Bassman 135 blowing fuses when turning off

[Guran]

After that I recapped bass player's '81 Bassman 135, I ran into a weird (to me) problem. It hadn't been used for like 15 years and it hummed terribly. All the electrolytic caps where replaced, then the fun started. It sounded great, but after turning it off and later on again it didn't make sound.

Tubes were glowing but no sound. Turned out that the 630 mA fuses were blown. These fuses are not in the schematic, and I haven't traced them yet, but I suppose they are on the B+ circuit. Another anomaly is that the version for the Swedish market does not have a standby switch.

Alright, I replaced the fuses and fired it up. Everything seemed to be fine, but after powering it off and firing it up again, no sound. The fuses were blown.

I cleaned and retensioned the tube sockets, and in the process I noticed that one of the power tubes showed traces of excessive heat, like red-plating or so. After testing the amp again, same thing; works fine until powered off.

A new set of power tubes should fix that, right? Well, no, still the same thing. Works fine for two hours off loud rehearsal, but once powered off, the next start the fuses are blown.

Now, I haven't checked if the fuses actually blows when powered off, but I believe so based on the fact that new fuses never blow when turning power on. I may pop by the rehearsal place tonight, just to check the fuses before next power-on.

Does anybody have an idea of what might be happening here? I feel a bit confused!

 

[Jon Snell]

Some people think that replacing the main smoothing capacitors with higher values will help with the hum, not correct. What is does is take more of a surge to charge up before the output valves begin to work and indeed blow the HT fuse.
If your HT fuse is before the main smoothing, (it shouldn't be for the reason you are getting) it must be a slow blow fuse. The fuse, if fitted, must be where the old standby switch is and should be fast blow.
The mains fuse, 3A Slow Blow will cover any issues from the rectifier failure etc.
Are the main smoothing capacitors wired in serie or parallel I wonder. Easy mistake to make.

 

[Guran]

The smoothing board before and after.

Orientation seems right to me. There's 22 uF instead of 20 uF because that's what I could get. The two on the left are 70 uF. The schematic says one 60 and one 80, I believe the caps that were there were 68 uF. The sit on the bench at home, I can check tonight.

The total capacitance increase for this is like 1%, give or take, which I think should be acceptable.

 

[Guran]

Alright, I found some images I took before I ordered the caps. The fuses (slo-blo) are on the red wires from the PT to the rectifier.

How much of difference is there when it somes to internal resistance in modern caps vs vintage ones? I'm talking speced difference, not age related.

A standby switch would reduce the startup surge. After all, the amp was originally designed with a standby switch...

 

[Jon Snell]

It looks fine, no need to double check the values. Electrolytics are normally +50 -10% so ideal. The fuses are taking a high level of peak current and are completely in the wrong place.
I would remove the home made fuse links. Place one where the standby switch possition was, if you think Fender got it wrong not to have one there and leave it at that.

 

[andrewRneumann]

I did a little research on this. Hammond sells a transformer for the Bassman 135 with a whopping 1A rating on the HT secondary. Merlin Blencowe recommends multiplying that by 1.5x to 3x for the HT fuse(s). Might it be as simple as increasing the rating of the fuse? Is 630mA written down somewhere as the correct rating?

Does this PT have an HT center-tap (for voltage doubling purposes) like the schematic? In that case, I think I would go with one fuse on each side of the HT secondary like it is currently wired.

 

[dsutton24]

If I'm following correctly the fuses in question are between the transformer and the rectifier. I'd check the diodes and caps in the bridge rectifier. I can't imagine why they'd fail from changing your filter caps, but there you go.

Nothing leaps out at me. Barring a short circuit somewhere from a solder glob or something it's time for some good old-fashioned troubleshooting.

 

[Guran]

About the fuses and lack of standby: It came from Fender that way. Probably only to the Swedish market, because of some quirk in national electricity safety regulations back then. Another example is my '86 Marshall 2204 that, instead of a glass fuse had an automatic fuse that is very prone to wearing out. I have replaced it with a standard glass fuse holder, but it looks kind of funny as I have mounted a round holder in a square hole.

Back to the Bassman: Yes, I'm considering rebuilding it to schematic spec. Of course, I could try 800 mA there and see if they hold up (it says 630 mA on the chassi). Normally a terrible way of troubleshooting, but in this particular case it would be interesting to see. Anyway, a standby switch would take the OT's core out of the startup equation and that would reduce the surge, I think.

The amp works for hours, once turned on. Sounds great, very little hum and other noises. If the problem was in the rectifier area I don't think that would be possible. As said, the fuses blow during either powering off or on. If it happens during power-on, which I think is most likely, then the surge is not worse than that the fuses hold up for one start, but not the next one.

I didn't have time to go by rehearsal place last night, so no hard fact news. I want to go check if the fuses are blown or not right now. We did two loud hours on Saturday, then the amp was turned off. If they're blown now, it happened during shutdown. If not, which I think is far more likely, it happens when fireing up.

