18Watt.com

during my searching the internet , ive noticed that when a schematic only uses 1 preamp tube, there has been a couple of different ways its been drawn up, just curious which is the way to go?
1) just use half and leave other half unconnected
2) on a 12ax7 pins (2 and 7), (1 and 6) (3 and 8 ) are all connected
3) a 3rd wiring ive seen however with several components connecting the unused half but not connected to guitar signal these parts predominately connected to ground.

That generally depends on the circuit you're looking at, and the position within the circuit.

It's not arbitrary, different things are happening, and you also have to be aware of the direction of the signal path.

If you take a look in the downloads section of this site, you see the same thing. Some circuits use both halves or only 1 half of the preamp tube.

Most often on single-channel amps (on this site), only one PI input has signal passing through it (you can see the cap is earthed on one side). In this case, sometimes the unused input of the PI can be used for other signals to be passed through it (reverb, fx loops, etc...). Or on two channel amps, the two halves of the PI are used separately to establish the two channels.

Thanks,
Josh

PS. If you want to discuss what you're actually doing with the tube, you should specify the circuit and position. If it's a preamp tube, for example V1: how you use or connect the halves affects the amount of gain. If you use only one half, it's less gain than using both halves. How you use/connect both halves also affects the amount of gain.

here are the two different ones i came across

I'm not sure what circuits those are, but here's what it looks like is going on to me:

In the first image, they're only using half the tube for input. The unused half of the tube is being grounded to avoid issues. Single input.

The second image they're using both halves in parallel of the tube for input. This is a very popular method. It also has hi and lo inputs.

Here's another option: storage/18wattLite2b.gif
That version of the Lite 2b has 2 inputs for either single or parallel input. Yet another fun way to wire the input tube.

Thanks,
Josh

Eventually a valve with a dead side becomes less efficient I believe, so it is good practice to adopt the circuit in the first diagram or make use of it in some way.

I built the single/parallel combination for the two inputs into the normal channel of my original version. But I have to say I seldom use the parallel other than it has become habit to plug my Gretsch with its lower output pickups into that input. I should give it another chance.

Dd

thanks for the info !

Daviedawg wrote: ↑

Wed 01/23/19 3:24 am

Eventually a valve with a dead side becomes less efficient I believe, so it is good practice to adopt the circuit in the first diagram or make use of it in some way.

I built the single/parallel combination for the two inputs into the normal channel of my original version. But I have to say I seldom use the parallel other than it has become habit to plug my Gretsch with its lower output pickups into that input. I should give it another chance.

Dd

In single half use, most often I see people (for whatever reason) worrying about the valve wearing unevenly in case they want to reuse it in a different application. They don't generally want one half to be significantly worn more than the other. I think that's kinda silly personally, that's what the box of other tubes is for. ...but in cases where the unused half is still getting heated, it shouldn't be so uneven anyway.

The parallel input supposedly thickens up the tone compared to single input, I
never built the parallel/single input so I never A/B'd them. I'd love to hear your opinion about that if you A/B them.

Thanks,
Josh

Josh. I've done a recording. With no A/B switch I just paused the recording and moved inputs. It's been my view that the parallel is just louder with no benefit to the tone of the single input. I tried to do something which lets you hear what is going on.

But I can't seem to add an MP3 file which I think I have done before.


Dd

I enabled mp3s. It should work now. What kind of guitar/pickups? I'm guessing if it would be noticeable (besides gain), it would have be single coils or thinner sounding pickups...but voicing the tube might make it unnoticeable anyway.

PAF style humbuckers. If I get time today I will do another with my Shawbucker which might be interesting anyway. Also I will do it at a higher volume to see what that does to differentiate them.

Dd

At 12 seconds and 1:13 there's a mildly perceivable difference in tone, but that could just as easily be your fingers being more warmed up. I'm still curious about singe coil pickups.

I'll definitely do that when I have the house to myself as it should really be at a higher volume to see what that does to the two tones.

Dd

Here you go. Using middle pickup on Fender Strat Shawbucker (Fat51?) straight into the 18 watt. In the room I just detected louder, but listening to the recording which is with no great recording gear, there is a fatness to the second part which is of course the parallel input. See what you think. Vol is at 9 o'clock so just breaking up.

Dd

yeah i've listened to both tracks through good speakers and it's definately a fatter tone in parallel, i would say a touch more breakup as well in parallel,you can hear it with the single coil.
that's some neat fingerpicking Dd! can you play the banjo? we could have a duel!!

+1 I hear it too. 0:21 is not as thick as 0:45. And obviously it's pushed a little more too. Much more noticeable on the SC pickups, as suspected. Cool stuff, thanks for posting the comparisons!