Author Topic: Voltage sag problems in tube amp (SS rectifier)  (Read 7216 times)

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Offline ELS122Topic starter

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Voltage sag problems in tube amp (SS rectifier)
« on: June 03, 2020, 03:59:47 pm »
I have a terrible sag in volume after I play something through the amp and it "flubs out" basically, I measured the voltages and found that:
the bias supply is ok
all the B+ supplies are sagging a LOT and even before the choke.

so could the 4uF capacitor before the choke be of too small value? I don't have an LCR meter and I've heard that if you don't know the value of the choke it should better be left alone (the capacitors left alone)
so should I increase the value? and to what value?

here's the schematic of the power supply



maybe the rectifier diodes are dead? I used the only ones I could find and they were some Russian 1N*something* and put 4 in series. since I couldn't stand to wait shipping  ;D
and also it will decrease the pretty high B+. I could use a tube rectifier like a 5Z4S, but I've heard that for 2 6L6's or 6P3S's in this case, it's underpowered and will steal a lot of output power. and also I just don't really want a tube rectifier, I don't really see the point. plus my power transformer got uncomfortably warm when I had the tube rectifier instead of the SS one.
 

Offline greenpossum

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #1 on: June 03, 2020, 04:20:14 pm »
I wonder if your diodes can really handle the current without causing a lot of drop. Do they get warm? Is the 4uF capacitor the original value? It seems a bit low. Pity you don't have a scope to look at the waveform after the diodes. If the diodes or capacitors are inadequate there will be a lot of ripple.
 

Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #2 on: June 03, 2020, 04:27:44 pm »
diodes are ice cold. the 4uF and the 20uF caps before and after are the original value. choke is the same too.

although it originally had the 6P3S's cathode biased, so it wasn't designed for this power with fixed bias. but the OPT by the look should easily handle ~40W or power, and it's also ice cold.
what seems weird is that into an 8 Ohm dummy load it outputs at the absolute max about 10W, but into a 12Ohm speaker it goes up to 48W. maybe my multimeter is measuring some transients or something and coming up with all that voltage (~24VAC) but it doesn't seem like it.

the OPT is 24:1 so 8Ohms should output more power than 12Ohms.
« Last Edit: June 03, 2020, 04:31:53 pm by ELS122 »
 

Offline T3sl4co1l

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #3 on: June 03, 2020, 04:30:18 pm »
What is the choke value, what is the nominal minimum and maximum load current, and what is the design B+ voltage?

Tim
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Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #4 on: June 03, 2020, 04:33:58 pm »
What is the choke value, what is the nominal minimum and maximum load current, and what is the design B+ voltage?

Tim

don't know the value, don't have an LCR meter. haven't calculated the load current, and don't have any schematic of the original amp to know the designed B+ voltage, I'll remeasure the AC voltage before the diodes and calculate the theoretical voltage output.
and then calculate all the current draw.

the AC voltage out of the transformer is
380V-0-380V.
so if I calculate the DC voltage by 380V * 1.4 = 532V and the diodes are about 1V voltage drop so, like 528V
I checked and the diodes are 'D226B'
they're rated for 300mA (ummm.. I think my amp is right up there  :-[ ) and 400V reverse voltage (that's why I put 4 in series)
« Last Edit: June 03, 2020, 04:48:34 pm by ELS122 »
 

Offline duak

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #5 on: June 03, 2020, 04:36:33 pm »
Please also provide the DC resistance of the transformer and choke windings.
 

Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #6 on: June 03, 2020, 04:42:05 pm »
145Ohms for the choke, and 105 Ohms from each side to the center tap on the power transformer
 

Offline greenpossum

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #7 on: June 03, 2020, 04:46:40 pm »
Not 1.6. Should be sqrt(2) =~ 1.4.
 
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Offline T3sl4co1l

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #8 on: June 03, 2020, 04:51:05 pm »
Welllll... pop the choke out of circuit and measure it with a signal generator and voltmeter?

For example 1H will resonate at with an impedance of 1kohm; if DCR is dominant, then it'll have a Q about 7, and a series resistance of R1 = 10kohms can be used to set up a bandpass filter to measure the resonance.
https://www.seventransistorlabs.com/Calc/RLC.html#frq
(in this case, R2 will be the equivalent parallel resistance, a combination of core loss and a transformation of DCR.)

