Light Bulb Current Limiter with 5E3 Amp

[@patrick4228]

I can only repeat what others have said here before: if one is interested in (guitar) tube amps, your website is a gold mine of valuable information. A really big thank you Rob!

[@robrobinette]

Thanks Patrick.

[@BoltOnzs]

Rob. I just wanted to thank you for your incredible website. The amplifier information is absolutely priceless. Thanks so much for the time and effort. RC

[@robrobinette]

Thanks RC, I'm glad you like the website.

[@wamgoc3637]

Different wattages of bulb can be used for different purposes,as can heaters. I went to electronics school in the 80s and tubes were gone but in the old days the benches had an AC supply that had a variac, an isoformer, and outlets and jacks for AC with metering. There was a series outlet off the isoformer with a shorting switch so you could use a light as a current limiter. It had a 6.3v fil supply and a bias supply as well. It was much like the old Lab Volt panel but more versatile.

[@robrobinette]

I forgot to mention the main use of a light bulb limiter is to safely test suspect electronics. I use it primarily to start up newly built or repaired amplifiers. It can also save fuses when troubleshooting equipment that blows fuses immediately after turn-on.

[@robrobinette]

The video camera makes the bulb look bright but it's actually so dim at all times you can see the lit filament inside the frosted bulb. If my amp had a short the bulb would burn at it's full 75 watt brightness but the current is limited to how much a normal 75 watt bulb can draw which is about 0.6 amps. So with a max of 0.6 amps running through your amp even when it is shorting out it most likely won't damage any of the amp's components. This allows you time to troubleshoot and find the circuit short. Using a 150W or higher bulb will make the difference between a normal amp and a short circuit--a normal amp will only make the bulb filament glow red where a short will light it to its full brightness.

[@billhart4710]

Rob Robinette Ed

[@jimtalltheislandbrothers6639]

I was wondering cause it looked like the amp was shorted with the bulb ligthing up. So it is just glowing which is normal.

[@soloventa9233]

Hey Rob, I love you website..... congratulations for the all information.. incredible, thank you for everything, .. all the information is very useful, greetings and thanks from Caracas-Venezuela

[@60retired]

Hi Rob, I to would like to thank you for that beautiful job your’ve done on your web site. I have learned so much from you and really enjoy working on my vintage Fender amps. What size incandescent bulb do you recommend using for the dim bulb test on old Fenders. I have seen everything from 75w to 250w. Read something about a 250w bulb having lower resistance and causes less damage to the amp? It has totally confused me. Thanks

[@robrobinette]

For smaller amps like 1 and 2x6V6 a 60 watt is good but is still adequate for a 100 watt 4x6L6 amp. There's no need to ever go over 150 watts. A 250 watt bulb will be so very dim with a properly working amp.

[@southamericanrocker]

@@robrobinette respectfully, that the whole idea!

[@robrobinette]

@@southamericanrocker a 250 watt bulb can flow 2 amps which is enough to damage a 2x6V6 amp with a problem.

[@johnward3874]

I love your site Rob and thanks for all the effort. Do you have anything on the 1962 fender bandmaster 6g7a circuit? I saw you annotated the ab763 as well as others. I’d like to install switchable a tube rectifier in the vacant tube spot on the chassis. Thanks for the bucking transformer info!

[@robertsiskaii5111]

Hey Rob, wanting to build a 5E3 clone and your website is incredibly useful for all sorts of mods. Any knowledge on installing a VU meter or a good resource? Love the aesthetic look of one on a faceplate.

[@robrobinette]

Here you go: robrobinette.com/Generic_Tube_Amp_Mods.htm#Add_a_VU_Meter

[@robertsiskaii5111]

@@robrobinette Did you just post that?! Wow, didn't see it before but I'm glad it's there!

[@robrobinette]

@@robertsiskaii5111 Yes, just posted it for you.

[@robertsiskaii5111]

@@robrobinette That's amazing, I'll try to post something when it's done.

[@johnhanson4970]

I am confused here. I thought the current limiter bulb should come on and then after the vacuum tube warms up the limiter bulb dims down or goes out. Why is the limiter bulb drawing current (bright light on bulb) when amp is powered on? On my new 5e3 build I have a standby switch the goes between pin 8 on the rectifier tube and the 250 ohm/10 watt resistor and filter cap 22uf. I experience full limiter light in standby and when switched back and the power switch "on" I dont have any light in the limiter bulb. Any response would be appreciated.

[@MrBasslayer2]

Here in México it's hard to find 65 and 75 watt bulbs, only a 100 and only at places where they sell electrical products, the gobement prohibited the manufacture of those bulbs

[@tommyfoley]

You just made my day. Thank you!

[@goanglingblues]

I built one of these and use it all the time - thanks! - one question - I have about 10 orphaned power transformers with unknown values. I hook them up using the current limiter. With my DMM I am able to identify all the leads but want to measure the current in say the 6.3V winding to be sure it has enough current for my application. Should I remove it from the current limiter to get accurate value?

