Configuring parallel triode 12AX7

[alerich]

I would like to configure the unused triode of V1 in my amp for switchable parallel triode use. The amp has only one input jack. I do not wish to drill a hole for a second input jack for switching purposes. I can fashion a bracket on the rear of the chassis to hold a switch to perform this function. The existing triode of V1 has a 1500 ohm cathode resistor with a bypass cap. I would also like not to unsolder or otherwise disturb that.

Can I simply hard wire the plate of the new triode to the plate of the current triode, hard wire the cathode of the new triode to ground with its own dedicated cathode resistor and bypass cap of the same values as the current triode and then use a SPDT switch to connect the grid of the new triode to the existing grid or ground it when not in use?

[kagliostro]

Can you show the schematic or which is the value for the plate of V1 ?

K

p.s.:

In the attached images you can see the way Vox do it in the AC15HTHV1 Handwired and in the AC30CC2

[alerich]

I would assume the plate load resistor for V1 is 100K. I don't have a schematic drawn up for this amp (someone else's home brew amp) and don't have the amp opened up at the moment. I have been doing more reading this morning about parallel triodes. It seems there are not many hard an fast rules about the accompanying components. It's more of a "season to taste" type of thing. Off to do more reading. Thank you for the post!

[sluckey]

Can I simply hard wire the plate of the new triode to the plate of the current triode, hard wire the cathode of the new triode to ground with its own dedicated cathode resistor and bypass cap of the same values as the current triode and then use a SPDT switch to connect the grid of the new triode to the existing grid or ground it when not in use?

That'll work, but I'd change it slightly. I would also jumper the two cathodes together and put a 1500Ω on the new cathode. Both cathodes will use the same bypass cap and the total resistance will be 750Ω.

I would also consider replacing a volume control with a combo pot/pull switch to keep a very clean look.

Look at this for reference...
http://www.el34world.com/charts/Schematics/files/marshall/marshall_18watt_schem.pdf

[kagliostro]

Hi Sluckey

Arranged that way if the jack is inserted to the Hi (J2) input V1a and V1b are paralleled

Nice trick !

K

p.s.: Is that circuit deliberately missing the 68k resistors ?

[sluckey]

p.s.: Is that circuit deliberately missing the 68k resistors ?

I think so. I believe Pedal Monkey was trying to stay close to the original Marshall which had no grid stoppers on the normal channel. See pic...

[alerich]

I would also consider replacing a volume control with a combo pot/pull switch to keep a very clean look.

That's a good idea. I'll cobble it together with a switch to test drive it. If it's a keeper the switched pot is a go.

[sluckey]

I'll cobble it together with a switch to test drive it. If it's a keeper the switched pot is a go.

I'll bet you like it so much that you won't even need a switch.

[jojokeo]

One of my amps is always either in dual triode mode or when switched they are cascaded. Mucho mahor / much better this way. In this amp cascading doesn't give the gainy distortion you would think rather a signal increase but with more carity & strength to it that really kicks the amp up a couple of notches which increases touch sensitivity also. A very nice option to have and blows away a single to dual/paralleled triode option. Check it out alerich I highly suggest it. Oh, and you have to keep the triodes seperately biased when you do this...no biggie.

[kagliostro]

Sluckey

Thanks for posting the original schematic

Jojokeo

something like Merlin's proposal ?

K

[alerich]

I'm really not trying so much to gain up an anemic preamp as I am just trying to explore some additional tonal options. This is an 18 watt amp built on a Hammond AO-35 chassis. I really love the current tone. Lots of chime and sparkle and all of those other intangible adjectives. I was intrigued by another thread where parallel triodes were being discussed. I have a spare triode so I started (over)thinking about it.

One objective is to preserve the current tone I have untouched. If I have to compromise the current tone I love to add any additional options I'll pass and leave the second triode idle. This is why I am trying to wrap my head around parallel triodes from a theoretical standpoint. In theory, I've read that when the triodes are paralleled the cathode and plate load resistors are halved and any cathode bypass cap is doubled to preserve the same frequency response across the pair that existed with the single triode. One question I have is with this wiring scheme would the original triode operate as it did by itself with the second triode's grid simply switched to ground? Or do you only halve the cathode resistor and double the bypass cap while leaving the plate load resistor the same? I've seen that configuration, as well. Using the smaller resistor values is supposed to be the reason that parallel triodes have lower noise.

