6SL7 Express/Rocket tweaks

[passfan]

Wanting something a little more tame than the Express and noting my passion for octal preamp tubes I have built a new Rocket turret card to drop into my express. I retained the presence, added a mid and did away with the cut. Some things are occuring now that were unexpected so I'll give it to the best minds I know and see what they think about it.

Volume is lower than I would expect (less than a Rocket) with virtually no breakup even wide open.

Bias current swing on the ouput tube is down to a few ma's either way (the express swung over 70ma's and most of you know).

The ouput tubes are Mesa 415 STR's 6L6GC's and so they're naturally clean but when I was running the Express board it was LOUD!!!.

I figure the drive from the Rocket board has to be much lower than the Express board and am leaning toward going back to the cut control (instead of the presence) and switching to cathode bias.

Here are some pics

[M Fowler]

Looks good which octal preamp and PI tubes did you use?

[passfan]

When it was an Express I used a 6sn7 in v1 and 6sl7's in v2 and v3. Since converting it I went to a 6sl7 for v1 as well looking for more gain. I just realized I swapped the 100k feedback resistor for a 47k. Would this cause my gain to drop?

[breakfastbuddy]

yes you killed it , you need more gain

[Darkbluemurder]

I retained the presence, added a mid and did away with the cut.
Volume is lower than I would expect (less than a Rocket) with virtually no breakup even wide open.

I figure the drive from the Rocket board has to be much lower than the Express board and am leaning toward going back to the cut control (instead of the presence) and switching to cathode bias.

The Rocket - being a tweaked AC30 - has quite less gain compared to an Express. It also does not have negative feedback which the Express has. I would ditch the negative feedback and rewire the presence to a cut control. That should give the amp more gain and apparent volume. The clean volume should not be lower than the clean volume on an Express, and if at all, not by much.

Cheers Stephan

[passfan]

Yes I just came back in from confirming what you have been generous in telling me. I swapped out the 47k for a 100k and my gain increased. So, I guess it's time to change the feedback to a cut and let it rip. Thanks to all.

[PlinytheWelder]

You might want to put the feedback on a switch or make it variable. That would be fun to play with...

[passfan]

I removed it completely and went back to a cut and the amp got too dark. It sounded like crap so I went back to the 100k on a feedback loop , and put the presence back in it. I've built the Rocket circuit with these preamp tubes and it sounds really great so it's either the fixed bias or the SS rectification causing it.
On another note, I see we're almost neighbors, cool.

[Jackie Treehorn]

The 6L6's need a lot more signal swing than EL84's and the 6sl7's have less voltage gain and higher headroom than 12ax7's. Plus the negative feedback also increases the drive requirements.

You can get some more signal by increasing the resistor on the cathode follower to 100k if it isn't already. Then you could bypass the cathode of the second stage. I guess you've already got the 1 meg volume pot from the express. You could try raising the voltages on the 6sl7's, too, increase the plate resistors. Then there's that unused triode...