Tuning Bias Supply Range couple of questions...

[keithrick]

I have finished my ODS Clone and have been tweaking the bias supply to acchieve the proper bias range. I used the typical Fender Bassman iron with the 50v bias supply. Now here is my quandry....When using the Hybrid A bias supply values I am only able to achieve 35ma of idle current on the tubes. Which is acceptable 60% of dissappation but I would like to have more adustment value.

Per my understanding from the Hoffman Amps Website you adjust the range of the pot by either increasing or decreasing the 1.2k 1watt resister to get your pot in the correct range while the 15K resister to ground is a safety measure so you do not redline the tubes.

So.....

1. I would need to raise the 1.2K dropping resister to allow more idle current?

2. What is a minimum safe value for the 15k resistor to ground.

3. Can the 15K resister be changed in liue of the 1.2K dropping resister in the circuit?

Thanks in advance for any comments!

[glasman]

What size pot are you using?

I would say that reducing the 15K to maybe 10K would be a reasonable approach to reducing the bias voltage. I typically use a 100K bias trim and set the range from about -25volts to about -65 volts. I also hang a 22uf cap on the bias output to make sure that is stays at AC ground.

Gary

[keithrick]

Hi Gary thanks for responding. I currently have a 25K pot with a 1.5k dropping resister and 1k resister to ground. With this set up I am getting 38 ma of current.

I am confused and asking about this even after reading the Hoffman site that clearly spells it out because I originally had the 1.2k dropping resister and 10k pot with a 15K resister to ground and was only able to achieve 17ma of idle current.

Am I on the right track by raising the 1.5K to around 2.7k?

[glasman]

Was is the voltage at the top of the pot. It should be around -50 volts if even thing is as you say.

Given the 1K tail and 25k pot you should have from -2v to -50v. You should be able to cook the tubes. This assumes that you are taking the voltage from the wiper.

With the tubes removed. Check the range of the voltage as measured at the bias feed point. Also where are you taking the bias voltage? Wiper or top of the pot (pot wired as a variable resistor)? If you are taking the voltage off of the top of the pot, you will need to play with the resistor values. I normally grap the bias votlage from the wiper.

Gary

[keithrick]

Given the 1K tail and 25k pot you should have from -2v to -50v. You should be able to cook the tubes. This assumes that you are taking the voltage from the wiper.

Gary this is exactly what I am getting 2v to 55v!

Here are the stats of my amp as of last night. I think I may have a blown output transformer......

Negative voltage sweep: 2-55v Wiper supplies voltage

Pin3 to CT - 38ma idle current
Ct to ground: 457v at -2v negative voltage from bias supply

V1 (1)217.5v (3)2.19v (6)223v (8) 2.098v
V2 (1)227v (3)2.07 (6)231 (8) 1.99v
V3 (1)448v (3)5.31v (6)363.4v (8) 0v
V4 (1)334.6v (3)59.5v (6)326.9v (8) 59.5v

The reason I am concerned with the bias is because of a few issues I have:

1. No clean headroom and lack of volume on clean channel, I have to dime everthing to get sufficient volume.

2. 2 burned 1k ground resistors from the power tubes (pin1 and8 to ground) Both still measure 1k.

Note that I do have the artificial center tap for the heaters grounding at the same point on the first 6l6 and the reverb is grounded at the second 6l6 tube. Could this have caused my burnt resistors?




[/quote]

[glasman]

Given the 1K tail and 25k pot you should have from -2v to -50v. You should be able to cook the tubes. This assumes that you are taking the voltage from the wiper.

Gary this is exactly what I am getting 2v to 55v!

Here are the stats of my amp as of last night. I think I may have a blown output transformer......

Negative voltage sweep: 2-55v Wiper supplies voltage

Pin3 to CT - 38ma idle current
Ct to ground: 457v at -2v negative voltage from bias supply

V1 (1)217.5v (3)2.19v (6)223v (8) 2.098v
V2 (1)227v (3)2.07 (6)231 (8) 1.99v
V3 (1)448v (3)5.31v (6)363.4v (8) 0v
V4 (1)334.6v (3)59.5v (6)326.9v (8) 59.5v

The reason I am concerned with the bias is because of a few issues I have:

1. No clean headroom and lack of volume on clean channel, I have to dime everthing to get sufficient volume.

2. 2 burned 1k ground resistors from the power tubes (pin1 and8 to ground) Both still measure 1k.

[/quote]

I think your problem is the 1K resistors. Those should be 1 ohm. You are developing a bucking voltage on the cathode that is clamping the tubes.

Use a 1 ohm 1 or 2 watt. I am SURE that this is where the problem lies.

gary

[dogears]

Keith,

Hang a 150k to 180k off of the last resistor in your string to ground. That will lower your V1 and V2 plate voltages. You are too hot.

Lemme know how it sounds!

[glasman]

Me thinks he needs to fix the power amp first

Gary

[dogears]

Yah, but you already got that one figured out!

Me thinks he needs to fix the power amp first

Gary

[mlp-mx6]

I think your problem is the 1K resistors. Those should be 1 ohm. You are developing a bucking voltage on the cathode that is clamping the tubes.

Use a 1 ohm 1 or 2 watt. I am SURE that this is where the problem lies.

gary

Keith,

I am kidding with you when I say...
Congratulations! You just created the first "silverface" *umble!

(The AA1069-series Fender amps used an odd combination of fixed and cathode bias, with 150R resistors on the cathodes in addition to the normal fixed bias. These were the amps that gained the "silverface" reputation for sounding cold and sterile.)

[keithrick]

Gary, Are the 1 ohm resisters optional? If they are I will just run a wire to ground.

Dogears- I thought I was one step ahead of you ....but I bought 150ohm 2w instead of 150K

Mlp-mx6 -

[mlp-mx6]

Yes, the 1-ohm resistors are optional. They are used to measure idle current through the tube for setting the bias.

[dogears]

Keith,

Just use a plate resistor!

[glasman]

Yes, the 1-ohm resistors are optional. They are used to measure idle current through the tube for setting the bias.

I would say since the original poster was using the transformer shunt method for measuring current the 1 ohm resistors are not really needed.

But, I will say that putting a 1 ohm 1 watt (or even 1/2) does add a bit of blow out protection for the output transformer. Just in case a tube shorts a resistor is cheaper than an output or power transformer. A 500ma fuse in the HV will also provide the same protection.

Gary

[keithrick]

Hey Guys,

Thanks! I am in business! I removed the 1k resistor and was able to bias and have sufficient range. Of course I did wake the baby up when I flipped it off standby, it screamed like a banshee, so after reversing the speaker leads it sounds well.....great! Still need too tweak though!


Gary, can I steal a idea I got from my Express build and use (3) reverse diodes per tube from pin3 to ground for protection?

I am going to try to lower the voltages tonight!

Scott, I just want to clarify that the you mean to hang that 150k plate resistor from the positive end of the 1.2k 2w (b+4 to b+5) dropping resistors on the filter caps to ground?

Thanks again for the help