Determining current consumption of tube circuits

[H.R. Shove and Stuff]

Hola,

 

 

 

I've been looking at the two tube circuits that G Forrest Cook has on his Solorb site and I've been trying to read up on how to determine the current needed for them. I'd like to put both circuits in an enclosure to save on space as well as money with only one transformer. I've been having a hard time figuring it out, so I thought I'd post my analysis and see what smarter brains make of it.

 

 

 

Let's start with the liquidator

 

 

 

http://solorb.com/elect/musiccirc/liquidator1/

 

 

 

I've found you can determine max current that can be passed by doing supply voltage/plate resistor. In that case, going from left to right on the schematic, we get 1.9mA + 18mA + 18mA + 18mA + 3.9mA + 2.6mA = ~63mA.

 

 

 

I've read that these numbers are the max the tubes could draw, but it's unlikely that they will ever draw that much. How would I determine what is a likely number? Do I split it in half and hope for the best?

 

 

 

The second schematic has me more baffled as it employs a push pull type poweramp using a 12AX7.

 

 

 

http://www.solorb.com/elect/musiccirc/fuzznikator1/index.html

 

 

 

I would forgo the last triode as I would just switch between clean and distorted instead of blending them, but the first two come in at 2.5mA using the method above.

 

 

 

Cook mentions 10k resistance on the primary OT, so I assumed it would be something like 180V/5k, just guessing on the voltage for B+2, would be 36mA for both.

 

 

 

That puts us at 140mA total, excluding heaters. Is that reasonable ? It sounds unreasonable, but I don't know anything. Thank you for any guidance!

[WRGKMC]

The transformer uses a 1 amp fuse so the entire circuit is drawing less than that. From what I'm understanding, you're wanting to run two of the same circuits off the same transformer.

 

You're dealing with two different sets of supply voltages for the two circuits. You need to realize its more then just current you're dealing with. You have Voltage, Current and Resistance in DC Circuits. The first one requires 255 & 185V DC bias and 6.3 DC filament voltage plus a full wave bridge rectifier to get the 7.3v dc voltage for the LFO circuit. The second uses 250V bias, a decade resistance for variable biases, and a 6.3 AC filament voltages.

 

The two transformers are the same but you'd be adding the filament load and you'd have to redesign the entire power supply circuit. You cant just connect the two to the same transformer leads and expect each to work properly. Think of it this way. What happens when you connect two speakers in parallel or series. In parallel, the total resistance drops (it becomes more conductive), In series, the resistance increases (becomes less conductive) If you connect the two circuits in parallel, you will make the transformer voltage feed through two different paths and the current draw will be the sum of the two. Same fro the heater circuits.

 

You'd need to integrate the two supplies into a single conversion circuit and all the resistor values would change in order to create separate B+ supply voltages for each circuit. You'd be better off buying a Cad program designed for electronics and then you can just plug in the proper values and come up with total load parameters. Doing it the old fashioned way using electronic formulas and a calculator takes allot of experience. I haven't done those kind of computations for a good 35 years. I'd need to breadboard the circuits to come up with real world parameters. My education predates these newer electronic Cad programs being used today.

 

You need to get the Transformer specs too and see if it can handle the additional loads. Hammond makes some good transformers that last decades, but running them below maximum specs is usually the norm. I don't know if that transformer will run up to 2A for the additional heater voltages or not but that's where you'd want to look first. Otherwise you'll need a heftier transformer to do the job and that adds to the weight which is what you're looking to avoid. Unfortunately this doesn't give you the exact answer you're looking for but it does reveal the complexity of what you need to address.

 

The transformer only has a 71ma secondary. running two complete circuits may blow the secondary if that second circuit draws as much as the first.

 

Specifications: [TABLE=class: bullet specification_table]

[TR]

Size: [TD]43 VA[/TD]

[/TR]

[TR]

Dual Primary: [TD]115 VAC, 125 VAC, 60 Hz.[/TD]

[/TR]

[TR]

A.C. high Voltage Secondary RMS: [TD]380V.C.T. @ 71mA[/TD]

[/TR]

[TR]

Filament Winding #1 [TD]6.3 V @ 2.5 A[/TD]

[/TR]

[TR]

Filament Winding #2 [TD]-[/TD]

[/TR]

[/TABLE]

 

[H.R. Shove and Stuff]

Thanks for the reply WRGKMC.

