Bias Circuit Question

[trem]

In this Blackstar, I cannot figure out how to bias the amp.

If I measure Volts across the Cat Resistors, I get about 0.80mA (0.0008A)


I measured CT to each side of the OT and got (round numbers)
220 Ohms
185 Ohms
But When I switch the meter to VDC, I get about 200mV (0.20V)
That is about the same current I get using the above Cat Resistor numbers.

I have no idea how this Bias/Bias balance circuit works.
The power tube grids are at about -45 VDC.

How do I check the bias on this amp.?
Thank You

[überfuzz]

After a brief look at the schematics. Please double check this.

It seems you have two resistors on the cathode side of the power tubes, R207 and R221. Measure the voltage drop over these and use ohms law to determine the current flowing throw the power tubes. Then you multiply the calculated current with the plate voltage, SP16 and SP17.

[trem]

After a brief look at the schematics. Please double check this.

It seems you have two resistors on the cathode side of the power tubes, R207 and R221. Measure the voltage drop over these and use ohms law to determine the current flowing throw the power tubes. Then you multiply the calculated current with the plate voltage, SP16 and SP17.

I did do that (second sentence).....I got similar numbers.
The heaters have normal voltage.
The plates are at about 380 VDC.
I must be doing something wrong somewhere.?

[Mick Bailey]

I can't recall with this particular amp, but most Blackstars have to be biased with a plug in the input.

The OT imbalance is normal due to the way the transformer is constructed.

[Jazz P Bass]

Direct from the mouth of Blackstar.
"HT Studio 20. The bias settings for the el34’s should be 11.5v across R221 resistor".

'Balance Adjustment': "The trim pot labeled PR1 adjusts the balance between the two tubes to achieve minimal hum."

[trem]

Direct from the mouth of Blackstar.
"HT Studio 20. The bias settings for the el34’s should be 11.5v across R221 resistor".

'Balance Adjustment': "The trim pot labeled PR1 adjusts the balance between the two tubes to achieve minimal hum."

Hmmmm.....wow.
I need to check a few things.
Not exactly sure of the history of this amp...but I am sure it WAS working. Hard to believe the bias would be turned all the way off. I'll give it a twist and see what happens.
Just thinking out loud.....can Power Tubes be completely dead (both of them) not drawing any current, but not shorted anywhere.?
Will try another pair of EL34, and see what I get.
Perhaps I am making a fundamental mistake somewhere.....I just installed a new PT. Seems like all the normal voltages are present.......
I'll retrace my steps.
Thanks

[eschertron]

Direct from the mouth of Blackstar.
"HT Studio 20. The bias settings for the el34’s should be 11.5v across R221 resistor".

'Balance Adjustment': "The trim pot labeled PR1 adjusts the balance between the two tubes to achieve minimal hum."

11.5vdc across R221 (cathode resistor) yields about 52ma per tube. Seems good for a plate voltage of ~380vdc, considering the hybrid biasing scheme. The valve data sheet at BM amps indicates that the cathode-grid potential (the actual bias voltage) will be about -30vdc. Grid to ground would then be closer to -40vdc.

I agree. Confusing

edit: so the measured -45vdc grid to ground is not all that far off, by my reckoning.

[Dave H]

I can't recall with this particular amp, but most Blackstars have to be biased with a plug in the input.

And a plug in a speaker jack. The BIAS_ON signal is gated with the input and speaker jacks.

[Tom Phillips]

Some things to check.

Is screen voltage present?

Is R221 good and properly grounded?

Do the power tubes have heater power?

Does the amp work at all now?

[Dave H]

...but I am sure it WAS working. Hard to believe the bias would be turned all the way off. I'll give it a twist and see what happens.

Don't do that. The bias is turned all the way off unless there is a plug in the input and one of the speaker jacks.

[trem]

Oh...OK.
I missed that above.
I have a load on the OT, but nothing going in.
Thanks for reiterating that.
Will see what that does.
Thank You

[Mick Bailey]

I didn't know about the plug in the speaker jack, though you wouldn't try to bias an amp without a load. Good protection against a disconnected lead

[Gingertube]

Interesting protection circuits.
For those newbies trying to wade thru' the schematic to see whats going on then:

Sheet 1 of 4, IP_JACK is at 0V with nothing plugged in to the guitar input jack, goes to +3V3 when jack inserted.

Sheet 4 of 4, SPKR_OK is at 0V with no speaker plugged in and goes to +12V if any of the JK5, JK6, JK7 Speaker Connections is used (via diode OR function, D13, D14, D17)

SPKR_OK goes via CONN 2 to CONN 3 (pin 11) and is level shifted to +3V3 logic level and inverted by TR5 to become SPKR_JK

SPKR_JK and IP_JK are combined via IC12-A (inverter) and IC12-B (NAND Gate) to produce a logic low (0v) at IC12-B pin 6 when BOTH the Input Socket and at least one speaker socket are used.

IC11-B and IC-11A are a comparator and an inverter (respectively) to produce the BIAS_ON signal which is -12V when BOTH Input Socket and at least one speaker socket are used (else it is at +12V)

Sheet 3 of 4, The -12V on BIAS_ON pulls the base of TR4 down to -12v (via D15) which turns ON TR4 to give normal bias, adjustable by the bias set pot.
When either there is no input connection or no speaker connection then the +12V on the BIAS_ON signal allows TR4 to turn OFF (D15 reverse biased).
When TR4 turns OFF then the bias voltages go to MAXIMUM NEGATIVE Volts (set by the negative supply) effectively turning the output tube idle currents to MINIMUM.

NOTE that the -12V on BIAS_ON also turns on the audio input to the power amp phase splitter (TR1 and TR2) via FET21.

Good mostly "Fail Safe" design.

Hope this is of benefit to some of you.
Cheers,
Ian

[trem]

Wow.....that is a great review, and exactly describes what I was seeing.
Thanks

[Tooboob]

I like that circuit protection idea very much.