specs for driving 147 family leslie amps?

[Guest]

I am trying to use a small tube amp to drive a 147 amp and have no need for the preamp pedal because I have already installed solid state motor control circuitry. I understand that I could use a 10W amp with the console load resistor set to 8Ohms, was wondering what kind of amp it would take to drive the 147 with the console load switch set to open. I have read this "Any source capable of delivering over 6 V into a load of 2.5 kohm, or less, will be able to drive the Leslie amp to full power with the "console load resistor switch" set to "open"." Which would tell me it would only take a 0.0144 Watt amp, but this doesn't seem correct to me. Basically I am looking to use the lowest power tube amp possible, while still providing the correct input requirements..any help would be great !

[Model E & Me]

Re: specs for driving 147 family leslie amps?



What you read is correct. You need a voltage signal, not a power signal, to drive a 147 with the Load switch open. So any type of preamplifier capable of delivering the required output voltage will work. Power output rating is irrelevant, and non-existent, for a preamplifier.</P>


The Load switch is a convenience for those who need to feed the Leslie from an instrument already having its own power amplifier, because without the load switched inthe instrument's power amplifier would basically be feeding almost an open circuit, which is OK for a voltage amplifer but not for a power amplifier. But the load is strictly a "dummy" -power is not needed to drive the Leslie.</P>


We could probably be more helpful if we knew what the original signal source is that you are feeding to the Leslie, and what kind of output voltage it has..</P>

[Guest]

Re: specs for driving 147 family leslie amps?

That is my problem, I'm not sure what to get as the original source to bringthe low level out of my keyboard up to suitable level for the 147. I am somewhat adept in fabrication and circuit design (have an Associates degree in Electronics and Computer Technologies). Basically a circuit to replicate the combo preamp (I got the schematics for the combo preampand removed the second input and speed switching parts which I do not need) but am not sure if this is the way I want to go. I am a little confused however..... I was under the assumption that a low level amplifier would be appropriate as long as it did not force too much power into the leslie, but you are saying that power is not a factor, it is voltage output which I assumed went hand in hand with power. Anyone know what the combo preamp's voltage output is? looks to me like it's around 35V, but my calc. are probably off.

[TheAdmiral]

Re: specs for driving 147 family leslie amps?

You might check the PreSonus Tubepre preamps. I have one but haven't tried it yet but it's supposed to be good.

[johnny b3]

Re: specs for driving 147 family leslie amps?



Interesting topic. </P>


I currently use an L-100 power amp to drive my 147...it's built into an enclosure but is bulky.In the endI have no qualms with using/not usingthe load resistor switch. </P>


I have a little hobby box 5x3x1 (inches) in which i've fitted my 6-pin socket, 5A fuse, and a quick release plug for the speed switch pedal. An AC cable exits the box to the nearest wall socket and the audio cables also exit, terminating at some flimsy marrets. Currently, the connecting cables leaving the marets are the typical green/black wires which terminate atthe matingL-100 output plug. </P>


So you can imagine its a bit of a flimsy setup. If i were to screw the hobby box to the amp enclosure it would be a bit better, but i'm still left with 2 AC cables (one for the leslie, one for the amp), yet its not my biggest problem.</P>


At the last gig (used with a VK7), with the leslievolume at 7 or 8I was getting a crunchy tube distortion which sounded amaing, but I would like to figure out why I can'tget aclean signal at loud volumes. This of course does not happen when the leslie is hooked up to my L-100, so i'm leaning towards the idea thatthe VK7's output is overdriving the L-100 amp.Howdo I achieve stage volume with this setup but with the option of cleaning the signal a little bit?Am I unknowingly stressing one of my amps?</P>

[Model E & Me]

Re: specs for driving 147 family leslie amps?



OK, so you want to feed a keyboard output into the Leslie. Do you know what the output voltage of the keyboard is? </P>


The front end of the Leslie is a voltage amplifier, and it needs a voltage signal. Sure there is some power involved, but it is miniscule. You need a preamplifier that will amplify the voltage output of the keyboard up to the voltage level needed by the Leslie. You do not need a power amplifier.</P>


Isthe keyboard feeding any kind of amplifier now?</P>

[johnny b3]

Re: specs for driving 147 family leslie amps?



was your reply for Justin or me? didn't mean to hijack this thread, seemed to be on topic.</P>


The owners manual does not specify the output voltage. If a pre-amp simply ups the voltage, why do you hear a difference between tube and solidstate units?</P>

[Model E & Me]

Re: specs for driving 147 family leslie amps?



