unexpected capability discovery in my amp

[Koreth]

Re: unexpected capability discovery in my amp

Just some off the wall guessing here, perhaps the reason you're not getting the hum you expect out of the mismatch is because the amp is in a really clean environment, power and electrical noise wise? Have you tried running the mismatch in an environment with known disgustingly dirty power EM noise out the wazoo (like some of the bars some of us play gigs in?)

 

[Brown Note]

Re: unexpected capability discovery in my amp

In a perfectly balanced p/p power stage, all line noise is common mode and thus cancel in the output transformer just like in a humbucker pickup. Now that the balance is being skewed, ie. the matching is less than perfect, the cancellation is only partial. If you put enough regulation(active, capacitive or inductive or any combination of them) there is no line noise to appear in the primary of the output transformer, thus no hum in the output even if one tube was completely off.

 

[glassman]

Re: unexpected capability discovery in my amp

Just some off the wall guessing here, perhaps the reason you're not getting the hum you expect out of the mismatch is because the amp is in a really clean environment, power and electrical noise wise? Have you tried running the mismatch in an environment with known disgustingly dirty power EM noise out the wazoo (like some of the bars some of us play gigs in?)

My shop power is as dirty as power gets...I'm in the middle of an industrial area and have been going around and round with the power company to get it cleaned up a bit. It actually works to my advantage sometimes; bad power makes power associated issues with amps in for repair much easier to diagnose. Crappy power has no negative effect on the hum level though it can...when it'a really bad, make any amp sound like it has a phasor in it.

In a perfectly balanced p/p power stage, all line noise is common mode and thus cancel in the output transformer just like in a humbucker pickup. Now that the balance is being skewed, ie. the matching is less than perfect, the cancellation is only partial. If you put enough regulation(active, capacitive or inductive or any combination of them) there is no line noise to appear in the primary of the output transformer, thus no hum in the output even if one tube was completely off.

I'm not doing any fancy tricks at all and my filtering would be considered conservative by anyones standard.

I have chosen to not list this "attribute" on my web site or white paper at this time. I really feel I need to understand this phenomenon a little better before I make it standard procedure. My web site is complete and now my stupid a$$ has to figure out how to configure my FTP server so I can make it live. Hopefully that will happen in the next few days!

The clips turned out great. We changed it considerably from the original plan though...I only have five clips each with a different guitar. Each track has a lead (panned hard right) and a rhythm (panned hard left). All are done with the "as delivered" tube compliment and at neutral settings (the same for all guitars). I will add more tracks in the future based on feedback.

 

[Brown Note]

Re: unexpected capability discovery in my amp

I'm not doing any fancy tricks at all and my filtering would be considered conservative by anyones standard.

You more than likely have 400+ voltage on the anodes and even 100µF worth of filtering is enough to rid of pretty much all of the ripple at idle. If you scope the anode voltages you'll see a couple hundred millivolts of ripple, at most. As the output transformer is a step down device with a ratio of 40 or so, the couple hundred millivolts make almost no audible noise in the speaker output. The feedback also helps. Hardly any amp has insufficient filtering, as hum becomes a nuicance long before any intermodulation, like ghost noting, happens.

If you have a choke, the so called "pi" filter (two shunt caps and a series inductor) cancels easily 40-60dB of noise all by itself, thus every anode voltage behind the screens can be considered noise free. This makes the centre tap feed of the output transformer the only source of line related noise which p/p transformer inheritly cancels out.

No noise is hardly ever a problem :naughty: