Hiwatt 200 (DR201) bias pot + general questions

[boroman]

I recently got in my hands beautiful dead-stock DR201 from 1980s or late 1970s. It's the 6xEL34 version, even original Mullards (AX7s and 34s) are there. Sounds HUGE, not a single hum or hiss.

I've tried to get an information how to add a simple bias pot as these does not have any... Is there a bias resistor I could replace for the pot in series? I did it with a lot of amps, old ampegs and stuff. Caps look fresh but have little bubbles (pic), better change them? Also, Switches work the opposite way, when its in the up postion amp is off (and sandy is off too), when pushed down, it powers on. What's even stranger, the main power led lights on "a bit" when main power cable is connected to the wall socket. Its light intensifies when you hit it to make amp on. Are all biacrown-made Hiwatts like that?

Pics:

[drewl]

next to the blue cap on the top board of the top picture is a 22K resistor to ground.\
That is your bias resistor, add a pot in series or resistor and pot to get enough bias range.

The main filter caps should be replaced, especially leaking like that.
They may work for years to come, or die. Not worth the risk.

As far as the pilot light, somehow voltage is leaking through the switch.
it may have dirt or gunk inside causing partial conduction.
You could try some contact cleaner, but I would measure across it to see if voltage is coming from somewhere it shouldn't be.



https://el34world.com/charts/Schemat...200w_dr201.pdf

[boroman]

Thank you @drewl. It was super clean inside, no dirt, no spidersweb etc. But I'll check voltages there, after a bit of pot/switches cleaning.
By the way, why there is a recommended voltage_doubler mod to this amp? It's because the bias is too low, mostly?

[drewl]

Yes.
Just Google Hiwatt voltage doubler mod.
I'm too lazy to post it.

[Randall]

The switch position is a British thing I think. Marshalls are like that for example.

[Enzo]

A "dirty" switch does not mean mud and grease, it means electrically dirty. Dirty switches can look good as new.

[boroman]

The pilot light (which light up even without hitting power switch) has like 90V then.. I think the + resistor could bring that down a bit.
What is suspicious, that I get 372V DC plate from pin3+g. Tubes have around 18mA so I will bring them up with variable bias pot somehwere in the future, but still, 372V?

[Helmholtz]

that I get 372V DC plate from pin3+g

Not sure how to read this.

If it means power tube plate voltage, pull the the PI tube and measure again.

[boroman]

I sorted out the pilot light thing, just soldered to the other lug of the switch and now it's perfect.
What's the deal with pulling the phase inverter tube to measure PV?

[Helmholtz]

What is PV ? I hate guessing and mistakable abbreviations.
Apart from that you didn't answer my question.

Pulling the PI tube means interrupting the PA NFB loop. This stops parasitic oscillations caused by connecting the meter leads to the power tube plates, which often results in wrong plate voltage readings.

Just do it and report back.

[boroman]

Thanks, I'll try it.
But measuring plate volatge from pin3 gives you actual reading that tube gets in operation, which leads to set bias right etc.

[Enzo]

And in many amps, when you put your meter probe on a pin 3, it causes the amp to oscillate. So pulling the PI tube stops that oscillation, but doesn't alter the plate voltage.

[Helmholtz]

Thanks, I'll try it.
But measuring plate volatge from pin3 gives you actual reading that tube gets in operation, which leads to set bias right etc.

Sure, pin 3 is the power tubes' plate terminal. But connecting any wires (like meter leads) to the plates may excite oscillation meaning a deviation from idle, causing lowered plate voltage. Any signal (wanted or unwanted) at the plates lowers DC voltage.

Generally it is completely sufficient to use plate supply voltage measured at the OT CT for setting bias.

Pulling the PI tube won't change power tube (idle) DC voltages.

[g1]

Can you also measure DC voltage at pins 4 and 5 of power tubes and post them.

[Helmholtz]

A sustained condition where there is a DCV difference between plate and supply voltage of say 200V is impossible as it means that the OT as well as the power tubes would have to dissipate several kWs - more than enough for meltdown.