Amp design question
Question:
I think the Super Reverb has the nicest tone of the blackface Fenders for jazz. Unfortunately, it just does not have enough clean headroom. If you put a Twin Reverb chassis in a Super Reverb, would the result sound like a Super Reverb or like something between a Super and a Twin? In other words is the circuit the same sonically? (forget impedence matching for the moment) Thanks, Jim Visit – rec.music.theory – to discuss all aspects of music theory. The author is singularly responsible for the contents of this message; they do not reflect any opinion of the organizations sponsoring this net account.
Response:
> I think the Super Reverb has the nicest tone of the blackface Fenders > for jazz. Unfortunately, it just does not have enough clean headroom. > If you put a Twin Reverb chassis in a Super Reverb, would the result > sound like a Super Reverb or like something between a Super and a Twin? > In other words is the circuit the same sonically? > (forget impedence matching for the moment) > Thanks, > Jim
I’m pretty sure the Twin Reverb chassis will be too wide to fit in the Super Reverb chassis. I know Elliot Randel had his Super Reverb modded to run 4 6L6 tubes to be an 85 watt amp. Rather than punching two big holes for the 2 6L6s you might concider running 2 KT88s, it would probably mean changing the transformers. Either way you would ruin the value of the amp. Kelley
Response:
- Hide quoted text — Show quoted text -> I think the Super Reverb has the nicest tone of the blackface Fenders > for jazz. Unfortunately, it just does not have enough clean headroom. > If you put a Twin Reverb chassis in a Super Reverb, would the result > sound like a Super Reverb or like something between a Super and a Twin? > In other words is the circuit the same sonically? > (forget impedence matching for the moment) > Thanks, > Jim > I’m pretty sure the Twin Reverb chassis will be too wide to fit in the > Super Reverb chassis. I know Elliot Randel had his Super Reverb modded to > run 4 6L6 tubes to be an 85 watt amp. Rather than punching two big holes > for the 2 6L6s you might concider running 2 KT88s, it would probably mean > changing the transformers. Either way you would ruin the value of the amp. > Kelley
Well, the point is, I will build my own amp, have the cabinet built by one of the capable builders here, then build the chassis myself. I can follow a Super Reverb schematic or a Twin schematic. My question is, it I follow the Twin schematic but keep the box dimensions and the 4 10s of the Super Reverb, will it sound like a Super Reverb or a Twin (which have very different tonal character)? Perhaps the transformer will largely change the tone since the Super Reverb transformer will not work for the 4 6L6s and the Twin Reverb transformer will not work for the 4 10s (I’m presuming). So I guess I’m dealing with three parameters: the circuit design, the transformer, the cabinet/speakers. Any idea on how to get the power of a Twin with the tone of a Super Reverb? Thanks, Jim Visit – rec.music.theory – to discuss all aspects of music theory. The author is singularly responsible for the contents of this message; they do not reflect any opinion of the organizations sponsoring this net account.
Response:
- Hide quoted text — Show quoted text -> I think the Super Reverb has the nicest tone of the blackface Fenders > for jazz. Unfortunately, it just does not have enough clean headroom. > If you put a Twin Reverb chassis in a Super Reverb, would the result > sound like a Super Reverb or like something between a Super and a Twin? > In other words is the circuit the same sonically? > (forget impedence matching for the moment) > Thanks, > Jim > I’m pretty sure the Twin Reverb chassis will be too wide to fit in the > Super Reverb chassis. I know Elliot Randel had his Super Reverb modded to > run 4 6L6 tubes to be an 85 watt amp. Rather than punching two big holes > for the 2 6L6s you might concider running 2 KT88s, it would probably mean > changing the transformers. Either way you would ruin the value of the amp. > Kelley
the tone stack is different in a super reverb closer to a bassman if you like super tones convert the tone stack in your twin probably no further mods are necessary pat
Response:
> Perhaps the transformer will largely change the tone > since the Super Reverb transformer will not work for the 4 6L6s and > the Twin Reverb transformer will not work for the 4 10s (I’m presuming).
