Frequently Asked Questions: Please read these to see if your question is answered
Also, be sure to visit our Facebook page where we will discuss these issues from time to time.
1. When did the GT-100 first come out?
Guy Hedrick built the first one for himself in 1986, recorded and played out with it until 1995 when he 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.
2. What are the Dimensions of the GT-100, and GT-212 cabinet?
GT-100 Amplifier Head
HEIGHT- 11.00" (including handle, and feet)
GT-212 Closed Back Cabinet
HEIGHT- 21.25" (Including handle and feet)
GT-212 Open Back Cabinet
HEIGHT- 21.25" (Including handle and feet)
3. 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 due to Ohm's Law, 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!
4. 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 bias system, similar to what is used in the popular 18 watt amplifiers.
By 1998, we were getting a much wider spread of tube values from our suppliers. Since the GT-100 greatly benefits "tonally" from having a perfect bias adjustment, Guy added the ability to adjust the bias, while keeping the tonal character of the Cathode bias system. Other than the perfect adjustment of the circuit, there is no difference in tone from the original.
5. My amp just doesn't sound right, should I re-tube it?
This has even happened to us here at Guytron so watch for it! 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 amp's 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.
Our advice is to keep an extra set of EL84 power tubes 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! One trick I like is to buy a matched quad of EL84s and then set the bias for the first pair. Then your spare pair will be plug & play if you should ever need it on a gig since it would use the same bias setting!
6. 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 available 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 the original selection is still excellent.
We load current GT100 F/Vs with Ruby EL34BSTR or Winged C EL34s. (small additional cost for Winged Cs)
The Sovtek EL-84 and the JJ EL84 are both excellent and we use them interchangably.
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 that is all we use for preamp tubes on all Guytrons currently. We have tried many other brands, most of them far more expensive but none sound as good as the Chinese 12AX7s to our ears.
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. Download the tube chart and work with the tubes that affect the channel you want to work with at the present time. For example, for the A channel, play with V2 only. For the B channel, play with V1 and V5. If you want the master tone section to sound differently, then you would play with V4 and possibly V8 (efx loop recovery) and V9(EL34 phase invertor).
7. 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.
8. 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.
9. 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. 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)
10. Should I play my Guytron all the time?
11. Should I ever sell my Guytron?
No. Seriously, if you are thinking of selling, contact us
and let us know what you are thinking and why. We
might have some ideas about how to address your
issues, including trading in for a new GT100 F/V.