hmm, is this amp tech no good?

[popthree]

i have a little fender champ that i re-tubed awhile back. i've never had the amp gone through by an amp tech...so i decided to take it to a little place here in town and have it looked at... figuring i may need to have a few caps replaced, and have the amp biased.

so they took it in, kept it for 6 weeks, called me and said it was ready. i go pick it up and the ticket reads "amp is class A, self biasing" caps check good.

they only charged me 20 bucks.... but did they do anything ? i thought i'd read here on the amp forum from many sources that champs do require biasing. specifically, mine is a late 70's silverface champ.

[skeksis]

my $0.02 would be that you've got a surprisingly honest tech. too bad it took 'em so long. probably because he's got a ton of work from word-of-mouth recommendations :-)

my understanding is that there's no need to bias a champ, or at least not in the sense usually meant for larger push-pull amps. it's been awhile, though, so I may be remembering wrong. if the caps check good, you'll probably want to have it gone through again in 5 years, but I wouldn't sweat it much now. just enjoy.

[Billm]

I'd say your tech is old school; probably doesn't believe in replacing caps until they're smoking or there's goo coming out of the vent. After 30+ years, I'd change out all of the electrolytics.

Yes, a single-ended amp is self-biasing, but you can change the bias point with a cathode resistor swap. There's also the issue of whether or not yours has a screen resistor (probably not, but probably a good idea).

[Durtdog]

6 weeks for that? He may be honest, but that's a long time to give a little amp like a Champ the once-over.

[pedenphoto]

The tech is a professional and his bench time is worth something even if his conclusion is no work is needed. Seems like he's honest and not terribly expensive. Most techs I know charge $75.00 to $80.00 for an estimate which of course is credited if there is work to be performed. J Peden

[JimiBryant]

you've got an honest tech, that's like GOLD in my town!!

[younkint]

Seems like a fair price for an honest opinion to me.

[popthree]

all good to hear. thanks.

i was really scratching my head about it because i could've sworn i've read here on the amp forum that a champ needs to be re-biased after a tube swap. guess i was reading wrong.

[giginthesky]

if you buy matched tubes, would you still need a bias?

[Guitarslinger1]

Originally Posted by popthree

all good to hear. thanks.

i was really scratching my head about it because i could've sworn i've read here on the amp forum that a champ needs to be re-biased after a tube swap. guess i was reading wrong.

Actually what you read is correct. For the best tone and tube life, the bias should be checked on every amp, including a Champ, after a power tube swap, and adjusted if necessary. Doing so can be a bit of a pain on a cathode biased (self biased) amp like a Champ, because the power tube cathode resistor value must be changed to set the bias correctly. The good part is, that you only need a volt/ohm meter to check the bias on it.

Adjusting the bias properly (or improperly for that matter) can have a fairly dramatic effect on tone, even in a single 6V6 Champ.

[trdlasvegas]

Originally Posted by giginthesky

if you buy matched tubes, would you still need a bias?

If you buy matched tubes you still won't need it biased, and you can take one of the matched tubes and keep it as a spare since the Champ is single ended amp and ONLY uses 1 output tube.

-Tony

[Guitarslinger1]

Originally Posted by trdlasvegas

If you buy matched tubes you still won't need it biased, and you can take one of the matched tubes and keep it as a spare since the Champ is single ended amp and ONLY uses 1 output tube.

-Tony

Hey no doubt. Buy a matched pair, install one and bias the amp, and you're good to go on the next tube change. Providing you remember where you stashed the spare tube...

[bradpdx]

Specialty and I have gone around this one before, but I'd say your tech was being honest. The Champ wasn't meant to be biased and does use a very traditional self-adjusting circuit - always did.

The only reason to consider changing bias (which involves swapping resistor values and some trial and error) is to correct a condition in which the tube is running at excessive power (heat) dissipation. Fender's design does run them hot, and I can see this as an issue with cheap 6V6s. Back in the days of all USA tubes Fender figured it didn't matter, and judging by all the Champs they sold, it didn't.

But other than that it won't really change the sound of the Champ. The output is totally asymmetrical no matter how you slice it, and you would only be tweaking that asymmetry. They are, after all, pretty crude little amps.

[Guitarslinger1]

Yes Bradpdx, we have been through this before. And I'm surprised you're still perpetuating this myth after I posted all of those links from revered amp experts.

I posted links to several websites, and have several books, that all discuss the need to properly bias cathode bias amps, including single ended amps such as Champs.

Looks like I need to repost those links. We really need to put this myth to bed that biasing doesn't matter in Champs.

