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1966 Ampeg B-15N issue - capacitors

#21
Jack-Jack said:
There's just one minor issue. The tubes are so hot that the lucite plate on their cover melts down. I played for like 15 minutes and the plate bent and there are some air bubbles inside. I'm going to get it replaced with a glass one, I don't think I'll be able to use the amp without the glowing logo. You don't know whether the logo plates on the heritage b-15 are glass, do you? I have a friend who engraves things in glass with laser, could that be helpful?
Secondly - the cables in the back also seem to be melting from the tube warmth. Should I be worried?
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You have a problem. It shouldn't be melting anything.

Is your wall voltage 230 VAC 50Hz? Measure the output voltage on your step down transformer just to make sure that the voltage is correct. Look for the output to be in the 115-120 VAC range.

The logos are plastic on all the amps, not glass. I run mine without the logo in place as it blocks the air flow. If the amp is against a wall, like in you image above, you should move it away. There needs to be at least a couple of feet of space around the amp for air to circulate.

If you have a small desk fan you can try using it to circulate the air around the amp. Position the fan so that the air is blowing onto the back of the tube cage.
 
#22
2.6kHz said:
It should not be getting that hot. It's running at the correct AC line voltage, and not trying to do 110v on a 220v or anything like that? If the line voltage is right, there's a serious problem with bias of the power tubes, they are getting way too much current somehow, incoming B+ could be too high or bias not set right, could be a short, lots of things really. I'd get it on a bench hooked up to a dummy load and get a look at the power section plate voltages and go from there. I assume the power tubes are red plating?
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I got myself a transformer, which converts 230V into 110 V.

Yes, the power tubes are redplating at high volume settings. Is this bad
The bass can be heard on standby as well. The power tranny is very hot as well. The amp is fixed bias.

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#25
You haven't broken anything. These amps are pretty tough.

The transformer is running a bit low, 230 to 115 would have been a better choice. The thing is, sometimes they are off when they wind the transformer. That is why I said that you should take a voltage reading to confirm that it is 110VAC. At 110 VAC you are still within the acceptable limit of the power voltage range of 105-125 VAC, it just isn't optimal. The lower voltage will affect the high voltages (what they call B+) within the amp.

The bias voltage is affected by the line voltage. Your tech should have set the bias at the voltage that your step down transformer delivers which is supposed to be 110VAC. I don't know how the bias was set but if you look on the schematic for the 1966 B-15NF, they indicate -50V DC for a line voltage of 117VAC. If your line voltage is 110VAC, the bias voltage needs to be changed accordingly. If he didn't do this, you may need to have the bias changed so that the power tubes are operating within their safe operating range.
 
#29
Standby switch cuts B+ voltage to the power and preamp tubes, but allows filament (heater) voltage to still get to the tube. So you should not hear any sound out of the amp when it is in standby. All voltages besides the filament voltages should be zero on all tubes in standby mode. So if the plates/grids/cathodes of the tube are reading voltage in standby, there is a wiring issue somewhere.
 
#32
It is redplating.

The mains voltage at my home is 240V. Every device is functioning properly. The external voltage transformer I got for the amp reduces the voltage value down to 123V.

So, is this the voltage that is causing the redplating or should I find the solution elsewhere?
 
#34
123VAC is good. One potential problem down.

The next step is to check the bias the amp. As I mentioned the bias is changed by selecting a resistor. There are different ways to select the optimal bias voltage. This is a procedure that your tech could follow. It involves reading the plate voltage and measuring the cathode current. Having what is called a bias probe makes the procedure safer. These are available from amp parts supplies or one can be built. Based on these readings, a resistor is selected and soldered into the bias circuit.

It is important to note that the bias setting doesn't have to exactly match some calculated number. You want the amp to sound the best and have the tubes operating within their safe operating range.
 
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#36
My dad has a degree in experimental physics and tube electronics. I think that may save me a trip to the tech (120 miles away).

We also suspect that there may be some cold solder joint within the amp. I will translate the procedure for my dad and we'll see if we can go on from there.

PS the amp was functioning properly @120V at my tube tech's
 
#37
The good thing about your dad checking out the amp is that he can show you what to do. It isn't that difficult, you just need some basic tools and to know how to do these sort of things.

If the bias voltage was properly set at 120VAC it should be close enough now that you are at 123VAC. Since you are red plating, it is still a good idea to check the cathode current. You might find this helpful: [Invalid or Expired Link Removed]. They also have a video on the web page.
 

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