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24.27.41.140
I have several good transformer sets that are right for 6L6 tube types. I also have a good number of NOS 6L6GB tubes. When I look at the specs for these tubes, it's difficult to reconcile the differences between manufacturers' data and the voltages that are sometimes present in commercial amplifiers.
For example, early Fender amps like the Bassman 5E6A used the 6L6G. This tube is rated for a maximum of +360 anode and +270 screen. However, the Fender schematic shows +405 on the anodes and +410 on the screens. The slight over-voltage on the anodes is probably no big deal, but the screens are a different matter. If anyone here knows, were these old Fender amps troublesome from the standpoint of tube failure or maybe short lifespan? And did other manufacturers do this as well?
On a related issue, have there been any studies published online regarding the effect of screen voltage on THD? It seems that tubes like the 5881 and 6L6GC are happy in terms of output power with screens running anywhere between about +250 to +400. I'm sure the exact voltage chosen for the screens affects distortion, but I don't recall ever seeing any discussions on this topic.
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Buy Chinese. Bury freedom.
Follow Ups:
At a constant g2 voltage, the value of the voltage is the 'matching' quantity to set the g1=0V operating envelope to the load line( what ever that happens to be), based on a-a reflected load, and Class( where it might go from a-a/2 to a-a/4 in an AB amp). The Class A is of course effectively stuck at a-a/2.If the g2 supply is not stiff enough, then you have an open, difficult to predict, can of worms. Model-able for sure, but a level of complexity slightly elevated from a rock hard g2 voltage, as the performance envelope changes dynamically...
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Edits: 04/21/21
As I'm sure you know, guitar amps tend to have designs that push tubes much harder than typical Hi-Fi amps do.
Whatever happened to FenderLover? I believe he was the moderator on the Vintage board but he doesn't seem to post anymore. Hopefully, he is OK. I would think that he could tell you about the reliability of Fender Bassman amps using the 6L6G.
I would also suggest that the key spec is not voltage but dissipation.
I've never seen a Fender Bassman that used 6L6G. The early Tweed amps used small bottle 5881's.
Sometime in the early to mid 60's, they switched over to 6L6's, but 6L6GC was available by that time.
This is the Fender schematic I found. I also looked up some of the ealy Bogen amps using the 6L6G and 6L6GA. Voltages in the Bogens are much lower, more in line with the tube's rated limits.
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Buy Chinese. Bury freedom.
IMO that tube sounds superb at 275 on the plate and 250 on the screens with a cathode bias resistor of 200.
The Mind has No Firewall~ U.S. Army War College.
I don't recall ever seeing a 6L6G with +440 in a commercial product. I'd be interested to know the model(s) so I can look at it.
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Buy Chinese. Bury freedom.
You're right, I stand corrected. I was not aware Bassmans ever came with 6L6G.
Looks like the first Bassman to use 5881's was the 5F6.
And it's a safe bet that the 410V B+ shown on the 5E6A schematic is closer to 430V when operated on 125VAC. The PT was designed for 110VAC.
Hi
are two different topology of circuit.
You can't see only the HT voltage but also the type of bias.
On is autobias (Bogen) and other is fixed bias.
The Bogen maybe is in class A for a good range of power output
The Ibias of Bogen is 100 mA for both 6L6, 50 mA each one
It has also a partial cathode feedback ( 33 ohm) on output stage
Walter
Your observation is correct. Self-bias in the Bogen circuit means it has even less standing voltage from the anodes and screens to the cathodes than the Fender. It's very close to the maximum ratings, whereas the Fender is way over.
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Buy Chinese. Bury freedom.
You have to look always at the maximum ratings.
For 6L6GC the RCA says 500 Vdc for Anode and 500 Vdc for G2 in a p-p
Same for GE
Then, in a p-p, with a proper load and a reasonable Ibias you stay safe.
Walter
450V anode and 400V screen seem possible for some models.
Omnes feriunt, ultima necat.
The 6L6G is rated +360 / +270. The 6L6GB is rated +400 / +300. Datasheets for the 6L6GB to show typical operation in pentode AB1 to be +360 anode and +270 screens, just like the "G".
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Buy Chinese. Bury freedom.
and there are late model GB that I am sure are actually of the GC line...or perhaps even the 35W 7581A line... :)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I've read somewhere that the 6L6GC is quite similar to the 807 but without a top cap. I've seen you recommend the 807 before Douglas - are the 6L6GC as good?
I'm being lazy on the top cap aspect - and have 6L6GC on hand... hence the draw......
I think the 'GC has also a higher plate dissipation rating( 30w vs 25 at the ccs usage ). Not quite sure that translates well to the 6L6GC that is for sure at least a 30W plate.
I need me a ham fest or two; I need more 3/8" plate caps.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I'm still trying to find the plate clips that were used in ARC-5 transmitters. I know there are 100,000 of these in a back corner of a warehouse somewhere...
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Buy Chinese. Bury freedom.
If you can tolerate open, go with fuse holder clips. That is the best I have found for the small ones like a 3D21A.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Guess I haven't seen those. The 6L6GB types in my stock look like 23W tubes. The anodes aren't as large as the GCs.
To tell the truth, I'm not too concerned about the issue of screen voltage from the standpoint of ratings. I'm using VR tubes in series from the B+, so the actual voltage at the screens can be adjusted merely by plugging in a different VR. I'm more concerned about distortion at various screen potentials. I'm surprised this topic isn't better represented in technical publications online. Maybe it's in one of the RCA publications...
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Buy Chinese. Bury freedom.
The distortion performance is not due to g2 voltage, but what g2 voltage does to the performance envelope, and where the load line is driven across the characteristic plate curves.
It could be this was treated as 'everybody knows that' sort of topic, and therefore effectively ignored.
Call up a solid data sheet, likely a GE one, and for a given idle point, examine what changing the g2 voltage does to the load line location. Stay entirely resistive( a straight line, not an ellipse) and examine a 6CL6 data sheet. I have no doubt the lesson is transferrable to your 6L6 work.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I've always believed screen current introduces its own non-linearities, and that this modifies the optimum ratio when looking at small-signal distortion. Given the vagaries of real world loads, this might be too minor to be worth pursuing. I'll just use the loadline as you suggest and maybe adjust the screen voltage empirically after the amp is built.
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Buy Chinese. Bury freedom.
Seems like a plan; the 'bunching up' of the plate curves at low current( gm shrinkage ) is unavoidable. Driving the load line through the knee where they bunch up again is avoidable.
I put together a 6AV5 pentode linestage once. g2 at 75V, B+ about 250 and filtered the heck out of the B+. Ran a 510R load, idling at 50 mA. Nearly vertical as far as the tube was concerned. That load line went nowhere near the knee, and it sounded quite good( as well as letting me use a 0A3 to shunt reg the g2 supply ).
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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