DRRI...no bias current...resolved (bad filter cap)

[TubeGeek]

A buddy of mine brought over a DRRI that he is trying to fix for his buddy.

First thing I did was put my bias probes on and measured bias.  I am getting max 5mA per tube. Somethin ain't right with this one.  I have no swing on the bias pot, it goes from 0 to 5mA.

I measure 350 VDC on the plates and off of pin 8 of the GZ34 rectifier.   Voltages seem low.  I'll compare voltages to other dr's when I get back to my bench.

Tried new power tubes, same thing happens.

Tried new rectifier, no difference.

Replaced bias pot, no improvement.

Filter cap maybe?

Any ideas?

I'm mulling this over while at work, I'll get into the amp tonight but thought someone might point something out to me that will help.  I'm a little out of shape on repairs lately as it has been a few months since my brain has been put to the test on an amp.  

[phsyconoodler]

Remove the power tubes and measure the bias voltage at pin 5 of each power tube.If you have -35v or close then you are good.If not,you have a bias supply issue.
 I'm assuming it has very little sound. Those amps will have about 420v on the plates.
You indeed may have a filter cap issue if the voltage is that low.I would check the AC voltage at pins 2 & 4 to see if they are at least 350v AC(something like that)Remove the rectifier tube to make this voltage check.

[TubeGeek]

Yeah I have proven the bias circuit out also...I do measure typical -35 on pin 5.

I was expecting around 420 VDC on the plates too.

I am betting on bad filter caps the more I think it through.

Playing guitar through the amp does indeed sound like a filtering issue as it's out of tune and sounds like dog crap.

[stratele52]

Schematic on 65 Deluxe Reverb reissue show 396 plate voltage ;Your reading at 350 is not bad

No is not the B+ filter caps . Bias cirsuit only .

if you have -35VDC  on pin 5 , bias circuit probably work OK .

Remove power tubes and check if bias pot work with reading on pin 5 each power tube. If you can read let say -25 to -40 ; Bias circuit is good. If not check resitor , diode.

If bias is good , check your bias probe ; or tubes socket , broken solder / contact  pins 8 , 5 and all. Check if heater work , check pins 4 to have close to 396 and 470 ohms résistor .

Il look like your power tubes don't drive power and  current that's what you read only 5 ma, like someting is disconnected , Output transformer OK ?

[phsyconoodler]

Schematic on 65 Deluxe Reverb reissue show 396 plate voltage ;Your reading at 350 is not bad

 They are usually over 400v so 350 is an issue for sure.

[TubeGeek]

The culprit was c32, a 47uF 500V filter cap was dead.

Replaced that and all is well.  Proper voltage on the plates and bias current is now within spec and set up.

[stratele52]

The culprit was c32, a 47uF 500V filter cap was dead.

Replaced that and all is well.  Proper voltage on the plates and bias current is now within spec and set up.

Hard to believe that C32 was the problem with what you write before. Yes very hard to believe.

Good if the amp is OK

[phsyconoodler]

Hard to believe that C32 was the problem with what you write before. Yes very hard to believe.

It's the most likely culprit because of the voltages.Something had to be loading it down.The first thing after the rectifier is the Filter Caps.

[stratele52]

Hard to believe that C32 was the problem with what you write before. Yes very hard to believe.

It's the most likely culprit because of the voltages.Something had to be loading it down.The first thing after the rectifier is the Filter Caps.

The B+ voltage Tubegeek give us was OK or very close to schematic . Not defective to give 5ma bias.
That why I write ; " Hard to believe ...with what you write before.

Is that Tubegeek to repair something else working without noticing  ?

[TubeGeek]

I don't think I repaired anything else in the amp, the same voltage on the rectifier and plates led me to look at the filter cap. I knew that the plate voltage was low so once I tested the caps with my meter, c32 tested faulty.  I just went with it.

[stratele52]

I don't think I repaired anything else in the amp, the same voltage on the rectifier and plates led me to look at the filter cap. I knew that the plate voltage was low so once I tested the caps with my meter, c32 tested faulty.  I just went with it.

Thank's for the answer.

[phsyconoodler]

The B+ voltage Tubegeek give us was OK or very close to schematic

That's not very close at all,that's why it's wise to find out where the extra load is coming from.5ma is incredibly low for bias current so something was definitely loading it down.He found the issue.

FYI: I have never found a Deluxe Reverb reissue to have less that 415v on the plates when operating.I was a Fender warranty tech for 4 yrs.Even when 396v is listed,350v is way too low.