Ampeg B15 Build -- Bias supply question

[g1]

Here's another Piazza drawing, this time with a 100uf bias filter shown. I'm pretty certain 100uf is the correct value and 10uf is a typo.

[The Dude]

Here's another Piazza drawing, this time with a 100uf bias filter shown. I'm pretty certain 100uf is the correct value and 10uf is a typo.
[ATTACH=CONFIG]24928[/ATTACH]

Ahhh,...now your previous comment makes sense and now that I think about it 10UF is a bit low. That said, there are plenty of old Fenders that use a 25UF, so I would think 33UF would be sufficient.

[Enzo]

33uf will I am sure work as well as the 25uf in a Fender. Nothing critical here. However, when i replace the 25uf cap in a fender I pretty much always install a 100uf in its place. Plus speaking of Piazza errors, Fender on many schematics and parts lists specifies a 50vf cap with 55 volts writtin on the schematic as the circuit voltage. SO I use 100v caps too.


There is nothing cosmic about the K on the drawing. It is a standard drawing convention. You want to connect two points in the schematic together without drawing lines all over, so one point has an arrow to a letter, and another point has the same letter with the line continuing. Exactly the same as Fender using X and Y on MANY layout drawings for a wire on the eyelet board. Example:
http://www.webphix.com/schematic%20h...b763_schem.pdf

Note on the later Piazza example in post 16, the heater balance which had been point K is now shown wired, but the bias supply refers to point B, with another B up at the power tubes. Contrast the earlier drawing whch showed the tubes wired.

[bob p]

I didn't think the K was magic, i just couldn't find the other K that was hiding in the heaters. never expected it there.

fixed bias? i always use 100/100 regardless of whether the schematic uses something smaller with a lower voltage or not. i find it easiest to just keep a supply of 100/100.

[Enzo]

Dude was thinking it was purposed, like as a test point. I was trying to point out it was a drawing artifact, not something that exists on the amp. You had already mentioned looking for the other K, so I knew you understood.

[loudthud]

These bias supplys that run off the HV winding have a really long time constant. With a 100uF cap it can take 30 seconds to come up to the final bias voltage. In this case the time constant is a little quicker because the 56K brings the equivalent series resistance down to about 36K.

[Tom Phillips]

Hylaphone,
Now that you have sorted out your circuit problem I suggest that you consider making the bias supply adjustable. In the old days the power tubes were much more consistent. However, the currently available power tubes can require significantly different bias voltages to set an acceptable bias current and subsequent power tube dissipation value. You do, of course, have the choice to test select the resistor values to set your bias but a pot would be more convenient.
Cheers,
Tom

[hylaphone]

Thanks everyone for the interesting thread. I have some 100uf caps on the way, and am busying myself cleaning up the grounding in the meantime. Definitely have some 60hz hum, we'll see how it sounds with the new bias cap.

Speaking of which, on a filter cap how close to the rated wattage to you all care to go? The first cap is 600v, no worries there, but that second one feeding the 6l6 screens is rated 500v, and hovering around 470v right now. Another 600v would be a tight fit in that location..

Tom, thanks for the suggestion -- I actually had a bias pot in there all along, but wanted to get the circuit working with the stock resistor before messing with the pot.