Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bonsaijodelfisch on April 23, 2022, 12:03:34 pm
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Hello forum!
I'm new here and with a question, I hope I'm not posting in the wrong area, but I couldn't find a forum more suitable or this problem being posted before.
A couple years back my amp, the Laney 50 pro tube in question, broke down. If i remember correctly the signal was suddenly cutting out and there was a distinct burnt smell, so I quickly switched off and disconnected it. Now life got in between actually having a look at the amp for a number of years, but this week I finally opened it up to look what's what. First glance the problem is fairly apparent, one resistor is completely burnt and blown (pic1).
Now here's the question, the resistor in question was connecting one of the power tubes (from latch no.7) to the chassis or ground (pic2, right tube is where I removed the resistor, left it is still connected). My idea was simply replacing the blown one with a new and be done with it, but when I was looking at the schematics here, there doesn't seem to belong a resistance there in the first place. If I am reading the schematics correctly (https://el34world.com/charts/Schematics/files/Laney/Laney_aor50.pdf), and I might not, I am very green in terms of electronics, schematics etc., this connection should just go straight to the ground. This also is in line with pictures of other AOR insides online, both the series 1+2 and both the 50W and 100W versions
(https://ampstack.files.wordpress.com/2012/05/aor-series2-004.jpg ,
https://ampstack.files.wordpress.com/2014/01/dscf1828.jpg).
Looking at the still in place resistor on the other tube (pic5), this seems to be brown/black/gold/gold, which should be 1 Ohms if I'm not mistaken, so fairly minimal. I've ordered a replacement, but my question is:
this seems to be a mod from a previous owner, would I be better off instead just putting a wire there, directly connecting the tube latch no.7 to the chassis instead, as was probably the original configuration? Plus, if this was a mod, what actually did it do? Is that a good idea and I should better recreate it?
Any help or advice would be greatly appreciated!
Cheers,
bonsai
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Adding 1 Ohm cathode resistors is a very good idea. Do all the power tubes.
That resistor did NOT blow itself. Something else failed. Probably the tube.
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Yes, what PRR said.
Also we don't call the tube pins a latch. We call them a pin. The 1Ω resistor is actually connected to pin 8 not pin 7. Don't go by the numbers stamped on the socket. They can be misleading. Instead, use the notch in the tube base as the start key. Looking from the bottom of the socket, start at the key and count pins clockwise. The first pin is pin 1. And so on. The last pin before you get back to the notch key will be pin 8, and that's the correct pin to connect the new 1Ω resistor to.
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Thanks a lot for the recommendations! I'll replace the resistors with new ones (and at the correct pins) and get a new set of tubes and hope to get this old box running again :)
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Welcome to the forum!
I am very green in terms of electronics
Tube amps have high voltages, even when turned off. Make sure to unplug the amp and discharge capacitors, when working on it.