I recently purchased a used Carvin V3 amp from guitar center. When I bought it, it had the stock el34's but one of them was blown. The staff changed them out for 6l6's since it was all they had and the amp has a fixed bias switch. My question is if i need to have the amp re-biased for new tubes? i thought "fixed bias" ment you just put the tubes in and forget about it, but i was looking on carvin forums and read people saying they had to get there's bias and stuff.
Im new to tube amplification so i've got alot of confusion.haha
I recently purchased a used Carvin V3 amp from guitar center. When I bought it, it had the stock el34's but one of them was blown. The staff changed them out for 6l6's since it was all they had and the amp has a fixed bias switch. My question is if i need to have the amp re-biased for new tubes? i thought "fixed bias" ment you just put the tubes in and forget about it, but i was looking on carvin forums and read people saying they had to get there's bias and stuff.
Im new to tube amplification so i've got alot of confusion.haha
You should have the bias checked and adjusted if it needs it.
From Aiken Amplification:
"There are two main types of biasing: fixed biasing and cathode biasing. Fixed biasing does not mean the bias is not adjustable, in fact, it usually means the opposite. Cathode biasing is usually fixed, and not adjustable, and fixed biasing is usually adjustable with a small trimmer potentiometer, or "trimpot". It is no wonder the subject is confusing to people!
Fixed biasing means the tube is biased by means of a DC voltage, which is usually a negative voltage applied to the grid of the tube with respect to the cathode. As the negative grid voltage is adjusted, the bias current will increase or decrease, depending upon the direction the bias voltage is going. In general, as the bias voltage becomes more negative, the bias current becomes smaller, and the tube is biased "colder". As the bias voltage is adjusted less negative, towards zero volts DC, the bias current becomes larger, and the tube is biased "hotter". This is because a tube is a "normally on" device; that is, it allows current to flow from the cathode to the plate when the grid is at zero volts with respect to the cathode. The tube can be turned off, and the current flow stopped, by making the grid voltage negative with respect to the cathode. The tube can also be biased by referencing the grid to ground, or zero volts DC, and applying a positive DC voltage to the cathode. This is the same as keeping the cathode at ground and applying a negative DC voltage to the grid, because it is the grid voltage with respect to the cathode that determines the amount of bias current in the tube.
Since vacuum tubes are "normally on" devices, a trick can be used to bias them without having to supply a negative DC voltage source to the grid. If a resistor is placed between the cathode and ground, and the grid of the tube is referenced to ground (usually by connecting a large value resistor, such as a 1Meg, from grid to ground), the tube will try to conduct a large current from cathode to plate, since the grid and cathode are initially at ground potential. However, this cathode current flow will cause a voltage drop across the cathode resistor, making the cathode voltage positive with respect to the grid. Since the cathode voltage is now positive with respect to the grid, the current flow will decrease, and the tube will head back towards cutoff. A point of equilibrium will quickly be reached where the increase in current is offset exactly by the increase in cathode voltage, and the bias current will stabilize at some particular value. It will remain at this value unless the resistor value is changed, or a different tube with different characteristics is plugged in. This allows the desired bias point to be set by varying the value of the cathode resistor."
Playing guitar and never playing for others is like studying medicine and never working in a clinic.
I can't speak for the Carvin forums or different models in the Carvin Lineup, but if your bias is fixed, then it's fixed and that's that. Change the tubes, but only per cross-reference charts, often in matched sets, and plug away. http://thetubestore.com/
Like a bird on the wire,
like a drunk in a midnight choir
I have tried in my way to be free.
I can't speak for the Carvin forums or different models in the Carvin Lineup, but if your bias is fixed, then it's fixed and that's that. Change the tubes, but only per cross-reference charts, often in matched sets, and plug away. http://thetubestore.com/
Jay, maybe you misread the quote above? FIXED bias uses a potentiometer to fix the bias to one value. Every tube has a different bias value for best performance, and this value should be re-adjusted for every new set of tubes to make sure the tubes are performing at their best. Can you get away with not doing it? Yes, but be aware that it may shorten the life of your new tubes if the current value is too high, or cause the amp to lose it's volume and fullness of tone if the value is too low.
Cathode bias is self-adjusting within a certain range of values. In this type of amp, tubes of the type specified for the amp are indeed "plug and play" as you've described above.
It's a weird nit-picky thing, but an important one for owners of tube amps to understand if they want the best performance from their amps.
Playing guitar and never playing for others is like studying medicine and never working in a clinic.
Aiken = word.
Read that thing up above again.
It's pretty unlikely that the same bias voltage will work with both 6L6s and EL34s.
To add more confusion, fixed bias can either be fixed or adjustable. It's still fixed bias, because the bias voltage on the signal grids stays fixed wherever it's set. Lots of older amps and some modern ones were built with nonadjustable fixed bias. Cathode bias voltage changes with the average current flowing through the cathode resistor, so it tends to keep that current at a nearly constant level.
"A cheerful heart is good medicine."
I own a v3 head as well.
You should flip the switch on the back to el34 or 6l6 appropriately when you change the tubes. But that is just the starting point. I've read that Carvin biases them very cold. Users can bias these amps. If it is used, I would definitely rebias it or have it done professionally because you won't know what the previous user had it set at.
Great amp btw! Congrats! Do you have the Diamond grille on it or it is a combo?
I have the head version w/o the diamond plate grill. It does seem to be a pretty decent amp (especialy considering in only paid 600 for it.) I've got it driving a egnater tourmaster cab w/ v30's and it sounds damn good. but i think mine has a build quality issue. on the 2nd channel, if i touch the mode switch, the volume goes up for a second and then back down. not really a huge problem but..unsettleing(spelling?) none the less.