Why not? Just use 4 x 16 ohm speakers in parallel. Am I missing something here? Alternatively, there must be multitapped output transformers out there for 4×6L6GC configurations….dunno of any but then again, I haven’t really looked. Sorry, can’t help you on what it’ll sound like. Cheers Andrew —
Response:
- Hide quoted text — Show quoted text -> > I think the Super Reverb has the nicest tone of the blackface Fenders > > for jazz. Unfortunately, it just does not have enough clean headroom. > > If you put a Twin Reverb chassis in a Super Reverb, would the result > > sound like a Super Reverb or like something between a Super and a Twin? > > In other words is the circuit the same sonically? > > (forget impedence matching for the moment) > > Thanks, > > Jim > I’m pretty sure the Twin Reverb chassis will be too wide to fit in the > Super Reverb chassis. I know Elliot Randel had his Super Reverb modded to > run 4 6L6 tubes to be an 85 watt amp. Rather than punching two big holes > for the 2 6L6s you might concider running 2 KT88s, it would probably mean > changing the transformers. Either way you would ruin the value of the amp. > Kelley >the tone stack is different in a super reverb closer to a bassman >if you like super tones convert the tone stack in your twin >probably no further mods are necessary
The only difference between the tone stack on a Super Reverb and a Twin Reverb is the cap that connects to the junction of the bass and mid pots. The cap is .022 in the Super and .047 in the Twin. I modded a silverface twin by changing the .047 to .022, and by changing the resistor from 100K to 33K (of course, I also changed the mid pot from 10K to 30K ). If you wanted 100 watts out of a Super Reverb chassis, you’d have to change the output tubes to 6550 (an AIMS amp gets 106 watts RMS out of a pair). The Electronic Supply sells these) and a 3.4K ohm CT output (ARS Electronics carries one called an OT-100). The 800VCT transformer does not have an 80 volt tap, so you’d have to get a 100K (or so) series resistor for the bias circuit and connect it to the HV winding. The screen resistors for the tubes would have to be changed to 1K 5 watts. You’d also have to change the dropping resistor to the plate resistors on the driver tube (probably about 3.9K) so the plate voltages remain the same in the preamp and driver stages. Antique Electronic Supply also has a 4.3 K ohm CT output transformer (T-1650N) that is rated at 60 watts in the catalog, but the sticker on the actual unit says 80 watts (with 4.3K , you’d get 80 watts and the 6550’s would like this better than 3.4 K). Your B+ would be 545 V vice 435 V. There is 40mA going through the 1K 1 watt resistor, so figure a little E=IxR. 3.9K/5W would yield a drop of 150V at 40mA. Red Rhodes did the above to a Fender Princeton sometime back in the early seventies (Rusty Young’s pedal steel amp) according to an article in Guitar Player. He got 85 watts RMS. Agree that doing the output tube mod would kill the value of the amp. Furthermore, 6L6’s have a nice tone
Response:
- Hide quoted text — Show quoted text – > I think the Super Reverb has the nicest tone of the blackface Fenders > for jazz. Unfortunately, it just does not have enough clean headroom. > If you put a Twin Reverb chassis in a Super Reverb, would the result > sound like a Super Reverb or like something between a Super and a Twin? > In other words is the circuit the same sonically? > (forget impedence matching for the moment) > Thanks, > Jim > Visit – rec.music.theory – to discuss all aspects of music theory. > The author is singularly responsible for the contents of this > message; they do not reflect any opinion of the organizations > sponsoring this net account.
Not a bad thought. Check out Elliott’s site. http://www.elliott-randall.com/ Regards, Rich Koerner, Time Electronics. http://home.att.net/~rich-karl/
Response:
> Thanks for posting this Mark. I’m interested in trying (emphasis on > trying) to write my own program to model this stuff – so here’s whats > been going through my head. I (think I) know enough circuit analysis > stuff to be able to take the tone stack by itself and calculate transfer > functions for various frequencies.
Hi Scott, How are you planning to calc the transfer functions? Kirchoff? > That’ll give me a feel for how these > things actually work, but I’m a bit concerned about leaving out the > pre-amp tube that drives it, especially in view of your comments about > loading it under some circumstances.
You can assume that it’s a plate res in parallel with the load resistor, for practical purposes. Kirchoff loop analysis just places this in series with the first loop. > (Problem being I’m not certain at this point how to model the tube > itself – does a simple model not accounting for plate capacitance, > transconductance, and all the other tube parameters, even exist, and how > useful would it to be?