If you just want the amp to make a sound, any sound, and aren't concerned about tube life, then sure, biasing doesn't matter.

But if you want the amp to sound as sweet as possible, and get as much tube life as possible, it is necessary to check the bias when a new power tube is installed, and correct the bias if needed.

With the variation in the B+ voltages due to component tolerances, even in the same model amplifiers, variances in line voltages across the country, and the wide variance in tubes, it's silly to think that the bias shouldn't be checked and adjusted if needed in these amps.

The bias greatly effects the tone, ranging from (1) thin and wimpy, to (2) sweet and chimy with great dynamics when played clean and a great cranked tone, to (3) flat and raw sounding even at low volumes.

I don't know about you, but I'd prefer (2) out of the above scenarios. But, to each his own.

Originally Posted by bradpdx

They are, after all, pretty crude little amps.

They may be designed from "crude", simple circuits but they don't have to sound crude.

[11 Gauge]

Originally Posted by popthree

so they took it in, kept it for 6 weeks, called me and said it was ready. i go pick it up and the ticket reads "amp is class A, self biasing" caps check good.

they only charged me 20 bucks.... but did they do anything ?

Yeah - you were charged a 6 week storage fee!

I'm mostly kidding - 20 bucks is fair for someone to pull a chassis, spray some contact cleaner and vacuum, and hook up a meter...

...Still - 6 weeks for a Champ? You could ship it ground to any number of techs and get it done in a fraction of that time. Not that I recommend that, but still - 6 weeks is unreasonably long especially for a Champ, IMO.

Also - the "caps check good" part doesn't really satisfy me. You can visually inspect them, throw a meter on them, etc., but that doesn't take age into account. Unless they could pull a year of manufacture off of the caps, I'd be a little suspicious. You can check this yourself - the champ has a cap can in the middle of the chassis, underneath. The originals were typically made by GE and stamped as such. Replacement cans typically have a sticker or silkscreening on them.

Something along the lines of rebiasing - the B+ should be checked. With modern wall voltage (averaging 127 VAC), my B+ was around 420 VDC, or 70 volts higher than Fender's original spec'd amount.

[Teletale]

tt

[tele-martini]

IMHO I think the tech was honest and depending on what was mentioned when you brought it to him he may have done the right thing. In other words if you told him look ,I don't want to sink much money into this amp he may have done the right thing. If you mentioned that you wouldn't mind spending a few bucks to re-fresh caps and make sure you have continued reliable service then he should have changed the electrolytics. Not a huge job or expense.

I have a question on the bias issue. If the voltage is to high (from increased line voltage from wall socket) would you replace cathode resistor with a higher value resistor to lower DC voltage at B+

[Teletale]

tt

[Guitarslinger1]

Originally Posted by Teletale

The only reason to change the bias point in a champ is if the tube red-plates when it's dimed.

A 6V6 can be dissipating more than 12 watts in a Champ, which sounds like hammered donkey kong, and not be red plating.

I happen to have two un-matched NOS RCA EL84s right here that I bought for my new single ended GA-5 Ri. With one, the plate dissipation worked out to be 10.9 watts and it sounds sweet. With the second, the plate dissipation worked out to be 13.3 watts and it sounds raw and not-so-great. I ran with the first one since I could, but if I only had the second one to work with, I'd need to adjust the bias on the amp to get that plate dissipation under 12 watts.

Originally Posted by Teletale

but remember that tubes are often only matched for mutual conductance not for headroom/threshold.

They are typically matched for current draw as well, which is what comes into play here.

Unfortunately, so many cathode biased amps, including single ended, are improperly biased, making them sound bad, that many people think that's how they're supposed to sound.

Oh well.

I suggest that anyone interested in this subject do some research on the net.

[Teletale]

tt

[Guitarslinger1]

Here's some info from the Aiken Amp site.

http://www.aikenamps.com/

What about cathode-biased amplifiers?

Do cathode-biased amplifiers need to be biased? The short answer is yes. The cathode biasing method is self-regulating, to an extent, because increases in cathode current create a larger voltage drop across the cathode resistor, which in turn, creates a larger negative grid-to-cathode voltage, which counteracts the increase in current. The tube will reach a stable point of equilibrium and stay there. However, just as different tubes from different manufacturers will draw varying amounts of current in a fixed-bias amplifier, the same is true of a cathode-biased amplifier. For this reason, the bias should always be checked, even with cathode-biased amplifiers.

