Voltage Question

[suchtonemuchloud]

Hey guys, you are probably going to think I am a lunatic so bear with me here. I have an '84 Marshall 4103, which is just about the loudest, most rock n roll amp on earth. That thing with a clean boost on all the time is basically my sound, but it's impossible to get the power section working at home. I've been running it with 2 power tubes pulled for a while because one burned out and I don't see the point in hurrying to replace my old sovteks when I don't need the full 100w from the power section.

I was thinking about to the EVH interview where he said he got an amp from England that was set for 220v and when he plugged it in, once it came on it got "that sound" but just way quieter and allegedly that is how get got the idea to use a variac. So I said what the hell, and I switched the amp to 220v and turned it on. It took a minute or so for the heaters to start glowing, and the amp actually had a fair bit more power than I was expecting but oddly enough... it sounded good. I played for a few minutes and nothing blew up, so I turned the amp off and I switched it to 240v mains. Again, it still sounded pretty good, just with less volume. 
I was actually kind of surprised that running at right about half voltage the amp doesn't seem to mind it. I suspect that if I had a full quartet of el34s like I should, the amp would probably be far less happy with the arrangement, but as it actually had a bit of clean headroom, if desired. I didn't max out the master but I would imagine it's putting out at least a decent amount of power with the mains switch set to 240v, almost enough to gig with.

My question is, what if I were to run this amp like this for longer periods of time? Would it damage anything? If I'm not mistaken what I am doing is really no different than using a variac except I only have 3 settings for the power. I noticed that the tubes cooled off pretty quickly once I turned the amp off, which would imply they aren't getting as hot as they do at the higher voltage. I'm aware that the tube heaters are supposed to see a certain amount of current and that they likely aren't getting that when running the whole amp at a lower voltage, but is that actually going to damage the tubes?

[shooter]

is that actually going to damage the tubes?

if you're running them outside "normal specs." probably, do you care 

knowing the ACTUAL PA tubes current and the filament volts might help

[suchtonemuchloud]

is that actually going to damage the tubes?

if you're running them outside "normal specs." probably, do you care 

knowing the ACTUAL PA tubes current and the filament volts might help

I had the same thought myself but I don't have a working multimeter at the moment.

[mresistor]

What is the impedance of your cab?  You night want to read through this ..    https://www.premierguitar.com/articles/20713-ask-amp-man-removing-output-tubes-to-reduce-power


I wouldn't run the amp at higher voltage settings.   I might use my variac,,  but I rather like the sound of an amp the way it was designed.  I would be concerned about the filament voltages being too low.

[kagliostro]

I don't know whic is the line voltage in your country, however, as mresistor say

if you give less voltage than what is set in the voltage selector you'll obtani a lower B+ (thing that can be accepted from the circuit) and also lower filament voltage, this isn't a good thing for the tube and can result in damage (the tube life time however is difficult or impossible to be established a priori)

give a look to the datasheet of the tubes present on your amp, you will notice that many datasheet, for heaters, give a max voltage and also a minimum voltage allowed and if they give you this data there is a reason

--

However, in some old Geloso Tape Recorder, it was practice to subfeed the first tube on the signal path, with a lower heater voltage to decrease the noise injected on the circuit

--

To avoid this problem you must use a VVR to reduce only B+ (note that your amp is a 100W fixed bias and require an adequate VVR circuit)

Franco

[Willabe]

I noticed that the tubes cooled off pretty quickly once I turned the amp off, which would imply they aren't getting as hot as they do at the higher voltage.

I'm aware that the tube heaters are supposed to see a certain amount of current and that they likely aren't getting that when running the whole amp at a lower voltage, but is that actually going to damage the tubes?

I say yes, it can/will damage the tubes.

There has to be a reason that tube manufacturers list a heater acv/dcv AND +/-% of that listed voltage. And they made controlled warm up rectifier tubes that took time a little time before they released any dcv to the B+ power supply.

All tube are designed to have a certain heater acv (or dcv). That acv (or dcv) heats the tubes cathode (K) hot enough so electrons are released (thermionic emission) in an electron cloud around the K and are drawn to the high dcv on the tubes plate. If the K doesn't get hot enough, the dcv on the plate will still try to draw electrons from the K and can/will strip the K's coating, killing the tube. The tubes plate dcv has to be high enough for it too happen. The higher that plate dcv, the faster it will strip the K, if the heaters aren't hot enough. Big transmitter power tubes (radio/TV stations, etc.) can run plate dcv's at 1Kdcv and much higher, 3Kdcv. (I don't know the limit.) Guitar amps use receiving tubes with much lower plate dcv's. But a cold heater will not release enough electrons, but that plate dcv is still trying to suck them up, just a smaller vacuum.   

If you want to dial in the B+ on the tubes plates for less output power that's what power scaling/VVR is for.

[suchtonemuchloud]

What is the impedance of your cab?  You night want to read through this ..    https://www.premierguitar.com/articles/20713-ask-amp-man-removing-output-tubes-to-reduce-power


I wouldn't run the amp at higher voltage settings.   I might use my variac,,  but I rather like the sound of an amp the way it was designed.  I would be concerned about the filament voltages being too low.

I halve the impedance setting on the amp when I'm running just 2 el34s.

