Fender Blues Deluxe Help

[donegone]

Low levels/voltages at output. Unwanted distortion at low levels. Lows and highs are cut. Output voltage from 6L6 plates is low and unbalanced (roughly 12Vac on tube 1 and 6Vac on 2 with 4mVac signal applied at input as per Fender test spec). Phase inverter bias network only receiving 30Vdc when 42.8Vdc is spec. Higher gain on phase inverter output (roughly 7Vac from side A and 6Vac from side B). Measured for faulty coupling caps and plate/grid resistors. Nothing wandering or out of tolerance. I would assume the problem is in the phase inverter but I have replaced every tube numerous times with the same results. Preamp stages measure fine. The inverter bias resistors test fine. No leaky/shorted caps causing low gain. AC test points measure according to spec until the phase inverter outputs. DC voltages including B+ measure accordingly until the long tail pair bias network. Tested with output tubes biased at 30mA (plate 385Vdc). Connected another output transformer without the negative feedback connection and received same poor response. Suggestions?

[Enzo]

did you connect that other OT to the jack on the panel? Or used clip wires to its leads?

My first reaction was that you had your speaker plugged into the extension speaker jack instead of the main speaker jack right next to it.

But in your schenario, is plate voltage present on both sides of PI? Open plate resisotrs in Fender PIs happens more than I would think. One of the first things I check any more. That would cut the current in half for the shared cathode resistance, so the voltage would be lower. Check that and for way off value.


And welcome to the place.

[donegone]

Thanks for the response. I am running a resistive dummy load 50w 8ohm into the correct output. When I swapped transformers I was jumping it into a Blues Deville cab (same output transformer) but I had forgot to connect the negative feedback loop. So when it sounded poorly I figured it would have to be in the phase inverter section. I tested the plate resistor off of side A of the inverter tube but may have neglected the voltage divider portion (Fender spec sheet was too small to read and it was exhausting me). A drifting resistor could be whats causing my low DC bias voltage on the phase inverter. I dont have the amp in front of me till next Monday. I appreciate the advice and will get back to you.

[Enzo]

Just look at pins 1 and 6 of the PI tube, are the DC voltages there pretty even? Or is one missing? Open resistors would be a common failure mode. A film resistor drifting so much as to throw the whole works off seems unlikely.

Not sure you got my question. When you swapped output transformers, on the new part, did you connect its secondary wires back to the jack in your amp, or did you leave its secondary leads flying and use clip wires to the remote cab? One way might expose a shorted jack, while th other would not.

[donegone]

Well I made a rookie mistake. I wasn't even paying attention to the output jack wiring and had the speakers connected to the ext. jack and not the 8ohm. The amp sounds fine but my measurements were still off according to the schematic. I went and measured them again injecting a 4mV signal (connected to the correct output jack this time) and there still was a large signal difference on the plates of the output stage (roughly 14v on one side and 8v on the other) The amp sounds fine. What do you think could cause this?