Tube amps are specialized amplifiers that are known for their musical reproduction. Although distortion is generally higher to much higher in tube amps, the nature of the distortion is not generally displeasing.
It is amazing that, when a guitar amp is fed a source of hi-fi audio, and plugged into a hi-fi speaker, what a great job it does and how good it sounds!
The guitar amp has had a comparatively long history in modern technology, and it is surprising that the Vacuum Tube (Valve) Amp is still so much sought-after.
I'm not a guitar player. I'm a technician who loves the sound of guitars. So most of my time is spent on repair, design, and listening. I've been doing technician work since the mid-70's, in audio, digital, radio, and computer electronics.
I repair vintage, classic, and modern tube amps inside the Loop in Houston, Texas.
Repair time is about 2 weeks, but it can be as long as a month including the wait for an opening on my bench.
Be sure and let me know when your next gig is, and I will try to work you in.
Same-day repair is available if possible. Priority is given to working musicians or others with a need to get their amp back in a hurry.
If your amp needs a special-order part or a large (expensive) part, I need at least half up front to order it. Sometimes I will give you the information to get the part and you can do that part. Then, when you have the part, the repair process can start.
Rates are as follows, based on a semi-flat rate.
>Minimum Charge with 1/2 hour labor: $47.25
>Normal Checkout and Estimate with 3/4 hour of labor: $70.88
>Minor Repair with 1 hour of labor: $94.50, parts extra.
>Most often billed is 1.5 hour of labor: $141.75, parts extra.
All repairs are concluded with a final test:
- Low volume
- Noise, Hiss, Microphonics
- Function of Features, including Tone Controls
- Audio Quality at Normal and High Volume Levels
- Measured Output Power at Clipping
Rarely does the labor on a repair exceed $200. Most repairs including parts are less than $200!
I can rebuild, re-cap, or restore your amp into whatever shape you want it to be. I can just repair what you want fixed, as well.
I NEVER put in parts that you don't need, unless it is part of the diagnostic process. If I do put in a part that does not contribute towards the repair of your amp, I don't charge for them, and all old parts are returned.
Mods: If you don't like the way your amp sounds, we can discuss what mods would help it. I don't really like doing other people's mods. If you want a "insert specific name here" mod, take it to him. I can generally come up with my own mods, and they generally work out better.
References available.
Call
.The "Amp Repair Guy."
Need parts? I'm trying to make an outlet for parts. Don't call for parts, please. Use my new Amp Parts link.
Shameless plug for my friend... See Cathy's Theatrical Costumes for any purpose.
Article: Tube Biasing - A Complex Problem Made a Little Easier
Copyright webmaster@TubeModules.com 2009
Tube biasing is made all the more confusing by the wide variety of printed material on the subject.
One thing that said is true from all the sources: biasing is the one part of setting up your amplifier that needs to be at least close to where it should be.
First of all, bias circuit designs comes in two main varieties: self-biased (also known as cathode bias) and fixed bias, which is more common. You will see self-biasing on most amps that use EL84's, and some 6V6's.
"Fixed bias" means that it is set for a certain value and left there. It can be adjusted with a potentiometer, or set by a resistive divider and not easily changed.
A schematic is necessary to be sure of what you are doing, or some good directions from a source you trust.
The designers of any particular amp had a certain design goal when they published the bias settings for their amp. Some amps require that you set the grid bias for a certain voltage. Whenever I see this value in a schematic, I set the amp for that value. If there is no grid voltage given I use the method I describe here.
A Little Theory...
Class A operation makes both halves of the push-pull output section draw current constantly. This reduces crossover distortion, increases linearity, and makes the tubes stay close to the point where hard playing runs them into the rounded-top distortion that makes that "tube-y" sound tubes are famous for. The problem is, this constant current draw wears out the tubes, strains the power supply, and makes anyone with even the slightest bend towards energy conservation cringe.
Class AB, on the other hand, is a good compromise when done right. One tube is always drawing current, and sometimes both, but always in a small amount. AB is more like an electronic see-saw, where when one tube is up, the other is down. Careful adjustment of distance from the ground (bias) and position of the balance of the see-saw board (bias balance) makes for a wonderful result. Almost no crossover distortion, quite linear, and far enough from the tube's current limit that you have plenty of headroom. Oh, and idle current draw is not much over what goes into making music. Very efficient. Power supplies are happy, tubes are happy, and electric bill is not outrageous. And your tubes will last much, much longer.
