Differences/advantages of a 12AX7 in parallel verse cascaded

[erl]

I have a couple of question about a build I’m doing, and I’m hoping the experts’ here can shed some light for me. I’m building an 18 watter with an EF86 and TMB channels. Since I added the EF86 I have 2 12AX7 to use for the TMB, and my first question is, what would be the differences between running the first 12AX7 in parallel verse cascading it? I have built a test head using a 12AU7 for output and everything works great, but there is a noticeable, but livable, difference in volume between the EF86 and TMB channels. On this one a cascaded the first tube, but looking at the TMB-lite schematic I’m wandering if parallel would be the way to go.

Also I have a post PI master volume, so if I where to not have both the volumes on a normal TMB, which one should I use, one feeding the tone stack, or one after?

Thanks,

Eddie

[zaphod_phil]

The paralleled triodes will give you *way* less gain than cascaded. Basically, if you want to have a full TMB tone controls, rather than a simple single-knob tone control, then you will need to have the cascaded preamp to overcome the pretty huge losses in the TMB stack.

I presume you're refering to the "18W TMB SuperLite" schematic I hacked together...

[erl]

Yes I had looked at that one, as well as this one, http://www.18watt.com/files/tmblite_pre.jpg, which I was thinking of using since I will have 2 12AX7's available for the TMB channel.

Thanks so much for the info,

Eddie

[s2]

Wiring the triodes in parallel only gives you about a 30% increase in gain over a single triode. You can get a little more out of it if you give each triode its own cathode resistor.

Phil is correct about the TMB. If you want to use a full TMB tone stack, you must cascade the triodes.

[Mr_dB]

Wiring the triodes in parallel only gives you about a 30% increase in gain over a single triode. You can get a little more out of it if you give each triode its own cathode resistor.

Well, technically, there is no gain increase in parallelled triodes, if you scale the plate and cathode resistors to account for the resulting change in rp. BUT, if you just parallel, say, a 12AX7 into a circuit without changing any of the component values, then it's the same as if you double the plate and cathode resistors for a single triode.

[markd]

Wiring the triodes in parallel only gives you about a 30% increase in gain over a single triode. You can get a little more out of it if you give each triode its own cathode resistor.

Steve,
Don't you mean give each triode it's own plate resistor?
markd

[JonFrum]

As a hacker, I'd love to see someone do the math on this one. Randall Aiken, where are you when we need you?

[s2]

Wiring the triodes in parallel only gives you about a 30% increase in gain over a single triode. You can get a little more out of it if you give each triode its own cathode resistor.

Steve,
Don't you mean give each triode it's own plate resistor?
markd

Ummm... Yes.

[markd]

As a hacker, I'd love to see someone do the math on this one. Randall Aiken, where are you when we need you?

How about a quick approximation from another hacker. Like Mr_Db mentions above putting two stages on a single 100K plate resistor is the same as doubling the plate resistor on a single stage. Now work the Voltage gain calculation for a 100K plate resistor and a 200K plate resistor. Not taking the following stages imput resistance into account(I'm lazy), I get a gain of 61·5 for a 100K plate and 76·2 for a 200K plate. That is the increase in gain when using a shared 100K plate resistor. Close to the 30% S2 mentioned above. If you have your 18watt Normal channel imputs wired to parallel the two gain stages, you can hear the increase in gain as you change from the low to high imput.

If you put a separate 100K (or even a 200K) plate resistor on each gain stage, you should see that gain approx. double.
If you have a JTM45 you can hear this effect. The first preamp gain stages on each channel of a JTM45 have a shared cathode resistor, but serarate plate resistors. Jumper the channels and compare the increase in volume when using one channel to using both channels. If it's not double, it's close.
hth, markd

[zaphod_phil]

Yes I had looked at that one, as well as this one, http://www.18watt.com/files/tmblite_pre.jpg, which I was thinking of using since I will have 2 12AX7's available for the TMB channel.

That one just allows someone to build a TMB without a Normal channel, and still use up the extra triode. As you will see this one also has cascaded triodes in the preamp to give the extra gain needed for the TMB controls, exactly as with the regular TMB preamp. The "TMB SuperLite" gives you a TMB 18W with the same tube count as a Lite II.

[JonFrum]

As a hacker, I'd love to see someone do the math on this one. Randall Aiken, where are you when we need you?

How about a quick approximation from another hacker.
hth, markd

I don't doubt the outcome, but I still don't understand what's going on at a conceptual level. I understand the effect of increasing the plate resistor in one triode stage. I think I understand that two triodes in parallel see a 100K resistance as 200K. That's where I break down. Are the two paralleled triodes acting as one triode with a 200K plate resistor? Excuse me if the answer is obvious.

[erl]

Thanks for all the help/input; the clouds are starting to clear. I have many ideas to try out now, should be fun!

Thanks again, you guys rock

Eddie

[markd]

I don't doubt the outcome, but I still don't understand what's going on at a conceptual level. I understand the effect of increasing the plate resistor in one triode stage. I think I understand that two triodes in parallel see a 100K resistance as 200K. That's where I break down. Are the two paralleled triodes acting as one triode with a 200K plate resistor? Excuse me if the answer is obvious.

I might be able to help you understand it in the simple way I understand it. Seeing concepts represented visually makes it easier for me to understand them. Maybe that would help you too.
Learning to read loadlines for common preamp gain stages will help anyone understand how changing the plate resistor value, or power supply voltage, affects the gain and distortion of that stage.

