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Dirty-South Blues Harp forum: wail on! >
tube amp questions
tube amp questions
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Hobostubs Ashlock
1729 posts
Mar 11, 2012
3:16 AM
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well I got me a fender super champ xd a few weeks back,And I have been messing around firguring out diffferent settings,I got it mainly for guitar,But come to find out its not a bad harp amp,I know some people on here dont like em ,But compared to the other 2 practice amps I had befor it really is nice,I love how tubes sound its my 1st tube amp,Ive known for years that there were great for guitar but never had one,And tonight I hooked up my green bullet and cuped her tight with some delay and man she aint that bad for harp,well at least From what i was using,but anyways ,Like I said I never had a tube amp.and I got to reading where If you switch out the power tubes and dont go back to the factory specs,they need to be biased,I looked up that,and came to the conclusion that I would probelly just buy the factory tubes,But was wondering how much does it usually cost to have a amp Biased? and How often do you need to change the tubes,i heard every year aint bad,I got the last superchamp they had in the store and it was the floor model so it probelly has some milage on the tubes. When do you know that its time to replace.To me she sounds great,But i never relised how much extra trouble having that warm tube sound can be maintiance wise,I did find a tube shop online that had tube sets made for the superchamp xd in 3 models depending on how much you wanted to spend with the basic setup for around 45$ Is there a certain place anyone recomends to get tubes in the US,any help is greatly apreaciated and thanks
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Hobostubs
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5F6H
1097 posts
Mar 11, 2012
4:02 AM
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Unlike tweed, BF & Silverface Champs, the Super Champ XD is "fixed bias" as opposed to "cathode-biased" (the output section owes more to a tolex Princeton design than any classic champ). Fixed bias amps usually require the idle current (bias) to be monitored & sometimes adjusted when installing new tubes. Cathode biased amps are incorrectly referred to as "self-biasing", cathode bias is generally more forgiving and often only "needs" to be adjusted in extreme circumstances.
The good news for you is this, the SC XD runs its power tubes at fairly low voltage, fairly low idle current, has a bias adjust pot fitted and idle current monitor points...if you wanted to adust the bias, it would only cost you the minimum bench fee from a tech - getting into the amp takes more time than setting the bias.
In short, if the amp sounds good as it is, it will take several years of hard gigging before you NEED to change them.
Your perception of "tube amp maintainece schedules" is greatly exaggerated. 1000 to 2000 hours of use is a rule of thumb for guitar amps, for tube replacement Your amp does not stress its power tubes that hard, so I'd go for AT LEAST 2000 hours - which equates to 3.5 years, gigging 200 nights a year, 3 hours/set...if you were working that much, then $45 every 3.5 years isn't much $ to stay on the road ;-)
If it was my amp, I'd look to change them every 5 years of hard use at the outside, just noodling at home? You might get a decade! Most people would change their tubes due to curiosity, rather than necessity, during this kind of timeframe.
6V6 powered Fender amps typically come fitted with Electro Harmonix 6V6 tubes, these are pretty good 6V6s.
You only have a "problem" if you want to have one ;-) Relax & enjoy.
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Hobostubs Ashlock
1731 posts
Mar 11, 2012
6:03 AM
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thats cool,and thanks for all the info,I enjoyed the read,;-)
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Hobostubs
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hvyj
2247 posts
Mar 11, 2012
6:34 AM
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You know, in my Fenders, I replaced the stock tubes (which were Groove Tubes or Ruby Tubes) with JJ Electronics Tubes which I think sound warmer and fatter for harp. I did that for tone. But since then, it's been years and i haven't had to replace any tubes.
But the guitar player in my Blues Band who plays through a custom Fuques amp just had to replace power tubes and that amp is less than 3 years old--maybe about 2 years old. His amp kept blowing fuses when the power tubes went bad.
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jbone
824 posts
Mar 11, 2012
6:41 AM
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i have a silvertone 1482 which i've played for about 9 years now and have not changed a power tube yet. this amp has been used at home and out at clubs. i'd estimate it's run about 50 3 hours gigs and dozens of 45 minute single sets in that time, not to mention a lot of live rehearsals. i have yet to have an issue with it. i figure when it craps out it'll go to the shop.
one q i do have about amps in general is, does using an amp at higher volumes put more wear on the tubes?
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Hobostubs Ashlock
1732 posts
Mar 11, 2012
7:49 AM
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I was wondering that myself
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Hobostubs
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5F6H
1098 posts
Mar 11, 2012
8:52 AM
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Good question Jbone - the answer is "it depends" :-I
What kills components is heat. The primary source of heat in a tube is the amount of idle current. Cooler bias means longer tube life (figures for the SC XD suggest factory bias is set on the cool side - 20mAx375vc = 7W per tube, or 50% dissipation for a 14W 6v6).