Thanks for the replies everyone, I am guessing that the voltage drop is too low to continue running like this without killing my set of tubes, but I had to satisfy my curiosity and see how it sounded.

[punkykatt]

I read that vacuum tube heaters can operate at +/- 10% rated voltage. Get a good meter and measure the heater voltage. If below 5.7v its a no go.

[kagliostro]

I am guessing that the voltage drop is too low to continue running like this without killing my set of tubes

Which is the line voltage in your area ?

Franco

[Williamblake]

If you convert to choke input filter you would have less output and only have to reduce the load and have the heaters still right. Would probably be still too loud but much less. On the other hand you could wreck your hearing and be done with it.

[mresistor]

If it were my amp.. I'd install a brand new matched quad..  and then buy and use a weber mass attenuator..

[j_bruce]

seems like a separate transformer for the heaters and then do what you want with the voltage selector might be a worth while experiment if the heaters are what you are worried about.

On a related note, waaay back in about 1985 I was learning to play a bit of guitar (ended up playing bass mostly) and I bought a Vox AC10SRT head at a yard sale for $5. I did not know a darn thing about it. The guy who sold it to me said the voltage selector was to change the output watts from 120 up to 240 and he kept it on 240 but the reverb never worked so he was selling it.

Had I known . . .

[suchtonemuchloud]

Franco, line voltage is probably around 125v. I've never measured the wall where I am now.

A new quad is in the amps near future, but I am not really sold on attenuators. I don't want to beat up my output tubes and transformers just to play at home, but the master is so touchy and the amp has so much headroom that is hard to use without going over 100db in the room. I am somewhat resistant to modding this amp, but if I wanted to install london power scaling, would the sv1 alone do the job? Or would I need to get the rbx with it? It is basically an '84 2203 but I think the input jacks are mounted to the board since it's a combo and they are positioned horizontally like the later model heads. It has the earlier filtering setup though, not the later one that only had 3 caps iirc. The amp has a DI output that is basically useless, since the amp has a pre pi master I should only need to add one knob, it can go in the hole where the DI jack is. Modding old Marshalls is not really kosher in my opinion, but if it were easy to reverse I think I could justify it.

I have been considering just building an amp that is a similar circuit but with power scaling or vvr built in. If I did a low B+ build with 4 el34s, cathode bias, and vvr, could I actually get down to talking volume with the vvr? My only reason for thinking the cathode bias route is that it would make it a lot easier to implement the power scaling, I could just copy the hall setup and not worry about the bias tracking.

[92Volts]

Some Music Man amps used this as their low-power switch. It's a little less drastic than cutting the voltage in half, but it still confused me to see the pilot light being brighter or dimmer depending on the power setting. I thought something was modified or broken until I took a closer look at the schematic and actually, it should drop the heaters to 4.5v or so. It is doing something just like this -- wiring the power transformer to "expect" 180v or so but only feeding it 120v.

So it's not unheard of. Is it bad for the tubes? Maybe, the Music Man amps are known for killing tubes. Hard to say if it's because they run the heaters at low voltage on the low setting, or because they put over 700v on the plates in the high setting. Probably both 

If you're willing to risk reduced tube lifespan, I don't see a reason not to do this. It's not stressing your power transformer or anything like that. Personally I would remove any old-stock or nice tubes and put in the cheapest, most easily replaceable tubes I could find. Might be a good idea to put in a lower-value fuse since the typical current and peak fault current should both be lower if you're running it this way.

[tubeswell]

I halve the impedance setting on the amp when I'm running just 2 el34s.

That’s even worse for the tubes. You need to double the load impedance if you have 1/2 the number of output tubes (as well as adjust the bias a bit).


And even with 1/2 the number of output tubes, you won’t get that much of a volume reduction.


Also, if you’re replacing output tubes in a quad, then you should replace the whole quad (especially in a Marshall Super Lead). Otherwise the hotter pair of tubes will suck all the current when you drive the amp hard, and they will quickly go south.

[pdf64]

I halve the impedance setting on the amp when I'm running just 2 el34s.

That’s even worse for the tubes. You need to double the load impedance if you have 1/2 the number of output tubes (as well as adjust the bias a bit).

I don’t see a contradiction, both saying the same thing, just phrased differently 
Halving the amp’s impedance setting (eg amp set to 4 ohms with an 8 ohm cab) works out the same as, eg using an 8 ohm cab with a 4 ohm amp

But yes, it’s a somewhat pointless exercise in regard of sound pressure level reduction. And the now super stiff HT will push the remaining EL34s harder.

As their HT winding has a CT but FWB rectification, it’s eminently feasible to fit a 1/4 power switch to many 100W Marshalls. Thereby eliciting a slightly more useful 6dB SPL drop. But they make rather unwieldy 25W amps!

[tubeswell]

I halve the impedance setting on the amp when I'm running just 2 el34s.

That’s even worse for the tubes. You need to double the load impedance if you have 1/2 the number of output tubes (as well as adjust the bias a bit).

I don’t see a contradiction, both saying the same thing, just phrased differently 
Halving the amp’s impedance setting (eg amp set to 4 ohms with an 8 ohm cab) works out the same as, eg using an 8 ohm cab with a 4 ohm amp

True.