Bias Voltage Question

[bbaug14]

I can't seem to get both the bias current and the pin 5 voltage in line with what should be expected.

The exact voltage on pin 5 doesn't matter. The voltages on pins 3, 4, and 5, and the current you measure will all be varying as you change the bias pot, but all that matters is that you hit the dissipation figure (Ia x Va) that you are shooting for.

Okay. What I'm going to try later today is 180k(is). That should give me the swing for that bias current that looking for.

I'll try a fresh set of tubes first too.

I'll report back after those checks.

[dorrisant]

The exact voltage on pin 5 doesn't matter.

Amen! This is most important. Even if you have seen people bias this way... Forget about it.

[bbaug14]

Yeah, I understand. I'm just surprised that the two values that seem to be the most common (150k and 220k) both aren't getting me to the correct bias range. Oh well though. We'll see what happens with both different tubes and a value in the middle. Thanks.

[rooster]

If your B+ is around 400V then the rest of the secondary voltages should be pretty much as expected. If you want more idle current, increase the 220k, or decrease the 47k in front of the bias pot. Either way, you need to make the pin 5 voltage higher, meaning a bit less negative.

Wha? This is pretty total jibberish. What should have been said is 'If you want more bias voltage decrease the 220K resistor.' The last sentence is accurate enough, you need to 'make the pin 5 voltage higher'. To do this, continue in the direction you were going, 220K>150K>120K>100K>etc. Going to 180K (from the 150K value) is going back the other way, reducing the voltage at pin 5 - you don't want this.

[bbaug14]

Okay, I put some JJ EL34s in it. 395V at the plates. Pin 5 reads -24V. Voltage across the 1 ohm is 41mV. This is just over half the bias pot sweep. So with the JJs it looks like it's working just fine. Maybe I did have a marginal power tube?

[martin manning]

You have a 1R resistor on each cathode, right? Are the tubes drawing similar currents?

[martin manning]

If your B+ is around 400V then the rest of the secondary voltages should be pretty much as expected. If you want more idle current, increase the 220k, or decrease the 47k in front of the bias pot. Either way, you need to make the pin 5 voltage higher, meaning a bit less negative.

Wha? This is pretty total jibberish. What should have been said is 'If you want more bias voltage decrease the 220K resistor.' The last sentence is accurate enough, you need to 'make the pin 5 voltage higher'. To do this, continue in the direction you were going, 220K>150K>120K>100K>etc. Going to 180K (from the 150K value) is going back the other way, reducing the voltage at pin 5 - you don't want this.

I don't think so... To get more current to flow you need higher (less negative) voltage. A larger resistance before the rectifier will shift the bias voltage range closer to zero, less negative, or higher.

[bbaug14]

On, I only had one 1ohm resistor on hand so only one tube has it. I'll have to get another one from work on Tuesday to really check. The tubes are matched of course though.

[martin manning]

Yes do that. I've had varying degrees of "matched" depending on the vendor, and they may drift a bit after some running time.

[bbaug14]

Oh, totally agree. I just tagged the 1 ohm in to make sure I wasn't going crazy. I'm going to grab my Fluke and a couple more 1 ohms on Tuesday. Until then, im hoping tomorrow to finally hear this thing, with my Power Station as it's really really loud and I have children... and I like the paint how it is... on the walls.

[bbaug14]

Just reporting back. Got the 1 ohm's in and biased up properly and finally got to jam on this thing. Sounds friggin awesome! What a fun amp to play. I really like the way it feels, especially with the highest bright cap engaged with my LP. A little too bright, but so much fun. I think going to a 250pF or something in that range would be about perfect, but I'll let the amp settle in for a bit before I think about tweaking anything.

Thanks again to everyone who chimed in here. I appreciate all the help!!!

[martin manning]

Good to hear. I was wondering about the hum you mentioned earlier and if there was some other issue to resolve, but apparently not. Where'd you end up with the resistors in the bias circuit?

[RJ Guitars]

Great report, thanks for posting this all up for us to learn a little from.

I was also curious about that hum. I occasionally get a minimal transformer coupling hum from a Wreck build - inherent in the design but usually you can only hear it using the headphone trick. You can also get some hum from mismatched tubes if they are way off...

Glad you like the sound. The band will no doubt feel your presence in a new way from now on.

[bbaug14]

Good to hear. I was wondering about the hum you mentioned earlier and if there was some other issue to resolve, but apparently not. Where'd you end up with the resistors in the bias circuit?

I think the hum was due to having the chassis open on my bench, which is in a room completely lit by flourecent lighting. When I put the aluminum shield on the amp and turned the lighting off, it was just a hiss coming through the cab like would be expected.

The bias resistor is 220k as I stated with. Both tubes are at 41mA and 42mA respectively. No issues there at all. Basically, I was chasing a tube issue the whole time. Had I just tried a different tube set to start, I probably wouldn't have made a thread.

My negative max bias with no tubes installed is still on the low end of the spectrum, but it isn't a concern since I've got ample range now in my bias circuit with tunes installed.

I really like the sound with the 500p bright cap engaged. It's too bright, but the attack and texture is perfect. I think a darker tube in V1, more burn in time, and maybe a darker PI tube would be ideally suited with my LP, which is a little on the bright side.

[dorrisant]

Use the tone control on your Les Paul... It is much more effective with this circuit than with others.