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Parallel triodes

3/9/1999 11:26 PM
tubegirl Parallel triodes
More questions for you wonderful people.  
1.Can anyone tell me the relative merits (or otherwise) and tonal characteristics of running two 12AX7 triode stages in parallel ? This is something Matchless have at times done for their first input stage. I'd be very interested to hear how this affects the tone as compared to traditional approach.  
2.Any IMHO's on the sound/feel/playability of the Matchless Spitfire.  
3. I've heard that if you're interested in tone rather than instant high-gain, running 12AX7's at an abnormally low voltage (say 50VDC or less)can create an intensified "tube" tone due to the build-up of the electron cloud. Any thoughts ? Could this strategy be utilised as a preamp first gain stage (which would also enable lower first stage hum), building up the voltages of later stages in a cascade method to achieve same amount of gain?  
OR, would lowering that initial high gain remove one of the classic beneficial trademarks of tube amp sound ( reference to clean playing).  
Gasp...forever questioning. tg.
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3/10/1999 12:17 AM
Benjamin Fargen

Running the triode stages in parallel boosts the drive(you already knew this!)and has the benefit of possibly lowering noise due to the increased signal level.  
As far as the tone goes...I really liked it at first(probably because it was my first amp project that worked!haha!)  
I found the tone stack to be way to limiting and the high end just a wee to shrill, But don't let this scare you away from building it! Its a fun project and you might come up with a different opinion of it than me.  
In regards to your last question, my only comment would be to look at the voltages that VOX ran for their preamp sections. Lower than most and listen to the warm early breakup you get. I guess it depends on your taste.  
3/10/1999 5:34 AM

1. You have double the transcondutance (gain) and half the plate resistance of a single section, so you can use smaller resistors for the same gain (and get less noise).  
3. A lot of preamps, multi-FX, and pedals like the Real Tube, Tube King and Paia SIAB use this principle (starved plate supply) with voltages as low as 12V.  
Many people think the "tube" effect is too exagerated. Anyway, you loose headroom - that means it's hard to get a cool clean sound out of it, it won't be much touch responsive and the old trick of backing the guitar volume pot to clean the sound won't work very well. That's why you see ads of preamps and pedals where the use a "normal" B+ is emphasized as a plus.  
The Real Tube ties the grid to B+ instead of ground, so that the grid is biased at 0V and grid current flows. If such a stage is preceeded by a low output impedance stage (eg opamp) this can increase the clean headroom a bit.  
There's another company that uses a (transistor) current mirror as the plate load so that the plate voltage will remain constant, and again this can increase the clean headroom.  
Other thing you can do to get more voltage swing out of a given B+ is to use chokes as plate loads, but I've never seen it in any practical starved plate supply circuit.  
Other people think that lowering the voltage of the first stage as you suggest is a great way to get "brown" sound.  
I think you can experiment and search for a compromise.  
Many people report that lower gain triodes like the 12AT7 and specially the 12AU7A will sound (a lot) better than a 12AX7 in starved plate supply designs.  
3/10/1999 6:09 AM

Hello, folks. Concerning tube B+ supplies, a fun thing I've done in the past while breadboarding preamps is to hook the thing up to an adjustable B+ supply (a Lambda 0-450V tube model, bought at a hamfest for $10) and note how the tone changes when the voltage is changed. On a Marshall-type preamp, the tone gets browner and loses headroom as the voltage goes down. When the voltage is increased, the tone gets cleaner and harder (these are general observations when the voltage is varied aboveelow 250V). IMHO, the supply voltage is very important when voicing a preamp, as the tonal changes over voltage are not at all subtle.
3/10/1999 1:42 PM
Your Right MKB, Thats why some fenders[or any amp] sounds cleaner with higher voltage on the preamp plates,The way you kept the same circut and just changed voltages is a good experiment. So if you could find the fine line that has the best tone would be the way to go, and then fix the voltage to run at this setting. We don't see to many asking these questions, but its a good one. about the starved plate voltage and tone it gets, or cathode following type circuts we need some more thoughts on this.We all could learn something. The amps I've seen with preamp voltages of 120 to 150 seem to sound the best. [Richie]
3/11/1999 1:23 AM
Well thanks. I sure will try low voltage and parallel preamp experiments when I get the time.  
Another dreamer project I've had in the back of my mind for a while is the idea of constructing a guitar amp that is entirely "differential" or "balanced", from first stage right through to the output push-pull. Basically, the ultimate hum-cancelling amp. I'm sure we've all noticed how quiet amps are when inputting signal directly to the phase-inverter/driver. My thoughts were to convert the unbalanced guitar input to a balanced signal (RIGHT at the input jack) using a high-quality mu-shielded (grounded) balanced mic. transformer. The output centre-tap would become circuit ground, and the rest of the amp would be built strictly symmetrically around this with its two "sides". Disadvantages would be: Preamp triodes might have to be "matched" as well(?); any midway i/p's o/p's such as fx send/return would have to balanced (limits fx use to high quality units. A bad thing??); have to be careful about design of tone controls so that settings of such do not excessively unbalance the whole arrangement.  
Any ideas ? Anyone done it before ? Did McIntosh do something similar in hi-fi or am I misremembering ? Is this too far to go in the quest to remove that last smidgeon of hum ?  
Thoughts from an overactive tubular imagination. tg.
3/11/1999 7:15 AM

Hey, not a bad idea, but you are correct that there'd be some head scratchin' needed to implement tone stacks and FX loops.  
And yes, McIntosh's usually had a differential configuration from input to output.  
I'm not sure it would be worth the hassle vs the benefits, but I commend you for pushing the envelope of thermionic thought!  
What I do is wear two hats, guitar and hifi. Then I can explore all topologies where appropriate and realize cross over ideas when they occur. (but I haven't built that 300B guitar amp yet!)  
... Whit

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