Been using tube amps for 50 years and the only time I blew a fuse is when I switched the power on when the standby was in "play". Only happened once, I guess I don't rock hard enough but I still use the standby because the chassis runs cooler when in idle. I have a Marshall DSL that will give a little pop in the speaker if you shut it off with just the power switch so I find the the standby switch useful. Old habits.

Love these repair videos. Also dig, how you correct yourself and take responsibility for any verbal mistakes you make. Very humble of you....and I really appreaciate being given the best info you have on hand. I have learned a bunch from you and Uncle Doug. Maybe one of these days....I will actually try some small mods on my own units. Thanks again for all the information you have passed on to the public with your efforts. Big love for the Guitologist from Austin Texas!!!

Yeah, the whole misunderstanding with using the standby switch came from high voltage / high current use. I was an engineer for a X-ray mfr and we would recommend warmup for our tubes, but they saw voltages of 120,000 volts and the inrush current was carefully limited by circuit design (for dose control as much as equipment protection). The relative low voltage of guitar amps makes this switch not only useless but actual detrimental to the amp's longevity.

Hi brad please don't stop making repair videos they are very informative i love the older tube amp repair videos i have always wondered about standby switches and now i know

I'm so glad you're putting out slightly more content now! I hope KZfaq Keeps the tap on. Maybe they'll turn that tap on for me one day too (not likely). Anyhow thanks for the great video. This is great info. I've always wondered about this.

Brad ...I am blown by ALL THE KNOWLEDGE ABOUT VALVE AMPS AND THEIR REDESIGN YOU HAVE !!! It continuous to AMAZED ME ! YOU ROCK MISTER !!!

Great video. Super informative...thanks for your hard word.

So, based on the schematic for my 2204 clone, the standby switch only has the potential to harm the rectifier and transformer because the filter caps will charge as quickly as they can, resulting in a current spike upon closing the switch. All subsequent power stages are current-limited by resistors. It seems to me like a current limiting circuit would be the best course of action. A simple constant current circuit set at 1A should suffice. If the amp is drawing less, it's wide-open; if the amp tries to draw more, it cuts off. Though the BJT version that I know of probably isn't feasible at 120-240VAC. Your thermistor solution may be the best solution without over-modifying an existing power section.

All of us who miss the amplifier videos are rewarded with this video..This amp keeps having problems. And the guitoligist to the rescue.

Awesome video! Very informative! I'm glad to see you in my KZfaq feed again. I wasn't getting any notifications of new videos you've posted.

This standby debate has always confused me... All my old Marshall, Hiwatt, and Fender amps have a standby switch which I've used for decades. I haven't noticed any diminished tube life and never had any electrolytic cap failures. Some of my amps still have the original caps and tubes going on 60 years of use!

I have a Seymour Duncan convertible that I'd like to keep around for a while. Good advice. Here's to many more years of warm tube sound!

You could also consider a resistor to limit the higher current that FB pot on the back causes. it wouldn't probably take very much away from the amp. I was surprised how much the current draw changed when you varied that pot! I assume more neg FB (lower pot resistance) *decreased* the current?

Couldn't agree more, Brad. Any modern build, even a clone of an old 'classic', should have a built-in thyristor inrush limiter (and to my mind, internal fusing where appropriate) and a thermistor to prevent voltage spikes from damaging your PT; if the design incorporates a master volume (a la Trainwreck or Dumble), then turning down said control is essentially the same as the function of a 'mute' switch, which is where Leo and others of that time period were headed anyway. None of these additions will do anything to diminish the magical 'tone', but they will add years of usable life to the amp. Don't get too down about not remembering every detail of a particular tube's specs; I can remember when I was a kid, watching the TV repairman come over with his box of goodies (which did not include a digital multimeter or calculator; loved that miniature oscilloscope of yours, BTW) and sort out why the picture was so wavy in short order. In retrospect, don't you think most of those cats were near-geniuses on some level?

Great solution Brad, I am with you and I too wonder what that power transformer’s spec at.

Thanks 'for the video Brad. I've heard about the record flooding in Kentucky I hope all is well where you live. Stay safe

This is one of your best videos. I’m going to build an ab763 Deluxe Reverb, single tremolo channel only. I’m using a Princeton reverb chassis. I’ll install a reducer in the first octal tube socket to accommodate the extra 9 pin tube needed. The remaining octal socket openings will receive the 6v6’s. I’ll use solid state rectification WITH the thermistor you pointed out in this video. No standby switch as well. Perfect.

