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Old 9th February 2006, 08:08 PM   #1
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Default Help needed lowering B+ for DIY amp

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:
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Old 9th February 2006, 08:45 PM   #2
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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
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Old 9th February 2006, 11:38 PM   #3
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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.
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Old 10th February 2006, 02:14 AM   #4
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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
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Old 10th February 2006, 06:38 AM   #5
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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
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Old 11th February 2006, 08:21 PM   #6
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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
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Old 11th February 2006, 09:23 PM   #7
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to be honest i would aim for a B+ of about 430 - 450VDC, shoudlnt be too hard to get it down to this and it should run quite nicely. My 6L6 Guitar amp is running 440VDC B+ and it is a very nice amp!
Hope this helps,
OWen
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