Amp Hum and Modulation of Distorted Frequencies.

[HaroldBrooks]

Just read a very interesting article that discusses the role 60 and 120 cycle hum in a guitar amp play in the generation of intermodulation distortion and discordance in a distorted amp.

I believe I solved just such an issue in my Gretsch 6162 amplifier by reducing the hum (both 60 and 120 cycle) at a few points in the circuit. 6162 schematic http://www.prowessamplifiers.com/sch...tsch_6162.html

First I would like to say that I play a rather large range of cover music, and that requires everything my amp can muster, from near perfectly clean, to highly distorted tones.

First thing I did to reduce hum was to increase the size of the V1 Cathode Bypass cap. Normally I would never go super big, but in this case I did at 220UF, and it paid off big in reducing hum. I also tried many different 12AX7 tubes in V1 and V4 (second preamp stage) until I found the ones with the lowest leakage hum. Same with using different 6973 tubes until I got a pair that played nice with less hum. Next I swapped out the 6EU7 PI tube (online schematic is wrong) for a more conventional 12AX7 (like the one used in the schematic) using an adapter. I am not sure if the new 12AX7 tube itself reduced hum, or if it was the change in the PI tubes orientation (higher up away from the very close power transformer) that reduced the hum. I also made sure I filtered as much of the below 80hz signal as I could in the amp circuit by choosing the correct combination of High pass filtration along the way, starting at just after the input.

All this has led to the amp sounding much smoother in very high gain. Finally, I bypassed the rather peculiar Tone control in the amp, in favor of using a Parametric EQ in front of the input, and that has also worked quite well, but no change in hum.

What does everyone here think about the content of the below article, specifically Section #4 that deals with Power Filtration ? Should I use a larger cap than the stock 20uf one currently in the amp (perhaps 40uf), or should I just leave well enough alone ?

Thank you for your help !

Article :

http://sound.whsites.net/valves/clipping.html#s4

[loudthud]

The article has a factual basis, but it's somewhat academic with no 'been there done that'. I don't think Rod plays guitar. What the article doesn't mention is that when you beef up the power supply in an amp, it feels different. The long and short of it is: try adding capacitance, if you don't like it, you can always take it out or put it on a switch. In fact, you may not be able to hear or feel the difference unless you can rapidly switch between to two.

Neil Young (user of a tweed Deluxe) is famous for hearing the change in mains frequency when touring in Europe. It is a subtle effect that you need to train your ear to hear it.

[HaroldBrooks]

Thanks for your reply loudthud. I think I understand what you mean, as I did add extra filter capacitance to one of my other amps, changing it from 10uf to 20uf, and didn't feel or hear any difference in tone or hum.

So I think it depends on other factors in the circuit as well. I will take your advice and 'Tack' in another primary filter capacitor and see how it goes in the real world.

It's true, I can do just about all the math associated with amp design, but what it often comes down to is getting something in the ball park on paper, and then actually playing it to see if it makes any noticeable difference.

I remember not too long back I found out that Marshall amps use negative feedback in a very effective way, but when I tried wiring in negative feedback on one of my vintage Valco amps, it worked... But was a horrible change that altered the character of the amp in a direction I did not anticipate, essentially it changed the touch sensitivity and made the amp feel flatter and all wrong, when actually playing it. I was going into this thinking it was going to make my vintage amp into a rock monster, but I quickly and independently found out negative feedback was a disaster for that particular amp, so I reversed it shortly after and never looked back (for that amp).

So any mods need to be tested in context. That is the final step that determines the viability of the change, not what you thought or hoped it might accomplish.


“This above all: to thine own self be true" - Polonius