HD130 blowing fuses

well about 20 years ago i was using my HD130 in a musical and when i went to turn it on one day i found it blew fuses when i took it off standby. now i'm trying to put the amp back into service and my tech friend and i looked at it and established it's still doing this. anybody know what that might be? i vaguely remember hearing way back when that this was a common problem.... anyway, if you have an insights or if you know someone in southern cal who really knows these amps, let me know

Does this amp have the 12ax7 Phase inverter tube? These amps(I have two) had a bad habit of melting tubes and smoking the fuse 25 yrs ago. That was the reason they went to the solid state driver and removed the 12ax7. You will have to start with that. Remove all of the power tubes and turn it on and take it off standbye. Blows? Then its trans/power supply or a dead short somewhere in the primary. If it doesn't then chances are the power tubes are smoked and you will have to replace them. These amps had a bad habit of eating tubes when the tubes were a lot more robust than what we have today. If you bias them like MM said to with the 1/2 volt at point Y, you will be pretty hot. I have mine biased at 21mA of idle current and it survies with JJ E34L's for a long time. I do NOT bias by the shunt method or the crossover distortion for this reason. With almost 700v, a little cool on the bias won't hurt. Mike.

in case anyone cares, i was right in remembering this was common (or at least familiar to a music man expert)... well it was about $1.50 in parts (bad diodes in the power section i think). so that was easy! martin at custom audio (bradshaw) figured it out really fast and kudos to him.....

Great to hear! Sometimes, these amps are easy to fix and other times, it's something expensive(like a transformer!). Glad to hear you got it up and running. MIke.

Hi,
I also had the problem with my 212-HD 130 that the fuse sometimes blew but only when turning on the amp, not when taking it off standby. Since once the amp was running it didn't make any problems I decided to try and put in a stronger fuse (2.5 instead of 2.0 Ampere). I haven't had any problems ever since but as you can imagine I don't feel entirely comfortable with this solution. What do you suggest could be the reason of blowing the fuse and what should I do? Could it simply be fluctuations in my grid? I once had a tube short circuit and after replacing the power tubes my amp technician didn't observe anything wrong with the amp but he also said he had to put in a resistor that didn't exactly match the original...
Cheers,
Stephan

If it does it when you first turn it on and the standbye switch is not on, I would look at the switch area itself first. It could be something as simple as an arching switch or even a loose wire in the back that touches. Also, if your amp has a cap to ground on the input, that could be passing enough voltage to blow it.

Thanks, I'll check that. What are other possible reasons? What are the dangers of using the stronger fuse?

You know, come to think of it, the MM's that I have spec a 5 amp fuse with the 130 watt amps that use the 12ax7 PI tube. Which amp do you have?

Mine has a 2 amp 250 Volt fuse (export model), also with a 12ax7 tube and I'm currently using it with a 2.5 amp fuse.

Sorry-didn't think of export models. Not used to WWW being so world wide. I'd look at the transfomer supply coming out and heading for the filaments. If it's blowing before going on, with just the warmup in place, the only things running are the heaters. You could have one barely touching and blowing on startup. Let us know if we can help. Other than that, it's most likely tranny if not in these areas. I'd look at any caps on the input side though before I panick. Mike.

Er, what is a "tranny" in this context? Transformator, transistor...? I've never opened my amp myself before. Do you think I can do anything there myself without knowledge of amp electronics? Can you give me specific instructions how to perform these checks or should I better get it to the service guy? Your help is very much appreciated!

The reference is to a transformer.
If one isn't familiar with the internal workings of high voltage devices, and how to propery protect oneself from the LETHAL VOLTAGES found inside - I advise you to find a good technician
who can.
Even though an amp may be 'off', voltages approaching 750V can still be present. Such
voltages, if contacted, can stop your heart,
or if you're lucky just throw you across a room.

I agree with Nic-even though I used a "slangy" kind of term, it's pretty universal. I dodn't mean to degrade but it sounds to me like you should have someone else look at it. Those amps do have over 700v on the plates when running and the caps will retain a lot of that shut off. I would suggest a tech or someone familiar with high voltage. Mike.

Actually, I have the exact same problem, the amp is blowing fuses the moment I turn it on. It's a 410HD Onefifty and it ran perfectly for about 1/2 a year, but two weeks ago it was dead because of the first fuse was burned. Replaced it with a fuse with the same specs (2A, 250V because of export model). It worked fine throughout the rehearsal, but the week after it was the same again. Replaced it with a 2,5A 250V (bought by misstake) and it has now worked fine for two rehearsals.
Like someone said, I don't feel comfortable with having a different fuse.
Haven't taken it to an amp-doctor yet. Thought I could gather some more info on the web first.
My first thought was that the valves probably need to be replaced, but I don't hear anything different in the sound.

You COULD have a power supply transformer pulling too much juice. I don't have a schem for a 150 here but what does it use for a bias supply? You could have a bias cap going south and running the tubes too hot. I'd pull the tubes and check the current at the fuse with and without the tubes first. You have to start somewhere. I don't remember if they use a ss driver like series II amps or not. Let us know. Mike.

I turned out to be completely wrong about the fuse spec's. When this started i just leaned over the amp from the front-side and took the blown fuse out for replacing it. I read on the fuse and since then I have thought that the correct spec was 2A - 250V. Yesterday I took a closer look on the back-side and noticed the text saying 2,5A - 250V.
I admit that I felt quiet stupid not having seen this text before.
Now I don't have to feel anxious for having the wrong fuse, and the box of fuses "bought by mistake" has suddenly become useful. Hopefully the amp will run a little longer though.
I will also follow the advice to run it on low-power. I have heard that this function was bypassed on export model but I'm not sure. It is hard to hear the difference. Does anybody know about this eventual bypass??

Not sure about exports but I run mine on high all the time-I just bias it a little cooler and I've never had any problems. These amps run hot and need to be cooled down a little.