5F2A To Fixed Bias

[Morgan]

Okay, okay. I know, two threads on the same build? What an idiot! Everything stock on the amp is great, but my "mod" ideas are causing a little havoc at the moment.

So I put in a DPDT switch to switch in a fixed bias circuit for a different sound coming from this amp when wanted. To create negative voltage, I stole AC from pin 4 of the rectifier.It runs through your standard fixed bias circuit (dropping resistor, diode, bias caps, load resistor, pot) which is left open until the switch is thrown. The switch lifts the ground of the 220K grid resistor and connects it to the bias supply.

Everything seems to do what it's supposed to. When the switch is thrown, I get negative voltage on pin 5 of the output tube. I can adjust the bias on the tube appropriately.

However, the fixed bias circuit is injecting a lot of noise and distortion in to the circuit. Super loud hum and very gravely distortion when the amp is played. The hum is so loud that I'm afraid to keep the amp on for very long even though the voltages are checking out okay.

Any ideas? What have I got my self into? Some of the wire runs seem fairly long. I left the 220K in place and ran a wire from it to the switch (underneath the board, beneath the filter supply). The switch is located between the rectifier and the power tube. The negative voltage bias supply is glued to the chassis between the board and the power transformer.

Thanks!
Morgan

[MWJB]

You say the switch connects the grounded 220K to the bias supply but you don't mention what happens to the 6V6 cathode? This should be grounded in fixed bias mode.

Your bias cap(S) are correct polarity (+ to ground)? How many are you using, value, etc? Your description mentions "bias caps" but there only seems to filtering from one point in your description? The conversions I am aware of use 2 bias caps and have an additional dropping resistor between diode & load resistor/pot (tweed bassman/brown Concert/Pro style).

Load resistor/pot & bias caps should be grounded to main filter ground.

[Morgan]

Sorry about that - should have been more descriptive.

Throwing the switch does ground the cathode (through a 1 ohm resistor for biasing).

The negative voltage bias supply goes like this: Pin 4 rectifier, 220K dropping resistor, 1N4007 diode (cathode facing the 220K), junction of 8 uF bias cap (+ to ground) and one end of 50K bias pot, wiper of pot connects to 220K grid resistor and another 8 uF bias cap (+ to ground), last end of bias pot connects to 10K load resistor to ground.

The grounds go to the same lug on the PT that the mains are grounded on.

On a suggestion, throwing the switch lifts the ground of the cathode bypass cap, but leaves the cathode resistor in place. I am going to play with lifting the ground of the cathode resistor as well to see if that is where the noise is coming from.

Anything else I'm overlooking?

Thanks,
Morgan

[Morgan]

Any ideas, anybody?

I've gone back and re-checked my wiring and cannot find any faults. I tried lifting the ground of the cathode resistor when in fixed bias mode, but it did not do anything.

I did notice today that when I adjust the bias, the nature of the hum changes very slightly.

What will typically introduce this kind of hum into a fixed fias circuit? Again, the amp works fine in cathode bias mode.

Thanks,
Morgan

[Ray Ivers]

Morgan,

If the amp works well with cathode bias, IMO the problem has to be noise introduced by your bias supply. The supply you described sounds somewhat similar to a 50W Marshall 1987-model bias supply, which works well in that amp - but keep in mind that a fixed-bias circuit for a single-ended amp had better be dead quiet.

You could use a full-wave-CT bias supply (two diodes) for easier-to-filter 120Hz ripple, but I think a Zener-regulated supply would probably be the way to go - if this seems like something you want to try but don't feel comfortable working out the component values, let me know and I can figure them out for you. Also, the bias circuit ground should be to the same point as the output tube cathode resistor.

Ray

[Morgan]

I, Morgan, do pledge, from here on out, to properly trouble shoot all amp builds before asking for help on this, or any, forum.

I went back and began troubleshooting by checking the resistance of each ground point. I found that while the two bias caps were indeed tied together on the ground side, somehow I did not succesfully tie them in with the load resistor to ground. I tied them in and the ugly, ugly noise went away.

Leason learned: I now know what ungrounded bias caps sound like!

Ray - thanks for your input and for your offer to work out the values with me.

Morgan.

[Satamax]

Hi everybody!

Morgan, been following the discussion, and the other one. I have one thing to say, it would be far easier to draw a litle sketch at least, when asking for help. Ok, may be just cos i am french, and have hard time following lenghty descriptions about which lead goes where. And may be link to the schemos when you have them availlable, so people don't have to dig out.

Bye.

Max.