Ampeg B-15N Schematics

[txstrat]

Great description, thanks.
Do you encounter a hum cancelling position pf the volume or any other pot? I mean where the hum (while turing the pot up or down) is less on a single spot.

[dpacek]

not that i have noticed... definitely hiss increas when treble is up but I think i'm still hunting
hum in the splitter output stages... to verify this i disconnected the lead from the pre-amp to splitter
input and grounded the splitter input line. I think i'm gonna look for a second hand O-scope so I can probe
around and find the hum faster, unfortunately my current scope from the 70's doesnt seem to be able to
handle high voltage.

[txstrat]

The hiss in my build is not that bad even when all pots are full open. It's more the buzz bugging me - either under 4 or over 6 on the volume pot dial.
A friend gave me a very old scope that only works for 15 minutes. Don't know if I can manage to find the hum in this time (else I would have to fix the scope first). Seems like I have to dig more into it too. It came with no probes so I think I have to buy some probes in the near future.

[dpacek]

well I do have that 100ohm 1watt pot stradlinging my heater lines with the wiper going to my bias supply (-60v) and it does help
and i do have to change positions when i move the amp from one room to another.
the biggest hum reduction so far has been when i when i went from 1 to 4 caps on the bias supply line to ground and added a 330k 1 watt resistor.
the difference was amazing.

another noise reductuction thing i did was to use coax from the input jack to the preamp tube and also from the preamp tube to the splitter tube
I used ordinary RG-6 that I use on my sattelite dish hookup.

[txstrat]

another noise reductuction thing i did... ...and also from the preamp tube to the splitter tube

That sounds like it's worth a try. At the moment I'm using shielded wire just for the input of V1.
On the bias I'm using one 100uF/100v cap. What's the capacity of your caps on the bias line?

[dpacek]

I've got 4 22uf 450v caps (+ side to ground) in parrallel with a 330k 1 watt to ground all connected to the main
bias line, which also goes to the virtual centertap on the heater lines.

[dpacek]

The latest hum killer I added was a KOC mod from his B15 schematic in TUT3 on the Feedback line from the 8 Ohm tap he has a 270ohm 5watt resistor to ground .
I had to use a 250 and a 20 (both at 5 watts) to achieve that. he also uses a 8.2k resistor to feedback to splitter. I substitited a 6.2k inline with a 4.2k pot with enter tap
to a cap for presence control. Still some his when treble is up all the way but this thing is very quiet otherwise.

[txstrat]

I've put the ground of the volume pot to the PI filter cap ground and almost all of the buzz has gone. Still have heater hum.

[dpacek]

I would try doubling up on the cap that u use on the bias
add one or more of the same value cap in parallel and see what happens.

the final kicker was that 270ohm 5watt resistor on the feedback line to ground that seems to do the trick.
what are you doing for virtual center tap on the heater leads ? where is it connected ?

I'll clean up my schematic and repost soon

[txstrat]

I already have 100uF/100v. You think still more would be better? Usually there are one or two around 10uF. Although I could clip another one parallel to the first and see what happens.
Thanks

[dpacek]

It's definitely worth a try, I started by adding a 22u 450v cap and a 330k 1watt in parallel to ground and kept adding caps of that value until i was getting no more benefit

[txstrat]

I got my 100Hz buzz sorted out and remedied. Still have a low frequency hum like the 3rd fret (E-String) on a bass guitar.
I've added a 22uF and a 47uF parrallel to the existing bias cap. Not the slightest change. What's the 330K to ground for?

[dpacek]

Never mind the 330k .. I was trying to do this from memory but i just opened mine up again and updated my schematic.
I think the big difference here is that I am using a hum balance pot across the heater mains with the wiper attached directly the bias supply
raising/lowering the ground on that circuit to -60v and I have 4 22uf 450V caps at that point going to ground. That hum balance pot really does work in this config.

Paceman100DV-v2.pdf

one last thing you may try swap the brown and blue leads to your OT i saw that note in the weber B15 schematic and tried it and it did reduce hum a bit as well

[txstrat]

one last thing you may try swap the brown and blue leads to your OT i saw that note in the weber B15 schematic and tried it and it did reduce hum a bit as well

I've tried that but the hum got worse. I believe I had the better way already. After some last changes (shielded wires) the amp is quiet enough now.
Thanks

[dpacek]

Latest change

Turns out that the new Sovtek 6SL7s i was using in pre-amp and splitter
were, crap. After reviewing Fliptops.net I learned that do NOT recommend
those tubes for Ampeg designs as they induce hum and in my case random
and bursty hiss. The recommendationis for Tungsol or better versions.
I just purchased a pair of JAN-6188s from tubestore.com and installed them.
What a difference !!!


I also mated the unit to a slightly use Ampeg SVT-410 HLF cab.
Loud !!!