6sj7 input tube

[andrew]

Are there any pros or cons to wiring a 6sj7 input tube as a grid leak bias or cathode bias? Thanks for any advice.

[David Root]

You mean wiring it as a triode?

[andrew]

http://www.harmonicappliances.com/images/Route6V6.jpg
http://home.comcast.net/~psymonds/GA20.htm
Hi David, The signal from the guitar going to the 6sj7 is wired differently.

[Cliff Schecht]

Grid leak will get you more gain from a single stage, at the cost of linearity. Of course with more gain comes a higher miller capacitance at the output which in turn decreases bandwidth, but probably not below audible frequencies so this might not be a huge concern.

My little Silvertone 1331 is a single grid-leaked 6SJ7 feeding a 6V6 and it's a cookin' little amp. I use it often for blues/slide stuff live and in the studio. It's puny 6" speaker kicks out some nice bass and it mikes up really well.

[martin manning]

I see no reason to expect more gain from a grid leak biased stage vis-a-vis a cathode-biased stage with a fully-bypassed cathode resistor. The input impedance will be higher (because of the large grid leak resistor), and so the source won't be loaded as much, but it may not make any real difference. The biggest disadvantage to grid leak bias is that it is less predictable (tube-to-tube) and less stable as the tube ages.

[VacuumVoodoo]

Grid leak will get you more gain from a single stage, at the cost of linearity. Of course with more gain comes a higher miller capacitance at the output which in turn decreases bandwidth, but probably not below audible frequencies so this might not be a huge concern.

Effects of Miller capacitance at the output of a triode, or triode strapped pentode, is usually negligible. It's the Miller capacitance at the input (roughly = anode-grid capacitance x voltage gainn) that is troublesome. In a pentode Miller effect is greatly reduced.

[Alexo]

My real-world testing revealed a gain of right around 100 with a 220K plate load, 2m2 screen dropper running cathode bias, and about 110 in grid leak bias.

Not a huge difference, but every drop of gain counts in a single gain stage SE build like the 5C1-ish amo this was in.

Subjectively, I feel like you get a little more character and bite out of grid leak.

[andrew]

The grid leak in in now. Thanks for the advice all. This amp is a cut and paste job and it sounds good. It is a 270v PT with 6v6s- Gibson GA20 paraphase PI (6sL7) with an added plate balance pot (mo louder)- Route 6v6 volume and tone controls- 6SJ7 with the 2.2M/ 220k setup. I didn't have a 1k/1uf for the 6SJ7 so I left it out and went to ground like the GA20. I used the GA20 grid leak input. DC heaters on the preamp.The OT is a Hammond 7.6k. I tried an old Magnavox alnico speaker and a WGS ET65 but I liked the Jensen C12N reissue better in this amp. Any more ideas would be appreciated.