Adjustable fixed bias resistor substitutions

[deiseldave]

Hello, I am building a Mark Huss-esque Plexi6V6, except (unlike Mark's) my PT had a 45V bias tap. So I planned on using the adjustable fixed bias scheme shown on the EL34world site (10K pot with 27K going to ground). The problem is, I just realized I forgot to order a 27K resistor... The closest values I have are a 22k and a 33K. Can I sub one of these? Thanks.

[David Root]

You could parallel a 68K and a 47K for 27.7K.

What bias voltage are you looking for? With a 10K pot and 27K tail and -63V raw bias (45x1.4) you will have a range of about 17V or so on the pot, depending on what initial dropping resistor, if any, before the diode, that you use.

[deiseldave]

You could parallel a 68K and a 47K for 27.7K.

What bias voltage are you looking for? With a 10K pot and 27K tail and -63V raw bias (45x1.4) you will have a range of about 17V or so on the pot, depending on what initial dropping resistor, if any, before the diode, that you use.

Thank's for the response, David. I am biasing 6V6's. Like the EL34world scheme, I have a 470R resistor before the diode, and the rest past the bias adjust portion is just like Mark Huss' scheme (100K & 5k6 before pin 5 of each 6V6). To be honest, I am not sure how to calculate the voltage on this. This is my first build, and I am trying to understand bias section.

[ToneMerc]

Hello, I am building a Mark Huss-esque Plexi6V6, except (unlike Mark's) my PT had a 45V bias tap. So I planned on using the adjustable fixed bias scheme shown on the EL34world site (10K pot with 27K going to ground). The problem is, I just realized I forgot to order a 27K resistor... The closest values I have are a 22k and a 33K. Can I sub one of these? Thanks.

For 6V6's you will need an even smaller tail resistor. IIRC I typically use these values in a generic 6V6 bias circuit; 45V>680-750R>10K pot>12-15K.

Here's the scheme from a Fender BF Deluxe, 45V>470R>10K pot>10K.

TM

[deiseldave]

Hello, I am building a Mark Huss-esque Plexi6V6, except (unlike Mark's) my PT had a 45V bias tap. So I planned on using the adjustable fixed bias scheme shown on the EL34world site (10K pot with 27K going to ground). The problem is, I just realized I forgot to order a 27K resistor... The closest values I have are a 22k and a 33K. Can I sub one of these? Thanks.

For 6V6's you will need an even smaller tail resistor. IIRC I typically use these values in a generic 6V6 bias circuit; 45V>680-750R>10K pot>12-15K.

Here's the scheme from a Fender BF Deluxe, 45V>470R>10K pot>10K.

TM

Thanks. So, since I already have a 470R soldered in, just use a 10K tail?

[ToneMerc]

Hello, I am building a Mark Huss-esque Plexi6V6, except (unlike Mark's) my PT had a 45V bias tap. So I planned on using the adjustable fixed bias scheme shown on the EL34world site (10K pot with 27K going to ground). The problem is, I just realized I forgot to order a 27K resistor... The closest values I have are a 22k and a 33K. Can I sub one of these? Thanks.

For 6V6's you will need an even smaller tail resistor. IIRC I typically use these values in a generic 6V6 bias circuit; 45V>680-750R>10K pot>12-15K.

Here's the scheme from a Fender BF Deluxe, 45V>470R>10K pot>10K.

TM

Thanks. So, since I already have a 470R soldered in, just use a 10K tail?

Yes, a 10K tail.

TM

[deiseldave]

Thanks,

[roberto]

Always use the bias pot to ground, not on the raw bias voltage side.
If the pot fails you'll find the raw voltage on grids, so low bias current and amp safe.

If you use the pot on the raw bias voltage side and it fails, you'll ground the grids. Tubes and trafos won't be happy.

[deiseldave]

Always use the bias pot to ground, not on the raw bias voltage side.
If the pot fails you'll find the raw voltage on grids, so low bias current and amp safe.

If you use the pot on the raw bias voltage side and it fails, you'll ground the grids. Tubes and trafos won't be happy.

If I am understanding what you said correctly, I think that is what I have. Is the scheme at this link http://el34world.com/charts/Biascircuits.htm how you are recommending?