Well my jury is in on this mod and I LOVE IT.
I am running a 300-0-300 transformer with a GZ34 rectifier through a 130 ohm sag resistor to my filter string, and I left the choke in. Also running 82 ohms on the cathode which gives me 30ma's on each tube. With a humbucker I get the cleanest AC/DC crunch that just hits you and with a single coil it is driven blues with an attitude. Roll your guitar volume around to 8 and it cleans up and gets chimey. The humbucker reminds me of an old 72' Marshall head and a 69' Les Paul type of crunch. If you love really good vintage crunch and the cleanliness of the Rocket you'll love this mod. If you love good clean Rocket and you think the 50 ohm cathode resistor was too much and changed it to something higher to clean it up even more...stay away from this mod.
Honestly this mod transforms the rocket into something different, almost more express-ish but still able to take pedals and without all that sizzle of a wide open Express.
Funny thing, I checked it when I took the sag resistor out and it was pulling 389 volts in standby and 285 volts on. It struck me as kinda odd because with diodes I usually get 400 volts loaded out of this transformer. The gz34 is a bit close to the line on the tester. Could it be dropping due to a weak rec. tube. I didn't take any readings when I re-installed the sag resistor. It just sounded horrible without it and with a 50 ohm cathode resistor. I will have to take some measurements tommorrow.
Nick got it right when he did the Rockit using switchable 260 and 290 secondaries. It is using 50R and 68R bias resistors.
Not much difference between 290 and 300 and the transformers could be actually less or slight above the rated voltages.
But you must be running hot liking the tone and will need to replace tubes more often. Take some measurements and will need to decide how hot you want to keep running the EL84s.
The Dizzy 30 is also 290v secondary and using 62R and 68R bias resistors.
Both are using a 220uf/160v bypass cap.
You could also run a bias switch between SS and tube not much difference in rectified voltage but perhaps different feel or tone. The sag resistor can be switched out as well.
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M Fowler wrote:Nick got it right when he did the Rockit using switchable 260 and 290 secondaries. It is using 50R and 68R bias resistors.
If I understand Nik's layout correctly, with the two 50R and 68R cathode resistors, it allows for switching the two outer EL84s out of the circuit then running the cathode resistor at 118R for the pair.
But wouldn't you then need to run a 16 Ohm speaker load into the 8 Ohm OT tap, or an 8 Ohm load on the 4 Ohm tap, to shift the 4.3k primary (with the Dynaco 470 as an example) up to 8.3k - which is more like what a pair of EL84s in PP wants to see. Or am I missing something here?
(I am currently powering up a wild looking bread-board of this circuit, no smoke yet..)
I tested Nik's 50R / 68R combination on a bread-board, but I did not gain any more versatility with respect to the subject of this thread.
To summarize, it is similar to an option RJ mentioned a while back:
the switch gives you either the stock 4 valves with the 50R cathode resistor situation, or just 2 valves with an 118R cathode resistance (the 50R+68R in series) and those two outer valves seem to me to be hanging in the air still seeing B+ and Heater current. So I guess you are supposed to pull them. Then you still have the load mismatch to the OT to deal with. I used the 2 valve set up for the first checks with two old lame EL84s. Then, when all was fine, I went up to four good valves.
In my case switching back to the 2 valves made the B+ rise 30V which messed the tone up. If you then drop the B+ down to where it should be it sounds OK again, but I think one would be starting to loose the plot to start by compensating for that in a Rocket.
This may be getting off-threaad:
the 68R, or higher, as a switchable alternative to the 50R, could be worth considering - if only as a "go easy on the valves" option. In my case the 68R took the plate dissipation down by about 19%. The tone was still fantastic and a touch more polite.
