Fender UL Twin Going into OD Too Soon.

DeaconBlues

Member
Messages
2,981
I just recapped a Fender 135 watt TR. The amp came to me with a very high noise level at idle. It had recently been retubed with all JJ tubes and they all test to be good. So, I recapped the amp and replaced the plate resistors. The amp idles very quietly now and sounds great at lower volumes. But, now if you dig in on a chord the amp easily goes into OD. It's actually a very pleasant sounding OD, but my customer plays country and wants it clean.
What could possibly cause this amp designed for ultra cleans to push into overdrive so easily?
 

Blue Strat

Member
Messages
30,066
Measure the values of all the resistors you replaced, trust me! Also, measure all voltages that are marked on the schematic and compare. Pay particularly close attention to the PI circuit. If one side of the PI tube isn't operating you'll get very early distortion. This is troubleshooting.
 

DeaconBlues

Member
Messages
2,981
Thanks, Mike. The PI was suspect and on my list to troubleshoot. I'll also measure the resistor values.
 

zenas

Member
Messages
8,127
Might be worth a shot to swap a few tubes in, I'd start with the PI. Might get lucky unless you're like me, I never get lucky.
 

pdf64

Member
Messages
6,855
Measure the amp's power output. If it's clean up around 100 watts, it may just be that the regular eminence oem speakers are pretty weak, so try it through a different cab.
 

DeaconBlues

Member
Messages
2,981
pdf64, yes the speakers were my first thought. They are JBL Fenders, btw. I tried the amp with a cabinet loaded with Jensen Neos.
 

J M Fahey

Member
Messages
2,149
Glad you solved it :) , but the first (and fastest) thing you should have done is to feed it 100mV 1kHz tone into it and scope it, trying various volume and tone settings to check whether you could get clean 20V RMS into 4 ohms at the output.
And if not, go backwards until you found the clipping stage.


Grid resistors were 470 M Ohm!
:eek::eek::eek: where did you get them from?
Not a common or easy to get value nd very unlikely to find in s regular Tech parts bin :confused:
 

Tone Meister

Member
Messages
3,268
I misordered/fat fingered the wrong value. Entirely my fault, nothing to do with the distributor.
I'm late to the party so Mike K beat me to it. Did that once with a screen grid resistor on a Fender tube socket - 470K on one socket instead of 470R.
 




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