Blown Power Tube But Not The Fuse?

MrKite89

Member
Messages
1,653
Have you ever blown a power tube without blowing a fuse too?

Yesterday I was playing with my EC Vibro-Champ: I was pushing it with a treble booster and the volume on 10/12. When I hit a strong high note the amp ceased to work and produced a relatively loud "hum", accompanied by a bad smell of burned electronics.

So I immediately turned the amp off. When I opened it I noticed that the smell was coming from the power tube base\socket, but no other component (including fuses) was damaged. I tested the tube with my tester and was fried, so I replaced it with a JJ 6V6S I had lying around. The Gold Lion that blown was pretty old, so my fault here I guess, but sometimes this amp (this model, not just my unit) presents this problem when you push it very hard. The weird thing is that all the fuses are safe, which is not good for the amp I guess...have you ever experienced this?

Thanks, cheers...
 

xtian

Gold Supporting Member
Messages
2,389
Yes, definitely. High voltage can burn thru screens and screen resistors, without drawing too much current.
 

Vanyu

Member
Messages
738
What size fuse do you have in there?

Tweed amps spec 2A while later amps just 1A. 2A is just silly for an amp this small, and I’d wager that it’s big enough to do some really big damage.

Now let’s think about that 2A number for a second. For some perspective, 2A is what I use in all my 50w amps. Regardless of what Fender specs for your setup, there’s no freaking way a 5w 5F1 needs the same fusing as a 50w Marshall. Just 1A fuse should be more than sufficient for a Champ, and I believe that’s also the value Fender used after the Tweed years.

If you have a later Champ that specs a 1A, then maybe try backing down to 750mA. You really don’t want your components to act like a fuse like you’ve experienced, it can be quite costly and destructive if you’re unlucky.
 

VICOwner

Gold Supporting Member
Messages
1,845
And remember that a 1amp fuse with 120 volts =120 watts of energy. So when the 1 amp slo-blo opens that 120 watts of energy/heat went somewhere for a moment before it blew.
 

Laurence

Gold Supporting Member
Messages
8,301
Yes, twice. Once in a 6G6B when one of my beloved RCA Black Plate took a dump, and once in a Musicman
210-RD when one of the Philips 6L6GC tubes fell victim to the high voltage.
 

MrKite89

Member
Messages
1,653
Thanks for sharing your experience guys.

What size fuse do you have in there?
1A, 250V: mine is a 220V amp, since I live in Italy.

Probably I just shut down the amp quickly enough to "save" the fuse, and the amp seems totally fine: certainly I don't want this to happen again and again, so I will replace the out tube more often in the future.

A weird thing...I KNOW that I CAN blow any new, perfectly working tube in this amp if I really try: seems like it doesn't like to be pushed above 10 (out of 12) or with powerful boosters, especially if they cut lows and so reduce compression. In fact, it's easier to blow a tube by hitting a strong HIGH note, while low notes compress and the amp survives.

I know that it's not just my amp: I had another one before (same thing) and you can read similar experiences on the web.

One thing to note is that this is NOT a 5F1 circuit: it is based on the 5F1, but it's actually a mix between a tweed Champ and a later Vibro-Champ. The input stage is a 5F1, the second stage (after the vol. control) is from a later Vibro-Champ. There's one more 12AX7 for the tremolo and less overall negative feedback than on a pure 5F1.

I guess it's just a "critical" circuit, a bit fragile, with a lot of stuff in a little space, but the tone is simply perfect, especially for recording: a great little amp that sounds much bigger than what it looks like. :)
 

pdf64

Member
Messages
7,247
mine is a 220V amp
Have you measured your wall outlet voltage?
230V is the nominal standard EU domestic voltage.
Fender amps for our region can usually be readjusted for 220, 230 or 240V.
It's a good idea to check the amp's heater voltage, ie where it sits in relation to the 6.3Vac nominal.

The prime reason for the regular fuse in an amp or any plug in electrical equipment is to to prevent house fires and protect the electrical installation. Any protection for the equipment itself is purely incidental / secondary, it really isn't there to protect for a resistor getting too hot (the probably cause of the 'bad burned electronics' smell noted).

However, I think your amp has a HT fuse, F4; that should protect the transformer from being damaged by a tube short but unfortunately the specified type (T250mA) may be rather too slow and high a rating to be much use. I suggest that F150mA may offer better protection.
 
