correct value cathode resistor for an 18 watt rocket

[rock_mumbles]

Since you've used quite a bit higher voltage PT, you have a 580V PT vs the Rocket 520V, so I'd think your voltages will be quite a bit higher than those on the Rocket voltage chart ...

[M Fowler]

Yes the voltages will be higher for the noted voltage charts but a 5u4 recto tube can be used if you have 3A 5V tap. Or use 5Y3 2A recto.

290-0-290 is what some 18w Marshall builds use or Vox AC15 build.

I attached my 18w Rocket build voltage chart using 290-0-290 OT and the layout of my amp.

I gave this amp to my oldest son and I think I will change it over to 260-0-260 to get the plate voltage down. He doesn't use the amp much but still that would help on tube life.

Mark

[wsaraceni]

thanks for the voltage chart. it'll be nice to compare mine to yours. after this whole build, i sometimes wish i went with my toneslut express transformers and just build the 30 watt version. but i think the 18 watt version will work fine for me. is there a huge difference in voltages with a 280-0-280 through a gz34 and a 290-0-290 through a ez81? I can't ever seem to find the link that shows the math.

[wsaraceni]

so here are the voltages i got. and just so everyone is aware. this is closer to a z-wreck 18 than a rocket 18.

V1
pin:
1)136.2
3)1.199


V2
pin:
1)252.8
2)158.6
3)157.8
6)158.7
8 )1.086

V3
pin:
1)237.7
2)39.9
3)57.6
6)239.7
7)39
8 )56.97

EL84s
cathode (pin 3) 11.74
pate (pin 7) 322
screen (pin 9)329

mains 122.8
secondaries 286
B+:
1:334.6 off of standby switch
2:320.7 off of choke
3:297 off of 27k resistor
4:251.6 off of 22k resistor
5:313.1 off of 20k resistor

to be honest, this is about as far as my technical knowledge takes me. hopefully you guys wouldn't mind helping the tuning process!

[wsaraceni]

so do i have this right? the resistor i am currently using is 150ohm 5 watt so the current is 11.74/150 or .078 amps. so each tube is using .039 amps. at 322 volts or 12.6 watts each? which puts them at just over 100% dissipation? or am i way off?

[overtone]

As I see it you have under 12 Watts plate dissipation per tube.
The plate "sees" the voltage between the plate and cathode 322 - 11.7 = 310.3V
You have some screen current in there too, so it will be more like 11W plate dissipation per tube. That's a quick guesstimate as I did not take on board your screen B+ and resistor value.
Once I rolled my own Rocket and it came in at 11.5W plate dissipation per tube. Sounds good.
Best, tony

[wsaraceni]

thanks. i didn't realize that. i did notice that my voltages are close to, but slightly under, the spitfire schematic and they are biased with a 120ohm resistor.

[M Fowler]

You can increase power tube bias resistor to 150R to lower bias range.

Your dropping string can be adjusted by where you tap the voltage from.
To lower the voltage increase the dropping resistor.

Mark

[wsaraceni]

Im at 150r now with the above voltages. I'm looking up ac30 /ac15 voltages to compare as those are closer in design. Did you happen to take voltages on your zWreck clone in the speed setting? I'm guessing that's where I need to be. From what I can see on the messy ac15 schematic I have I'm not far off. Just unsure about the preamp mostly.

[wsaraceni]

does it make sense that i get a lower voltage off of the 22k resistor than i do the 27k resistor since they are parallel to each other, not in series?


also, for the ac30 top boost preamp schematic i've found. they are looking for 290v B+ going into V1 and V2. I am getting 297v for v2 B+ and 313 for v1. i'll probably change the resistor for v1 from 20k to 22k and take the readings again.

should i leave the power section biased as is? it seems the ac15 ran slightly higher voltages AND a 130ohm resistor which would be much hotter than this setup.

on a side note, i plugged it in to my crappy cab but had a chance to crank it a little bit. I'm pretty excited. it sounds pretty good and i'm going to consider this my first real amp build!!!! serial #001

good feeling to finish it up today except for some minor tweaking!!!