Annoying overwhelming brightness. Power amp problem?

[Phil_S]

You can tap the HT secondary where it enters the bridge. Just run a lead out to the bias circuit. Do it like this http://www.webphix.com/schematic%20heav ... pa_45w.pdf

For 6V6's you want a range of -15 to -25, maybe a bit wider depending on the plate voltage.

[Dingleberry]

That Marshall PT has a center tap, mine doesn't...
Does it really work like that with full wave bridge also?
Let me think... Ok the two "lower" diodes of the full wave
bridge makes sort of artificial center tap, am I right?
Then it should work ok. Am I correct?

-T

[Firestorm]

Look at the Ampeg V2 or V4. Those tap the bias supply off a full wave bridge with a .05 cap, then the diode and resistor network. Never thought about why ...

[rp]

No need for a separate tranny. I often tap off the secs, works great, hardly any current needed so it's fine. I usually copy the Marshall style, IIRC there's also a 6V6 Fender that did this - BF Princeton? I think you want around -30V.

[Phil_S]

Something like this. You may want to raise or lower the 220K dropping resistor to get voltage into the correct range. Get the voltage from where one leg of the high voltage secondary enters the bridge. -30V seems about right. I was thinking about a single ended 6V6 which needs about -12.5V. 50v or 100v rated caps should be fine.

[KT66]

SInce you are using an LTPI you could try strapping a "fizz" cap across the PI plates like in a plexi to tame the highs, or add an AC30 / Rocket style cut control.

[rp]

BTW I read somewhere that best to keep the bias caps low at 10-20mf. My instinct was to up them to 100mf, figured in old days caps were bulky and expensive and that was why they used small values. But big caps take longer to charge so you'll be running without bias for a few secs longer than necessary. Makes sense to me, thought I'd pass it on.

[surfsup]

BTW I read somewhere that best to keep the bias caps low at 10-20mf.

I think this was in case someone went to stby then off stby quickly... maybe somone can confirm.

[Dingleberry]

Made myself a bias board yesterday morning, spending rest of the day cleaning our house with my girlfriend... Now she's away for two days to see her Granny so I have a plenty of time to get into my amp.

As you might notice from the picture the ac-dropping resistor is still missing because I have to try few different values and see which gives me a good range of adjust. The board is from old tube radio and I used what components I had on hand... Trimmer looks cool with it's plastic dust cover.
If I remember correctly I disassembled it from some old non working measuring eguipment.
I used 22uf caps because those I had on hand and they just fitted there.
Off to the workshop.

-T

[Dingleberry]

Ok.
Thanks for replies and suggestions, but the thing seems to be that you can't tap other leg of the secondary for bias supply if you don't have a center tap in your PT. I don't, as I stated already in my previous posts, and it just doens't work. The case seems to be that you can't get positive and negative voltages from same winding (ok you can but it's more complicated than just use external bias transformer...) if you use bridge rect. It actually makes sense to me if I think of it now.
Gotta use an external transformer for bias supply in my case.
Back to workshop...

-T

[Firestorm]

No you don't need an external transformer. But to generate the negative rail off a full wave bridge, you need a capacitive divider rather than a resistive divider. Change the dropping resistor to a cap (try .047); the larger the cap, the more negative the derived voltage. Or leave the dropping and load resistors, but put the cap in front of the reversed diode from that junction. Ampeg did this kind of bias supply for years. Look at the V2 or V4 schematic to see how they did it (it's not the only way). If you need, I can PM you an alternative approach.

[Dingleberry]

Shit... My bad.

V2 schematic opened my eyes!
Somehow it was easier to realize than the V4 schem that I came across to.

I'll try that arrangement! Ive got some MKT caps up to 1uf/1kV. They should do fine in that posision.
It is good to realize that there's always something to learn every day.
Thank you Firestorm, and all of you guys too.
It's 23.46 here right now so it'll be tomorrow when I get back to bench.
And nice to realize that it was actually a good thing that I didn't drill the holes tonight for that bias supply transformer that I found...
Peace and love and good night.

-T