1973 Super Lead problem

[Dries]

I've recently acquired a '73 super lead. PT was once replaced with a Dagnall T4154. I replaced all tubes, set the bias at 34mA and wasn't very satisfied with the sound, there was a small buzz on top each note, that fades out, just like cold bias or something.. Then saw that the original bias split resistors were 82K, but the bias circuit resistors were all EL34 values, so i've replaced them with 220K ones, but it didn't sound better, maybe even worse, so i've tried two 220K's parallel for comparison, and liked this a bit more, but the buzz is still there. All B+ decoupling resistors seem to be stock, but are two 15K's in parallel, for all of them. So each dropping resistor is 7k5.

Filter caps are still the old ones.

Here are some voltage's and pictures,

B+ : 483V
heaters : 6.53V

V1:
pin 1: 174V
pin 2: 0.02V
pin 3: 1.07V
pin 6: 222V
pin 7: 0V
pin 8: 2.02V

V2:
pin 1: 186V
pin 2: 0.01V
pin 3: 1.25V
pin 6: 315V
pin 7: 185V
pin 8: 187V

PI:
pin 1: 228V
pin 2: 0.01V
pin 3: 25V
pin 6: 40.6V
pin 7: 27.4V
pin 8: 40.6V

V4,V5,V6,V7:
pin 1: 32mV
pin 3: 481.6V
pin 4: 474.6V
pin 5: -41V
pin 6: 479V


http://i43.tinypic.com/viq0.jpg
http://i42.tinypic.com/fvvekx.jpg
http://i41.tinypic.com/2uswtxv.jpg
http://i43.tinypic.com/v7t6k8.jpg
http://i42.tinypic.com/2l9o0ft.jpg

[stef]

pre amp tubes
The NFB wire purple is to close to the mid, bass pots, it should go under the edge of the board (oscillation!)
lots of piggyback'ed components

[Dries]

Thanks will try this !

About the piggybacked components, most of them are done by marshall. I've found a topic where someone has completely the same amp as mine, turns out the production date of mine is one day later.
http://forum.metroamp.com/viewtopic.php?f=3&t=38285

Probably some resistors and caps were out of stock in this period!

[neikeel]

The usual answer in a tube amp is to try known good ones before you do anything else.

Interesting that your B+ is lower than I would expect of a stock PT but will not give you any issues of the type you describe.

PO is something that happens at high volumes higher gain situations so that would surprise me.

Do you have particuarly hot pick ups?

Has the amp stood unused for a while?

Does the amp hum?

Suggest you plug it in and leave it on but with all controls up but volumes off and connected to a load for a couple of hours, see if it improves, if not and you do not know the history of the amp I suspect a new set of filter caps might be in order?

[Dries]

Have tried moving the NFB wire and connecting it to the 8 ohm tap, no difference. It is most noticable with higher output pickups, not so much with low outputs like strats and filtertrons. B+ was even lower before, it was wired on the 240V tap, now on the 220V. I also think the amp was unused a couple of years.

And about the humming, sometimes it hums quite a bit on high volume, when a other transformer is plugged in next to it, but most of the times it's quiet, and very little hiss. The amp has also a fair amount of gain, maybe even a bit more than my 2203.

[Dries]

It's like the buzz is always present, no matter what the volume control is on. It's just something on top of the sound. Set the amp on now for a while, will look if it will get better. Will order a new set of filters though

[Dries]

While chopsticking the amp there was loud plopping when I hit the B+ terminal and first decoupling resistors, like the solder lugs are microphonic.. Then when measuring the B+ voltage the amp started humming, it gets loud when the channel volume is turned up. But now it doens't plop anymore while chopping the B+ terminals. I have a strong suspicion the filter caps are bad.

Will now try to replace them with the old ones of my 79 super lead, they were good when I took them out.

[Dries]

Replaced all filter caps except the bias caps, and no improvement, but the hum is gone. Amp is really quiet now, but the fizz is stil there. I've made a soundclip where you clearly can hear the fizz/buzz overtone on top of the chords and notes.

http://www.soundclick.com/player/single ... 67230&q=hi

[stef]

Did it sound the same with old tubes?
I had the same problem with my SL and it was a faulty preamp tube (old mullard)

[Dries]

Swapped all tubes 5 times, no improvement..

[Dries]

I obviously made a mistake in measuring the PI voltages, here are the right ones.

pin 1: 228,6V
pin 2: 25,8V
pin 3: 41,82V
pin 6: 222,5V
pin 7: 27,4V
pin 8: 41,7V

I also saw some amps , including mine, have a electrolytic cap .0022uf coupling cap. Could this one be drifted?

Thanks,
Dries

[neikeel]

If the problem persists I would check that the V1 cap or the cathode resistor is 2k7 and not been changed has not gone out of spec.

You may well be correct, but it would not do it on normal channel input

[Dries]

Thanks will check this ! Ordered today a whole new set of F&T filter caps and bias caps, new MO B+ decoupling resistors and other stuff. Will take the pots out of the chassis and lift the board for easy repair. Will keep you noticed!

[Dries]

Yesterday I received new f&t caps and some other stuff. Renewed the biascaps too, and installed new b+ decoupling resistors, a sozo .0022uf coupling cap instead of the old electrolytic and new original value 82k bias feed resistors.
Problem gone! Very great sounding.

Ive seen they once changed the channel II coupling cap to a .047uf instead of the original .022uf. It sounds really good too!
Pictures comming!
Thanks

[Dries]

Here are some pictures

http://i41.tinypic.com/k9h5s.jpg
http://i40.tinypic.com/33z7ibs.jpg
http://i39.tinypic.com/2aewneo.jpg