Several good suggestions for using that idle triode and I appreciate them. I don't currently have a schematic for this amp so tomorrow I'm going to open it up and at least map out the current front end and clip in a few jumpers and see where that leads me. Thanks for all the input and responses.

[jojokeo]

Sluckey

Thanks for posting the original schematic

Jojokeo

something like Merlin's proposal ?

K

You could do that or here's what I did. This gives a better response though.

[jojokeo]

1) In theory, I've read that when the triodes are paralleled the cathode and plate load resistors are halved and any cathode bypass cap is doubled to preserve the same frequency response across the pair that existed with the single triode.

2 )One question I have is with this wiring scheme would the original triode operate as it did by itself with the second triode's grid simply switched to ground

3) Or do you only halve the cathode resistor and double the bypass cap while leaving the plate load resistor the same?

4) Using the smaller resistor values is supposed to be the reason that parallel triodes have lower noise.

5) I don't currently have a schematic for this amp so tomorrow...

Answers:
1) YES
2) Yes, only with the second triode's grid switched to ground but not when it's in parallel - you need to do as stated in question 1 above for that.
3) No - also halve the load resistor too for same response as one triode only
4) No - just having smaller resisistor values is not the reason
5) Here's a schematic also w/ basic mods from someone trying to make a "sort of" 18watt out of an A0-35 Hammond chassis

[SILVERGUN]

I would like to configure the unused triode of V1 in my amp for switchable parallel triode use. The amp has only one input jack. I do not wish to drill a hole for a second input jack for switching purposes. I can fashion a bracket on the rear of the chassis to hold a switch to perform this function. The existing triode of V1 has a 1500 ohm cathode resistor with a bypass cap. I would also like not to unsolder or otherwise disturb that.

Can I simply hard wire the plate of the new triode to the plate of the current triode, hard wire the cathode of the new triode to ground with its own dedicated cathode resistor and bypass cap of the same values as the current triode and then use a SPDT switch to connect the grid of the new triode to the existing grid or ground it when not in use?

Great thread,,,,I was looking for more info. on this subject and here it is...just like that.....way to expand on a theme alerich

I'll try to keep good notes on my experience with this type mod,,,, and report back soon

[SILVERGUN]

I paralled my third gain stage, with the unused triode in V2, and created a fourth gain stage, but because I already have a ton of gain, I can't really descibe the effect....
It DIDN'T create any more noise, and did provide some boost/gain, but I'm gonna have to tame my front end and install a PPIMV to give a better assesment...
Be back soon......anyone know where I can get a nice set of new ears???

[kagliostro]

I paralled my third gain stage, with the unused triode in V2, and created a fourth gain stage

Paralleling a tube you don't have one more gain stage, so I'm not able to figure what really you mean

please can you explain ?

K

[SILVERGUN]

I paralled my third gain stage, with the unused triode in V2, and created a fourth gain stage

Paralleling a tube you don't have one more gain stage, so I'm not able to figure what really you mean

It was my understanding from what I read that it would add about 30% more gain, without any additional noise, and I pictured that as being another gain stage.....my terminology might be incorrect.......please help me to correct my thinking....thanks K

[Willabe]

Ia a gain stage has, lets say a gain of 10 and you add a second gain stage of 10 you get 10 x 10 = 100.

If you have a gain stage of 10 and parallel another triode with it you get 10 + 3 (30%) = 13.


                          Brad       

[kagliostro]

Ach, so

You mean you have increased the gain of one stage, not added one other gain stage

Now I've understand you

K

[SILVERGUN]

It worked OK for me but didn't really give me the gain BOOST I was looking for so I turned that 1/2 triode into a cascaded gain stage,,,and that's much more along the lines of what I was trying to accomplish...

So,,,, I fold