Help with Fault finding on valve amps

[blindboybenton]

Hi

Im in need of some help. I have trouble when im trying to repair some valve amps and some SS amps

I have some electronic training at college. at present repairing computer equipment during the day

What im looking for is a Schematic that has scope waveforms on so i can see the signal going though the amp. it seems to be around the PI tube is when i have trouble.
On the whole i think i do ok Whats the best way of checking the tone control are working using the scope and a signal gen. whats the setting and how does it effect the waveform. I enjoy the mucking around with them and prefer to spend lots of time on the amps and learn at the same time

Ive built another clone amp learning from the last mistakes, but this time ive got some strange things going on. everything is wired correctly. want to follow it though to the bitter end

Also. Help with finding out if the amp has too much ripple. Scope setting ? Sometimes i check a signal or voltage line for ripple, but im not 100% sure if what im seing is correct or not

many thanks for any help in advance

bassman1965

[soundguruman]

"it seems to be around the PI tube is when i have trouble."
the first thing any of us would do, is put in a new phase inverter tube...
However, you need to describe the "trouble" with plenty of details.

"a Schematic that has scope waveforms on so i can see the signal going though the amp."
It's all a sine wave, sine wave in...sine wave out...

"finding out if the amp has too much ripple"
If it has too much ripple, the amp will hum loudly...

Tube amps are dangerous. It's not a 12 / 5 volt power supply for a computer. It's lethal.
I'm not sure if a person who does not understand basic operation of an O scope, should be working on a tube amp.

I am thinking that you need some basic core knowledge of electrical safety and test equipment before you try to repair a tube amp.
From what you have said, I would not recommend you get into it.
I recommend you take some courses first...and avoid self electrocution.

[The Dude]

I'll add: Don't just go randomly poking around with a scope in a tube amp until you know the voltage rating of your scope and what the voltage is at the point you want to test. I've seen a few o'scopes smoked because someone stuck them on the plate of a tube and the scope was not rated to take that kind of voltage.

[nickb]

Hi

Im in need of some help. I have trouble when im trying to repair some valve amps and some SS amps

I have some electronic training at college. at present repairing computer equipment during the day

What im looking for is a Schematic that has scope waveforms on so i can see the signal going though the amp. it seems to be around the PI tube is when i have trouble.
On the whole i think i do ok Whats the best way of checking the tone control are working using the scope and a signal gen. whats the setting and how does it effect the waveform. I enjoy the mucking around with them and prefer to spend lots of time on the amps and learn at the same time

Ive built another clone amp learning from the last mistakes, but this time ive got some strange things going on. everything is wired correctly. want to follow it though to the bitter end

Also. Help with finding out if the amp has too much ripple. Scope setting ? Sometimes i check a signal or voltage line for ripple, but im not 100% sure if what im seing is correct or not

many thanks for any help in advance

bassman1965

If I gave you an annotated schematic it will help you, but only for the one case. My best advise to to go Merlin's site How to design valve guitar amplifiers and read all the excellent explanations that can be found there.

Something else you can do, it to get a copy of ltSpice, a free circuit simulator and learn the basics. You can then draw schematics and examine the wave forms for yourself. The great advatange is that is costs nothing and is quite safe to work on

[blindboybenton]

Hi all many thanks for your comments

I have been to college here in the uk for electronic servicing. Im touched that you are all concerned that i may be found dead some day touching a filter cap
I know i do computer stuff during the day, this has and still does inc repair to switchmode psu with here in the uk means 330v dc. I have been repairing valve amps for about 2 yrs now
and i think i do ok. Just do it for me, Buying some from ebay , repairing and selling on or just repairs for friends. All good fun for me. I try to repair myself and learn as i go without asking for help to offen. I only discovered this site a few months ago. Sometimes i get caught out by going down the wrong route. Had two amps with motorboating faults. tested on scope. was not 100% sure if what i saw was correct or not. So i replaced the filter caps. Still the same. Fixed it in the end and amps are gone off to the owners.
How do i check the ripple on a scope ?