Depends if you want to model the tube, or just the tone circuit. The tone circuit is pretty easy. Tubes are tough, but there are spice models around. > I don’t necessarily want to get bang on accurate > values, but I’d like to be able to mess around with stuff in the tone > stack and get at least a qualitative feel for whats happening).
Then it’s simple…just assume linear voltage going in. Say 1 volt at the other side of a 40k resistor (or whatever that parallel value turns out to be). It’s not real critical, as it just contributes some hf loading. MGarvin
Response:
>>> I’m pretty sure the Twin Reverb chassis will be too wide to fit in the >> Super Reverb chassis. I know Elliot Randel had his Super Reverb modded to >> run 4 6L6 tubes to be an 85 watt amp. Rather than punching two big holes >> for the 2 6L6s you might concider running 2 KT88s, it would probably mean >> changing the transformers. Either way you would ruin the value of the amp.
Elliott has posted here before. I believe he talked about that amp. You could look for the post via dejanews. Or ask him to post about it. He’s a nice guy, and he loves to talk about his amps and guitars. >the tone stack is different in a super reverb closer to a bassman >if you like super tones convert the tone stack in your twin >probably no further mods are necessary
The mid ‘notch’ in a Super is higher than most other Fender amps. I don’t remember a Bassman with the same circuit as a Super. In fact, the Bassman amps with two .1 caps in the tone circuit will move the notch downward…opposite the Super. >The only difference between the tone stack on a Super Reverb and a Twin Reverb > is the cap that connects to the junction of the bass and mid pots. The cap is > .022 in the Super and .047 in the Twin. I modded a silverface twin by > changing the .047 to .022, and by changing the resistor from 100K to 33K (of > course, I also changed the mid pot from 10K to 30K ).
If you like the sound of the circuit, that’s what important. The Fender tone circuit was not exactly designed for hifi response. Given that tone prefs are completely subjective, there are still some generalizations that can be made regarding those changes: The Fender circuit was designed to take a big scoop out of lower mid frequencies, centering around 300 hz or so. Close to the fundamental of the guitar’s high E string. Though the response will be down by about -24db at that point (-21db rel to high harmonics), the string’s fundamental is ‘reassembled’ by the human ear due to the way your hearing works. In short, the harmonics provide enough info to tell the pitch, even if the fundamental is completely nulled. What that mid notch does is to remove enough energy from the guitar signal so that the amp can be pushed a bit harder without overdriving the output stage as much. In other words, more high-freq energy is available at the expense of richer mids. Fender amps attenuate some of the bass freq’s too, much for the same reason…and because that’s what makes them sound like they do. That mid notch, and the way that the treble control is ’tilted’ has a *lot* to do with Fender’s sound. Changing the notch frequency upward allows a lot more bass response, but you may notice a change in the nature of the mids and how they are phased. Re the 33K ’slope’ resistor: The output impedance of the previous tube stage is around 30K to 40K, so a 33K slope resistor will load it down a bit. This will result in decreased treble response. Nothing real serious, but you’ll lose a few db’s at the top. Yep, Marshall did this in some of their amps. You may notice that the mod results in a shift toward Marshall-esque phasing in the mids. A subtle thing, but it’s there. Also, the 30K mid control with a 33k slope res will restrict the usable range of your bass control when mid is turned up. Again, if you like it… I wrote a program to simulate response of various tone circuits (tho there *are* easy ways to determine notch freq by simple math) and I recently adapted it to model the Fender-type config. I ran your values thru to confirm loading, etc. Here’s what to expect from the 33K slope with .022 mid cut cap: Bass response at low E (82.4 Hz) will be boosted by almost 8db (!) This may be just what you want, but could also cause your output stage to sound ’sputtery’ at high levels. The mid notch freq. is moved upward (by over an octave) to around 740 hz. This also means that the treble control sweep is effectively moved upward. (Not as much treble range, but that probably does not matter). The max mid notch depth will be only -20 db or so, relative to high harmonics. The stock Fender circuit has a -30 db notch, which contributes to their chimey, clean tone. More mids are good for jazz or Chicago blues sounds. Around 3db is lost over the entire treble spectrum. This is due to loading of the previous stage, as described above. Comments above relate to mid setting of 0. When mids are turned up, the Fender circuit will still be notched considerably. This allows the bass control to maintain some usable swing. The Fender mid control only varies mids by about 12 db max. Changing the slope res to 33k and mid control to 30k (where did you find a 30k pot?) will result in a mid swing of around 17 db, but will limit the range of the bass