KT66 & Cathode Bias?

[Gene Ballzz]

Has anyone here ever used KT66 tubes in a cathode bias configuration? I've not yet noticed such a use.

The reason for the question is brain storming (maybe brain "farting") a JTM 45 style build. My thought is to build a JTM45 style amp, but centered around having the option of utilizing KT66 or 6V6 power tubes in a cathode bias manner.

Now, while I realize that cathode bias does not really capitalize on the full output the tubes are capable of, in this day and age of striving for "Big Boy" amp sounds and response at "Little Boy" wattages and volumes, this may be a benefit.

I'm assuming cathode biasing the KT66s will drop the nominal output wattage of an otherwise standard JTM45 to about 30 to 32 watts (maybe a little less) and then simply swapping to 6V6s, dropping that another 35% to 50%. Given that both tube models seem fairly happy with an 8K primary output transformer and both will tolerate similar voltages, it seems fairly doable. I realize that the cathode resistor and cap may need to be switchable to different values for the two different tubes. Is there something else I'm missing?

Ya see, here's my theory: In todays "gin mill" performance environment, an amp needs to provide the desired sound and response, but in a package that is used only as an on stage monitor for the performers. The amp is then easily miked, not for volume, per se, but instead to more evenly distribute that miked sound to fill the room in the same mix as the rest of the band and vocals. The big trick seems to be getting that great "Big" sound at lower, more controlled volume levels.

Please Share Your Thoughts?
Gene

 

[neikeel]

Sounds like a good idea, probably don't need to vary the cathode cap and resistor combination.
I guess it depends on your PT voltages and if you are going for individual Rk/Ck per tube?

 

[2L man]

KT66 won't produce much more power when it is used in 6V6 optimized circuit . Operative voltage and OT impedance has most effect to output power. You need to half OT impedance changing loudspeaker to next higher impedance output and then power almost doubles.

Cathode biased AB class push pull amp produce about as high peak power as fixed bias but when playing long loud notes power drops when cathode voltage tries to climb. Obviously using higher value cathode resistor slows down this phenomenon.

 

[South Park]

I don’t think so you can’t bias power tubes the same way as preamp tubes . Power tubes have control grids for the bias .preamp tubes are biased on the ground no control grid . Power tubes will not work at a lower power levels you can burn up your output transformer

 

[yladrd61]

You can also try a properly rated variable wire wound cathode resistor and adjust until you find a value that sounds good and is within the operating spec of the power tubes you are using.

 

[Ivan H]

I don’t think so you can’t bias power tubes the same way as preamp tubes . Power tubes have control grids for the bias .preamp tubes are biased on the ground no control grid . Power tubes will not work at a lower power levels you can burn up your output transformer

You very much can cathode bias power tubes. Both power tubes & pre-amp tubes have control grids, so called because this grid is used to control the electron flow (this is the grid that the signal is applied to). When we cathode bias, we use a resistance in the cathode (between cathode & ground). Current drawn through the resistor causes a voltage drop across it, thus making the cathode somewhat "positive". Cathode being positive with respect to the control grid equals the control grid being negative with respect to the cathode. Same function whether power or pre-amp tube. Cheers

 

[Gene Ballzz]

You very much can cathode bias power tubes. Both power tubes & pre-amp tubes have control grids, be so called because this grid is used to control the electron flow (this is the grid that the signal is applied to). When we cathode bias, we use a resistance in the cathode (between cathode & ground). Current drawn through the resistor causes a voltage drop across it, thus making the cathode somewhat "positive". Cathode being positive with respect to the control grid equals the control grid being negative with respect to the cathode. Same function whether power or pre-amp tube. Cheers

^^^^^^^ What He Said! ^^^^^^^

Many popular amps have cathode bias power tubes! The first two that come to mind are the 5E3 Tweed Deluxe and the whole Marshall Studio Series, SV20, SC20 & 2525. There are many, MANY others!

Just Tubin'
Gene

 

[ibmorjamn]

The 6L6 is a better choice to go with the KT66, check out this thread=https://www.marshallforum.com/threads/do-6l6s-and-kt66s-bias-the-same.23563/

 

[Gene Ballzz]

Sounds like a good idea, probably don't need to vary the cathode cap and resistor combination.
I guess it depends on your PT voltages and if you are going for individual Rk/Ck per tube?

Well sir, going off previous builds, I have a 5E3 Tweed Deluxe that uses a ClassicTone PT 40-18021, 355-0-355 VAC and with a 5Y3 produces about 372 VDC at the OT center tap. The 6V6s run happiest right at that voltage. I'm suspecting that for the KT66s to run as happily, I may need to use a different rectifier to get me closer to 390-400 VDC? Obviously that Tweed Deluxe PT is woefully shy on the current capabilities, but the voltages seem OK to start with. I'm finding that EDCOR has a pretty wide variety of transformers, though the wait time can be a bit lengthy! This one looks like a good candidate:

https://edcorusa.com/collections/tu...cts/xpwr155-700v350-0-350-200ma-6-3v-5a-5v-3a

And this looks like a good candidate for an OT:

https://modulusamplification.com/JTM45-Output-Transformer-Based-on-RS-Deluxe-P5295837.aspx

Of course another option is to just buy a Modulus kit and simply modify it to cathode bias?

https://modulusamplification.com/Plexi-45-65-kit-35w-Output-P4377451.aspx

I'm assuming that even considering the currency conversion and shipping to the Used To Be United States, it will likely be a cheaper way to go, than sourcing all my own parts. Also, I've not heard any complaints anywhere about the transformers that Modulus provides? Probably save a little by getting my own tubes, but bump the price back up a bit with F&T caps and CTS pots.

