Frequently Asked Questions: The Continuing Story!

Thank you for waiting so patiently while I get around to doing something with this page. As you can imagine, I have been perpetually swamped with the production of the GT-100 amplifier, and the general duties of running a business, and staying in touch with everybody.

My intention is to add to, and improve this page as often as I can, and to keep the answers brief enough so that navigating the resource is a reasonable thing. I am still undecided as to what to do with subjects which require a more "in-depth" treatment than this format will allow. There has been talk of a book, or a series of articles which might serve this purpose, as well as to wade through some of the misinformation I constantly hear about "out there". We'll see. Clearly, the interest is there, but for now, we can just address some of the "Guytron specific" questions which I seem to get the most often. Again, thanks for waiting so long, and thanks most of all, for supporting Guytron all these years.

Tonefully Yours,
Guy Hedrick

When did the GT-100 first come out?

I built the first one for myself in 1986, recorded and played out with it until 1995 when I decided to make the concept available to the public for the first time. That's when it took the form of the self contained GT-100 amp. The original from 1986 was a separate rack mount pre-amp and power-amp design.

What are the Dimensions of the GT-100, and GT-212 cabinet?

Here's the dimensions you asked for:

GT-100 Amplifier Head

• WIDTH- 28.25"
• HEIGHT- 10.00" (Plus 1/4" for handle, and 3/4" for feet)
• DEPTH- 9.50"

GT-212 Cabinet

• WIDTH- 29.25"
• HEIGHT- 20.25" (Plus 1/4" for handle, and 3/4" for feet)
• DEPTH- 12.50"

What are the bias settings for the GT-100 amp?

The bias is set at idle, with no audio signal flowing through the amp.

• EL-84 tubes get 45ma per tube (V6,V7)
• EL-34 tubes get 32ma per tube (V10, V11, V12, V13)

*NOTE-1: For the qualified technician's convenience, the GT-100 has a 1-ohm resistor (like a built in bias probe) adjacent to and in line with each power tube for just this purpose. The current for only that tube runs through it, therefore if the voltage drop across this resistor measures 45mv (for instance) with a digital millivolt meter, then the current running through the tube is 45ma (milliamps).

*NOTE-2: There is always a debate about how to best set the bias of an amplifier. Well, not here at Guytron! A technician can reliably set your bias as described above without any risk to the tubes, circuit, or inconsistencies in the tone. When the internal specifications of the amp and tubes are known, the best all-around method is to set the mathematical parameters where they were designed to be. Anything else is deviating from the specifications for "artistic" reasons. Some of these deviations can be hazardous to any amplifier.

EXAMPLE: One of my customers was advised by a popular tube supplier that he could run KT88 tubes directly in place of his EL-34 tubes, and safely bias them as high as 80ma per tube. WRONG!!! From a tube-only perspective, this might fly, but your clean head room goes right out the window!

Now, let's look at this from the amp's perspective for a moment... 80ma (times 4 tubes) = 320ma total idle current, times 450VDC plate voltage = 144 WATTS! Do you think it is healthy for a 100 watt amp design to be trying to idle at 144 watts? This is before you even hit a note? Nobody would!

The power transformer literally ran so hot you could have fried an egg on it! This was a bad situation, and all from some guy giving out advice without thinking about it. You have to be careful where you get information!

What is this "Hybrid Biasing system" I have heard about?

This is an upgrade included on the Rev. C design introduced in 1999. When the original GT-100 Rev-A, and Rev. B models were in production from 1996 till mid 1999, the EL-84 tubes had a Cathode only bias system, which was biased at Class A for a standard Russian EL84 tube.

By 1998, I was getting a much wider spread of tube values from my suppliers. And since the GT-100 greatly benefits "tonally" from having a perfect bias adjustment, I just added the ability to adjust the bias, while keeping the tonal character of the Cathode bias system. I actually prefer the tone because of the ability to get it perfectly where you want it. Other than the perfect adjustment of the circuit, there is no difference in tone from the original.

My amp just doesn't sound right, should I re-tube it?

This has even happened to me in the past. So watch for it!

Suddenly, one day I realized my amp was just plain sounding bad. It had been working fine all along, and nothing has happened to it, and it was still working fine, but I just didn't like it, and I wondered why I ever did?

All tubes do this, but the EL84/6BQ5 is the best at fooling people. The tube just sounds "ratty" and "sour", in general it starts to produce different "nonmusical" harmonics. It makes you hate the amps tone. Just like the way having dead strings sneaks up on you, until suddenly you cannot even tune your guitar anymore. It's the same thing with tubes. They don't always blow out, especially with a Guytron.

In the GT-100 design, this is further exacerbated by the fact that the signal is "post amplified" by another 100 watt power section before you hear it. So the very thing that gave the GT-100 such an incredibly complex sound when it was new, is now giving it a sharply contrasting effect in the opposite direction. El-crappo!

When this happens, you need to remember one four letter word that starts with an "E"; ( No! not "E-BAY") you need to remember EL-84. Keep an extra set of them handy. It's not that they are unreliable, quite the opposite, they are great little troopers. But the contrast between a new one and a tired one is startling to say the least.

