Tube Amp Design: Rectifier Tube Capacitor Sizing.

This is another video I've wanted to make for a while, but it was difficult to explain what is happening here, even though I finally understand it. I hope this makes sense, as there is more than just the inrush current working against a rectifier tube!
Hope you find this informative and I plan to do more of these as we work on this 12AX7/EL34 amp!
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[@zebdeming]

I'm working through building my first tube amp, and while reading that you shouldn't use a big first cap, there was never a description like you just presented here. Absolutely fantastic way of describing what is happening, thanks so much for taking the time to do what you do, it is very helpful!

[@DrGIzmoBRad]

What an excellent tutorial on first capacitor sizing in a C-L-C filter. You clearly explained the concept of keeping the ratio of in-rush current divided by in-rush time (in-rush energy) as low as possible to preserve the life of the rectifier tube. I've the same 6X4 feeding a 6uF->0.55H->500uf C-L-C power supply hooked to a low noise 6DJ8 RIAA pre / SRPP line stage for 13+ years now. Looking forward to Pentode Load Lines next🙏:)

[@panheadchopper]

Great video and explanation Stephe,! I'd like to add one thing to the explanation that may help people when thinking of designing a power supply. The choke/inductor part of the supply is sorta 2 devices in one, similar to a resistor and capacitor, It acts like an electrical accumulator and shock absorber.. It cushions the current flow by way of it's internal copper resistance and also stores and releases current in the form of magnetic field energy it builds up and releases through it's magnetic core

[@SkunkieDesignsElectronics]

Great tip! And why I like using a choke if I can rather than just a resistor/cap filter.

[@johnwilliamson467]

Well done . The description is good enough for the intended viewer . Need not be down in the weed about the subject . Good advice about how to make a long lasting tube power supply .

[@SkunkieDesignsElectronics]

I just try to explain things as I understand it. Maybe not in enough detail to design the tube, but we aren't doing that :)

[@garagepunkfan]

Another very informative video. Thanks for putting this together. The "buckets" and "water tower" analogy is perfect

[@SkunkieDesignsElectronics]

Glad it was helpful!

[@debilthomes501]

Great video! Right up there with Uncle Doug 🤠

[@SkunkieDesignsElectronics]

Thanks 👍

[@kubockferre5532]

You are an exellent teacher i understand very well the topic thanks a lot

[@MikeGalusha]

Very well done, this should help a lot of folks. Another thing to keep in mind that I don't see mentioned much is the plate supply resistance, this can vary with the voltage but if it's too low, the tube can be damaged. Often the secondary has enough resistance in the winding to satisfy this, but not always and it may require some resistance between the transformer and plates/anodes. The GE data sheets for 5AR4 and 5Y3 have good info on this. Keep up the good work!

[@cobar5342]

Your best video yet, Stephe. Brilliant.

[@SkunkieDesignsElectronics]

Wow, thank you!

[@interesting7906]

It's also useful to look at the corresponding current diagram that shows the spike you're talking about. Also, it's important to point out why it's not smart to tube roll rectifier tubes without knowing how the amp is put together.

[@balkanart8596]

Excellently explained👍👍👍

[@SkunkieDesignsElectronics]

Thanks!

[@simongrove3276]

Great video and explanation, such a good series, very much enjoying your recent videos.

[@SkunkieDesignsElectronics]

Glad you enjoy it!

[@kb6dxn]

By changing the inductor or cap you can have either the voltage lead the current or the current lead the voltage, it's called the time constant/ phase angle. Tube devices need the current to lead the voltage. Make sure you check for the discharge rate of the cap as well, it does make a difference.

[@eugenepohjola258]

Howdy. Spot on. I would imagine there is an ambition to replace the rectifier tube with Solid State diodes. That would be OK as long as nothing else is changed. There will be an inrush at switch on, though. Someone may reason that with Solid State larger filter capacitors could be used. Perhaps the choke could be omitted. That would not be wise. The transformer current will be really spiky generating much reactive power (VAr). What may happen is that the transformer power rating (VA) is exceeded, possibly resulting in a power transformer burn out. High Regards.

[@hornsby5533]

Thank you for your explanation. I didn't find that in German.

[@RobertKohut]

Great info... 🙂

[@mattrenaud7573]

Nice video. Good explanation of rectifier conduction angle and peak repetitive rectifier current. Might want to do a followup talking about how that high pulse current can put a lot of wear on the power transformer as well. Keep up the good work.