I do still have another build that needs to get finished, before starting on this one!

Still Plannin'
Gene

 

[Georgiatec]

Totally do-able. The Astoria amps are Cathode bias with KT66's factory fitted.

 

[Gene Ballzz]

The only thing I don't like about the Modulus kit is that it appears to only have a 2A/5V tap. I'd really like a 3A option, for monkeyin' with different rectifier choices! Maybe go with the kit minus PT and go for that EDCOR PT I linked?

Still Thinkin'
Gene

 

[2L man]

The only thing I don't like about the Modulus kit is that it appears to only have a 2A/5V tap. I'd really like a 3A option, for monkeyin' with different rectifier choices! Maybe go with the kit minus PT and go for that EDCOR PT I linked?

Still Thinkin'
Gene

You can use GZ34 which diode voltage loss is lowest for rectifiers which filament use max 2A. Then you can simulate less efficient rectifier tube installing series resistors to both anodes and sag effect comes stronger.

 

[Pete Farrington]

And still benefit from the nice slow ramp up of HT voltage provided by the GZ34.

 

[william vogel]

You can probably get away with setting the bias resistor for the 6v6 and it’ll be a more conservative bias for the KT66.

 

[Pete Farrington]

The push pull cathode biased triode conditions on p2 indicate that cathode resistors of somewhat over 600ohms may be necessary. Chinese KT66 have a rep of requiring an even higher bias voltage (ie and so require higher value cathode resistors).
https://frank.pocnet.net/sheets/086/k/KT66.pdf

 

[neikeel]

My calculations suggested 1k4 for a shared cathode KT66 in push pull with 8k primary with voltages of 450v plates and 440v screens. Seemed a bit high to me.
In particular I recently restored a very early 60s Selmer Bassmaster 30 that had been converted from cathode bias to fixed bias (like the TnB 50 series).

I redid the fixed bias properly but have been contemplating back to cathode bias. Obviously 480v plates, 4k OT and EL34s very different but this would run 500R and 100uF combo

 

[Pete Farrington]

My calculations suggested 1k4 for a shared cathode KT66 in push pull with 8k primary with voltages of 450v plates and 440v screens. Seemed a bit high to me.

My thinking is that cathode bias has to idle at much higher current than we’d use here for fixed bias. Otherwise the degree of bias shift with signal would be crippling. So the HT voltage will get pulled down by the higher idle current.

TBH, when Ive tried a fixed / cathode bias switch on a fixed bias 6L6 amp, the results in cathode bias were somewhat disappointing. I think cathode bias works better with lower HT voltages than are normally used in fixed bias amps. Even biased crazy hot, there’s still a lot of bias shift as signal level increases, the high power waveform becomes more like a triangle wave than a sine, and the overdriven tone becomes somewhat thin and reedy.
However, the 8k OT here may mitigate things; I was using a typical 4k OT and didn’t think to try different loadings.

 

[2L man]

My calculations suggested 1k4 for a shared cathode KT66 in push pull with 8k primary with voltages of 450v plates and 440v screens. Seemed a bit high to me.

Perhaps that resistor value is for one tube bias and for PP it is halved to 700 ohms?

Still very cold bias and loadline travels far below the power curve...

 

[Gene Ballzz]

Most datasheets for the KT66 in Tetrode Connection/Class AB1/Push-Pull/Cathode Bias seem to stress the phrase "*It is essential to use two separate cathode bias resistors" although some sheets word it differently. Can I assume this would also necessitate two separate bypass caps? Can I also assume that the most critical part of this setup would be determining the correct bias component values? FWIW, the most detailed and in depth datasheet for KT66 seems to be the G.E.C. from 1963. I sure wish I could wrap my brain around reading "load lines" and understanding what they actually mean!

https://frank.pocnet.net/sheets/084/k/KT66_GEC.pdf

The idea of also being able to also use 6V6 as output tubes is not a truly necessary option, however it seemed that with the similarities of the B+ and OT primary requirements, it might be viable. I do surmise that use of the different tube models may not be a truly "plug n play" situation and might necessitate a means to "switch" between different cathode treatments. The 6V6 that I would be planning for (if attempted) would be the "technically more robust" current production units from JJ.

Please understand again that I am by no means a true technician or amp designer. I do however understand safety protocols and have the skills to be a good amp "assembler," once a circuit has been well planned and ironed out! I am also aware enough to notice similarities in various components and imaginative enough to dream up combinations that have not yet been widely messed with.

I must embarrassingly admit to having a previous build still sitting in my amp cradle, waiting for completion. My workbench has become infected with and buried beneath multiple household and "honey do" projects waiting for parts and finalization for completion. Of course, part of this is getting off my arse to tackle cleaning it all up! If/when you folks help lead me by the nose through these planning stages, I promise to get the fire lit under me and finish that other amp and get going on this one, so your time will not have been wasted!

I really appreciate you fine folks taking the time to travel with me down these less frequented rabbit holes!

Thanks Again!
Gene

 

[william vogel]

I don’t see the need for separate resistors as long as a bypass cap is used. Without the cap, separate resistors would be needed. Also the value of 280 ohms is quite close to the value used with 6v6.