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I just realized today that the first 5E1 I built I put a 470/5W cathode resistor on the 6V6. I'm getting ready to build another, and noticed that the schematic doesn't have a power designation, just 470 ohms. Is there enough power dissipating at that point to require a large resistor? Would a 1/2W burn out pretty quickly?
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Yes, it needs to be higher than 1/2 w - go 5W. You might get away with 3 W, but why push it? That resistor can get hot depending on how much current you have in the 6V6. The calculation is P = I squared * R, and then you usually double that for safety. -
Thank you sir!Comment
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Depending on plate voltage, you could be developing 20-30vdc accross that resistor...let's take the worst case scenario..
30v*30v=900
Divide 900 by resistor value in ohms (470) gives us 1.91W of dissipation, double that for safety and we get 4W rating min, 5W are often easier to find. So you see a 1/2W is not really up to the job.Comment
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Not to hijack the post, but I think my question is related if the OP is designing a 5E1...Originally posted by MWJB View Post
The old data sheets list max plate voltage of 350V for 6v6, but it seems like we can get away with more than that. What is the rule on this? Are there more modern datasheets out there? What are the real limits to plate voltage and current that we should be designing for?Comment
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In cathode bias, SE, 21W-ish dissipation seems safe enough. For NOS 6V6s I'd stay under 425vdc (measured from plate to cathode), Current production, Hi voltage, 6V6 like the EH, JJ are unlikely to have voltage issues in a typical champ build.
400v effective plate (to cathode) voltage if you want to leave a safety margin, for any kind of 6V6?
21W plate dissipation.
60mA plate current.
...whichever is the limiting factor you hit first.
You would be very unlikely to find a Fender that runs a single 6V6 at <350vdc.
In cathode bias PP - I know of a guy who builds reliable amps that run around 400vdc (plate to cathode) @ 30mA. Some old 5E(F?)3 Deluxes might exceed 18W per tube (but probably won't have the original OT!), best to stick to 14-16W max. I'd try and stay <40mA per tube, and >30mA (in some old cathode biased amps biasing to less than 14W with the stock PT means running in the late 20's mA-wise, doesn't always sound so good and can push voltages to the high side for NOS tubes).Comment
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The original amps had a tiny carbon comp 2 watter and I have not seen many that had a bad one after 50 years.Originally posted by wcrankshaw View Post
You'd be OK with a 2 or 3 watt resistor but 5w resistors are so easy to find, I'd just use one anyhow.
I'd use a 510@5w ohm resistor myself.Comment
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Yikes, 21 watts is brutal are you actually thinking an old 6L6 not a 6V6?Originally posted by MWJB View Post
I have three old Deluxes here right now for refurb with American made 6V6s (they aren't NOS now but were just a couple months ago) and with my variac set to 122vac, the 5B3 is running at 11 watts per tube, the 5C3 is about +12 watts and the 5E3 is the hottest at around 13.5 and 14 watts per tube... nos 5Y3GT rectifier in each.
Plate to cathode voltages all seem to be hovering around 335dc to 355vdc in them with the stock 5 watt 250 ohm biasing resistor.Comment
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"Yikes, 21 watts is brutal are you actually thinking an old 6L6 not a 6V6?" 6V6 - It's typical enough for a higher voltage SF champs with a 470ohm cathode resistor.
I have seen 410vdc from plate to ground in a late 5E3, with stock PT & 250ohm cathode resistor, had 50mA per tube...indeed not typical, nor desirable, but OP was asking about limits rather than yardsticks.Comment
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