Strange Herbert noise, retube question.

[doctorgonzo]

howdy!

I've got a Herbert, and it suffers from a weird hissing, or static noise that was once relegated or confined to channel 1, while channels two and three were fine. Lately, however, it seems that the other channels have become infected with this noise. Its amplitude is volume dependent and is only obnoxious at bedroom volumes, but is unbearable and unplayable at anything much louder than that. Incidentally, if the input to the amp is unplugged, the hissing sound disappears entirely but returns at a lower amplitude than before when the guitar is first plugged back in, but the loud hissing noise returns within a few seconds.

After reading in and around this forum, I concluded that my issue might be related to a preamp tube issue, so I decided to bite the bullet and invest in a new set of preamp tubes sourced from Magic Parts, per Diezel's current recommendation, but I opted for Genalex Gold Lion KT77s instead of JJs. A Weber Bias Rite was also procured.

So first, does it seem that I'm on the right track to fixing the symptom? Secondly, I am aware that strumming the guitar while the Bias Rite is plugged in will ruin the unit, but does an instrument need to be plugged into the input jack in order to accurately get the amp up to idle current, or is this unnecessary?

Thanks for your time.

-dg

 

[bananaladonpcp]

When you bias with a bias-right it is very important to leave the input on the amp UNPLUGGED. Also if you think it is pre-amp tube problems why are you buying power tubes? Also ruby's are pretty great tubes. they are just re-branded tubes but ruby tests them for low noise and for equal output and gives you a warranty.

-Alex

 

[doctorgonzo]

Thanks for that vital info. No compelling reason for the power tube swap, it's got a set of EH 6L6s in there now, and I simply wanted to try a set of KT77s out, and used my eBay Bucks to get a smokin' deal on a set of Gold Lions. Thanks again for the help.

 

[King Crimson]

A Weber Bias Rite was also procured.

DG, which Herbert do you have and does it have the internal/external bias test points? If you have the bias test points on your Herbert then there is no need at all for a WBR.

 

[doctorgonzo]

It's biased internally. Not that I'm unlucky to own this amp (quite blessed actually), but the guys with the external bias test points are really lucky! I've got a bear of a work schedule: I work seven days consecutively, four of those 15-hour shifts, but I'm off for seven starting Friday, at which point I'll retube and bias the Herbie. I'll post again to report whether my noise difficulty was solved or not. Thank you for all of your help, gentlemen!

 

[Peter Diezel]

Do you replace tubes every Friday or how often would
you need the external bias points ?

We had to move the points inside because of the law.

Peace,
Peter

 

[King Crimson]

We had to move the points inside because of the law.
Peace,
Peter

 

[doctorgonzo]

I'll grant that you make an extremely valid and well reasoned point, Peter. As infrequently as I play it may be two or three years, or longer, before I have to retube.

But what arcane creation of a demented Politician forced you to go to the trouble of moving the external bias points inside the chassis?

As Henry David Thoreau once said, "'That government is best which governs least'; and I should like to see it acted up to more rapidly and systematically. Carried out, it finally amounts to this, which also I believe--"That government is best which governs not at all".

I'll descend from the soapbox to state that I am happy to report that my amp is, for all intents and purposes, fixed. I was going to perform the complete tube swap tomorrow, but curiosity got the better of me so I changed the V1 position tube (only) with the fresh Ruby Diezel Selection 678+ 12AX7 tube, and I believe the amp is cured! Channel 1 formerly was the worst, and the loud hissing or static sound has now subsided to a very gentle barely audible hiss, probably not worth mentioning, probably normal. Channels 2 and 3 are nice and quiet. I played for a good hour before I put the guitar down, needless to say this is a crazy-loud, bassy, versatile and beautiful-sounding guitar amp. I'll report back once I've retubed the whole amp and biased it. I'm stoked! Thanks to all!

 

[Peter Diezel]

Dear Doc,

thank you so much for the kind words

To explain:
The bias points, in order you remove the tube fault fuse,
are a break between the cathode of the power tube and
ground. Normally there is a voltage of 20 to 30 volts which
is not a big deal. If a power tube is totally damaged, what
never happens, there "can" be a high voltage on the bias
points. That´s the reason the authority don´t like any
serviceable parts outside the metal chassis.

Cheers,
Peter

 

[Me]

Dear Doc,

thank you so much for the kind words

To explain:
The bias points, in order you remove the tube fault fuse,
are a break between the cathode of the power tube and
ground. Normally there is a voltage of 20 to 30 volts which
is not a big deal. If a power tube is totally damaged, what
never happens, there "can" be a high voltage on the bias
points. That´s the reason the authority don´t like any
serviceable parts outside the metal chassis.

Cheers,
Peter

That's annoying. Could you not argue you have to take the rear metal plate off to get to those? Or put little rubber lids on them or something so people can't poke conducting things through the grille? Then you can say it's only for techs and is already inside (although it would be general knowledge how to do it)!

I didn't think you could get to the bias points without taking that back plate off anyway. I can't remember though as the back plate lives off mine. If I'm at a gig I usually have a pair of spare tubes and a multimeter just in case! It lives in a full live-in rack so is pretty well protected normally without it as long as you don't stick your hands in the back and burn them on the tubes! Since it's never on I'm considering making a Diezel shelf out of the spare back and a pair of L-brackets for my guitar polish/pot of picks/slide/etc! I can always remove it and put it back on if needs be though and will look cool in the meantime (if I can remember at all where I put those bolts)!