zener diode trick to lower B+ volts

[tubenit]

Can someone provide a schematic on how the zener diode trick works?  I am interested in the PT  CT with zener diode to ground method.  Not sure which direction for the zener diode though?

I did a search on the forum and the internet and surprisingly could not come up with a schematic and read conflicting information about this.

Q:  Is the schematic I've attached showing the correct orientation of the zener diode? 


I found other ideas using mosfets also.  (OK, found something with zener orientation showing, so hopefully I am understanding this correctly?)

With respect, Tubenit

[kagliostro]

Ciao Tubenit

May be here there is something of your interest

I remember other articles on the matter, but at the moment I've only this link under hand

see zener back-bias circuit

http://www.aikenamps.com/index.php/what-is-back-biasing



With this architecture of course you get also a drop in B+ that is the thing about you are searching for

Franco

p.s.: In that position you can also use an amplified zener, so you don't need a high W zener to drop B+

[MakerDP]

In the "Simple Voltage Reducer" schematic you posted above, if you replace R1 with a 1M pot and tweak R2's value a bit (100k to 220k, depending on minimum voltage you want) you basically have a VVR (variable voltage reducer) I just put one on my 18-watt build and absolutely love it. Lets me get quieter while keeping the full character of the amp. I am going to implement it on my other builds as well. It's so easy to implement that I think I will put one in all of my builds moving forward. If you use a switching 1M pot, you can use it as the main power switch too!

[kagliostro]

This is the amplified Zener connected to CT as to drop B+ that I mention before


the trimmer is necessary only if you want to adjust the voltage


you can omit it and connect directly to the junction of the resistor and zener with the transistor base





Franco

[tubeswell]

Can someone provide a schematic on how the zener diode trick works?  I am interested in the PT  CT with zener diode to ground method.  Not sure which direction for the zener diode though?

I did a search on the forum and the internet and surprisingly could not come up with a schematic and read conflicting information about this.

Q:  Is the schematic I've attached showing the correct orientation of the zener diode? 

Yes that's essentially correct. You can also stack a number of zeners in series to make the voltage drop add up.


I'm positive I've posted the zener reduction schematic numerous times in the past. Here it is again.


(Edit: Of course, if you have a fixed-bias output stage, you'll want to take into account the power supply source for the bias voltage when considering dropping the HT voltage with this zener method. Some PTs have a separate winding for bias supply, and in that case the bias voltage will be independent of what you do to the HT winding)

[Joel]

Resurrecting an old thread to ask if anyone who has used this amplified zener trick to lower B+ has had problems with noise?

I tried to implement this trick in my latest junkbox build.  I used a circuit very similar to the Merlin one placed at the Power Transformer center tap, but I used a MJE3055 and two series 24V zeners.  The result was loud buzzing.  Taking it out of circuit resulted in a dead quiet amp (but with B+ a bit too high for my liking).  All semiconductor devices were tested as serviceable prior to building.

I commented on this noise issue in my post https://el34world.com/Forum/index.php?topic=29299.0

[mresistor]

I used the plain old zeners, several in series, that look like regular diodes and there was no noise.   


example:  https://www.ebay.com/itm/254081173645?var=0&mkevt=1&mkcid=1&mkrid=711-53200-19255-0&campid=5338590836&toolid=10044&customid=325f39c8fd36109b97e85aa61a7e99ce

[RadioComm]

I lowered my voltage over 20 years ago with my first amp build. Can't remember the voltage/diode numbers, but I used a chassis mount style diode:

https://www.amazon.com/Pairs-70HFR-Spiral-Rectifier-Diodes/dp/B0859C48LQ/ref=asc_df_B0859C48LQ?tag=bngsmtphsnus-20&linkCode=df0&hvadid=80470624769002&hvnetw=s&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4584070152943951&psc=1

Found the article I used to lower voltage. The attached is from Dan Torres's book "Inside Tube Amps". Awesome book! Learnt so much from it. Would recommend this book to everybody.

Hope this helps.

ttfn

[Joel]

Update.  I just went and bought 5W 15V zeners.  3 in series in the PT center tap.  Works well, no noise.

[marshallguy]

I lowered my voltage over 20 years ago with my first amp build. Can't remember the voltage/diode numbers, but I used a chassis mount style diode:

https://www.amazon.com/Pairs-70HFR-Spiral-Rectifier-Diodes/dp/B0859C48LQ/ref=asc_df_B0859C48LQ?tag=bngsmtphsnus-20&linkCode=df0&hvadid=80470624769002&hvnetw=s&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4584070152943951&psc=1

Found the article I used to lower voltage. The attached is from Dan Torres's book "Inside Tube Amps". Awesome book! Learnt so much from it. Would recommend this book to everybody.

Hope this helps.

ttfn

-I did the exact mod you listed but had to add in series another zener. I placed a cap across each zener to as I thought filter out noise. Do I even need the caps?  I did scope out these little spikes coming into the preamp stage and they amplifying the further down the audio line I look. The background noise looking at the input to the MV pot has these spikes that look like a very short squeezed sign waves at 120hz intervals. The only place I see big enough 120hz spiked that could be finding their way into the preamp are on the zener diodes. They're on opposite sides of the chassis and my grounding seems to be sound. I have no other explanation as to the possible source other then the CT zeners. It does work, lowers B+ pretty easy mod to do but how do you eliminate or suppress the 120hz?

[sluckey]

I'm pretty certain the zeners are the source.

Well, use a gator clip test lead to connect the PT center tap to chassis. Should prove it one way or another.

[marshallguy]

I'm pretty certain the zeners are the source.

