SE rocket crash

[andrew]

My project sounds good at LOW volume but it won't get clean enough for a good volume. It distorts to much and sounds bloaty. If I work the treble it gets harsh. The PT is a 270FX hammond 275vt -150m. The power supply is the Doug Hammond Octal Fatness. I followed the Rocket through to the 220k--.047 combo to a 2.2k to pin 5 of the 6v6. I have a 22uf-250 ohm 5 watt off pin 8 to ground on the 6v6. How can I get more headroom before too much breakup? (Also I am running two 6v6 in parallel) Thanks for any advice!

[RJ Guitars]

Andrew, can you lead me to a Rocket schematic? I think I've got the one for the Octal Fatness. I've had similar experiences with a couple of my SE amps, but I'd like to look at the schematic before I say anything...

Might be good to have your voltage numbers too.

rj

[andrew]

Thanks for the reply RJ but I've already turned it into a Liverpool through the .1 cap then into the 1.5k into pin 5 of the 6v6. ( should this be a 2.2k? I'm running two 6v6 in parallel) The other end of the .1 has a 100k to B+3 and from .1 to pin 1 of the 12ax7. I'm using the Ceriatone websites' Liverpool layout. Do the values look right in your SE experience? This setup does have more headroom and less breakup than the Rocket. I'm still using the Octal Fatness power supply . Thanks for any additional ideas!!

[RJ Guitars]

Andrew, If it sounds better, you're on the right track. I tend to swap stuff out and listen for the differences until it sounds right and eventually it all makes sense to me. The better way to do things is probably to study and get a thorough understanding and change components for more logical reasons...but I'm pretty impatient with that...

I'm still not following the schematic completely but let me verify what your up to... you're using a Liverpool preamp into a set of parallel single ended 6V6's?

I've made a handful of single ended amps and one thing I have found is that when I worked at lower voltages (under 300 B+), I drove the output tube into distortion very easily. I believe my problem was from two sources. 1) Blocking distortion and 2) my output tubes couldn't handle all the gain I had.

I read about blocking distortion on the AX-84 web site and found even more info on Randall Aikens's web site. This convinced me to use a couple big resistors to knock down my signal between each gain section and before the output section. As I did my amp got louder with more headroom... plus the tone sweetened up. It was altogether non intuitive and I never really understood the problems I was having.

Now yours may be entirely different, but it's worth reading the Randall Aiken notes and you'll be amazed at how big of resistors he recommends throwing in there... I'm still curious what your B+ is on the output tubes.

rj

[andrew]

Hello RJ, Yes. A Liverpool into SE paralleled 6v6. For the 6v6 pin#3- 300v, pin#4 -220v, pin #8-25v. B+1-360v, B+2 325v, B+3 220v, B+4 160v. I followed the Aiken advice for the grid stopper resistor on pin 5 of the 6v6 raising it from 1.5k to 20k. Also instead of the .1 (the last cap before the 6v6) I used a .047. I tried higher resistors in the preamp cap/resistor combinations but couldn't tell any difference. I kept the cap values there at 22uf. I'm wondering if the 5w 250ohm / 22uf coming off 6v6 pin 8 is correct. I have bass on full , mid at two and no treble. I use the 500pf at the volume control and use a 12" Jensen alnico. Overall it's a nice change from the Champ that it started from. I wish I could relay the information in more technical terms. The Ceriatone site has a great color layout for the Liverpool. Any of you SE knowledge would be great. I have another tranformer set for another build. Thanks RJ! Andrew

[Zippy]

Andrew - Give the Rocket it another shot. I'm betting that all you need to do is drop a lil' signal going into the final stage to clean it up and get the full range of dynamics.

[RJ Guitars]

Andrew, It sounds like your are getting real close... I tend to agree with Zippy... just a little drop in gain might clean it all up. If you put in giant resistors in the signal path (like 1 Meg or so) your high frequencies will start to drop out so avoid going too far... but you should be able to hear when that happens.

You aren't working the 6V6 especially hard with the B+ voltages you've got, although that's higher than the territory where I've been working. The specs on that tube are generally pretty conservative and many folks crank them up around and even over 400 volts.

I figure you've already done this... but I'll say it anyway. Look at your bias voltages and make sure you're in the ballpark.

Do you have any negative feedback... when I added a little it brought out a lot of fullness in the tone for me and cleaned up some of the low volume distortion. Then I went back and tweaked the preamp a little more.

I also removed the cap across the cathode resistor on the output tube... that's kind of a taste preference thing and you might want to do this using a switch to get a couple different flavors.

rj

[Zippy]

Good call on the cathode bypass cap, RJ.

Another thought, Andrew, is whether you are swamping your output transformer. I looked up some of the Octal Fatness builds and all of them tend to be single output tubes running at 5-7 watts. That leads to two things - whether you are overpowering the OT and whether you have the correct impedance.

A quick check would be to pull one of the two 6V6s and run at lower power. If it sounds cleaner at higher levels, then you probably need bigger output iron if you want to keep the SE parallel configuration.

[andrew]

Hey guys, it's getting better. I put a 2.2M on the #2 pin of the second 12ax7. Also I removed the cap from pin #8 of the 6v6. I's not as "powerful" sounding but much cleaner. I'd like to try the negative FB but where should it connect . My OT is a Weber SE 15watt 5K wired for 8ohm. Thanks for all this help! Andrew

[RJ Guitars]

Andrew,

I think it's pretty cool as you see an amp come into alignment... so to speak. I'll attach a Fender Champ 5E1 schematic that will show you how where to hook up the negative feedback... it's the 22K resistor at the top of the page. Try a few different values greater than the 22K and see how it sounds. Again, you can switch this in and out if you want a couple variations...

rj

[andrew]

I think I found it. Is it the 10k resistor coming off pin#3 of the second 12ax7? Andrew

[andrew]

I forgot to add I was looking at the liverpool layout.

[RJ Guitars]

Andrew - I just looked at the Liverpool schematic and I believe you are correct... add your negative feedback onto pin 3 of that 2nd preamp tube (between the tube and the 10K resistor)... let us know what it sounds like... if it takes too much edge out of the tone try a bigger resistor until you like it.

rj