What would we think of a generally stock JTM45, with KT66s and the twist of cathode bias? And what what need to be done/added to also support the use of 6V6s. Both tube models seem fairly happy with similar B+ voltages and both seem to be well suited for an 8K primary output transformer.
Just Thinkin'
Gene
Cathode Bias JTM45/KT66?
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The Ballzz
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Re: Cathode Bias JTM45/KT66?
Out of curiosity(*): Isn't this kind of power section supposed to add a significant amount of character to the amp?
I don't know enough about Marshall amps to tell you wether it will be too much or not - but if the preamp is fairly simple in terms of gain stage number, why not?
(expecting the "because of..." to be objected)
_
(*) with apologies if this message is of little help to you
I don't know enough about Marshall amps to tell you wether it will be too much or not - but if the preamp is fairly simple in terms of gain stage number, why not?
(expecting the "because of..." to be objected)
_
(*) with apologies if this message is of little help to you
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Stevem
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Re: Cathode Bias JTM45/KT66?
In terms of running 6V6 tubes in this amp.
Cathode biasing done to any amp that was fixed bias would 1) lower the output wattage of the amp some.
2) soften the attack as the availabile power supply current started to level off.
3) give the amp a touch more overall sustain.
4) dump more heat into the chassis since you would need close to a 20 watt cathode resistor .
5) you would need to play around with the value of that cathode resistor bypass cap to get the bottom end you want and the added gain and output stage break up point that you like .
This assumes you run the amp loud enough to get to the break up point, but the cathode biasing itself will get your there sooner.
I would use a 50 volt rated cap.
6, other changes) you will need to ground the junction of the 220k resistors on the output side of the PI tube.
To keep from cathode biasing this amp with its high plate voltage into to class A so you do not burn up the tubes I would start with a 1000 ohm cathode resistor and then see how much cathode current your drawing.
To measure this you will need a 1 ohm 1% 3 watt resistor that goes in series to ground from the cathode resistor .
I would also for starters up the two 470 ohm 5 watt screen resistors to 800 ohms .
Cathode biasing done to any amp that was fixed bias would 1) lower the output wattage of the amp some.
2) soften the attack as the availabile power supply current started to level off.
3) give the amp a touch more overall sustain.
4) dump more heat into the chassis since you would need close to a 20 watt cathode resistor .
5) you would need to play around with the value of that cathode resistor bypass cap to get the bottom end you want and the added gain and output stage break up point that you like .
This assumes you run the amp loud enough to get to the break up point, but the cathode biasing itself will get your there sooner.
I would use a 50 volt rated cap.
6, other changes) you will need to ground the junction of the 220k resistors on the output side of the PI tube.
To keep from cathode biasing this amp with its high plate voltage into to class A so you do not burn up the tubes I would start with a 1000 ohm cathode resistor and then see how much cathode current your drawing.
To measure this you will need a 1 ohm 1% 3 watt resistor that goes in series to ground from the cathode resistor .
I would also for starters up the two 470 ohm 5 watt screen resistors to 800 ohms .
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Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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martin manning
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Re: Cathode Bias JTM45/KT66?
I think that's the point, no?
And disconnect the negative bias, of course.
I'd just use the measured value of the cathode resistor.
For 6V6 I'd use the OT as normal for 8k primary, but for KT66 or EL34 I'd miss-match the load to get a 4k primary. With the larger tubes the power section would then be something along the lines of a Bad Cat Hot Cat 30.
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The Ballzz
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Re: Cathode Bias JTM45/KT66?
My sincerest thanks to all for humoring me on this trip down a somewhat less traveled rabbit hole. Please understand that, as I've mentioned here before, 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.
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.
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 and efforts will not have been wasted!
Thanks Again Folks!
Gene
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.
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 and efforts will not have been wasted!Thanks Again Folks!
Gene