(continued top of next column)
First, what is the magic number?
Where is the perfect bias point? I can only tell you what I believe as a technician.
Data that goes into this decision comes from two places: the tube data sheet and the amp's information, often just the schematic. But I always try to get an actual reading of the B+ or plate voltage myself, since there is a hidden factor that you don't often read about that makes it necessary to check that voltage.
Amps from earlier years were designed to work from 105, 110, 115, 117, or 120 Volts AC (USA) at the wall socket. That was the standard, nation-wide. The earlier the amp, the lower the voltage was.
These days, line voltages are higher. Mine is anywhere from 120 to 125 Volts AC. What this means is that since most amps do not have regulated power supplies, bias voltage and current requirements published with the amp do not apply with today's line voltages. These older amps need to be adjusted for today's line voltages. Also, an amp that sounds great in the city will sound different at that end-of-the-wire roadhouse in the country, especially when the A/C comes on. The voltage can be as low as 110 volts at the end of a long run of country wire. You need to adjust it to the line voltage it will be used with.
How to figure what the bias should be.
Look up the tube specs from the manufacturer, and find Maximum Dissipation in Watts.
Look up or measure the place voltage or B+ voltage of the amp in question.
Example: 6L6 in a Fender Twin 50 W = 460 V * ?
1. W/V = A 50/460 = 0.1087
2. 25% of A = 0.1087/4 = 0.027 or 27mA of bias current
The source of the 25% figure is a little hard to explain. I think it is more a rule-of-thumb than any hard fact. I would think that you would not want any more than 1/4 of your tube's capacity wasted on just keeping it active.
Now, these figures were pulled from memory. but they just happened to come real close to what Fender says the Hot Rod DeVille should be set for, that is, 60mA for two tubes in parallel.
This can be done the same for any amp and tube combination.
Purists will argue (correctly) that there are factors I did not take into consideration, but really I haven't gotten to the good part yet. It is more complicated than just what I stated.
After all, how is this measured? Where do you measure it? Under what conditions? What factors are you taking into consideration?
The formula above figures very roughly the plate dissipation at a certain current and voltage. It doesn't take into consideration the DC resistance of the output transformer primary, or the screen grid current, which is often a sizable percentage cathode current during operation. But at idle, the screen draws little current. So we can neglect that safely.
But even if these things were not factors, the B+ or plate voltage would be. The figures say you need less idle current when the voltage is high, and more idle current when the voltage is low, to stay within the tube's maximum wattage (and therefore current) with proper headroom.
I measure the plate voltage at the plate.
I measure the current at the cathode, with a 1 Ohm resistor to ground. So do most amp companies that have a place to measure it.
I measure when the tubes are warm, about 15 minutes after turning the amp on. I check it again later after more run time.
Some schematics only show the bias voltage on the grids. There is no problem with this, either. The designers had a target for the best compromise in headroom and overdrive, and that was it.
(I am sure a lot of effort went into this figure. Probably they made 100 amps, had the techs adjust them, then measured the ones that sounded the best. That was the new goal. Repeat until it works on all of them. Meanwhile, make sure the current is not excessive, while good-sounding, because amps would fail.)
Conclusion.
Read the tube data from the manufacturer. Get the maximum dissipation figure.
Find the schematic, measure, or read somewhere what the plate voltage is for your amp.
Do the little math to get a target current in mind.
Buy or make a bias current reader. I just use a 1 ohm resistor to ground if there is room.
Set the amp's bias circuit to measure your target current, then try it out. Tweak for sound, measure for safety.
Don't go too high for idle current, unless you like buying tubes often.
Be safe. Observe rules about measuring high voltages, which can absolutely kill you or at least ruin your day.
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The whole amp, from preamp to output section, is built on the top of an
old-fashioned "breadboard." This board has the whole amp on it,
including the gain and tone controls, the phase inverter and output
sections, and they can all be changed to the values you want to try. 