control to only 6 db (when the mid control is at max). Overall, that circuit change will work for some who really want to increase bass or mid response, or just want to overload the amp more easily. Caveats about sputtery sound above. I recommend staying with a higher value slope resistor and simply decreasing the value of the mid cap. That avoids loading effects and keep treble freqs intact. Here’s another idea: if you don’t use the bright switch (I don’t), you could reconfig that switch to change the mid-cut cap. Dave Cigna (who doesn’t come here much any more) also wrote a tone control simulator for Fender, Vox and one Marshall-type circuit. Nice work, and he makes the program available (sorry…mine was written for modelling some proprietary circuits). If you are interested in seeing how this stuff works, check Dave’s web page: http://www.phy.ohiou.edu/~cigna/amps/index.html Yep, this post took a few minutes, but I love this stuff. MGarvin
Response:
- Hide quoted text — Show quoted text – > The Fender circuit was designed to take a big scoop out of lower mid > frequencies, centering around 300 hz or so. > What that mid notch does is to remove enough energy from the guitar > signal so that the amp can be pushed a bit harder without overdriving > the output stage as much. In other words, more high-freq energy is > available at the expense of richer mids. Fender amps attenuate some > of the bass freq’s too, much for the same reason…and because that’s > what makes them sound like they do. > That mid notch, and the way that the treble control is ’tilted’ has > a *lot* to do with Fender’s sound. Changing the notch frequency > upward allows a lot more bass response, but you may notice a change > in the nature of the mids and how they are phased.
I noticed that the Rivera Amps concern themselves with the mid-notch issue also. Their rack series amps have "slope" and "TMEQ" controls, while their other M/S series and other amps have a pull knob for mid-notch shift. I have no idea how effective their designs are. Steve From their web page: * * * * * * * * QUOTE: Middle/Notch shift The MIDDLE control is a passive adjustment that influences the depth of the notch (dip in amplifier response) between the BASS and TREBLE controls. Because the MIDDLE control is passive, it does not boost midrange frequencies. Therefore, increasing the setting of the MIDDLE control fills in the notch but does not cause a bump in frequency response at the notch point, even when turned up to 10. The interaction among controls gives you more flexibility in fine-tuning the timbre. For example, if the sound has too much bass or treble even with the BASS and TREBLE controls set at 1, you can turn up the MIDDLE control to balance the sound. The MIDDLE control also incorporates a NOTCH SHIFT switch (a notch is a dip in the frequency response). When the control is pushed in (normal position), the notch is at 550 Hz. This is a warm sound typically associated with British amplifiers, and is great for instruments with single-coil pickups. When the control is pulled out, the notch is shifted to 250 Hz. This provides a good rhythm sound typically associated with American amplifiers, and it is great for Instruments with hum-bucking pickups. The NOTCH SHIFT is also useful for compensating either for lack of warmth or for excessive boominess in different speakers. * * * * * * * RACK SERIES: QUOTE: Slope (All) Until Rivera Research and Development began its exploration into the inner workings of guitar amplifiers, most musicians were unaware of the existence of midrange notch in their amplifier’s frequency response, or of how modifying the notch would open up new worlds of sound for them. The SLOPE control was designed to exploit the behavior of the notch. Settings of the SLOPE control vary from 1 to 5. In position 1, the notch is most deep, making the sound more thin in the midrange. This is the sound most often referred to as a "pre-CBS" sound. As the control is turned upwards towards its maximum setting of 5, the notch is simultaneously filled in and moved lower in frequency. This has the effect of fattening up the sound, as well as giving the amplifier more overall gain. Because the controls of channel 1 are passive they are interactive. When the SLOPE switch is at its "maximum fat" position there is substantial midrange, and the BASS and MIDDLE controls are affected in their range. If the TREBLE control is on position 10 at this point, the effectiveness of the bass control seems less apparent. As the TREBLE control is turned down past position 6 or so, the bass control again becomes effective. Experimentation with the controls and their wide range of sounds and versatility will yield the best results. Example 1. The user is playing an Instrument that has an excessively fat sound and requires a thinner timbre (absence of midrange). The SLOPE switch would be set at position 1, the MIDDLE control turned to I, TREBLE pulled out (for Bright timbre) set between 8 and 10, and BASS adjusted for taste (probably between 3 to 6). Example 2. The musician is using a thin sounding guitar and needs more body on the top strings without boomy bass. The SLOPE switch would be set to position 3 to 5, TREBLE set 6 to 10, BASS set 5 to 10, and MIDDLE on 10.
