Sick Randall head

Ghostman

Ghostman

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The new tubes showed up two days early which is great. I put the new set into my Randall 667 and tried to bias up the head.

One thing I noticed right away is that the tube position that popped on Friday that warranted the new set, looks visually brighter on the heater. It glows brighter than the other 3.

When I go to Bias it up, I can dial in 3 of the 4 tubes. Position 1 I can only go from 0.0mA up to 12mA. The others I can dial them into the 32-38mA that the manual shows for 6L6's.
The other issue is that the controls are backwards from the other 3 bias pots. For the good 3, the current increases with Clockwise turns, but the weird position 1, it's backwards. If I increase the Pot, Clockwise, the bias goes down.

Where would one start looking?
 
Checking heater voltage in the chassis, with the tubes in.
Pin 2 - Gnd - 80vDC
Pin 7 - Gnd - 0vDC
 
Take
Ghostman said:
The new tubes showed up two days early which is great. I put the new set into my Randall 667 and tried to bias up the head.

One thing I noticed right away is that the tube position that popped on Friday that warranted the new set, looks visually brighter on the heater. It glows brighter than the other 3.

When I go to Bias it up, I can dial in 3 of the 4 tubes. Position 1 I can only go from 0.0mA up to 12mA. The others I can dial them into the 32-38mA that the manual shows for 6L6's.
The other issue is that the controls are backwards from the other 3 bias pots. For the good 3, the current increases with Clockwise turns, but the weird position 1, it's backwards. If I increase the Pot, Clockwise, the bias goes down.

Where would one start looking?
Click to expand...
Unplug power.
Take out the tubes.
Beware that the filter caps may store voltage and discharge the power supply.
Measure the screen resistors that connect to pin 4 of each socket, with an ohmmeter.
These are large 5 watt resistors (probably), and you might have a bad one.(on the socket that won't adjust)
Replace the bad resistor.

2. Set meter for DC volts.
Connect black probe to chassis.
Connect red probe to pin 5 of output tube socket. (especially the socket that won't adjust)
Turn on power, but leave standby in warm-up position. Do not turn standby to "operate".

While watching the pin 5 DC voltage, adjust the bias pot up and down.
Do this for all output sockets.
Note the voltage on each output tube socket, pin 5 and report that voltage range to us.

Do not put the tubes in. Do not try to play the amp.
Do not turn the standby to "operate."

What did you find?

Do all the measurements (we guide you) first. After everything is fixed and working:
Put the tubes in LAST.
 
Amp Mad Scientist said:
To be truthful:
randall amps are rather difficult to work on, becuase of the design.Very time consuming.
Very similar to Mesa Boogie.
Click to expand...
I've seen the boogies and this is a clean amp comparably. I can pull every board in this amp faster than my old DSL100. Barring the 12,000 knobs, but it's actually very simple layout. I reached out to a tech I know but he's in Canada on the East coast, and he's getting me a referral in Seattle.
 
Randall head came back home today. One of the bias pots was toast. They replaced the pot, checked the health of everything, set the bias, and gave it a clean bill of health.

Got it home and it sounds glorious. Just shy of $200 which didn't help much, but it's a small price to pay for no headaches.
 
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