Amp doesn't produce sound until I turn it past 6. Any suggestions?

[Spectrum II]

I have what I suppose is just a 1200s that is having a weird issue. I can't get it to produce any sound until I turn the volume past 6. From there I turn it down to my normal playing volume of 4 & it works fine but will occasionally cut out. The light on the power switch seems to flicker in & out, which I'm not sure that it did before. All of this began after I had the power tubes replaced & the amp rebiased. Any clues as to what my problem is? I just spent a shit-ton of money on tubes & the guy charged  me $75 dollars to rebias it because I needed it the following day, so I'm not really into spending more money to have some guy dick-around with my amp. Funny thing is, I had all of this done so I wouldn't have any problems while out on the road for a month.

[JoeArthur]

$75 for 10 minutes of work. That's $450 per hour!! Maybe I should start hyping the need to bias...

Sorry, got distracted buy the tons of money to be made...

It sounds like you have two problems.

The flickering of the pilot light only means something if the flicker is in time with a loss of volume with the amp. If it isn't, then it only means that the light is about to die. If it is, then it means that the power switch is about to die. Could just be a loose connection that needs to be re-soldered either way. Most power switches are sealed which means no way to clean them.

The volume control problem is something different. It could be nothing more than a dirty control - like you are pushing a spider's egg sack out of the way before you are able to turn it down. (Don't laugh, I did extract a spider's egg sack out of a control once).

If it's not a dirty control, then the volume control could have lost tension between the center lug and the resistive strip. It happens, pots don't last forever.

Clean it first. If that doesn't fix the problem, then replace the volume control.

At $450 per hour, replacing the control could be more expensive. It might take 15 minutes or so.

Whatever it is, it's not a major issue with the amp. The major issue is with your tech and what he wants to charge you.

[paulcampo]

So Joe, are you saying you dont need to bias the amp when changing the power tubes? What about a rectifier tube? I thought that was the way to get the most life out of your tubes....

[Isaac]

I think he's saying that the tech grossly overcharged for the job, not that it didn't need to be done.

[JoeArthur]

In the case of fixed biased amps, changing output tubes is not going to change the bias voltage applied to the grid of the output tubes. The bias voltage is a function of the bias power supply, adjustable or otherwise, not a function of the tubes. The bias voltage is not going to change if you simply take out old tubes and put in new ones. It doesn't "wear out" with the tubes.

The bias voltage is specified by the manufacturer of the amp to work with a particular output tube type and handle the range of tolerances within that type and to provide an acceptable balance between tone and tube life. The only real reason for amps having an adjustable bias in the first place (those that have them) is so that the manufacturer can easily set it during production without having to hand select individual components.

As long as you stick with the manufacturers tube recommendations there is no real reason to bias a normal production tube amp during tube replacement. People changed their tubes without a bias adjustment requirement for roughly three decades without any problems.

If you change output tube types (e.g. 6550 to EL34) then the amp does need to be biased to work correctly with the different parameters of that tube. That also goes for rectifier replacements, but only if you change tube types or use a solid state replacement, as this may affect the B+ voltage which has a relationship to the bias voltage. However, when you start talking about changing tube types, then you enter the realm of "modification" and none of this applies.
 

[Spectrum II]

Thanks, Joe. He initially told me $40, but charged me $75 when I went in because "the chassis was hard to get out of the cabinet.". Bullshit, I know, but I didn't feel like arguing. It's so hard to find a guy that does a great job, isn't crazy, & has reasonable prices. I had a good experience with him when he fixed my Dual Showman Reverb, so I thought I'd let him at the simple task of rebiasing my Sunn. Ehh...

[EdBass]

It's so hard to find a guy that does a great job, isn't crazy, & has reasonable prices.

They exist, but you have a better chance of finding the Holy Grail!