Is This a Bias Trim Pot?

[bnwitt]

Ok, you should be reading negative DC voltage and you should be getting something like -5 volts to let's say -40 volts. Make sure your meter is set on DC voltage.

[bnwitt]

After 15mins my range on both sides increaces to 299mlv-312mlv

Sounds like you're on the wrong pin.

[cudamax2343]

O.K On pin 4 both sides I'm getting a range of this. -.04 to-49.5

[cudamax2343]

O.K a slight confusion on pins. First I think this explains them backwards
http://prewaramps.com/library/octal.jpg
Because if I START WITH THE RIGHT OF THE TIT AS #1, I'm good and my readings are -.o4 to -49.4
So what do I do now to finish-up?

[bnwitt]

Leave the bias voltage at full negative -49.4, put your tubes back in and see if you blow a fuse. If not, you can bias the tubes. Do you have bias probes?

[bnwitt]

O.K a slight confusion on pins. First I think this explains them backwards
http://prewaramps.com/library/octal.jpg
Because if I START WITH THE RIGHT OF THE TIT AS #1, I'm good and my readings are -.o4 to -49.4
So what do I do now to finish-up?

That picture shows the pins from the wiring side not the tube side. As I explained in my earlier post its counter clockwise starting from the pin on the right of the keyway when looking at the tube side of the socket. It would be a mirror image of that picture on the tube side of the socket.

[cudamax2343]

It has been blowing Fuses with the pot at its full neg position (fully counter-clockwise) and what do Bias Probes like like?

[cudamax2343]

O.K To the right, I'm a little slow. no what?

[bnwitt]

Take the power tubes out and see if it blows a fuse.

[cudamax2343]

It does not.

[bnwitt]

It does not.

Buy some new power tubes. If you don't know what a bias probe is you're going to have to take the amp to a tech to have the new power tubes biased.

[cudamax2343]

What, your Kidding right. You can't tell me where to Hook-up and get some live readings to set the Idle valve values?

[bnwitt]

Your amplifier has no place to "hook up" for biasing according to the schematic and you need bias probes to take a bias reading. One or both of your power tubes has a short or you would not be blowing the fuse with full negative bias voltage present on pin 5. The typical bias voltage for EL34's is -15 volts DC and with -49.4 volts DC present on the pin 5's of your tube sockets, the tubes would be running so cold you couldn't possibly blow a fuse. You need new power tubes and you need bias probes to set the new tubes idle current. You can't set tube idle current without them unless you have 1 ohm resistors present on the cathodes.

[bnwitt]

Ok, I just saw your previous thread on this amp regarding the wiggling around of the power tube to get it to work. Have you tried to tighten the pins in the power tube sockets with a pointy instrument with the power off of course? Also, you might have a cracked solder point on the power tube sockets at the pc board. you'd have to drain the amp, pull the board and use a magnifying glass to find the crack or cracks on the pin solder joints on the board. Can you solder? If one of the pin 5 solder joints is cracked you'd lose the negative bias voltage at the tube and it would redplate and blow a fuse. you're still gonna need a set of bias probes to bias the amp.

[bnwitt]

Here is a dual bias probe on eBay:

http://cgi.ebay.com/AMP-HEAD-Dual-Bi...lenotsupported