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Post subject: Hot Rod Dlx Omega Mod Schematic
Posted: Mon Dec 09, 2013 1:10 pm
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Anyone know where this might be available or is it a proprietary thing?
I'd like to have it done locally, but...
Thanks all.


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Post subject: Re: Hot Rod Dlx Omega Mod Schematic
Posted: Tue Dec 10, 2013 12:13 am
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I don't know exactly all of the changes involved in the "Omega" mod, but what I do know is that there are some good changes and some that are unnecessary/redundant, and some that are just plain stupid.

You can make changes in stages, starting with the best, most significant changes, which are also the easiest changes, and you can do yourself if you are so inclined. The best and easiest things that you can do to improve the tone of the HRDlx:

- First, try biasing the current output tubes that you have, they are usually very cold from the factory and re-biasing can improve the tone, sometimes quite enough to be satisfying. Typically the factory bias, checking at the test point, is around 60 mV or less. Assuming about 430 Plate volts, check to be sure, a better setting would be at least 70 mV, and up to about 85 mV.
- Proper lead dress of the transformer wiring and DC supply wiring, can lower the hum/hiss produced by an amp.

If you are not happy after resetting the bias, you can try:
- Change V1 tube to a 5751, some even prefer a 12AY7
- Some people also prefer a 5751 in V2 to tame the drive channel (I don't)
- Change V3 to a 12AT7
----- (My preferred preamp lineup is; V1 - GE 5751, V2 - GT 12AX7-C, V3 - GT 12AT7. A JJ 5751 is also nice in V1)
- Your preferred 6L6 output tubes, properly biased. I like the GT 6L6 S with a high rating, 8 to 10, for great clean, and with the above preamp lineup, very nice drive.
- Many people get a great change in sound by changing the speaker to one that they like better. I still use the stock speaker and have modified my amp a little as described below, and tubes indicated above.

The next changes are more difficult, and involve removing the main circuit board:
- Add another 47 µF cap to the first filter stage in the power supply. There is already a slot for the extra cap, just be sure to put a jumper in the holes below the new cap which connects the positive of the cap to the positive of the existing cap next to it. Alternatively, swap the existing 47 µF cap for a 100 µF, which is about the same as two 47s in parallel. This will tighten the low end a little bit, tighter bass with less flabby sound.
- Install a jumper on the mid pot, connecting the wiper of the pot to the end of the pot nearest the bass pot. This will allow more control of the mids in particular.
- If you want a different tone response, try changing the values of the tone stack caps. I changed the Bass and Treble caps to a different value, and found that the only difference was where the pots were set to get the tone that I liked. There are various kits for tone stack caps.
- I also changed the value of the screen grid resistors from 470 ohm to 2K ohm, which gives a slightly more compressed tone, just a bit smoother and warmer, not dramatic. Be sure to use flame-proof resistors, I used two-watt.
- Changing the factory input jacks to Switchcraft metal jacks can be a reliability change, but will not change tone. Since the factory jacks are isolated from the chassis, I used fiber shoulder and flat washers to isolate the new jacks.

There are more mods, such as reverb mod, presence mod and others, most of which don't give any more change than you can get by simply adjusting the associated pots.

Now for the unnecessary/redundant/stupid mods:
- Replacing the factory caps with caps of another brand but the same value does nothing for tone, but in some cases might improve reliability. I replaced my power supply caps only after the factory caps started leaking. I used F&T, a better brand than the factory IC.
- The amp already uses an audio taper pot for the Master, despite what some want to believe, no need to replace that pot with the same thing. Check the schematic, check the amp, it is an audio taper pot.
- Replacing flame proof resistors with carbon comps is not only a bad idea, but it doesn't change the tone one bit, with the possible exception of Plate resistors in certain amps, only when the amp is cranked wide-open. It is a myth that using carbon comps will improve tone, it does not, and it lowers the reliability and safety of the amp. Yes, I did try it just to prove a point that not even the Plate resistors being CC changed the tone one bit. The myth probably comes from the fact that vintage amps have the CC resistors and some wrongly assume that they must be great since the old amps sound great. The fact is, all that was available back then were CC and that is the only reason that they were used. We now know that CC resistors can catch fire when they fail, so flame-proof are a much better choice. I have even read ridiculous comments, by someone who claims to know about amps and electronics, about using CC for screen grid resistors, such as "they will fail shorted, protecting the output tubes". <---that is as stupid as it gets, absolutely untrue. :wink:
- Changing the 5 watt power supply resistors from 470 ohm to 330 ohm is really stupid, and will produce more heat in that circuit. This is another myth started by some less-than-knowledgeable person, possibly after seeing the 330 ohm in the Devilles. The Devilles have a different AC supply voltage than the Deluxe, that is why the lower value resistors. This change has zero effect on tone, but can potentially cause problems. Again, read comments by a so-called "guru" that the lower value resistors "allow more current, therefore reducing heat". More current? Yes, Ohms law. Less heat? No, again Ohms law. This same "guru" claims that Fender changed the value to 330 ohms in the new HRDlx III, which is not true either, they are still 470 ohm. To prevent possible circuit board damage from the heat given off by these resistors, install new ones of the same value, using some method to raise the new resistors off of the circuit board, such as a dab of high temp silicone between resistors and board. My HRDlx, 14 years old, shows very little signs of heat on the circuit board. The HRDlx III uses thicker circuit boards, so this is not a problem.

These are suggestions based on my experience, and I'm sure that there are other opinions......but only mine counts. :lol:

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Post subject: Re: Hot Rod Dlx Omega Mod Schematic
Posted: Tue Dec 10, 2013 10:02 am
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Thank you! That was a really detailed response and I appreciate the time you took to write it.
I'll start with the tubes and maybe the speaker. I have some Dumble pedals that are really nice, so with a sweet clean platform...I'll be good. It'd be nice to have a really nice drive channel, but I can get by without it.


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Post subject: Re: Hot Rod Dlx Omega Mod Schematic
Posted: Tue Dec 10, 2013 2:33 pm
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The preamp lineup that I use, makes for a very nice drive channel. The lower gain tubes really smooth things out, and the high gain tube in V2 keeps some good drive sounds. When switching to the drive channel, half of V2 is added to the circuit, so if you still find the drive to be too much using lower gain tubes in V1 and V3, changing the V2 tube can affect the drive channel noticeably. It is fairly inexpensive to experiment with preamp tubes, and you get a lot of the amp's sound character from those three tubes, so a lot of bang-for-buck difference by simply changing tubes.

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Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Post subject: Re: Hot Rod Dlx Omega Mod Schematic
Posted: Thu Dec 12, 2013 10:43 am
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Apparently the George Benson model of this amp, besides the speaker(?), puts a 12AT7 in V1 to let George get the sound he likes from his humbuckers. I might try that after I get my amp.

You are correct, Shimmilou about very real changes from preamp tube arrangements and brands. At least from my newbie-to-tubes perspective.

Much of my information coming from you and few other gems here at this forum. :shock: As well as experimenting with the advice offered. :D

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