807’s replacing 6L6gc’s
I was looking at the tube data sheet for the 6L6gc and it states that the 807 shares the same curves and voltages but has a different pin out. I have an AR ST-70-C3 that takes 6L6gc’s and has individual tube biasing. Would it be possible without too much component changing to just wire in some 807 sockets and caps? Do you think there would be much more then that involved since all the voltages appear the same?
I traded my 6L6gc RCA clear tops for a nice set of Mullard EL34’s for an other amp and only have some crappy Russian tubes to run the C3 on and would like to hear some different tubes run on it. On a side note the guy I traded the clear tops to, on the first day he had them in his guitar amp he was pushing it on stage and the wheels hit something and the amp fell over destroying 2 of the tubes. R.I.P RCA Clear tops.
I was looking at the tube data sheet for the 6L6gc and it states that the 807 shares the same curves and voltages but has a different pin out. I have an AR ST-70-C3 that takes 6L6gc’s and has individual tube biasing. Would it be possible without too much component changing to just wire in some 807 sockets and caps? Do you think there would be much more then that involved since all the voltages appear the same?
I traded my 6L6gc RCA clear tops for a nice set of Mullard EL34’s for an other amp and only have some crappy Russian tubes to run the C3 on and would like to hear some different tubes run on it. On a side note the guy I traded the clear tops to, on the first day he had them in his guitar amp he was pushing it on stage and the wheels hit something and the amp fell over destroying 2 of the tubes. R.I.P RCA Clear tops.
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Just a little more on the original question;
The 807 internal structure is exactly the same as the 6L6, but the latter has been ruggedised over the years, and the 6L6GC is now a 30W tube compared to the 807's 25W. Internal mounting has also raised the allowed maximum Vg2 to 450V (500V in case of UL) instead of the 270V (under correction) for the 807.
You mention "trading" 6L6GCs for EL34s, so it is a done deal. Still, for general interest I would mention that the two are not really inter-replacable, apart from ratings. Re ratings the EL34 can take 800V anode, but is only Pa = 25W compared to 6L6GC of 30W, also with almost double the heater current. Not asked, but I would venture to say that I found the 6L6(GC) a little more tolerant design-wise than the EL34. (Difference between beam tube v s. pentode and all that.)
The 807 internal structure is exactly the same as the 6L6, but the latter has been ruggedised over the years, and the 6L6GC is now a 30W tube compared to the 807's 25W. Internal mounting has also raised the allowed maximum Vg2 to 450V (500V in case of UL) instead of the 270V (under correction) for the 807.
You mention "trading" 6L6GCs for EL34s, so it is a done deal. Still, for general interest I would mention that the two are not really inter-replacable, apart from ratings. Re ratings the EL34 can take 800V anode, but is only Pa = 25W compared to 6L6GC of 30W, also with almost double the heater current. Not asked, but I would venture to say that I found the 6L6(GC) a little more tolerant design-wise than the EL34. (Difference between beam tube v s. pentode and all that.)
807 vs 6L6GC
One of the main improvements of the 6L6GC over the earlier 6L6's (6L6, 6L6G, 6L6GA, 6L6GB) as well as the 807 and 6BG6G/GA (which are essentially 6L6's with plate caps and higher plate voltage ratings) is that the 6L6GC uses a special 5-layer plate material that has copper in the core, so that the heat is spread out. This helps keep the plate from glowing orange right in the center, the way the old tubes would if they were overloaded.
One other difference when comparing the 807 and 6L6GC is the different rating systems each tube uses. The 807 uses the "Absolute Maximum" rating system, where the max ratings should never be exceeded. The 6L6GC uses the "Design Maximum" rating system, which for an average tube, should not be exceeded. This is about 10% more conservative than the absolute maximum system. The older 6L6s use the "Design Center" rating system, which is the maximum for an average tube under average operating conditions. This is about 10% more conservative than design-maximum ratings, or about 20% more conservative than absolute maximum ratings.
So, lets compare plate dissipation specs:
6L6 - 19 watts (design-center)
6L6GC - 30 watts (design-maximum)
807 - 25 watts (absolute maximum)
Now to compare apples with apples, lets normalize the wattage to all be absolute maximum ratings (multiply the design maximum by 110% and design center by 120%):
6L6 - 22.8 watts
6L6GC - 33 watts
807 - 25 watts
So, normalized to the same rating system, the 6L6 isn't that much wimpier than the 807, but the 6L6GC is quite a bit more rugged than the 807.
These rating systems are nicely described in the front section of the GE Essential Characteristics manual.
