Fender 5f6a build low output & distortion

[Phil_S]

I'm not convinced you are metering the sense resistor correctly as 5.4mV just seems wrong. The procedure is to measure the voltage drop across the resistor. This means putting one probe on the cathode (8 ) pin and one probe on the ground side of the resistor. Because it is mV, it can be very difficult to get a good reading with hand held probes. I always use clips. Clips allow a hands free reading. Hands free means you get a solid connection to the test point and no wiggle because you can't hold your hands 100% still. Wiggle and contact are critical factors in getting a good reading.

As to the 5% question, yes, it makes a difference. Since the value of the 1R is so low, you can only verify the actual value of that 1R with a high quality calibrated meter. So, we trust the manufacturer to make it to spec. At 1%, the range is 0.99 to 1.01, narrow enough to ignore. At 5% the range is 0.95 to 1.05, which is probably wide enough to make a pronounced difference. Do the math yourself....in just a bit I'll cover it.

Remember, we use a 1R resistor to take advantage of Ohm's Law, to measure voltage instead of current. We do this because it is simple and it is comparatively safe. Measuring current (Amps) is comparatively dangerous and many of us prefer to avoid doing it where not absolutely necessary. It's a simple short cut. Ohm's Law: V = I * R. In this case, where R = 1, V = I. So, we measure volts as if it is amps.

On to the tolerance question. Let's say you see 40mV. If the sense resistor is exactly 1 Ohm, you know it is also 40mA. If it is a 1% tolerance resistor (remember, you have no easy way of knowing the precise value of the 1R) then your range is 39.6 to 40.4. This is tight enough, especially when comparing two tubes, that you can ignore the small spread. By contrast, the 5% spread is 38.0 to 42.0. That is a 4mV (mA) spread.

Let's say your are working with two tubes and 5% sense resistors. One tube meters at 42mA and the other at 38mA. Well, you don't know if one is 46mA and the other 34mA or if they are both at 40mA or something else. They could be a perfect match or they could be 8mA apart. The latter case may be too much of a spread to give you good sound/performance. It might sound OK anyway, but, generally speaking, you probably don't want a 10% spread.

I just used 40mA because it is an easy number to work with. As Firestorm suggested, the probable appropriate number is in the low to mid 30's mA and this amp should be loud. I am guessing bias voltage should be around -35V. At -57V the tubes are probably turned off, and it's no wonder you can't hear much.

I think you said you moved bias voltage to -41V and got the amp to pass some OK sound. I think I might make the bias voltage a bit more positive (going towards -35V) and see if the amp sounds better.

5% tolerance resistors will give you a rough idea of what's going on, but it may be too rough to be clear about what to do, if anything. I think you can see 1% resistors are what's needed if you are going to the trouble of doing it.

As to your question of eliminating the 1R, it is not a problem. They are there only for the purpose of evaluating the tube's current draw. There is no need for them for the circuit to operate correctly.

I don't think I'd run out and buy new tubes just yet. Yours may have some life left. For a second, forget all this technical mumbo-jumbo. How do they sound? If manage to get the biased into a reasonable range, and they sound dull and lifeless, then you should replace them.

[steves3972]

thanks Phil, some good info there. Looks like it's worth getting some 1% resistors. When I test the cathode the way you suggest it yields the same reading as just measuring pins 1 or 8 to the chassis. With the bias set to -35 I get cathode readings at about 10mv. Somebody on the ceriatone facebook page suggested the following:
"you should turn the biaspot to increase the voltages on the cathode of your 6l6s. Slowly increase them to say 20mV, if bias keeps stable after 1hr increase to desired level".
If I increase the voltage on the cathodes to 20mv the bias is up to about -30. That seems a bit hot right?
The amp is sounding good to me, but I have to turn it past 2 & 1/2 before I hear anything. Maybe I wired the inputs incorrectly? That was something I wasn't certain of when I did it.

[Firestorm]

Don't worry about the actual amount of the negative voltage; different tubes may want different amounts. But do understand what you're doing by changing it. The negative voltage on the grid controls how much current the tube will pass at idle, which is a percentage of the maximum amount of current the tube can deal with without hurting itself. [EDIT: I didn't say that quite right: the tube can handle a maximum amount of heat, in watts.] For a 6L6GC, this maximum amount is 30W. When you get cathode readings of 10mV across a 1-ohm resistor, that equates to 10 milliamps of current. Since power (watts) is voltage X current, with a plate voltage of 491 and current of 10mA, your tubes are idling at 4.91 watts (491 x .010 = 4.91).

This is much too cold. You should shoot for a cathode voltage (across your 1-ohm resistor) of around 32mV. This will sound "Fendery." Bear in mind that as the tubes draw more current, the power supply will sag a little so your B+ voltage will fall. You'll need to adjust the bias a couple of times to get it dialed in.

[steves3972]

ok so the 1% resistors are on their way. I have adjusted the bias but the max on the cathodes I can get is about 25mV. I've turned the pot as far as it can go. The amp is sounding better and louder now but I have to turn it up to 2-3 on both channel's volume pots before I hear anything. When I get to 4 it's plenty loud for playing at home. I still don't think something is right here.
Could this have something to do with the bias or something else? I think it'd be the latter.

[Jana]

It's hard for me to tell from that one picture, but is that a 470R or a 47K resistor for the screen resistor on the power tube?

[Structo]

Yeah, he said he had the wrong value on the screens and corrected it to 470R.

[steves3972]

ok, so now all the resistors are the correct value, and I replaced the 22k in series with the bias pot to a 15k and I now have the tubes running at about 34mV. So the only thing not quite right is the volume output, I still have to turn past 2 1/2 before I hear anything and can run with the channels jumped both volume pots on 5 or 6 and it's pretty loud but not as loud as it should be, it is still very very clean, no distortion at all. I checked all the pots and they are all the correct value.
anybody please, ideas?