Amp Tech Thread / Ask a tech Q

[Hemisaurus]

OK, can do the power handling, but you're going to have a 10 ohm cab.

50W 8 ohm drivers, all in parallel       = 2 ohms
200W 8 ohm drivers in series parallel = 8 ohms

connect the two sets in series            = 10 ohms

The 2 ohm section can handle 4*50W   = 200W  (2 ohms / 10 ohms total) =20%
The 8 ohm section can handle 4x200W  = 800W (8 ohms / 10 ohms total) =80%

As you can see the percentage and the power matches, 80% of the power goes to the 200W drivers and 20% to the 50W drivers.

You can safely drive 1000W into your cab, if you can match your amp to 10 ohms.

[liquidsmoke]

Does a tube amp run hotter and/or harder on the tubes, transformers, etc at 4 ohms vs 8 or 16? I'm talking 4 ohm output to 4 ohm cab vs 8 or 16 ohm output to 8 or 16 ohm cab of course, not running things wrong.

[Hemisaurus]

No. A tube amp, if the impedance switch is set, delivers the same amount of power regardless.

The higher impedance setting (16 ohm) will deliver a negligible amount of extra power, due to the cable losses being minimised (read explanation earlier in this thread).

[liquidsmoke]

No. A tube amp, if the impedance switch is set, delivers the same amount of power regardless.

The higher impedance setting (16 ohm) will deliver a negligible amount of extra power, due to the cable losses being minimised (read explanation earlier in this thread).

Ah thanks. I'll probably make my new cab an 8 ohm then. If I need a second in the future I'll be running 4 ohms which is more power for solid state amps. The only thing is that I'm not sure all tube amps run 4 ohms but I don't have one now that doesn't, just thinking ahead.

[VOLVO)))]

All of mine do!  The T does 2, as well.

[Hemisaurus]

No. A tube amp, if the impedance switch is set, delivers the same amount of power regardless.

The higher impedance setting (16 ohm) will deliver a negligible amount of extra power, due to the cable losses being minimised (read explanation earlier in this thread).

Ah thanks. I'll probably make my new cab an 8 ohm then. If I need a second in the future I'll be running 4 ohms which is more power for solid state amps. The only thing is that I'm not sure all tube amps run 4 ohms but I don't have one now that doesn't, just thinking ahead.

you'll need the hemisaurus speaker looper, which allows speaker cabs to be chained in series and parallel

new low price.

[eyeprod]

Regarding my sunn concert lead head and problems it was having which were solved by this little fix I learned. I figured this handy bit of info would fit into this thread...

I've got a '76 red faced model. It was having some issues with low output and a generally unstable signal. I took it into the local repair guy. He told me that a common problem on these is a corroded contact on a switched jack in the back of the amp. It's the preamp in jack, I believe. The amp isn't in front of me at the moment, but it's either that one or the power amp jack. They are both right next to each other on the back panel.

Take the amp out of the sleeve (4 screws on top) and check that jack. You can see that it's got 3 terminals and a little switch where the contacts are always made unless you plug something into that jack, which causes the contacts to break. Those contacts were oxidized on my amp. I sanded them until they were shiny and now the amp works great. Problem solved. You plug into the front of the amp, right? but I guess the guitar signal comes in and goes through this switch at some point. I hope my description makes sense. If you're having any issues with this amp, I suggest cleaning this switch and see if that solves them. I would assume that some other solid state sunn amps in the concert series, and possibly others will also have a similar signal routing and switched jack.

[Hemisaurus]

sounds fair, I've got this exact same problem on my T-Max.

2 quick n' dirty fixes are:

1. Spray contact cleaner in the jack socket, work it in and out with a jack plug
2. Run a patch cord between the pre out and the power in, very handy if it starts crapping out during a show

[SpaceTrucker]

^had this happen to my cabinet, It wasn't working and then some contact cleaner later its running fine.

[rayinreverse]

I have some tube amp woes right now.
My mesa nomad seemed to lack power the other night. Admittedly the tubes are probably 3 years old, and have had 100's of hours of high volume play on them. I am assuming its just power tubes starting to die yeah?

I pulled out my Butcher and plugged it in. I stopped using it because it just wasnt loud enough. though I have been doing some reading, and they are supposed to be wicked loud. My nomad was a lot louder, and did have a tone I liked so I just switched back.
I havent fired up the Butcher in quite some time. It was super hissy, and noisy. Extremely so actually. However the volume was great. It had lots of balls and sounded killer, but the hiss and noise when my volume knob was down was so loud I couldnt talk over it. I am assuming this is a microphonic pre amp tube. (Relatively new tubes, with little use) As the night went on though, the volume and punch started to drop in the amp as well.
Anyone have a clue? I dont want to run out and buy 300+ dollars is tubes if I dont need to.

[justinhedrick]

I have some tube amp woes right now.
My mesa nomad seemed to lack power the other night. Admittedly the tubes are probably 3 years old, and have had 100's of hours of high volume play on them. I am assuming its just power tubes starting to die yeah?

I pulled out my Butcher and plugged it in. I stopped using it because it just wasnt loud enough. though I have been doing some reading, and they are supposed to be wicked loud. My nomad was a lot louder, and did have a tone I liked so I just switched back.
I havent fired up the Butcher in quite some time. It was super hissy, and noisy. Extremely so actually. However the volume was great. It had lots of balls and sounded killer, but the hiss and noise when my volume knob was down was so loud I couldnt talk over it. I am assuming this is a microphonic pre amp tube. (Relatively new tubes, with little use) As the night went on though, the volume and punch started to drop in the amp as well.
Anyone have a clue? I dont want to run out and buy 300+ dollars is tubes if I dont need to.

could be:
bad tubes

bad caps
dirty pots/jacks

something else

take your pick.