Response:
> http://www.phy.ohiou.edu/~cigna/amps/index.html > Yep, this post took a few minutes, but I love this stuff. > MGarvin
Thanks for posting this Mark. I’m interested in trying (emphasis on trying) to write my own program to model this stuff – so here’s whats been going through my head. I (think I) know enough circuit analysis stuff to be able to take the tone stack by itself and calculate transfer functions for various frequencies. That’ll give me a feel for how these things actually work, but I’m a bit concerned about leaving out the pre-amp tube that drives it, especially in view of your comments about loading it under some circumstances. How overly simplistic is it to just take an input signal, apply it to a parallel combination of plate resistance and plate to B+ resistance (to model the output impedance of the tube?) and then into the tone stack? Or am I way off base here? (Problem being I’m not certain at this point how to model the tube itself – does a simple model not accounting for plate capacitance, transconductance, and all the other tube parameters, even exist, and how useful would it to be? I don’t necessarily want to get bang on accurate values, but I’d like to be able to mess around with stuff in the tone stack and get at least a qualitative feel for whats happening). Regards, Scott H. (P.S. You’re not the only one that figured it was getting pretty stinky around here).
Response:
- Hide quoted text — Show quoted text ->If you like the sound of the circuit, that’s what important. The Fender >tone circuit was not exactly designed for hifi response. >Given that tone prefs are completely subjective, there are still some >generalizations that can be made regarding those changes: >The Fender circuit was designed to take a big scoop out of lower mid >frequencies, centering around 300 hz or so. Close to the fundamental >of the guitar’s high E string. Though the response will be down by >about -24db at that point (-21db rel to high harmonics), the string’s >fundamental is ‘reassembled’ by the human ear due to the way your >hearing works. In short, the harmonics provide enough info to tell >the pitch, even if the fundamental is completely nulled. >What that mid notch does is to remove enough energy from the guitar >signal so that the amp can be pushed a bit harder without overdriving >the output stage as much. In other words, more high-freq energy is >available at the expense of richer mids. Fender amps attenuate some >of the bass freq’s too, much for the same reason…and because that’s >what makes them sound like they do. >That mid notch, and the way that the treble control is ’tilted’ has >a *lot* to do with Fender’s sound. Changing the notch frequency >upward allows a lot more bass response, but you may notice a change >in the nature of the mids and how they are phased. >Re the 33K ’slope’ resistor: The output impedance of the previous >tube stage is around 30K to 40K, so a 33K slope resistor will load >it down a bit. This will result in decreased treble response. >Nothing real serious, but you’ll lose a few db’s at the top. Yep, >Marshall did this in some of their amps. You may notice that the >mod results in a shift toward Marshall-esque phasing in the mids. >A subtle thing, but it’s there. >Also, the 30K mid control with a 33k slope res will restrict the >usable range of your bass control when mid is turned up. Again, >if you like it… >I wrote a program to simulate response of various tone circuits (tho >there *are* easy ways to determine notch freq by simple math) and I >recently adapted it to model the Fender-type config. I ran your >values thru to confirm loading, etc. Here’s what to expect from the >33K slope with .022 mid cut cap:
I have an amp that I built that uses the 33K slope resistor, the .022, and a 30 K mid. But what I have going from the wiper arm of the 30 K pot to ground is a 250K pot. To disable mid boost, I merely ground out the 250K pot. I also changed the treble cap to 470pf. The cap for brightness is 470 pf vice 120 pf. I assume you have PSpice. I ran this tone stack on PSpice with all controls maxed and got flat response with about 1 dB loss overall. Works like a charm!! I got tonal versatility.
Response:
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