New Rocket, el84 screens a couple volts higher than plates

[Matt D]

I'm finishing up a new half power Rocket build. I have tried a couple of different pairs of power tubes and I measure 300-303 on the plates and 303-305 on the screens.

I'm guessing my output transformer is dropping more voltage than my choke. I'm using a Triode z565-48 OT and an oversized potted choke that measures about 300dcr. My screen resistors are 470r. I have done a bunch of searches on this and come up with some different and in some cases contradictory prescriptions for this situation. Do you guys recommend any of these:

1) Don't worry about it. Under operating (non-idle) conditions it doesn't matter.

2) Up the screen resistors to 1k or more.

3)Put a 5 watt resistor between the choke and the screen supply to drop a couple of volts.

4)Use a different choke with a higher DCR. I'd like to avoid drilling the chassis at this point but if I need to change the choke that is an option.

One thing that I found in my searching is that with the EL84 in particular the slightly higher screen voltage might cause some fizzy artifacts in the sound. I"m not noticing anything like this at around the house volume levels but I'll have a chance to really turn it up at practice in a couple of days and report back. Thanks.

[kwijabo]

what's your B+1 and B+2?

[rp]

I'm curious what you think about that Dynaclone ST35 OT for guitar, if too clean. Report back.

[Matt D]

With B+1 at 308 and B+2 at 305 I got 302 on one plate and 300 on the other. 303 on the screens.

The Dynaclone definitely doesn't sound sterile. Tomorrow I'll have a chance to play it with a band and compare it to both a a full power Rocket using a Hammond OT and a 90's AC30 reissue. I'll post about how the shootout goes.

[Matt D]

It's also odd that the plates are at slightly different voltages. With a pair of Motorola 6bq5's I measure 300v and 302v. With Phillips Holland EL84s I have 305.5v and 308v.

[Matt D]

Well, I bumped the screen resistors up to 1k and that puts the screens a volt or two under the plates. I'm using this power transformer:

http://www.edcorusa.com/p/670/xpwr074_120

For filtering I'm using 60uf at the first node and standard Rocket filtering after that. I'm using a 5AR4.

Sounds pretty good so far! I'll be able to give it a full band-volume workout tomorrow and try to get some pictures posted.