Voltage drop resistor value

[MacGwyn]

Question for one of you math guys... I'm converting a Marshall 18 watt clone to a Trainwreck Liverpool, so I'm working with the 18 watt power trans.  Step one was converting from tube to diode rectifier.  With that done, I'm measuring 383 volts dc from the diodes.  The Liverpool requires 331 volts at the first cap, so I was hoping someone could help me figure the resistor value needed for that 52 volt drop.  It calls for a 5 watt resistor and I may have to combine two to get the value necessary.  I have one schematic in my collection that shows a 22k drops voltage around 100 volts... so maybe a 10 or 12k would do?   

[PRR]

Try a 10k.

But I think you are missing an important point and data.

[MacGwyn]

Try a 10k.

But I think you are missing an important point and data.

Please elaborate... as a rookie builder I need the input.

[thetragichero]

V = I * R (ohm's law)
figure out the current draw of the whole amp (use the power tube data sheets for plate and screen current at your desired plate voltage and ot primary impedance), figure a couple ma for the little bottles
also keep in mind your unloaded b+ will be higher than when it's loaded with tubes

you can determine resistor power rating by combining ohm's law with the power formula to work with the known values P = I * V = V² / R = I² * R, although it's best to use a resistor rated for at least twice the power

[MacGwyn]

Occurred to me after I first posted that I should have specified that this will be a 2 EL84 amp, not 4 as in the original Liverpool.

[pdf64]

The Liverpool requires 331 volts at the first cap

What voltage were you getting with the valve rectifier?

[labb]

The Liverpool requires 331 volts at the first cap

What voltage were you getting with the valve rectifier?

Good question. Been a while since I looked at the Marshall 18 watt but I seem to remember that they ran about 330 on the EL84 plates and that would have been with the tube rectifier...I would think that using an 18 watt PT with diodes would give one about 400 volts.

[thetragichero]

maybe i'm being stupidly simple but.... why not build the liverpool with the tube rectifier if it gives you the voltage you're looking for?

[MacGwyn]

The tube rectifier was putting about 365 volts to the first cap... still higher than the 331 volts the Liverpool schematic requires.  I did build the Liverpool preamp and PI within the 'guts' of the Marshall chassis and it was working.  At this stage, I'm moving on to build the Liverpool power supply section and any corrections/additions on the 2 x EL84 power amp stage.  Thought changing from tube to diode rectifier might affect the amp sound in a positive way... as in "less sag", etc.

[shooter]

being just a tech;
the difference between 365 and 331 through a SS rect. on "tone" is 0.
the difference between 365 and 331 through a tube, might have a tonal impact, since "sag" is a thing

[Willabe]

Thought changing from tube to diode rectifier might affect the amp sound in a positive way... as in "less sag", etc.

Not if those EL84's are being run in class A, or mostly class A.

In class A the amps power supply is already sagged.

[pdf64]

What do the mains (120V?) and 6.3V heater circuit voltage actually measure?

[thetragichero]

Thought changing from tube to diode rectifier might affect the amp sound in a positive way... as in "less sag", etc.

what would be the difference between a tube rectifier's internal resistance and a fixed resistor under load? fender excelsior, for instance, used two big resistors (one on each transformer ht secondary leg) to simulate sag from a tube rectifier

the other option is to find a power transformer that will output your desired dc voltage with solid state rectification and be done with it