 

The idea was to put both the liquidator and the fuzznikator in the same circuit to save on money, using one transformer, as well as space. The transformer I was thinking of using is from Antek, specifically this one: Antek 50VA 240V Transformer

 

It's got two 6.3V windings at 2A each, which I figured I could use one winding for each individual circuit, or perhaps parallel them and supply them all off the parallel windings.

 

Then use the 220V tap to get roughly the rectified ~250V needed for both circuits B+1. The B+2 and 3 are already in parallel in the liquidator, so I thought I could simply tap another parallel voltage out for the starve voltage for the fuzznikator.

 

I had been told that each circuit will probably only draw about 20mA each, possibly 50mA total, for B+ voltages, so I was thinking the 220V 0.1A winding would be able to power both circuits without a problem, as well as the heaters handling a little more current from whatever LFO I put in there, especially if I parallel them to distribute the load.

 

Does any of that sound right to you? Hopefully I've explained the plan a little better than in my first post. I do have LTSpice, although I've tried to simulate tubes.

[WRGKMC]

It's got two 6.3V windings at 2A each, which I figured I could use one winding for each individual circuit, or perhaps parallel them and supply them all off the parallel windings.

 

Definitely do not parallel them. You're not thinking about the transformer itself. Its not a regulated supply with buffered outputs that can be combined. If you parallel the winding you'll be shortening the secondary wind and will blow the transformer. The transformer's secondary may have taps, but its still one long wire with a specific internal resistance. If you parallel the winds, you shorten that wire length and the resistance reduction would be enough to blow the secondary.

 

Then use the 220V tap to get roughly the rectified ~250V needed for both circuits B+1. The B+2 and 3 are already in parallel in the liquidator, so I thought I could simply tap another parallel voltage out for the starve voltage for the fuzznikator.

 

Ohms law comes into play here. You need to crack out your calculator and do some computations. Voltage and current change when the load changes. E = I x R Additional circuitry will have more resistance in parallel. If current consumption goes up your supply resistors have to change.

 

 

 

 

 

 

I had been told that each circuit will probably only draw about 20mA each, possibly 50mA total, for B+ voltages, so I was thinking the 220V 0.1A winding would be able to power both circuits without a problem, as well as the heaters handling a little more current from whatever LFO I put in there, especially if I parallel them to distribute the load.

 

The B+ isn't directly connected to ground. If you look at the B+ going to the tubes and follow any branch it terminates at the Plate or a cap. Once the circuit is charged with B+ there is essentially no additional load on the power supply with it setting idle. What does create a load is the AC signal when its making the tubes conduct. AC will pass through caps. DC wont. Just how much load the SC puts on the DC supply depends on signal strength and the field effects going on inside the tubes and caps. The 12AU7 tubes have a 22ma rating each. You have three of them in the first circuit in parallel plus an additional tube at the end which is likely in the same range.

 

The first three tubes can produce a maximum of 66ma. You usually give it a safety factor of 10~20% minimum. Of course these tubes only consume that much at maximum. Normal operating occurs between 30~70%. 70.7 is an RMS value. 50% is pretty typical for many clean tube circuits. So running the tubes clean would need a minimum of 11ma each. with the 4th tube they may require 44ua most of the time up to a maximum of 88.

 

Filiment current can be between 150 to 300 ma. Lets just use the maximum there for now. 4X300 = 1200ma. That's too high for this transformer. If we use the minimum of 150X4 that's 600ma for add you maximum plate current and you're looking at 680ma.

 

If the secondary is .1A (1000ma), when you add in the heater current from two circuits, all parallel, you can be well over the transformers ability to handle both circuits. I haven't looked at the actual current draw by adding in the power supply resistance. I'm sure thay would drop down the current in the B+, but the heaters are hard wired and they would be the deciding factor.

 

You're looking at 7 heaters. If each draw the minimum of 150ma that's 1050ma just with the heater circuit. If it was 300ma, that's 2100ma. Neither will work with a 1000ma transformer without blowing it.

 

You may be able to get a separate heater transformer? Small but can handle the current?