[Hemisaurus]

A lot of people observe their tube amps seem to lose their punch as the night goes on, 99% of the time this can be attributed to stage volume hearing loss, the same scenario that causes musicians to demand their monitors get turned louder and louder as the night progresses.

Especially if the amp sounds fine again the next day, it's not the amp, it's you I've done the above dance a few times with people, until we worked it out, in one case it involved an evening of having the guy sit outside the practice room, whilst someone else played his amp.

Microphonic tubes are vibration sensitive, take a plastic pen or something non-conductive, and rap on the tube, or the can of the tube if it's canned, do you hear it? If so, then yeah it's a microphonic tube. Noise is in the ear of the beholder, it's oftener bad power or bad cables, than bad tubes, if you've left the amp powered off for a really long time, like a couple of years or more, then it could be the caps, tubes tend not to age if they're not used, hence the high market value of NOS tubes. Tubes are sensitive to harsh use, and harsh handling, so if the amp has been thrown about the practice room, tubes could be bad. If the amp is quiet with the volume up or down, and no cable plugged in, then it's likely bad cable or extraneous noise.

[VOLVO)))]

The Butcher is very similar to my JCM800 in terms of design, those "low" input is in parallel with the high, so if the low isn't passing signal, you'll be losing volume. Use the cable/contact cleaner trick to clean it out. Clean all of the pots with the same contact cleaner, spray them and work 'em to and fro. Dirty, or loose-tensioned tube sockets also can cause intermittent bullshit noise. You can clean the tube sockets by spraying the base of the tube, and all of the pins with contact cleaner, then working them in and out a few times in circular fashion. Sockets need to be re-tensioned after a couple tube changes, it's the same idea as stretching out a spring, once it's "sprung" it won't ever be the same again.

Perhaps we should do a legit "tube amp maintenance" thread, and sticky it... I can do the 800, and the Model T... which pretty much covers the Marshall-style and the Bassman style of amps... 

[rayinreverse]

The noise exists only in the peavey. The exact same setup cables,cab and all do not produce the noise on the mesa.
My mesa was noticably quiter and lacking punch. Even my band members noticed.

[VOLVO)))]

My old Butcher, now owned by nonoman, was dead quiet while I had it. The cleaning should clear up any noise in that amp, it's too simple not to do, IMO, it'll eliminate many of the common noise makers in amps. If it doesn't work, your next bet is microphonics, then caps.

The Mesa is most likely due to tired old tubes...

[Hemisaurus]

Different amps can have different susceptibility to noise. I can get away with using my crappy $1 cables on certain amps I own, and on some of the others it's terribly audible. With tube amps, different amps of the exact same type can act differently, due to differences between tubes, and of course as the life of the tubes goes on, the amp sounds different again.

Here's some checks:

Is it noisy with no cable on the input?
Is it noisy with a shorted jack on the input?
Is it noisy with a shorted jack on the FX loop return?

shorted jacks are wonderful things for finding a fault, or narrowing it down, get a jack, solder the terminals together, done.

Sunn, most amps have a lot of common ground, it would be a good idea to group things, like retensioning tube sockets, cleaning pots, etc together, and then have a separate post for amp specific gotcha's like preamp out / power amp in or fx loop gotcha's and that kind of thing.

That's what make tube amps fun, learn one, learn 'em all, start seeing where the similarities lie

[Hemisaurus]

If the Butcher's tubes are so new, why not put them in the Nomad? It's all 6L6's right?

Mark each tube as you take it out of each amp, so you can return it to the same place, and mark the amp chassis, say use letters for the Butcher and numbers for the Nomad, so you can return everything to it's original state.

If it's just crud buildup on the tubes, even the act of taking 'em out and putting 'em in again helps.

Check that generic biasing video on eurotubes.com for a visual on how to clean tube sockets.

[VOLVO)))]

I gotcha, You want to make the thing or do you want me to do it, and you add your insight? It'll be a few days before I get a couple minutes at the bench, I have some other projects on it. If you've got a tube amp, and a camera, you can do the basics...

[Hemisaurus]

My tube amps might scare people

[rayinreverse]

If the Butcher's tubes are so new, why not put them in the Nomad? It's all 6L6's right?

Mark each tube as you take it out of each amp, so you can return it to the same place, and mark the amp chassis, say use letters for the Butcher and numbers for the Nomad, so you can return everything to it's original state.

If it's just crud buildup on the tubes, even the act of taking 'em out and putting 'em in again helps.

Check that generic biasing video on eurotubes.com for a visual on how to clean tube sockets.

That's actually the plan tonight.

[Hemisaurus]

That's actually the plan tonight.

Good luck

[VOLVO)))]

Ive got 911 on speed dial!

[Hemisaurus]

Mesa's are all fixed bias, as long as you've got a good set of matched power tubes, should be OK. Then again, most noise is in the preamp tubes anyways. Good preamp tubes are more important than good power tubes, you can limp along on older power tubes, but crapped out preamp tubes are going to make the whole thing sound bad.

[liquidsmoke]

For financial reasons I'm going to swap the Governors in my 2x12 for Swamp Thangs rather than buy a whole new 2x12 loaded. I want the cab to be 8 ohms. Do I get 16 ohm speakers or 8 ohm speakers and how do I wire it? Honestly I haven't Googled this yet but I can. I just want to make sure I do it correctly. I haven't soldered in 20 years but I'll figure it out or get an electronics friend to help.

[VOLVO)))]

Two 16 ohm speakers in parallel = 8 ohms

EDIT: FUCK YOU DROID X