Fuse blew, now it hums - any advice?

[therealjoeblow]

Hi,

I have a 1979 DR103 (2-input) Canadian Import with the "post-factory" high-gain mod that the importer typically applied, other than that no mods. The Poweramp tubes (EL34's) were replaced with a matched set about a year ago and have probably around 50 hours on them, all at normal practice volumes, never an issue at those volumes. I believe the preamp tubes are all original as are the capacitors, etc.

Last night I cranked her up to approx 50% (master volume at 100%, preamp volume at 50%, input into the normal channel) and the 4A fuse blew after about 10 minutes of that, no odd sounds or smells, it just went silent while playing. The Canadian model runs at 120VAC/60hz and has a 4A and a 1A fuse. The 1A fuse was fine.

I changed the 4A fuse and turned back on the main power (standby off), and it immediately hums through the speaker fairly loudly. So I turned it off right away, because under normal conditions it's dead silent when I turn it on and even after I turn on the standby after letting it warm up for about 2 minutes.

Any ideas as to where to start trying to repair it, or did I cook something important like a transformer that's beyond simple repair?

Many thanks
The REAL Joe

[mhuss]

I suspect one of the output tubes. Do you a spare set to try?

--mark

[therealjoeblow]

I do not. Can I test in pairs (2 outer, 2 inner) with no ill effects?

Thanks
The REAL Joe

[OldSchoolDave]

I do not. Can I test in pairs (2 outer, 2 inner) with no ill effects?

Set your impedance selector to half the actual value when running two out of four tubes, to be safe.

Dave

[therealjoeblow]

Without looking at the amp, I believe the options are 16/8/4 ohms, aren't they? I'm running a 2x12 cabinet which is 4 ohms, so I can't go down to 2 ohms on the selector, can I?

The REAL Joe

[OldSchoolDave]

I'm running a 2x12 cabinet which is 4 ohms, so I can't go down to 2 ohms on the selector, can I?

Nope. You'll have to run it at 4 ohms. Only run it this way for brief, troubleshooting periods.

Dave

[mhuss]

Actually, IMO matching the impedance is less critical when you're running at half power.

--mark

[therealjoeblow]

I've had no luck in sorting this out.

I removed the two outer power tubes, and as soon as I power on, the loud hum is still there. So I put back the 2 outers and removed the middle two, and then when powering on, everything was silent so I thought all was well, but after I switch the standby on, it's still silent, ie, there's no signal making it through the amp at all, at any volume.

Any suggestions on what else to look at?

Cheers and thanks
The REAL Joe

[mhuss]

The standby switch might have gone bad, but there is no way that the amp should make any noise while on standby.

Try removing the HT (1A) fuse and see what happens.

--mark

[Zells]

I removed the two outer power tubes, and as soon as I power on, the loud hum is still there. So I put back the 2 outers and removed the middle two, and then when powering on, everything was silent so I thought all was well, but after I switch the standby on, it's still silent, ie, there's no signal making it through the amp at all, at any volume.

You need to have tubes installed in the two middle sockets when using only two power tubes on a DR103. Move the outer tubes to the middle and try those. In the meantime, order a fresh quad of matched EL34s.

[therealjoeblow]

Thank you gentlemen for the assistance. I checked the 1A fuse and it was also blown. I replaced it, and now with the 2 outer tubes the amp works fine at low volume. I haven't tried cranking it up, nor tried the 2 inner tubes (that cause the hum) yet because I wanted to ask further advice.

Do you have an idea why the amp worked fine with all 4 tubes for ages at low volumes, but blew both fuses and presumably cooked one (or both?) of the center power tubes when cranking up to about 1/2 volume?

And, if I replace those 2 tubes, is it likely that something on that half of the amp is Fubar now and will just blow the new tubes immediately when I turn it on? I don't want to waste time and money experimenting if not necessary.

Thanks again
The REAL Joe

[mhuss]

Tubes fail, even if you're not abusing them. Modern power tubes particularly are not really very sturdy. "They don't make them like they used to" certainly applies here. Unless money is really tight, I would replace all four...one failed, another may be ready to.

--mark

[OldSchoolDave]

And, if I replace those 2 tubes, is it likely that something on that half of the amp is Fubar now and will just blow the new tubes immediately when I turn it on?

Most likely, no. As mark suggested, buy a new quad and hang onto your working pair as spares.

Dave

[therealjoeblow]

Resurrecting my old thread here...

I don't play this amp real loud very often, almost never since I only play it at home. After having replaced the tubes and fuses as described earlier in this thread, everything worked as normal again for ages at low volumes, so as suggested, I just suspected it was a marginal set of tubes and went on my merry way. However, the last time I cranked it up to about 1/2, it did the same thing again, blew the fuses. I have not checked the power tubes to see which if any blew at the same time. Before using it any further, or doing any testing I wanted to seek some more expert advice here, as I am not able to find a local tech who has a decent amount of experience with Hiwatt amps.

History of this amp's use: It's a 1979 2-input model, I bought it 1 year old/used in the early 80's and used it extensively in the 80's and 90's, at which point I stopped using it due to lack of time to play and just stored it at home. I started using it again in about 2010 for low volume playing at home as described, so it was completely un-used for about 15-20 years.

It still has the original filter caps in it. Having looked at them, they do not appear (to my untrained eye) to be bulging or leaking. I spoke to a local tech, and although he admitted he is not particularly experienced with Hiwatts (has lots of experience with Fenders and Marshalls), he suspects that either:

1) the capacitors are dried out and need to be either "reformed" or replaced, or

2) the output transformer has an issue with the insulation which creates a short when driven hard (I believe this is unlikely given what I know about durability of the Partridges, and he agrees, but still he says its a 1/1x10^6 chance).

To verify if it is in O/T or not, he suggested taking out ALL 4 of the power tubes, and then turning up the volume to see if the fuses blow. He says if I can get it to max volume without blowing the fuses, it is the capacitors that need replacing; and if the fuses do blow without the tubes, then the O/T is faulty.

Question: Is this safe to do, and the right way to narrow down in which section the problem lies? I've looked through much of this board and the vintageamps board, and have not seen anyone suggest troubleshooting by running an amp at full volume with no output tubes installed.

Many thanks
TRJB

[mhuss]

I think the suggestion is a little off; if there's an intermittent problem with an output tube (or an arcing socket ,etc.) it may still only blow fuses with the amp cranked and tubes in. However, pulling the output tubes and running it at full volume will not hurt anything; I just think it's highly unlikely you'll blow any fuses in that configuration. The only reasonable path for excessive current that's directly related to output level will be in the output section - tubes, sockets and , yes, OT.

--mark