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mojotone JTM 45 build - ALL DONE!
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tangelolemon



Joined: 26 Jan 2009
Posts: 2691
Location: Brooklyn, NY

PostPosted: Mon Feb 24, 2014 5:55 pm    Post subject: Reply with quote

Bear in mind (in case you don't already know), lower bias voltage = "hotter" bias (more current through the tube), and a higher negative bias voltage = "cooler" bias (less current through the tube).

Not sure what spec is for bias on that amp, so just throwing that out there.
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ajeffcote



Joined: 09 Nov 2009
Posts: 2184
Location: Baytown,Tx

PostPosted: Sun Mar 02, 2014 3:38 pm    Post subject: Reply with quote

Running the tubes cooler will make them last longer. But too cool or too hot is not good. How cool is too cool is what you have to figure out. really, get a couple of precision 1 Ohm resistors and solder 1 end to the pin 8 and the other to ground. Check the voltage across them and that is the current. then all you have to do is a little math. Check the Aiken amp site, look at hid White Pages under tech support for a great and detailed explanation of bias and lots of other good info.
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greysun



Joined: 31 Mar 2011
Posts: 421
Location: Chicago

PostPosted: Mon Mar 03, 2014 3:14 pm    Post subject: Reply with quote

I'm going to take it to an amp tech to bias it with the right equipment and just to be sure nothing is going wonky.

I plan to run all my distortion via pedals, so I don't foresee a need to really crank this amp (I'm told that at about noon-1oclock the JTM45 starts to break up into a distortion territory) - running the tubes a little cooler is fine by me in that regard...

I'm also told that it's a pretty loud amp even with the lower wattage - someone compared it with a solid state amp with twice the wattage and was able to get similar volume from each - but I don't know the accuracy of that statement or what it would mean at a show... Most every venue I go to mics everything through the house PA, and the few that haven't aren't exactly big spaces...

If I can get within the range I need it and there's nothing wrong inside the amp, and I can get a good 2x12 cab that sounds good, then I'm cool with it as is. Hopefully I don't have to replace that resistor to 82k from 47k.

Here's a general question about tube amps, tho - I often play at home and live in a condo building - can you play a tube amp at low volume and have it be fine, or is it better to run more volume to keep the tubes healthy?
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greysun



Joined: 31 Mar 2011
Posts: 421
Location: Chicago

PostPosted: Sun Mar 09, 2014 8:16 pm    Post subject: Reply with quote

She's done. I took her to the local repair technician (a place called chicago music exchange, which is a musicians wet dream of a place) just to get a once-over and a good biasing. He found that the resistor right off the 1n4007 was too high (I think it was a 220) and he was getting around 40 for bias, which is pretty hot... he swapped in a 110 and it biased to the right number. He complimented how well I did compared to some of the home-brew he's seen before, so that was nice to hear *pats self on back* Smile

Funny thing happened, tho... He played through it, biased it, everything was fine. I took it upstairs to buy a cabinet (pic below) - The second I threw the standby switch, a fuse blew. Now I can only think of 2 reasons for this:

1) The guy plugged it into a 16ohm cab while the amp was on an 8ohm setting. I don't think that would blow the fuse, though - it's just bad for the amp in the long run, right?

2) I had tried to do the jumper thing between high treble and normal volume by hooking up a jumper from lower input 2 to upper input 1 - Up until this point, I had only plugged into lower input 1... But I don't see how that could cause a short in the fuse - any insights? I just wanted to extend the range of the amp, but it sounds perfect through input 1 only with the cab I just bought...

either way, I took it back downstairs and he popped a new fuse in, tested the tubes and everything was fine - so he really doesn't know what happened. Who knows!?!

Here's a pic of the amp in it's new house and with its new cabinet (an Orange 2x12) and some of my home-built pedals (a couple big muffs, a fuzz factory, an overdrive and a delay) - I will do sound samples next weekend, but this by far and away blows any new JTM45 reissue I've played through - the tone is warm and open and I honestly couldn't be any happier with it. Smile


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tangelolemon



Joined: 26 Jan 2009
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Location: Brooklyn, NY

PostPosted: Mon Mar 10, 2014 8:58 am    Post subject: Reply with quote

Of the two conditions you described, I'd say it was probably the 16 ohm cab. It's actually a good sign that the fuse blew so quickly in that circumstance.

Congrats on your great work!!
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greysun



Joined: 31 Mar 2011
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Location: Chicago

PostPosted: Mon Mar 10, 2014 11:42 am    Post subject: Reply with quote

really? They said that the amp should be able to deal with the load (of having the 8ohm selected but plugged into a 16ohm cab)... I remember reading someplace that doing that isn't great for the amp, and that's the first thing I checked. I'm a little surprised the guy didn't set it up right, to be honest... I'm just glad they fixed it for free right then and there.

