First time biasing and I have some questions

[professormudd]

Greetings,

I have attempted to search, but finding answers through the massive amount of results shows that I do not know enough to even form the best search query. So, I apologize in advance if this is a redundant question.

Please refer to the layout.

I just completed the assembly of a Brown Note D-Lite Blue Monkey 44. I was preparing to start it up for the first time and I was ready to set the bias on the power tubes. Below are a list of my steps along with questions:

• I was to believe that this amplifier can handle 2 6L6 or 4 6V6 power tubes. I installed 2 apex-matched JJ 6L6GC power tubes. One in V6, the other in V7. Are these the correct tube sockets to install the tubes?
• I measured the voltage from pin 3 on each of the tubes. V6 was 505v, v7 was 507v. This seems high, but I do not know.
• I used the calculator from AmplifiedParts.com to get 100% bias point of 59.41mA for V6 and 59.17mA for V7.
• I inserted my multimeter probes into the bias tip jacks for each of the sockets. I set V7 using the first bias pot (B1). When I inserted the probe into the tip jack for V6, it was only reading between 10-17. This is the point I stopped because I am confused.

Would anyone be so kind as to walk me through adjusting bias on this particular amplifier?

Did the readings I take make any of you raise eyebrows?

[professormudd]

FYI: :

These are the pots for the bias adjustment

I was not entirely confident in how to wire the pots as the diagram did not match the numbers on the actual pots.

I followed the logic of orienting the pins the same as they are in the layout, and decided to connect pin 1 on each of the pots (as numbered on the pot itself) to the resistors that go to ground..

[pompeiisneaks]

Greetings,

I have attempted to search, but finding answers through the massive amount of results shows that I do not know enough to even form the best search query. So, I apologize in advance if this is a redundant question.

Please refer to the layout.

I just completed the assembly of a Brown Note D-Lite Blue Monkey 44. I was preparing to start it up for the first time and I was ready to set the bias on the power tubes. Below are a list of my steps along with questions:

• I was to believe that this amplifier can handle 2 6L6 or 4 6V6 power tubes. I installed 2 apex-matched JJ 6L6GC power tubes. One in V6, the other in V7. Are these the correct tube sockets to install the tubes?
• I measured the voltage from pin 3 on each of the tubes. V6 was 505v, v7 was 507v. This seems high, but I do not know.
• I used the calculator from AmplifiedParts.com to get 100% bias point of 59.41mA for V6 and 59.17mA for V7.
• I inserted my multimeter probes into the bias tip jacks for each of the sockets. I set V7 using the first bias pot (B1). When I inserted the probe into the tip jack for V6, it was only reading between 10-17. This is the point I stopped because I am confused.

Would anyone be so kind as to walk me through adjusting bias on this particular amplifier?

Did the readings I take make any of you raise eyebrows?

1. if you are going to only install 2 of the 4 tubes, you just need to ensure one tube per half of the OT windings. I think, if you're saying that the tube count starts at the first preamp tube is V1 and there are 4 preamp tubes, V1,V2,V3,V4, then V5,V6,V7,V8 going in that line are the 4 power tubes.
you coudl either use V5,V7, or V6,V8, or V5/V8 Meaning the 'pairs' that must not be 'alone' are V5/V6 or V7/V8.... any other combination works...

2. If the amp isn't warm and isn't outputting any sound etc, or if it's on standby, it will be higher voltage until the tubes start conducting (also if you only have two power tubes and not all preamp tubes it may run higher as well, each tube adds an overall voltage drop to the B+ rail)

3. yes but I"m hoping you don't want to set it at 100% but instead are going to set it at about 70% dissipation which is the correct setting for grid biased tubes.

4. That reading makes no sense. If you're setting your multimeter to milliamps, then you're doing it incorrectly... you need to set it to read voltages. The 1R resistors then give you an ohms law value of mA directly from the mV reading:

V = I*R

V = I * 1. = V = I

so maybe you're on milliamps reading and just pulling a few milliamps from around the cathode 1R resistor and getting bad readings?

