Guitar Amp- Tube Change

January 26, 2007 3:33 PM

Replacing power tubes in a guitar amp

I’ve been considering swapping out the power tubes in my amp. I have a 100 watt Carvin 3212 2x12 combo with Groove Tube 5881s, and I’m wondering if I could get a more Marshall-like sound by using EL34s instead. Spent some time on Google but the information I’m getting is pretty technical; I really need some more straightforward answers to the following:

1. Has anyone here done a similar change? Did it make a significant difference in the sound you got from your amp? Any unforeseen problems?

2. The amp has a bias switch to go from 6L6s or 5881s to the EL34s. A lot of what I have read talks about bias adjustments that would need to be done by a professional, but I’m not sure whether the switch would take care of that or if further adjustments would be needed. Can I just flip the switch and install the tubes myself, or should I take it to a pro? What’s a normal price for that service?

3. The amp is switchable down to 50 watts, which is how I usually play. Right now the switch is broken and I’m locked in at 100. Can I just pull out two of the tubes to get to 50, or should I get that switch fixed? What do you think it would cost?

4. Does anyone know a reliable amp repair shop in the LA area, preferably somewhere between Redlands/San Bernardino (where I live) and Whittier (where we practice)?

posted by InfidelZombie to media & arts (12 comments total)

It makes a noticeable sound difference.

Your amp will likely need to be re-biased when the tubes are swapped -- have a professional do it, for sure. As long as a pro does it, and it's biased properly, you shouldn't have any problems.

The bias switch is helpful, but not always precise -- have a professional do it. As far as I know (IANAAmp Tech) the most common problem with improperly biased amps is that they burn through the tubes fast. But maybe there are worse problems, too?

Getting the switch fixed while you're getting the tubes swapped out and re-biased shouldn't cost much. A few months ago, I took my 20 year old amp to a great amp shop in San Diego to get a speaker fixed and when I picked it up, the guy had repaired my tremolo unit and a switch and done a tune-up on the chassis -- no charge above what the speaker repair had been estimated at.

It was the first time I've taken an amp to be repaired, and I should have done it years ago. It was inexpensive, and my amp sounds incredible now.
posted by JekPorkins at 4:35 PM on January 26, 2007

You can just flip the switch and install the tubes yourself. If you buy tubes from someone, they ought to do this for you for free; it shouldn't take more than one minute.

At the risk of starting a holy war, I think that 90% of the guys out there who want to "rebias" your amp by hand don't know an electron from a hole in the ground. Tubes need to have their plate voltage in a certain range to operate properly with good life. If the plate voltage is wrong, you shorten your tube life. A lot of guys are like, hey, let's make the plate voltage wrong, I like the way it sounds. Fine, whatever. That shortens your tube life and if taken to an extreme can lead to operating your output transformer out of its rated spec.

The reason your amp has a bias switch is simple: 6L6s, EL34s, and 5881s all require different bias voltages.

Depending on how your amp is wired, it might actually be OK to pull a pair of tubes, but wouldn't it be easier just to get the switch fixed? What's wrong with it?
posted by ikkyu2 at 4:57 PM on January 26, 2007

Bias is applied to the control grids of a vacuum tube, so that the signal it recieves can not force it out of its designed range of linear operation. For power beam tetrodes like the 6L6, which are generally designed to be operated in Class B push-pull output circuits, the application of bias to force Class B operation also results in lower noise and reduced harmonic distortion due to the suppression of secondary plate electrons. The original 6L6 tube was a metal jacketed tube, rated for something like 17 watts at 360 volts plate, and 270 volts screen. Most often, they are replaced these days by the physically larger 6L6G glass enveloped variant, which has a slightly higher plate power rating of 19 watts at 360 volt plate and 270 volt screen.

The 5881 tube is a slightly higher power variant of the original 6L6, and is also known as the 6L6WGB. It's a bigger glass tube than the original 6L6, and carries a maximum rating of 23 Watts output at 360 volts plate, and 270 volts screen. Your tube "bias" switch may be attempting to set these different bias resistor networks to maintain proper bias voltage for changed operation from stock 6L6 tubes at the higher plate and screen currents these lower impedance tube variants create.

There are now even higher power variants of the 6L6 family, the 6L6GC, and the 7581A, which achieve their 30 watt plate power ratings through plate voltage increases to 500 volts, and 450 volt screen voltage. These are physically bigger tubes, that also generally drive bigger output transformers, and need much beefier power supplies, to work right. You can't typically substitute these tubes in an amp like yours without damaging your amp, so don't try.

It's so hard to have conversations with muscians about operating amps in distortion, that I doubt anything I say here isn't going to be immediately countered by war stories from many others. Believe then, what you will. But I advise you to not run an amp with "empty" power tube sockets, as you'll be shortening the life of various capacitors that should be filtering for tubes that aren't there, and you'll be well off the bias curves if your amp was correctly biased for operation fully loaded. The "power" switch is most likely a gain control network, that switches in a padding resistor network on the signal driving the final stage of the amp. Your tubes will last a lot longer running in this mode, and you sacrifice only 3db of "head room" in terms of absolute volume. So that's the right place to run, and the right way to do it.

