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 Post subject: Cathode resistor question
PostPosted: Jan Tue 30, 2018 3:25 am 
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Location: Bristol, Rhode Island
Hi all,

Building a slightly modified Fender 5D5 circuit, which has two inputs/two channels. I'm building it one input, one channel. My question is about the cathode resistor. Should I increase (4x?) its value to account for three of the four triodes not being used?


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 Post subject: Re: Cathode resistor question
PostPosted: Jan Tue 30, 2018 11:16 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
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Location: Fayette County, Pa
You would need to increase the value, but you may find it is not really 4X. The easiest way would be to build up the circuit and measure the cathode voltage on the single tube version and compare it to that called out on the original schematic. They should be the same, all other factors being equal. (B+ voltages and plate drop in particular.) My guess is you will be somewhere around a 1.5 K 1/2 W resistor. (That is common for the input stage of many Fender amps)


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 Post subject: Re: Cathode resistor question
PostPosted: Jan Wed 31, 2018 2:39 am 
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Location: Spring Hill, FL
Someone double check my thought process here, but I would think you would actually change the plate resistor or the dropping resistor in the power supply B+ rail.

When you remove a tube from the circuit (or add one for that matter) you are changing the total plate current that the dropping resistor sees, and those are picked based on that current and the desired voltage drop (which is figured based on the plate voltage and pale current.) If you remove the other 3 triodes, that throws off the total amount of plate current, which will change the plate voltage. If you change the dropping resistor to one that is 4 times the value that's there now, that should maintain the same plate voltage. From there, the single triode and it's cathode values would remain the same.

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 Post subject: Re: Cathode resistor question
PostPosted: Jan Wed 31, 2018 2:53 am 
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First, the cathode resistor is set by the desired plate current and grid bias voltage. R = Eg / Ip. If the tube is a tetrode or pentode, then add in the screen current.
By this logic, the size of the cathode resistor would exactly scale with the number of tubes in parallel---ASSUMING that the plate voltage remained constant.

So, yes, if there is a shared load resistor, it would have to be scaled. But the cathode resistor also has to be scaled

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 Post subject: Re: Cathode resistor question
PostPosted: Jan Wed 31, 2018 7:54 pm 
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Location: Vancouver, WA
You cropped too much off of your picture. Each pair of triodes shares a 100k plate load resistor. So if you are only using a single triode then you would have to double the plate load resistor as well.

You could fiddle with the with the B+ resistor if you want but it really wouldn't make any difference. Below you can see the load line for the stock configuration in green. The red line is an estimate of what it would be if three preamps were removed (and plate load doubled). The yellow line is for a 1.88k cathode resistor and the blue line represents a 1.5k cathode resistor.

Image

The plate voltage is nowhere near the maximum rating so there is no need to drop the B+ for that.

With any of those combinations there is no effect on gain and there is more than enough grid swing to accommodate even the hottest humbucker pickups. This is one of those times where you don't need to sweat the details. Slap it together with a 200k plate resistor and whatever 1.5-2K cathode resistor you have on hand. As long as the bias voltage ends up around a volt or more then you are good to go.


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 Post subject: Re: Cathode resistor question
PostPosted: Jan Wed 31, 2018 10:01 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 501
Location: Fayette County, Pa
It is true you would have to adjust the plate resistor, assuming the circuit uses a single resistor for multiple tubes. I also assume you have duplicated plate current ad voltage with the original multiple rube circuit. Each tube must be the same as your single tube version. That said my previous statements hold. But if you have made any changes, B+ or other factors the of course you will have to compensate.


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 Post subject: Re: Cathode resistor question
PostPosted: Feb Thu 01, 2018 4:58 am 
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Location: Vancouver Island, Canada
I know you are wanting to calculate your own resistor values, but for reference, check out schematics at AX84.com.

Here's an example, one input: http://ax84.rru.com/media/ax84_m37.pdf

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 Post subject: Re: Cathode resistor question
PostPosted: Feb Sat 10, 2018 5:27 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Gainesville, Florida
tons of reference amp schematics

https://www.google.com/search?q=fender+ ... 336NwxCzgM:


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