June 9, 2012 at 2:20 am #5163
Has anyone done this? Besides maybe a 15inch speaker, what other changes would make this a good harp amp? Is it ok to use a 15 inch speaker? Is there enough power?
Thanks.June 9, 2012 at 2:55 am #5645
I’d change the preamp tube (V1) to a 12AY7 and try a 6V6 power tube to start with. I need to think on other circuit mods…..
A 15″ speaker will work as well other sizes with the 2 Stroke. Just as with a guitar, the output and tone will vary with the speaker’s voicing and efficiency. I bet a Eminence 12″ Cannabis-Rex would be a good harp speaker.June 9, 2012 at 4:33 am #5646
The 12AY7 cause it has slightly lower gain? and the 6V6 because it has slightly less power?
thanks.June 9, 2012 at 11:28 am #5647
The mic/harp are going to drive the front end a lot harder than a guitar signal so using the 12AY7 will be an attempt keep some headroom. The 2 Stroke does not have much anyway. The power tube choice is more of a guess, the 6L6, 6V6 and EL34 all breakup differently, so trying them with a harp will be a good experiment. Adding more filtering to the power section seems like it might be good. Also, changing the grid resistor might be a help too.June 9, 2012 at 10:02 pm #5648
thanks, thats great information. The tube filtering changes sound like great experiments. That might be my plan for Two Stroke #2 (assuming I get my soldering iron soon and actually start building #1…)
Is cloth-insulated wire better than the standard plastic insulated? I think it looks better, but maybe not for the twisted heater wires?June 10, 2012 at 12:39 am #5649
Cloth insulation looks vintage, I’ve used it a fair amount. If you want to build a authentic looking 50’s or 60’s style amp, it the thing to use.
You need something like this with sharp blades:
Otherwise getting the cloth to trim neatly is a real pain.
In terms of performance, the cloth wire is no better or worst that PVC or Teflon insulated hookup wire. Teflon is good because it does not melt easily.
In a tube amp, 20ga is the usual recommendation for everything except the heater circuit, which should be 18ga. I’ve done several twisted pair 6.3v circuits with cloth insulated wire, it works fine. Just don’t over twist the pair or the cloth covering will split open, not big deal because of the plastic insulation underneath, but it ruins the look. Cloth covered wire comes in a number of colors but if you don’t want to spring for a bunch of rolls, buy a roll of white and color it yourself with a Sharpie.June 11, 2012 at 11:11 pm #5650AndyKeymaster
Good tip on the sharp cutters. I resorted to an exacto knife on the last build because I was unhappy with the way the cloth was stripping.September 15, 2012 at 3:25 am #5692AndyKeymaster
I’ve been thinking on this a bit and looking at some other places that make some modifications to see what they are doing.
I think that removing the 68K Negative Feedback resistor will make it a bit less bright. As I understand it, removing the highs that might cause feedback with a mic are critical.
Here is an interesting read. I’m not sure what I think about the change to the Rectifier Tube without checking the specs though.
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