Great video!! Very informative, thanks!

I always wondered what those thermistors do .. thanks for sharing

I was raised as a tag-along kid in my dad's TV repair shop back when folks did that sort of thing and ended up getting a EE degree. I think the power supply filter cap changes, thermistors and biasing mode you've made are very good to improve safety as well as long term reliability of this amp.

Hey Brad thanks for the heads up.

Good to see you Brad

Great info Brad!!! I've been gigging for years now, I never use standby switches! I just turn my amp off after the first set. All my amps have thermistors as well.

Great video Brad.

Are you going to demo the amp? I was looking forward to hearing you play through it

Hi Brad, Glad to see you back on the channel. But your comment did raise a question with me. See I have been hard wired to believe that with a tube amp the stand by switch acts as a Power tube cool down after playing I have always switched the play mode to stand by to cool down the power tubes And to eliminate any crackle pops if accidental chord disconnects. And you are saying that the stand by switch is no longer needed. Thanks brother

what Merlin Blencowe suggested was also put 2w/5w resistors across the standby switch to create a low current flow that partially fills the filter caps caps, which is then shorted when flipping the switch.

Doesn't the use of the "inductor filter" directly after the rectifier mean that the voltage and current will be quite a bit higher on power up than the more common "capacitor filter" designs? That is what I have found when testing power supply designs. Thank you for the idea of adding thermistors. I will update the design I'll currently working on.

Awesome I always wondered about standby and why solidstate usually doesn't have one, but does this all still apply to more modern tube amp designs like a dual recto, 6505, newer marshalls?

Thak you so much for another brilliant video

Good video, Brad. I agree about the Standby switch. The Thermistor needs to cool down a bit before you re-apply power. When the Thermistor is hot its resistance is low, so it is not offering you much in current limiting. It was an excellent demonstration of what happens when power is applied to a hot Thermistor.

Haven’t seen the whole vid, but thumbs up! Reason, ppl always told me you should put a tube amp on standby. I know I’ll hear the real best thing to do with my amp. Trust a tech that works on actual amps more, than that other guitar guy that also has a tube amp. Also love your vids! I keep coming back to learn. Wish I had your knowledge, I always wanted to build something, meaning one of those build your own amp things. But I get so scared, I have a brother that knows more how to put things together but far apart. I try and try to learn electronics and…..my mind goes kinda mush… lol. 😂😂 not that smart for that, but hope one day I can put those “build your own kits” together by myself. But man. I’d hate to mess it up and not the tools to find what went wrong. Sorry just a lil story. Much mahalos 🤙

I have seen other videos explaining about how standby makes little difference in spikes for solid state rectification and it makes some difference for tube rectification but is not really necessary as well as that cathode poisoning/stripping is not really a concern for guitar amplifiers because they don't use massive voltages and such compared to some other tube devices. Lower wattage tube amps often don't have a standby switch at all because they have an automatic standby function or just let the tubes warm up as normal.

PLEASE READ: This video was meant to discourage amp designers from designing in Standby Switches. It applies more to tube rectified amps than solid state rectified amps as the tube rectifier takes time to warm up, and needs to charge the reservoir capacitor from cold to prevent arching due to excessive inrush current brought about by flipping a Standby Switch with a fully heated rectifier tube. But even amps with a solid state rectifier don't need a Standby Switch! The only reason to use Standby would be to prevent overloading a power capacitor or blowing fuses. As long as the power caps have been properly spec'd with voltage ratings exceeding the maximum voltage of which the power transformer is capable (something the amp designer should have considered already) and fuses are slo-blo and properly spec'd, this should be no problem. The best preventative measure for ensuring the life of components is for amp designers to design in ROBUSTNESS in capacitor specifications and use thermistors to help ease inrush currents. I had been meaning to do a video touching on this subject for some time, and this amp finally reminded me. For a detailed discussion on this topic, see this article from the great Merlin Blencowe: www.valvewizard.co.uk/standby.html Note that in his article, Merlin points out that if a Standby Switch is used in a design, the reservoir capacitor should be wired BEFORE the switch, not after. Also, someone asked about choosing a thermistor. For this I went with a CL-70 Amphenol. Most Fenders use CL-60. These thermistors have cold ohms ratings and current rating in amps. Both are important considerations. Here is a datasheet for comparing thermistors: www.mouser.com/datasheet/2/18/AAS-920-325D-Thermometrics-NTC-Inrush-031814-web-1315885.pdf

Brad - Love your tube diagnostic and repair vids. Learn a shit-ton every time. Damn, need to send you my original Silver Jube (50W) for a "tune up." It's making too many damn noises.

