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Tweed Twin-Amp clone too much overdrive

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12/10/1999 4:41 PM

Tweed Twin-Amp clone too much overdrive

I've recently "finished" my first project amp, and I'm pretty happy with the results. It's a 4-6L6 tweed twin, 'ept I'm using a 15" alnico speaker. It sounds loud and bitchin' at volumes under about 3, but at higher levels distorts very harshly. I've messed around with the preamp tubes, (subbing 12ay7, 12au7, etc.) with no noticeable improvement. I've heard that tweeds tend to overdrive easily, is this a "normal" characteristic of these circuits, or is there some tweaking that I can do? I'd appreciate any input. Thanks!

Keith

12/10/1999 5:02 PM

Benjamin Fargen

Keith,
What tweed Fender did you model it after? Did you make any of your own revisions to the circuit while building it?
If you are using a 12AY7 in the first preamp stage......it is going to distort real easy.
Did you use 1.5K grid stopper resistors before the power tube grids.......you could be getting some blocking distortion by driving the grids of the
power tubes with to much signal.
Kick down some more info and with the suggestions here you'll have it fixed in no time!

Benjamin

12/11/1999 2:47 AM
SpeedRacer	I think Ben meant if you have a 12AX7 in the first position, you'll have gain to burn. Go with a 12AY7 in the first position, and use input # 2 (low gain) if you need bigger headroom. There are lots of tweaks that work well with these amps.. which circuit did you build?

12/11/1999 3:05 AM

It's basically the 5F8-A schematic, 'cept I'm using two 5U4 rectifiers istead of the GZ34. The schematic shows a 1k5 grid resistor on two of the four output tubes. Should I maybe connect 1k5's to the other two as well? I saw this mod in Tom Mitchell's book. The speaker impedance is matched to the OT, and I'm using a Power Soak, so I don't think I'm driving the speaker too hard. Thanks in advance for the input! I'm itchin' to finish this beast, since the brown tolex and oxblood griile cloth just arrived the other day!

12/11/1999 3:28 AM

Ditch the power soak and then see how the headroom is!! They have a real tendency to make the amp break up earlier than usual.
Definitely do the other 1.5k grid stoppers..
Remember the 5F8-A is just a 4x5881 5F6-A Bassman, and as such has big potential gain (some might even say Marshallesque.. ;-)

;-)

so start with the "Stock" layout first and work your way up from there. Also try a 2x12 cab insted of a 1x15. You'll get more volume etc from a 2x12.
Definitely lose the soak while you're dialing in the amp though. IMHO that's key.

12/11/1999 5:58 PM
Benjamin Fargen	DUH!........thanks Speed!

12/10/1999 6:03 PM
Jim S.	Alnico speakers are usually rated for relatively low wattage. A Tweed Twin can easily generate 60 or more clean watts. I'm wondering if the 15" speaker is breaking up badly. Also, is there any impedance mismatch between the speaker and the output transformer?

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