quick question about biasing a 5150

[MitchK]

I'm about to bias my 5150 but I can't find where the ground wire for pin 8 is? i'm reading the lord valve bias FAQ and it says to disconnect the ground wire and replace it with the 1 ohm resistor but I just can't find where it actually goes to ground, its all pcb and hard to trace stuff. what would happen if I just disconnected the main ground wire with the 1 ohm resistor?

also what I was wondering, is can't I just connect the pin directly to ground with the resistor? how does that work theoretically? everyone always uses teh plumbing analogy, and if I think of one big pipe (the mains circuit) connecting to a basin with water under pressure, if I put another pipe in (the 1 ohm resistor), water is going to flow through that too. how does the path of least resistance play into the whole thing?

[Enzo]

The sockets are on a circuit board, there is no ground wire for pin 8. In fact it is difficult on these to isolate a pin for such a purpose. This is a case where a bias probe really makes sense. I am guessing the Lord Vavle instructions are generic - covering amps in general, rather than the 5150 specifically.

[MitchK]

hmm well in the meantime, if I disconnect the main ground and put the 1-ohm resistor there, will things catch on fire. I have a 10 watt resistor, but yeah its the main ground and the back of the amp says 400watt...

[Enzo]

"Main ground?"

The point of a 1 ohm resistor is it needs to be in series with the plate current of the power tubes, and nothing else. With pin 1, we accept the addition of the relatively small screen current to the total. A small but predictable error it represents. You could in fact even measure the screen currnets and subtract them.

I don't know what you mean by main ground. If you mean the ground on the power cord, that would tell you zero about tube current.

[Thorny]

A 5150 does not have an adjustable bias control, so it has to be modified. Do you know how to do it? I have done it, but I don't recall the specifics (have to look at the board). I don't have one around. Also, I am speaking of the 5150 head, as it is really pretty easy to modify - the 5150 combo is not (avoid the combo).

But, get yourself a bias probe and use that to measure the current. Don't do the 1 ohm thing. You have to measure the voltage as well. Then do your calculations and adjust the bias to around 15-16 watts static dissipation. For tubes, get you some of those brown base Military Shuguang 6L6GCs that Lord Valve sells, and for the preamp tubes get some JJECC83s (at least the first couple tubes). Do that and your 5150 will kick! And the clean tone will sound MUCH better as well.

[MitchK]

"Main ground?"

The point of a 1 ohm resistor is it needs to be in series with the plate current of the power tubes, and nothing else. With pin 1, we accept the addition of the relatively small screen current to the total. A small but predictable error it represents. You could in fact even measure the screen currents and subtract them.

I don't know what you mean by main ground. If you mean the ground on the power cord, that would tell you zero about tube current.

Maybe I just have no idea what I'm doing but this is what was going on in my head, If I measure pin 8 with my red probe, and the chassis with my black probe I'm measuring cathode to ground. with the cathode resistor method I need to put a 1 ohm resistor between the cathode and ground, in series of course as you said because if its parallel it will be bypassed. SO since I can't get at the sockets soldered into the PCB I was thinking that I could connect the 1 ohm resistor between where the circuit connects to the chassis..

anyways at this point I'm taking the sockets out of the PCB and re-mounting them in a riser that I'll attach to the chassis so the tubes are spaced further apart so I can fit 4 kt88s in there.

I have already installed an adjustable bias, I'm trying to put the resistor in so that I can measure the bias via the cathode resistor method because I can't seem to get the OT shunt method to work. I also have a mediocre meter...

[Enzo]

If you are dispensing with the pc board, then include the 1 ohm resistors in your new wiring. There is no easy way to add one to the stock pc board. Likewise, the stock board is REALLY difficult to modify for EL34s. Hard wired sockets, just jump pins 1 and 8 together.