 

[Jon Snell]

Fuses, without exception, do not blow when powering down.
Those fuse holders, to me, do not look original. Can't really tell without a photo of the panel.

 

[Guran]

Fuses, without exception, do not blow when powering down.
Those fuse holders, to me, do not look original. Can't really tell without a photo of the panel.

I agree about fuses, or at least I did up until this...

Why did I start questioning this statement? Because with new fuses it works up until next start. So could it be the power-down? My not fully developed thoughts went along lines like "what if bias voltage drops faster than B+..." or such. If I had checked the fuses after turning off I would have known. I didn't, because at first I thought it was the tubes, then it just slipped me.

As soon as I can go by rehearsal space I will take a photo of the rear panel. I haven't looked at it myself since I recapped it in January, but as I recall it, it looked original. I didn't pay much attention to it then, though.

 

[Jon Snell]

"So could it be the power-down? My not fully developed thoughts went along lines like "what if bias voltage drops faster than B+..." or such. If I had checked the fuses after turning off I would have known. I didn't, because at first I thought it was the tubes, then it just slipped me."
~~~~~
The fuses are in the AC supply to the bridge rectifier. When powered off, there is no current passing, regardless of what the bias or current draw of the output valves do.
...

 

[2L man]

Get a multimeter and after next power down remove fuses and measure if their resistance is about 1 ohm or less.

 

[radiocaster]

I also suspect that the fuse is not original, and I also suspect that it is placed where the standby switch was. I don't think there was a "Swedish version", just an export version. In any case, do put the voltage selector on 240V as most of Europe now runs on 230V.

Also, you may want to check that bias thing. It's not a real bias, but balance, and you may have it skewed too much to one side.

Your amp will actually also run on only two power tubes since there are parallel pairs. The output impedance will be 8 ohms in this case.

If all else fails and you have converted it to the original schematic, you can also check the power section from the bridge rectifier all the way down to the V1 supply. You could even have a faulty resistor.

 

[Guran]

Get a multimeter and after next power down remove fuses and measure if their resistance is about 1 ohm or less.

I said in OP that I would do that.

 

[Guran]

The fuses are fine after turning it off. Didn't try to start it as I was short on time.

Here's a photo of the rear panel. I read up on history: The Swedish agent, at the time Muskantor, had to do certain mods to get it approved by Semko (the authority that approved electrical products for the Swedish market at the time (see the blue sticker with the (S)). The power switch had to be lit, hence the red rocker switch, no standby and those fuses. The T by the ratings "mAT" means "trög" which is the swedish word for slow (at least for fuses, the actual meaning of the word is more like inert).

Another thought: as you see, it's rated for 220 V. In 1988 Sweden "switched" to 230 V, but that was more of a change in definitions, as we already had that. 220 V was what you were guaranteed to have, even far out by the end of the line. Anyway, if the PT is wired for 220 V and the new caps charge a little quicker, maybe that could be enough to blow the fuses on second power-up.

There seems to be 220 V and 240 V taps (no voltage selector accessible from the outside) but I can't see in the photos if all wires are there. If they are, switching to 240 V could be enough.

Also, from examining the earlier photos, one of the fuses seems to be at the standby switch location. It's rellay hard to see though.

Closeup:

 

[Jon Snell]

The S mark is a voluntary procedure but CE is slightly different;
Electrical products sold throughout the EU must bear the CE marking. However, as CE marking is self-declared, impartial testing is not required. Products bearing the S Mark (a voluntary certification) show that Intertek has independently tested and certified that the products fulfill valid safety requirements. With the S Mark, your product may be sold throughout the EU without further safety testing or certification. The testing and certification regimen for the S Mark is the basis for CE marking and the required documentation.

Interesting stuff.
I cannot find a reason under 'safety' to install these extra fuses but think that as new type electrolytics have a much lower ESR, that is the issue.
A 10R NTC on the mains input would solve this issue, in series with the thermal fuse.

EPCOS B57127P0100M301 Thermistor 10Ω, 31 x 7mm | RS Components

uk.rs-online.com

 

[Guran]

The S mark is a voluntary procedure but CE is slightly different;
Electrical products sold throughout the EU must bear the CE marking. However, as CE marking is self-declared, impartial testing is not required. Products bearing the S Mark (a voluntary certification) show that Intertek has independently tested and certified that the products fulfill valid safety requirements. With the S Mark, your product may be sold throughout the EU without further safety testing or certification. The testing and certification regimen for the S Mark is the basis for CE marking and the required documentation.

Interesting stuff.
I cannot find a reason under 'safety' to install these extra fuses but think that as new type electrolytics have a much lower ESR, that is the issue.
A 10R NTC on the mains input would solve this issue, in series with the thermal fuse.

EPCOS B57127P0100M301 Thermistor 10Ω, 31 x 7mm | RS Components

uk.rs-online.com

That's the current situation, but Sweden did not join the EU until 1995. Before that the S mark was required for electrical products sold in Sweden. Semko was the testing/approving part.