Songwriter Prototype

[passfan]

Here is my prototype. I realized I hijacked the dongbiter pics thread and felt I should get this all out of there. This amp just......... something, it's hard to describe. You play it and it just feels like it wants to take off (feedback and sustain) but it doesn't. It's like it's right on the edge but it doesn't break , it's the coolest thing. It's not chimey but you hear with clarity. I'll hold a note and it will just fade out feeling like it's going to feedback but doesn't. I have an 820 ohm on the v1 cathode so I need to go back and change that out. Might have something to do with the amp breaking up like mad at full volume. I'm pushing the preamp too hard. I also have the 250k bass pot which is wrong. The effect is minimal there so I need to change that. I like it better with humbuckers but I've yet to tweak it any yet so all I have are first impressions . And , I have a small signal buzz which is present anytime a cable is plugged into the input (I was scrounging for parts and such so a proper jack will eliminate that, no worries).
B+ is running at 365 and output tubes are running about 50 ma's apiece.

[passfan]

Got a small opportunity to do some voicing this a.m. I changed the v1 cathode resistor to 1k5 , changed the PI coupling caps from the .047 to .005 and .1 off Aikens site for the 6SL7. It now has backed off the earlier tendancy to break like all hell at full volume. The changes to the PI have cleaned up the mud in the bottom and made my 250 kb bass pot more responsive. I am understanding the crappy NY amp show demo after having played with this thing overnight. The only way to play this thing seems to be wide open with a light pick attack. That is where all the magic is. At lower volumes you get the NY amp demo.....nothing special. I went from a Sylvania 6SL7GT and an Amperex 12AX7 to a GE 6SL7 short plate and a Mullard (Blackburn). This combination makes me want to play this thing all day. I've swapped out the 6SL7 with an RCA 6SN7 short plate (black) with excellent results. The breakup almost disapeared and I was left with all the "good stuff" ( I was able to be a bit more aggressive with my pick attack at full bore while still avoiding all out breakup). I wouldn't hesitate to build this version out and perhaps someone will finish one up with some EL84's and report about it. As soon as I can get my recording rig back up I will see about posting a clip or two.

[passfan]

Now I know why Ken named this the songwriter, I got so caught up in tinkering up and down the neck looking for harmonical magic that before I knew it I was writing new licks. This thing really lures you to things you wouldn't normally do. It doesn't seem to matter what kind of guitar you're playing when this thing is wide open. You'll find yourself looking for something. I need to fix the buzz and build a proper cabinet.

[andrew]

Did you keep the 100k anode values for the pi or lower them? I have a 6cg7 I may try. Thanks

[passfan]

I kept the 100k plates and used a 6SL7 which allows a 6SN7 to drop right in. Not sure about the pin outs on a 6CG7. Isn't that a medium mu twin triode miniature? Have you used those before in a guitar amp. I've seen them in stereo amps but not recently. Aikens PI has 82k plates , 680 ohm bias with a 47 k tail (.005 in and .1 to ground). I may go to 82k on my plates but not right now. Once I find the noise I'm going to burn her in for a while.

[Reeltarded]

Very good! Thanks for the description. I don't think it's an amp for me if it's so clean, maybe. I thought it would be though.

I like the description of almost taking off.

[andrew]

The 6cg7 has too low a gain factor. Even at full volume it was weak. A 6sl7 is in now with the Aiken values and it's plenty loud. I changed the 250k audio cut to a 250 linear and I like the way it gradually changes the frequency cut. For me the 250A may as well have been an on/off switch.

[passfan]

Try your 6SN7 now Andrew. It will remove the rest of the breakup at full volume and leave you the harmonic artifacts.

[andrew]

I rechecked my wiring on the 6cg7 and tried it again. This time it worked. It is plenty loud and very nice sounding. Clean and clear but not harsh and I have a 500pf bright cap in. I'm using a WGS ET-65 but I have a Tone Tubby alnico I want to try too.

[passfan]

I've made a few changes that I think are for the better so I will share them here. I am running a .047 to the bass control instead of a .0047. Parralel to the I am using a 100 pf instead of a 47. This has opened up the amp a lot. Before it tended to be muffled sounding in the bottom end and I originally went to a 500 pf but that was too trebly and brittle. My bass pot is a 1 meg ; I tried a 3meg reverse but it acted more like a gain control instead of a bass control. I may retry that again now that I've unmuffled the bottom. My plate resistor is 220k and my cathode resistor is 1.5 k. This is the cleanest option for V1. I have tried it with 100k and 820 ohm respectively and every combination of the four and any other way the gain is too dirty. This is supposed to be a clean amp so I kept it that way. I am running Aikins 6sl7 PI and so the feedback into the tail resistor through the extra 100k to ground bothered me. I have used Aikins PI in the Rocket so I know it's clean and chimey at 47k to ground. The problem is you start losing headroom when you raise this resistance to ground and the extra 100k was doing just that. Initial experiments with a 10k there sound very promising. I noticed a big difference even though this would seem to affect the feedback circuit. The amp is starting to sparkle a bit more at lower volumes now as opposed to shining only at full blast. My goal here has changed somewhat from figuring out what Ken did circuit wise to pumping the most out of this simple design. Try these out for yourself and you be the judge.

[roberto]

The amp is starting to sparkle a bit more at lower volumes now as opposed to shining only at full blast.

Yes, this is normal if you decrease NFB so much, you should obtain a warmer sound too.

My goal here has changed somewhat from figuring out what Ken did circuit wise to pumping the most out of this simple design.

I totally agree and I think it should be the goal of everyone.

[passfan]

Do you mean decreasing the feedback resistor thereby increasing negative feedback ? This would , in effect lower the output of the amp wouldn't it ? Which , in my case , allow more voltage swing of the PI giving me back the volume I lost. It seems this is what has occurred as I can not discern any noticeable drop off in volume , just the increased chime and some warmth at lower volumes now. I wonder if I could have just swapped the ground and the negative feedback wire to opposite ends of the 100k resistor on the board instead of changing the 100k to 10k ?

[roberto]

I refer to the 100k you lowered to 10k (that's 4k7 in Marshalls):

I am running Aikins 6sl7 PI and so the feedback into the tail resistor through the extra 100k to ground bothered me.
Initial experiments with a 10k there sound very promising.

With the same 100k from the 16 Ohm tap (or 47k from 4ohm tap, it's the same) and a 10k to ground (or maybe even 4k7 as in Marshalls) you have way less NFB than with 100k to ground and you raise the bias of the PI at the same time. So you should notice a sound with more harmonics at lower volumes and a warmer sound.

[passfan]

Okay, I understand now, thanks.

[passfan]

A simple centertap using 100 ohm resistors to ground for the 6.3 volt filament and the buzz is gone. I have installed a dumble OD circuit with a MV fed through an effects loop. I will need to do some tweaking on the OD as it is brutally brittle and has to be adjusted everytime you switch back and forth. It looks promising enough to to install a footswitching relay and take it off of the switch I am using now.