Telecaster Guitar Forum - View Single Post

EMan · April 6th, 2003, 02:06 PM

Hi Med. Gm is a tube parameter associated with AF (Amplification Factor). It is: (Delta) Plate Current/(Delta) Grid Voltage, and is expressed in "mho"'s. It is a number that describes the ability of the the tube to amplify (it's efficiency).

Ip is the Plate Current. Keep in mind, that the parameters will change as the tubes 'age'.

The idea of matched tubes is to insure fidelity; to insure that each have of the PP amp is running the same. Very important in HiFi applications where absolute fidelity is important.

In guitar amps...we generally think about the 'distortion' or non linear qualities as being desirable. In that case, you might expect a slight mis-match to be "toneful".

As mentioned above, matched sets are common these days, and why not just buy them that way.

PP amps have ripple voltage (AC) on the B+ supply, and is a function of the load. The OT's are designed so that the two tube's currents are flowing in opposite directions. This is what "cancels" that AC voltage (so you can't hear it). When these individual currents become too different, the "hum" is audible through the speaker.

However, a "bias balance" (ot output tube matching) pot in your amps bais supply circuit can eliminate most all the hum from lesser matched tube sets.

Okay...I'll **** up now....

April 6th, 2003, 02:06 PM	#9 (permalink)
EMan Tele-Holic Join Date: Apr 2003 Location: Dallas, Texas Posts: 939	Toobs Hi Med. Gm is a tube parameter associated with AF (Amplification Factor). It is: (Delta) Plate Current/(Delta) Grid Voltage, and is expressed in "mho"'s. It is a number that describes the ability of the the tube to amplify (it's efficiency). Ip is the Plate Current. Keep in mind, that the parameters will change as the tubes 'age'. The idea of matched tubes is to insure fidelity; to insure that each have of the PP amp is running the same. Very important in HiFi applications where absolute fidelity is important. In guitar amps...we generally think about the 'distortion' or non linear qualities as being desirable. In that case, you might expect a slight mis-match to be "toneful". As mentioned above, matched sets are common these days, and why not just buy them that way. PP amps have ripple voltage (AC) on the B+ supply, and is a function of the load. The OT's are designed so that the two tube's currents are flowing in opposite directions. This is what "cancels" that AC voltage (so you can't hear it). When these individual currents become too different, the "hum" is audible through the speaker. However, a "bias balance" (ot output tube matching) pot in your amps bais supply circuit can eliminate most all the hum from lesser matched tube sets. Okay...I'll **** up now....