Why does my tube amp make a scratchy, popping sound?

[longdrive55]

After my amp warms up, it gets a random scratchy sort of clicky, popping, static sound. The sound most reminds me of the sound of a needle in the grooves of a dirty album. It's not consistent either. It sort of comes and goes and can get pretty loud at times.

The sound only starts once the map gets very warmed up (1/2 hour to an hour after start-up).

I have tried changing tubes with no luck. All of the capacitors are new in the amp, except the Cap Can in the power supply. Also, there are a number of older carbon composition resistors in the signal path.

All the voltages are correct in all the right spots in the circuit, so there are no obvious issues.

Any ideas anyone?

 

[Wavebourn]

Any carbon comp resistors inside?

 

[quadtech]

Did you check for loose sockets?

 

[longdrive55]

Yes, there are Carbon Comps in the circuit and in the signal path. I know it may be time to replace those.

Also, it did seem that the RCA jack might be the culprit. This is an older console amp with an RCA jack that is riveted in (not screwed in). As I messed with this, it seemed I was able to totally stop the static sound, but I'm betting it will return.

I'm thinking its time to rebuild the circuit entirely (including carbon film resistors, new cap cans and new RCA and speaker jacks) throughout. Just have to bite the bullet and get started.

Thanks

 

[Brit01]

Yes, there are Carbon Comps in the circuit and in the signal path.

How do you recognize CC resistors?

 

[Matt BH]

They normaly look something like this:

http://www.apexjr.com/images/APEXJRABCARBONRESISTORS1.jpg

Just a picture from a google search.

Older ones have leads that come out at right angles to the body and painted spots rather than bands. You dont see many these days:

http://www.mcclellans.com/images/Philco37-61/RepaintedDogbones-1.jpg

Cheers Matt.

 

[Gorgomat]

Hi,

have you checked the rectifying diodes ( or bridge rectifier) and the cap can in the power supply ?

Best regards,

George

I had a similar problem, one Diode 1N4007 was faulty..

 

[Wavebourn]

If you can't fix it you may bring it here, I'll help.

 

[pointy]

if it only happens when the amp gets hot I would think its thermal expansion.
which can make and brake dry joints or the wire to ring connection in wire wound resistors, it can also happen in side tubes.
it can be a right ^£&%" as you can miss it if testing with the amp cold so you may need to run the amp until the problem starts then disconnect from mains and discharge real quick, and then run tests.

maybe...............?

 

[limono]

Try replacing anode resistors on driver tubes. I had this problem with new Dale 1W resistors .I replaced tubes , sockets , bypass caps etc before I've got to those anode resistors.Pull the driver tubes first and check if the problem is still there .If yes its going to be related to output stage or PSU.

 

[longdrive55]

Thanks for all the replies and the offer of help Wavebourn.

I think the thermal expansion is right on and I also think it has something to do with the input jack and the input sensitivity variable resistor. Truth is that I've been planning to do a complete rebuild of the circuit for some time (and have all the parts), so I just need to suck it up and do the rebuild.

Also, this amp is tube rectified. It's a dual 6973 power tube, EZ81 rectifier with a EF86 Gain stage and a 12AT7 phase splitter. 16 glorious watts of tubey good sound.

Thanks again all.

 

[richwalters]

Job for a hot soldering iron and go through every joint. Anything chassis rivetted for electrical connections should really be replaced by nut & bolt & tag.
In the earlier days it wasn't unusal for a rectifier tube to be doing strange things at the end of it's life.

richy

 

[Zap]

FWIW, I had an issue of this sort that was related to the preamp tube's socket in my amplifier. I adjusted the pins and it went away, though I may end up having to replace the socket as it may have a defect.