#1
Hopefully someone can help me with this:

I'm designing a high-voltage boost pedal that utilises a 12BH7 valve (similar to 12AU7). Naturally, the gain of either triode is just over 11 before clipping. With both triodes in series this multiplies to over 100.

With a not-too-ridiculous signal of 2vPk-Pk, the audio signal is amplified to about 200vPk-Pk at maximum gain and volume. Isn't this a bit ridiculous for a guitar pedal? Am I missing something?
Guitars & Gear:
Parker Nitefly M
Sumer Metal Driver
Ibanez RGD2120Z
AMT SS-11B
Two Notes Torpedo CAB
Last edited by Emperor's Child at Nov 12, 2013,
#2
You have to take into account your power supply. If you don't supply the tube with enough voltage to get to 200vP2P, it will just clip at the maximum voltage. If for some reason you supply it with enough voltage to get to 200V P2P, then yes, that is quite ridiculous.
#3
Like TJHague says, you'd need a high voltage supply to make that happen. I bet it'd sound like crap going into 99% of the amps out there though - if the frequency response built into the amp topology isn't expecting 200 Vp-p in, it's probably going to go into blocking distortion and "fart out" pretty quickly.
Quote by kcdakrt
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#4
Thanks for the replies so far guys. Thought I would add that I am simulating these schematics on Proteus VSM with a voltage of 250v.

On the basis that the 2x 12BH7s provide the desired amount of clipping in their current setup, is there a good way to reduce the gain whilst retaining the same amount of clipping? Or a way to reduce the gain after (other than a potential divider)?

TLDR, How do I reduce gain without reducing clipping?
Guitars & Gear:
Parker Nitefly M
Sumer Metal Driver
Ibanez RGD2120Z
AMT SS-11B
Two Notes Torpedo CAB
Last edited by Emperor's Child at Nov 13, 2013,
#5
Depends on how much gain you want to dump, but probably just a voltage divider. For something like this, I'd probably end up with a 500k resistor in series with a 50k pot (as the "send level") or maybe a 25k pot.

BUT REGARDLESS - for the love of all that's holy, run that through a buffer of some sort before it goes out. The output impedance of a normal tube gain stage is likely going to be too high to push a signal to the next box without tone suck. Since you have a HV source, a Supertex LND150 set up as a source follower is a really nice possibility. IRF820 as well.

Unless your last tube stage is a cathode follower. Then you're likely fine without.
Quote by kcdakrt
DLrocket89 makes my ug experience better!


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Kit Amp Building Tutorial
#6
Yes, I was already planning on using an IRF820 as a MOSFET follower. I was planning on putting the divider after the MOSFET follower but if I am adding a passive tone stack (bass, mids, treble) would it even be worth adding a second MOSFET follower after the tone stack or is this overkill?

Again, thanks for all the help!
Guitars & Gear:
Parker Nitefly M
Sumer Metal Driver
Ibanez RGD2120Z
AMT SS-11B
Two Notes Torpedo CAB
#7
Quote by Emperor's Child
Yes, I was already planning on using an IRF820 as a MOSFET follower. I was planning on putting the divider after the MOSFET follower but if I am adding a passive tone stack (bass, mids, treble) would it even be worth adding a second MOSFET follower after the tone stack or is this overkill?

Again, thanks for all the help!


I'd set it up like a "traditional" preamp topology:

last gain stage > tone stack > volume (the 500K resistor plus 50k pot I mentioned before) > IRF820 source follower > output

The plate resistance of the 12BH7A is 5300 ohms or so (holy smokes, that's low!), so the output impedance is even lower. It shouldn't lose a lot driving a tone stack by itself, so I don't think one between the last gain stage and the tone stack is needed.

Especially because what the follower is going to do in that circumstance is just make you lose less signal going through the tone stack..and you have too much signal, not too little!
Quote by kcdakrt
DLrocket89 makes my ug experience better!


Member of the official GB&C "Who to Listen to" list

Kit Amp Building Tutorial