Ampeg sj 12 T 6L6 and EL34 Biasing

[Smitty02]

By the way... These tubes are very close to the speaker magnet, i seen several other make that are like this also what is yall's take on this. Does it effect the tubes , seems like it would.

[Enzo]

The schematic says adjust for slight crossover notch at full out. It also sayd line current draw will be about 0.65 to 0.7A. That means the mains - the power coming in from the wall. There is no test point for that. You either plug the amap into a metered outlet - recommended - or otherwise insert an AC current meter in the AC power. You COULD pull your mains fuse F1 and connect the current meter there. SO when properly adjusted, the amp will typically be drawing about 7/10 of an amp from the wall at idle.

That is how they adjust it it the factory, no fancy bias meters, no calculations, just quick and dirty, no test points involved.


Bias is not a critical adjustment, this is a guitar amp, not a NASA space probe. They say -38v is typical, Yours is set at -42, so your tubes will be running a little bit cooler than at -38v. I don;t know what current will be trough the tubes at that poinbt, probably less than the 70% rule of thumb you hear all over the internet. If the amop sounds good and the power tube is not getting red hot, you are fine. Really.

As was mentioned above, the 38v on the schematic is bias VOLTAGE, not tube current. All those current readings you need to calculate dissipation are totally different. now 38ma of current is not unreasonable, but it is a total coincidence that that number resembles the voltage number. Kinda like my car has 18" tires and the battery weighs 18 pounds.

I think you are probably confusing several factors. You may have installed a 1 ohm resistor, but were unable to get a reading across it because the ground connection was still there. SInce tube dissipation is plate volts (406) times tube current, I wonder how you got 51.7. If your current reading was zero, what did you use. And to compare any calculation with the 38v reading tells me you are not yet sure of things.

Having said that bias can be OK over a WIDE range of setting, I don't mean to say ignore it. Even if precision is not needed, there is nothing wrong with doing a lot of overly precise figuring as a learning exercise. For example watching the relationship between increasing the negative bias voltage and the resulting decrease in tube current. And due to the nature of things, as you lower the tube current, the plate voltage will tend to rise from the reduced load on it.

[Enzo]

No, the speaker magnet is not an issue to the tubes.

[Smitty02]

Thanks Enzo yes i think I am over thinking some, specially after your explanation. I didnt have no idea what that crossover notch business was all about, now I understand. Is that how they build them ? rather than spending time biasing them they do it that way cause the amps are all the same and then they get QC'D if ok out it goes ?? The 51.7 is the reading i got from the Weber bias calculator. I was checking out several of the bias calc's sites have. I was planning to install a 1 ohm resistor actually one for each tube but i usually use Eurotubes with good results do you think I could just install one resistor if the tubes are well matched from a good dealer? Or would you recomend one for each tube. I did roll the bias adjustment on the tubes in it, down to -38v. I have a friend that had bought me a twin reissue that he took to GC and they managed to sell him 2 of those new Groovetube 6L6 GE"S expensive stuff, when he brought his amp over they had them 2&2 instead of alturnating them new old new old. His amp really needed a good cleaning and a retube, he ordered a full set of JJ"S those old GT's were worn out. I fixed him up and told him to hold onto the 6L6 GE's, they were only a few days old and i may have a buyer some time for him. Well I was thinking of getting those GE's for this Ampeg, do you think those will be suitable for this amp ? as I can get them for $35 dollars, they cost twice that or close to it and from what Ive read they are decent tubes and made in America again I read. Thanks for your explanation , I do understand what you are talking about, I think I mentioned I wanted to do the bias resistors so he could run EL34 or 6L6's with a easy way to adjust the bias as these amps I understand are capable of both as the bias adjustment pot has the range. From what Ive read anyways and they originally came with EL34's AND does that crossover notch apply here for the 2 different tubes or is that something I dont need to worry about. Thanks for your words your a born teacher LOL Bruces mention of just telling Eurotubes what amp I got and have him get me a set suitable for this amp is the easiest way but I wouldnt be learning all this If i did that. Also on a last note when you engage the trem on this you can hear the lopeing sound of the tubes, I thnk this is normal for these amps isnt it ? Its not a issue ( to loud) bu is noticabe. Thanks again for your input.

[Enzo]

The calculator is a fill in the blanks math program online. You got 51.7, but 51.7 WHAT? And to get that you had to enter the plate voltage and the tube current, so with the tube currents at zero because of the wiring shorting your 1 ohm resistor, what number did you use for current?

They measure mains current and if within a range they determine, off it goes, because the main variable in mains current will be the output tube currents, and yes, they are all similar enough for that to work.

Most guys do not concern themselves with crossover distortion unless it becomes objectionably large. That is why you hear the "70% rule" all the time instead of crossover notch. Crossover distortion is bad for a hifi amp, but guitar amps are not hifi amps.

SOme trems are noisier than others. Get your amp running right first, then sweat the small stuff.

[Enzo]

The calculator is a fill in the blanks math program online. You got 51.7, but 51.7 WHAT? And to get that you had to enter the plate voltage and the tube current, so with the tube currents at zero because of the wiring shorting your 1 ohm resistor, what number did you use for current?

They measure mains current and if within a range they determine, off it goes, because the main variable in mains current will be the output tube currents, and yes, they are all similar enough for that to work.

Most guys do not concern themselves with crossover distortion unless it becomes objectionably large. That is why you hear the "70% rule" all the time instead of crossover notch. Crossover distortion is bad for a hifi amp, but guitar amps are not hifi amps.

SOme trems are noisier than others. Get your amp running right first, then sweat the small stuff.

[Smitty02]

51.7ma....The Weber bias calculator has at the very top a section that asks for Class, tube type, then plate voltage then below that one they have a more defined one for lack of a better term. I was just looking at some of the calculators out there , so I put in class AB 6L6GC and plate voltage of 406 pin 3 and 8, right. It came up with 51.7ma sorry for not making that more clear..