Adjusting Forplay III heater supply resistor for different tubes

[Doc B.]

Here's some info from PJ regarding changing the resistor in the heater supply of a Foreplay III to handle different types of tubes. Add up the total heater current draw of the tubes you plan to use and adjust the resistor to the appropriate value listed below. The resistor comes before the filter cap, so its value is not linearly related to the current. Here are some values as calculated:

0.3 amps 1.37 ohms
0.6 amps 0.62 ohms (as used in stock Foreplay III)
0.9 amps 0.35 ohms (close to the upgraded Foreplay III)
1.2 amps 0.22 ohms
1.5 amps 0.15 ohms
1.8 amps 0.10 ohms (as used in the S.E.X. amp)

Two 6SN7s in a stock FP-III want 1.2 amps; three in an upgraded FP-III will want 1.8 amps.

[baddog1946]

Hey Doc:
I was working on my "Foreplay III line stage this afternoon and while wiring up the selector switch I found a horrendous error in the build manual.
the copy of the manual I have shows two connections for the red wires to terminal (20) and.
This is definitely incorrect but the photo shows it correctly. If one was to follow the written instructions first you would be making a mistake.
I think you should check the manual text and advise everyone who bought the kit recently to check this.
I don't know how old the edition of the manual I am using is but I ordered my kit on Feb 17, 2010 and received this manual with it.
this is FYI and anyone who has the edition I have. Be careful! Hope no one else has already run into this.
Regards
baddog1946

[Lynxo]

Why 0.62ohms for stock tubes (600mA load) The tubes see around 8.9V even after the series resistor, not 6.3V.

Thanks.

[Paul Joppa]

If you are measuring 8.9 volts across the heater pins with the tubes drawing current, then something is very wrong with your amp. When properly operating and connected to a 120-volt power line, it measures 6.3 volts DC.

If you are calculating 8.9 volts as the raw winding voltage times the square root of 2, that would be the voltage with no load and with perfect diodes (no voltage drop). It is not the operating voltage.

[Lynxo]

If you are calculating 8.9 volts as the raw winding voltage times the square root of 2, that would be the voltage with no load and with perfect diodes (no voltage drop). It is not the operating voltage.

that is correct, just calculating. I was determining tube heater resistance at 10.5ohm, 6.3V/600mA (stock 12au7 x2).

8.9V - .9V (1N5820 voltage) drop, about 8V to .62ohm and 10.5ohm(heater), approx. 7.5V. I'm just trying to get a better understanding.
Maybe I cannot use the 10.5ohm as heater load to calculate final voltage? or the trafo really drops 1.2V during the load?

Thank you.

[Paul Joppa]

The resistor is installed before the capacitor, so the resistor is only effective during the short interval when the capacitor is being charged. The current during that interval is many times higher than the average current, so the resistor drops much more voltage. It reduces the net voltage many times more than you might expect.

This is a very common mis-perception. You can read classic texts on power supplies, but for many people it is more valuable to see it visually. You can download for free the "PSUD" program from the Duncan Amps site; it is an excellent simulation program which can be surprisingly accurate. It will take some effort to learn to use it, but that effort will pay major dividends in understanding. And once you understand how to use it, you can try different configurations easily and broaden your understanding in ways that would take years if you just built things.

http://www.duncanamps.com/psud2/index.html

On the Audio Asylum, especially the Tube DIY forum, there are many threads on using PSUD. My own advice is to be wary of threads that go on very deep, as they are too often flame wars, but otherwise there is a wealth of hard-earned knowledge available there.

[Lynxo]

The resistor is installed before the capacitor, so the resistor is only effective during the short interval when the capacitor is being charged. The current during that interval is many times higher than the average current, so the resistor drops much more voltage. It reduces the net voltage many times more than you might expect.

This is a very common mis-perception. You can read classic texts on power supplies, but for many people it is more valuable to see it visually. You can download for free the "PSUD" program from the Duncan Amps site; it is an excellent simulation program which can be surprisingly accurate. It will take some effort to learn to use it, but that effort will pay major dividends in understanding. And once you understand how to use it, you can try different configurations easily and broaden your understanding in ways that would take years if you just built things.

http://www.duncanamps.com/psud2/index.html

On the Audio Asylum, especially the Tube DIY forum, there are many threads on using PSUD. My own advice is to be wary of threads that go on very deep, as they are too often flame wars, but otherwise there is a wealth of hard-earned knowledge available there.

Simple R-C, simple math but there is more to it.  I actually installed psud2 the other day and tried to sim the heater R-C. got errors  regarding delay timing. have to play with it some more.

Thank you Paul for taking the time to explain.

[porcupunctis]

I just read through this thread and downloaded PSUDII.  I used a load resistance of 6 ohms (two 12au7 heaters in parallel) but I do not know the value for the transformer resistance and that seems to have a fairly big impact on the resulting chart. 

My goal was to answer the question: "What is the specific purpose of the 0.62 ohm resister in the power supply?"

Am I correct that the answer is to limit the amps going to the heaters in order to prevent them from overheating? 

My other power supply question has to do with the jumper going from pin 9 of the 12au7s over to the 150K resistor connected to the 0D3.  Does this mean that the 0D3 is providing regulation to the heater supply as well as the B+?

I have a pretty good "general" understanding of the foreplay III schematic but now that I've gotten mine built I've decided to try to understand things a little better.  Seems like the power supply is a good place to start.

[Grainger49]

I just read through this thread and downloaded PSUDII.  I used a load resistance of 6 ohms (two 12au7 heaters in parallel) but I do not know the value for the transformer resistance and that seems to have a fairly big impact on the resulting chart. 

My goal was to answer the question: "What is the specific purpose of the 0.62 ohm resister in the power supply?"

Am I correct that the answer is to limit the amps going to the heaters in order to prevent them from overheating? 

As Doc says it is not purely a current issue.  The transformer will put out a little more voltage if the current "draw" is lower and vice versa.  The purpose of the small value series resistor is to shed the slightly higher voltage making the voltage across the heater windings within specification (in the FP III case it is 12.6V heaters "folded in half" working as 6.3V heaters).  Look at the table, as the current goes up the resistor value goes down.

My other power supply question has to do with the jumper going from pin 9 of the 12au7s over to the 150K resistor connected to the 0D3.  Does this mean that the 0D3 is providing regulation to the heater supply as well as the B+?

I have a pretty good "general" understanding of the foreplay III schematic but now that I've gotten mine built I've decided to try to understand things a little better.  Seems like the power supply is a good place to start.

The OD3 doesn't regulate the heater voltage.  But there is no ground wire on the heater circuit.  The center tap of the two 12AU7 heaters is "biased" by that wire keeping the heater safer than if the heater were grounded somewhere.  There is a specification that concerns the maximum voltage there should be between the heater and the cathode (which is what the heater heats).  This DC bias makes the heater safer.

[porcupunctis]

OK, I couldn't imagine how the 0D3 could manage two different voltages anyway so I knew that couldn't be it.  You also answered my next question which was about that voltage differential spec on the 12au7s.  There is certainly a lot more detail in this power supply than I'm used to seeing in the old guitar amps that I've worked on. 

Also, thanks for the generous comments on my FPIII build.  I will try to get a shot or two of the "guts" as soon as I get a moment when the power is not on.  I'm in the middle of "Interstellar Overdrive" right now.  I think OSHA would not approve if I handled it very much with the power on. 

Thanks again,