[@SkunkieDesignsElectronics]

Great suggestion!

[@Lu_Woods]

Thanks!!

[@bob_mosavo]

Thanks 👍

[@300poundbassman]

Perfectly Tech talk. Aka All rect tubes have sag. Vs current. 😎🎶🎵

[@SkunkieDesignsElectronics]

Yes they do

[@el_arte]

I don’t think you failed at explaining the importance of not over sizing the first cap. I am just saying the “Limiting Resistor” line on the data sheet is also important.

[@SkunkieDesignsElectronics]

Well the video was about capacitor sizing :) I've found when teaching, it's important to explain one thing at a time and try not to over complicate things. This was never intended to enlighten folks who already have an engineering degree.

[@el_arte]

@@SkunkieDesignsElectronics Valid. Not trying to rattle any cage here. Just noting that a lack of limiting resistance could also cause rectifiers to arc. Carry on, it was a great video for 99% of the population.

[@pstock49]

Maybe learned something today. Thanks. Let me know if I'm correct: If you use the larger cap at the beginning of the smoothing component chain you'll risk depriving the base frequencies of the voltage surge capacity needed for a full and detailed low frequency response. Someone once mentioned to me that for tube amps the big filters that go on the power supplied to the power transformer end up being counter productive because they cut the responsiveness of the amplifier's power supply. It may not be the same with a solid state amplifier power supply.

[@SkunkieDesignsElectronics]

I do know the type of first cap can absolutely impact the bass response. I recently did a video about discovering this.

[@bob_mosavo]

In a KT88 amp build playlist from Blueglow Electronics, he uses a negative temperature coefficient (NTC) thermistor (on the AC side) to limit inrush current.

[@SkunkieDesignsElectronics]

But you can't do that and then go ahead and use a huge 1st cap. That's the point of this video, there is more than just the initial >power on< inrush current at play here.

[@bob_mosavo]

@@SkunkieDesignsElectronics How are you defining a "huge" capacitor? What uF?

[@SkunkieDesignsElectronics]

Over 33uf in a higher current amp.

[@hardball107]

What your describing is that with a higher value cap you have cut the time allowed to recharge with the SAME load as when you were using the lower value cap increasing output load on the rectifier tube because it has to do the same work faster.

[@SkunkieDesignsElectronics]

Yes, that is exactly what is going on. Without the constant load, going to a larger cap would only impact the inrush current filling the cap. But that isn't the only thing at play.

[@Jimantronic]

Really helpful explanation, thank you. I have an amp with 5U4G followed by CLC using one of those dual 50uf+50uf caps with half either side of the choke. Looks like I would be better off with 2 separate caps, say a smaller 22uf before the choke and a larger 100uf or so after it. I’d be interested to get your thoughts and see some testing on different types of caps used here too. Maybe an electrolytic first, then a film cap after the choke would work well?

[@SkunkieDesignsElectronics]

I typically use a film cap for the first if there is room, but I recently discovered the brand used can impact the sound! Some of this is really just trial and error on what it's going to sound like.

[@Jimantronic]

Yeah, apparently so. I've seen a few recommendations for Kemet C4AQ DC Link film caps as a really good combination of sound, price and size. I'm going to give them a go on my amps@@SkunkieDesignsElectronics

[@SkunkieDesignsElectronics]

I don't know about those DC link caps. See this video kzbin.info/www/bejne/rWOkqqupbLd6os0

[@munja2111]

I don't understand why solid state rectification isn't used more often. First, it is cheaper, secondly, there are not as many losses as in the case of rectification through a rectifier tube, and there are no strict limits on the value of the first capacitor in the network. The only drawback is that there is no soft start, but this can be solved with a B+ power delay circuit. Please explain to me why not to use solid state?

[@keeper1855]

More voltage stability using solid! Tubes can't receive that amount of current that solid can that why my amps with solid are stronger with better bas punch using bigger caps you have less distortion from power line but people are blind and can't hear about that...

[@miles5681]

The GZ34 solves so many start up problems due to its B+ delay. But, I agree, SS rectifiers also eliminate the heat from the tube rectifier and the heat from the filiment load on the transformer. Look at some Fender schematics to see the various ways they wired in the Standby Switch. Simple way to delay. It's a manual switch, but what the hey....

