Ampeg B-15N Schematics

[dpacek]

thanks again that helps a lot.

I'm going with the Hammond 291FX
for a PT.. no center heater tap.

I do want a hum balance control
but if I use a pot I'm unsure where the
center of it (wiper) connects .. piazza design
shows it connecting to the bias control

[txstrat]

The wiper of a hum balance control usually goes to ground. Except the developed hum is too strong. In this case you could try to cure the hum by "lifting" the ground to some 40 - 80 volts.
I've made best experience with two 100 ohms resistors to ground. A pots wiper could fail easier IMHO.

[dpacek]

The wiper of a hum balance control usually goes to ground. Except the developed hum is too strong. In this case you could try to cure the hum by "lifting" the ground to some 40 - 80 volts.
.... That must be why the Joe Piazza schematics have the pot wiper connected to the bias circuit


I've made best experience with two 100 ohms resistors to ground. A pots wiper could fail easier IMHO.

sounds easy and simple.. I'm going with that .

[dpacek]

Getting close but no joy

OK good so far.
1. For the same frequency response as the B-15 I'd go for the .022 but I'd use a 600v rating.
2. The Fender label of the negative feedback is indeed 820 ohms but same as before I'd go for the Ampeg value and if you don't like it (or the amp don't behave) you can change that to different values.
3. If your PT don't have a heater center tap you're in the need of a hum balance either as a pot or two 100 ohms/2w resistors from each heater wire to ground (virtual center tap). If neither cures the heater hum you might have to lift the heater ground to a higher voltage level (we'll discuss that if necessary).

Ok Build is up and running .. kinda ... lots of HUM ... corected substntially
with 2 470-Ohm 1 watt resistors from heater taps to ground (didn't have any 100ohm handy). The hum certainly sounds like 60hz... i'm hunting for loos connections
and signal line proximity to heater lines but so far no luck.
>>> ok found some 100ohm 10watt wirewounds and installed them from
heater lines to ground... quieter but still lots of hum.

Perhaps I need to do as you suggested and lift the heater ground.
I did create a floating ground switch (isolating signal and secondary winding ground from chassis/safety ground as per KOC TUT3 design but no real effect on hum.
>> ground lift does have an effect when nothing is plugged in
to input. noisier on float
(BTW, I am using insulated jacks and a galaxy ground pattern)
current bias setting appears to be a bout 35ma per tube

I ended up using transformer and choke replacements from Classictone
via Amp Parts Direct

(Vintage Fender 4Hy, 90mA DC Style Choke 40-18003
Vintage Fender Style 100W Power Transformer 120V 40-18004
Vintage Fender Style 100W Output Transformer 4/8/16 ohm 40-18013)

I built the power supply of the FenderShowman AB673


Btw here is my schematic
Paceman100DV-v1.pdf

with amp in standby mode the B+ voltage is +460
in Play mode the B+ drops to 455
but what's strange is that the voltage drop across the choke is zero
so in play mode the screen voltage AND the plate voltage
for all four power tubes is 455.

It seems like I really need to get the screen voltage down by at least another 5 V
not sure the best method.. maybe get rid of the choke and add a resitor ?

[txstrat]

hum certainly sounds like 60hz...

If you play a note on the guitar (E string 3rd fret) it'll sound very close to 100Hz. That way you can determine if you got 60Hz or 100Hz hum/buzz.

Perhaps I need to do as you suggested and lift the heater ground.

This definitely helped with the hum on my build.

I did create a floating ground switch (isolating signal and secondary winding ground from chassis/safety ground as per KOC TUT3 design but no real effect on hum.
>> ground lift does have an effect when nothing is plugged in
to input. noisier on float
(BTW, I am using insulated jacks and a galaxy ground pattern) ´

The galaxy ground is what I'm gonna try next, too.

current bias setting appears to be a bout 35ma per tube

Fine

It seems like I really need to get the screen voltage down by at least another 5 V
not sure the best method.. maybe get rid of the choke and add a resitor ?

It's not THAT important to get the screens down another 5volts as long as they're not much higher than the plates.

Separate the grounds for the preamp and PI filter caps. That helped alot with the hum in my build.

