From: Mark Beckles Willson Date: Thu, 01 Oct 1998 22:36:54 +0100 Subject: Re: amp re biasing and "starved plate" operation
At 16:36 01/10/98 -0700, you wrote:
As Don implies biasing amps is a controversial subject and is better thrashed out on alt.guitar.amps than on harp-l but, in the absence of Mike C's expertise, let me try to remember in words of one syllable, or thereabouts.
First of all, most of the valve (tube to you in the US) pre-amp/overdrive/distortion pedals which I have taken apart are based round a dual triode such as a 12AX7, the most common guitar amp pre-amp tube, run at a relatively low voltage in comparison to, say, a vintage Fender. As a result their tube characterstics are severley limited and, unless the bass frequencies are seriously pushed, they don't tend to sound as warm as you would hope a tube device would. I think that low plate voltage rather than bias is the issue here.
Going back to bias, and talking about fixed bias amps like the Bassman, you need to remember that what is referred to as the bias voltage is a NEGATIVE voltage applied to the control grid to set the current flow through the tube when there is no signal. The greater (ie more negative) this voltage, the more biased the valve is. The amp will probably sound stiffer (if that isn't too much of a mixed metaphor) but tubes will last longer. Steady state current will be lower - Don's 20mA for example. Reducing the bias voltage (ie making it less negative), on the other hand, will allow the steady state current to be higher giving a fatter sound but more stressed tubes. People who know more about hot rodding than I do have compared this to setting the idling speed on a car.
If you reduce the bias voltage too much, the increased idle current will start to make the plates heat up and glow. Remove it altogether and the tube will burn out. I once plugged in a 100 watt amp having failed to reconnect the bias supply - fortunately EL34s were cheaper then.....
Hope this helps.
Mark
>> From: John Thaden >> Date: Thu, 01 Oct 1998 11:53:32 -0500 >> Subject: amp rebiasing and "starved-plate" operation >> >> In another thread, Don D. wrote >> >> >On a stock RI Bassman, there is no way to >> >easily rebias the amp, and Fender usually biases the amps "cold" to save >> >on tube wear and to cut down on warranty replacement on the cheap, >> >imported tubes they use. This will tend to make the amp sound "thin" or >> >"lifeless." If you replace the rectifier and power tubes with good US >> >NOS tubes and rebias the amp a bit hotter, you'll find you get a big >> >improvement in tone. >> >> I'm not sure I know what Don means by 'cold', but I've read that some amp >> designs run the output tubes with less plate voltage than spec, because >> they then distort at lower volumes and have a round tone. Also, that the >> single 12AX7's in some of the tube distortion pedals are set up this way. >> My question is, does 'bias' refer to the plate voltage? If so, wouldn't >> lower (colder?) bias give a nice tone at lower volumes? >> >> >> John Thaden -Gil Franklin Band >> Little Rock, Arkansas, USA > > The subject of biasing amps is very a very complex one at best and I'll >be the first to admit I don't know all the electronic theory behind the >subject. Mike Curtis, where are you? And a discussion of the matter >could get very long also, but what I was refering to is current or >amperage. Most amp techs would agree that 6L6 tubes sound best in most >amps when the bias current is set to 35-40 milliamps. I was recently >involved in a discussion with someone who had the bias checked on his >stock RI Bassman and found that the bias was running at around 20ma. >Sure, it's fine and the tubes will last longer, but you'll find that >most amps will sound fatter and richer when the idle current is a bit >higher in the 35-40 ma range. Of course you can over do it and if the >plates on the tubes start glowing red, you know you have too much >current, this situation would most certainly shorten tube life >considerably. > As far as your question about voltage and it's relationship to current >in biasing, I'll leave that question to the electronics experts on the >list. I do know that extremely low biasing current will cause a tube to >"clip", but this kind of clipping creates a "buzzy" distortion, not the >harmonically rich compression that most harp players think of when you >overdrive an amp. I don't know if this is the same thing you're >refering to, but most of the tube distortion pedals I've heard have a >very harsh sound to them. > Hopefully some of the more knowledgeable electronics wizards on the >list can help clarify your questions. > >Don D. > >
