Marshall 610x 30th Anniversary problem

[ValleyArts]

Hi there. It's my first post, so please be nice. It's great to see a forum like this.

I have changed the power tubes on a marshall 610x amp replacing old 5881 tubes with EL34 ones as I was told that was possible. Then I used the BiasMaster from TubeAmpFactory to help rebiasing the tubes. All worked well for like 6 months, but now the amp does give sound for 1 minute before blowing my main fuse (and sometimes the speaker output fuses). Normally when a tube is dead, it blows the main fuse instantly so I'm wondering if there is something else than the power tubes that can be defect? (Btw. it should be exactly the same "time constant" on the fuses blown by previous tubes and the fuses I use now.)

[Amp Kat]

These amps are notorious for shorts in the HV supply. Inside the amp if you go to to main board you'll see three wires next to a rectifier that has WO5 on it and the 3 wires are Black,white & yellow in a row. Pull those three connectors off and fire it up again and see if the power led comes back on and doesn't blow the fuse and we'll go from there.

[Enzo]

I'd be betting on a bad power tube. Pull the power tubes, now does it still blow the fuse after that minute? if the fuse holds with the tubes removed , it is a pretty good bet that the tubes are at fault.

[Amp Kat]

Yes and thank you Enzo for pointing that out to do first as it could be tubes. The ones I've seen are results of what happened from bad tubes.

[ValleyArts]

Thank you so much for your answers, I will first try the testing without power tubes. I'm a complete newbie at this so I have some stupid questions for you:

1. When testing the amp without power tubes, what happens when I press the standby switch off? Do I need to have speakers connected in order to not damage the amp?
2. Can a defect (shorted) speaker cable destroy a power tube?
3. I did get a strange current reading (much lower) from one of the power tubes when I replaced them 6 months ago, but it did work anyway until now. Should I complaint to the tube dealer about such things or could there something with the amplifier electronics that may cause the tube getting wrong current?

I've got tons of more questions, but I'll hold back for now.

[Enzo]

With no power tubes, there is nothing going on in the output transformer, so no there doesn't need to be a load. However, it is a good habit to always connect a load to an amp, so you won't forget when it does matter.

Bad cables could damage a tube, but it is more likely that an intermittently open speaker cord would cause damage than a shorted one.

If you have multiple power tubes, and one measures low, move it to a different socket. Now does the same tube still measure low in the new socket? If it does, then the tube is not right, or at least is a serious mis-match for the others. And also, put a different tube intoi the sovket where the first tube originally measured low. Does that tube now read OK, or is it low too? We want to decide if the tube is faulty or if the amp has a problem around that socket.

[ValleyArts]

Thank you for the tips both Enzo and Amp Kat, I really appreciate it.

I turned on the amplifier without power tubes and no fuse blew. I noticed that one of the tubes were a little burned, but changed all four and (eh) rebiased the amp, all tubes giving about the same measured value.
If anyone has rebiased the 6100 amp with EL34 tubes using the biasmaster (which I would guess measures something like the idle current on the tubes?!), I would gladly appreciate some good values to use.
Now I left it at 15mA simply because it sounded best in my huble opinion. It also gave less humming from the loudspeaker, but that might just be another story.
Any comments are appreciated.

[Amp Kat]

The problem with 15ma's is that is pretty cold and will most likely cut out on you when you go to crank it and go into crossover distortion. The EL-34 is normally a 25 watt tube and a good dissipation rating is around 70 to 75 % so say your plate voltage is 450vdc (all the tubes in ) then 70% of 25 =17.5/450 =.038 or 38 ma's. If your plate voltage is 500vdc then 17.5/500=.035 or 35 ma's. It's ok to bias at 15 ma's if you play at low volumes as your tubes will last longer with compromise of better tone and crossover distortion.

[Enzo]

15ma is not far off from the current in a stock PV 5150. A pretty cold setting, but not harmful to the amp.

[ValleyArts]

Hey. Thanks again for your swift answers.

This is hard to describe, but I'll give it a try.

You are right about the amp sounding a little strange on high volumes. The first 30 minutes it sounded very compressed and distorted on a low gain setting. When playing a chord it were immediately clipped and increased in output volume as the input signal (chord) attenuated.
After the first 30 minutes, the sound got less limiting of the input signal. I will try to increase the current a little bit to maybe 20-25mA and describe the effect of it...

Btw. would anyone happen to know if the amp is using slow or fast fuses? My guess would be slow fuses as that's simply most common, but I bet you know better. The manual says replace fuses with the ones described, and that's fuses called T2A for main fuse (~230V). Does the T stand for something?

[Enzo]

T means "Timed", so it means slow blow.