odd circuit ideas

[Mystic]

Any one tried any non standard circuits? I was wondering what running double preamp tubes in parallel would sound like. (2 V1's) (2 V2's) I suppose V3 has to be only one tube. Or is this just a dumb idea that can't work. My knowledge about guitar amps is very limited.

[MWJB]

Do you mean like running 2 channels in parallel? You can do this with a Tweed Bassman style amp, or Tolex Fender that doesn't have reverb, by running a patch cord from Normal 2 to Bright 1 & plugging your guitar into Normal 1.

If you are just talking about wiring a single tube's 2 triodes in parallel, then this has certainly been done many times with respect to V1. It is usual to halve the plate resistor & cathode resistor values that you would have for a single triode, to retain a consistent tone.

Large scale manufacturers tend to steer away from such designs because of the higher parts count. But you can be assured that nearly anything you can think of has been tried at one time or another.

[Enzo]

To agree, there are not many circuits that have not been tried. And standard circuits are standard for a reason - they work well and reliably.

But that is not to steer you away from experimenting. After all, even if someone has already done something, that doesn't mean you ever heard it. And if you come up with a design you really like, does it matter if Brand X did the same thing in 1962? Just don't be surprised when someone points it out.

Remember that vacuum tubes was a mature technology 70 years ago. They were making radars and things with it 65 years ago. Books about tubes written in the 1930s are still perfectly valid today.

[GibsonLover]

For a good helping of the parallel triode plan, check out the Matchless Chieftain schematic. It sounds devine, and there are several different techniques in that amp compare to the standard guitar amp models.

[rathmann]

Triode gain stages in parallel are additive as opposed to the multiplicative relationship of series stages, hence two triodes in parallel will yield twice the gain of one. Whereas cascaded stages yield gains in multiples of 10s and 100s. You will also get a lower output impedance and lower noise floor.

Paralleled triodes are cool, however the net result is rather subtle compared to other things you could do with that extra stage like making it an oscillator for a tremolo, a buffer for a line out/effects send, or cascading it to get overdrive. If you're talking about cascading two parallel pairs, or paralleling two cascaded pairs, well, same as above but more so. Note that most high gain preamps utilize three to four stages of gain in series (dumping quite a bit along to way, btw)

I've done parallel/series switching in a number of builds now. I've even set them up with different biasing and bypass caps and used a pot to be able to adjust the blend of them. The effect is noticeable but really less complex/interesting than you might hope. I typically set the mix dead center. The more effective experiment was to have the switch between the 2nd and 3rd stages of a four stage preamp. The effect is something like a "vintage/modern" distortion mode. Would it sound much different if the switch just bypassed one of the stages? Probably not. One thing I've thought of trying at some point would be paralleled triodes each with a local negative fedback loop. One would have an active treble control, rolling highs out of the NFB (a la "Presence" in most power amps) and the other a similarly configured bass control.

My overall feeling on parallel triodes is that if your design winds up with a "spare" stage and you really don't want a ton of extra gain go ahead and parallel it with something. There aren't really any negative performance consequences, and there are some slight advantages. I just wouldn't expect it to be an untapped secret to magic tone.

As already mentioned - just about everything has been done. The standards are such because they work well, but also sell well - the sweet spot of good sound at reasonable cost to manufacture. Check out Ken Gilbert's BAGA (big ass guitar amp) or Fred Nachbaur's Dogzilla for some interesting ideas of things to do with tubes in a guitar amp without consideration for mass production.

[TheTinMan]

IIRC the "Blues Preamp" on AX84.com has a later gain stage with parallel triodes - using LEDs in place of cathode resistors.

Also, tubenit here has several designs using parallel triodes for the first gain stage. See Hoffman Amps forum and search for "CBS" or "Carolina Blues Special" for starters.

Hope this helps,

Chip

[mooreamps]

IIRC the "Blues Preamp" on AX84.com has a later gain stage with parallel triodes - using LEDs in place of cathode resistors.



Chip

Yes, they sort of get the jest of it on that one.

-g

[Chuck H]

As "non standard" circuits go, there are a lot of circuits that aren't (or weren't) commonly applied to guitar amplifiers. Some for good reason, but others are valid. For example, I really like using a summing circuit that is commonly called a "virtual earth" mixer. It was used in a ton of old colsole systems and other tube gear that required multiple channels. It works very well for guitar amps IME, but it's never been used in any guitar amp (save for home brews) that I know of. There are some diode circuits circulating that allow for bias manipulation and avoiding grid loading that have become useful. These are fairly new but catching on quickly. Power scaling is a new idea (reletively speaking) that is going balls out.

Hmmm... Non standard?.. Look to pre 1965 hi fi and other home audio product schematics. There are a lot of non standard (as guitar amps go) and useful circuits for guitar amps in that era of home audio.

Parallel triodes are not as non standard as you might think. Of course, you said parallel tubes. So that would mean either quadrupled triodes or parallel triodes for both the first and second gain stages. Parallel triodes do have some advantages, but they are really negligable for guitar amps. The difference can be wonderful in a careful design, especially if you stagger each triodes characteristics, but more often the difference is so small that you may wonder if your imagining it.

One of the reasons certain circuits are implemented most commonly is because they work so well. As was mentioned, tube technology was a bigger deal than transistors once upon a time and has enjoyed a lot of attention. There are only so many "notions" about how to use an amplifier (tube, triode stage etc.) and almost anything you can imagine has been tried. We keep the good stuff and don't bother with the rest because it's not as good. It often turns out that there is a good reason a circuit is "non standard".

All in all tubes are basically simple devices (though non basically very complex). What would you do to improve a BLT sandwich for example. I had one with avacodo once that was pretty good, But I still order the regular sandwich more often than not. The challenge to any artist (creator/designer, etc.) is to create something new without creating something strange and worthless. Tube technology is very mature, So...

The point is, use circuits that will serve you best and deliver on tone. Don't go in for "non standard" circuits just because they're "non standard". You may as well go out and get a MetaSonix amp if that's your game

Chuck