Well, use a gator clip test lead to connect the PT center tap to chassis. Should prove it one way or another.

I'll dig up a pair of KT90's and swap them in beforehand since they can handle the higher screen voltage, re-bias, then take a look see before I report back with the results.

[marshallguy]

I'm pretty certain the zeners are the source.

Well, use a gator clip test lead to connect the PT center tap to chassis. Should prove it one way or another.

Sluckey,

I bypassed the CT zeners, soldered directly to ground completely removing them, threw in a pair of KT90’s, biased them, and lo and behold nothing improved.

I’m using a 2246 4 ch. 100Mhz with a 20 mhz switch for ch. 1. With BW at 20mhz, those little spikes dancing around on the noise floor are clear to see spaced @ 120hz. I don't see any measurable ripple on any of the power supplies any where in the amp except the zeners and they been eliminated as the cause. Either channel, and Clean channel with reverb at 0 only using V1 and V4 before the P.I. they're there too but a lot less noticeable. OD channel much more noticeable....

Coupling caps are all Sozo 500 to 630v rated (except for the .002 at V2) per previous schematic.
Thanks to your suggestion I eliminated the Zener diodes as the cause…… Back in goes the 6CA7 / EL34 CEI Japan and zeners.

With the all the pots at 10, preamp, vol, eq, on the OD, with fluke on hz setting, I'm getting a resonate freq around 2.6k +/- 100hz with no input, might be what I'm hearing chiming, ringing under the notes while playing the guitar. 

Any other ideas?

"failure is not an option”.

[Satchmoeddie]

Sometimes you get switching noises from almost any kind of diodes. Shunt each diode with a capacitor. If the diodes are switching fast enough to make noise the caps charging and discharging will pretty much eliminate all of that switching noise. .01 uF 400, 600 or 630 volt DC work fine.  Running back across zeners to the CT I'd probably opt for 630 volt caps just in case. Shunting the bridge rectifier diodes with caps was very common in lots of amps. There is also a cap multiplier that can smooth out more ripple too. A couple of transistor increase the current, remember BJTs amplify current instead of voltage, so the B+ is allowed to put out more current, and say a 20uF cap with 10x more current available will act like a 200uF cap. I very rarely see that cap multiplier trick used. The zener referenced MOSFET regulator is another good way to reduce the B+. Another neat trick, if the amp uses a rectifier tube is use a different rectifier tube. A 5R4 rectifier can shave off 20-30 volts over a 5U4, and 30-45 volts over a GZ34 and those plug in and unplug. Then there is burning off B+ voltage as heat. Compute the amp's B+ current draw and divide the desired voltage drop by the current which might be >100-300mA. You will need a 20-40 watt power resistor. It will get hot too. If you use an aluminum gold brick resistor bolt it onto the chassis, in a safe place!

[marshallguy]

Sometimes you get switching noises from almost any kind of diodes. Shunt each diode with a capacitor. If the diodes are switching fast enough to make noise the caps charging and discharging will pretty much eliminate all of that switching noise. .01 uF 400, 600 or 630 volt DC work fine.  Running back across zeners to the CT I'd probably opt for 630 volt caps just in case. Shunting the bridge rectifier diodes with caps was very common in lots of amps. There is also a cap multiplier that can smooth out more ripple too. A couple of transistor increase the current, remember BJTs amplify current instead of voltage, so the B+ is allowed to put out more current, and say a 20uF cap with 10x more current available will act like a 200uF cap. I very rarely see that cap multiplier trick used. The zener referenced MOSFET regulator is another good way to reduce the B+. Another neat trick, if the amp uses a rectifier tube is use a different rectifier tube. A 5R4 rectifier can shave off 20-30 volts over a 5U4, and 30-45 volts over a GZ34 and those plug in and unplug. Then there is burning off B+ voltage as heat. Compute the amp's B+ current draw and divide the desired voltage drop by the current which might be >100-300mA. You will need a 20-40 watt power resistor. It will get hot too. If you use an aluminum gold brick resistor bolt it onto the chassis, in a safe place!

Other than the lower voltage power supply diodes or reversed bias ones for protection, I do bypass the rest of the higher voltage diodes with caps. I posted a schematic on a previous post. I made a couple of minor changes and I did remove the diode across the B+ fuse. It took some time rerouting grounds trying different points but I eventually removed what was getting into the preamp.

[mresistor]

Radio Comm   fortunately the 50w zeners I see today don't have to have 5/8ths size holes to drill.

[wdelaney72]

Can someone provide a schematic on how the zener diode trick works?  I am interested in the PT  CT with zener diode to ground method.  Not sure which direction for the zener diode though?

I did a search on the forum and the internet and surprisingly could not come up with a schematic and read conflicting information about this.

Q:  Is the schematic I've attached showing the correct orientation of the zener diode? 

Yes that's essentially correct. You can also stack a number of zeners in series to make the voltage drop add up.


I'm positive I've posted the zener reduction schematic numerous times in the past. Here it is again.


(Edit: Of course, if you have a fixed-bias output stage, you'll want to take into account the power supply source for the bias voltage when considering dropping the HT voltage with this zener method. Some PTs have a separate winding for bias supply, and in that case the bias voltage will be independent of what you do to the HT winding)

Bringing this discussion back up, as I'm retrofitting a Marshall / Friedman Circuit in a Peavey Dual 212. The PT secondaries are 275 each, so I'm getting 565-570 plate voltage unloaded...need to drop it down closer to 500.


Can I do the zener series diode on Center tap if it's connected to the middle of the 2 mains caps like a Marshall 100W superlead or does this require the CT to be directly to ground?