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How to lower the voltage on power transformer?

Niblos

Member
Messages
434
Two years ago a buddy managed to dig up a '62 fender bassman 6G6-b right outside of town. Everything was original except the power transformer that had previously burn up at some point. My tech originally though the amp was a 6G6-a due to the incorrect label inside the head. We ordered a specially hand-wound transformer for the 470 voltage specs. However it was soon discovered it was actually 6G6-b which runs around 435 voltage. Is there a way to safely lower this transformer to this level?

I have searched over a year for an original 125P7A transformer but to no avail. The only one I did manage to locate was lost in the mail due to the sender taking the cheapest way out - parcel post with no insurance. :messedup
 

thilton59

Member
Messages
80
You could use a variac to alter the wall voltage which would then correspond to the other voltages throughout the amp. One idea anyway.
 
Messages
6,116
Two years ago a buddy managed to dig up a '62 fender bassman 6G6-b right outside of town. Everything was original except the power transformer that had previously burn up at some point. My tech originally though the amp was a 6G6-a due to the incorrect label inside the head. We ordered a specially hand-wound transformer for the 470 voltage specs. However it was soon discovered it was actually 6G6-b which runs around 435 voltage. Is there a way to safely lower this transformer to this level?

I have searched over a year for an original 125P7A transformer but to no avail. The only one I did manage to locate was lost in the mail due to the sender taking the cheapest way out - parcel post with no insurance. :messedup

There are few different ways. One is to stack diodes in series,
and then put them in between the rectifier output and first filter.
It takes 2-3 1N4007's to drop 2 volts, so you would need
to run around 60 to do the job. Sounds like a lot, it is.
(you techs can use a small perf board)
This is not a job for a consumer, best thing is to run the correct transformer.
 

mbratch

Member
Messages
2,381
What about the old zener diode(s) on the center tap trick ?

I've also done this successfully using a string of 2 or 3 smaller axial zener diodes (11V @ 5W) on a 5E3 that has a PT spec'ed at 384-0-384 and I wanted it closer to 350v. The 5E3 is a fairly low wattage amp, so the amount of heat generated by the diodes wasn't significant in this case.
 

paolojm

Member
Messages
68
What about the old zener diode(s) on the center tap trick ?

I've also done this successfully using a string of 2 or 3 smaller axial zener diodes (11V @ 5W) on a 5E3 that has a PT spec'ed at 384-0-384 and I wanted it closer to 350v. The 5E3 is a fairly low wattage amp, so the amount of heat generated by the diodes wasn't significant in this case.
This is a good option.

best thing is to run the correct transformer.
This would be the best idea, if possible.

You could use a variac to alter the wall voltage which would then correspond to the other voltages throughout the amp. One idea anyway.
This is not a good idea at all. Running valve heater circuits at an incorrect voltage is never a good idea.

My 2c
 
Messages
227
I'm using one of Weber's zener diodes to drop my B+ by 40 volts. As Weber mentions, you will have to adjust your bias. In my case, I had to change the resistor on the bias pot to shift its range. Not difficult but something to keep in mind.
I'm pleased with the results. I wanted lower B+ for a homebrew Brown Vibrolux/Deluxe (6G11/6G3) I built.

Mark
 

MKB

Gold Supporting Member
Messages
8,209
Here's another way to do it. It works amazingly well, as well as a variac but is much cheaper, safer and convenient. I have one permanently installed in my 50W Marshall clone.

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

However, keep in mind this will lower the other voltages in the amp as well (such as the filaments). Watch those voltage tolerances...
 

Husky

Member
Messages
11,818
Two years ago a buddy managed to dig up a '62 fender bassman 6G6-b right outside of town. Everything was original except the power transformer that had previously burn up at some point. My tech originally though the amp was a 6G6-a due to the incorrect label inside the head. We ordered a specially hand-wound transformer for the 470 voltage specs. However it was soon discovered it was actually 6G6-b which runs around 435 voltage. Is there a way to safely lower this transformer to this level?

I have searched over a year for an original 125P7A transformer but to no avail. The only one I did manage to locate was lost in the mail due to the sender taking the cheapest way out - parcel post with no insurance. :messedup
You can use a small transformer in buck mode as long as it has the correct VA. You can use a big zener, Power scaling but the easiest would be to replace it
 

skipm45

Member
Messages
297
Here's an cheaper method than the Vdump which combines a small zener and a MOSFET. Connect between the PT center tap and ground.

Sort of like a fixed "powerscaling" approach as suggested in the previous post, but the MOSFET source is at ground potential and can be bolted directly to the chassis without an insulator.

http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm


Scroll about 2/3 of the way down the page for the circuit.

Skip
www.skipzcircuits.com
 

MKB

Gold Supporting Member
Messages
8,209
Here's an cheaper method than the Vdump which combines a small zener and a MOSFET. Connect between the PT center tap and ground.

Sort of like a fixed "powerscaling" approach as suggested in the previous post, but the MOSFET source is at ground potential and can be bolted directly to the chassis without an insulator.

http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm


Scroll about 2/3 of the way down the page for the circuit.

Skip
www.skipzcircuits.com
I've done this too, it works very nicely, but you have to heat sink it well as it can get VERY hot. It also requires chassis modification in some cases; you can bolt the mosfet to the chassis but you need a hole for the bolt. I did this and the RG Keen transformer method, and ended up using the transformer method as it doesn't generate as much heat.
 

Niblos

Member
Messages
434
Man you guys don't know how much I appreciate everyone's input. I did contact Heyboer about a custom built 125P7A and I am also considering the diode divider.

I understand the output transformer has more to do with the TONE than the POWER transformer. Since everything else is original I guess as long as the correct voltage is being supplied it will probably won't be too much of a dfference.

This board is simply amazing! THANK YOU!!!
 
Last edited:

BCJek

Member
Messages
903
I knocked ~50 volts off the B+ of an amp with a Zener on center tap. Worked like a charm.

Michael
 

thilton59

Member
Messages
80
Yes, it is. If the heater voltages drop too low, but that's pretty low. If you operate it at 110ACV(which was standard wall voltage when that amp was built) you won't run into problems. There's some room to move around there.
 

Structo

Member
Messages
9,559
I used a 120v 50 watt Zener on the center tap of a 1958 Hammond organ amp I converted to a guitar amp.
The PT was putting out 460v!

The Zener works well, just make sure you get a high enough power (watts) one that is a stud mount and mount it through the chassis using heat sink compound.

The heater voltage was in the ball park so I didn't touch that.

The problem with a variac as mentioned is that if you go below spec for the primary voltage, you lower all the secondary voltages including the rectifier 5v and the heater 6.3v.
 




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