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Can 5AR4 be substituted with 5R4G variants?

grindfix

grindfix

Full time hobby
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This is for amp build from organ parts. Original 5AR4 + 2 series connected chokes put out 520v.
I'm looking to get 450v or close to it. It looks like 5R4G/GY/GYA can drop additional 57v.
I get 428vac to center tap from transformer. Amp uses 2 x 7247 and 4 x 7591 tubes.
Thanks,
Dmitriy
 
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The best way to get a good idea of what your B+ will end up at is to use PSUD2. it is a free download. I don't think you have provided enough info to make a good estimation. Personally,, if you want to drop volts, make your PS choke input. This is better than a cap input ps and it is also easier on the tube rectifier with the voltages you are considering.
 
I mentioned that it is choke input in original post. 2 chokes in series.
I do not know transformer or choke specs to use psud2.
 
The 5R4 will certainly give you the voltage drop that you are looking for,or pretty close to it,depending on how much current you are pulling.

But keep in mind that this is a directly heated rectifier,and the DC output will be there almost instantly.Unlike the 5AR4,which will ''ramp-up'' almost in step with the other tubes coming to life,there will be no delay with the 5R4,which may stress your filter caps,depending on their voltage rating.
 
Amp is pulling about 150ma at idle at 35ma per 7591 in PP. This is with 300 ohm resistor in first voltage stage to achieve 450v at 122vac input.

I looked at both rectifiers data and looks like both are cathode heater common. What makes 5R4G different to 5AR4 in terms of output timing?
 
grindfix said:
Amp is pulling about 150ma at idle at 35ma per 7591 in PP. This is with 300 ohm resistor in first voltage stage to achieve 450v at 122vac input.

I looked at both rectifiers data and looks like both are cathode heater common. What makes 5R4G different to 5AR4 in terms of output timing?
Click to expand...
The 5R4 series is directly heated, meaning there are just the two plates and a filament. The 5AR4, on the other hand, is indirectly heated, as it has a cathode between the filament and plates. The cathode allows for a controlled warmup compared to the 5R4, which warms up and starts conducting much quicker. Another indirectly heated rectifier out there is the 5V4, though you might want to check the ratings to make sure that it's not going to be overloaded in your circuit.
-Adam
 
The 5V4 is an interesting tube,and is usually available for very small money. Unfortunately,it has a really poor reliability record that is rather well known.Some manufacturers/versions are ok,but who's to know for certain which ones to trust.I have always regarded this type as similar (little brother?) to the 5AR4 due to it's indirectly-heated cathode and it's low voltage drop.Not good for your application,as you'll only loose 25 volts or so with this type.

The early (and rare) metal type is rated for 125ma,the later glass type is good for 175.
 
One would hope you have a multimeter that can measure resistance? You can then measure the PT winding resistances - that is the main measurement, along with secondary winding voltage which you gave in post #1, and an estimate of output stage idle current (from the valve datasheet design centre max anode power dissipation), to provide a PSUD2 estimation. It is well worth doing, given that you have a few options with how to set up the power supply (and can check out whether the 5V4 is ok or not), and you are pretty much 'designing' the amp and not working from someone else's schematic.

If you post further info then we can guide you through the first use of PSUD2.
 
trobbins said:
One would hope you have a multimeter that can measure resistance? You can then measure the PT winding resistances - that is the main measurement, along with secondary winding voltage which you gave in post #1, and an estimate of output stage idle current (from the valve datasheet design centre max anode power dissipation), to provide a PSUD2 estimation. It is well worth doing, given that you have a few options with how to set up the power supply (and can check out whether the 5V4 is ok or not), and you are pretty much 'designing' the amp and not working from someone else's schematic.

If you post further info then we can guide you through the first use of PSUD2.
Click to expand...

Absolutely, measuring resistance is not a problem. PSUD2 is a little complicated and confusing for me. I poked around and at one point I entered transformer measurements into it. Not sure if I did it correctly. I don't know manufacturer's nominal data program is asking for. I selected 5AR4 rectifier with choke input filter and got some kind of error that I have no clue what to do with. Also, results on the left, below the drawing are in Greek to me.
Could you possibly walk me through it?

Thank you!
 
120vac (122vac measured on average) 60hz mains
Primary is 60ohm
Secondary is 84ohm
Secondary voltage with rectifier pulled (unloaded?) is 422vac against center tap (ground). That would make 844vac, correct? My meter limits me to 700vac.
 
For a 'new' project, you see a default power supply schematic. Right click on the left side of the schematic to 'yellow' select the power transformer and diodes region. Right click that yellowed region to 'change' and select vacuum tube and full wave. Double click on the transformer symbol and you get "edit transformer properties' dialog box. Click the small "..." box beside Ohms to bring up a Source impedance calculator box. Insert the primary winding voltage and resistance. Insert the secondary details (this is for one 'half' of the secondary winding (ie. 422V and 84 ohm - assuming 84 ohm is the resistance from one anode to CT), and 'ok' back to the main schematic.