This really applies to any amp with the EL84 tube in it, but especially the Guytron GT-100 design. So before you make any rash or cash decisions with your coveted hand made amplifier, no matter who made it, change out your El-84's. And YES, matched sets are better!

What tubes do you recommend using?

The original design was tuned to readily available parts. And at the time, the best and most consistent supply I could find was:

• Svetlana EL-34 tubes for (V10, V11, V12, V13)
• Sovtek EL-84 tubes for (V6, V7) regular style, not military ones (Do Not Use NOS EL-84's)
• Chinese 12AX7A tubes for (V1, V2, V3, V4, V5, V8, V9)

Since that time, others have come on the market which are great, but I still like the original selection.

The Svetlana EL-34, (In the GT-100 amp), has the purest sound to my ear. I have listened to others which all sound pretty good, some smoother, some tighter, but the Svetlana EL-34 has a great fullness at low volume levels, and hangs tough at higher levels, sounds very musical, and is very reliable and consistent.

The Sovtek EL-84 is the toughest and lasts the longest, but I think the Tesla /JJ sounds fuller and smoother when it is new. However, it does not last as long, and when it goes weak, it really sounds bad to me. Where, the Sovtek EL-84 would still be sounding good, and be more reliable. I have had the Sovtek EL-84 go for up to 4 years getting the crap beat out of them without causing a failure!

The Chinese 12AX7A from 1996 was a great staple item until they quit making them. You may also like the Tesla /JJ 12AX7 for it's buttery flavor. Thankfully though, they are making the Chinese 12AX7 again, and the latest samples I have heard seem to exceed even the quality of the older ones. Too bad the prices have doubled!

Of course, you can always dabble in NOS tubes when it comes to 12AX7's, as well as the 5751 military, 12AT7 12AY7, 12AU7, etc. Just don't change all of your tubes at once.

Why shouldn't I change all my tubes at once, and what's a good replacement schedule?

This is why: ( a common mistake)

"Hey Guy, I am having some trouble with my GT-100 When I first got this amp it sounded unbelievable. Now, after a retube, it has not reached the same level of great tone."

Why would you retube the entire amp? As a rule, the Guytron would not require a complete retube. This is the schedule I recommend:

• The EL84's (V6, V7) get changed every 6 months to 2 years depending on use and individual tube quality.
• The EL34's (V10, through V13) get changed every 1 year to 4 years depending on use and individual tube quality.
• The 12AX7 tubes get changed individually (1 tube at a time), and ONLY when they begin to sound bad, because they can last years. The best way to check them is by substituting with a known good tube. If this produces a better sound, then it must have been getting tired sounding, so leave that one in place, and try the next. If a new tube does not produce better results, then save the money, and put the old one back in place and move on. This way you don't change so many variables at once, which makes it impossible to know which variable it was.

*Unless the tubes are so old, that it's pretty much guaranteed that most of them are bad, you should not assume they all need replacing with any amp. And even then, you should change them one at a time. Not only does it waste money, but the new tubes may sound markedly different from the last ones you put in. Over the years, tube quality and supplies wax and wane, so cover your bases and keep your old ones until you are satisfied.

Can the effects loop be adjusted to be fully serial, or is it parallel only?

Yes, the effects loop is fully serial when it is adjusted full clockwise. In this position it only listens to the loop, therefore the parallel signal is effectively removed from the signal. This is useful if you have the blending worked out already on your effects unit, or if the unit you have has phase issues which might conflict with a loop that is 100% accurate.

After playing the Guytron Amp for a few hours switching back and forth through the two channels, I switched to the B channel and I got a real boomy, slightly louder, muffled hi gain sound...it lasted about 5 seconds and then sounded right. The tubes weren't glowing strange or anything.

*That is a Rev-B amp and it has some connectors in it which may need cleaning. There are a bunch of brown Molex connectors which connect the PCB to the various off-board components. They need to be checked.

BEWARE OF ELECTRIC SHOCK HAZARD!!

Be sure the amp is unplugged for at least 15 seconds before working on it. These connectors can be cleaned with contact cleaner as follows:
1) Remove the Molex connector plug
2) Spray some cleaner into a shot glass
3) Dip a Q-tip into it and apply cleaner to the pins on the PCB header (where the plug fits onto)
4) Replace the plug and remove it again 2-3 times to "work" the connection

**This cleans the connectors for 1 to 4 years depending on climate.

*The other thing you should do is gently pull on each of the wires on each connector. Just to be sure that none of them are loose. If so, then repair as needed. These do not loosen up with time, it would have to been overlooked at Guytron when it was originally built. Needless to say this is a rare occurence, but it has happened a couple times over the years.

*The other thing is to check / tighten all the input and output jacks and switches, etc. These are very important, especially the Input Jack, and the Speaker Jacks. A loose jack can cause many different symptoms, including hum, signal surges, crackling, etc. The jacks will spin, so you have to hold the backside of the jack while tightening the nut. This way the wires attached to it do not get twisted off when you tighten it.

Do these things every fifty thousand miles, and she'll run fine. (about every major tube change)

Should I play my Guytron all the time?

Yes.

Should I ever sell my Guytron?

No.