No manufacturer of musical instrument amps uses matched tubes, with the possible exception of Groove Tubes.Īs discussed in the TUT-series, matched tubes will drift out of balance over time due to electrical imbalances in the circuit and the different response of each individual tube to mechanical stimulus. tones, then you simply plug in the tubes you have, check the bias and play. If you are trying to achieve vintage Fender, Marshall, Vox, Silvertone, Gibson et al. Then they talk about matched preamp tubes, and I don't know where to get those.Ī: You don't need matched tubes of any kind in your guitar amp. Q: What's the deal with matched tubes? Some experts say they're very important, and these days it doesn't seem to be too expensive to get matched tubes. Sorry for the long post, and thank you so much for helping me, I really appreciate it! Or Maybe I could swap the tubes places every other month so the OT saturation evens out? Guessing wildly here! There has got to be a better cheaper way.maybe installing a pot or something to balance the tubes. Is there a way to balance the tubes DC against each other, so I don't have to buy matched tubes.I've avoided matching tubes because of reading stuff about canceling even order harmonics and because I can't really afford buying two tubes if only one breaks. Should I replace the screen resistor for tube A or should I get a larger value cathode resistor or both? I´m wondering if something went along with it.īy the way the screen resistor is the 1KΩ one that's connected directly to pin 2 right? Thats the one I've use for screen current calculation. Do you think the screen voltages are too wide apart? Maybe i need to replace the screen grid resistor? When the tube in the left slot died it kind of made some fireworks and smoke, so So it's not related to the tube it seems. These numbers are the same even if I swap the tubes slot places. Tube A is still to hot it seems.the screen voltage is lower on on this tube 0,14 V compared to 0,64 for the other tube. Switching to 240 volts made the plate voltage drop and at least one of the tubes is now within the 12 W rating. Is it not bad/dangerous to mismatch the mains to the PT? maybe 20V doesn´t matter? Sorry if this question is stupid, but I'm in curious mode! I measured 220 on my mains voltage supply. Plate current = (cathode current - screen current)Ĭathode current = (cathode voltage / cathode resistor value)Ĭathode current = 11,5 V / 303 Ω = 37,95 mA Plate dissipation = (Actual Plate voltage x plate Current)Īctual plate voltage = (Plate voltage - Cathode voltage) Ok I´ve changed the PT tap to 240 V setting, Could this be harmful to my output transformer? How do I balance these tubes so the core of the OT doesn't get saturated? Should I measure cathode voltage while playing through the tubes also? So if my calculations are right I should get a larger value cathode resistor, am I right? If so what value resistor should I put instead?Īll of the voltages were measured at idle since I didn't have a guitar at hand for testing. Plate voltage: 395 V (measured from pin 7 to ground)Īctual plate voltage: (395-12,8) V = 382,2 Vĭissipated power: 382,2 V x 0,04224 A = 16,1 WĬathode voltage: 12,3 V (measured from pin 3 to ground)Ĭathode resistor: 311 Ohms (measured in circuit)Ĭathode current: 12,3 V/311 Ohms = 0,03955 AĪctual plate voltage: (395-12,3) V = 382,7 Vĭissipated power: 382,7 V x 0,03955 A = 15,1 WĪm I doing this right? Is it OK to measure the resistors in circuit or should I take them out?ġ6 and 15 watts is above the maximum dissipation of 12 W for EL84 tubes. This amp is cathode biased with one cathode resistorĬathode voltage: 12,8 V (measured from pin 3 to ground)Ĭathode resistor: 303 Ohms (measured in circuit)Ĭathode current: 12,8 V/303 Ohms = 0,04224 A Tell me if I've got things right or wrong that would be so great! To help by reading my calculations and thoughts and I've never done this before so if someone feels an urge I'm wanting to bias the power tubes in my Dynacord MV17 amp,