KT66 Rocket Build

[Blind Lemon]

Ok, so were do the standoffs behind the pots for the bus bar come from. That's a good idea. I hate soldering to the back of the pots.

BL

[Kregg]

I don't like soldering to the back of pots either. I recently built an express with P.E.C pots and ended up attaching the bus to the board (as done by another member).

Here's my adaptation: [IMG:639:286]http://i141.photobucket.com/albums/r54/ ... C_0255.jpg[/img]

The bus wire is soldered to wire terminals and bolted to the board at each end. Though only one end terminates to the chassis through a board standoff at the end near the caps.

[Structo]

Kregg, I didn't know you built an Express.

How do you like it?

TweedDeluxe, where do you find those ground lugs that mount on the pots?

[rawnster]

Hey tweedeluxe, great looking build. How do those big bottles sound? I've got a thang for big bottles. I'm curious about how it sounds, especially if it stays clean up to 1 o'clock.

[tweedeluxe]

OK, I ran into a little problem, maybe some of you smart guys can help me out.

I was checking the voltages last night after an extensive preamp tube swap session, and noticed that the screens are about 7 volts higher than the plates at idle: Va=~310V, Vg2=~303V. I'm getting a 10 volt drop across the OT primary, but only a 3V drop across the choke and screen grid resistors.

Can anyone tell me if I'm going to destroy the screens running the amp like this? Or will Vg2 drop below Va when signal is applied? I'm having trouble understanding the role of the choke in this - will it keep the screen grid currrent under control?

Without having the amp in front of me (wish I had accurately measured screen current) I'm estimating about 4.5ma screen current per tube at idle - so I'd need to use at least 1k8 screen grid resistors to bring their voltage potential below the plates', or add a shared 1k.

What do you guys think I should do?

More voltages:

Mains: 120V
Secondaries: 510V
B+1: 313.5
B+2: 311.3
B+3: 229.0
B+4: 197.1
B+5: 188.8

V4,V5
Plates: 303.7
Screens: 310.6
Cathodes: 25

So - cathode current in each tube is ~62ma, and power dissipation is about 18W - if that makes a difference.

Structo, I used these: mouser # 523-31-759. I built a little jig so it would be easier to drill the tiny hole for the buss. Allyn has the pre-fab ones for sale in the buy/sell section.

RJ - The OT is great - it stays cool even after cranking the amp and wailing away for 30 minutes - now I'm worried that it's because the screens are eating up all the current!

The .0047 cut cap is a cornell DMT polyester from mouser, and the .047 coupling caps are panasonic polyester film - available at digikey. I'll see if I can find the PN# for those later. They are not the ones listed in Ron Worley's BOM.

Thanks for the comments. The amp sounds spectacular - it makes my strat souds like grand piano, so Vg2 being off is a real bummer. Hopefully someone here can help me straighten it out.


EDIT: Could the current leaking through the spike protection diodes be dropping Va at all?

[tweedeluxe]

rawnster - the amp sounds very, very much like the clips you posted.

I'm slowly gathering the parts for an "accurate" EL84 Rocket build - I'll let you know how the amps compare when it's finished.

[Kregg]

Kregg, I didn't know you built an Express.

How do you like it?

I'm still working on it, I'll start a thread on it tomorrow.

[Blind Lemon]

So do you have to know a secret handshake to find out...........

BL

Ok, I'm a dumbass!!! I didn't see it up in tweedeluxe's post above. Thank you TD. My bad...............

[tweedeluxe]

Any comments about the screens being 7 volts higher than the plates? Should I try to drop the screen voltage before playing the amp again, or is it not that big of a deal?

I generally do not like like the change in tone/feel when screen grid resistance is increased (ie 470 up to 1k in fenders), and I love how this amp sounds (hopefully that isn't because the tubes are about to go nuclear), so I would prefer not to change anything.

I just can't find much information on this topic, other than that some production guitar amps did operate this way (early 50w marshalls and maybe some Gibsons?).

I understand that having the screens at a higher potential than the plates can cause runaway screen current, but shouldn't the beam-forming properties of the KT66s ensure that the electrons will fllow through the screen grid anyway?

And what about the role of the suppresor grid? Isn't one of its purposes to prevent primary and secondary electron emission from flowing backwards towards the screen grid?

Also, does the relatively low operating voltages in this amp make screen meltdown less likely?

Anyone?

[Blind Lemon]

Screens should be lower than plates.

The choke is for filtering, smoothing out the ripples and will drop some voltage. What do you have on for screen resistors on the KT66s?

Also check to make sure you don't have B1 and B2 swapped.

BL

[tweedeluxe]

Screen grid Rs are 220ohm compared to 100ohm for the el84 rocket.

The choke drops about one volt as do the 220R screen grid resistors.

The OT primary, however, drops about 10V.

[Structo]

I noticed on my recent build (not a Trainwreck) that the screens were at the same voltage as the plates.
I have a 5H choke and 470R screen resistors.
This is on 6V6 tubes. I wonder if they are different in that way from 6L6 tubes.

But having the screens higher than the plates does suggest possibly that B1 and B2 are backwards.

[tweedeluxe]

B1 and B2 aren't backwards, unfortunately. I'm thinking that the resistance of the primary winding is a little higher than normal maybe?

[Blind Lemon]

"primary winding is a little higher than normal maybe?"

You might be right, If I'm right you should see about 100 ohms to the CT per side. (5K)

BL

[Blind Lemon]

I'd measure the screen rizes also.

Bl