Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: texwest on August 16, 2020, 06:36:18 pm
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I'm looking at a new 5e3 build trying to make one with more headroom and wondering about mounting the screen resistors over the tube sockets from pin 1 to pin 4?
I know fender mounted the grid resistors from pin 1 to pin 5 and screen resistors from pin 6 to pin 4, but I'm curious about doing the opposite.
It looks to me like my lead dress will be neater if I wired it with the grid resistors from pin 6 to 5 and the screen resistors from pin 1 to 4.
Is there any reason to not do this??
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That's fine.
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I was wondering if there was a reason to not put the screen resistors there. If putting them over the socket would expose them to excessive heat buildup?
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There are no screen resistors in a 5E3 and Fender mounted the grid resistors between pins 5 and 6. That's the only proper way to do it. :icon_biggrin:
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Seriously, I'm building a 5e3 that will be using a 5ar4 or 5u4 and trying to push the clean headroom a little higher. Are screen resistors still not needed? Do they change the tone too much?
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Here's how I did my 5E3. The tone is wonderful and the headroom is just right.
http://sluckeyamps.com/5e3/5e3.pdf
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Seriously, I'm building a 5e3 that will be using a 5ar4 or 5u4 and trying to push the clean headroom a little higher. Are screen resistors still not needed? Do they change the tone too much?
Rg2 can be useful to eat up Ig2 peaks under big signal conditions. But in a 5E3, the Vg2 is sitting 50-60V below the plates anyway (because of the ~5k (4k7) screen supply resistor). In addition to lowering Vg2, the supply resistor eats up peaks in Ig2. The screen supply resistor also lowers Vg curves so the load line is closer to crossing the knee of the Vg0 curve, so that under big signal conditions, there is less risk of harm from excess Ig2 anyway.
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Here's how I did my 5E3. The tone is wonderful and the headroom is just right.
http://sluckeyamps.com/5e3/5e3.pdf
That looks like a standard 5e3 circuit except for the .047 cap on the tone control on both the schematic and the layout. It looks like you have 470's on the screens in the layout but not on the schematic.
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Seriously, I'm building a 5e3 that will be using a 5ar4 or 5u4 and trying to push the clean headroom a little higher. Are screen resistors still not needed? Do they change the tone too much?
Rg2 can be useful to eat up Ig2 peaks under big signal conditions. But in a 5E3, the Vg2 is sitting 50-60V below the plates anyway (because of the ~5k (4k7) screen supply resistor). In addition to lowering Vg2, the supply resistor eats up peaks in Ig2. The screen supply resistor also lowers Vg curves so the load line is closer to crossing the knee of the Vg0 curve, so that under big signal conditions, there is less risk of harm from excess Ig2 anyway.
Thanks, sounds like the 5k resistor makes the 470's unneeded. I guess I could try them and see if I like it or not.
{EDIT: untangled quote- PRR}
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Using pin 1 as an anchor point for the screen resistors is a potential problem if the amplifier ever uses vintage metal-can 6V6's. The exterior surface of these 6V6's is designed to be grounded via pin 1. Having the screen voltage accessible on the exterior surface of the valves ( tubes ) is an obvious safety issue.
Metal-can 6L6's also use the same connection.
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Using pin 1 as an anchor point for the screen resistors is a potential problem if the amplifier ever uses vintage metal-can 6V6's. The exterior surface of these 6V6's is designed to be grounded via pin 1. Having the screen voltage accessible on the exterior surface of the valves ( tubes ) is an obvious safety issue.
Metal-can 6L6's also use the same connection.
There we go. That's interesting. Of course this could also be a problem if we put the grid resistor there too.
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FYI, back when Fender was building new 5EX amps there were plenty of metal jacket 6V6s and 6L6s, so pin 1 was always connected to chassis and the grid resistor mounted between pins 1 and 5. Later on when the AB763s were introduced (and metal jackets were phasing out) Fender quit grounding pin 1 and began mounting the grid resistor between pin 1 and pin 5. Still doing so.
It really makes for a neat wiring to put the grid resistor directly between pins 1 and 5 (close to the tube base) and the screen resistors between pins 4 and 6 (about 1/2" above the socket pins).