Clark Belmont Reverb troubleshooting

[mat]

My friends Clark dropped volume and got a nasty distortion on the sound while gigging. He says the amp works ok for a while but after about ten minutes it starts to give these symptoms.

I got little time to look at the tubes. With eye they seem to be ok. I opened the chassis and the components seem to be ok. No burned marks to be seen. The voltages (anode/cathode) are:
V1 199/1.4 205/1.4
V2 201/1.4 205/1.4
V3 (reverb) 455/11 455/11
V4 201/1.4 203/1.4
V5 (trem) 449/0 399/80
V6 (PI) 212/80 196/80

V7 (JAN Philips6L6WGB) 462V
V8 (JAN Philips6L6WGB) 461V

I did not measure the tube rectifier.

The 2x10'' (8ohms) gives 4,6ohms readining. I chop sticked the amp trough and got very minor pops at V2 plates.

Btw. He uses the amp with an 4 ohm attenuator.

I haven't got chance to hear the problem on the amp but if anyone has ideas where to start the trouble shooting I would be very happy to hear.

Next I try to get the problem on and start swapping the tubes one by one.

Btw. Can I sub the 6L6WGB's with 6L6GC's ?

Thanks,

[Structo]

According to my data the 6L6WGB has a max plate voltage of 400v with a maximum screen voltage of 300v.

So according to your readings I wouldn't use those tubes in the amp.

Most likely it is a tube related issue.

Try a known good tube and try it in each socket until you find the problem.

[M Fowler]

The rectifier is suspect and I am sure you can run those voltages on a good 6L6.

[mat]

According to my data the 6L6WGB has a max plate voltage of 400v with a maximum screen voltage of 300v.

So according to your readings I wouldn't use those tubes in the amp.

Most likely it is a tube related issue.

Try a known good tube and try it in each socket until you find the problem.

You are right Structo. I also read few articles saying that the JAN (military) Philips version handless 450V's.

My friend had put the amp on again and after about 15 minutes the other of the 6L6WGB's redplated really bright. So it might be the relatively high plate voltage plus using the attenuator and running the volume at 8 has been to much for the power tubes. Btw. every resistor in this amp is cc.

http://mattitee.kuvat.fi/kuvat/clark/IMG_0668.JPG

[M Fowler]

Well then you would have to ask the originator of the Clark circuit to explain why he is using 6L6s and the specific voltage he/she choose.

From what I get from this post the amp was working great until one day it starting acting up during a gig, right? Now you say it's red plating, it would have red plated from the time it was first fired up if it truely is a design flaw in HT voltage and dropping string values.

Put in a pair of EL34's then.

[mat]

Well then you would have to ask the originator of the Clark circuit to explain why he is using 6L6s and the specific voltage he/she choose.

From what I get from this post the amp was working great until one day it starting acting up during a gig, right? Now you say it's red plating, it would have red plated from the time it was first fired up if it truely is a design flaw in HT voltage and dropping string values.

Put in a pair of EL34's then.

Sorry M Fowler, I dont quite understand Your post (maybe language barrier). I think the former owner installed the 6L6WGB's without knowing (or maybe intentionally) they do not take 460V as easily as other 6L6's and while my friend running the amp quite hard the tubes have just came to the end of their life.

Sorry, I'm still not so good on these.

[Dr-Joned]

Contact the manufacturer @ mikeclark@clarkamplification.com

[M Fowler]

Mat,

Sorry about the language barrier being from the Dakota's we get that a lot.

All I am saying is I understood the Clark amp was meant to run 6L6 so if the amp suddenly starts red plating now, there must be another reason for the amp to all of a sudden start red plating. If you think it is due to 6L6's that are not rated for such plate voltage then yes you guys better fine some that are.

Good advice from Dr-Joned in my opinion. From what I have heard the Clark amps are very good.

Mark

[mlp-mx6]

Any time there is red plating the first thing I suspect is the bias.

[mat]

Contact the manufacturer @ mikeclark@clarkamplification.com

I did already, waiting for the answer..

[mat]

Any time there is red plating the first thing I suspect is the bias.

Bias was ok before the red plating. I did not measure while the red plating occured. I'll try the amp with JJ 6L6GC's I have.

[mat]

From what I have heard the Clark amps are very good.

Mark

The workmanship is the best I've seen yet