What happens above 330 ohms

[atmars]

OK, you'll just have to take my word for it on this... I actually have some experience with these amps.
B+ of 360v to a max of 380v is right at the limit for these with a 250 to 270 ohm biasing resistor.
If you use a cathode biasing resistor that is more then 330 ohm (to an absolute max of 360 ohms) will just make the amp start sounding and feel completely different then the classic 5E3.
Yes, you can run very high B+ and use a high resistance biasing resistor to keep the tubes idling at or below their 14 watt DC rating....but it won't have the classic 5e3 sound.
The trick 5E3 set up is to bring the B+ down, use no more then 270 ohms and a pair of matched power tubes that are not overly soft, (that means wanting to draw high quiescent current with the same bias and B+).
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How does the character of the amp change as the cathode resistor gets beyond this (330) cut off point? Has any one tried it and can report on the change in sound? I am at that point with my current build and I am wondering what to listen for to tell if I have gone too far. Also, i am just interested to know what this change is and what causes it.

Thanks

[tubeswell]

With 330R instead of 250R or 270R, you will be increasing the bias, which will make the output stage run a bit cleaner and will lower the tube current, making the amp sound a bit sterile.

[MWJB]

"which will make the output stage run a bit cleaner and will lower the tube current, making the amp sound a bit sterile." Not necessarily, it really depends where you are voltage-wise to begin with. Some older 5E3s run well over 380v/40mA with the stock 250ohm (some current kits also run borderline, like the TAD at 385v, going up on cathode resistor value will bring plate dissipation back around the 13W to 14W mark, especially if using a 5V4 rectifier (relatively common tweak), or the Sovtek "so called" 5Y3 that comes with some kits & returns considerably more voltage than a NOS item. As the quote suggests bringing down B+ is an option, this is commonly done with zener diodes.

Plate voltage will rise and the amp may sound stiffer as cathode resistor value rises.

If your 5E3 runs <385v at the plates it's probably not necessary to rebias from the stock 250/270ohm. On the other hand you might feel you need more headroom and use a tighter rectifier, requiring a rebias? What voltages do you actually have?

As the quote states (it covers the topic pretty comprehensively), it is perfectly feasible to run higher cathode resistor values (as long as resulting voltages don't exceed tube & cap ratings, under 30mA plate current per tube is probably not a good idea) and you 'may' like the sound you get, BUT your amp won't have "classic" 5E3 tone...which is what most people would assume that you do want, having built a 5E3 in the first place.

So your first port of call is really to record voltages/plate current with a NOS 5Y3 in the amp.

[atmars]

What voltages do you actually have?

This build is a conversion from another old non-guitar amplifier.

Old stock 5Y3

Voltage measured across cathode: 25.5v
Cathode resistor value: 350R - actual measured value 344R
DC measured between plate and cathode: 402v

Weber calculator gives me 14 watts dissipation with 35mA . 14 watts was pretty close to my hand calculation.

I've never heard a real 5E3 in person, but this thing sounds pretty awesome. Is this situation reasonable, or is voltage reduction the way to go?

[MWJB]

"Is this situation reasonable, or is voltage reduction the way to go?" this sitiuation is perfectly reasonable as far as tube life is concerned...if you like the way it sounds then that's cool.

Voltage reduction using a couple of 25v 5W Zeners between PT centre tap and ground would allow use of the stock 250ohm cathode resistor...easily reversed if desired too.

[clyde]

In my Mission 5E3, using a nos 5y3, think it runs about 320 volts, last I remember measuring. Even tried a russian "5y3". That put b+ at about 390. The only thing about high voltage like that would be tube life, as MWJB stated. Unless the tubes are red plating or having other issues, go for it.