If you've set it to VDC and are getting 10-17mA then your'e biased very cold and would need to dial it up until you get ratings you'll expect/want.

BUT an amp is going to work optimally with the 4 power tubes that the OT was designed for, becuase the OT reflects the right impedance back from what was expected. On the other hand if you want to, you can adjust the speaker load accordingly. I can never remember if it's up or down, but basically if you're halving the impedance of the power tubes I think you need to inversly increase the speaker load to bring the impedance ratio back up...

Therefore, if you have the impedance of the speaker set to say 8 ohms, and you're only using 2 tubes you should likely use 16 ohm speakers on the 8 ohm setup (hopefully someone corrects me if I have that backwards)

~Phil

[xtian]

Yes, you put the two power tubes in the right sockets. Plate voltage is very high. I remember telling you you may want to used the lower voltage PT secondaries. Now is the time to switch them over.

You do NOT want 100% dissipation. Aim between 50-60%. So, using the bias calc, and assuming you'll have about 450v plates, you'll want 33-40mA per tube. After adjusting the bias, your plate voltage will change (because the current draw thru the power tubes will cause the power supply to sag). So, recheck the plate voltage and refine the bias voltage.

I inserted my multimeter probes into the bias tip jacks for each of the sockets. I set V7 using the first bias pot (B1).

OK, but you don't say what bias voltage you got.

When I inserted the probe into the tip jack for V6, it was only reading between 10-17. This is the point I stopped because I am confused.

Do you mean to say 17mA is the max you could get by adjusting the bias pot? You may have to adjust the bias circuit.

[professormudd]

1. if you are going to only install 2 of the 4 tubes, you just need to ensure one tube per half of the OT windings. I think, if you're saying that the tube count starts at the first preamp tube is V1 and there are 4 preamp tubes, V1,V2,V3,V4, then V5,V6,V7,V8 going in that line are the 4 power tubes.
you coudl either use V5,V7, or V6,V8, or V5/V8 Meaning the 'pairs' that must not be 'alone' are V5/V6 or V7/V8.... any other combination works...

Yes, there are 4 preamp tubes.

2. If the amp isn't warm and isn't outputting any sound etc, or if it's on standby, it will be higher voltage until the tubes start conducting (also if you only have two power tubes and not all preamp tubes it may run higher as well, each tube adds an overall voltage drop to the B+ rail)

All 4 pre-amp tubes are installed. All knobs set to 0. Speaker cabinet plugged in but nothing in the input. The amp is idle.

3. yes but I"m hoping you don't want to set it at 100% but instead are going to set it at about 70% dissipation which is the correct setting for grid biased tubes.

I want 65%. I should have said that. I only wanted to find 100% so I could derive the value I wanted. The calc only offered increments of 10%.

4. That reading makes no sense. If you're setting your multimeter to milliamps, then you're doing it incorrectly... you need to set it to read voltages. The 1R resistors then give you an ohms law value of mA directly from the mV reading:

My apologies... I was reading mV. See attachments.

[professormudd]

Yes, you put the two power tubes in the right sockets. Plate voltage is very high. I remember telling you you may want to used the lower voltage PT secondaries. Now is the time to switch them over.

Bummer. At least they are easier to reach than other stuff.

You do NOT want 100% dissipation. Aim between 50-60%. So, using the bias calc, and assuming you'll have about 450v plates, you'll want 33-40mA per tube. After adjusting the bias, your plate voltage will change (because the current draw thru the power tubes will cause the power supply to sag). So, recheck the plate voltage and refine the bias voltage.

Yeah, don't fret. I just wanted to know 100% so I could derive on the fly.

OK, but you don't say what bias voltage you got.

Do you mean to say 17mA is the max you could get by adjusting the bias pot? You may have to adjust the bias circuit.

I was checking mV. Sorry.

[pompeiisneaks]

I'd double check the other ones for completeness, maybe you cross wired v6/v5 or something to the bias points? If not that means something on that tube is restricting conduction severely. Maybe a solder joint that's bad etc?