There is a good explaination of drawing and reading loadlines at the AX84 site.
http://195.178.239.50/ax84/media/ax84_m226.pdf

I have also put together a short explaination of drawing loadlines for 100K and 200K plate resistors.
http://45watt.18watt.com/gallery/loadline
It might help you understand what's happening with two gain stages connected to a shared 100K plate resistor. The explaination below won't make much sense unless you read this first.


It only takes 3mA of current to drop 300v across a 100K plate resistor. With two gain stages connected to a shared 100K plate resistor, they would draw only 1·5mA each to drop 300v. And for any point along the voltage swing they would be drawing only half the current that a single gain stage would draw with a 100K plate resistor. That means each gain stage would be effectivly operating as if it had a 200K plate resistor. That is what accounts for the increased gain when paralleling two stages with a shared plate resistor, IMO.
hth, markd

[ServiceTech]

Just thought I would add the valves' input capacitance would be doubled along with the total transconductance and the plate resistance would be halved if wired in parallel. That is plate-to-plate, grid-to-grid and cathode-to-cathode.

Regards,

[JonFrum]

That means each gain stage would be effectivly operating as if it had a 200K plate resistor. That is what accounts for the increased gain when paralleling two stages with a shared plate resistor, IMO.
hth, markd

Thanks for all the help.

Regarding the statement above, which gets to my confusion.... is each gain stage operating as if it had a 200K plate resistor - 2 gain stages, each with a 200K plate resistor, summed.... or are the two paralleled triodes with one 200K plate resistor acting as a single tube stage with a 200K plate resistor?

Or, what is the difference between

a. two triodes sharing a 100K plate resistor, and
b. a single triode with a 200K plate resistor, and
c. two triode stages, each with it's own 200K plate resistor, mixed after the plates?


Am I making my point clear? If not, I'll drop it.

[zaphod_phil]

... what is the difference between
a. two triodes sharing a 100K plate resistor, and
b. a single triode with a 200K plate resistor, and
c. two triode stages, each with it's own 200K plate resistor, mixed after the plates?

Assume a single stage with a 100k resistor has a "gain factor" of one
a. has a gain factor of around 1.3 to 1.5ish
b. has a gain factor of around 2
c. has a gain factopr of around 2, but allows you to mix to separate inputs or channels.

All of these are affected by the impedance of whatever comes after the gain stage in question. If it has a low impedance (eg a TMB tone stack) it will knock down the effective value of the load resistor, and slug the gain.

Edit: And there's the catch! For "c" it really depends on how you go about mixing the two outputs together. If they are fed into two isolated hi-impedance mixer inputs, then the 2X factor holds true. If not then each output will have a loading effect on the other and the overall gain will be reduced. At the other extreme, if both outputs are mixed by having their coupling caps joined, then in effect you have load impedance of 100k (two 200ks in parallel) and you're back to a scenario "a" situation. This is why we often use mixing resistirs to provide a crude form of isolation, and the gain factor will then be somewhere between 1.3 and 2.

[markd]

... what is the difference between
a. two triodes sharing a 100K plate resistor, and
b. a single triode with a 200K plate resistor, and
c. two triode stages, each with it's own 200K plate resistor, mixed after the plates?

Assume a single stage with a 100k resistor has a "gain factor" of one
a. has a gain factor of around 1.3 to 1.5ish
b. has a gain factor of around 2
c. has a gain factopr of around 2, but allows you to mix to separate inputs or channels.

All of these are affected by the impedance of whatever comes after the gain stage in question. If it has a low impedance (eg a TMB tone stack) it will knock down the effective value of the load resistor, and slug the gain.

Phil,
I hate to disagree with you, so if you think I'm wrong, be gentle. ha! Doing the Voltage gain calculation for a single stage with 100K resistor, and with a 200K resistor, shows the gain factor for choice b. is more in the 1.3 range.

For "c" it really depends on how you go about mixing the two outputs together. If they are fed into two isolated hi-impedance mixer inputs, then the 2X factor holds true. If not then each output will have a loading effect on the other and the overall gain will be reduced. At the other extreme, if both outputs are mixed by having their coupling caps joined, then in effect you have load impedance of 100k (two 200ks in parallel) and you're back to a scenario "a" situation. This is why we often use mixing resistors to provide a crude form of isolation, and the gain factor will then be somewhere between 1.3 and 2.

This is a really good point. No sense in using two plate resistors to get that last bit of gain without making sure it isn't loaded down by the following stage.

Jon,
The practical application of this info ,IMO, is that if you only have a single gain stage for the Normal channel(say in a TMB) use the 200K plate resistor. You will still get the voltage gain of the paralleled stages on a single 100K resistor. You won't get the doubled current and power out you get with the paralled stages, but the effect on tone isn't too drastic.
If two stages were available for the Normal channel(like the originals), I would want to try choice c., with the separate 200K plate resistors. They would have to be mixed properly, as Phil mentions above, but the extra voltage gain would be worth giving it a try, IMO.
markd

[zaphod_phil]

Doing the Voltage gain calculation for a single stage with 100K resistor, and with a 200K resistor, shows the gain factor for choice b. is more in the 1.3 range.

I freely confess I have very poor memory for figures and numbers. Concepts stay with me much better. Off the top of my head I seem to remember that in the gain equation, the gain is directly proportional to load impedance - assuming the preamp stage is feeding into an infinite impedance.

Now I'm going to have to go over to Randall Aiken's site to make sure I haven't mis-remembered the gain equation...

Edit: Hmmm, looking on Aiken's site, it appears the internal anode impedance also plays into this. So if it's value is much lower than the load resistor, increasing the value of the load resistor won't have a huge effect on gain. It's all a matter of proportion.

[JonFrum]

Thanks for the answers guys.