cathode bias vibroverb style amp. need help for choke and cathode math

[pietro_moog]

hi guys. i want to build a small 2x6L6 combo, cathode bias, fender style preamp + reverb, no tremolo, no NFB , no normal channel.
like a vibroverb, but with cathode bias.
i have a lot of spare parts from other builds, like a PT of 300-0-300 and 2 JJ6L6GC.
i wanna go for a couple of diodes as rectifier, since i don't wanna spend for a tube and i could use some space.

i need help to calculate the value of the bias resistor, but i think that a 270Ω 10w resistor, like the 5E5, is a good start.
do i have to choose another resistor because of the bigger absorption of the 6l6gc?

i have found a cheap vibrolux OT, rated 35w. i think it's a good option, right?

i need help to choose a choke. i'd love a choke, and a light one. i can buy an Hammond 125x one, but i'm not sure which.
it would be awesome if i could use a 400v one, because they are cheaper, smaller and lighter. i would expect 420v with no load,
but i have no idea about what real voltage i would see. the 6L6gc are not mine, i never used the big bottles.

any help is appreciated. thanks guys

[pdf64]

The VB+ would be about 425V which may be high for good results using cathode bias.
My experience is that its not possible to obtain a happy balance between static dissipation and crossover distortion at that level, so only use cathode bias with VB+ well below 400V.
Probably better to use an output transformer with a primary impedance greater than 4k, to help with the above.
I think such tweed amps tended to use around 5k or 6k.
I use a Hammond 157J choke (400V rated) at about 450V but it's an amp for personal use and the B+ winding is fused (that tends to be omitted with Fender type amps).
Pete

[jazbo8]

Here is a standard textbook design, puts out about 18W-27W (depending on Ra-a), so the OPT rating could be relaxed further.



First two columns of the GE datasheet:

[pdf64]

I've often wondered (but never bothered to investigate) how the screen grid supply was obtained for the tables in manufacturer's tube info, ie how to get a reasonably well regulated supply that much lower than the main B+.
I had assumed that OC3 type voltage droppers were used but now I know (a separate choke input B+ specifically for the screen grids)- thanks jazbo8!

Further to my previous post, crossover distortion (resulting from cathode bias at highish VB+) can be mitigated by the use of zener diodes at the cathode to limit voltage rise under high signal conditions but that can be seen as just an alternative implementation of fixed bias.
Pete

[Steve Conner]

The manufacturer would have used a couple of big regulated power supplies to give the voltages that showed the tube at its best, without too much thought to how the customer would actually go about getting them.

This is (imo) why most tube amp designs ignored the recommendations and went for the cheapest option, a screen voltage similar to the plate voltage, leaving the users to handle the fallout of frazzled screen grids.

[jazbo8]

Gingertube just posted this design on diyaudio for screen grid at ~ 1/2 B+.

[pietro_moog]

The VB+ would be about 425V which may be high for good results using cathode bias.
My experience is that its not possible to obtain a happy balance between static dissipation and crossover distortion at that level, so only use cathode bias with VB+ well below 400V.
Probably better to use an output transformer with a primary impedance greater than 4k, to help with the above.
I think such tweed amps tended to use around 5k or 6k.
I use a Hammond 157J choke (400V rated) at about 450V but it's an amp for personal use and the B+ winding is fused (that tends to be omitted with Fender type amps).
Pete

oh no. so you are saying that a fixed bias would be a better option. i loved the idea of a 25-30w cathode bias, but maybe fixed bias won't be much more powerful. that's ok, i guess. i'm not sure of the efficiency of this PT toroid, so i guess i'd expect something lower than 425v. i really hope. is there some diode with more voltage drop than the 1n4007 that i can use?
a bigger choke would be a pain in the ass, basically because of the weight. a 156m weights 1/2 pound, and that would be very nice.

the 4k OT is cheap, i think i'll go with that instead of something with bigger impedance.
i hope i can find a better PT.

another thing i'm worrying about is how this combo might sound. it has a single 10" speaker. my goal is to benefit from the lower efficiency of the speaker for a more manageable volume at home, and plug an extension cab for something else. do you guys think this is a bad concept?

[pietro_moog]

i was wondering: what happens if i wire the PT primaries at 240v, while i have 220v on the wall? the math tells me i would have like 550v on the secondaries. that would be cool. is this a good idea? what happens to the 6,3v secondaries? is it safe?

[jazbo8]

With higher voltage and fixed bias, you can get quite a bit more power out of a pair of 6L6GC's, but it doesn't have the compressive effect that a cathode biased stage can deliver. So it depends on what you prefer... As for wiring the primary to the 240V tap, it is safe, the filament Is a tad low at ~5.8V but you can likely get away with it.

[pietro_moog]

mmm.. so what i'm getting is that i need a tube rectifier. and that my reverb is off the chassis because i have no space. and that i have to move to a
cathodyne PI. i'm feeling kinda sad

[jazbo8]

Perhaps you can consider using other tubes for the output that are better suited for cathode bias?

[pietro_moog]

after a check on my stocks, i had an idea. my PT is rated 300-0-300 at 300mA, 6,3v at 5A, 5v at 3A. i could use a rectifier, like the gz34 or the 5u4g, to lower B something below 400v. 2x6L6gc cathode bias, and 2x12ax7 as preamp+cathodyne. and i could add a 6bm8 i have, as reverb driver & recovery stage. i think i could make room for these tubes, and the total heather current is 3,2A which is about 65% of the total.
the 300mA could cover the 50mA of the preamp and the output relatively easy, i think (but i'm not sure. can it?)

in this scenario i could use a 400v rated choke, which is light and cheap. the Hammond 156R is rated 200mA. is that enough?
the amp might be just below 30W, with great tone coming from the cathode bias behavior and the cathodyne. reverb might be super-cool.

can this solution work?

[pietro_moog]

Perhaps you can consider using other tubes for the output that are better suited for cathode bias?

my combo is currently using 2x6v6 and it sounds good, but after building a 50w 2x6L6 for a friend i felt the need to change.

[frus]

Why cathodyne?

[jazbo8]

Take a look at the EL34 (think Marshall), very doable with cathode bias.