[@kubockferre5532]

tube rectifier sound better

[@adamphillips1263]

SS is used on almost everything built new. Tubes are used here because it's an all tube amp. If you want solid state, build a solid state. I'm sure there are tons of builds on YT you could watch. There is nothing wrong with using SS rectification on a tube amp but that is not what this video is about. Please share your build with us!

[@keeper1855]

@@kubockferre5532 haha nice 🤣 specjały when they are weak and can't deliver power to rest of amp.Yea you have absulutly right..

[@carlespy7470]

Thank you for this explanation. I'm a visual learner too. Why isn't the first cap an electrolytic and the second is electrolytic? How do you determine the cap values and the choke value?

[@SkunkieDesignsElectronics]

The first cap can be an electrolytic. PSUD2 software is great for playing around whith choke and cap values and looking at the resulting AC ripple. I'll do a video on that sim software soon.

[@justins.4839]

Just curious, now that youre opening back up the 6sq7 amps and looking at the schematics again for the 12ax7 build, in the videos you made for the 6sq7 build the 2nd cap looked like 270uf not 470uf. What would be the difference between using a bigger or smaller cap in the second position?

[@SkunkieDesignsElectronics]

Yeah I would have to go watch the video myself, I sold that amp and don't have it anymore. And yeah 270 sounds more reasonable :) Edit: Yeah that was a typo, I used a 270uf.

[@Jimantronic]

Just thinking this through, could a too large first capacitor and it’s associated short/high current charging spikes also be responsible for a power transformer getting hot?

[@SkunkieDesignsElectronics]

Absolutely it could.

[@h7qvi]

Leakage inductance of the transformer will limit the narrowness of current peaks to some degree

[@adamphillips1263]

I don't think this video was intended to be educational for Electrical Engineers. It is a basic explanation of how a 1st cap influences rectifier behavior for folks that do not have an in-depth knowledge of electronics. I understand what is going on but feel Stephe did an outstanding job of presenting the information. GOOD JOB!!!!

[@SkunkieDesignsElectronics]

Which is deeper into the weeds than most people need to go :)

[@h7qvi]

@@SkunkieDesignsElectronics If there's enough leakage inductance, you can use a much larger first capacitor, so it's worth measuring.

[@WayneKnight_Rider]

What clamp would you recommend for mounting a large Solen cap (22 or 33 uF) inside a chassis?

[@SkunkieDesignsElectronics]

I am using rubber lined metal clamps that have been carefully pre-shaped so they only very lightly clamp the cap.

[@WayneKnight_Rider]

@@SkunkieDesignsElectronics where are you buying them?

[@SkunkieDesignsElectronics]

Amazon

[@thomasleong9401]

The rectifier tube data sheets will indicate the Max first cap value! Dr Z amps with 5AR4 tubes use 60uf as first cap, and I hve played it for 8 years without rectifier failure. Could it be because it has a Standby HT toggle switch?

[@kubockferre5532]

yes the standby switch is a must for tube life ,warm up 2 min for 6.3 v perfect for eliminate cathode stripping

[@SkunkieDesignsElectronics]

And the cap size is related to the current draw of the amp, as also described on the spec sheet. As others commented, there are other deeper things at play, leakage inductance and the DCR of the transformer windings, possible resistance added to the power supply circuit etc etc. This wasn't meant to be the replacement for an engineering degree, just my attempt at explaining why the size of the first cap matters.

[@Yarach]

@@kubockferre5532 Cathode stripping is not real. IF it were to be real it would be talked about more often as tube ampd esign is here for more than 100 years now in the same way we now know carbon film resistors can get less resistive over time. I have built my own amp without it. The onyl reason a standby switch BEFORE the rectifier was used on amps is back in the times voltage ratings on caps were not sufficient to deal with rush inlet current which is mitigated once the tubes are conductive. Example: My Marshall JVM 210 has a standby switch, but has no use for it. The maximum inrush current never comes above the voltage rating of the capacitors. It spikes to 560, and quicklu settles to somewhere in the 500 range. I demonstrated it to fellow guitarists multiple times. My gear runs just fine many years later. The capacitors are rated for 650 volts.

[@kubockferre5532]

Hot switching on rectifier is really bad for the tube ,if you switch with cathode only ,with a standby its ok . Heat tubes 2 min ,before to put h.t. its the best.Regulation on tube make the tube life longer. .@@Yarach

[@antoniojoseandreomartinez9710]

👍♥️🫂

[@KlaweKlapki]

In simple bigger cap = bigger current in shorter time and tube rectifiers dont like that