Last edited:

MrKite89

Member
Messages
1,653
Have you measured your wall outlet voltage?
Yes. It tends to float between 228 and 234VAC.

I bias my fixed biased amps at the higher voltage, or with a little "margin" just to be safe. On my cathode biased amps I select the output tube(s) for the right bias (close to ~100% of plate dissipation) measured across the cathode resistor or with my Eurotubes probe.

The Gold Lion 6V6 that blown was at about ~86% of plate dissipation last time I checked (if these are truly 14W tubes), but was pretty old and worn.

I suggest that F150mA may offer better protection
Once I've blown that fuse (blown power tube) and replaced it with the original value: my only concern is that since this amp almost works on the verge of breaking up, a lower value and\or faster fuse could cause shutdown when not really necessary. It's only a hypothesis. ;)
 

pdf64

Member
Messages
7,247
I KNOW that I CAN blow any new, perfectly working tube in this amp if I really try: seems like it doesn't like to be pushed above 10 (out of 12) or with powerful boosters
It may be the issue here is that heavy overdrive of the preamp sends an strongly asymmetric signal to the power tube, such that it's conducting heavily for much more than 180 degrees. When the plate voltage is pulled down low, the screen grid will then also conduct heavily, stressing both it and adding to the total tube dissipation.
It may be that the absence of a grid stopper on the power tube is exacerbating the condition, so initially I suggest to fit one, and in view of the issue, make it a bit larger than is typical, eg 4k7 - 10k.
While in there, check R14 and R11 (for visual condition and tolerance to nominal); they were probably the source of the burning electronics smell noted.
If that doesn't help, then try increasing R14, eg 12k 2W.
Schematic https://ibb.co/GkQFLT0

And with the amp set to ? nominal wall Vac, and the actual measured wall Vac of ?, the amp's actual measured heater Vac is?

The Gold Lion 6V6 that blown was at about ~86% of plate dissipation last time I checked (if these are truly 14W tubes)
If they are, it's a design max rating; design max ratings put the onus on the equipment designer to derate the tube as required, in order to accommodate supply and component variation.
As that's outside that scope of techs / owners, I think it best to use the design center rating of 12 watts for 6V6, and assume a design center rating of ~26 watts for 6L6GC.

Once I've blown that fuse (blown power tube) and replaced it with the original value: my only concern is that since this amp almost works on the verge of breaking up, a lower value and\or faster fuse could cause shutdown when not really necessary
Firstly, it should be applauded that Fender have started fitting HT fuses, at least on some models :aok:banana
There's always a difficult balance when choosing a fuse spec, as there's a very strong incentive to avoid it blowing spuriously, especially for an internal fuse during the amp's warranty period, but it absolutely needs to blow to avoid things catching fire and killing people, and for most foreseeable fault modes, it would be really nice if it could blow before expensive parts like transformers and circuit boards get damaged.
Your experience indicates that the HT fuse spec value of T250mA is too slow / big, the balance would seem overly weighted in favour of avoiding it blowing spuriously.
Where there's a tube rectifier and no stupid hot switching standby, a F / quick blow HT fuse can be used.
And as you've happy and competent to replace it yourself, there seems no reason not to trial lower values, 150mA should be way more than HT should ever be, even as a peak / instantaneous value.
 
Last edited:

MrKite89

Member
Messages
1,653
While in there, check R14 and R11
R11 was good, measuring 467 Ohms. I will check R14.

And with the amp set to ? nominal wall Vac, and the actual measured wall Vac of ?, the amp's actual measured heater Vac is?
I will check.

I think it best to use the design center rating of 12 watts for 6V6
Except for the JJ 6V6S (maybe). If so, that tube was running at ~100% of dissipation. Anyway I'll buy another one of these: extremely (mechanically) rugged tube, with a sweet sound. It didn't rattle even after the "accident" and, at least, 4-5 years of heavy use and dragging the amp around. Pretty well made, much better than the EHX\Tung Sol (cheaper) counterpart. The JJ is realiable too, but I don't like the tone as much in a Champ. Much better in my Deluxe Reverb. The Champ needs a "sweeter" tube.

Thanks for all the info, I will try a smaller\faster fuse and we'll see... :)
 




Trending Topics

Top