• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

On Line Tube Learning for newbies....

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#186
Test Tube

Hi, I am totally new to working with tube amp circuitry. I have a Marshall 8080 and I am trying to repair it. I turn it on and no sound comes out when I play. Could this be due to a broken tube? If so, how do I test the tube with a multimeter?
 
#187
Your best approach is to find somebody local with a tube tester, just to rule out a bad tube.

You can't really check them with a DVM other then to see if the heaters are intact. Find a "datasheet" or google "pinouts" for your particular tube.

In this case you only have one tube in the amp which is a 12AX7, the heaters are pins 4,5 and 9.

You likely have a buddy who you can snag a known good 12AX7 to try out.

Read up on safety before diving inside the amp.

If your tube is OK then time to start learning a little more about it. Your is is a Hybrid so you have twice the fun ahead of you. 🙂
 
#190
Decades, even a century if properly designed and constructed. I have some some RCA-3 45 push pull amplifiers from early 1920 that still function flawlessly. The capacitors may dry and and need replacement or the resistors drift and need replacement, but the basic amp should last a long time.

Note: This is not necessarily true for circuit board based tube amplifiers. Traces lift, the material degrades and starts conducting, audio sound gets worse and worse, and eventually it is trash. Point-to-point is what lasts.
 
#192
coffeedj said:
...
Note: This is not necessarily true for circuit board based tube amplifiers. Traces lift, the material degrades and starts conducting, audio sound gets worse and worse, and eventually it is trash. Point-to-point is what lasts.
Click to expand...

Hmm... While I don't necessarily disagree that point to point is probably better in the long run, especially better than PCBs without solder masks, but I have plenty of first generation PCB based amplifiers, and none have lifting traces, nor degrading/conducting materials, and they certainly don't sound like trash, or even worse than I remember they did decades ago.

I suspect your scenario assumes very poorly treated (dirty, mishandled) or poorly designed (burnt areas) products.
 
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#193
regarding safety....there is a small capacitor on switching the amp on & off.This are MPX 2X capacitors for EMI surpression and this small thing is saving other capacitors and semicondactors in the amp and all equipement conected with the amp from dangerous overvoltage spikes caused by sparks when switching amp on and off...the question is..does anybody knows can I use a biger capacitance ( the original has value of 0.01mf) or can I use even 0.1 mf instead ?
 
#195
coffeedj said:
I typically use .1uF x 630V, and have seen even .22uF used. However, I'm not sure if there is any real advantage in going larger.

On a related note: Most important is the quality of the switch. Even a small amount of residual resistance can cause high frequency EMI.
Click to expand...

Thanks....only I use 275 v as I found this value in the amp...and..so true..the quality of the switch is very important..this EMI values can rise to more then 6 KV and damage all capacitors - from electrolitics to blocks in a sgnal path..this damages are very special as all capaciots are showing nominal values but once you put signal throug them you have very unpleasant highs,no deep bas and the sound is just to loud....only cheramic capacitors are less sensitive on this EMI
 
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