- John Atwood
One of the main improvements of the 6L6GC over the earlier 6L6's (6L6, 6L6G, 6L6GA, 6L6GB) as well as the 807 and 6BG6G/GA (which are essentially 6L6's with plate caps and higher plate voltage ratings) is that the 6L6GC uses a special 5-layer plate material that has copper in the core, so that the heat is spread out. This helps keep the plate from glowing orange right in the center, the way the old tubes would if they were overloaded.
One other difference when comparing the 807 and 6L6GC is the different rating systems each tube uses. The 807 uses the "Absolute Maximum" rating system, where the max ratings should never be exceeded. The 6L6GC uses the "Design Maximum" rating system, which for an average tube, should not be exceeded. This is about 10% more conservative than the absolute maximum system. The older 6L6s use the "Design Center" rating system, which is the maximum for an average tube under average operating conditions. This is about 10% more conservative than design-maximum ratings, or about 20% more conservative than absolute maximum ratings.
So, lets compare plate dissipation specs:
6L6 - 19 watts (design-center)
6L6GC - 30 watts (design-maximum)
807 - 25 watts (absolute maximum)
Now to compare apples with apples, lets normalize the wattage to all be absolute maximum ratings (multiply the design maximum by 110% and design center by 120%):
6L6 - 22.8 watts
6L6GC - 33 watts
807 - 25 watts
So, normalized to the same rating system, the 6L6 isn't that much wimpier than the 807, but the 6L6GC is quite a bit more rugged than the 807.
These rating systems are nicely described in the front section of the GE Essential Characteristics manual.
- John Atwood
As mentioned here, and in several other threads, the 807 shares its design with the 6L6GB. In some cases they have EXACTLY the same internals. The 6L6GB is a 25 watt tube. There were a lot of vendors making 807's in the WWII era, and some were more robust than others, but I haven't met any that I would use at dissipations above 25 watts, and some exhibit a noticable glow at the 25 watt level. Most 6L6GC's can handle 30 watts without and signs of color on the plate, and some can take far more than 30 watts.
The amp shown in the picture currently has Sovtek 6L6WXT's in it. These can handle 35 watts without even sweating. If your amp runs the tubes hard, the 807's will have a tough time replacing those.
As stated above 6BG6's are also cast from the 6L6GB mold. Ordinary 6BG6's are also 25 watt tubes. The 6BG6's sold by SND are not ordinary. They are from the end of the vacuum tube era when many manufacturers (especially Sylvania) stuffed glass envelopes with whatever guts they had to fulfill contracts. These 6BG6's appear to have guts from the 7027A or 6L6GC-STR line. They can eat 30 watts without sweating either.
The amp shown in the picture currently has Sovtek 6L6WXT's in it. These can handle 35 watts without even sweating. If your amp runs the tubes hard, the 807's will have a tough time replacing those.
As stated above 6BG6's are also cast from the 6L6GB mold. Ordinary 6BG6's are also 25 watt tubes. The 6BG6's sold by SND are not ordinary. They are from the end of the vacuum tube era when many manufacturers (especially Sylvania) stuffed glass envelopes with whatever guts they had to fulfill contracts. These 6BG6's appear to have guts from the 7027A or 6L6GC-STR line. They can eat 30 watts without sweating either.
Johan Potgieter said:
The trade was not for the same amp. I have ST-70-c3 that uses 6l6gc's and a modified ST-70 that takes EL-34s. I was not happy with the sound of the RCA clear top 6L6gc's. I think they were later tubes and had the kinda grey black plates. I have a set of really old RCA 6L6gc's (non clear top) with almost frosty dark black plates and they sound far better so I traded the clear tops for a quad of Mullard Xf2 El34's for the St-70.
I got a quad of nice RCA 807's for 10 bucks and want to try them in something so I figured that I could just get some sockets and caps and wire them up in the St-70-c3.
Fair enough, Pyre,
Just note that optimum bias for 6L6 differs from that of EL34 (I think that the ST70 used EL34/6CA7.) I am not sure how stringent the 807 max screen voltage of about 300V (?) is (ST70 is a UL amplifier). I know these ratings are quite condition dependant - one can increase.
Once again I would like Tubelab to advise; he has a lot of practical experience here.
Just note that optimum bias for 6L6 differs from that of EL34 (I think that the ST70 used EL34/6CA7.) I am not sure how stringent the 807 max screen voltage of about 300V (?) is (ST70 is a UL amplifier). I know these ratings are quite condition dependant - one can increase.
Once again I would like Tubelab to advise; he has a lot of practical experience here.