Sorry, j b3, I was replying to Justin. Iwas trying to better define just what it is he needs to do.</P>


By all means, join in the discussion - welcome! As to your question, a "pure" preamplifier raises the voltage from a low level to one useable by a downstream device. Many commercial units may also have tone-shaping circuitry or other goodies thrown in. I would guess most guitar units do this, although Iadmit that I am completely out of my leaguewith guitar amplifiers.</P>


As to an aural tube/solid state difference, on really well-designed amplifiers there isn't any, butsince solid-state voltage gain is so cheap, it isn't usually worth the designer's effort to do so. It is much easier to builda solid state high-gain amplifier with lots of negative feedback to cover any shortcomings. Also, such solid state units do not overload graciously. </P>

[Guest]

Re: specs for driving 147 family leslie amps?

Maybe it'l help if I specify the signal chain before. I am using a Fatar Waterfall Controller MIDI'ed to a laptop running Plouge Bidule hosting Native Instruments B4II. The signal comes out of an M-Audio Firewire Solo who's maximum line output is 1.26Vrms balanced (stated in manual not measured), this runs into a presonus tube pre, who's output I am measuring at 270mV nominally, 3.10V near clipping. I also found something wierd, I still get a reading...albeit a different one with only one test lead connected to the DMM, ?????.anyway, after this the signal is sent to an 85W solid state amp whose output I measured at 12V. I am looking to replace this solid state amp with a 147 amp which I do not possess (I will probable get a 47 or 145 or 45 becuase speed switching is not an issue)

[Guest]

Re: specs for driving 147 family leslie amps?

oh, by the way.... doesn't have to be tube :)

[Model E & Me]

Re: specs for driving 147 family leslie amps?



So you've already got about a 3 volt signal into your present power amplifier, and you need about a 6 volt signal for the Leslie.</P>


Did you ever consider an audio interstage transformer? You only need a 2:1 or at most 3:1 voltage ratio, if your preamplifier is happy with low impedance loads.</P>

[Guest]

Re: specs for driving 147 family leslie amps?



Never even crossed my mind......just a passive interstage tranny eh?</P>


something like this http://www.surplussales.com/transfor...udTrans-2.html</P>


or perhaps a microphone transformer http://www.jensen-transformers.com/mic_in.html</P>


the interstage tranny's seem kind of rare eh?</P>

[Guest]

Re: specs for driving 147 family leslie amps?



would any of the other trannys on that 1st page http://www.surplussales.com/transfor...udTrans-2.htmlwork? </P>


there are only two with listed ratio's, labled as "driver transformers".</P>


there is an assortment of others also, say </P>


<SPAN class=smalltext>Raytheon audio transformer.</SPAN></P>
<BLOCKQUOTE>


<SPAN class=smalltext>• Primary: 600 ohm (130 ohm DCR)
• Secondary: 1000 ohm (25 ohm DCR)
• 9mw, 175 wv
• Dimensions: 1-3/4" x 1-5/8" x 2-5/8"H</SPAN></P>


<SPAN class=smalltext>would this have a ratio of 1:1.6 ??</SPAN></P>


<SPAN class=smalltext><SPAN class=smalltext></SPAN></P></SPAN></BLOCKQUOTE>

[Model E & Me]

Re: specs for driving 147 family leslie amps?



For a voltage gain of 2 or 3 without any appreciable power,atransformershould work well.</P>


A lot of the Surplus Sales transformers have big iron cores. The excitation current on those might be a bit much for your amplifier output.</P>


The microphone transformers look like a better bet, but many of those would be overdriven and saturate with the signal levels you need. The 13K7 might work all right. It has a 1:5 ratio and will take a +7 dBu input, which translates to about 1.75 volts rms. The secondary impedance isn't bad compared with the 147 input impedance. The question is whether your amplifier will be happy feeding a 150 ohm load (or less, because the 147 input impedance is lower than 3800 ohms, according to your earlier post). Its also kind of pricey for experimentation. But those are the kind of specs you would need.</P>


While you are shopping around, you might notice thata lot of audio line or interstage transformers just list impedance ratios. You probably know already, but the voltage ratio is just the square root of the impedance ratio.</P>
<P mce_keep="true"></P>

[Model E & Me]

Re: specs for driving 147 family leslie amps?

In case you are wondering about the concept of using a transformer for input to a Leslie, most Leslie kits for connecting 122s (or 31Hs) to Hammond organshave an interstage audio transformer. They are used mainly to isolate the dc control voltage to the Leslie from the organ preamplifier output, but they also step up the signal voltage by 10% or so to give the Leslie a bit more range on its volume control.