paralleled triodes?

[lovedat800]

What sonic advantages do you get from paralleling both sides of a AX7 instead of running them in series? i realize you have less gain this way, but what sonic advantages does it have? Thanks.

[snoof]

IIRC, you will get more high end rolloff due to Miller effect when you parallel. I think it sounds a bit fatter.

[zaphod_phil]

Sounds fatter generally. A lot less gain than two stages cascaded but around 1.3 times the gain of single non-parallel stage.

[lovedat800]

thanks, gonna try this.

[echuta13]

They have a lower output impedance (good for driving a tone-stack), and are considered lower noise due to less internal resistance within the circuit (great for a first stage where noise control is more critical).

***info blatantly ripped from Merlins book***

[WesKuhnley]

If both triodes are share the same plate resistor, but have seperate cathode resistors that are different values, you can get this great mid-range swirl effect going on. Not sure what causes it, obviously some sort of interaction between how much current each triode is pulling at different points in the waveform. Cool effect anyway.

[zaphod_phil]

+1

[Eric]

Sounds interesting...hey, how about a volume control for each triode?
In order to blend the different sound of each triode together, with varying intensities ?

[echuta13]

If both triodes are share the same plate resistor, but have seperate cathode resistors that are different values, you can get this great mid-range swirl effect going on.

My best guess would be subtle variances between the triodes? Most triodes are mis-matched anyways...

Sounds interesting...hey, how about a volume control for each triode?
In order to blend the different sound of each triode together, with varying intensities ?

Sounds a like a Marshall 1987 preamp to me(just paralleled).

[zaphod_phil]

My best guess would be subtle variances between the triodes? Most triodes are mis-matched anyways...

No, that's not how it works. The triodes have to be biased hot and cold relative to each other. Then they operate in different parts of their characteristic curves, but at the same time their anodes are tied together, which means both anodes must have equal voltage on them. So the result they're kind of fighting against each other, with the anodes of the two triodes alternatively feeding current into each other in alternating directions. This seems to add a nice shimmer into the tone.

Sounds interesting...hey, how about a volume control for each triode? In order to blend the different sound of each triode together, with varying intensities ?

Sounds a like a Marshall 1987 preamp to me(just paralleled).

Yes, that may be a little tricky when the anodes are tied together. Otherwise, like echuta13 said, you have a Bassman or Plexi preamp, with their bright and dark inputs.

[guitarmike2107]

With the anodes of the two triodes alternatively feeding current into each other in alternating directions. This seems to add a nice shimmer into the tone.

sorry to get technical but what is shimmer? Odd order Harmonics?
I have an SE amp that has a pararell triode in the 3rd stage, I wanted to play about with setting one for grid current limiting and one for cut-off.. just havent got (the motivation) there yet. but my thinking is that it will likely add more odd order harmonics as the output wave will be cliped more evenly.
Mike

[Merlinb]

sorry to get technical but what is shimmer?

Tricky one. I would venture to say that it is a continually varying spectrum of strong even-order harmonic content and lower-level odd harmonics, with all the associated intermod' distortion also changing of course.
In parallel triodes using a different type of bias on each causes the load seen by each triode to continually change, since each triode's effective internal resistance is not only non-linear, but now is also varying in time with the audio signal. The result is that the triodes are not really fighting but working into an elliptical load line, just like a power amp. Basically, everything is changing in time with the audio signal. Grid current, headroom, Miller capacitance, THD, IMD, output impedance, everything!

[bustyman100]

I have been using a F*nd*r Pro Jr converted to a lite2b for a while. I definitely noticed a HUGE reduction in hissing when I changed the preamp tube wiring from series to parallel. Since I only had space for one input jack, the triodes run on parallel all the time.

I recently bought another used Pro Jr for real cheap so I could experiment with other mods. Right now I am thinking of turning it into a super clean lite2b to complement my other dirty lite2b. So I will probably be keeping the fixed bias for the power tubes and the split-pair thing for the PI, amongst other things.


Anyway, to get back to the point of this thread. For the preamp triodes, how would you guys set it up for optimum cleanness? I know parallel triode is the way I want to go because of the lower noise, so I am basically looking for an alternative way to bias the 12ax7.

Would it be okay to lower the joint plate resistor to 50K and bump up the joint cathode resistor to 1K? At what point will it begin to sound "sterile"?

