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Help needed lowering B+ for DIY amp

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I am new to this board and hope I have placed this question in the correct area.

I am building a guitar amplifier that uses a 5U4 rectifier, 2ea 6L6 tubes in push pull configuration and 12ay7 and 12ax7 tubes in the preamp. I am using a schematic obtained from the internet for a fender super amp model 5E4.

My project is to build this amp from scrapped parts of an old 1950 TV set, using the chassis, power transformer and some of its other parts and componets. The big old heavy power transformer has a high voltage output secondary centertap winding measures approximately 400 - 0 - 400 volts. (800 volts end to end)

I have completed the wiring of the power suppy circuit (filter caps and choke) and have measured the B+ voltage at 580 volts (without any tube or circuit load). I beleive that this is far too high for the B+ voltage for the circuit that I need which the schematic calls for about 385 vdc. However, what I don't know is how much of a voltage drop will there be when the circuit is loaded down.

What is the best way to go about reducing the B+ voltage to drop it to get it down to where I need it? I just don't know how to calculate the correct resistance needed to drop the voltage or if there is another method to get the B+ down. Should I scrap this idea of using this higher voltage transformer and try locating the proper transformer or does someone have a possible solution?

Thanks for reading my post any advice is graetly appreciated.

Hankster:
:confused:
 
frugal-phile™
Joined 2001
Paid Member
You can use psud to sim it... http://www.duncanamps.com/psud2/

Due to it running under Windows (and my not having the time to waste to put up with window's clunkiness) i usually just make an educated guess based on ohm's law (V=I(1R1+R2+...+Rn)) and the approximate draw of the tubes, and then fine tune the last R in an RCRC...RC chain to get the right value...

dave
 
Hankster - It's not easy to reduce the B+ voltage with plate current being so high. IF you use a tube rectifier, you should have as much as 60V or more drop across the rectifier under normal idle current conditions. You can use a choke input filter which can drop 50V or more (depending on the inductance). This would be LCLC! With large chokes, you will be able to drop the B+ to whatever you want it to be. I wouldn't use resistors to drop the B+ to the output tube plate. The V. regulation will be terrible. The lower B+ voltages for the rest of the amp will be dropped by several series resistors.

I use Electronics Workbench (Multisim) to model circuits, but I am not sure it would be useful in this case. Do you know the plate idle current? This value in mA is very important in the simulation.

Richard C.
 
Hankster,
No cause for panic yet.

The 400-0-400 you measured is NO LOAD.

Once you start drawing High Voltage current and Tube Filament Currents from the transformer this will drop to something like 370-0-370 (or maybe even 360-0-360).

Then when you start drawing High Voltage Current the voltage drops across the 5U4 will increase significantly (from paractically 0V to about 50V) dropping the voltage again.

You will end up with a voltage much less than the 580V you see at present. The 385 Volts specified for that design is not "hard and fast", the circuit will work very well with voltages up to 450 volts or even a little higher and its my approximation/guess/best estimate that you will end up with about 430V when the circuit is all up and running.

Close enough for tube work.

However - make sure your filter caps are capable of handling the full no load voltage - I guess they are or you would have had a big smelly bang by now.

Cheers,
Ian
 
Gingertube is right I'm also building push pull 6l6 guitar amp (for my 17 yr old son). My mains tranny gives 375 - 0 - 375. Using a 5AS4 rect. a choke with 200ohms dc resistance and 100uf filter caps I get 394V B+ with all valves plugged in (2x6l6 4x12ax7). 6l6's are biased at about 55mA each. When testing it I placed a 5k 10W resistor across the B+ and turned it on just long enough to measure the B+. Fender bassman's run 450v B+ so anything up to 450V should be ok
 
Thanks to all who have replied to my post. I have finished up with the buildout of the guitar amp today. The B+ voltage under the full load of the circuitry has dropped down some but it is running about 480 vdc, too high for the design. I am going to try replacing the existing choke with a larger one or may experment with adding another choke in the circuit. Looking back, it would have been much easier if I used the correct value transformer, but I wanted to show off using the existing old huge transformer and chassis from the old TV set. Now I can say I turned the old TV into a guitar amp, with a "little help from my friends"

Again, many thanks for the education.

Best,
Hankster
:)
 
what is the secondary DCR of your power trans. Also what is the values of your filter components including the inductance and DCR of the choke. Knowing those numbers and a approximation of your total current load it is easy to run the psud simulation and come up with variations to fit your amp.
 
Capacitor input supplies, i.e. CLCRC tend to go as high as 1.414 times as half of the High Voltage secondary.
But that does not take into account the voltage drops of the rectifier (due to the load current), the voltage drop in the choke DCR (due to the load current), and the voltage drop in the R (due to the load current).

Choke input supplies (that have at least critical inductance) i.e. LCLCR tend to have up to 0.9 times the secondary voltage.
But that does not take into account the voltage drops of the rectifier (due to the load current), the voltage drop in the choke DCR (due to the load current), and the voltage drop in the R (due to the load current).

Capacitor input supply with 400V half secondary: 400V x 1.414 = 566V (plus the other drops above).

Choke input supply with 400V half secondary: 400V x 0.9 = 360V.