Now run that 6V6 at 90% dissipation in a fixed bias amp & I'd expect the part life to decrease...sharply.
Also be aware that for a given bias setting, an overdriven guitar signal, pushing the output section well beyond clean rated RMS (a "50W" amp putting out 60-70W), will kill the tubes quicker than a harp & mic at those same bias settings, because with harp we are usually using the power section at, or less than, full rated power output (a "50W" amp at 30-50W?).
Bias is always set at idle (no signal) but it is really the average signal current (whilst playing), less the power sent to the speaker, that is the work that the tubes do. Cold bias lets everything cool back down at idle (gaps in playing). If the amp only exceeds rated W RMS occasionally, then *you may* get away with hotter idle settings. If you constantly run the amp past the max W RMS rating, at regular/high idle currents, your tubes wear quicker (like HYVJ's pal using guitar with a Fuchs - tube current kills fuses when the tubes ask for more than the fuse can handle).
It's rarely beneficial to run fixed bias amps at very hot currents (one or two amps buck this trend for secondary reasons, like controlling plate & screen voltage that may be excessive for the tubes at lower currents) even for guitar. For harp I tend to aim for "just enough" idle current to give the tone you like.
All the above applies to "push-pull, fixed bias" amps. Cathode-biased amps & cathode-biased single-ended amps are a different story & run high idle currents/lower voltages as part of their design...a single-ended, cathode biased SF Champ might do a decade on a single 6V6 despite running at 150% dissipation! :-) This is due to the peculiarity of its design.
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Last Edited by on Mar 11, 2012 10:22 AM
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Greg Heumann
1526 posts
Mar 11, 2012
10:12 AM
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Hobo - why would you change the power tubes in the first place? MOST of the time when we talk about tube swaps to make them more harp friendly, we are talking about swapping the PREAMP tubes out for lower gain ones. IF that's what you meant, you do not need to re-bias the amp at all. That is only done for the power tubes.
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/Greg
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5F6H
1099 posts
Mar 11, 2012
10:30 AM
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Hi Greg, power tubes have gain too, bias & brand of power tubes can well affect an amp's "harp-friendliness" too (though this doesn't seem to be Hobo's concern with his SC XD - if within reasonable perameters & stock tubes, should be just fine), but indeed, you have a wider choice of workable preamp tube options for some amps (not all amps react well to lower gain preamp tubes alone).
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Last Edited by on Mar 11, 2012 12:21 PM
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hvyj
2249 posts
Mar 11, 2012
10:52 AM
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HEAT....My Fender Super Reverb RI and my Fender Princeton Reverb RI have better tone after the tubes get HOT. So, i never use the standby switch on my Super. I let it cook in between sets so the tubes get nice and hot. Maybe I'll need to replace them sooner rather than later, but so far, so good. The Princeton doesn't have a standby switch.
My guitar player's Fuchs has a power selector switch so it can run at 50 or 100 watts. It also has cascading and selectable gain stages controllable with switches on a pedalboard. So, i assume different combinations of tubes are in use at different times. I dunno--but his amp sure sounds great when it's not blowing fuses.
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5F6H
1100 posts
Mar 11, 2012
11:32 AM
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HYVJ, both your SR & Princeton come stock wih GZ34/5AR4 rectifiers which have a "slow warm up", the standby is pretty well moot.
All amps get hotter when you play them, signal causes the plate current to rise, even amps that are biased cold, get hotter in use...the critical factor is how hot. Unless they are biased to unusually high idle currents, tubes will last the usual lifespan (JJ tubes are very sturdy). Fixed bias amps can be left on for days, if correctly biased, as I noted before, they can cool off (relatively) when not being played, rather than get hotter.
Preamp gain stages affect the tone, not necessarily the power the amp makes. For a 50-100W power change it is more likely that the power section works just as hard, just uses less power tubes?
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Hobostubs Ashlock
1733 posts
Mar 11, 2012
5:03 PM
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thanks guys for the info.
No Greg I was just wondering when they needed to be kept maitained,Im happy with the stock tubes in it,
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Hobostubs
Last Edited by on Mar 11, 2012 5:08 PM
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Littoral
496 posts
Mar 11, 2012
5:17 PM
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I have an early 60's Harmony that sounds fantastic and I assume the tubes are original -every one has the Harmony logo stamped on them.
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