You should then get 422V and 801 ohm showing in the main schematic. Double click the valve and change to 5AR4. For starters, just double click on the capacitor and change to 50uF. Go to Options menu and click on 60Hz in Mains Frequency, and ensure soft start is not ticked. Save the .psu file to your project directory.

Choose 300ms and 0 secs delay in the simulate area. Click the Simulate button, then tick the V(C1) and I(R1) result parameters, and the output waveforms should show output voltage and current rising to 400VDC and 80ma.
 
trobbins said:
For a 'new' project, you see a default power supply schematic. Right click on the left side of the schematic to 'yellow' select the power transformer and diodes region. Right click that yellowed region to 'change' and select vacuum tube and full wave. Double click on the transformer symbol and you get "edit transformer properties' dialog box. Click the small "..." box beside Ohms to bring up a Source impedance calculator box. Insert the primary winding voltage and resistance. Insert the secondary details (this is for one 'half' of the secondary winding (ie. 422V and 84 ohm - assuming 84 ohm is the resistance from one anode to CT), and 'ok' back to the main schematic.

You should then get 422V and 801 ohm showing in the main schematic. Double click the valve and change to 5AR4. For starters, just double click on the capacitor and change to 50uF. Go to Options menu and click on 60Hz in Mains Frequency, and ensure soft start is not ticked. Save the .psu file to your project directory.

Choose 300ms and 0 secs delay in the simulate area. Click the Simulate button, then tick the V(C1) and I(R1) result parameters, and the output waveforms should show output voltage and current rising to 400VDC and 80ma.
Click to expand...

422v is voltage from each anode tap to center tap
84ohm is resistance from one tap to another, not to center tap (CT I assume?)
40.8ohm one anode to CT
44.7ohm another anode to CT
 
OK, so go back to the source impedance calculator box and choose 45 ohm for the secondary winding. That will change the total effective resistance to 763 ohm.

60 ohm primary winding seems pretty high - you may want to double check that measurement.

The 7591 has a 19W dissipation rating. A quad would perhaps dissipate 4x15 = 60W ballpark once idle is set up. 400V at 80mA is only 32W. If you double click the load resistor and change from 5k to 2.5k you get 340V and 135mA, so about 45W.

If you left click on the capacitor region in the schematic and right click the 'yellowed' area and choose 'change', then select LC filter, then the main schematic will show a 5H choke before the 50uF cap. If you 'simulate' again, then the plot will show the output ramping to 280V and 110mA.
 
My mistake. I took primary measurements at CL-80 limiter.
Primary is 1.7ohm.
5AR4 tube data suggests 10H choke. I don't know what resistance to select for choke and for capacitor
Error in the picture.
 

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thats a much better primary resistance! do you know the inductance and resistance of the 2 chokes you have?
 
Well you can just aim to use the chokes without knowing the inductance, and use final measured ripple voltage at idle as an indicator of whether that suits your amp. That series resistance of the chokes is very low, so I'm not sure they are really meant for B+ filtering. The simplest way I've found to measure choke inductance at dc operating currents is in the linked article:
https://www.dalmura.com.au/static/Choke measurement.pdf

My sim now has 57 ohm effective secondary resistance. With a 5AR4 and capacitor input filter yes will get about 520V. With 130mA loading and a 50uF cap, the diode plate continuous rms current is circa 200mA (so well below 825mA max from datasheet), but the first pulse after turn-on reaches 4.5A which is somewhat above the allowed 3.7A datasheet rating. A 30uF cap would keep the initial peak current pulse under 3.7A.

If you operate the output stage with cathode bias, and identify the idle dc voltage drop across the output transformer primary winding resistance, then the idle plate-cathode voltage would be somewhat under 500V. Using extra RC or LC filter for each output stage B+ would drop a few more volts, so getting down to 450Vak.

5R4 appears to be more sensitive to initial peak current - there is no datasheet rating I can see so it would need to be inferred from the boundary conditions - and so a CLC or CRC filter with small first C may well be needed.

The use of PSUD2 for that initial pulse assessment is quickly described on page 6 of linked article:
https://www.dalmura.com.au/static/Renovating PA amps.pdf
 
Amp is not cathode biased.
It has individual bias adjustment per output tube. Current is measured across 10 ohm cathode resistor.
I will have to study links you've referenced to tomorrow. Getting late here in US. I'll have questions ready for you when you wake up :)
Thank you!
 
Maybe just me, but had no luck here substituting a 5R4GYS for a 5AR4. Amp wouldn't bias up properly with it in. I could get the mV on a cold start, but it'd fade fast as it hated. Got lucky and scored a couple Mullard GZ37 and have been running those. Some say the spec isn't much better than that of a 5AR4, but I find them pretty much bulletproof, driving them hard with a set of KT120's with VT231 drivers at smokin' spl's ...
 
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