Bias is trimmed by a technician, to balance slight manufacturing differences in tubes, so that running in a push pull Class B configuration as these tubes are designed to do, the tubes do equal work, and the waveforms presented are amplified without clipping distortion. Bias needs will change slightly over the life of the tube, as the cathode, plate, and screen grid coatings change, and as as any mercury vapor in high power tubes is adsorbed into the tube elements. The bias changing switches on your amp are intended to allow you to continue to play safely if you can only substitute variant rated versions of tubes in a gig situation, not as the best means of trimming the amp for good sound and full power operation. That kind of bias adjustment needs to be done on a bench, with a low distortion signal source, and both harmonic and intermodulation distortion analyzers. People who tell you they can "power tune" by ear are generally BS artists, who will shorten the life of your tubes, and probably your amp, substantially.

If you want to play with "distortion," you shouldn't do it by trying to muck up the operation of the final power stage of an amp. It's both a lot better for the amp, and a lot more musical, if you distort the low level signal fed to the power stage, and let the power stage amplify that cleanly.

You can buy hand matched tube sets, which are pairs of tubes that have been hand tested and selected for having very, very close electronic performance, and are from the same manufacturer, usually made in the same manufacturing runs, and which are expected to age identically in service. You pay a lot for such tube sets, because of the hand testing and the hassle of maintaining and selling matched pairs as inventory items, but they can sound sweet in a well designed amplifier. In musical instrument amps, I doubt anyone would know the difference, but if you keep track of hours on your amp, and re-tube on a schedule for reliability reasons, it can make sense to install matched sets when you retube, and it generally makes it easier to set bias properly and maintain it, over the life of the tube set.
posted by paulsc at 5:45 PM on January 26, 2007 [4 favorites]

ikkyu2: the sliding part of the switch fell off the last time I went to change the setting, right now you just see a couple copper strips. You're right, I should forget being cheap and just get it fixed.
posted by InfidelZombie at 6:54 PM on January 26, 2007

Excellent answer, paulsc. You put a lot of effort into that response and I, for one, enjoyed reading it!



Can I just pull out two of the tubes to get to 50...?


No, InfidelZombie. That would be like taking two wheels off your car to make it go half as fast!
posted by FauxScot at 7:09 PM on January 26, 2007

If it was a Mini, and they were the back wheels, that would probably work.
posted by flabdablet at 7:18 PM on January 26, 2007

These will allow you to install el34's.

I put them in my Fender (class AB) to make it sound more Brit (class A).

Totally a lay person operation.
posted by sourwookie at 7:23 PM on January 26, 2007

Link directs to the Yellow Jacket Converter, for those disinclined to click.
posted by sourwookie at 7:26 PM on January 26, 2007

I'm not an EL34 guy (6V6 is more my style) but I've heard great things about the Shugang ones and they're cheap at $20-25/pair. If you can't find them locally I'd recommend Lord Valve in Denver. I've bought tubes and speakers from him before and he's got good stuff; burns in and tests his tubes thoroughly.

You can just flip the bias switch to the EL34 setting but I'd probably heed the suggestions of having a tech check the bias unless you're interested in learning to do it yourself.

You can pull two tubes for 50 watts. Getting the switch fixed is worth doing, though, and it'll make resale higher if you ever need to sell the amp. For "best" results you'll want to set your impedance switch one lower, ie set the switch to 4 ohms for an 8 ohm cab, or 8 ohms for a 16 ohm cab. 50 watts is still damn loud, though.
posted by 6550 at 7:51 PM on January 26, 2007

Man, oh, man, InfidelZombie--there is a ready-product that does just as you wish--for very little money and no tools.

No biasing, rewiring or nothing. Just get the (Bruce Zinky designed--as far as I know) Yellow Jacket converter and go.

You will lose output power, but in class A terms how much do you need (an AC30 is too loud in most venues)? Even if you dropped to 25 watts class A you will still piss off sound guys. Later on, drop in a pair of bluebacks and you will have the ghetto Vox.
posted by sourwookie at 12:50 AM on January 27, 2007

It pains me to have to point this out, by the way, but nearly everything in paulsc's answer above is wrong or not applicable to guitar amps. Guitar amps rarely if ever operate in a pure class B setup - many bill themselves as entirely class A, although strictly this probably isn't accurate - and there is no desire among guitar players to minimize distortion in the power stage. This is in stark contrast to audiophiles with their $20,000 tube receivers; most of paulsc's comment is accurate with regard to the way these folks run their KT-88's.

Power stage distortion - with properly biased tubes - has been desirable to certain guitar players since the 1960's and, modelling aside, its character cannot be replicated by altering the signal source prior to the power amp. You can google "recto sag" for one example of why.
posted by ikkyu2 at 3:44 PM on February 20, 2007

Put a set of EL34's in the amp yesterday (Groove Tube E34L-S) and it sounds fantastic. We were all amazed at how much it changed the sound of the amp, and how much better it cut through the mix in practice. Next on my list is some new preamp tubes...

Thanks everyone for the help.
posted by InfidelZombie at 8:52 AM on February 28, 2007