question about tube amps???

ORIGINAL: derekzoolander

hey guys, i've got a 50watt JCM800 head with Mesa tubes in it. The problem is that it doesn't seem to be as loud as it used to be?? The power tubes are brand new and they didn't seem to help. My clean channel is fine, but when I switch to distortion it is nowhere near as loud. Any ideas??..........peace and chicken grease

-zoolander

did you bias the amp when you changed the tubes ?

Hammerheads right dood, you gotta rebias when you revalve. That's the first thing to do then you should be good to go. Also need to match the pairs and pay attention to your preamp valves in case they're tired.

Hope this helps and cheers!

It's not a GIVEN that it needs re-biasing. But it could be. I would definitely replace the preamp tubes WITH the power tubes, though. That's kind of strange that both sets weren't replaced at the same time. That's probably not helping your problem.

about biasing, it is not always needed. Boogie does not make provisions for rebiasing on their amps. They say that biasing was only needed back in the day, when there was more variability in the quality of power tubes. Boogie amps come with a fixed bias now. Bias is only the heater voltage after all. I am curious. the old tubes where el34s or 6550s ? has naything else changed (impedance of cabinet, impedance selector switch setting?, effects loop.)

about biasing, it is not always needed. Boogie does not make provisions for rebiasing on their amps. They say that biasing was only needed back in the day, when there was more variability in the quality of power tubes.

Indeed. However, I'd like more info - how old is the amp? Has it had any tube changes previously? JCM 800's go back...how old is the amp?

Getting the bias checked out might be a good thing to do, but it is a falacy to believe the amp needs to be rebiased after every power-tube change! The important thing is that they're in matched pairs; unlike preamp tubes, you can't mix-and-match them (at least, you shouldn't). Think of power tubes as light bulbs - you don't need to check your wiring every time you screw in a new light bulb.

Speaking of preamp tubes, they last way way longer than power amp tubes. Preamp tubes can go ten years before needing replacement - though I wouldn't wait that long. Whereas under gigging use the power tubes should be changed every year, I usually will go about 3-5 years before changing the preamp tubes on my main amp. My Blues Junior still has the original preamp tubes, and I've had that thing forever!

Anyways, a simple power amp tube replacement should not drastically alter your power and tone like that. Either you got some bad tubes, or something's not right with the amp. I'd try another set of tubes, then take it to the shop.

Dudes, I mean no disrespect here, but there are some incorrect statements flying around here. I am no expert. I’ve owned custom built Marshalls since I was 12 or 13. Two of my buddies that I jammed with growing up built a bunch of amps. I’m trying to learn by reading everything I can. I am confident that what I am saying is true, based on my friends common sense approaches and not the tall tales that seem to plague tube amp beliefs. Let’s take an end user, not a techie point of view. That power transformer in your 800 puts out enough juice to kill you dead and the filter caps can store it for days after you turn it off. One mistake, DEAD, NO B.S.

Every time you change output tubes, regardless of brand or type of bias supply circuit, the bias should be checked. It doesn’t always need to be adjusted. There is a range of tolerance that the tube should operate at. It is okay towards the high or low side of these tolerances. Most say without any effect on tone. It doesn’t make your amp louder just by biasing the amp hot. If you are a stud tech you could play with the entire output section to make it drift into the class B range (instead of the stock class AB) and possibly get more volume or distortion or whatever. WAY outside the scope of changing tubes and checking to make sure everything is cool. Bias is the grid supply voltage, not the filament/heater voltage. Filament voltage is typically 6.3 v and is set by a separate tap on the power transformer. Again, people play with these, still way outside our scope as an end user.

It is recommended by most to buy matched sets. This is practical in trying to keep the amp stable and even a bit anal retentive. For years I’ve bought tubes randomly off the shelf from the local electronics store without a problem due to mismatched tubes. Plenty of problems for other reasons though. Again, people experiment with mismatch for tone. Not us as beginners.

It is not required to change the preamp tubes at the same time as the power tubes. A lot of people change the phase inverter (preamp tube usually closest to the output tubes) at the same time for good measure since it can get beat up. The other preamp tubes that are in the tone circuit can last a lifetime. So you are probably throwing away a good tube. When your output tubes go bad they will let you know. They will short and blow a fuse.

Moral of the story: put your old tubes back in until you can take it to someone qualified to check the bias for you. If you want to learn, there is an abundance of internet and printed resources to help you.

Pat