Peavey - Classic 30; BIAS issues

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I've read a lot on the BIAS issues on the (most older) Classic 30 amps from Peavey. As I haven't found a clean and clear way to BIAS this amp, I thought I shared my insight on this topic!

  

 


 

Recently, I had a Peavey Classic 30 come in that was missing the powertubes and a proper power plug. In the online manual I read that a BIAS of this amp is not necessary as the BIAS is set quite low to easy swap old tubes for new ones. So I added a quartet of EL84's to get it going; soon 2 of the tubes died on me. As due to the construction of this amp a clear way is biasing is not possible.


The circuit

If you remove the amp chassis from the combo in a Peavey Classic 30 you'll notice that the PCB is folded in a U form. This is a common way Peavey amps are build; the tube sockets are on the lowest plain, on the side are the relais and other components and on the top there are the big caps and the potmeters. Servicing such an amp is tough and biasing such an amp is quite impossible. There is no BIAS potmeter and you can't reach the powertubes for good measurement. Maybe with a BIAS-probe something can be measured (a bias probe is simply a tube socket that goed into the tube socket. A probe has a single 1 ohm resistor hidden somewhere to take the BIAS readings). Due to the dying of two powertubes I investigated the power supply too.




The only way to service the amp is by removing the PCB from the casing. Most wires are connected by connectors that can easily be removed. To remove the folded PCB though:

  1. Unplug the amp
  2. Drain the power on the caps
  3. Unplug all connectors on the PCB.
  4. On the backside of the amp two jacks are visible for the footswitch and an extra speaker. Inside the amp is a small PCB for this. Remove the nuts and washers and place the board away from the big PCB. The small board is connected to the big PCB via a ribbon cable
  5. Remove all screws on the bottom of the board at the tube sockets
  6. Remove all knobs, nuts and washers from the faceplate. Also remove the input jack nut
  7. Fold the top side of the PCB (the location where all the potmeters are) down to get the PCB free
  8. Move the PCB out of the chassis,

In the amp I was working on the big caps all looked old and dry. Measuring them gave information that most of them needed to be replaced. For reference I used this schematic.

I replaced most power supply caps (C39, C40, C41, C42, C43, C45, C46, C47, C48). I also replaced all power supply cap in the BIAS power supply (C44). 


To make the BIAS adjustable, I removed R64 (33k) and replaced that with a 25k trimpot and a 22k resistor in series with each other. I bolted a small turrent to the chassis to hold the new trimpot and resistor and used two wires to connect this to the original position of R64.
(I started with 25k trimpot and a 10k resistor to get a value of 35k but this value set the BIAS already too high on even the lowest BIAS potmeter setting. With 22k in series I could dial in a proper BIAS).

To make the BIAS measurable I removed the ground connection from the powertubes' kathode and replaced them with a 1 ohm resistor. On three powertubes the ground connection is a small jumper that can easily be de-soldered. One powertube had a trace going to ground; I cut this trace with a knife. Also, on one powertube a small cap is located to ground that is now not connected to ground due to the 1 ohm resistor (C56). There is no issue when this cap is grounded through the new 1 ohm resistor but you can always cut the trace from the cap to the kathode of the powertube and add a new ground line via an extra wire. 
(I placed all 1 ohm resistors on the solder side of the PCB. You can solder them directly to the tube socket or to the leftover place of the jumpers as mentioned above)
To easily measure the BIAS I connected the kathode of all powertubes through four wires to a big turrent I bolted on the chassis in which all tubes have their own measure point. Through this point you can directly monitor the current of a single powertube and set the BIAS accordingly.

I won't go into how to bias this amp or at which value as this is different per amp tech, but this setup give you the ability to measure and adjust the BIAS according to the tube. 





When I would adjust the BIAS to factory specs (so set the value of the trimpot to make a 33k resistor) all tubes would work at 134%. This is also why two tubes died on me; the BIAS was set too high. By adjusting the BIAS to a lower value all tubes cooled down and no tubes would suffer or die. 
Why the manual state that there is no problem to swap, I don't know. Maybe due to the rise in voltage from the AC line? Drifting of parts? New ways tubes are build? Bad luck? I read this issue on more Classic 30 amps so it isn't limited to this unit. So if you have a classic 30, please take care of your powertubes! They already need to work so hard.

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