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Half Power Mod


 
10/20/1997 5:01 PM
Jason
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Half Power Mod
Hello all. Would it be practical to consider installing a half-power switch on a '66 blackface Showman head with 4 6L6's? Also, how about a variable impedance switch so I could use either the rated 8 ohm load, or a two-cabinet 4 ohm load? I have heard that models from that year did not have a certain circuit element that later models do, supposedly added to keep the impedance more constant when utilizing the "ext speaker" output jack. Suggestions? Thanks!!
 
10/22/1997 2:22 PM
Doc
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Jason:  
The Showman amp has a single output winding which is looking for a speaker load of about 4 ohms. So there's not much you can do in regards to an impedence selector swich. It's safe to operate this amp as-is with an 8 ohm load, but max power will be reduced, and it will sound a little different.  
I've installed 1/2 power switches in these by interrupting the grounded cathodes of one push-pull pair of output tubes. If your amp has a 3-wire cord installed, you can use the old hum switch location for your 1/2 power swich so as to not deface your blackface.When two of the four tubes are taken out of the circuit, using the same OP trans, the remaining two tubes are now looking for an 8 ohm optimum load.  
The latest SF models of Twin Reverbs had output sections run in ultralinear mode (had screen grid taps on the OPT) which is less sensitive to load impedence variations than straight pentode mode. Ultralinear is great for Hifi, works good for bass guitar, but it's "too clean" for most guitar.  
There are many amps which do have transformers with multiple output taps, switch-selectable (like Marshall), or some have automatic switching via the extension speaker jack internal contacts to compensate for the parallel speakers .  
The half power switch is an easy mod to your amp. It will not, however, result in half the volume level this amp achieves now. If you're really looking for a noticeable drop in volume capability, you could run one pair of tubes in triode mode, then switch back to all four in pentode (stock setup) for full power.This is similar to (but not exactly) what is done in a Mesa Boogie SimulClass. There are some switching schemes in one of Kevin O'Connor's "Ultimate Tone" books.  
Regards,  
Doc
 
10/24/1997 4:59 AM
Steve Ahola
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Doc  
.. Could you elaborate a bit on Mesa Boogie SimulClass? So they aren't using Class A at all, just pentode/triode tricks? Is that with cathode bias or fixed?  
.. With fixed bias, do you need to change it when you disconnect 2 of the tubes?  
.. Any info on SimulClass would be appreciated.  
Thanks,  
.. . Steve Ahola
 
10/24/1997 1:04 PM
Jim S.
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I'll try to answer this one...  
 
The idea of SimulClass is to combine a push-pull pair of power tubes running in fixed-bias class AB with another push-pull pair of tubes running in class A triode. It turns out that there's more hype than reality when you look at the actual circuit.  
 
It turns out that BOTH pairs of tubes are run using fixed bias (negative bias voltage applied to the control grids). Now the "class A" pair does run hotter than the class AB pair - this is accomplished by a voltage divider that both reduces the negative bias voltage AND the drive signal going to the control grids. However, this pair of tubes is running much cooler than maximum plate dissipation. Typically, the class AB pair is run at around 25ma per tube idle current and the "class A" pair run around 35ma per tube, all tubes having a plate voltage between 440 and 480 volts (depending on what amp model). What supposedly limits crossover distortion in the "class A" pair of tubes is the fact that the drive signal is attenuated to such an extent that it will clip (in the driver stage) before it can cause either tube to be driven into cutoff. Thus, the claim of "class A" operation.  
 
The pseudo triode connection (I say "pseudo" because tying the screen to the plates is not really the same as using true triode power tubes, like certain audiofile amps do) does help to soften the sound and aid in the illusion of class A tone.  
 
It's interesting to note that the classic class A guitar amp output circuit - the AC30 - is completely different than the Simulclass "class A" circuit. In the AC30 (and many other new amps that use 2 or 4 EL84's in class A) the tubes are run in full pentode cathode bias mode. This creates a great, classic tone that many guitar players feel is unequalled. IMO, the SimulClass circuit sounds nothing like this, even when switched to class A only operation. (I remember reading a Ken Fischer article on this subject - his opinion is that when the SimulClass circuit is driven into distortion, the sound "turns to poo poo". I'm not sure if I agree it's THAT bad...)  
 
It would be interesting to see if anybody ever comes out with an amp that uses 2 or 4 6L6GC tubes in a class A cathode biased pentode configuration. I wonder what it would sound like? (Come to think of it, I did once work on an old Supro which was basically a copy of a Tweed Deluxe, except it used 6L6s and had a 15" speaker. It probably ran at class A or at least pretty near. Once it was repaired the amp sounded great!).
 