I just watched the DLab video about the standby switch. On a scope the Marshall spiked the same if you used the standby switch after the tubes warmed up, or just left the standby on and just turned the amp on. My whole life, I’ve been told I was going to ruin my tubes from not using the standby switch.

I have started using a variac in all of my amps to 115 volts instead of 120volts. I rebiased my amps and everything seems to run much better and even sound better at that voltage. Johnny Hyland country guitarist on KZfaq talked about using the brown box variac. It really does sound and play better.

I couldn’t agree more. Just plug into the wall outlet. So much tone!!!!!

A great watch!🤘🤘🤘

Hi Brad, Great idea to add the thermistor, but a question for you. Rather than add in two, why don't you just add one after the standby?

Brad, I have a F.R.E.D. rectifier in my Bassman 100. It must be heard and played to be appreciated.

The advantage of the GZ34 tube over a 5u4 is the the go is an indirectly heated tube-the warm up time for them is longer that the 5u4 which has a directly heated cathode-it’s warm up is faster. With tubed rectifiers a standby switch isn’t needed at all.the slow arm up of tubed rectifiers makes them safer for tubes than a solid state rectifier which’s applies power instantly-long before the circuit tubes have warmed. And the sis can be harder of filter caps-a standby switch or in the case of transmitter s I deal with a time delay relay in the primary circuits of the hv rect transformer. Seen the gz34 in hi-fi amps.

Hey Brad hope all is well with you and the family. Crazy times.

I have been using Ampeg svt's for years. I've always used standby on the vintage amps as they were hardwired with a standby tube and the new vr has an automatic 20 second standby.

standby switches have the advantage of reducing thermal shock to the valves. the heaters come on first and you can hear the 'tinkling' noise as the internals expand. then the H.T is applied and the anodes begin to heat as electrons crash into them. upon switch off its more severe because the anodes are usually quite hot. you can hear them cool. i usually give it a few minutes then switch off the heaters and you can hear that too as they cool. it's surely better to cool more gradually to reduce stress on the internal welds and parts. on the subject of cathode stripping : it seems to apply to very hight h.t voltages only. several thousand volts. i had an amateur radio transmitter with 700 volts on the anodes and when it was switched to receive only, the heaters were turned off and the anode voltage remained. this never caused any issues

Been playund for decades, never use the standby... Just unplug or zero volume... Thank you for this video...

Hey Brad thanks for the video. Was really nice to hear another one of your videos especially on amps. And I learned a little something which is always priceless at least that's how I was brought up anyway hope things are well with you buddy and your family your girls your wife and everyone else that's important to you LOL just wondering though I have a cheap too bad really cheap one it's a stage right 15 watt and it seems like when I cut my EQ down off you know base mid and treble sounds like the almost all the volume is gone and I heard somewhere a term called boy I don't even know if it's the right term it's just what pops up in my head or a negative EQ I don't know if that's the right term to be honest I don't even know if that's a thing lol. But if so could that be why it's just kind of Acts differently than other ramps I've experienced two but otherwise or maybe this is something just indicative to this amp I don't know but you know the only stupid question is the one I asked right buddy I do like the amp and I don't have a huge experience with two bands in relation to guitars I did buy in the early eighties at a yard sale I think it was a 1950s Epiphone electoral zephyr amplifier which needed work but I was able to use it anyway and that's my only experience I mean it was so old it had electromagnetic speaker really cool grill though but yeah so I you know this whole foray into electric electric tube world is very new to me and again I like it it's really basic and simple you know and for me that's like really important by the time I could strum a note of course and get to the right knob on an amp any amp at this point sometimes the sustain is gone before I get to the knob which becomes really frustrating trying to set a tone that you want so yeah the less knobs the easier it is the same does have a tone knob I guess that's probably like a bright knob or something like that from what I've tried to find out anyway just glad to have listened to another one of your videos and you don't pop up into my feed all that often as you used to but I'm assuming that now that I clicked on this one I'll start getting more of you in my feed you know the saying how to sight out of mind out of ears out of mind all right buddy hope you keeping cool on top of all this oh yeah there was nothing you said you won your first the court case as a lawyer really cool to hear congratulations I did catch that video I guess and yeah pretty exciting I guess no I would think so pretty exciting and something to be proud of so congrats sincerely Bob the blind bedroom guitars

