Zener diode

[staxmotownjunkie]

Zeners in vintage amps ?????

if you have a 340-0-340 PT that means it comes from an old amp that maybe had a self in the HT suply line or a speaker with excitation coil , acting as a choke-self and lowering the Dc to acceptable values .

so why dont you use OC3 gastubes to lower the tension , they lower for about 90 / to 100 Volts and can deliver max 30 mA ,

But the correct solution is this : you put a self-choke first after the rectifier 's cathode and no Capacitor .. put the elyt cap after the choke ..result will be lower Dc and less riple .
if you are useing modern HT suplys without a choke in the rectifiers line it is usually the AC x 1,4 that the DC will be ; in your case 340 x 1,4 = about 475 Volts ..
with power on lowering to around 425 /450 V depending the current you are drawing from the PT .

I don't see the point in using Zeners in a vintage amp .. waste of power and heat .. wrong technic . a choke in series with the rectifiers cathode to the first electrolyt is the correct technic , no extra heating , no extra warming up of your PT


If I read this thread diagonally it seems to me you are all youngsters that never have seen old amps from the 30's ,40's and 50's am I right ?? the high voltages on the early PT's was because it was common to use a speaker with an excitation coil instead of a fixed magnet . these where used as excitation to build up the magnetic field for the voicecoil of the speaker , the power needed was between 10 to 20 watts depending the type and model .. see old Jensen speakers from that area . these excitation coils where high restance coils DC resistance something between 1000 and 1500 Ohms and a high Inductivity , acting in the HT line as a self coil to get rid of the ripple , in those old years the HT caps where oil filled and not large capacity between 4 and 8 or 10 microfarad , so they needed a big self with high inductivity to get rid of the Dc riple in the Ht line.. ( some of excitation speakers had a lower DC resistance and where used in the cathodes of the output tubes as a common auto bias resistor 50 to 200 ohms Dc ) but yours seem to have come from a high impedance excitation coil of the speaker .. so it is quit normal that if you use it in modern typetube amps the HT ++plus is much to high , in the region of 450 to 500 Volts ..

take a look at old amps and you will see what I mean .. you don't use zeners or fets or any other modern crap .. to try and get it functioning .. you need to use the old technics !

look at old amps from the years 30 and 40 and you will see what I mean . take a look here at L1 in the suply :

http://captain-foldback.com/Hammond_...R40_QR40_1.gif

http://captain-foldback.com/Hammond_...JR20_early.gif

If you take a close look you will see how it works .

cheers , Ben ( 69 years old) and colectioner of old gear ( 1935 - 1960)

International Hammond Festival | De Casino ? Concertzaal

[SoulFetish]

I like choke inputs. But they need to be rated for the full load current which usually makes the obnoxiously large. This is often not practical when retrofitting. Plus it changes the dynamics of the power supply, which may or may not have the desired results. The best solution is a power transformer properly designed for the particular amp. But short of that, your going to pay a price in real estate, money, or heat. A zener diode on the gate of a mosfet is cheap, reliable, and takes up minimal space. It needs a proper heat sink.
RG, for some reason I get database errors when I click the Mosfet follies Geofex link. I've attached a schematic of your circuit adapted for a bridge rectifier which I dug up. It's credited to you and Geofex, I hope you don't mind the adaptation.

[staxmotownjunkie]

Just an extra hint ; if you don't have a big Selfinduction choke at hand you can try to use an old OPT and use the primary connections , leave the speaker side open .. it will work .
Have fun with your experiments . Cheers , Ben

[SoulFetish]

Just an extra hint ; if you don't have a big Selfinduction choke at hand you can try to use an old OPT and use the primary connections , leave the speaker side open .. it will work .
Have fun with your experiments . Cheers , Ben

Wouldn't you need to use a single ended OT?? I would think you would need the air gap to compensate for the DC.
No?

[Leo_Gnardo]

Wouldn't you need to use a single ended OT?? I would think you would need the air gap to compensate for the DC.
No?

I'm sure that would help. As if there are lots of large SE OT's hanging around looking for something to do besides being paperweights. In the previous rant, we are all making the dreadful misteak of not using field coil speakers. Forward - - - into the past! What next, we should all be building back bias amps with directly heated triodes.

[SoulFetish]

I'm flattered, but I don't know if I still qualify as a "youngster". They keep telling me I'm old enough to know better.

[Leo_Gnardo]

I'm flattered, but I don't know if I still qualify as a "youngster". They keep telling me I'm old enough to know better.

If you were born after the invention of the transistor, you're a youngster. If you started your hobby or profession by heating up your soldering iron in the coal stove then you're qualified as a veteran. Extra credit if you mined the coal yourself.

[stokes]

"If I read this thread diagonally it seems to me you are all youngsters that never have seen old amps from the 30's ,40's and 50's am I right ??" Dead wrong.Been working in tube circuits since 1969 when I WAS A YOUNGSTER,working in the local TV repair shop.Stop reading the thread "diagonally" and you will see I am building a Brown Princeton clone of sorts,not an organ amp.No choke involved,and no field coil speaker.Besides,I am using a small chassis and have no room for any choke.The PT I am using is one I had on hand,close but I need the voltage lower.The OT is also something I've had on hand,thats why I'm using 6BQ5's.

