Boob Tube V1.5 output impedance

[Iknownothing]

Hi, I am new to the forum. Thank you so much for this amazing resource. I want to build a pedalboard amp using the LM3886 as the power amp and the boob tube as the preamp. The input impedance from the power amp schematic is 22k. I was just wondering what the output impedance is on a boob tube V1.5? If this wouldn’t work, are there any other suggestions someone might have?   Thank you so much!

[Neil mcNasty]

Cheers and Welcome!

I think the impedance is pretty low (around 1.2k seems normal for tube circuits, but somone please correct me if I’m wrong), so you should be good. I use the a BoobTube style circuit as my no.1 go to adapter for getting guitars up to Line Level, either it be similar to your case with a power amp, or going direct in to a mixer/recording interface. (I’ve pumped the plate voltage up to around 44 volts)

Have you tried using them together?
If No: Why not?
If yes: Does it sound good?
If it sounds good, then you are good… Anything that sounds good, is good, no matter if it is wrong or not.
Don’t forget that the majority of guitar circuits are regular circuits done wrong! The first Fuzz was a faulty mixer channel…. Guitar pedals is all about doing things wrong in order to see what happens.

Impedance mismatch is something we often mess with in order to get a desired effect, as it can not damage anything. It only affects the carracter of the sound.
Very few actually cares about it unless they notice a problem (loss of highs or low end depending upon which direction you go) and mainly become relevant in the studio; you exploit impedance in order to get different character, frequency response and variations out of our preamps and microphones.
Low impedance goes in to high impedance, is pretty much the main rule of impedance. Preferably to a ratio of 1:10 as a default/rule of thumb.

We put synths and keyboards through guitar pedals all the time. That is a often a pretty large mismatch, but it still sounds awesome. We put guitars through line and phono preamps (AMZ-Mini Booster/ZVex SHO) and we play through small modified transistor radios, we go directly in to a Tascam/Fostex mixer with the gain cranked to make fuzz, and so on… pretty much everything is a result of doing it wrong!

Which makes Rule no.1: Never ask permission to do anything!
Just do it and tell us if you stumble upon something cool…
That is how new and exciting pedals/amps are made!

Cheers!
Hope this inspires you to go nuts! 😎

[Neil mcNasty]

Also while I'm at it:
The Tone Control of the Boob Tube is pretty much pointless, as your main issue will not be too much highs, but rather too much low end (flabby bass response) at lower voltages (hence my increased plate voltage, as I like it sparkly, and not too overdriven)
So instead of a Tone pot, I would rather suggest that you add a passive tonestack from one of your favorite amps. That way you will get a lot more variety out of your amp, as just going straight in; can result in a too flat response with too much Low Mids, compared to what we normally get from guitar amps.
A Tone Stack also brings down the Volume a bit, so that you do not slam the front end of the power amp too hard (gives better gain range), although you still get great high gain sound from doing that too, as there is plenty of volume on tap...
You can add it with a bypass switch as well to have both options. This will create a jump up in level, therefore acting as a boost.

I have also reduced the input cap to 22nf on my build to reduce excess lows, which makes it more focused.

Personally i often use the Boob Tube as a preamp to blast the front-end of small transistor radios that I modify with a Jack Input, going directly into the Amp section. A cool studio trick I like to use that sounds huge!

I also built the Tiny Giant Amp sporting a Celestion Vintage 10 speaker at 8-Ohm (about 12watt output)
It sounds amazing when pushed hard with a Boob Tube. Very Marshallesque with more lower Mids.
Mine sounds best overdriven like this with the Tone Stack bypassed, as it creates a lot of extra higher harmonics/distortion that balance out with the extra Low Mids (Flat EQ)