Tube amp volume drop....

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psychepool
Posts: 260
Joined: Fri Apr 18, 2014 8:29 am

Tube amp volume drop....

Post by psychepool »

I made a blackface based tube amp. with the circuit on this page.
http://s935.photobucket.com/user/psyche ... sort=3&o=2

It operates well and sounds good but recently, it has a volume drop.
To be precise, the volume level is down and maintained at a certain level.
And if give it a certain shock, the volume level come back again.

It's the same symptom as what happens when the tubes get old, so I changed to another one that works well and result is the same.
I do not think it's a vacuum tube problem.

Now I remove the caps at the volume knob and put the bright switch.
When I turn the switch on and off, sometimes the volume level is come back.
And unplug input jacks and plug them back in again, the volume level is come back.
When I hit the guitar hard, it also come back.
On the other hand, when the volume level is normal, turning the Bright switch on and off may cause the volume to drop.

Where is the cause of these symptoms?
The most certain method of returning to the original position is to remove the input jack and reinsert it. I wonder why.

The "pushed switch" switches the filtering cap value. Is this the cause?
Or is there a problem with the input jack?
Or where the shorts ...


If you have any clues, please advise me.
pdf64
Posts: 2688
Joined: Sat Mar 12, 2011 9:23 pm
Location: Staffordshire, UK

Re: Tube amp volume drop....

Post by pdf64 »

Photobucket seems to have become unusable, plagued by ads, some of them dodgy - one just took over the entire photobucket tab purporting to be a message from google that a critical chrome update had to be downloaded!
Can you try a different image hosting site?

A problem with Fender type builds can be the link wires, component leads, solder blobs etc between the 2 boards causing shorts etc.

I suggest that you feed in a suitable test signal and scope and measure the signal along the various stages.
Do that in the normal and fault modes and you should be able to identify the stage where the issue lies.
psychepool
Posts: 260
Joined: Fri Apr 18, 2014 8:29 am

Re: Tube amp volume drop....

Post by psychepool »

Thanks for your reply.

Here's another link. Can you see it?

http://www.pedalparts.co.kr/shop/board/ ... %26div%3D0

It is like a one channeled deluxe reverb.
I refered to rectoverb clean channel.

Unfortunately, I am not an electrician. So there is no professional testing equipment. (such as an oscilloscope)

Are there any anticipated areas that can be checked without test equipment in this symptom?
psychepool
Posts: 260
Joined: Fri Apr 18, 2014 8:29 am

Re: Tube amp volume drop....

Post by psychepool »

Could it be due to power supply electrolytic capacitors?
ChopSauce
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Location: So Paris, France

Re: Tube amp volume drop....

Post by ChopSauce »

Maybe you can check this by measuring voltages when the trouble occurs? Sorry if this is dumb. I'm still a noob in the fog.

About oscilloscopes, I could read someone - here - recommending a software/plugin to run a PC sound card as an oscilloscope. I've never used that, though :wink:
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sonicmojo
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Location: Oahu, HI

Re: Tube amp volume drop....

Post by sonicmojo »

To me this sounds a lot like a faulty solder joint. Try using a wooden chopstick to lightly tap around the board and tube sockets while on but be careful. Sometimes upside down compressed air (not too much) will expose those harder to find cold solder joints.
---------
Bryan
psychepool
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Joined: Fri Apr 18, 2014 8:29 am

Re: Tube amp volume drop....

Post by psychepool »

It was not easy to check the voltage by waiting for the volume to fall.
I haven't been able to check the B + voltage yet.
The bias is measured, but there is no difference compared to when it is normal.

I'm checking the faulty solder joint steadily, but haven't found it yet.


I have found the most obvious case where volume is restored.
When I turn the power switch off and on then always the volume level turns back.

What are the possible reasons for this symptom?
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pompeiisneaks
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Re: Tube amp volume drop....

Post by pompeiisneaks »

I would guess a solder joint, it's about all I can think of, but that doesn't mean it's the only possibility. Basically the current running through some connection causes it to heat up, and shutting it off lets it cool down enough to be fine again. It could also be a component I guess that's nearing failure and when it gets warm, starts to misbehave. Have you chopsticked the crap out of every component when the volume drop happens?

~Phil
tUber Nerd!
Firestorm
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Location: Connecticut

Re: Tube amp volume drop....

Post by Firestorm »

Check your grounds. A dodgy input jacks ground can do exactly this sometimes. When you flip switches, hit loud power chords, bang on the amp, etc., the signal spike can punch through whatever unintended resistance is present. This how those Christmas tree light string fixers work.
psychepool
Posts: 260
Joined: Fri Apr 18, 2014 8:29 am

Re: Tube amp volume drop....

Post by psychepool »

It would be better try to check the soldering condition first.

When I hear the advices, I decide soldering problem seems to be the most persuasive.
It is also likely to be a cost-cutting measure to check this issue before replacing parts.

Until now, I have only checked my eyes and wooden chopsticks, but have not found any particular problems yet.
I will try to solder all the joints one more time.
Especially, I need to remove the coaxial shielded cables and replace them with normal twisted pair. The shielded cable has been feeling uneasy from the past.

Before working on all the points again, try replacing the input jacks and watching the results. Thanks for the answers!
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drew
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Re: Tube amp volume drop....

Post by drew »

Did you build an effects loop into the amp?

Focused, well-lit photos of the amp make it easier to offer troubleshooting assistance.
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