[staxmotownjunkie]

Hey Stokes building amps in '69 ?? then you are a transistor man and printed circuits ,
I happen to be working on this stuff at least 10 years before you ..
I built my first modern tube amp in '58 at 10 yrs young with the then modern EL84's in Europe , 6BQ5's in the US, and still point to point soldering , Now if you are building some sort of a Princeton clone why are you using a power transformer from an oldtimer that was at least 25 years older then your age , this is not a PT from a modern amp ; but from the age of field coil speakers , since it is giving firm over 450 volts Dc and in my opinion there where in that time only some weird tubes with topanodes like 807's that are ok for high voltages certainly no EL84 or 6BQ5 , even standard cokebotle 6L6's start already sweating at 400 Volts or you should go for the better 6L6 GC or the 7189A from RCA those can just take near to 475 Volts without risks of starting fireworks inside , anyway what is wrong with Vintage organ amps ?? they where made by Webster , same guys from the cine scene and theatre PA .. long before a guitar freak started even thinking of amplifying his instrument
and I havent' seen Brown Princetons with Fets or Zeners .. yes with a couple of 6V6's and running on around 300 Volts Ht not far over 400 Volts .
I just gave you the hint to use an old OPT as a choke to lower your DC remember ? old OPT's are everywhere lying around and can be useful as such . Ifd you don't like somebody you never met to be a bit helpful Ok then do what you have to do .. I leave you in wisdom fidling with transistors , fets and Zeners .. Have nice weekend man ..

By the way , being almost 70 gives me the freedom to call everyone who is at least 10 years younger then me, as being a youngster hahaha a 10 year older then me is classified as a dinosaur and hope he is still not suffering of Altzsheimers . Can you stand a joke man ?? I know I got a weird sense of humor .. and English is not my mothertongue so sorry about that , but none here understands my language . Just try that Choke now and fidle it somewhere on the bottom in your box , your DC problem will be solved for peanuts and don't worry . I still use my flame heated soldering copper for heavy tinning work
Have a nice weekend

[staxmotownjunkie]

Leo Gnardo .. your friend Fender named Leo used at first a 'fieldcoil ' speaker in his first amp remember , nothing wrong with that ,
efficiency ?? between 98 and 100 dB

[Leo_Gnardo]

Leo Gnardo .. your friend Fender named Leo used at first a 'fieldcoil ' speaker in his first amp remember , nothing wrong with that

Wasn't much choice in 1946*. Fender trained as a radio repairman, a very practical guy. Use the current technology, build amps that could be fixed by any competent radio/TV repair shop, now there's a good idea!

*The same could be said for early Rickenbacker "lunch box" amps starting 1931. They had field coil speakers made by some upstart tyro, James B. Lansing. Yes, plenty efficient! Bright and clear too.

[stokes]

Never worked with transistors and I absolutely hate PCB's.You are not quite ten yrs older than I.Never said anything was "wrong" with organ amps,they are just very different than guitar amps.A Brownface Princeton runs the 6V6's at about 315vdc back in the day,a bit higher with modern voltages today,and the Blackface runs the 6V6's over 400v's and if you look at the PT it is 340-0-340vac,hmmm,exactly what I'm using.IIRC,the PT I have is from the late '50's,not from the 1920's as you assume,you make a lot of assumptions instead of knowing the facts.If you go back and read,again,not diagonally,I said I dont have room for chokes or extra OPT's in the chassis I am working with.I didnt find anything about your first response being a "wierd sense of humor",but rather condescending,assuming people here are "youngsters" "and never seen old amps" even though you dont know anybody here,since you just signed on here in the last month.If you stick around,you will find many of the members here are very experienced and well respected in guitar tube amp knowledge.I am not trying to make a "true to the original" clone and as I said tube guitar amps are a very different animal than an organ or PA amp,so it would be more appropriate to assume you know very little about guitar amps and I think your response and solutions are irrelevant to this situation.

[R.G.]

Never played with Mosfets,I'm listening.

Most of the MOSFETs available today are N-channel enhancement devices. That means they hook up with a positive suply on their drain, the more-negative supply on their source, and the gate to source voltage controls things. A typical N-channel needs its gate about 4-5V more positive than its source, and after that, it conducts about 1A for every volt more positive on gate-source.

You could make a source follower, analogous to a cathode follower, by connecting a MOSFET drain to B+, source to some resistor to ground, then holding the gate at some DC voltage in the middle. The gate held at, um, Vgate, means that if the source sags more than about 5V below the gate, the drain lets through LOTS of current, and shuts off as this current pulls the source up to with about 5V of the gate. It's a follower, but follows about 5V down from the gate.

Now imagine you hook this thing up with the drain grounded, the source to the CT of your power transformer, and a 30V zener from drain to gate. As the source goes negative, nothing happens until the source gets about 35V more negative than the drain. At that point, the zener conducts, pulls up the gate, and the drain lets through current. It acts like a zener of a voltage equal to the zener you actually use, plus the threshold voltage of the MOSFET gate-source. But it acts like a zener with a SHARP conduction knee, because the merest whiff of the zener conducting turns on a lot of current.

So you get your high voltage "zener" which can be set to any actual zenering voltage by picking a different small zener to set the cutin voltage. There are some other parts needed - a 1K resistor from the zener to the gate to stop VHF oscillations, a 12V zener to protect the gate from transients, but otherwise, you get an "active zener" that very well might be cheaper than a real zener of equal voltage and power rating.

[stokes]

Interesting,R.G.,but I've already ordered 2- 10 watt 30v zeners for this build.It seems Mouser doesnt carry them anymore.I put in the part number for the NTE zeners I have and comes up "not found".Found them at a place called Vetco.

[trobbins]

Just to clarify if you don't mind:

How were you measuring the 'voltage drop' - as an rms voltage across the zener, or a difference in B+ before and after?

Is the amp operating with a normal idle loading current on B+ for the test?