Your setup's even sweeter now ^^

I enjoyed this - some good info. Sort of agreeing with what you were saying - I am not saying for certain, but I had recently a bad batch of JJ GZ34s - they worked for a while but promptly failed after very limited use. Unfortunately, I bought several at a time and by the time I use them and they fail I have had them too long to get my money back on them. I hate eating the cost of crap tubes. I quit buying JJ rectifiers for a little while and was buying Sovtek. But I have also heard they (Sovtek) had bad batches too (from other folks) and they are harder to get/more expensive due to the current war going on. I never liked Chinese rectifiers, but I may have to consider them. Interested in who made the GT you decided to use. Use american or english if you can find one! But GZ34s are crazy expensive for NOS. I have used a .01 or .02 mfd 630v polyprop cap (like those little brown ones you like to use) across the standby to eliminate pop and prolong the switch life - and it does a good job for that. Curious if it affects the inrush current as I've never taken time to measure it. I expect it may because that is likely what causes the POP sound to begin with. I have never tried the varistors in the standby I certainly have replaced a good number of them in power section in newer amps. I also am a proponent of putting vintage correct values of capacitance in tweed era designs. Messing with the vintage values (adding more mfd) will mess up the tone and breakup in the amp. After all, the lower filtering is part of the design that affects the tone of the amp. Now if you are building an amp to be cleaner or to provide more thump - filter away. Also, get a chop stick/spudger and point things out - please don't use your finger. I know you know better but makes me a nervous wreck. One accidental brush against something nearby where you are working and it will light you up.

Wouldn't using a scope to check the inrush current show a more realistic peak current value by holding the value ?

Thank you, Brad, and Root Mean Square at al. Until the next time: To You and Yours. Rab 👋

Yeah ! thermistors are great for overcoming inrush current ptoblems. You ended up setting the bias so it is pretty cold when overdriven.... That amount of crossover distortion is pretty audible when You see that degree of X-over distortion, and thin brittle sounding. I think the main problem is the power transformer, is overpowered, and the filaments represent close to zero ohms when they are cold thus all the inrush current. It would be better to put 44 uf's at the 1st filter cap. The bouncing waveform when overdriven is intermodulation distortion with 120 HZ ripple in the supply, caused by less than optimum filter capacity. Also that lovely red paint was probably lacquer with drying agents in it, which are nitrogen compounds, which can act like an electrolyte, and can cause leakage at high voltage. It is a really bad idea to paint a circuit board or rusty transformer with lacquer spray paint. By the by, I have also run into too many bad 5AR-4's lately, that arc.

I have not used my standby switch at all since i got my jmp 2204 ... just power up, and tune the guitar and quick warmup of fingers, and then switch of the tunerpedal and go ... no problems for 6 years

Its been a kind of moving target for you brad, great job tho and fingers xst nil desperundum!😊👍

Hello, sorry for asking but I am a newbie. I just purchased a tubes Bugera V55 and it has the stanbie switch. Is all that has been discussed here to ne applied to my ampli? Thanks in advance.

My main amp is a professionally made Blackface Deluxe clone and my understanding is that the rectifier tube releases voltage slowly to the power tubes so I would never use the standby switch because there is no need. This question plaqued me for decades before I learned this.

ohh that's the amp that had all the caps painted up... I remember that one now, you got it sorted, sweet

To make sure I understand, I should turn both switches to “on” when I start my amps? RedBear MK60 (2204 clone-ish), 5153 EL34, Mesa 50/50 But once the amps have been running for a while and I go to take a break, THEN it’s okay to turn it on standby?

What kind of voltage is coming from the standby switch? Did you use a second CL-70 on it as well?

I don't have a stand by switch on my amps ,but I was wondering if using a RX brown box would help with the situation you are describing here. I usually drop the voltage down by 4 volts on the brown box , unless I use it to keep the voltage coming in to 120 volts from the wall socket .

