Ampeg B-15N Schematics

[dpacek]

Killed the last Hum bug

Turns out the bias supply circuit i copied from the Fender showman design was just to
noisy ( single bias tap + single diode = half wave noise). Ditched that and used KOC recommended full wave design (but without the Zener .. cause i didn't have one handy)
and this thing is now totally quiet at idle.


On to the next project - DIY 300W SVT

[ThermionicScott]

Maybe the cap was bad? Lots of people bump up the bias cap to 47 or 100uF and increase the voltage rating while they're at it.

- Scott

[dpacek]

the components were all brand new, I probably should have tried the 100uf but all I had handy was a bunch of 22uf 250v. I had 4 of them in parallel on the old circuit at one point and it did take the hum down considerably but never totally gone and adding more didn't help so I started from scratch. I will take your advice next time I run into that type of bias circuit. Thanks for the tip.

[txstrat]

Turns out the bias supply circuit i copied from the Fender showman design was just to
noisy ( single bias tap + single diode = half wave noise). Ditched that and used KOC recommended full wave design (but without the Zener .. cause i didn't have one handy)
and this thing is now totally quiet at idle.

Great to hear you sorted it out.
Do you have a layout plan of your build? Or maybe some gut pics?
I'm curious if I could adapt something for my B-15 copy to kill the little remaining noise I'm still having.

[dpacek]

Sorry no layout plan (thats probably why it's so messy) but it is modular.
I'll take some pics or perhaps a short video and narrate some key areas for you.
Probably later this week. What issues are you still having ?
I found my bias problem when I looked at the supply on the oscope and saw a .5v 60 hz ripple, which I didn't think was bad over a -60v supply
but I had a portable variable DC power supply handy, so I disconnected the bias supply and wired in the DC supply and set it to -60v
and Voila! no more hum. Thats when I went to TUT3 to look for inspiration.

[txstrat]

I'll take some pics or perhaps a short video and narrate some key areas for you.
Probably later this week.

That'd be great.

What issues are you still having ?

Well, I have two sorts of hum/buzz which change with every bit of changing the layout. So far I got the best out of it but it's not as quiet as I'm used to with tube amps of all sorts.

There's a deep frequency hum as soon as I switch the power on (std by in off position) which is only noticeable when you get close to the speaker and I would consider it as quite normal (actually I started a thread about it cause I was thinking it's the PT and many of our fellows here on the forum helped me to make the best out of it). But it gets even louder when I switch std by to "on" - then it's too loud for my taste but all in all I could live with it.

Then there's a buzz I can't live with. It starts when I turn up the volume and/or the treble pot and gets way too loud for my taste especially when the brigth switch is engaged.
Seems like a ground loop cause changing the position of the pots grounding lugs changes the amount of buzz.

I've copied the complete TUT3 layout (except I'm using fixed bias instead of cathode bias) and the buzz is still there. My first attempt was the '68 B-15 layout which is different from the TUT3.
When you wrote about the bias rectifier being the culprit in your amp my face brightened. Might have to try it. Haven't put a hand on that amp for many weeks.

Thanks for listening.

Matt

[dpacek]

do you have a hum balance pot on your heater lines ?
I have one and definitely need it... radically reduces buzz and hiss.
I actually adjust mine depending
on what room/plug I'm using.
Can't figure out how you can get
hum in standby mode.
I'm using fender psu layout which basically
kills all plate voltage when in standby mode.
I'll go look at the TUt 3 book and see if something pops out
Did you install the 270ohm 5watt resistor on the feedback line ?

[txstrat]

do you have a hum balance pot on your heater lines ?
Did you install the 270ohm 5watt resistor on the feedback line ?

First off you might wanna read through this thread: http://music-electronics-forum.com/t22693/ - That's what I've done so far.

I tried a hum balance pot to no avail. Seems like it's not heater hum.

No I haven't installed the 270 ohms 5W resistor. I thought it was only for protecting the OT if you play the amp with no cab plugged in. Mine has a shorting jack like older Fender amps usually have, which opens when you plug a cab in.

[dpacek]

I suggest the hum balance pot because it seems to be
necessary to deal with buzz and hiss.
I suggested the 270 ohm 5watt because while I was tweaking
my design I found that it actually reduced hum
as well.
Both of my transformers have bells and are inside
steel cans and the are a good 6 inches apart.
Can you scope the blue or brown leads to the
OT and see if the noise signal is present there ?

if you are having hum even when u pull all the tubes
then the bias supply can't be involved.
you may have noise on the B+ line.
scoping the b+ line may show the problem.

also if you are getting hum with all the tubes pulled
AND you are in standby mode (which should mean B+ = 0v)
this basically means the primary of the OT is connected to
nothing. you can always disconnect the B+ from the primary
to test this.

