Vintage Tube Amp Repair

and Custom Amp Building

I repair vintage, classic, and modern tube amps inside the Loop in Houston, Texas.
Wait time is about 2 weeks, but it can be a month.
Be sure and let us know when you need it, and we 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.
I have a good parts-supply chain if I need to order something, but I have many parts in stock already.

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 at high volume levels and check of all functions and features. We normally clean all controls and jacks as well.

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, 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.

References available.

Call

.
The "Amp Repair Guy."
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A word about our sponsors: I buy a lot of stuff from Parts Express. I've never received any bad parts or junk. Try them out at the link nearer the bottom of the page.
I recently added Vann's. Check them out. Even though they are in what, Montana? Wherever they are, they ship, often FREE, to you and their prices are still lower than local sources.
I actually buy from some of these advertisers, so I choose people I would like to buy from.

Article: Tube Biasing - A Complex Problem Made a Little Easier
Copyright @ 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.

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 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 now (2009) is anywhere from 120 to 123 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, 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.

Introducing...

The Open-Design Guitar Amplifier

Change your amp's design and components with only a screwdriver!
The Open-Design Amp is made to be changed!
Nifty View of the open-design amp board

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.
The amp comes completely built, and has drawings with it that detail the construction to the point that you can rebuild the amp back into its original configuration.

Configured as a classic generic 15 Watt push-pull guitar amp, this design can be changed to values that represent other famous and classic amp designs. The included drawings name the function of each component, so you can easily change the "slope resistor," for instance, without struggling to find it.
The amp is provided in a push-pull "fixed-bias" configuration that can be easily changed to self-biased (also known as cathode bias) mode. It can also be made single-ended.
The target price for the Open-Design Amp is about $475.

It comes in a nice wood box, and the amp itself slides into it from the back. Volume and tone controls are in the front.

Caution! This design, while having safeguards to prevent shock, can still be dangerous.
There is almost 500 Volts on the power supply, and that is enough to kill you if you circumvent the safeguards!

I am now building custom amps. Here are some of the amps that I have built:

Click on image for closer look of the RR Special amp!

The R.R. Special Guitar Amplifier
- This amp is an all-tube, one-input design, with two internal channels. The Normal channel is used all the time and can be run clean or dirty, depending on your settings. The Overdrive channel is mixed with the Normal channel via the Mix control, into the Master Volume control.
- This amplifier design uses dual outputs: standard 6L6's or the Hush Puppy outputs, so that the output power is selectable as either 45 Watts or 4 Watts. It can be switched on the fly, during play in fact.
- The Effects Loop Output is buffered and the level is adjustable. The Effects Loop Input goes right to the Master Volume Control input.
- The power supply is selectable for either a solid state diode or a tube rectifier. The Diode give more power, but the Tube gives more personality, and the amp responds more to your playing.
- There is a Feedback control that controls linearity. Completely off, and the amp can get real dirty. Turn it up, and errors (distortion) in the output are corrected, and the amp becomes more linear.
- The Ultralinear switch cuts in the Ultralinear windings on the output transformer, creating even more linearity.
- Bias for the 6L6's is switchable to self-bias (Class A-like), or adjustable bias. Bias is adjustable from the front panel, and is limited to extremes of Cold and Hot. The Hot setting would break up sooner.
- The Harmonic switch purposely unbalances the Phase Inverter to create even harmonics safely.

The Mr. J. Guitar Amplifier
The goal with this design was a Hendrix-like sound with plenty of gain and overdrive distortion. Power is from two 6V6's, with switchable Hush Puppies to reduce power. A reverb adds a nice dimension, and effects-box loop makes it expandable.

Contact me with your requirements and ideas, and I'll work up a quote. .

Servicing:
Acoustic, Alembic, Alesis, Allen, Ampeg, Ashdown, B-52, Bad Cat, Behringer, Carvin, Crate, Crown, Diezel, Earth, EBS, Eden, Electro-Voice, Epiphone, ENGL, Fender, Fuchs, Gallien-Krueger, Genz-Benz, Gibson, Hartke, Hiwatt, Hoffmann, Ibanez, Johnson, Kendrick, Korg, Krank, Kustom, Laney, Legend, Line 6, Marshall, Matchless, Mesa/Boogie, Metaltronix, Orange, Peavey, Randall, Rivera, Rocktron, Roland, Silvertone, Soldano, Sound Research, Sovtek, Squier, Sunn, SWR, Tech 21, Top Hat, Trace Elliot, Trainwreck, Traynor, VHT, Vox, Yamaha, Yorkville Sound.

Would like to see:
Andrews, Ashton, B.C. Rich, Blackheart, Blackstar, Bogner, Bruno, Burgera, Carr, Díaz, Cornford, Elmwood, Framus, Garnet, Heritage, Hughes & Kettner, Koch, Matamp, Mojave, Naylor, Reeves, Rick-Tone, Ross, Selmer, Session, Sewell, Soultone, Stephenson, Straub, Suhr, Supro, Synaptic Amps, THD, Tone King, Two-Rock, Valco, Yerasov

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