Boogie Studio .22 Bias

[selloutrr]

You might want to lower the wiper resistor to ~5K to give more range. It gives a minimum bias voltage when the pot is turned all the way up.

It'll give you ~ 20volts of adjustment.
i.e. your currently -14v fixed
Installed -6v to -26v adjustable.

[Gonzini]

Thanks l'll give it go.
Do you think the EQ is working correctly so only the graphic is enabled when switched to the high gain channel?
Still got pops and crackles to sort out as well...

[selloutrr]

the pops and crackles

bad solder points.
failing resistors.

If you have any ceramic caps - replace them with orange drops.

[Gonzini]

Ive run it up with out any of the front end valves in place so just the phase splitter and EL84's, crackle are pops are still there.
Its almost like you have to replace any resistors and caps that have HT anywhere near them.
So something in the power supply, graphic eq or output stage i guess.
I take it your not sure about the eq switching between the 2 channels?
Cheers

[selloutrr]

here's the manual it talks about the graphic eq it's auto switched with the lead ch....

[selloutrr]

POPS -

did you replace R132 and R123 when you did the caps?

also do you know the output tubes are Good? Tubes often have ticks and crackles even new tubes.

[Gonzini]

Hi
Yes those 2 resistors were replaced early on, the 5.6K was nearer 7K and the 10K was starting to burn up.
Ive replaced the filter cap in the bias circuit and replaced the 2 x 470R screen resistors tonight.
All seemed quiet but it tricked me last time i thought it had cleared up.
l'll leave it on few hours tomorrow and see what happens.
I was also going to replace the 2 x 100R resistors that act as the hum balance on the heater supply, they do look a bit ancient and discoloured.
The hum would just appear and then disappear randomly, its got better in general for changing the main filter caps but ive still heard the intermittent hum.
As for the output tubes their my test ones. Their not new but are fairly well matched going by the bias through each half.
I do have another pair i could try in there, though the crackles and pops were there with the original tubes (that do need replacing) so the problem didn't go after changing tubes.
The thing with the EQ was that i thought the main EQ on the knobs was still active with the graphic in one mode but its seems that its one or the other which does make sense switching between rhythm and lead so you have a different preset EQ.
I still need to sort the variable bias out but just want to clear the noise's up first so im not disturbing to much at any one time.

[Zouto]

You might want to lower the wiper resistor to ~5K to give more range. It gives a minimum bias voltage when the pot is turned all the way up.

It'll give you ~ 20volts of adjustment.
i.e. your currently -14v fixed
Installed -6v to -26v adjustable.

I just tried this in my trusty .22, just did not work, caused redplating.
The bias range gives -2.2 to -4.5 Volts, so I think the voltage divider across R111 with 4.7K is the wrong idea. I'll try replacing R111 with a 50K pot.
Is there any better circuit for adjustable bias in the .22?
Zouto

[selloutrr]

Increase the pot impedance. Or the entire resistor. I dearly you would remove the resistor use a decade box find the correct bias and solder in the correct or closest resistor value. Option two be Mesa boogies bitch and buy the over priced sovtek tubes.

If you replace the resistor with a pot make sure you ground one side invade of failure.


Use the same layout I posted on page 1. Use a 30k or 50k pot