.You could try a driver from an established design that uses an audio output tube with a similar mu factor and I think it would be close enough.
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Corrected PP 6CB5A Audio Schematic
Ooops! I made a mistake on the earlier schematic . . . the cathodes of the two 6CB5A are tied together so the two 1.1k 20w cathode resistors are in parallel.
The tubes are running at ~60mA and plate dissipation is ~19.5w. The data sheet lists maximum plate current at 90mA and 26w as maximum plate dissipation. I'm not sure if running them in triode affects those ratings though.
. . . Charlie
Ooops! I made a mistake on the earlier schematic . . . the cathodes of the two 6CB5A are tied together so the two 1.1k 20w cathode resistors are in parallel.
The tubes are running at ~60mA and plate dissipation is ~19.5w. The data sheet lists maximum plate current at 90mA and 26w as maximum plate dissipation. I'm not sure if running them in triode affects those ratings though.
. . . Charlie
Have another look, the way you had it the tubes were biased separately,now your showing the cathodes tied with the same resistors so the 1.1k will now be half that value. This is still correct for two tubes,with cathodes tied it will run the same current. Either way it's not an oops.
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Exactly. The two 1.1k resistors in parallel equal 550 ohms nominally. Actual measurement is just slightly higher. The output stage is based on the PP 6CB5A design which Thomas Mayer has on his website. His shows a shared 500 ohm resistor and the closest I had in stock that could handle the dissipation was the pair of 1.1k resistors. Reading posts about his designs was the inspiration to try building something with the 6CB5A. Mine is just a low budget version with the unusual paraphase 6GK5 front end.
Regarding the earlier info on dialing in negative feedback, I don't have a signal generator or scope, just a multimeter. If someone has some suggestions as to what value to start with I'm game to try it. On the other hand, Thomas Mayer's design doesn't use any feedback and the amp sounds damn good as it is. I'll try adding some cathode bypass caps over the weekend and see what it sounds like.
. . . Charlie
Regarding the earlier info on dialing in negative feedback, I don't have a signal generator or scope, just a multimeter. If someone has some suggestions as to what value to start with I'm game to try it. On the other hand, Thomas Mayer's design doesn't use any feedback and the amp sounds damn good as it is. I'll try adding some cathode bypass caps over the weekend and see what it sounds like.
. . . Charlie
The 6BG6/807 does tend to make more high order harmonics, therefore the "bright" sound. Doesn't mean you can't use 'em for music reproduction, as local NFB can fix that, as recommended in Beam Power Tubes: feedback 10% of the plate voltage to the grid, add some gNFB to take off the remaining edge, and they'll sound just great.
Used 'em myself (Class AB1; PP; 30W out) with excellent results.
The horizontal deflection power pents make some excellent audio finals. The 6BQ6GA(GTB) is another one that performs quite well, and without local NFB. Running open loop, they mainly give boomy, under damped bass, and an excessively "aggressive" or "bright" sound that can be cleaned up with gNFB. The main limitation is that a Class A Q-Point is well into red plate territory, so you must operate in Class AB.
It was popular in the late 1940s -- early 1950s, but fell out of favor. Seen the topology lots, but didn't hear the term "paraphase" either until relatively recently.
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Hey there Charlie have fun what ever you do,to me all this stuff is has been a really great hobby for me to kill my spare time and my Father was my teacher,when I was about 12yrs old.He was an educated man in electronics and I was very thirsty to learn from him in those days.I looked again on Thomas Mayer's site I can't see a PP version of the 6CB5 circuit, could you post the link Thanks Jim
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