Blackfacing a SF Bandmaster. Zener diode for B+ drop?

[fiveightandten]

I have a Silverfaced Bandmaster head that i'm going to be Black facing. Here are the schematics in question:

SF (.pdf):
http://www.schematicheaven.com/fenderamps/bandmaster_ac568_schem.pdf

BF (.gif):
http://www.ampwares.com/ffg/schem/bandmaster_ab763_schem.gif


Looks like i'll need to drop the B+ about 25V or so (based on the voltages before the rectifier). I haven't gotten in there to actually measure it yet though. I'm wondering what the best way is to go about this. If I were to use a Zener diode, what rating is typically safe for a SS rectified 2 x 6L6 amp?

Thanks in advance,

-Nick

[MWJB]

Personally I wouldn't get hung up over 25v, if that is the actual voltage difference you are measuring (do this first)? Fenders rarely conform to the voltages listed on the schematic, they are nearly always higher, especially with regard to 50's & 60's amps.

That said, Torres Engineering has the Zener you need, try 50W 50v, $25.

[Satamax]

A daft way to drop B+ is to add a tube rectifier :D I've done this before :D And it adds to the sound.

[stokes]

Mouser has them for about 15-18 bucks.I used the NTE5278AK which is a 56v 50watt.I paid less than $20.I connected it to the center tap and it dropped less than the stated 56 volts,its been a long time so I dont recall the exact drop I got,but I believe it was around 30v.

[Bruce / Mission Amps]

You can use three or four, 8v to 10v 5watt zener diodes in series.

8v @300ma = 2.5watts for each diode
10v @300ma = 3 watts for each diode

They only cost around a dollar each.

[jaysg]

Mouser has them for about 15-18 bucks.I used the NTE5278AK which is a 56v 50watt.I paid less than $20.Mouser carries almost the whole line and they used to be $14.18. Go online with that number, open the datasheet and then search on the one you want. If you want 25V off, you can get that or close, however, the final B+ will depend on how you set the bias.

[novosibir]

I connected it to the center tap and it dropped less than the stated 56 volts,its been a long time so I dont recall the exact drop I got,but I believe it was around 30v.
Theoretical those diodes should drop 71% of the declarated zener voltage, but in reality it's around 65% - so choose the zener 1.5 times of the intended voltage drop.

Larry

[jaysg]

Theoretical those diodes should drop 71% of the declarated zener voltage, but in reality it's around 65% - so choose the zener 1.5 times of the intended voltage drop.hmm...I suppose it depends on the type of rectification. I got more than the rating drop, however, that was on a full wave bridge instead of the full wave the original poster probably has. I think it's subtracting from the AC swing, not DC after rectification.

[Chris / CMW amps]

I have a Silverfaced Bandmaster head that i'm going to be Black facing. Here are the schematics in question:

SF (.pdf):
http://www.schematicheaven.com/fenderamps/bandmaster_ac568_schem.pdf

BF (.gif):
http://www.ampwares.com/ffg/schem/bandmaster_ab763_schem.gif


Looks like i'll need to drop the B+ about 25V or so (based on the voltages before the rectifier). I haven't gotten in there to actually measure it yet though. I'm wondering what the best way is to go about this. If I were to use a Zener diode, what rating is typically safe for a SS rectified 2 x 6L6 amp?

Thanks in advance,

-Nick

Just check the B+ you have in that amp.
My bf Bandmaster with export SR PT does have a B+ from about 450 Volts, last sf Bandmaster I did work on did have 420 Volts; somewhat sweeter sounding after blackfacing/etc. for the customer

[Alf]

If you use the 10 volt zeners at 3 watt ( 300 mA) do have to cool them in this type of amp ?
I used some 15 volt zeners in a lighter amp with 2 x el84 to drop about 45 volts and this goes fine without any cooling .
But in bigger amps do you use any trick to get rid of the heat ?

Alf

[novosibir]

You might look for the 'stud mount' zeners (with reversed polarity!), to get rid of the heat - but that requires to drill a 3/8" hole next to the PT.

Larry

[stokes]

The one I used and the # I posted is a reverse polarity.I've heard of some people drilling a hole thru a couple of pennies and bolting thru them as a kind of heat sink,if you are worried about dissipating more heat.