Checking the bias current in a cathode biased amplifier is easy, just measure the voltage across the cathode resistor and divide by the resistance value to obtain the cathode current. Note that if the output tubes share a common cathode resistor, you must divide the current reading by the number of tubes sharing the resistor. Note also that a common cathode resistor does not allow you to determine the individual currents of each tube, so if one tube is drawing more current than the other, you would not be able to determine which is causing the mismatch, and, in fact, you would not be able to tell there was a mismatch at all. You can add individual 1 ohm resistors from the cathode of each tube to the common bias resistor, but you must then measure across the 1 ohm resistors, not from the cathodes to ground, to determine the voltage drop, and thus the cathode current. You can also use individual cathode bias resistors on each tube. The value of the resistor will be double that of the common resistor if two tubes are used, or four times that of the common resistor if four tubes are used. Each resistor would also have to be bypassed with its own electrolytic bypass cap.

The difficulty with cathode-biased amplifiers is that the cathode resistor must be physically changed for another one of different value in order to change the bias current. Although it can be done, very few guitar amplifiers have adjustable cathode bias.

Cathode-biased class AB amps are usually exempt from the "70% rule", because their cathode voltage rises when a signal is applied, effectively reducing the bias, and shifting the amp further into class AB operation. This means you can bias them hotter than a normal fixed-bias class AB amp and the tubes will still survive. Having said that, you have to experimentally determine how hot you can bias them by finding out how far the bias shifts during signal flow.

If the cathode-biased amp is "true" class A, there will be no bias voltage shift seen on the cathode when signal is applied, so you can bias at max dissipation and not worry about it. If the amp is actually class AB, you might still be able to get away with biasing at max dissipation because of the large bias shift at full power that pushes the amp into the class AB region, but you should check the tube dissipation at all signal levels. Note that max dissipation may not occur at full power, rather at somewhere between idle and full power (usually around halfway), so you have to carefully determine the safest max idle current to avoid exceeding the dissipation at any point in the tube's operation.

Do I really need to bias my amplifier every time I change tubes?

You will occasionally hear guitarists say things like "In the old days, we just bought new tubes, stuck 'em in, and started playing", and "We didn't worry about biasing back then, and you don't need to now", or perhaps: "Biasing is a myth". In most cases, you don't have to bias your tubes when you change them. You can just plug a new set in and start playing, especially if you aren't too particular about setting up the amp for the absolute best tone. However, if the new tubes you have plugged in are different enough from the ones that were in there, with respect to current draw for a particular grid voltage, they may end up biased too hot for that particular amplifier. In this case, your new tubes will start to glow cherry red on the plates, either at idle or while playing, and they will soon be destroyed. In addition, the tube may short out and take out the output transformer in the process, leading to costly repairs. Tubes of the same type from different manufacturers will usually vary greatly in current draw at a particularly grid voltage, but even two different tubes of the same type from the same manufacturer can vary widely in their current draw. For these reasons, it is always best to check the bias after installing a set of tubes.

[chile]

I wonder whether Hendrixs guitar tech checked the bias on his new amps every time he put his guitar into an old one?.
I often wonder how up to speed all the old guitar heroes were on this stuff?.
Just thinking.

[Brent Hutto]

I think answers to two slightly different questions are being debated in this thread.

Question 1) If a non-technical guitar player with no test equipment or electronics tools changes tubes in his cathode-biased amp, should he consider re-biasing the amp a necessary step immediately after the tube change?

Question 2) If a professional techician has a cathode-biased amp opened up in front of him on his workbench is it a reasonable expectation that he'll check the cathode current and confirm that the tube is operating at a desirable voltage and dissipation?

For my part, the answer to Question 1 is that only an obsessive gear geek of a guitar player will lose any sleep over the possibility that he needs a 20% change in his bias resistor value.

And for my part, the answer to Question 2 is that yes the technician ought to confirm the operating conditions of the power tube and offer to change the bias resistor if it is indicated. How long would it take to measure plate and screen voltages and the voltage drop across the bias resistor, maybe a minute or so once the innards are fully exposed?

[JDO]

i think i'm more with teletale in practice.

in theory, yes the tubes have a data sheet and tell you what the max output is and blah blah blah. well, if we all believed that then start taking the PT's out of your deluxe reverbs. or since the amp is running so far out of spec and can't be brought in spec with it's design just give them to me.

if it sounds good to you and nothing is redplating or blowing up, it's fine. honestly, a simple rebias won't really work for a blackfaced champ. the voltage is too high for this single 6v6. again, if you want it to run in spec with a 6v6, get a different PT and then rebias. since we're not going to do that, just ask yourself if it's working and if you like how it sounds.

i have a vibrochamp with the stock biasing. there might be a better bias point but i don't care to find it. i'm happy with the amp as is and nothing is blowing up or redplating