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should be easy........what is the cathode currents of your tube? what is the negative grid voltage? knowing these two, you can Ohm your way to a suitable resistor, get the nearest standard value....
always remember, whatever voltage is dropped at your cathode bias resistor, then that is the negative grid bias...
take note too, that if you poke your voltmeter to your grid, you get a 0 volt reading, and that is because a positive voltage is developed at the cathodes by the cathode resistor.....
this is as easy as it gets...
always remember, whatever voltage is dropped at your cathode bias resistor, then that is the negative grid bias...
take note too, that if you poke your voltmeter to your grid, you get a 0 volt reading, and that is because a positive voltage is developed at the cathodes by the cathode resistor.....
this is as easy as it gets...
Page 2 of this datasheet states "it is essential to use separate cathode resistors"
http://frank.yueksel.org/sheets/084/k/KT88_GEC.pdf
Like Tony, I always use separate resistors. Even if you use high quality parts, the extra two resistors and two caps don't cost more than 20$. I used these in my KT88 amp as bypass caps for 10W Dale WW resistors.
PEG124PL347VQL1 KEMET | Mouser Canada
The 22,000 hour rating at 105c makes sense if they are mounted beside hot parts in a hot chassis.
http://frank.yueksel.org/sheets/084/k/KT88_GEC.pdf
Like Tony, I always use separate resistors. Even if you use high quality parts, the extra two resistors and two caps don't cost more than 20$. I used these in my KT88 amp as bypass caps for 10W Dale WW resistors.
PEG124PL347VQL1 KEMET | Mouser Canada
The 22,000 hour rating at 105c makes sense if they are mounted beside hot parts in a hot chassis.
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Thanks TonyTecson. I think this is an education and perspective thing.
I'm sure this is as easy as it gets... - for a seasoned amp engineer, who may have seen and resolved this type of challenge a thousand times.
For a self-taught newbie like myself, without any formal electrical training; without a dad who was a TV technician; and without a bloke up the road who shared his knowledge - it's really not so simple to understand.
I have learned to assemble tube amp kit-sets, learned how to use a multimeter and ERS tester, taught myself how to test and replace faulty components; and can follow a schematic to the letter.
So, I am here in DIYAudio to try and learn more - and do it myself. I appreciate that you are trying to help me, but the fact is, that I am not yet at an expertise level, that I can make full use of your data.
Baby steps for me...
Thanks for this kodabmx.
As below, getting the parts for this circuit change - or affording them - is a non-issue.
Figuring-out the right values and understanding exactly where to put them, are the issues for me.
Also, once the new components are in place, I need to understand what other changes I would need to make to the circuit, to de-activate any "auto-biasing" that is currently in play.
Questions, questions, questions...
As someone who was also self taught, I can say this is a great resource if you're starting out...
Basic Electronics, Volumes 1-5 (1955) : Van Valkenburgh Nooger & Neville : Free Download, Borrow, and Streaming : Internet Archive
I now design amplifiers... When I joined this forum almost 10 years ago, I hadn't even built a preamp as far as I remember.
Basic Electronics, Volumes 1-5 (1955) : Van Valkenburgh Nooger & Neville : Free Download, Borrow, and Streaming : Internet Archive
I now design amplifiers... When I joined this forum almost 10 years ago, I hadn't even built a preamp as far as I remember.
It's labelled R13 and it's in parallel with C5. This is the "shared" cathode resistor.
Ohm's law says we need to double the resistance to get the same voltage drop with half the current, so make R13 250R into two 500R resistors, each with their own cap, one set for each output tube. In a stereo push pull amp, that's 4 sets.
Totally agree. Also DF96 and AndrewT may they rest in peace. Great minds on this board.
Come to mention it, your posts are interesting too, Tony. You actually inspire me to learn how to wind a transformer. Just no idea how to do it for audio. I could probably wind a simple power transformer easily enough though.
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No.
You still want both tubes in a push pull pair to "amplify equally well", that
is have equal Gm.
Most serious vendors has little on no additional cost for matching as they
will sift through the tubes for test anyway.
Leave the 6SL7 tubes and remove the 100uf by pass cap on the 6Sl7' s cathode resistor. This will reduce the gain and decrease distortation.
Can anyone explain to me how leaving this bypass cap in place - as installed by the previous owner - delivers increased Gain and distortion, in this circuit?
My understanding is that having this 100uF cap - bypassing the original 100R resistor in this position - simply serves as a bypass filter, filtering (approx.) >160Hz frequencies.
So, what else is this capacitor doing here - and how is it doing it?
Or is the potential reduction in Gain and distortion - as proposed by hellbender - simply a result of returning back to the (originally designed) 6SL7 tube in this position; and the removal of the 100uF cap simply facilitates this return?
(Sorry - still learning...
)
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The 100uF cap removes any possibility to act as phase inverter.
Several people has suggested to at least restore to original "lampisator"
config.
Myself finds the whole design odd and faulty, but that is me. I also
lacks meaningsful data of the performance as lampisator suggests it
and with the 100uF cap (and other mods)
Discussing an amp that has no firm claim and no measured parameters
seems a little like religion to me.
Several people has suggested to at least restore to original "lampisator"
config.
Myself finds the whole design odd and faulty, but that is me. I also
lacks meaningsful data of the performance as lampisator suggests it
and with the 100uF cap (and other mods)
Discussing an amp that has no firm claim and no measured parameters
seems a little like religion to me.
cathode currents determine the tube transconductance, inside the tube there is a cathode resistor because of cathode current, this resistor do not show in schematics, and this resistor is in series with the external resistor to the cathode, if this external resistor is bypassed, it will look as if no external resistor was present, gain therefore is higher than when no bypass cap was used...plus the unbypassed external resistor acts as a local negative feedback.....
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