71' Marshall Bias Fixed!!!

[ptrickamp222]

Thanks to all for the input!!!! I first installed a 25K linear pot from Metro....perfect fit!!!!! Installed NOS Mullard's and WA LA!!!!! Biased at 65% at 432Vdc on the plates right in the middle of the sweep of the pot! I did not have to change the bias resistor....the 47K. The pot was bad inside and reading off the chart. Last nite I hooked up with temporary leads resistor's in parellel to get a idea of what to use.........did not have to! Awesome. I hate to alter a vintage pristine amp. Also took out the wire from the bias 220k to the rectifier and installed it to the hot side of the standby switch. Now when I turn on the power I get heaters and bias at the power tubes......much relief:wah Later this week I will install nice wire wound screen resistor's and some bias resistor's to the power sockets. This thing would eat tubes for breakfest:applaude Glad I had some extra Mullard's they can take it. Going to do it just for ease of tube and tranny. Played it today thru my 2x12 w/ Celestion Heritage 20 watter's and the 07' R9....This thing has some punch. With the laydown tranny it has 69 more volt's on the plates than my 72' with the upright tranny!!!!:wow

 

[ptrickamp222]

Just finished installing the screen and bias resistor's the 1k 5watt wirewound and the 5.6k carbon on the power sockets. Also couldn't resist to add a NOS .68 Mustard on V2 cathode. I like the extra midboost. The amp runs real quiet and I rechecked the Xf2's, didn't need to adjust. I haven't played it that much. Been getting it ready for playing. Will do a full tone check tomorrow now that all the work is done to the amp. Will be interesting to see what the difference will be with the resistor's on the power sockets and the .68 on V2. The amp was definitely more punchy and headroom with the laydown tranny and the extra 70Vdc on the plates than my 72'. I'am hoping to warm it up a tad with the screen and bias resistor's with the .68 cap for midboost distortion. The Heritage Celestion 20 watters really sound good for the old sixty's tone also.

 

[TBR623]

You can't go wrong adding the screen and grid resistors.Getting the bias voltage to the tubes really makes alot of sense too. Even though xf2's can take a beating-they ain't gettin' cheaper!Plus, the 5.6K's on pin 5 really help with stability.

I'll bet it kicks ass:wow :headbange