Fender Hot Rod Biasing Questions

[cogr]

I now this Marshall forum but I have a technical question regarding a Fender Hot Rod Deluxe. The other guitarist in my band purchased the Fender off of Fleabay and within about a month or so needed re valving. The said amp was taken to the repairer that he uses and came back a few days later with a full set of shiny new valves (6L6GC’s + preamp tubes). Things were fine for a few gigs until the clean channel once again distorted. I spent time doing a little research about the amp and came to the conclusion that it would be worth checking the bias, I also found a full set of schematics which detailed all of the test points and test voltages. One thing to mention I am a qualified electrical engineer (electronics and valves are not my specialty) so I am aware of the potential hazards and have do have a basic understanding of valve technology. I checked the bias voltage and found it to be about 32mv I tried to increase this but could only manage to get it up to about 48mv. The amp was returned to the repairer once again who fitted another set of valves; biased it and sent it back. I checked the bias to see what he had set it at and found it set to 40mv, I then increased this to Fenders recommended level. The amp was gigged an then blew and output valve. My question is as follows: Fender have provided a bias test point in the amp that when set should be about 60mv (states this on the schematic and every youtube demo). Does this bias voltage apply whether the amp is used within the US or UK or elsewhere (different supply voltages)? I assume that the power supply transformer secondary voltages are always the same irrespective of the supply (primary) voltage. The transformer fitted is selected for country in which it is to be sold. The amp repairer said that he sets the bias voltage at 40mv as the voltage in the UK is higher and that the values stated on the schematic are higher because of the lower supply voltage in the US. Comment or thoughts appreciated.

 

[Micky]

Idle current (bias) depends on a couple factors.
You need to know the B+ (plate) voltage, and then calculate the bias setting based on 65-70% dissipation.
Here is a web page that can help:
Weber Bias Calculator

 

[cogr]

Thanks for the info, more food for thought. Just a quick question, why do think Fender provide a bias voltage test point (measure in mv) if in reality you still need to measure plate voltage and calculate bias based on 65% -70% dissipation. I thought the idea would have been to make valve changes easier.

 

[Micky]

Once you have measured your plate voltage for your location, chances are it won't change.
I was just trying to be thorough and provide you all the relevant information.
You still need a test point, but in order to set it correctly, you need to know the other variables.