Valvestate 8040 transformer taps

[dweingarden]

I bought a Valvestate 8040 used. Looks like parts have been replaced before and the boost channel is not working.
The voltage on the tube's heater is about 6 volts between the two 68 ohm resistors. The transformer secondary voltage is 18VAC.
Tested the tube in another amp and it is fine.
The autotransformer seems to be OK when measured with an ohm meter
I'm thinking that since the transformer tapped in series (black wire connecting two middle taps = series) and running at 120v, the secondary is half of what it should be. This would put half the voltage on the tube heater. I'd feel better if someone could check and see how their transformer is wired. Even a picture would work.

1) For 120v primary, should the two windings be series or parallel?
2) What should the secondary voltage be ?

[Jon Snell]

For 120v see diagramme.
The TDA requires around +-25volts DC

[dweingarden]

Thank you Jon for responding. I think this transformer came from Europe. I sort of knew that was the problem but it helps to have someone else validate it.

[dweingarden]

Getting closer, the secondary voltage is 37VAC (RMS) and the tube is now glowing. Still getting very little output from the boost channel. Bypassing the tube by shorting from R33 to R20 allows this channel to work. The output of the autotransformer is 167VAC (RMS). Seems low, maybe autotransformer has shorted turns? It would be better if they would have printed some voltages on the schematic.
If it is bad, it doesn't seem to be available. Does anyone know a part that would work?

[Helmholtz]

Can I see the full schematic?

[Jon Snell]

Getting closer, the secondary voltage is 37VAC (RMS) and the tube is now glowing. Still getting very little output from the boost channel. Bypassing the tube by shorting from R33 to R20 allows this channel to work. The output of the autotransformer is 167VAC (RMS). Seems low, maybe autotransformer has shorted turns? It would be better if they would have printed some voltages on the schematic.
If it is bad, it doesn't seem to be available. Does anyone know a part that would work?

If the transformer had shorted turns it would run hot.

[dweingarden]

For those following along, this is the best schematic I can find
https://www.drtube.com/schematics/ma.../8040-iss8.pdf

[Helmholtz]

Getting closer, the secondary voltage is 37VAC (RMS) and the tube is now glowing.

What did you change?
What are tube pin voltages?

[g1]

The output of the autotransformer is 167VAC (RMS). Seems low, maybe autotransformer has shorted turns? It would be better if they would have printed some voltages on the schematic.

That number is good. The 'HT' supply will be around 185VDC and at the tube plates you should have around 160V pin1 and 100V pin6.

[dweingarden]

We went over the board again, and found something interesting... I had 160V RMS on the anode of D3 but nothing on the cathode. We replaced the diode but it still had no voltage on the cathode. The auto transformer does have an open connection. I must have measured some other load originally. Not sure if the 160 VAC may be capacitive. Now I need to look for a small transformer to replace it. If anyone has a part number please feel free to post. When I fix it I will post the result

[g1]

Now I need to look for a small transformer to replace it. If anyone has a part number please feel free to post. When I fix it I will post the result

We just had another thread about the same issue, you can start in at pg.3 here:
https://music-electronics-forum.com/...-channel/page3

[dweingarden]

UPDATE

I talked to an engineer at Triad magnetics about a substitute transformer. He recommended FS20-120 wired as an autotransformer. It works perfectly but doesn't fit on the board. I designed my own circuit board with screw holes for mounting (see photo). If anyone else has this problem, I have some extra. PM me.

[g1]

UPDATE

I talked to an engineer at Triad magnetics about a substitute transformer. He recommended FS20-120 wired as an autotransformer. It works perfectly but doesn't fit on the board. I designed my own circuit board with screw holes for mounting (see photo). If anyone else has this problem, I have some extra. PM me.

Can you show how it gets wired up, and did you measure any voltages after install?
Price is very reasonable, datasheet attached

[dweingarden]

The connections are as follows:
connect pins 2 & 3
connect pins 6 & 7
connect pins 4 & 5

pin8 = GND
pin1 = High voltage (measured 172 VAC)
pin4 = input 18VAC

These 3 signals are available from the pads of the old auto transformer.

Thank you everyone for you support