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Post subject: New guy with Hot Rod Deluxe question
Posted: Thu Dec 10, 2009 9:59 am
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I'm a relatively green guitar player, and I recently picked up a Hot Rod Deluxe to play on. Sweet amp! I've been taking lessons for 6 months after dabbling on and off myself for about 2 years, and I have a Kramer XLIII. I'm also an electrical engineer who loves tube gear. I collect and restore antique radios, and I have a 1975 Vibro Champ that I recently restored.

The only thing that bugs me about the HRD is why in the world Fender used a 12AX7 for the phase inverter. Looking at the schematic the voltage charts would indicate that it's pushing nearly 2 mA through each section of V3, and indeed, I measured it and confirmed it. A 12AX7 is only rated for 1.2mA per section last I checked. Since I have boatloads of tubes lying around I put a 12AT7 instead (good for 10mA), and it also made the volume control less sensitive. Still sounds terrific, but I can't think of any good reason Fender would do that, unless they just want to make sure you have to buy new 12AX7s frequently. Any tech guys here who might comment?


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Posted: Thu Dec 10, 2009 11:33 am
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Not a real tech guy but an old guy who had a HRDlx as well as tons of tube gear. I can repair the obvious, but that's really it. I like to tinker. I think it's all part of the Hot Rod marketing thing. It's the same reason they used a Linear Taper pot for a volume control. To make you think the amp is way more powerful than it really is. Some tube tweaking can really tame the beast. The gain channels on mine were like an icepick in the forehead. Mine sounded decent, but I had a ton of reliability issues with it so I sold it. Be gentle when you change out the tubes, the tube sockets are mounted right to a single sided PCB. Try not to wiggle it too much. If one of the joints fractures it's solder time.


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Posted: Thu Dec 10, 2009 5:13 pm
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fender used to put at7's for p.i. position in all their amps but today they try to use the whole idea that most people who play guitars nowadays think that an ax7 delivers more "distortion" . while it will when the "gain" is cranked up it also gets really loud in the process. most people don't even know what the gain control is for. they think it's a "distortion" control. i use ax7-c in v1 and v2 and an at7 for the p.i. my bdri amp sounds great. the ax7-c has a slow rise time and that allows more compression and the at7 gives it nice head room and allows more touch sensitive feel. the hrd amp uses the unused triode in the ax7 to deliver the more drive section. hit the button and that kicks it into full o.d. . try a 5751 in v1. it seems "gain" and "distortion" should be better defined. if it were fewer people would be dissatisfied with their amps. lower gain 12ax7's saturate sooner in the gain curve(5751 is in the ax7 family and have a 70% gain). preamp tubes are fun to play around with. a little advice, leave the 5881 power tubes in if you go with the ax7-c it helps to retain high end.thats what i discovered.


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Posted: Thu Dec 10, 2009 6:29 pm
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Yep Dave, you're right about the current handling capacity of the tubes. Most new production 12AX7s only have 70- 80% output when compared to the older Mullard, RCA, etc. So the use of a 12AX7 under load is kind of reasonable and Fender is no stranger to operating tubes over spec. The " why" seems to be a drive choice for the power section. Art

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Posted: Sat Dec 12, 2009 5:36 pm
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Thanks for all the info, I still think it's a decent amp. It just has some room for improvement. Still sounds pretty decent, but looking at the mods available I might try some of them.


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Posted: Sat Dec 12, 2009 7:59 pm
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Be careful if you do any soldering on the board, the traces lift quite easily even with a low wattage iron. I never had a problem with the way it sounded. It is a pretty fragile amp electronically, but decent sounding none the less.


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Posted: Sun Dec 13, 2009 7:45 am
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Thanks for the information.Its really helpful. 8)


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Posted: Sun Dec 13, 2009 7:57 am
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Yeah its really a nice discussion.


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