For the benefit of others who are curious (I couldn’t find this info anywhere when I built my amp) here’s a list of several modifications I’ve done to the stock two stroke and what they sounded like:
1) negative feedback loop pot: I put a 1M audio pot in place of the 68k resistor. I hear little difference when turning the pot to resistances higher than 68k (ie, progressively getting rid of the NFB).
Going less than stock starts to drop the volume and get more compressed sounding until it suddenly drops off and goes silent right before zero resistance. I’ve left this mod in since it occupies little space and does not affect the sound when left stock, but don’t envision I’ll use it much.
2) 3 way triode/standby/pentode switch: I detailed this in another post on this forum but the quick answer is that the standby is a really great feature to have, and the triode mode gives a noticeable volume drop, a slightly darker and compressed tone, and a little more hum (different amount with different output tube). I used a 1.5k, 1w resistor on pin 4 of the output tube and the triode mode connects this to pin 3 (while pentode mode keeps it on the filter cap power supply rail). I think it’s worth keeping, at least for me.
3) master volume: I replaced the 220k output tube grid resistor with a 250k log pot. it does what you’d expect, except that the preamp doesn’t seem to distort much in this amp (as might be expected). I suspect that the great distortion you get with this amp at higher volumes is due to the output tube distorting and the output tube further adding gain to and distorting the little bit of preamp distortion it is fed. I was curious what the relative distortion contributions from preamp/power amp were in this circuit, and to me it seems the answer is greater than 50% power amp/less than 50% preamp, maybe even 60/40 or 70/30. I have yet to try a clean boost up front to overdive the preamp with the master volume but suspect I won’t keep this modification since it is not very useful as a lower volume distortion control.
4) switch to select between the 330V and 300V taps on the Weber 022772 transformer: this lowers my plate and grid voltages about 30V, but does not affect the sound in any way (volume or tone).
5) speaker: I’m using an 8″ single speaker. I want a low volume amp. I think the amp sounds great through a weber 8″ alnico speaker by itself – much fuller and bigger than you’d expect. I have the amp in a 16x14x9 finger jointed pine cabinet.
6) grounding scheme: I couldn’t get the brass plate soldering to work (didn’t try flux which might have helped) with my 40watt weller soldering station. I gave up on the DH grounding scheme (which honestly confused me as it seemed unnecessarily complicated, but I know nothing about amp design). I did a star grounding scheme and directly wired each major ground connection to the terminal strip. this made for a less noisy amp than at first when I followed the book instructions (other than replacing the brass plate with a bus wire across the pots).
7) filter cap bleed resistor: why should this be a mod? I enjoyed the DH book, and loved building his amp, but I think he does a real disservice by not including a filter cap bleed resistor in this circuit. I put a 220k, 3watt resistor across the + and – sides of the 40uF cap. It has no affect on tone whatsoever, and can save your life when working with this amp. It drains the voltage down to 0 within 5 seconds.
I think it’s terrible that the DH instructions tell you to build the amp, turn it on, and if it doesn’t work perfectly, give up and go to an amp tech because you shouldn’t poke around in there after it’s been turned on because of lethal voltages. Those voltages don’t need to be there…it’s a 50 cent fix…I really would have appreciated a more thorough description of lead dress and wire positioning to trouble shoot hum in this amp but it’s non existent in the book because of this issue.
Thanks for a great forum which has helped me out a ton – I hope these help someone else down the road.
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) to try and suss out what issues a particular build might have to get it up and going. Now that your part of the team, I hope you’ll stay in touch with your input and observations.
