| Author | Message | ||
     edward solberg |
Recently I've been looking at a mod that simulates the tube rectifier sag of the old fender tube amp circuits. you know, the ones that allowed such great brown and over-driven tones for playing blues. one fellow, has set up his hot rod deluxe (reissue) with such a mod that also includes a cut out switch that allows him to go between altered and unaltered operating modes. He uses a DPDT (heavy duty, allowing for high voltage and current handling) that interrupts the DC power supply between rectifier and the first filter cap with a power resistor on one switch leg and on the other leg a bypass of the power resistor to return the amp to normal operation. [he also has another modification that adjusts the bias to the tubes' two different bias requirements in the altered and unaltered operating modes--because of the resulting decrease in plate voltage/cathode current--that we can discuss at another time--I'm primarily interested in the "sag mod."] The sag emulation mod requires the use of a heat-sinked, heavy (25 watt, inducting) wire-wound power resistor in series with the positive end of the DC power supply between the solid state rectifier and the first filter cap. the intent here is to simulate the voltage drop that occurs when the tube rectifier has more current demand than it can handle during the higher output tube demands of a volume increase. [btw, the author credits Kevin O’Connor with the idea in one of his Ultimate Tone books.] The resistance to current increase of the inducting power resistor allows for a more "tube-like feel" when the volume is cranked--instead of increased volume, we get a sort of compression when the rectifier temporarily denies increased current to the tubes' demand (or IOW: "sag"). This inductance after the resistor emulates the higher internal resistance of vacuum tubes to larger current demands (diodes have that fixed voltage drop that isn't really affected by the large changes in current). Though the inserted power resistor lacks the capacitance and inductance of a tube (it will not sound exactly like tube rectifier sag), the greater the resistance of the resistor the more pronounced will be the sag effect (even at lower volumes). The prospect of having this sag effect available in my super-clean musicman amp for blues solos is very attractive. The major question I have for our super-techs is whether or not the concept is theoretically sound and whether or not it is do-able in a musicman amp--truthfully, I don't care for the sound of pedal-driven overdrive and would really like to be able to call on the "distortion on demand" of this mod if feasible. so, thanks in advance to terry, steve and mike for considering my request for information[even if you guys don't choose to answer; you have often done so in the past —even stupid questions get answered— and deserve many thanks]. This board is the greatest resource available on the net for answers to the questions we have about our beloved musicman amps. e s This mod is discussed at the Unofficial Fender Hot Rod Deluxe Owner’s Manual (a web site at the University of Eastern Kentucky), by Justin Holton. It also contains several other modifications for the Hot Rod Deluxe (reissue) that are worth a look-see. | ||
| mike kaus |
Ouch! Interesting idea but I'm not sure if it's safe for a cathode driven amp. First, I suppose it's possible if you have the tube inverter model to TRY it(SS driver-I wouldn't even try it probably) without too much trouble but it's a long way to go for a little sag. I'm afriad you need somebody a little smarter than me for this one. Sorry. | ||
| edward solberg |
you know, Mike, it shouldn't be that hard to do. just adding a resistor and switch plus tweaking the bias to allow for the reduction in voltage on the B+ due to the resistor. then again, maybe I'll just content myself with my old tubescreamer (or turn the Twin up to 7.5--ouch!)for my distortion; it does a fair job of achieving the sound I want anyway. a mod I am going to do is find a good "line out mod" taking the signal off the preamp for my recording activities. I'm looking for a good one to install (any come to mind that you like?) and a good method for getting into the signal circuit after it clears IC7. my biggest question is one of technique to use at Point D--truthfully, I am not anxious to lift that damned PCB (as you said: 30 year old traces can break so easily...). but if I have to pick up the line on the PCB I'll need to know a good technique for working with one. once that is in place, I'll also want to decouple the output section and use the preamp's signal alone; even my neighbors have their limits and a 210 Sixty-Five screaming at 8 has always pissed them off in the past (especially during my favorite times to record: after midnight when the blood is hot and the iron is right for striking). otherwise, I hope all is well. thanks for the input. e} | ||
| Ed Goforth |
I think the resistor will work on the 12ax7 driver amp for a little sag, be sure its the inductive type though. Also consider Kevin's Power scaling for the amp, longer tube life and power tube compression at lower volumes. I built a London Power Scaling circuit and tried it in a Fender twin, it does what it says, but I would be sure to email Kevin and ask any/all questions you have. He has the 12ax7 driver Music Man amp listed, but he emailed me saying don't do this on the SS driver amps. I would like to try the Power scaling circuit on a HD-130/65 chassis. I have been contemplating building an amp inspired of the 12ax7 driver model with some of my own ideas. I really love the sound of the old HD-130 212 that I had years ago and I wish I knew then what I know now! So I might build something based on it. (530)-529-0389 Red Bluff Ca. | ||
| edward solberg |
hi Ed, sorry it took so long getting back to this thread (I had to make another business trip abroad to bail some stupid clods out of a jam of their own making....grrrr). I've looked at the power scaling idea but had thought it was not a mod we could do with our amps--I've got a 210 Sixty-Five with the 12ax7 PI--because of the 725 vdc B+. If possible, the power scale kit would be a pretty cool mod to try out on my hot little combo. In 1975 the ability to shatter glass with clean/brown volume was a real plus, but nowadays it gets a little loud for playing in the local clubs and pubs with friends' groups--so some sort of attenuation seems the best answer for my dilemma. I wasn't aware that Kevin O'Connor had a Music Man amp listed as a doable project; perhaps it is time to look into that option again. in truth, the rectifier-sag mod was thrown out into the lion's den to see what sort of reaction it would generate. my interest in the mod was mostly theoretical and/or whether it is feasible with a music man (a fender hot rod is one thing, a music man is a whole 'nother animal IMO). thanks for answering my question, ed | ||
| Ed Goforth (ed_goforth) Username: ed_goforth Registered: 06-2006 |
Hi Ed, I am just reading this thread so I'm sorry too for not getting back sooner. :^) That resistor should work well on the 12ax7 driven amp, the ss driver might have some problems and the bias settings would probably not work well... I am not sure, maybe Kevin or someone else here could elaborate on it more. I have found that using a Dumbleator type effects loop and placing the loop after the Master volume wiper off the .047 cap is the best place for a loop. The Dumbleator works well with this amp when setup this way, it acts like a power amp attenuator. The 2nd 12ax7 stage's output then goes to the return level, like a master volume and creats a powertube-like compression, allowing the amps master to be turned up, fattening the tone and less buzz. Use the return level as the master volume. This probably is not practicle if your not using effects though, but what a tone! There are schematics of the D*mbleator out there... |