Badcat 20 CLassic Deluxe output valves running extremely hot!

[taddy]

Hi everyone,

A friend asked me to bias his hand-wired Badcat 20 Classic Deluxe. Having studied the circuit, it seems to be very close to a Fender Deluxe Reverb. However, the anode voltages on the 6V6s are very high - 456V!

My mate has put new JJ 6V6S tubes in there which have a max dissipation of 14W.

The existing bias setting of -37V (I haven't tweaked it yet) produces a cathode current of 41.5mA (measured across a 10.4ohm cathode resistor which Badcat have conveniently included). This equates to a plate current of around 39mA and a plate dissipation of nearly 18W! As far as I know the previous valves were standard 6V6 s so this setting would seem even more inappropriate.

Readings are the same for both tubes and it seems to be fine otherwise.

Unless I've made a mistake with my calculations or I've overlooked something the obvious answer is to increase the negativity of the bias in order to bring the plate current down to around 27mA or less (apparently the amp is class A).

However looking at the plate characteristics for this tube (at http://www.jj-electronic.com/images/stories/product/power_tubes/pdf/6v6s.pdf) we are right in the bottom right corner of the graph and I'm wondering if that is healthy in terms of the sound? If I increase the bias significantly, am I going to get cross-over distortion?

[sluckey]

How much voltage do you actually measure across that 10.4Ω cathode resistor?

[PRR]

Is it eating tubes?

Does it have other problems?

A proper "bias" would start with getting that B+ way down. That would change the amp significantly.

Modern 6V6es "know" they will be run far hotter than 12/14W. Fender has run a recent Champ at 17W.

I say the amp is "biased" as good as it can be for the hot-rod design. Any hotter will burn tubes. Any colder and it will get "hoarse" on very soft sounds. Touch the bias screw and charge him $50.

[taddy]

How much voltage do you actually measure across that 10.4Ω cathode resistor?

0.432V across the 10.4ohm cathode resistor
V454 on the anode
-37.2V on the gird

The other tube was almost exactly the same.

[taddy]

Is it eating tubes?

Does it have other problems?

A proper "bias" would start with getting that B+ way down. That would change the amp significantly.

Modern 6V6es "know" they will be run far hotter than 12/14W. Fender has run a recent Champ at 17W.

I say the amp is "biased" as good as it can be for the hot-rod design. Any hotter will burn tubes. Any colder and it will get "hoarse" on very soft sounds. Touch the bias screw and charge him $50.

Hi PRR

I don't think it is eating tubes but it is hissing quite loudly and one of the last tubes burnt out.

Other than getting a new power transformer, how would I get the B+ down? Apparently we shouldn't put plate resistors on the output tubes.

I can't charge the guy! He's my best mate! But next time I bias an enemy's amp...

[92Volts]

Hiss is usually overheated resistors, more often in preamps (plate resistors).

I was going to suggest swapping in 6L6s if the power transformer could take the extra 0.9a heater current, looked for some info and it turns out the manufacturer says it can! https://www.badcatamps.com/bad-cat-usa-player-series-amps/classic-pro-20r-usa-player-series

So that's an option.

With 6V6 tubes lower bias is a trade of tube life vs. sound. Only way to know if it's worth it is for your friend to play it and see if they like it biased hotter. If they're willing to pay more (for tube replacements) to run it hotter... go ahead and run it hotter!

[taddy]

Thanks 92. It d be good to know what the normal operating parameters arefor this amp. Badcat didnt respond but if anyone else has cathode bias and plate readings...

[VMS]

Does it sound bad if you bias it colder?

[92Volts]

Arguably, it doesn't matter what "normal" current is for this amp-- we know how much power the tubes "should" handle and we can make our own judgement about exceeding those limits (or not) for better sound (or longer tube life) regardless of Badcat's intentions.

Consider JJ 6V6S is rated for 14W anode dissipation with no screen limit listed. Other 6V6 versions allow 2W screen dissipation, so these tubes may be good for a ballpark 16W total. The cathode current you're measuring is the combination of screen + anode current.

[VMS]

Here is one 6V6 amp with high plate voltage, notice that the grid bias marked from -55 to -45.


http://el34world.com/charts/Schematics/files/_Miscellaneous/jimkelly_facs.pdf