STR 415 bias reading too high

[Boogiebabies]

The MPD is around 83%, so not too far off. What you have to worry about it the rise in current under load.

What concerns me is why would you have 470V to the plates pulling 53mA ?
A 60W should be between 448-460V even with a tube duet pulling just 20-25mA each.
It may be time for a cap job, or check your outlet voltage.

Shoot a photo of the poser supply board near the two 50uf/75V caps, just to the right of the
red-red/blue/red cloth wires.

 

[Boogiebabies]

Your bias supply is stock. The issue is that the export transformer is set up for hotter bias due to its lower plate voltage, thus you have higher than normal plate voltage the bias is pulling a bit higher. Let me think about how we can adjust your bias supply without having to swap out resistors.

 

[Boogiebabies]

O.K., three of those five resistors are for the bias supply. I am having a few issues
getting to my C+ at the moment as well as my schematics.

To start, do not jump resistors in parallel. If you wan't to try to alter the bias it would be best to replace the 22K resistor with a 25K trim pot, but we still do not know if that would allow you to safely run yout STR-415's. The issue with the 415's you are having is called natural plate current. As the tube is made, the manufacturer has very little control over how much current the tube will naturally draw. In essence, they were born that way. They also react differently under higher plate voltage and over time they can drift and draw more current.

What is your voltage reading at pin 5 ? It should be around -47V.

 

[JOEY B.]

O.K., three of those five resistors are for the bias supply. I am having a few issues
getting to my C+ at the moment as well as my schematics.

To start, do not jump resistors in parallel. If you wan't to try to alter the bias it would be best to replace the 22K resistor with a 25K trim pot, but we still do not know if that would allow you to safely run yout STR-415's. The issue with the 415's you are having is called natural plate current. As the tube is made, the manufacturer has very little control over how much current the tube will naturally draw. In essence, they were born that way. They also react differently under higher plate voltage and over time they can drift and draw more current.

What is your voltage reading at pin 5 ? It should be around -47V.

Hi ,I already tried piggy backing the resistors and it didnt work it did in fact send the bias Higher I mistakenly thought it would lower it nevermind no harm done

Ive never taken a reading off pin 5 so Im not entirely sure how to do it correctly or safely
I got some of it but could do with precise directions to be sure .
I think Ive figured out so far and please correct me if Im wrong . that to take the reading I should have the amp on standby and the power tubes removed and I should Have the multi meter set to dc volts but....
how do I use the meter probes to do the reading off pin 5
is it pin 5 referenced to ground so the red lead goes to pin 5 and the black lead goes to the chassis ?

Red lead to pin 5, black lead to chassis ground. If I remember right, this reading can be taken while the amp is in "standby". Your amp should see near -47 DC volts if all is well.


JB

 

[Boogiebabies]

-42V is hotter than -47V as is is more negative. With the increase in plate voltage due to
the transformer output, it is running a bit hotter than stock.

Also, be careful with that circuit. It also supplies the EQ with negative voltage to cut and boost the frequencies.

Consider replacing the 22K resistor with a 25K Cermet trimmer. In the UK, Piher seems like a
readily available brand.

 

[Boogiebabies]

That supply also gets used for the LDR/Switching system as well. It gets converted back to positive voltage by a 1N4006 Diode, then dropped by a 680 Ohm 1W resistor into a 15V zener diode.

I would consider using just a 25K pot as any resisor could blow just as easily.
I have had a Cemet trim pot on my Soldano SLO-100 and it's made it since 1993.

 

[Boogiebabies]

Edit: I meant Cermet trim pot.