[ThermionicScott]

It's not THAT important to get the screens down another 5volts as long as they're not much higher than the plates.

People see the screens a couple volts over the plates at idle and freak out (especially in "E" series Fenders where the plates are after the choke), but it's a complete non-issue. The screens don't capture much of the cathode current until the plate voltage has swung WAY down, close to the diode line of the curves (under 50 plate volts for most beam power tubes).

- Scott

[dpacek]

Thanks, I guess I'll live with Screen=Plate voltage at idle.
Now focussing on chasing AC hum. I suspect the heater wiring.
in the process of replacing all the twisted daisy chain heater hookup wire
with foil sheilded twisted pair (with ground conductor twisted in as well)
in a home run hook up instead of daisy chain. Wish me luck.

[loudthud]

Two ways to tell if the hum is coming from the heater supply. It will be line frequency, 50/60Hz, not the buzzy 100/120Hz type hum. Secondly, the hum will slowly fade out when you put the amp in standby mode. The buzzy hum will cut off immediately when you put the amp in standby.

[dpacek]

It's definitely a 60Hz hum... my guitar tuner app on my iphone confirmed it.... not buzzy but defintiely constant.
The PT I am using has no center tap for the heater winding. did the 100-Ohm resitor on each leg trick to create a virtual center tap and changed
to individual twsted pair foil shielded runs for each tube . quieter but still not quiet enough. I'm looking at creating another +40~50VDC tap off the B+ line
to connect to the virtual center tap on the heater lines lifting the ground level. Anyone have any luck with that ??

[txstrat]

I have tried both ways in my latest B-15 copy. Elevated the virtual center tap of the heaters to 70volts fed from the B+ (with a voltage divider circuit).
Later I put it directly to the bias voltage (at the junction of the two 270K resistors like in Joe Piazzas schematic). Either way tamed the hum more than 75%.
The version with the bias voltage just spared me the voltage divider circuit.

[dpacek]

Success!!!

I tied the bias supply to the virtual center tap (actually the wiper of a 100Ohm pot across the
heater leads). Still had hum but I added a 25uf 250V cap from the bias output to ground
and killed 75% off the hum. I am now the proud owner of a 100watt version of B-15

[txstrat]

That's great to hear. Congratulations.
Do you have a layout plan and/or some gut pics you could share with us?
NICE build by the way.

[dpacek]

I didn't take too many pics along the way but I posted them here:
MobileMe Gallery

I used the Frank Piazza schematic for the most part (including hi/lo switches), but I added in a
a brightness switch and ground lift as per KOC suggestion and a Presence control as suggested in a weber schematic.
and of course individual bias adjust. The power supply is mostly the Fender Showman design.
internally I have 3 seperate sections of turret in the pics, preamp (upper left), power/splitter (lower left) and B+ and bias
supply lower right..with B+ taps on the upper right. as you can see by the end it looks pretty messy.
now that it's up and running I'm going to go back in and do some clean up.

[txstrat]

Could you describe exactly how you grounded the 6SL7 cathodes and the pots ground connections (each wire)?
I've got grounding issues (leading to hum) in my build discussed in a different threat and your grounding/wiring might be useful to me.

[dpacek]

I'm not sure if I'm at the end of my hum-bug killing myself but the unit is definitely quiet enough to play.
first of all i used insulated input and output jacks.
I created a signal ground bus on the pre-amp turret card this bus is then connected directly to the
power supply ground bus but I'm thinking of moving that connection to the ground of the B+ rail at the
neg side of the cap that feeds the 6SL7s.

I don't ground the cases of the pots (yet) but may try that as well to see if it does anything
on the preamp 6SL7 - Cathode 1 (pin 6) has a 5.6k resistor connected to it and Cathode 2 (pin 3) has a 2.2k resistor
connected to it. I twisted and soldered the ends of those resistos together and connected
a wire at that junction that goes to the pre-amp board ground bus

on the splitter 6SL7 -Cathode 1 (pin 6) has a wire to the splitter turret board splitter components
that has its own ground bus and Cathode 2 (pin 3) has a 200ohm resistor
connected to it and the other end of that resistor connects to the same point on
the splitter board as the first cathode.

hope that helps