Grounded grid preamp voltages

[nilsW]

I have a Grounded Grid preamp. The board is from Analog Metric in Hong Kong. When checking the voltages without tubes the voltages are substantially higher than specified.

HT is 16,9V. It is supposed to be 12,6V
+200V is 236v
-200V is 238V

All components seem to be the correct ones (I have however not measured them).

Is this big difference to be expected, or is something wrong?

 

[rayma]

The circuit DC voltages will rise without the tube drawing current. There is no regulation in the HV supply.
The same would happen if the tube was in the socket and the filament was disconnected.

 

[nilsW]

Great, thank you very much Rayma. Can not wait to put in the tubes!

 

[rayma]

Yes, be sure to insert all of the tubes on the board before applying power. There could be direct coupling between them.
Then check the DC voltages before connecting it to a system.

 

[nilsW]

I checked the voltages with tubes installed, I get 15V and +220V/-222V.
Tubes are old. (12AU7)

Can I live with that?

 

[Osvaldo de Banfield]

At first powering, I suggest to use a current limited power supply: remove the fuse in the primary of the PSU and wire there a filament lamp 1 to 3 times bigger power than the DUT. If anything go wrong, the lamp will bright increasing its resistance and protecting all including you. Note that no lamp other than filament: no fluoro nor led.

 

[rayma]

I checked the voltages with tubes installed, I get 15V and +220V/-222V.
Tubes are old. (12AU7) Can I live with that?

This is perfectly normal, with the DC HV supply voltages going down by 10% when tubes are installed.
You don't need to do any "lamp test" since you have already powered the unit successfully.

 

[as_audio]

Filament voltage is too high.

 

[rayma]

If the 15V is the filament DC voltage, that might be a problem.
Is the regulator/heat sink for the filaments? You may have to draw a schematic.
The LT1085 regulator should be more than capable of doing the job.

 

[nilsW]

Yes 15V is the filament.

The transformer I am using has 230V primary and secondaries are supposed to be 12,6V and 200V. Without load secondaries are14V and 216V. My mains are 233V.

Attached the schematic.

 

[nilsW]

Yes, the regulator/ heatsink is for the filament.

 

[rayma]

Yes 15V is the filament.
The transformer I am using has 230V primary and secondaries are supposed to be 12,6V and 200V. Without load secondaries are 14V and 216V. My mains are 233V.

The preferred resistor values are R20 = 125R, and R21 = 1.135k.
This is to ensure that the regulator always has more than the minimum rated load current.
I would change both resistors to these values, so the regulation does not suffer.

 

[as_audio]

Interesting schematic, but no "Grounded Grid" Preamp.

 

[as_audio]

Remark in post 12 is valid if the regulator IC is working and soldered correctly.

 

[rayma]

Interesting schematic, but no "Grounded Grid" Preamp.

Technically, V1B and V2B are operated as common grid stages, but with feedback returned to the grids.
The input is the cathode, and the output is the plate. So the grid is the "common" terminal.

 

[as_audio]

This is called a differential amplifier.

 

[rayma]

V1A/B is called a cathode-coupled amplifier.
V1A is a cathode follower. V1B is a common grid amplifier.

The cathode of V1A couples the signal to the cathode of V1B.
The tube section loading the common grid stage is sometimes called a cascoded cathode follower.

 

[nilsW]

Great, I will check R20 and R21 when I am back.

 

[as_audio]

Post 17 :
So you seemingly agree that the combination is called a differential amp,
in its classic form, but with a single output only (it can easily be converted
to balanced output). Both grids are input terminals, signal and feedback.

 

[rayma]

Great, I will check R20 and R21 when I am back.

If you change them to R20 = 125R and R21 = 1.135k, I suspect it will work properly.