Need for twin 5U4GB rectifiers in baldwin organ amplifier?

[RobCalm]

Hello all. I posted this on another forum, but it was awfully quiet over there. Hopefully someone here can help me out...

I currently have two identical amplifiers from two baldwin organs. They are each three channels - two channels using 12AX7 and 2x 6BQ5, and one channel using 12AX7 and 4x 6BQ5. My current project is to combine one of the power supplies (choke with 2x 5U4GB) with the one large channel (4x 6BQ5) from each amplifier on a nice new chassis. Seems simple enough.

My question is, are two 5U4GB tubes required? The power supply in these amplifiers also powered the motor for a leslie, and whatever else in the organ, so I figure that it has a bunch more to give than the audio amplifier requires. I was hoping that the answer would be 'no' due to chassis space issues, but having looked at tube ratings and stuff as best I could (I'm new to this), it seems like both rectifiers may be required.

I haven't measured the secondary voltage from the power transformer yet, so I just looked at maximum ratings from spec sheets...

Here's what I came up with:

5U4GB with filter input choke, supplied with 450v and outputing 175 mA at the anode should give 78 watts peak and 55 watts RMS.

12AX7 has Pa max (anode power dissipation) of 1 watt

6BQ5 has Pa max of 12 watts (8.5 watts RMS??)

So combined max power requirements (2x 12AX7 + 8X 6BQ5) = about 70 watts RMS. I concluded that if running the 12AX7s and 6BQ5s up to their max, the both rectifiers would be required.

I realise that my calculations are crude and don't take many things into account, but in general, am I correct?



Rob

[Ty_Bower]

I'd say go with two. Look here:

http://www.nj7p.org/Tube4.php?tube=5u4gb

Characteristics and Typical Operation
AC Plate Supply Voltage Each Plate (RMS) ...... 450 V
Input Inductor ........................................... 10 H
DC Half-Load Output Current ...................... 174 mA
DC Half-Load Output Voltage ...................... 355 V
DC Output Current .................................... 348 mA
DC Output Voltage .................................... 340 V
Voltage Regulation (Half-Load to Full-Load) ... 15 V

I figure a typical 6BQ5 eats about 35mA at idle. Eight of them will require 280mA. At full load you will need much more than 280mA. Under the operating conditions described above, a single 5U4GB can only provide 174mA at half load, which is less than what your output tubes need at idle.

[boywonder]

As Ty already mentioned, the limiting parameter is max forward current for the rectifier, assuming that you do not exceed the voltage rating. Exceed that, and sparks fly inside the tube, sometimes rendering it toast and sometimes rendering it still useful, depending on whether or not the arcing produces carbon tracks in the tube.

Are you planning on still using a choke input? With Capacitor input, the value of the first cap also has a large effect on the forward current of the rectifier. If the first cap is too large the rectifier will arc over, that's why rectifier data sheets call out max C for cap input. Choke input is a little easier on the rectifier IIRC.

In my incredibly limited experience, I had a 5AR4 and 5U4 both arc over with a B+ of 450V or so and 220ma of current, but I may have screwed something else up to cause the problem.......

SS rectification is another option..........

Since you have two of everything including power transformers, have you considered building monoblocks with 4 ea 6BQ5's?

Here is another datasheet for the 5U4GB:

http://tdsl.duncanamps.com/show.php?des=5U4GB

Looks like 270 ma is the max forward current.

[Brit01]

Maybe you're aware of this software.

Duncan Amp Tools : psud2

Great piece of freeware to simulate your power supplies. It will warn you when you exceed the tubes current rating.

[Ty_Bower]

Originally posted by Brit01
Great piece of freeware to simulate your power supplies.

Absolutely! Definitely a good piece of software to have around.

[RobCalm]

Thanks everyone for the input. I hadn't really considered current requirements as the focus (was thinking in terms of power, which I see isn't the usual approach).

I' m trying to recycle as much as possible to keep the cost down, so I plan to use the choke, and keep tubes in the power supply for now. I want to leave room for future tinkering too. I had briefly considered monoblocks, but the chassis I have are quite large and seemed overkill for a single channel... I wanted to create something tidy from something quite messy, if you know what I mean. Maybe I will reconsider. The power traffos are enormous.

Also, thanks for the PSUD idea. I have known about this software off and on for a while, but never got in there and looked at it. I'm motivated now.

Rob

[RobCalm]

Also, are the 'half load' ratings actually half of what can reasonably be drawn out of the tube on a continuous basis, or does it indicate a sensible operating point instead?

Rob

[boywonder]

Download and model your power supply in PSUD. It's easy to use, free, quite accurate and there are loads of folks here that can answer questions. You will quickly see if your solution will work, and with some playing around, you will see how close to the edge you are with current.

A couple of pointers to get started in PSUD:

Select full wave vacuum tube in the rectification box, and select 5U4GB.

Change the load from resistive to current, and use the estimated total idle current of all of the tubes (roughly 280ma for the 6BQ5's and whatever the idle current is for 2 12AX7's driver tubes) as the current load.

Measure the R of the power transformer secondaries and input that in the transformer specs box along with the rated voltage. This increases the accuracy of the simulation.

Run the sim with a timescale of 6 seconds or so and look at voltage at the load, and current thru the rectifier. If you exceed the forward current of the rectifier, PSUD will pop up a warning message "forward current has been exceeded at XX time".

After you do some sims, change the tube to 5AR4 for giggles, you should see a higher B+ voltage, since it drops less volts than a 5U4. The 5AR4 has a different forward current rating, so you can get a feel for the max current it can handle. You can also look at other flavors of 5U4's.

There are lots of other things that PSUD can show; start with the above to see how a 5U4GB will work in your circuit.

[RobCalm]

Thanks Boywonder,

I'll have a go at this in the weekend