MutantLeslie Amp

[knife edge]

Oh Great Titans Of Tubes, Legendary Lovers Of Leslies, Honorific Hoarders Of Hammonds. I present for your entertainment and chagrin, the MutantLeslie Amp Project.

The MutantLeslie, or ML, is a budgeted attempt to simulate the sound and tone of a Leslie tube amplifier.

To wit.

The base for this project is the A0-44 reverb amp out of an M-100. The M-100 was long gone by the time I bought this amp, so you can stop throwing rocks at my house. Also, at least a small piece of it lives on and is owned by a keyboard player.

I will not be using rotating anything, I need something that I can move easily in Mrs KnifeEdges SUV, sos I can gig with the youngin’s.

I first needed the front half of the 147 amp. Since I will be using Gsi VB3, on a Lenovo Miix 11 I3, the output is not balanced, hence the need for the 147 type front end. So to that end I will be using the venerable 12AU7A as the phase inverter. I will have only a volume control and on off switch.

As far as the back end “Power” section of the amp, instead of using the string pluckers’ choice EL84, or the ubiquitous 6550, I will be using a pair of 6L6GCs. The choice of tubes was not random or casual; obviously the EL84 is not suited for keyboard. The 6550 typical voltage is higher than 400V in AB1 PP Config. The Mutant’s power Transformer only puts out around 320 depending on household voltage. So, as we all know, Bass amps make good keyboard amps if you add an x over at 800 Hz, and a horn. So I found that the Ampeg portaflex 15 makes fine use of the 6L6GC tubes, the typical voltage of 360V puts this guy much closer to the Mutants power capacity.

I am having to graft the back end (Power) section of the 122 to the front end of the amp. The Mutant output transformer does not have a center tap on the Sec as the 147 does so the feedback; both global and local are handled differently. I tapped part of the feedback into the locations of where the 147 has them, because I am using the 147 front. The Original Spartacus Amp that this plan was taken from has a global feedback that runs from the OT Sec, back to the Phase Inverter. I was hoping to use this, with less filtering to perhaps gain more clean headroom, like the 147. But I do not want to change the base characteristics of the amp, that is for string pluckers.

The Power Supply is standard to the Spartacus A0-44 plan. I have to dump the Can Cap, because I need higher uFs to get a cleaner DC that what a Reverb amp would call for. But, if and when I build the 147 copy, sans motor control, I will use the cool looking can cap for it.

The heater circuit will be DC, this is due to the confined space and will keep and noise to a minimum.

So there it is, since I never took a Neets Exam, or have any FCC certs or licenses, I am open to all opinions and suggestions. The Budget for this is under 150 bucks and I am pretty sure it will be closer to 125. Anyway let the Perusal begin

KnifeEdge
(It is really a classical composition written by a Polish guy)

[David Anderson]

I'm not sure why the EL84 isn't suited for keyboard, but not worth having an argument about, I suppose.

It looks to me like you're building something like the Leslie 125 amp, so you should probably look up that schematic. Your B+ voltage will be lower, but you will probably need to increase the cathode resistor value since you will likely still burn up 6L6s with a 130Ω shared cathode resistor. I'd say at least 250Ω, possibly higher.

Normal line level will not be sufficient to drive the amp you show to full output. I'm guessing you know that.

This is a low-gain amp, so DC heaters will provide no real benefit in terms of noise.

Basing this on an AO-44, you have a power transformer designed for an amp that would put out around 12 Watts, so the PT may end up being your output power bottleneck. It may not allow you to obtain the maximum output from a pair of cathode-biased 6L6s.

[nonreverb]

I agree with David. The 125 amp would be a far better option. They are relatively cheap and reasonably plentiful.

[knife edge]

Hah, thank you very much David, that's why it pays to ask. The 125 looks like a good match for a first time amp build.

KE

[knife edge]

Ahaa, you thought ya’ll were rid of me, but like an Antibiotic resistant bacteria I am back. I’ve been working on some old school sound and led boards for my nephews and their friends. I’m hopin they get the electronics bug and expand their horizons. Anyway eventually I will put up pics and video.

So where am I on this Mutant amp? Well a couple of things, first, thanks to all for the Ideas.

1. The concept of the cheap 125 amp is, well, as far as I can see not much of a concept, I live in N.C. and have scoured everywhere and cannot find anything cheap/inexpensive, most of what I find is the complete speaker system, cabinet and all for around $500.00, but then you have to pick it up, so you add a couple of nights hotel to get to Cucamonga, plus food, etc, etc. You could easily get to 1k, just to get the amp. It appears that, like everything else “Vintage” there is a hefty premium, and this premium is, from what I can see higher than the cost of building the little Amp I want to.

A. I have $86 in this amp. With that price I have a power transformer, output transformer, rectifier tube, 9 pin sockets and chassis. The seller obviously took the neat little multi-function tubes out of it to resell, and for more than $50 a piece the seller has made more that break even on this deal. But anyway, as David stated 12w or so power is really not a whale of power but for a first time learning experience, I think it will be sufficient for me.