10/24/1997 1:57 PM
John Greene
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OK, let me test my understanding of how this works...  
 
What supposedly limits crossover distortion in the "class A" pair of tubes is the fact that the drive signal is attenuated to such an extent that it will clip (in the driver stage) before it can cause either tube to be driven into cutoff. Thus, the claim of "class A" operation.  
 
I don't believe the drive signal has anything to do with Class A operation. Class AB is when both tubes are conducting the signal at low drive levels (Class A) and one of them is driven to cutoff while the other conducts for higher drive levels (Class B).  
 
With Class A, the tube should be mid-point biased. This means when one tube is driven to cutoff, the other tube is being driven to full conduction. The reason for the attenuation of the drive signal for the Class A pair is that Class A has more gain. 3dB I would guess. I'm also assuming that it was the Class A pair that received the attenuated signal. (no schematic)  
 
The pseudo triode connection (I say "pseudo" because tying the screen to the plates is not really the same as using true triode power tubes, like certain audiofile amps do) does help to soften the sound and aid in the illusion of class A tone.  
 
Technically this is called Triode Connection. No pseudo about it. The data sheets publish Triode Connection data. It is a Beam Power Tube operating in triode mode. I don't think there was any reference that this was exactly the same as a true triode tube. It's just a mode of operation.  
 
It's interesting to note that the classic class A guitar amp output circuit - the AC30 - is completely different than the Simulclass "class A" circuit. In the AC30 (and many other new amps that use 2 or 4 EL84's in class A) the tubes are run in full pentode cathode bias mode. This creates a great, classic tone that many guitar players feel is unequalled. IMO, the SimulClass circuit sounds nothing like this, even when switched to class A only operation.  
 
Unfortunately I don't have my schematics with me. Is the simulclass amp using EL84s? If not, that would have an effect on the sound. Cathode biasing has a definite effect because the bias point changes with volume level. I believe the AC30 states something like 10volts on the Cathode resistor at idle and 12.5 at full volume. Dang, I wish I had my schematics.  
 
Anyway, I guess the point I'm trying to make is that there are several other factors involved that would affect the sound other than the Class of operation.  
 
--johng
 
10/24/1997 7:43 PM
Jim S.
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I DO have the schematics for the SimulClass versions of the Mesa/Boogie Mark IIC+ and Mark III circuits - so I know exactly how these operate.  
 
My main point is that the supposedly class A pair of power tubes (either 6L6GC or EL34 - they're interchangeable in those amps) are NOT biased for true class A operation. At best, they're biased for high class AB operation. The assumption Mesa makes (and which I disagree with) is that if you limit the drive signal going to a class AB power section such that the tubes never go outside of the class A operating region (no cutoff occurs), then you can call that circuit "class A". In other words, Mesa is basing their terminology on the premise that no significant crossover distortion is going to occur. My own opinion is that this is bogus.  
 
In addition, having had the experience of playing customers' SimulClass amps, I was left with the impression that, sonically, nothing magical was going on. There just isn't any of that sparkle and depth that players associate with good amps (such as VOX AC30 or Matchless) which use Class A circuits. I feel that SimulClass offers few sonic benefits that you can't get with a standard class AB amp that is properly biased.
 
10/25/1997 1:13 AM
Steve Ahola
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Doc, Jim and John:  
Thanks for your input- I printed out your posts to put in my binder on tube amps.  
Correct me if I'm wrong but I was under the impression that with a "true class A" circuit, the output tubes aren't hooked up in a push-pull arrangement like class AB, but are wired up roughly "in parallel" with a specially designed driver circuit and output transformer (like an AC-30).  
Mesa Boogie has a Maverick amp which is advertised as being true Class A and sounds better than their SimulClass amps (at least to my ears).  
... Has anybody here come up with a mod to add the "Tweed Power" option to a traditional Fender BF/SF amp? From a prior thread here, I got the impression that Mesa uses a power transformer with different primary taps, and would use the taps designed for 143 volts to reduce B+ approximately 16%. Does the "Tweed Power" setting effect the preamp tubes, or just the output tubes? Is there an aftermarket transformer like that which could be added to an older Fender amp such as a Pro Reverb? Or would it be possible to just step down the B+ voltage to the output tubes with resistors and caps?  
Steve Ahola
 
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