Blackface bandmaster reduce b+

[Quatro]

i got a hold of a mid sixties bandmaster that has the stock circuit but mostly new components. I figured out the power transformer is a fender period but not for the bandmaster. I believe its a 125P7A when it should be a 125P7D. This means to get the bias at about 70% puts about 500 volts on the plates. The schematic shows 440v. I know i can use a zener diode, but if i put it on the center tap this will screw up the bias supply, right? I'm also under the impression i can put a zener in line with the b+, but then I have to isolate the zener from the chassis while simultaneously heat-sinking it, correct? it's seems like it would be much easier to put a big resistor after the rectifier diodes (right before the standby?) Is this a bad idea for some reason? Will it cause sag? Ballpark value of resistor?

Thanks

[kagliostro]

but if i put it on the center tap this will screw up the bias supply, right?

yes

seems like it would be much easier to put a big resistor after the rectifier diodes (right before the standby?)

You can do that, if desired

One other method used on some old amp was to put a pair of resistors between PT and rectifiers (one for each branch)

Or you can use a buck transformer


Many way to obtain the same results, you must chose which one is more comfortable to you

K

p.s.: of course an increase in Power Supply resistance will give you SAG

[Quatro]

One other method used on some old amp was to put a pair of resistors between PT and rectifiers (one for each branch)

Why is it better to put two resistors before the the rectifier instead of one after?

[kagliostro]

Why is it better to put two resistors before the the rectifier instead of one after?

I don't know the exact reason as to do this way the thing

reducing the AC is intended as to have less stress on a tube rectifier

I don't know if the reason is the same also for SS rectify

and I've seen that also on amps with diode rectify

K

[Willabe]

One other method used on some old amp was to put a pair of resistors between PT and rectifiers (one for each branch)

Why is it better to put two resistors before the the rectifier instead of one after?

Well you only need 1/2 the wattage R in each leg.


                            Brad        

[Quatro]

i can check again, but I'm pretty sure that's the number i read off the transformer. And also the very high voltages. I believe the amp itself is a 65, but that's what i was told when I bought it. I haven't checked. As far as changes are concerned, in addition to the amp being in rough cosmetic shape ( very beat up missing corners, painted hardware, missing grill cloth , etc.) virtually every resistor and capacitor inside has been replaced. Not a lot in there old except the iron and the hardware. The high voltages made it hard to bias and it sounded pretty harsh. I'm trying to turn a beat up classic into a player.

[stratele52]

Don't put resistor after rectifier  this give some "sag" , voltage will change with output power. Low output = higher voltage, high output = lower voltage

Put Zener on center tap and readjust Bias voltage. This the best way IMO

[LooseChange]

Seriously, what is wrong with the 500v?

[Quatro]

I guess nothing is wrong with 500v, but there were a few reasons I wanted to change it. One -  i wanted to see how the amp sounds at the "correct" voltages. Two - tubes seemed to be running at the limit with no room for bias adjustment in either direction. Three - the amp seemed too harsh and bright. Preamp voltages were way up there as well. Another version might be to increase a dropping resistor to lower only the preamp voltages.

What I ended up doing was putting a 10 ohm fifty watt resistor on the primaries of the PT. Knocked everything down. Since the amp was calling for lower wall voltages when it was made, the heaters end up being just a touch lower than 6.3. Seems to sound a lot  better. I'm reserving the right to turn it back at some point.

LooseChange -  why are you a proponent of 500v?

[stratele52]

What I ended up doing was putting a 10 ohm fifty watt resistor on the primaries of the PT. Knocked everything down.   

Why do you want to use resistor instead of Zener diodes ? This resistor is more expensive thas few zeners and won't work well.

[rzenc]

Most Fender schems calls for 20% tolerances regarding voltages...
If schem is marked 440V:

+20%=1.2x440=528V
-20%=0.8x440= 352V

20% of 440V = 88V

Remember to also add today AC line voltage diferences compared to 1963/4

[Geezer]

You need to build this....

http://www.geofex.com/article_folders/vintvolt/vintvolt.htm

I built one using a 12v tranny & it works perfectly .....the 122v from the wall magically becomes 110v, and the B+ voltages go down 10% right along with it. That would get you in the 450v range.

$0.02

G

[jjasilli]

Everybody is right!  IMHO a variac is indispensable.  You can use it to drop or even increase the AC wall voltage to A/B different B+ voltages in the amp.  Your ears can tell you if a permanent voltage mod is desirable for you. 

In extreme cases a variac may mess too much with heater supply voltages.  This should not be a problem for a short period.  Another alternative is a bench power supply to temporarily serve the heaters.  The variac should be robust re its current rating compared to that of the amp under test.  Otherwise, it may may cause a voltage drop (sag), and skew your test results.

Also, a different B+ voltage will change the tone and performance of the preamp. That too should be separately A/B'd and adjusted to your preference. 

I'm a fan of bucking trannies, but they too may contribute to sag, because small trannies inherently have poor voltage regulation.  However, I'm a fan of sag -- a subjective preference.

VVR is another option.

However, IMHO, a series power resistor best satisfies the KISS factor for a voltage drop in the range you mention.

[DummyLoad]

as long as the heater voltages are within tolerances, then run it as is.

--DL

[kagliostro]

Automatic Boost / Buck circuit (for 220v line)



K

[kagliostro]

Will the step down transformer also work with the 220v?

Why not ?

the circuit I posted if the voltage is more than 220v buck the output

if the voltage is less than 220v boost the output

of course you never reach a perfect 220v level because the book and boost has fixed steps

but may stay within a range

K

[stratele52]

What I ended up doing was putting a 10 ohm fifty watt resistor on the primaries of the PT. Knocked everything down.   

Why do you want to use resistor instead of Zener diodes ? This resistor is more expensive thas few zeners and won't work well.

I did not read carefully all the answer since my first answer , but I  read a lot of very complicated solution, or very expensive for a simple problem of B +. Am I missing tips?

[stratele52]

Do not disagree that resistor is simple solution,
Step down transformers is a portable solution while being more complex. There has to be some Rube Goldberg in me. 

I guess one could make a zener or resistor just as portable.

Zener diode must be inside the amp ; this is a finish and good working mod for less than $2. I think you did not know what is a zener diode for talking as  portable

[stratele52]

drgonzom, we are probably the same age..... I remember avalanche Zener.

[Mars-Hall]

You need to build this....

http://www.geofex.com/article_folders/vintvolt/vintvolt.htm

I built one using a 12v tranny & it works perfectly .....the 122v from the wall magically becomes 110v, and the B+ voltages go down 10% right along with it. That would get you in the 450v range.

$0.02

G

I built one of these for my 61 Concert Amp (6G12-A), at times it was seeing plate voltages of 510v.  Leo really expected a lot out of current production 5881's of the time.  I didn't want to mode the amp itself, so this was the best solution for me.  Now the amp runs in the 440v range.  Much easier on the tubes.  However, with this setup, you do need to measure the affect on the heater voltage.