What about the output transformer (impedance, ratio and taps), and output tubes?  Bias resistors?  We can check those, and design backwards to find what it should operate at.

Tim
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Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #9 on: June 03, 2020, 05:09:08 pm »
Welllll... pop the choke out of circuit and measure it with a signal generator and voltmeter?

For example 1H will resonate at with an impedance of 1kohm; if DCR is dominant, then it'll have a Q about 7, and a series resistance of R1 = 10kohms can be used to set up a bandpass filter to measure the resonance.
https://www.seventransistorlabs.com/Calc/RLC.html#frq
(in this case, R2 will be the equivalent parallel resistance, a combination of core loss and a transformation of DCR.)

What about the output transformer (impedance, ratio and taps), and output tubes?  Bias resistors?  We can check those, and design backwards to find what it should operate at.

Tim

all the output components are good, I have the output FIXED bias not cathode biased. I have it biased correctly on BOTH tubes. tubes aren't matched and one of them is a bit weaker than the other.
the output transformer is also good. the RATIO is 24:1
it has another tap on the output but that is for a feedback loop, I am not using is and instead taking the NFBL signal from the output directly.
the phase inverter is a paraphase design.

I'll go try messuring the inductance of the choke whit that frequency calculation thingy

here's a shematic of the output


also I biased the amp at 75% max plate disapation
« Last Edit: June 03, 2020, 05:16:08 pm by ELS122 »
 

Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #10 on: June 03, 2020, 05:31:48 pm »
ok so I calculated about 8.4 henries of inductance at a resonant frequency of 88Hz, resonant resistance is 4.64k, loss resistance of 88.75k, and Q of 19.1
 

Offline TimFox

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #11 on: June 03, 2020, 08:07:04 pm »
When putting lower-voltage rectifiers in series for a higher reverse-voltage rating, it is mandatory to add equalizing components to avoid bad sharing of the reverse voltage due to unequal reverse-bias leakage currents in the individual diodes.  Back in the day, we used between 100k and 1 M with a 0.01uF disc capacitor at each diode. 
Also, the resonant frequency of 8.4 H with the series combination of 4 and 20 uF (3.3 uF) is about 30 Hz, which may be too high (I assume you are in the 50 Hz part of the world). 
There will be a lot of ripple across the 4 uF capacitor:  in 10 msec (100 Hz ripple), 100 mA will pull down the voltage by V = IT/C = (0.1) x (0.01) / (4 x 10-6) = 250 V pk-pk, neglecting the load through the choke.  Your current is probably higher than that.
If we neglect the 4 uF, this could be treated as a choke-input filter, which extracts the average voltage, rather than the peak voltage, from the rectifier.
Once you know the parameters of the transformer and the choke, a good piece of freeware to calculate the results of the rectifier and filter is "PSUD2" from Duncan's Amp Pages:  http://www.duncanamps.com/psud2/download.html
which shows the buildup of voltages, input surge current, and response to a step in load current.  The user interface is somewhat idiosyncratic, but once you figure it out is very powerful.
« Last Edit: June 03, 2020, 08:12:05 pm by TimFox »
 

Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #12 on: June 03, 2020, 08:17:23 pm »
well, I'm reading about choke filters on valvewizard but I'm dumb at math  ;D.
what does
'[2 pi f]'
mean in this equation?
'L = 1 / ( C × [2 pi f]^2 )'
 

Offline mikerj

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #13 on: June 03, 2020, 08:25:23 pm »
2*Pi*F  where Pi=3.1415 and F=frequency
 
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Offline T3sl4co1l

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #14 on: June 03, 2020, 08:44:07 pm »
That's 24:1 for total primary?  And, 8 ohm load, so that'll be 4.6kohm a-a.  In class AB, each tube sees 2.3kohm for the initial class A range, decreasing to 1.15k as the opposing tube goes fully into cutoff.

Seems pretty low, for two tubes and such a stupendously high supply voltage...

Yes, a 16 ohm load would probably be better suited, of course that will likely sacrifice some low-frequency response.  Unless the transformer is in fact designed for 9.2k to 16 ohms, in which case there you go, your load is wrong and that's that.