You can see why thread #3 above recommended you use a choke input supply.
It may have more ripple, but one more RC stage, or one more LC stage will fix that.

The Input Choke must have Critical Inductance.
Critical Inductance, Henry = 350/mA of Load current.
For example 350/100mA Load current = 3.5 Henry
In that case the Critical Inductance is 3.5 Henry
All of this inductance must be from the rectifier to the choke.
There can be no capacitor from the rectifier to ground (in a Choke input supply).

Even if you have a Choke input supply, if it is unloaded, it will rise to as much as 1.414 x the half primary voltage (i.e. 400V x 1.414 = 566V.

Be sure to use a good bleeder resistor across the first filter capacitor.
Otherwise when you open up the amp, you will be shocked (by 366V, 566V, etc.)
 
Hi

I use Duncan amps PSUD2 all the time in Linux using 'wine'. It works fine and is a very useful utility!

This is good to hear, thank you for mentioning it.

I also run all Linux here, and have PSUD running under wine - but was unsure if it could be trusted, due to the potential for timing issues etc. with the wine compatibility layer. Very good to know that others use it and all comes out as it ought.. maybe a silly concern, but you know how computer stuff can go. ;)
 
If your power supply is a CLC type, and thinking of it as C1 from rectifier output to ground (the 'reservoir capacitor'), L1 in series, then C2 from B+ to ground (the 'smoothing capacitor'), then try reducing the value of C1 to reduce the B+ voltage.

This will lower the B+, but it's not easy to predict what value of reservoir cap will yield what voltage. You'll have to experiment a little.

What value of reservoir cap do you have in there now? If it's something like 33uF or 47uF, you could try 2.2uF to start (it can be a film cap with high enough working voltage rating). If that doesn't drop enough volts, try 1.5uF. If that drops too much volts, try 4.7uF. Adjust until you get your desired B+ voltage.

Ripple will go up on the B+ if you do this, but in a push-pull guitar amp this may not be a problem.

6A3sUMMER's suggestions should be followed.
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One easy way to reduce B+ is to set up a buck transformer using a 2 amp 12.7v center tapped radio shack transformer. This lowers the 120 on the primary side to 106-114ish (center tapped secondary) which is closer to the 117 a lot of old tube transformers were designed for.
I use one on my 6l6 single ended guitar amp. I was over rating my capacitors till I did this.
It's more efficient to only transform the voltage you need, than to try and get rid of it later.
My power transformer gets hotter from absorbing rectifier tube heat, than the buck transformer which is barely warm to the touch.
I'm a lineman by trade, so transformer theory helped a little, but anyone who can work on tube voltages can figure it out.

Cory


Sent from my SAMSUNG-SM-G900A using Tapatalk
 
Wow.
Lots of advice.

But one of the only ones that made much sense were to insert an 'L' in front of the CLC power supply chain. The voltage drop of a large L can be substantial. Not because (so much) of series resistance, but just the RMS-averaging nature of an initial L instead of a “peak capturing” component like a 'C' capacitor.

The “buck transformer” idea ( of putting essentially a modest-sized inductor in series with the main transformer's primary winding ) is also “kind of OK”, except that ALL THE OTHER WINDINGS are dropped at the same time. This may not be optimal or easy to remediate.

My favorite is just to use the normal "CLC" power supply filtering section, and then follow that with a solid-state series regulator section. Sweetens (removes ripple) from the HV, which is a beautiful result; drops the voltage by nearly any (adjustable!) value, doesn't cost much, and can be removed with a bypass switch at any time. Of course, follow it with a modest-sized C reservoir to markedly lower the section's supply-impedance as seen by the rest of the amplifier circuit. Not obvious, except with experience.

Or, as I've written before: “never use a naked regulator”.

GoatGuy
 
Understood.
You mentioned using 1950s parts. Many of the era transformers are either labeled 115 or 117 volts for the primary side of the transformer. Nominal voltage was closer to 110/220 whereas today it is 120/240.
This change in primary voltage affects your all of your secondary voltages as a result of turns ratio. My 6.3 heaters were running at 7.2v, which is the upper limit for my 12ax7 tubes, and my B+ was 20v above the ratings of the capacitors.
It's not really a matter of 6L6 vs 6V6, I used my amp as an example because it probably consumes more power than yours, so the same transformer would definitely not be over rated in your application . There are several ways to lower your B+. Some involve resistance, some inductance, while mine involves an ac solution instead of a dc solution.
In the end, use what's best for your situation.
I made up an electrical box with a switch, a plug, and the 12v transformer. This way I can plug my 6l6 se, my 6v6 pp, or my 1970ish solid state, and they all operate at their design voltages.
Not THE right way, simply ONE way.





Sent from my SAMSUNG-SM-G900A using Tapatalk
 
My favorite is just to use the normal "CLC" power supply filtering section, and then follow that with a solid-state series regulator section.

That is a great solution. The one and only problem with that is the need to make space for a large enough heatsink on the pass transistor, so that the regulator is capable of delivering and withstanding enough current for a power amp. Figure the regulator will need to be able to pass 150mA plus its own current consumption. 50V dropped with 150mA current draw equals 7.5W dissipation (!!).

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