18Watt.com

Hi, I'm just wondering:
1: Can I use just one 8k2 x 1W resistor for each pair of 84s to pin 2, ie, one resistor to the first 84 and jumper to the next one and do the same for the other pair or is it mandatory to use one 8k2 per tube?
2: Can I use one 10W 130R resistor and one bypass cap for all 4 cathodes or should I use two 5Ws and two caps per pair of 84s?
3: Are 82R x 1W resistors as seen in the Ceriatone layout OK for the screens?
4: The resistor on the 32/32 can cap, should it be 1k 5W with the additional 100R screens or 82R as in the Ceriatone, or would 2.2k 2W with the addition of a 1k x 5W screens R as used in the Brown note 18 watt and what would the diff be.
Thanks

Deadwood wrote:1: Can I use just one 8k2 x 1W resistor for each pair of 84s to pin 2, ie, one resistor to the first 84 and jumper to the next one and do the same for the other pair or is it mandatory to use one 8k2 per tube?

You're meant to use one per tube, and they're supposed to be soldered directly to the tube socket pins at one end, to help prevent parasitic oscillation. However, you could always try the shared grid resistors if you like, assuming the tube sockets are very close to each other, and see if it sounds like c**p.

Deadwood wrote:2: Can I use one 10W 130R resistor and one bypass cap for all 4 cathodes or should I use two 5Ws and two caps per pair of 84s?

No way. For all four cathodes, you would actually need a value of 65 ohms. However, it's smarter to have separate cathode resistors for each opposing PP pair of tubes, so that you can pull one pair any time you want to run the amp at 18W. You also need to remember to change the OT secondary tapping the speaker(s) is connected to when yo do that. BTW I also wouldn't use anything less than a 150 ohm cathode resistor, as with 130 ohms, they'll really be frying.

Deadwood wrote:3: Are 82R x 1W resistors as seen in the Ceriatone layout OK for the screens?

Probably not. So once again I would recommend treating each PP pair in your 36W power stage separately, so you have the option of running at 18W any time you want. Then either run each pair of screens the traditional but very hot way with a shared 100 ohm resistor, or the Lite IIb way with a shared 1k resistor followed by individual 100 ohm ones, or even simply run individual 1k resistors to each EL84.

Deadwood wrote:4: The resistor on the 32/32 can cap, should it be 1k 5W with the additional 100R screens or 82R as in the Ceriatone, or would 2.2k 2W with the addition of a 1k x 5W screens R as used in the Brown note 18 watt and what would the diff be.

I tend to prefer the way it's done in the Lite IIb, to help prevent the screens getting close to meltdown every time you crank the amp. But some people prefer the sound of tubes melting down. So you can try different ways if you want and see which you prefer.

Thanks Phil, I won't be wanting the half power option with this amp, from what you've said I will use seperate 8k2s per tube on pin 2.
I'm still confused as to cathode resistor value/s. If I run 150R per pair, would that be good? and is 5W enough for each one? Or could I just use one for all four, you say 65R 10W? or better to use 2 x 150R 5W?
As for Q3, I think I'll run the lite IIb setup there with individual 1k 5W.
Q4: I'll go with the Lite IIb way again there. If you could please clarify my confusion with the cathode resistors I will be on my merry way.
I'll be doing the Lite IIb pre amp with SS recto and sag resistor as well. After my recent 18W lite I'm happy to just do this one the same but with the added power. If you could just clue me up on the cathode resistors mate. Thanks again.

Just use one 150 ohm 5W resistor per each PP pair of EL84s. That will also give a bit more safety factor.

And don't write off the ability to be able to run the amp on a single pair of tubes, if necessary. It costs you nothing and could at least get you by in an emergency situation, for example if one EL84 goes south at a gig, and you forgot to bring spare valves.

Many thanks