6P3S is rated 20W or so, so a 15W idle is 28mA at 530V, or 44mA at 340V.  In class AB, the peak current will be maybe 3-4 times higher, or 176mA tops (assuming enough screen voltage to draw that current at Vg1 = 0).  176mA into 1.15 to 2.3k is 200 to 400V.  Maybe 300V mean, since we're going between the conditions in class AB.

530V seems to be an incorrect supply voltage.

8.4H and Q 19.1 is very believable for a choke of that type, that's good.

8.4H resonates with 4uF at 27Hz, so it's not a resonant type filter.

Likely, if the transformer and choke were original and as intended, it was supposed to be a choke-input filter (which gives 0.9 * Vrms or 340V DC).

8.4H at 50Hz mains, full wave rectified, has a critical load of 7.56kohms, or 45mA.  That's one tube at idle, kind of marginal but should be okay.  Load shouldn't be much less than the class A setting so this should be okay.

The supply voltage will swell very high (530V) until the tubes come up.

So yeah, remove the 4uF, and increase the other cap if ripple is a bit irritating.

Tim
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Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #15 on: June 03, 2020, 08:58:39 pm »
That's 24:1 for total primary?  And, 8 ohm load, so that'll be 4.6kohm a-a.  In class AB, each tube sees 2.3kohm for the initial class A range, decreasing to 1.15k as the opposing tube goes fully into cutoff.

Seems pretty low, for two tubes and such a stupendously high supply voltage...

Yes, a 16 ohm load would probably be better suited, of course that will likely sacrifice some low-frequency response.  Unless the transformer is in fact designed for 9.2k to 16 ohms, in which case there you go, your load is wrong and that's that.

6P3S is rated 20W or so, so a 15W idle is 28mA at 530V, or 44mA at 340V.  In class AB, the peak current will be maybe 3-4 times higher, or 176mA tops (assuming enough screen voltage to draw that current at Vg1 = 0).  176mA into 1.15 to 2.3k is 200 to 400V.  Maybe 300V mean, since we're going between the conditions in class AB.

530V seems to be an incorrect supply voltage.

8.4H and Q 19.1 is very believable for a choke of that type, that's good.

8.4H resonates with 4uF at 27Hz, so it's not a resonant type filter.

Likely, if the transformer and choke were original and as intended, it was supposed to be a choke-input filter (which gives 0.9 * Vrms or 340V DC).

8.4H at 50Hz mains, full wave rectified, has a critical load of 7.56kohms, or 45mA.  That's one tube at idle, kind of marginal but should be okay.  Load shouldn't be much less than the class A setting so this should be okay.

The supply voltage will swell very high (530V) until the tubes come up.

So yeah, remove the 4uF, and increase the other cap if ripple is a bit irritating.

Tim

the 4uF cap, choke, and 20uF cap after the choke all are original to the amp. by what I calculated, a 22uF cap before the choke would be ideal.
24:1 is from one side to the center tap, not from plate to plate.

most Russian tube amps I've seen have really high B+ voltages with the 240V mains nowadays. for example, I have a Rigonda 102 that has 400V on the first B+ node.

I don't believe I ever said that the B+ is 530V, I said it's 490V or around that. and drops to 380V under max volume. but I swaped to a tube rectifier (5Z4S) and it now only drops to 420V. and also solid state diodes shouldn't drop nearly as much, well afaik they have 0 sag, but counting in the voltage drop on the transformer under load. I would estimate is to drop like 15V at that node but it drops more than 100V so I am pretty sure those solid state diodes are underpowered, dead, or something else.

I bypassed the choke but that didn't help the sag. so yeah I'm pretty sure I just need new diodes.
 

Offline TimFox

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #16 on: June 03, 2020, 09:05:38 pm »
well, I'm reading about choke filters on valvewizard but I'm dumb at math  ;D.
what does
'[2 pi f]'
mean in this equation?
'L = 1 / ( C × [2 pi f]^2 )'
As noted, that is the angular frequency for a sine wave.  That equation is for the inductor L that will resonate with that capacitor C at frequency f, not what you want in a rectifier circuit.
Note that a choke-input filter for a rectifier is a different concept than a resonant LC circuit.  With a true choke-input filter, the choke tries to keep the current through it constant, and the voltage swing at the node between rectifier and choke is rather large, with a reasonable ripple voltage after the choke.  Again, I highly recommend PSUD2 software, but you need to look at the textbook treatment of rectifiers and filters.  Putting a small capacitor before the choke is a somewhat recent idea;  back in the day, only the choke and output filter capacitor were used, sometimes followed by another L-C stage.
 

Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #17 on: June 03, 2020, 10:57:48 pm »
well again, the 4uF capacitor before the choke was original, it had no standby switch (if maybe it was some weird "pop" elimination cap or something). and all the solder joints were untouched.

either someone magically soldered a 4uF cap that also is the same type of capacitor as the other ones were, also was labeled "C39" and also made the solder joints exactly as the other ones look. then it's a clear sign that it was from the factory like that.
also, this is not an amp from like the 40s, as I've estimated it is from somewhere around 68-75
I am pretty sure a lot of tube amps by that time used capacitors before chokes. 
for example, the JCM800 super lead. it was made from 1959 and had a 100uF cap before a 5 henry choke.
 

Offline T3sl4co1l

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #18 on: June 03, 2020, 11:59:06 pm »
Doesn't mean it was designed properly, or at all.  Audio is full of "it makes sound, ship it!"

Clearly either the supply voltage or the output transformer is grossly mismatched to the tubes in use.  It would be marginal to OK for 6L6GCs, but not 6L6GB/6P3Ss.  Or at 16 ohm load if the transformer losses and frequency response are adequate.

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Offline greenpossum

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #19 on: June 04, 2020, 12:19:30 am »
You say the transformer ran hot when you had a tube rectifier. Maybe this was due to the extra heater draw and the transformer is deliberately underrated because they didn't expect it to be used continuously at full volume hence the droop. Maybe that's just the way it is.
 

Offline TimFox

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #20 on: June 04, 2020, 03:04:17 am »
If you do an actual analysis, you will find a huge difference between having 100uF or 4uF ahead of the choke.  In the heyday of tube amps, one either used a capacitor-input filter or a choke-input filter.  The choke-input filter was easier on the rectifier (peak current), but gave less voltage output, with reasonable efficiency and less droop under load.
« Last Edit: June 04, 2020, 03:07:28 am by TimFox »
 

Offline trobbins

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #21 on: June 04, 2020, 03:57:52 am »
You have the output stage screens at B+ (ie. circa 500V).  I'd suggest the B+ was purposefully designed to sag so as not to damage the screens - that was common for guitar and PA amps.  If you want to stiffen the B+ then you need to manage the screens.

Check your schematic against the amp around the input/PI stage cathodes.

You will need to measure the power transformer primary winding resistance, and use the calculator in PSUD2 to get the equivalent secondary winding resistance.  Prepare your PSUD2 simulation, including the choke resistance, and effective idle current from the output stage as load, and check that the simulated voltage is close to the measured voltage - and preferably post here as you may make some newb mistakes.   Then you can explore what the power supply provides when loading is increased, and confirm that is close to your measured sagged level.  Then you can work through any options you may have - and given you probably aren't aiming to replace the power transformer or choke, then that leaves the filter capacitance values, and the repercussions of changing them.

« Last Edit: June 04, 2020, 03:59:36 am by trobbins »
 

Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #22 on: June 04, 2020, 01:13:04 pm »
Doesn't mean it was designed properly, or at all.  Audio is full of "it makes sound, ship it!"

Clearly either the supply voltage or the output transformer is grossly mismatched to the tubes in use.  It would be marginal to OK for 6L6GCs, but not 6L6GB/6P3Ss.  Or at 16 ohm load if the transformer losses and frequency response are adequate.

Tim

while yes many tube amps old and new had useless or just weird parts of the circuit.
the output transformer AGAIN, is 24:1 from one plate to the center tap, not from plate to plate. that would mean that if measuring from plate to plate it would be 48:1

You say the transformer ran hot when you had a tube rectifier. Maybe this was due to the extra heater draw and the transformer is deliberately underrated because they didn't expect it to be used continuously at full volume hence the droop. Maybe that's just the way it is.

