Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Fiat_cc on September 12, 2019, 01:27:35 am

Title: Marshall 2204 style amp blowing HT fuse
Post by: Fiat_cc on September 12, 2019, 01:27:35 am

Thoughts on an amp blowing HT fuses...?
It’s a Marshall 2204 style circuit (I seem to keep having issues with these). It’s been running fine for a year, then just stopped mid rehearsal the client said. I immediately thought screens, since these amps are hard on screens. I put 2K2 5 watt screen resistors in to replace the 1K originals, and put in a new set of tubes. Amp fired up fine with just preamp tubes, but popped fuse as soon as I put in power tubes. I’d already checked that I had bias voltage on both grids (set max negative), and plate and screen voltage was about where I’d expect with no power tubes. Put in power tubes and fuse pops. I reflowed a couple of suspect looking solder joints around the tube sockets and grid leak resistors. Repeated process with same result. This time I saw a blue arc out of the fuse holder. No components look burnt or physically damaged, voltages all measure up ok.
Any thoughts?

Title: Re: Marshall 2204 style amp blowing HT fuse
Post by: PRR on September 12, 2019, 11:59:56 am

You didn't try different tubes?

I had a EL34 amp, one tube had a short, but *only* when hot.

Open G2 resistor would not blow fuses (current would be less, not more). Parts of the power supply would blow fuses, but you say it is fine without the EL34s. Which have bias.

Title: Re: Marshall 2204 style amp blowing HT fuse
Post by: Fiat_cc on September 27, 2019, 06:21:52 pm

I hasn’t come back to this for a while because I’ve been away in a work trip.

I tried different tubes. Same result. I have been using fast blow fuses (I know Marshall originally called for slow blow), but it worked fine for a year in that configuration.

I’m aware the screen resistors wouldn’t have been the issue directly, more that one of the screens may have shorted in the original set of tubes (the amp got dropped at one point too the owner tells me).
I’ll hopefully get some time to look at this amp soon, although I have two builds to get done too, before another work trip in 3 weeks.

Title: Re: Marshall 2204 style amp blowing HT fuse
Post by: shooter on September 27, 2019, 06:42:03 pm

Quote

This time I saw a blue arc out of the fuse holder

buy slo blo, 2 handfuls.
put in new fuse, pull PA tubes, power on
blow? if so replace pull PI
blow?
keep going 1 tube at a time, or all tubes out, if blows, isolate PS, if blows......

Title: Re: Marshall 2204 style amp blowing HT fuse
Post by: pdf64 on September 28, 2019, 05:21:02 am

Use a light bulb limiter to check if there’s a fault.