simms watt PA150 bias issue

[blindboybenton]

Good morning

I have a simms watt PA150 in. When the customer brought it ..it lasted about 30mins and then died.
He popped it too me with new Golden lion KT88 and 4 x new grid stoppers 1k 5 watt resistors. supplied by the previous owner .
I fitted the resistors and the put tubes in with a bias probe. When i turned on i can see the bias was going up over 70ma pt and rising . so i switched off and turned both bias pots to full. I turned on the bias was at 37-42ma but this is at bias voltage of -82 v
I changed all the caps and resistors / pots in the bias circuit ( Original ) still the same
The HT caps had all been changed before 4 x 470UF 450v with 100k resistors

Tried 2 x used quad sets of 6550's but still the same
I ordered some new JJ KT88. Fitted bias slightly lower at 35-38ma, but still at -82v

while i was checking things out i accidentally busted a pin on the tube base. So i decided to replaced all tube bases fitting 1ohm 2 watt resistors for bias monitoring

Power tube base voltages

Heater = 6.3v ac
pin 2 = 661v
Pin 3 = 656v
Pin 4 = -82v
Pin 6 = 663v

Now too me there is something not right here. All the 150-200 watt 6550s/kt88 amps i do the bias is at -45-50v . I have hunted for the schematic but no one has it

The owner is going to use the amp on a uk Tour this year, I need it to be right.

Thank you for reading .. any thought's

BBB

[pdf64]

...Power tube base voltages

Heater = 6.3v ac
pin 2 = 661v
Pin 3 = 656v
Pin 4 = -82v
Pin 6 = 663v

Now too me there is something not right here. All the 150-200 watt 6550s/kt88 amps i do the bias is at -45-50v...

What were their g2 voltages? Many high voltage/power designs operate g2 on a separate, lower HT than the main plate supply.
As Vg2 increases in magnitude, so must Vg1; it may be that with this amp, the very high Vg2 require a lot of bias on g1 to keep plate current in check.
See http://www.mif.pg.gda.pl/homepages/f...084/k/KT88.pdf
With HT of 560V, g1 needs 80V bias.
So noting the +/-25% for bias and the higher HT of your amp, you should have a max available bias of perhaps -120V.
That may require a voltage doubler for the bias supply, which is common with many vintage UK amps.

[Mick Bailey]

I fitted a Blankenship amp with 4xKT88 Golden Lion tubes and this had a bias voltage of -78v. Nowhere near your B+ and I think it was about 520v.

[blindboybenton]

many thanks Chaps. Ive learnt something today. I will do a voltage doubler circuit later .

BBB