But I definitely worry about plugging in a jumper again - I don't know if there's a way to short to ground or something that would cause that to blow the fuse, but maybe I did? I definitely want to hear how that sounds, though.

I plugged my guitar into lower 1, and jumpers from lower 2 to upper 1. Is there a different way to do that?

After all that I'm always a little nervous to flip the standby now - doesn't stop me, but still.

Also - I typically turn the amp on and wait a minute to flip the standby - I turn off the standby when I'm done and wait to flip the off switch. Is there some kind of rule of how those need to be turned on and off?

I know - it's a dumb question, LOL - but I want to treat this amp right, so even the dumbest question isn't off limits for me. Wink
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Bootsypratt



Joined: 01 Dec 2006
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Location: Columbia, SC

PostPosted: Tue Mar 11, 2014 6:26 am    Post subject: Reply with quote

When turning it on, you want it on stand-by for a minute or so before turning it all the way on. I believe, correct me if I'm wrong, you can just switch it all off at once if need be.
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Heavy Air



Joined: 04 Aug 2008
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Location: London, UK

PostPosted: Tue Mar 11, 2014 6:46 am    Post subject: Reply with quote

I put it on standby, then pick a guitar, plug in and dig around for a pick, then turn the amp on. So I guess a minute or so on standby, depends which pair of jeans my stash of picks are in Embarassed

Same thing when I turn it off, standby, put everything away then turn off.
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tangelolemon



Joined: 26 Jan 2009
Posts: 2691
Location: Brooklyn, NY

PostPosted: Tue Mar 11, 2014 12:32 pm    Post subject: Reply with quote

8 ohm output into 16 ohm cab is an "upward mismatch" which is the most dangerous kind for a tube amp. However, in this case, it's only a 100% difference, which shouldn't be fatal in any case. However, if the fuse blew, it could be a sign that the amp wasn't happy. This would be more likely if the amp was seeing lots of signal and doing lots of heavy lifting (i.e. loud, lots of bass, etc).

In any case, the outcome is not entirely surprising to me, given the circumstance. If it had been a 4 ohm cab (i.e. half the amp's output impedance) then it would be more puzzling, to me. Marshalls are known to be touchier about impedance than some other amps-- an unplugged cab (i.e. infinite impedance) can melt an output transformer in very short order.

I cannot conceive of a situation in which jumpering amp inputs could cause a fuse to blow. We're at a very low-level part of the circuit at that point, with no real flow of current (by the jacks). The only current in that part of the amp is what's generated by your guitar, and we know that's not very much! It's not until we make it to the first tube that we begin to be in a territory that could draw current from the power supply.

As for "standby"--

It's not as crucial as some would have you believe. In an amp with a tube/valve rectifier-- particularly one with a delayed rise time like a GZ34-- it's almost altogether unnecessary.

The purpose is twofold-- foremost, it allows you to shut the main B+ power off during set breaks, etc. without having to wait for the amp to warm up all over again. It also has the added advantage of allowing you to warm the tube cathodes before applying high voltage to the plates. People have started to conceive of the second purpose as the most important, but it's really not.

There's a phenomenon called "cathode stripping" which is alleged to occur when plate voltage is applied before the cathode is warm. This is a real phenomenon, but it mostly is observed in super high-powered broadcast tubes and such with kilovolt-range plate voltages. I've never seen evidence of it happening in a receiving (audio) tube.

Still, it couldn't hurt to give the filaments a minute to get warm before plate voltage is applied. Maybe it will extend tube life, who knows. I've never seen evidence that it does, but it "couldn't hurt."

In an amp with tube rectification, though, this feature is "built in" a bit. High voltage (B+) won't be applied until the rectifier tube gets warm, and then will rise gently and gradually. The other tubes are warming up while the rectifier is, so filament voltage will always "beat" B+ by at least a few seconds. GZ34/5AR4 have a longer time lag than some others, and so are even gentler.

But again, standby for 30 seconds to a minute before flipping the mains isn't a bad practice, and I do it myself, "just in case." I can conceive of zero advantage, theoretical nor practical, of reversing the procedure on power-down. I just shut 'em both off at the same time.
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ajeffcote



Joined: 09 Nov 2009
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Location: Baytown,Tx

PostPosted: Wed Mar 12, 2014 12:11 am    Post subject: Reply with quote

Shutting down, I'll leave the standby on if I'm going to swap tubes or if there's a chance I may open it up before I play it again, to drain the zap out of the caps. But I always let the tubes warm up a bit before flipping the standby to play.
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tangelolemon



Joined: 26 Jan 2009
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PostPosted: Wed Mar 12, 2014 6:35 am    Post subject: Reply with quote

Safety tip--

Leaving an amp on standby doesn't drain the caps.

"Standby" is just the filament supply-- literally ONLY the 6.3v heaters stay on when the amp is in standby. The rest is a hard "off" just like unplugging the amp from the wall. There may be some amps that do it differently, but every one I've ever seen is this way.
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