sssshhh! we better keep it low, the 6LU8 tubes that i got from the Rogalski's are now $4 each from a dollar per pop, good thing i bought plenty then...
i have the russian counterpart, 6J32...
i also have the 12by7, 6gj5,C3m, 18040, japanese 6R-R8 to try....
the russian 6j9/ef180/6688 rocks......
For a power amp noise is not an issue; signal levels are too high. Microphonics can pose a problem; however I have not had any problems whatsoever building them into power amps. Besides I would not be begrudging a couple bad units at that price And BTW the S on those 6BN11's is in excess of 11K uMhos. One can load them down with a 27K plate resistor and drive the 50K grid return resistor for a 6550 and still have a gain well over 100 and with 150V swing. An EF86 is not going to cut it as a drive tube!!! Note that most if not all current tube amp manufacturers do not provide tube manual value of 50K on their 6550s because their drivers are too whimpy!!!
And BTW the S on those 6BN11's is in excess of 11K uMhos. One can load them down with a 27K plate resistor and drive the 50K grid return resistor for a 6550 and still have a gain well over 100 and with 150V swing. An EF86 is not going to cut it as a drive tube!!! Note that most if not all current tube amp manufacturers do not provide tube manual value of 50K on their 6550s because their drivers are too whimpy!!!
Well, I'll agree on all points except noise. This device is probably adequate but not all signals feeding a power amp are large. Sounds good to me as a unit to explore. I have a bunch of BN11's and sockets on order and I will be ordering my pico scope later today.
BTW, how much plate and screen current are you pulling?
BTW, how much plate and screen current are you pulling?
Since I am running the twin pentodes in a long tail setup the biasing is automatic. IE the plate current gets set by the common cathode current. By experiment I found that having the SG dropping resistor at about 3X the plate resistor works very well. IE for the 30K plate resistor I have 100K screen dropping resistors. I also built the 6BN11's to an SE 6L6 amp using the 30K plate resistor and 100K SG resistor. This uses the UL EDCOR OPT; and is running the dual FB loops. I do not recall what I put in the cathodes; I think a 1K bypassed and a 100-Ohm feedback resistor to ground. With 375V to the plates and 3500 ohms plate loading she puts out over 10 very sweet watts per channel.
"sssshhh! we better keep it low, the 6LU8 tubes that i got from the Rogalski's are now $4 each from a dollar per pop, good thing i bought plenty then"
Last year I bought 50 6BN11 then checked very recently and they were still a buck a pop so I got 50 more. At this point I do not care, but they must have a ton of those things. I have tried running them in triode mode; they have a gain of about 60 or so. The really kewl thing about them is the hot heater wire that jumps between the cathodes on top of the spacer. Sockets are readily available on the Bay at about $2.50. Since they are video tubes they are designed for low noise and great linearity and high gain bandwidth factor. In short they are ideal drive tubes.
Last year I bought 50 6BN11 then checked very recently and they were still a buck a pop so I got 50 more. At this point I do not care, but they must have a ton of those things. I have tried running them in triode mode; they have a gain of about 60 or so. The really kewl thing about them is the hot heater wire that jumps between the cathodes on top of the spacer. Sockets are readily available on the Bay at about $2.50. Since they are video tubes they are designed for low noise and great linearity and high gain bandwidth factor. In short they are ideal drive tubes.
I think it was closer to 40 MHz but that isn't that important. Yes the IF is at a higher freq but linearity isn't as important in an IF tube. Just look at a remote cutoff pentode, they are used for AGC in the IF. As long as a 6BN11 is a sharp cutoff pentode it will be fine. My Pico Scope 4262 is "in the post" and will arrive next week. So now I must find my Heathkit IP-17's and get them out of storage.
My Coyne Television Servicing CYCLOPEDIA states that tubes employed in TV IF stages are high transconductance pentodes. They further state that AGC is employed on several stages. That AGC is moving the bias thus changing the transconductance of the sharp cutoff pentodes. Remote cutoff pentodes do not have the gain required on a 40 MHz if strip having a 6 MHz bandpass. In any case yes I was rather skeptical of the dollar specials and just dipped my toe in at first ordering a few. At first I was building them to floating paraphase drivers, then tried triode connected long tailed inverters. These eventually evolved to the circuitry posted earlier here. It is capable of taming any output iron with just a few small resistors and capacitors which do not impair the bandwidth of the amplifier. And it allows the use of local feedback as another tool to tame that iron. And then there is that kewl hot wire on top of the spacer.
Well I have no problem with using this tube, partly because it is a pair of sharp cutoff pentodes and because it has other features to make it a good driver, but regardless of whether it was a TV IF or an AA5 IF, remote cutoff pentodes are used in IF AGC. But we are not building a TV or radio so, who cares? I look forward to making some measurements on it, I am really interested in its bandwidth THD and output impedance.
A typical TV IF strip would use a remote cutoff valve for the first stage, and a straight valve for the second stage. For example, in Europe that would be EF85+EF80 (then EF183+EF184 for the last generation of valve TVs). AGC is applied to the first stage only. Because of the wide bandwidth of TV the dynamic range requirement is less than for sound broadcasting.
A video pentode can be just a beefy version of a straight RF pentode, but could alternatively be seen as an RF version of an audio output pentode (but typically a bit more linear).
A video pentode can be just a beefy version of a straight RF pentode, but could alternatively be seen as an RF version of an audio output pentode (but typically a bit more linear).
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