The Vox AC30 CC2 suffers _really_ badly from this exact problem. Have blown three rectifiers because of this issue. I changed the valve rectifier out for a solid-state plug-in replacement.

Hey Guitologist, I have a hand wired amp 1987 Plexi replica. On the back of the amp there's a b+ switch. It's a 3 way toggle switch. Please tell what it does. Thank you sir

What about a vtm120? This is kind of blowing my mind. Have no issues with amp but this intrigues me a lot.

Hey what size thermistor would I need to use in a 100 watt 1959 Marshall solid state rectifier

I might stop using my standby till I can get some thermistors soldered in or some way to prevent spikage think I do have a SS rectifier so good idea I guess, had no idea it spiked that high...

I wonder about my “modern “ tube amp. Line6 DT25. I’ve been using my standby switch since I bought it 12 years ago. It eliminates the loud click during power down and keeps the Soundman from pulling out his hair.

I don't have the tech qualifications, just a hobbyist. In any of the valve amps I've made, I have never used a standby switch. For higher draw amps, I've tended to use gz34, 5ar4, gz37, 5v4, with separate cathodes to the heater. I would even tweak the power supply to optimize for them. I find that there's a slower ramp up on the B+ as compared to 5u4g. With the 5y3, I've used on choke input. Another thing I've done, given as advise by other hobbyist 20yrs ago was to put a resistor before the cap input to make an RC filter which slows the inverse rush to the rectifier as we tended to use tx to hand and over spec'd for versality in future projects. Any of these unadvisable?

Thanks for the info, Brad. I've been thinking about building some kind of inrush current limiter for the soldano clone I built recently (sidenote: you and uncle doug inspired me and gave me the courage to take on such a complex amp for my first ever build). Seems like this would prolong the life of some components a bit. Good idea? What do you think, Brad?

Some makers use the standby switch to bleed the big filter caps prior to switch off. My VHT Special 6 for instance..

This is a really nice presentation. I build clones and convert the ground switch to a standby switch. I also never add the death cap. I love/build/play low watt amps, tweed Deluxe, Princeton Reverb, Brown Deluxe. I have never blown a fuse using a standby switch. Do you think its because I am running at lower voltage and amperage? Now that I think about it, these amps weren't even designed to have standby switches. The next time I crack one open, I'll get the readings that you just showed us. I understand the statement that we don't need standby when using a few tube rectifiers. The GZ34 is supposed to warm up and come up to working voltage very slowly. For now I will run with the standby switch closed passing current when I power up the amp. Thank you for the work that you put into these y-ts. Below is an explanation that I've never heard before. kzfaq.info/get/bejne/atSGn6eHnKfWqps.html

Haven’t used a standby switch in years going back to the early 80’s with Marshall‘s and I’ve had zero issues in that time. I turn my amp on at gigs and if I need to mute it for breaks or anything else I’ll use my tuner. Amp is ready to go at anytime.

Good video Brad. I gotta say, it sure seems like the wrong transformer is being used. Just a hunch.

Hey Brad, where can I get some multi meter leads like you have? The clip on. I'm digging the cigar box current limiter!

100% instructive. Thanks a million Brad

Very informative as always. I have a late-60's/early 70's Vibrolux--should I also not use the standby?

EveAnna Manley(Manley Laboratories) told me that the way they implement stand by in their tube amps is to have a tapped power transformer so you can go to half voltage on the AC side, before it hits the circuit. There is enough voltage to keep everything warm but stop any electron migration in the power tubes(another problem with stand by switches). I guess it also quells the inrush current too. I hope I'm remembering this correctly.

Hey Brad I heard you mention about using an attenuator on this particular amp I have a 1972 Marshall 50 watt the original output Transformer has been replaced with a replica and it has been recaped with j.js, currently running tongue soul el34 original power cord replaced with a ground and I'm pretty sure my guy bypass the polarity switch my question is, is it okay to run this amp with a Bugera PS1?

What's the difference in turning the power on without using the standby, or turning the standby switch off,and getting a rush of power doing it that way? Seems the same thing to me! In the Marshall booklet, they tell you to use the standby when the guitar;s not in use.

So the advice is power up by flipping standby before power to mitigate tube harm. I'm guessing that doesn't apply powering down? Powering down w standby first to avoid gross speaker thump should not cause issues right? Apologies if this was addressed in the video and I just didn't catch it.

