Other Amps

Blonde Bandmaster 6G7A issues

1

A while back I mentioned in a thread here that I had a friend's Blonde Bandmaster here, trying to figure out why it sounded like poo. Well I got it working. All the resistors measured ok, voltages were to spec - turns out the caps in the PI were leaky. So once I replaced them and a tonestack cap with lovely new Sozo blues the amp came back to life. The overdriven tones are ok but not amazing, but the overall sound of the amp is great. It really does have elements of BF and tweed in there. And the harmonic tremolo is interesting!

But I got a text from the owner yesterday. He says that after about 5-10 minutes of playing with the volume at around 6 the amp volume just dies. You can still hear it, but it is a lot lower. No crackle, no fuss, just all of a sudden the volume drops.

Let the amp cool down, start it up, same thing. Loud for 5-10 minutes then sudden volume drop.

Obviously something is warming up then failing. Has anyone had symptoms like this before? Any clues? My friend has tried replacing all the tubes with known good tubes but no difference. All the electrolytics look recent - I didn't have to replace the originals, it had already been done. I don't have the amp in front of me right now.

Suggestions?

2

Could be a bad solder joint that expands with heat and loses mechanical, hence electrical connection. I see that quite a bit. Poke around inside with the tip of a chopstick. Putting pressure on a bad solder joint usually reveals it.

Doing good solder work takes a quality iron and lot of experience. I get a lot of work as a result of people's poor DIY soldering skills. Solder blobs inside pots, melted caps, loose grounds, etc.

3

Ok I'l try that. I am a very anal solderer though! I guess the amp hasn't been used much in the past ten or so years and it IS older than me!

4

This happened on one of our TVs back in the day....Cracked resistor.

5

Yep, fractured resistors present the same way. The chopstick test will find those, too. Just don't use stainless steel ones.

6

If you don't have a chopstick, a Sharpie works just fine.

7

Well the amp is going to be here tomorrow, and thanks to you fellas I am confident we will get it sorted. Thanks guys!

8

Something to do with heat.

I knew a guy with a similar problem that turned out to be heat messing with a tube pin connection somewhere.

9

Well I got the amp back for a while. I tried the chopstick test - poked everything I could find! I was surprised that the noisiest parts were the little ceramic caps. But nothing really jumped out. In fact it was probably the quietest chopstick test I have carried out. So I reflowed some joints - any new joints which looked the least bit suspect got some new solder too. As usual, while it was at my place it ran flawlessly! I will check in with the owner to see if he has had any more issues.

10

I had a similar problem, volume drop once the amp warmed up, with my '63 Fender Tremolux. Took it in for servicing 3 weeks ago and the diagnosis was 3 poor solder joints. Got it back and all is OK now.


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