blues deluxe reissue problem

[shay]

Hi, i'm new to this site so please forgive any errors,
I have a blues deluxe reissue that has an ugly distortion and is humming.
On checking the bias test point its very low,around 10mV I am getting the correct voltage at pin 5 on the power valves .I 've tried different valves,I re -flowed solder on valve bases ,checked all the resistors and diodes around this area and am running out of ideas .
Any suggestions???

[MWJB]

10mV (5mV per tube) will most likely be the cause of the ugly distortion. Plate current is what is important the negative voltaeat pin 5 is incidental.

Cold bias does not typically cause hum however, how bad is the hum?

[shay]

hum sounds like mains 50hz , I'm in europe, Not so loud that you couldn't use it but still annoying

[MWJB]

Check the condition of the 2x47ohm resistors that form the heater circuit virtual centre tap (R80 & R81 on the schem I have?). If one or both of these have burned out you will get hum.

[shay]

both resistors are fine, thanks for the suggestion though

[Enzo]

I would think that with the bias at an average voltage setting, it would take some seriously unusual tubes to draw only 5ma.

Verify you have full B+ voltage on the plates - pin3. and also on pin 4, the screens. Without me looking it up, the voltage is probably 450vDC more or less? Set your meter to AC volts and measure it again. That measures the ripple. There should be very little ripple on the plates and none on the screens. If ther is susbstantial AC there, you may have a weak filter cap, or more likely cracked solder to one of them. SUbstantial ripple has the net effect of lowering your B+ - and reducing the current through the tubes. AND it will make things hummy.

[shay]

I've got 100v ac on pin 3 , I re-flowed the legs on C31 the filter cap so I suspect a failing filter cap . I will order one in and see what happens ,
thanks for your help

[MWJB]

What is your plate current now?

[shay]

I will not get the spare for a number of days, there is no local supplier ,i will have to ship it from the Uk as I am in ireland. I will update when it arrives.

[shay]

I replaced C31 the smoothing cap and everything is good. the amp is quiet, clean and drawing the correct bias.
Thanks for the help