My Orange TH30 head blows HT fuses a lot! I'm gonna try not using the standby switch man! Hope it solves that

Had to watch this as I now have a Lee Jackson designed amp that lacks a standby switch. Even my amps with standby I never use then because I'll forget and leave them on when I walk away. Good shit to know.

is the standby switch the warm-up switch? my victory amp asks me to warm up for 30 secs before full switch on

Is this why they put a standby switch on the Valco built Harmony 430, a near dead ringer for the Gretsch 6162 with the exception of a 5Y3(?) -can’t remember if that’s different on.schematic vs. reality. The 430 Schematic shows a 5Y3 but actually has a 5U4. The power transformer in mine actually took a bath and they installed a much smaller Hammond PT. I’ve also read that the standby switch in these valco’s are a source of noise. Im spinning plates trying to decide whether to ditch it or not. I never use it as such except when powering on. I need to install a 3 prong and eliminate the the polarity switch. I wasn’t told what fried the PT but this discussion might shed some light on it. I’ve got RCA black plates and my wallet gets a bit jumpy when I play for these very reasons. Cheers’

I had a Vox AC30 and soon learnt that leaving the standby button on saved the rectifier valve.

I had my Mesa rectoverb connected to a THD hot plate with the volume all the way up. I turned the standby switch off because I needed to disconnect my guitar for a second. I connected with my guitar back up after reconfiguring a few pedals. I then flipped the standby switch back on and then it was a huge pop and smoke😢

I have a Carvin V3 from like 2000 something.. the manual suggest to power on then switch on standby and do the opposite to power off.

Well, being none of my amplifiers ever had Stand By switches (including a cool, rather heavy, loud all tube/valve 1979 50 Watt Garnett), how could I?

I thought in the previous video the fuse blew because 1) 2 not 3 amp 2) not slow blow fuse 3) you eent straight off to on bypassing standby- I though you said THAT could cause the current spike too?

I'm now wondering if I should stop using the standby on my Marshall JCM 900 4100 from the mid 90's...

I’ve heard not to the standby switch if you have a rectifier tube But if you have solid state rectification like the voxac30c2 or hot rod deluxe, use the standby switch. I heard that from Psionic Audio. Another amp tech

Dude I blew some tubes on my AC30 twice and I've always used the standby and I literally thought you had to.

I think the voltage is coming from a voltage spike from the switch. A diode at the switch will keep that spike from happenning. The diode at the input and output side of the switch. I can't think of the word I'm looking for but diode on same line as the switch line but going around the switch or fuse with reverse polarity with the diode placement negative side on inby side. This will keep voltage spike away

I have mostly heard not to use standby switches. On dr z'channel he says only use standby if you have a solid state rectifier. Other than that i had an amp that said put it on standby for 2min. Mostly though techs say don't bother. Now standby or not my my current amp "fades in" at about the same rate

What about a valve-rectified (GZ34, to be specific) amp's standby switch after the reservoir cap? Same problem? Different problem?

Wow so oscilloscopes are a thing of the past? That is cool. It looks like the screen I have on my EuroModular kit build osc.

The Russian 5AR4's are more robust in my experience. However, I have a few JJ's in my project amps that have been hanging in there just fine. I think I'll be finishing another project amp tomorrow......... Also, in regards to the negative feedback loop........ when you're turning it down/off, it makes logical sense that you'd see a current increase, because the NFB effectively "turns down" the Pre-amp tube it's connected to. Sine wave testing is the most brutal thing you can do to an amp...... because unlike music with a break between notes, the amp gets no break... it's just a blasting one note infinitely.

Great post as usual. However let’s talk about Groove Tubes for a moment? Don’t forget to mention: Groove Tubes doesn’t make tubes. They’re just a distributor with a matching system. They use Sovtek/Electro Harmonix, Chinese and JJ tubes. Looks like you have either a JJ or Chinese rectifier

Would starting the amp with the volume all the way down help with longevity of the tubes?

Hi Brad, so if this is some kind of homebrew custom amp with an unknown power tranny, how do you know what the required fuse rating actually should be for the given circuit and the tranny? What if the fuse holder is mislabeled or rating is miscalculated by the designer and the amp indeed requires a higher rated fuse?

You can also slow down inrush current with a ... mosfet ? 😇