AGAIN, the amp was originally CATHODE BIASED. I changed it to FIXED BIAS. which would usually draw MORE POWER.
ALSO, I tried playing the amp trough another outlet in another house and it was nearly ice cold. this would make me think it may just be a bad power factor problem or something.
and ALSO, it is pretty normal for a power transformer to run warm when in use. BUT it's running a bit too warm for comfort. so it's either not made for a fixed bias ~40w amp (which it is not), or it's shorted, which if it was I am pretty sure it would be hot not just lukewarm.

If you do an actual analysis, you will find a huge difference between having 100uF or 4uF ahead of the choke.  In the heyday of tube amps, one either used a capacitor-input filter or a choke-input filter.  The choke-input filter was easier on the rectifier (peak current), but gave less voltage output, with reasonable efficiency and less droop under load.

yeah cause 1. my amp the 4uF capacitor is too small. it resonates at like 30Hz or something, also my choke is 8.4 henries while the one on the marshall amp is 5 Henries. by calculating the resonant frequency, it comes out to 7.1Hz resonant frequency.

You have the output stage screens at B+ (ie. circa 500V).  I'd suggest the B+ was purposefully designed to sag so as not to damage the screens - that was common for guitar and PA amps.  If you want to stiffen the B+ then you need to manage the screens.

Check your schematic against the amp around the input/PI stage cathodes.

You will need to measure the power transformer primary winding resistance, and use the calculator in PSUD2 to get the equivalent secondary winding resistance.  Prepare your PSUD2 simulation, including the choke resistance, and effective idle current from the output stage as load, and check that the simulated voltage is close to the measured voltage - and preferably post here as you may make some newb mistakes.   Then you can explore what the power supply provides when loading is increased, and confirm that is close to your measured sagged level.  Then you can work through any options you may have - and given you probably aren't aiming to replace the power transformer or choke, then that leaves the filter capacitance values, and the repercussions of changing them.



Yes I am thinking about moving the screen supply to a different node,


I already have concluded that I just had bad rectifier diodes.
 

Offline ELS122Topic starter

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #23 on: June 04, 2020, 01:48:30 pm »
I put a 100 Ohm resistor in series with the choke (before it but after the 4uF capacitor) and measured 4.7V across is when at idle. so 47mA and the voltage after the resistor was 485V at idle. at full volume it goes up to 15V across the resistor, so 150mA. when at full volume the voltage after the resistor was 385V.
this is with a 5Z4S rectifier.

by using PSUD2 I calculated the source resistance of 130.5 Ohms. turns ratio of 1.6

the load resistance, I have no idea how to calculate it  :-\

ok nvm, I figured it out. and even tho it was pretty clear that my diodes were the problem. it looks like it's not really the diodes fault. the voltage should drop about 100V in theory

now how do I fix this?}

nvm x2. PSUD2 is soo useful  :D
I found out that if I replace the 4uF cap with 22uF everything is better. the sag is MUCH lower and the ripple too.

also, I drew some load lines, and found out that I forgot to half the output transformer impedance when drawing it before. so yeah with an 8 Ohm load the max output is 8.5 watts class B1 and with an 16 Ohm load it's 27 Watts  class B1, on paper. so yeah it all makes sense now. thank's for making me recalculate the output tube output  :D and with the power supply problems  :-+
« Last Edit: June 04, 2020, 03:03:06 pm by ELS122 »
 

Offline TimFox

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Re: Voltage sag problems in tube amp (SS rectifier)
« Reply #24 on: June 04, 2020, 02:49:09 pm »
One detail about PSUD:  the transformer parameters used are the no-load voltage and secondary resistance value, although manufacturers normally specify the voltage at full load.  If you know the efficiency, or measure it directly, you can estimate the no-load voltage for the computation.  Note that if you use a proper choke without a capacitor before it, the voltage will be lower, but it will not fall so fast with load current.  There is a minimum value of current for this effect, for which I refer you to the textbooks (see "critical inductance").
Also, with a vacuum rectifier such as the 5Z4, there is a limit to the capacitance used in a capacitor-input filter, due to the stress limit on the peak cathode current.  The 1939 RCA data sheet recommends 8 uF max (unless you have a large source impedance), or 5 H minimum for a choke-input filter.
« Last Edit: June 04, 2020, 03:00:56 pm by TimFox »
 
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