Getting plate voltages just right?

[Garveyte01]

Hi everyone, I recently completed a Dumble clone 124. I am just fine tuning the amp now and trying to adjust but I found a few discrepancies

-the plate voltages at V1 are 206 and 205

~The plate voltages at V2 are 209 and 212

-the plate voltages at V3 are 312 and 319

The power tube plate voltages are 441 and bias to 46 milliamps

Any recommendations as to what to do, or should I leave it alone?

Thank you Tom

[martin manning]

Is the 38 mA cathode current? If so you are on the cold side but not extremely so. Adjusting the bias circuit to get hotter bias will lower your voltages, or you could also consider increasing the first dropping resistor. IMO V1 plates should be 190 or a bit less.

[Garveyte01]

Is the 38 mA cathode current? If so you are on the cold side but not extremely so. Adjusting the bias circuit to get hotter bias will lower your voltages, or you could also consider increasing the first dropping resistor. IMO V1 plates should be 190 or a bit less.

Hi Martin, yes that’s the cathode current, I guess my first issue I need to tackle is why the potentiometer won’t bias above 38 milliamps, this is the max voltage that’s available. I checked my bias supply I believe it was 68 volts but I need to verify that. Any reason why this is

[martin manning]

The bias tap voltage from your transformer may be high, or your output tubes may just require less negative voltage to reach the desired bias point. I would modify the bias supply circuit by trial and error to more or less center the bias pot at the required voltage.

[GAStan]

I've encountered a similar problem. What i did was remove the 27k resistor between the bias trim potentiometer and ground and replaced it with a 22k. Then I put a 5k on the other side of the trim pot, between the pot and rectifier diode. This kept the overall load on the bias circuit the same (~37k) while changing the bias voltage range.

Don't forget to adjust the pot for max negative voltage prior to powering it back up.

[ijedouglas]

To change the range of your bias circuit, adjust the value of the 3K3 resistor on the bias board. If you want the range to go higher, increase the resistor value and vice versa. A decade box works really well for this if you have one.

Your plate voltages are a bit high. Before adjusting the circuit, I would try a few different tubes in V1/2/3 and see if that makes a difference. I've found that tubes often have a bigger impact than the dropping string. Also, if it sounds good then I wouldn't worry too much about the higher voltages. As Martin suggests, you may want to put a larger value in your first dropper (2K7 or 3K) and see what that does. If it sounds better, you are going in the right direction

[GAStan]

To change the range of your bias circuit, adjust the value of the 3K3 resistor on the bias board. If you want the range to go higher, increase the resistor value and vice versa. A decade box works really well for this if you have one.

This would be better than my method. With my method I was unable to bias to less than several milliamps where before I changed it I could bias to nearly zero mA.

[jabguit]

To change the range of your bias circuit, adjust the value of the 3K3 resistor on the bias board. If you want the range to go higher, increase the resistor value and vice versa.

By higher, do you mean less negative??

cheers,

[martin manning]

LOL. Always a source of confusion. It's simple if you think of temperature... Which is lower, -10 or -15 degrees? Which is colder?

[ijedouglas]

To change the range of your bias circuit, adjust the value of the 3K3 resistor on the bias board. If you want the range to go higher, increase the resistor value and vice versa.

By higher, do you mean less negative??

cheers,

I tried to explain it in context of the question. The OP was having difficulty getting the bias higher. Technically you are lowering the bias circuit current (less negative) and therefore increasing the power tubes bias current.

[oldmacman]

Do you have the FET input installed? If not, an additional dropping resistor at the end of the string might bring the voltages down a hair to where you want them to be.

[jabguit]

LOL. Always a source of confusion. It's simple if you think of temperature... Which is lower, -10 or -15 degrees? Which is colder?

He mentioned "range", not temperature....

cheers,

[martin manning]

My comment was aimed at the tendency of people to refer to a more negative voltage as being higher than a less negative voltage.

[llemtt]

Hi everyone, I recently completed a Dumble clone 124. I am just fine tuning the amp now and trying to adjust but I found a few discrepancies

-the plate voltages at V1 are 206 and 205

~The plate voltages at V2 are 209 and 212

-the plate voltages at V3 are 312 and 319

The power tube plate voltages are 441 and bias to 46 milliamps

Any recommendations as to what to do, or should I leave it alone?

Thank you Tom

Plate voltages at V1, V2, V3 depend first of all on first supply voltage node (power tubes node), but also depend a lot on type/brand of the tube used, hence there aren't right and exact values.

Generally speaking there aren't many things that are right and exact about Dumble amps, they where built tailored to the owner. Let your ears be your guide!

So do you like your amp? Does it behave the way you want?

If it's so just be happy and play with it, if not you can ask this forum suggestions to better "tailor" the amp to you.

my .02
teo