I have always been under the impression that for a single triode set up, the "standard" is 100k plate and 1.5k cathode. Would the parallel triode "equivalent" to that be 50K joint plate and around 800R joint cathode? And if so, why does it seem that no famous amp uses such a combination for a parallel set up? I have seen amp makers rather not use one of the 12ax7 triodes than to hook it up in parallel (I think the Blues Jr has a wasted triode in it).

What kind of cathode bypass cap should I use for a nice rounded clean tone? I am already on the stock 50uF bypass cap on my first 18watt conversion, but I have read that 22uF is all you need for an essentially even gain increase across all the frequencies. Is the 50uF there because we need twice the uF for the 2 triodes in parallel?

Finally, to MerlinB and Zaphod Phil, I am intrigued by this separate cathode resistor shimmering thing. How hot/cold should I make each triode? Are we talking an 800R on one side and a 1.5K on the other? Will the shimmer increase if I have different value bypass caps on each cathode resistor?

Wow, I really got carried away here and asked a lot of questions. I hope I didn't just hi-jack the OP's thread with all this...

[rock_mumbles]

...

Anyway, to get back to the point of this thread. For the preamp triodes, how would you guys set it up for optimum cleanness? I know parallel triode is the way I want to go because of the lower noise, so I am basically looking for an alternative way to bias the 12ax7.

Would it be okay to lower the joint plate resistor to 50K and bump up the joint cathode resistor to 1K? At what point will it begin to sound "sterile"?

I have always been under the impression that for a single triode set up, the "standard" is 100k plate and 1.5k cathode. Would the parallel triode "equivalent" to that be 50K joint plate and around 800R joint cathode? And if so, why does it seem that no famous amp uses such a combination for a parallel set up? I have seen amp makers rather not use one of the 12ax7 triodes than to hook it up in parallel (I think the Blues Jr has a wasted triode in it).

What kind of cathode bypass cap should I use for a nice rounded clean tone? I am already on the stock 50uF bypass cap on my first 18watt conversion, but I have read that 22uF is all you need for an essentially even gain increase across all the frequencies. Is the 50uF there because we need twice the uF for the 2 triodes in parallel?

Finally, to MerlinB and Zaphod Phil, I am intrigued by this separate cathode resistor shimmering thing. How hot/cold should I make each triode? Are we talking an 800R on one side and a 1.5K on the other? Will the shimmer increase if I have different value bypass caps on each cathode resistor?

Wow, I really got carried away here and asked a lot of questions. I hope I didn't just hi-jack the OP's thread with all this...

OK, from my perspective, I built a Baby Will pcb LiteIIb, it was too "unclean" for me, it has its place for sure, but it wasn't a great blues amp, especially for our guitar player. Stock it had a 120k shared plate resistor, I think.

So I used about a 50k plate resistor (knowing that would be equivalent to 100k for each triode) and it was clean, but it was lifeless, thin, and anemic sounding. I liked the clean headroom, but not the tone. From this experiment I came up with a little saying:
"A gain sucking tone control is better than a tone sucking gain control!"

So v2.3 of the Baby Will came out, I built one for a friend, short story Sting used a "split load" plate resistor in his v 2.3 design for more clean headroom. It worked really nice, and if you wished you could jumper it to stock. By the way there's a high-end amp that uses a switchable "split load" plate resistor; there's a "normal" position (tapped at the bottom of the two resistors) for a fast response, or "split" with a pretty large attenuation for a "smoother" response. I do think that the split load plate does make the amp a little more "user friendly".

I put the split plate resistor on my Baby Will, I think I used a 56k to the B+ voltage and a 47k to the plate, then the coupling cap is connect to the junction of the two resistors. It was back to being a "real" amp, no longer did it have the "tone sucking gain control" sound. It had the clean headroom we wanted, and it retained the wonderful tone of the stock amp. The preamp tube is working (biased) just like stock, but the preamp output signal is attenuated so you have more clean headroom.

The other option I added a bit later was the 18Watt "Drive" control, a PPIMV (post phase inverter master volume) because of the way an 18W circuit works if you back off the "drive" control to about 3:00 o'clock you gain cleans.

Regarding the 50uf bypass cap, you are correct, that since the triodes are paralleled you need twice the capacitance for a "fully bypassed" cathode.

[bustyman100]

Thanks rock_mumbles! I never thought of using a split load plate pair on the parallel triodes... and I guess I should have since F*nd*r uses a split load plate pair on one of the PI plates- though it is a 91K with a 9.1K there.

I think I will give the split load a try once I settle into my new apartment. I probably won't have space for the PPIMV in my tiny chassis though.