[@robrobinette]

You can't really measure a transformer current rating. The high voltage current rating can be guessed by the transformer's physical size. You can hook up the transformer and see how it performs. If the 6.3v output is low then you're probably overtaxing the 6.3v winding. If the transformer doesn't get really hot--too hot to keep a finger on it--then the high voltage winding is probably ok with the current demand.

[@goanglingblues]

@@robrobinette thanks - that's what i'll do then -

[@thegolfnut812]

Rob, your How the 5E3 works is excellent. You have so many mods there I was interested to know if you've built this amp with all those mods? I have a BJ amp that I modified with the BillM mods. I'm thinking of building a 5E3 and was interested to know if some of the mods for the 5E3 would be applicable to the BJ.

[@osvaldorod2815]

Hi rob, Great material in your site ...just awsome! I want to start with the 5f1 amp, but here in my country we use 220v from the wall ..... what power transformer can i use to start the project?

[@robrobinette]

I recommend the Classictone 40-18019 www.classictone.net/40-18019.pdf

[@wesleymorris1]

If you change the bias while having the current limiter plug in will the bulb get brighter as you turn the bias up

[@robrobinette]

Yes, the bulb does get a little brighter with higher bias but of course it wouldn't make sense to try to set the bias with the limiter hooked up.

[@wesleymorris1]

@@robrobinette yea I know I was changing the resistors in my bias circuit and when I hooked everything back up I used the current limiter to check my work and turn the bias knob cuase I was curious, and yea it does get brighter when you turn it up, just wanted to make sure that was normal, I got it all back together, I've learned alot from your site thank you

[@brich2929]

tell me more about that amp.

[@robrobinette]

It started life as a "Proluxe", which is a 5E3 Deluxe with fixed bias and 6L6 power tubes. It's been modified with switchable mods to allow 6V6 power tubes and cathode bias so I can get close to standard 5E3 volume and tone but can still run 6L6's when needed. It also has 3-way negative feedback and a switch that cascades the bright channel into the normal channel for high gain. The speaker is a 15" JBL and the cab was enlarged and deepened for it. The cab is ported and the rear can be closed up so the cab can be used as a closed back extension cab. It also sports a pre-phase inverter master volume. More info is here: robrobinette.com/AmpBuild.htm

[@southamericanrocker]

Respectfully. You should use the highest wattage light bulb you can find, at least 200watt, otherwise the resistance of the bulb will be higher than the equipment you're testing and it will light up, just like in the video. This device will only protect you if the bulbs resistance is smaller than the device. I suspect this current limiter of yours will not protect you much. Take care! (here I found a more clear explanation that might help you kzbin.info/www/bejne/rYOpg6qFo7GmkKs )

[@burrmanchu687]

Pardon the ignorance here but that amp isn't anywhere near even 40w... Why would he be a 200w bulb?

[@stevet1578]

Thanks, this is the info I was looking for!

[@robrobinette]

For info on assembling a light bulb current limiter see: robrobinette.com/5e3_Modifications.htm#Light_Bulb_Current_Limiter I tried a 150 watt bulb in the limiter but the 5E3 Deluxe only made the filament glow orange. The video shows a 75 watt bulb that works well.

[@Cestlavie168]

Hi Rob, is it normal if you have a dim bulb during the amp is on?

[@robrobinette]

Yes, if the bulb glows bright like a normal light bulb then there is a short in the amplifier allowing too much current to flow through the bulb.

[@Cestlavie168]

Hi Rob, thanks for you answer, my problem is. I just build an Audio SE tube amp with 1x 5U4G, 2x 6sn7 and 2x KT88, when I first power up the amp without any tubes it was ok, no bright in the bulb, then I insert 2 6sn7 and power up again, it was no problem too, so I put all tubes in the amp than power up and few second later the light bulb grow slow up but not bright as in your video (actually it was dim), I know it must be something wrong in the amp so I turned it off at one, may be somewhere was shorted. I made this current limiter from UNCLE DOUG's circuit with 200W bulb, it was great and make me feel safe. when I saw you 5E3 video than I wonder why the bulb still bright when playing the guitar? is it because of the charging of capacitor?

[@robrobinette]

My video camera made my light bulb look much brighter than it really was. A dim bulb is good, a bright as normal bulb means there is a short pulling too much current so your bulb sounds normal and good.

[@zz-np2sr]

Rob Robinette quick question.I have a Valco style amp(octal preamps with pair 6v6's for power section).The PT is high voltage and lacking a field coil speaker(which the original used)a 10 watt/1K power resistor is substituted to drop it down to 310 volts or so.(it's after the rectifier/at the first filter cap)I was wondering your thoughts on using a bulb in place of the resistor.Any pitfalls you can see?(besides low impedance bulbs causing excessive B+ voltage)I was hoping to experiment with sag/feel of the amp.