Vintage Ampeg SVT = weird voltages

[Wasnex]

I suspect maybe an issue in the PEC part but i'm not sure and difficult to test.

A healthy C7 blocks DC from the PEC. Perhaps a PEC fault could cause the tube to oscillate?

I believe the symptoms you identify suggest high current through the plate resistor and low current through the cathode resistor.

I misspoke . Both the measured cathode and plate voltage suggest higher than expected tube current. The current calculates to a bit over 3.6mA (plate resistor) and 3.3mA (cathode resistor). The difference could be due to measurement and rounding errors. AFAIK theoretically the same current should flow through both resistors. Using the schematic values, the calculated current is 2.6mA plate and 2.1 cathode.

 

[beans-on-toast]

Hi,

i just did what you said: i reflow socket, put deoxit in tube socket, retentioned pin, and i got the same voltage on v3. The 47k is brand new vishay metal film resistor and test good. Before the (R24) 47k,voltage 300VDC. After (R24) 47k (Pin 1 V3) 140VDC. Here is voltage i measure :

V1 : 12DW7
Pin 1 : 300 V
Pin 2 : 180 V
Pin 3 : 190 V
Pin 6 : 180 V
Pin 8 : 1.73 V

V2 : 12AX7
Pin 1 : 185 V
Pin 2 :
Pin 3 : 1.7 V
Pin 6 : 237 V
Pin 8 : 2.23 V

V3 : 12DW7
Pin 1 : 140 V
Pin 2 :
Pin 3 : 5.2 V
Pin 6 : 177 V
Pin 8 : 20 V

V4 : 12DW7
Pin 1 : 300 V
Pin 2 : 123 V
Pin 3 : 132 V
Pin 6 : 123 V
Pin 8 : 1.26 V

V5 : 6C4
Pin 1 : 300 V
Pin 5 : 300 V
Pin 7 : 190 V
From Power Amp before 8.2k 2W resistor : 430VDC, after 8.2k 2W : 300VDC
My SVT is 220V stock version. Wall Voltage is 230VAC.

Fabien

Is R25, the 1.5K cathode resistor within spec? I see that pin-3 is high. The plate and cathode resistors are key here.

The power supply 300VDC after the 8.2K resistor is good and exactly where is should be.

 

[Fabmars13]

Is R25, the 1.5K cathode resistor within spec? I see that pin-3 is high. The plate and cathode resistors are key here.

The power supply 300VDC after the 8.2K resistor is good and exactly where is should be.

Thanks Wasnex, beans-on-toast,

yes, i changed cathode resistor R25 by vishay metal film (measured 1.25k exactly), R24 measured 47k, tried a brand new 0.1uF in C7 capacitor, still the same. I'm not an expert but maybe something wrong in Pin2?
In the Tube Amp Book , they troubleshoot a similar problem, see attached files.
Cheers,
Fabien

 

[Wasnex]

I'm not an expert but maybe something wrong in Pin2?

Theoretically +DC could be leaking through the Bass Select circuit from pin 3 of V1. Leaky capacitor(s).


I assume you already swapped tubes and got essentially the same readings.

 

[agedhorse]

Look for errors or modifications.

 

[Fabmars13]

Theoretically +DC could be leaking through the Bass Select circuit from pin 3 of V1. Leaky capacitor(s).


I assume you already swapped tubes and got essentially the same readings.

Hi Wasnex,

i will checked again for leaking capacitor coming from Pin3 of V1.
I have a lot of NOS 12DW7 stamped magnavox and yes same results when i swapped the tube.
I will let you know if i found a solution.
Fabien