At that point you can only be getting hum from the ground connection
on the 2ndary at the speaker connection, or your OT is
is picking up EM radiation from the PT. If you disconnect
the ground at the output you'll have your answer.
Didn't you say this was a custom wound OT you had someone build
for you? may need to try an off the shelf model.

[txstrat]

The amp is totally quiet when I pull the preamp and PI tube. Just a slight hum.
It is an off the shelf OT and I indeed suspect EM radiation to be the culprit since none of the transformers have end bells.
Actually I got the hum down to a level, where it doesn't bother me.

What's worse is the buzz from the (I believe) ground loop or wrong ground connections. Gets better or worse depending on where I put the ground connections on the bus wire to the final grounding point near the PT. Even with the copied TUT3 layout the buzz is still there when I turn up the volume and treble pot.

[dpacek]

I use coax on the signal lines
to and from preamp/splitter tube.

if you touch the insulation of any signal wires with your finger tips
and you hear an increase
in noise I would change that to a shielded wire

I still highly recommend the hum balance
100 ohm pot across the heater lines with wiper going to
ground or in your case bias. I tried both and ended upwith
ground connect.

and still recommend that 5watt on the feedback line

[jbefumo]

That’s where I was heading too… 4 6L6’s should give plenty of headroom without changing the tone too much. Did you stick with a tube rectifier or change it to solid state?

Any pictures?

Stumbled on this thread because I just acquired a (FREE!) Hofner bass, and was thinking of building myself an amp for it. I never played bass before, and find myself really enjoying it. My last bass player used a B-15, and I loved his tone, so I figured I'd start with that schematic. I have on hand a chassis and transformers from an old Sundown that made 100W from a pair of 6550s (which I have since replaced with Gold Lion KT88s), and it resides in a home-made 1X15 combo cabinet with and Altec (518, I think), so seems like a reasonable platform. I use the Baxandal tone stack in most of my guitar amp designs, so I'm pretty comfortable with that. Ideally, I would like something that can serve as a clean guitar amp as well.

I've noticed that some of the B-xx Ampeg schematics I've perused use a cathodyne phase inverter with an extra gain stage, whereas others use the more common LTP. Any thoughts as to how these differ in tone (particularly considering that I'll be driving KT88s)?

Thanks!

Joe

[dpacek]

Stumbled on this thread because I just acquired a (FREE!) Hofner bass, and was thinking of building myself an amp for it. I never played bass before, and find myself really enjoying it. My last bass player used a B-15, and I loved his tone, so I figured I'd start with that schematic. I have on hand a chassis and transformers from an old Sundown that made 100W from a pair of 6550s (which I have since replaced with Gold Lion KT88s), and it resides in a home-made 1X15 combo cabinet with and Altec (518, I think), so seems like a reasonable platform. I use the Baxandal tone stack in most of my guitar amp designs, so I'm pretty comfortable with that. Ideally, I would like something that can serve as a clean guitar amp as well.

I've noticed that some of the B-xx Ampeg schematics I've perused use a cathodyne phase inverter with an extra gain stage, whereas others use the more common LTP. Any thoughts as to how these differ in tone (particularly considering that I'll be driving KT88s)?

Thanks!

Joe

I haven't seen any B-15 schematics that have a gain stage between inverter and power tubes, I've seen them in SVT schematics though.
KOC has a 'simplified' SVT schematic in TUT3 that has splitter with a gain stage for driving 6 6550s. I have been planning on building that
this summer but with the true SVT pre amp schematic.

In my expanded B-15 (4 6L6s) I used the '68 B-15 splitter design as someone on here (or the music electronics forum) inferred that it was key to creating
the magic B15 tone. I used the Fender Bassman 100 power supply and 6l6 power stage layout schematic but added individual bias adjust.

[dpacek]

one last comment- if you want a clean guitar amp, this design is it. My son uses his for both electric and double bass
but uses a an amp with multi stage 12AX7 pre amp design for crunchy guitar sounds. Unless you use an external pre-amp
or a a very hot active pickup setup this thing is very hard to overdrive