I. Transformers
One thing I have found out is that transformers are pricey. New Classic Tone Leslie stuff $80 +. Used transformers are not that much cheaper, so for my purpose, getting the parts I did for $86, again for a Learner/Beginner, more musician than Tech, I am ok.

2. The choice of tubes.
I have done a lot of research since I first posted and I found that to properly use big tubes you need big V and I. Or rather, to get to certain power levels of output you need V and I, to make use of the V and I, you need bigger tubes. So as far as my tube selection, I believe that the el84s would be more suitable for this application due to their lower power requirements. I have not double checked the power output of the Power Transformer, and I plan to do so. But I did find someone who hooked it up to a modern Full Wave Bridge Rectifier and got 375 v DC out of it, obviously this little guy is maxed out at 375v. So my reasoning for using the 6v6 was that it is used by Ampeg in the Porta Flex. Generating good Bass is important to me, as is the mellow/complex sound of a tube amp. Getting the Higher frequencies is easy, throw a HF Tweeter, or Horn at it and you are done, but complex and rich bass is another story.

However, to make good use of the 6v6, you need to have 395+, at least this is what I have seen from most schematics, so it seems this is a “sweet spot” if you will for this tube. So looking at the max this transformer can put out, I really can’t see how it would efficiently drive the 6v6, and even if 375v was the sweet spot, it still means the transformer would be maxed out, with no fudge%. I don’t know how this would effect the transformer, and I really don’t want to experiment. Now my thinking comes from David’s comments, that got me considering the whole amp rather than just the tubes.

3. The Whole Amp
“Aye There Is The Rub” I found out rather quickly that the total design of the Amp is far more important than the tubes. It sounds elementary, but until you really sit down, research and then think about what you have learned, it will not be that clear. Leslie Amps are unique, not just in sound but in function. The design while sharing the basic elements of all Thermionic based amplifiers; the way these elements are assembled and used is unique. Of course this should not surprise anyone seeing as it is amplifying the most unique Organ ever made.

So I started looking at what is being done today with the el84/6BQ5. The Orange tiny Terror came up. One of the things I noticed was they power the plates of the phase inverter (12AX7) at 290v, as opposed to the 125s 12AU7 at 155v. The other thing I found was the power on the output tubes, they are putting 310v on the plates, obviously the Max v is 300v, but the peak is 550v, so they are going with the peak number, since the power draw in an musical amp is generally not constant. So comparing the two, not including the Oranges Pre amp, even though the two amps use very similar components, the design specs are totally different for two completely different purposes. I have heard the OTT and it sounds like a guitar amp, and that is how it is supposed to sound.

4. What is the Bottom Line?
So here are the specs and parts. I will post the schematic at some point.
1 6CA4 Rectifier
1 12AU7 Phase inverter
2 6BQ5 Power
A0 44
Power Amp
Output Amp
Can Cap
3 9 pin sockets

I am going to use 310v on the output plates and, as far as the output tubes it appears that 130 Ohms shared in P/P AB1 configuration is the correct figure for the cathode bypass resistor. Unless someone has a better suggestion.

155v on the 12AU7. I will be using the whole 125 circuitry for the 12AU7, again, unless there is a better suggestion.

Oh, one more thing, do I need a voltage regulator?

I am leaving a lot of room for educated suggestions on this project so chime in. In all, I believe I will have around $200 in this by the time I am finished, so that’s around what I was looking for.

Now I have another technical question. After going over, and over, and over, the NEETS modules 1-6, and re-watching Uncle Doug's videos on voltage drop, and calculating amperage, I have come to the conclusion that, either, as a mild dyslexic I just can’t do math, or I don’t know what I am doing, or the specs given for transformers may not be that accurate. So if someone could Opine, I would greatly appreciate it.

According to Classic Tone Transformers, the power transformer they sell for the Leslie 122 has a voltage rating of 420VDC and 190mA DC
So if I look at the 122 schematic I see the following. 420 VDC into the OC3, the OC3 being more or less the node for powering the rest of the Amp, obviously.

So the circuit in question is the Screen circuit for the power tubes. The lead comes out of the regulator, then to a 470 Ohm resistor, this gives the circuit a 110 VDC drop across the resistor. Now all of this is ok, except when I look at the amperage for this part of the circuit, using Ohms law I get 234 mA DC. Obviously this is higher than what the Classic Tone transformer specs out at. So there is something not quite right, either I am waaaaaay off, which is possible. Or the spec is wrong, or there is no problem with the circuit drawing this high of amperage, because the transformer has a “fudge” factor. So I would appreciate any input into this so I can see where I possibly went wrong.

Anyway, there will be more questions ahead, and thanks all in advance.

Knife Edge

[Tim_at_Jonas]

The AO44 OPT seems to be sizable, but I expect that is for clean low end. I doubt that there is anything to be gained by going beyond EL84/6V6 on the output tubes. I'd try it stock (except I expect you'll need to pad the input, or pull the transistor stage out), and if it doesn't have enough poop, sell it and get something else.