Hi Pyre,
I do not understand those figures. It is a long time ago that I have serviced an ST70. Bias current would be about 50 mA per tube, voltage on G1 -35V for 807/6L6. I have 807 data somewhere but will hunt in morning; now 01:50 am and I must go to bed. Others will probably reply before then, otherwise back tomorrow-night.
Regards.
I do not understand those figures. It is a long time ago that I have serviced an ST70. Bias current would be about 50 mA per tube, voltage on G1 -35V for 807/6L6. I have 807 data somewhere but will hunt in morning; now 01:50 am and I must go to bed. Others will probably reply before then, otherwise back tomorrow-night.
Regards.
The bias numbers do not mean anything to me either, since they are probably voltage readings across a resistor. The resistor value would be needed to determine the current, and the cathode to plate voltage needed to determine the dissipation.
The bias should be set to the point of best sound quality constrained by a maximum dissipation determined by the capabilities of the tube, and the power supply in the amp itself. This requires some knowledge of the particular amp.
A typical push pull tube amp is often set to a dissipation value that is 25 to 80% of the maximum value for a particular tube. This is a wide range, which could be narrowed by some knowledge of the amp in question. A pure class A push pull amp typically runs the tubes near 80% of maximum dissipation. The typical class AB1 amp uses 50 to 70%. An AB2 amp runs less bias, and more drive voltage while a Class B amp (rare these days except for screen driven sweep tubes) can run the tubes near cuttoff. Your amp will probably work in the 60% range which means 15 watts of dissipation per 807. Measure the voltage across the tube, then figure out what current gets you to 15 watts.
I just found out that my company is sending me to Chicago next week, so I may not be on the forums until next weekend, unless the hotel has free internet.
The bias should be set to the point of best sound quality constrained by a maximum dissipation determined by the capabilities of the tube, and the power supply in the amp itself. This requires some knowledge of the particular amp.
A typical push pull tube amp is often set to a dissipation value that is 25 to 80% of the maximum value for a particular tube. This is a wide range, which could be narrowed by some knowledge of the amp in question. A pure class A push pull amp typically runs the tubes near 80% of maximum dissipation. The typical class AB1 amp uses 50 to 70%. An AB2 amp runs less bias, and more drive voltage while a Class B amp (rare these days except for screen driven sweep tubes) can run the tubes near cuttoff. Your amp will probably work in the 60% range which means 15 watts of dissipation per 807. Measure the voltage across the tube, then figure out what current gets you to 15 watts.
I just found out that my company is sending me to Chicago next week, so I may not be on the forums until next weekend, unless the hotel has free internet.
Pyre said:
Depends on what you want to do with it. The 807 Application Report gives quite a few different operating points, from those that minimize THD to those that mainly give lotsawatts. (And then there are all the RF amp applications as well.)
Hi Pyre,
At this point I think you should regroup and rethink things a little bit. I think changing tube types would be a mistake in this amplifier. Keep in mind that AR has a certain type of sound. Don't try to make it into something it isn't. You can refresh this amp and make it sound like it should.
The 6L6GC is an excellent tube in general and previous comments made about the GC variant are correct. If you wish for a tougher tube (I don't know why), you can also use a 7581A. It has a 35W plate and matches closely with a 6L6GC. Same heater current as well (0.9A). An EL34 runs from 1.2 ~ 1.6 A. That's a big difference.
I have had great luck with the Electroharmonix brand with this and many other tube types. A 6L6EH from "thetubestore.com" should sound great. If it doesn't, there are other problems. That store is in Hamilton, Ontario.
Also, I am located in Georgetown in case you need some help. It's a nice drive out here. You can PM me if you want.
-Chris
At this point I think you should regroup and rethink things a little bit. I think changing tube types would be a mistake in this amplifier. Keep in mind that AR has a certain type of sound. Don't try to make it into something it isn't. You can refresh this amp and make it sound like it should.
The 6L6GC is an excellent tube in general and previous comments made about the GC variant are correct. If you wish for a tougher tube (I don't know why), you can also use a 7581A. It has a 35W plate and matches closely with a 6L6GC. Same heater current as well (0.9A). An EL34 runs from 1.2 ~ 1.6 A. That's a big difference.
I have had great luck with the Electroharmonix brand with this and many other tube types. A 6L6EH from "thetubestore.com" should sound great. If it doesn't, there are other problems. That store is in Hamilton, Ontario.
Also, I am located in Georgetown in case you need some help. It's a nice drive out here. You can PM me if you want.
-Chris
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