Rocket Voltages

[stevek66]

I have changed my Rocket from the low voltage taps w/50ohm cathode bias resistor to the higher voltage taps w/the 68ohm cathode bias resistor.
Just thought I would post the voltages for reference.Maybe it will be useful to someone and maybe not.Here it is.

[RJ Guitars]

Steve,

Can you tell us what you the sonic differences are? I have a Rocket built with a VOX AC-30 power tranny that has very similar voltages. It seems to have a bit more edge or you could say that it's not quite as smooth, but there are other differences besides the power tranny so I'm curious what your hearing?

Thanks,

rj

[stevek66]

Hey RJ,
I think the basic sound is the same.However with the higher voltage everything is bigger.The low end seems a little tighter and the amp breaks up a little sooner.Definetly a volume increase.
I need to look at the Bias however,seems pretty hot at 11.02 compared with 8.2 with the 50ohm resistor.I think I seen that you run a 100ohm on yours what do you get for Bias voltage off pin 3???

Thanks Steve

[RJ Guitars]

Steve,

Check my math - I get:

(11.02 / 50) / 4 = 55mA per tube
(8.2 / 50) / 4 = 41mA per tube

If I've got that right then yes it's pretty hot. I don't know what those tubes will take before they start to red plate, but at the least I imagine the life expectancy might be a bit shorter than the specs indicate... and of course they probably sound awesome.

In Gerald Webers book he mentions that you need to run the same tubes in a cathode biased amp hotter (higher bias voltage) than you do in a fixed bias amp... some of that is kinda intuitive but I'm not sure I understand the whole story there.

I would be curious how much difference you would hear in the sound if you lowered the bias down near that 44mA level?

I used a pair of 120 Ohm cathode resistors and split the output section to allow for a half power switch... so i think my values may be similar to the 44mA number, but that is a guess. I have not run the same math on my EL84 Rocket but I think this is an incentive to look into it at the very least.

Also, I was running the 260-0-260 Liverpool type power tranny so my answer may be even different than either of yours... I'll get that thing out of the cabinet and on the bench and get some numbers.

rj

[stevek66]

Steve,

Check my math - I get:

(11.02 / 50) / 4 = 55mA per tube
(8.2 / 50) / 4 = 41mA per tube

If I've got that right then yes it's pretty hot. I don't know what those tubes will take before they start to red plate, but at the least I imagine the life expectancy might be a bit shorter than the specs indicate... and of course they probably sound awesome.

In Gerald Webers book he mentions that you need to run the same tubes in a cathode biased amp hotter (higher bias voltage) than you do in a fixed bias amp... some of that is kinda intuitive but I'm not sure I understand the whole story there.

I would be curious how much difference you would hear in the sound if you lowered the bias down near that 44mA level?

I used a pair of 120 Ohm cathode resistors and split the output section to allow for a half power switch... so i think my values may be similar to the 44mA number, but that is a guess. I have not run the same math on my EL84 Rocket but I think this is an incentive to look into it at the very least.

Also, I was running the 260-0-260 Liverpool type power tranny so my answer may be even different than either of yours... I'll get that thing out of the cabinet and on the bench and get some numbers.

rj

Hey RJ,

In your math formula "(8.2/50/4 = 41ma) 50 is the cathode resistor,correct?
If so then with the highr taps i used a 68ohm cathode resistor.
That would mean ( 11.02/68/4 = 40.5 ma ).
Still hot with an increase of approx 40 volts w/higher taps.
The tubes aren't red plating and it sounds good.

Steve

[RJ Guitars]

Steve,

Correct you are.... so at 40.5 mA, you are probably in a very good spot.

I think Fishy Pete posted his voltages a while back and I know Ron Worley had posted his so there may be some other references. That will tell us what other folks have done and is useful especially when somebody built an amp that sounds really good.

I finally took the bother to read the tube specs on the Duncan Amps web site and was a bit surprised at the max cathode current for these tubes. http://www.nj7p.org/Tube4.php?tube=6BQ5 I am not entirely familiar with the subtleties of reading tube specs so the masses might want to get some real expert input before you cook your tubes, but if I read that correctly you can have 65mA on the cathode at a plate voltage of 300V. Stout little rascals I would say!.

rj