~Phil

[professormudd]

I'd double check the other ones for completeness, maybe you cross wired v6/v5 or something to the bias points? If not that means something on that tube is restricting conduction severely. Maybe a solder joint that's bad etc?

~Phil

Visually I do not see anything missing. I moved the power tubes to different sockets, and any tube in V5 or V6 reads in the teens for mV at the bias test points. I will keep inspecting for bad joints or shorts.

Are there any other checks I can perform to help narrow down where the problem is?

[pompeiisneaks]

Another test would be to swap the actual tubes from each side, to see if the fault follows the tube or stays with the 'side'

If it follows the tube, then you might have gotten a bad tube. If it stays on that side then something in the wiring between the bias connection at the left bias board, and the 220k resistor for that half may have a problem. I'd test in continuity mode between all the connections, ensuring they're good, then test the resistance to ensure you see say 220k as expected on that resistor, and that you're getting the expected resistance reading on the pot for the bias adjustment on that side etc.

~Phil

[xtian]

Are there any other checks I can perform to help narrow down where the problem is?

Yes, with the power tubes pulled OUT, measure the DC voltage at pins 5 on the power tube sockets--that's where the bias voltage is applied. Tweak the bias pots, see what range of DC you can get. If one side is way off, that's where your issue is.

[professormudd]

Yes, with the power tubes pulled OUT, measure the DC voltage at pins 5 on the power tube sockets--that's where the bias voltage is applied. Tweak the bias pots, see what range of DC you can get. If one side is way off, that's where your issue is.

I just tried this. I am not sure what level of detail is pertinent, so here is everything.

I noticed that the bias pots behave strange in that if I adjust bias pot 1 (B1) it only affects V7/V8 voltages at pin 5. However, if I adjust bias pot 2 (B2) it affects V5/V6, but it also has a bit of an affect on V7/V8.

Anyway, I tweaked B1 and B2 independently, meaning when I tested V5/V6, I turned B1 all the way CCW. And when I tested V7/V8 I had B2 all the way CCW.

• V5/V6: -57.5V to -62.8
• V7/V8: -57.8V to -21.3

[pompeiisneaks]

They should definitely not interact, something is connecting where it shouldn't. If you note the wiring in the layout shows different colors that connect the sides. There's only one 'common' place and that's the negative bias coming off the bias board that jumpers between the inputs of the two bias pots.
THen from there each side sends off with different colored wires to each side of the 220k resistors then off with different colored wires to each half of the power tubes, with a jumper from pin 6 to pin 6 of the other half of the power tubes.

there should be no other jumpers/connections that go between the pairs of V5/V6 and V7/V8 at all, on those pins, they only should have B+ in common coming to those 1.5k resistors

it seems like somehow some of the bias is bleeding over from the one side to the other but only on one side? Which is definitely weird.

~Phil

[xtian]

+1, they should not interact at all. Check the wiring. Again. Again! Once more!

[professormudd]

... If you note the wiring in the layout shows different colors that connect the sides. There's only one 'common' place and that's the negative bias coming off the bias board that jumpers between the inputs of the two bias pots.
THen from there each side sends off with different colored wires to each side of the 220k resistors then off with different colored wires to each half of the power tubes, with a jumper from pin 6 to pin 6 of the other half of the power tubes.

there should be no other jumpers/connections that go between the pairs of V5/V6 and V7/V8 at all, on those pins, they only should have B+ in common coming to those 1.5k resistors

Everything you said is the same logical conclusion I came to and I am certain that I followed. Just me following where the wires connect from the bias pots, to the 220k resistor, to the relay on the one side and to the capacitor on the other side... Could the issue be on the relay board?

it seems like somehow some of the bias is bleeding over from the one side to the other but only on one side? Which is definitely weird.

The interaction is +/- 2.5-5mV, if that is relevant.

[professormudd]

+1, they should not interact at all. Check the wiring. Again. Again! Once more!

I will keep checking.