My scope is good and ive had it for 2 yrs without blowing it up

The other area is PI. Got a marshall clone amp with what i think is a PI fault. Was hoping for some info on what the waveform should look like. I will post a picture on here.

Im going to spend some time with the next amp i get thats working. testing writing down some info on what i see

many thanks again. look foward to your comments

Bassman1965

[blindboybenton]

If I gave you an annotated schematic it will help you, but only for the one case. My best advise to to go Merlin's site How to design valve guitar amplifiers and read all the excellent explanations that can be found there.

Something else you can do, it to get a copy of ltSpice, a free circuit simulator and learn the basics. You can then draw schematics and examine the wave forms for yourself. The great advatange is that is costs nothing and is quite safe to work on

many thanks Nick b

I will look into itspice. I went and brought the torres book a few months ago. I find that very helpful

Bassman1965

[nickb]

many thanks Nick b

I will look into itspice. I went and brought the torres book a few months ago. I find that very helpful

Bassman1965

Excellent!

Here is a screen shot of simple PI done in ltSpice:


And here is the schematic file itself. I've included all the models in it and that is what all the text at the bottom is. Don't worry about all that for now (It's more common to use a library but I want to make this as easy as I can for you). Save it somewhere handy and unzip, install ltSPice, open the 12AX7-pi_inc_models.asc schematic and right click->Run. Now just click on the wires to see the waveforms.

Good luck.

[Chuck H]

I'll add: Don't just go randomly poking around with a scope in a tube amp until you know the voltage rating of your scope and what the voltage is at the point you want to test. I've seen a few o'scopes smoked because someone stuck them on the plate of a tube and the scope was not rated to take that kind of voltage.

Get a 10x probe and poke away.

[bob p]

It's still possible to blow up a scope with a 10x probe. You've got to be careful about where you put that ground connection.

I think that one of the most important things to learn is how to take differential measurements using two probes simultaneously.

[blindboybenton]

Many thanks nick

I use linux so i will have to get the laptop out later and install Itspice. Thanks for the help.

[blindboybenton]

Get a 10x probe and poke away.

I do and never had a problem so far even with my crap scope it didnt blow up. But i moved it from one part of the workshop to another during a reshuffle. It never worked again. Spent about 1 hr trying to fix it. in the end brought a nice kenwood 20mhz dual scope off ebay for not much

bassman 1965

[nickb]

Hi
<snip>.... Whats the best way of checking the tone control....<snip>
bassman1965

Honestly, the best way is probably your ears! They work surprisingly well


If you do want to check with a scope and wfm genny, then use a square wave, I suggest 100Hz and 50mV as a starting point. Monitor somewhere after the tone controls (duh!). Start with everything flat i.e the square wave looks like a square wave, roughly at least. As you increase the treble you will see overshoot on the leading edges, increase the bass and you get overshoot, change the middle and you get tilt. It gets complicated by other frequency shaping components in the amp so this is idealized but it should give you the idea:

Frequency response:


Waveforms:


Hope that helps a little.

Oh, for ripple just measure the RMS ripple volts using a DVM on ACV range.

Whatever you do, have fun.

[Chuck H]

Bob is actually right. If your ground is on an unchanging 0V then your alright. One caveat would be if, say, your checking the plate under clipping with an active load using a scope with an input rating of 200V. You could encounter spikes that would exceed 200V after the probes attenuation. Common advice here is to know what your up against (as dude suggested) and use a 100X probe for checking nodes with HV on them.

[Mick Bailey]

Never tried it, but I wonder what a MOV would do across the probe to protect the scope input. Thinking about a small box with an in/out and a MOV living inside. Perhaps there's too much capacitance or other unwanted effects.

[loudthud]

X10 and X100 probes will only work over a narrow range of input capacitance. An MOV would be way too much capacitance and probably too much leakage. An X1 probe might work but the bandwidth would probably be less than expected. Overload capability needs to be designed into the scope from the start, not added on.