Clean tube amp with compression

[Leo_Gnardo]

Just to reiterate as in my original post. I am AVOIDING audible overdrive/distortion. I am attempting to make a squishy CLEAN amp.

What you're looking for conforms more to bass amp design. Low gain preamp, lots of power. Take a look at Fender's white Bassman @ 1962, with a cathode follower instead of typical 2nd stage preamp, unique amongst Fender's designs. You may find leaving out cathode bypass caps in otherwise normal preamp stages helps limit gain, and you may need to use DC on preamp tube filaments to do away with filament-induced hum that cathode bypasses tend to eliminate. Choosing lower gain pre tubes such as 5751, 12AY7, 12AV7 may help. For more compression effect, try self-biasing output tube design rather than fixed bias. For instance I outfitted my 60's Vox AC100 with separate resistor/cap on each output tube's cathode. A bit radical perhaps but it tamed the amp.

Without a doubt, designing for clean & noise-free operation is a challenge. My clean-jazz customers have proved that!

[echuta13]

I'm thinking something like a Standel design would get you into the realm you are looking for.

Here a sample of one: https://app.box.com/s/d5tx0qg71jqw2k7icxaq

[NateS]

You could try to augment screen sag with a rectified and filtered signal driving a mosfet tugging screen grids down.

[bob p]

Did SteveA ever release those schematics to be posted on sites other than the Blue Guitar? Or are his pirated build schematics now being pirated themselves?

Just wondering. Now Howard is going to be really mad -- his amp schematics are everywhere now.

[lowell]

I want to exploit that clean squish to the fullest. I dig the idea of the MOSFET on the screens. Good idea. And I do believe the transient pick attack will be the main hurdle so I may very well need to implement a compressor at the input.

[Pedro Vecino]

And what exactly do you mean by the degree of powertubes ?

Alf

All Russian 12AX7 compressed similarly. WA / WC / WB / EH / WXT / LPS / Tungsol / Mullard reissue. The WB has a lack of extension in the upper zone that moves away from the others. Engl has used for many years as part of their amps for this purpose (first stage and driver). The LPS is the most open/linear of all.
Typical Chinese 12AX7 compresses less than any Russian, and also JJ.
The degree of power tubes is set by the relative plate current on the same circuit. If you take 100 EL34 and installed on the same circuit, current flow will different between them. In a range of 25 to 60 mA, for example. All have basically the same power capacity but low intensity units compressed more.
In Groove tubes you can choose the level between 1 and 10, and in TAD or Ruby Tubes, the PC (relative plate current) in a range of approximately 30mA. The margins vary between different models (manufacturing). For example, EL84JJ not have a limit as low as some Sovtek/EH units reach.

[Alf]

Thanks Pedro for the clarification , I already had an idea that that was what you meant but wasn't quite sure.

Alf

[Chuck H]

I'm thinking something that might be worth considering here. If you're after clean you'll want the widest clean range possible. To me that indicates a master volume type amp so that once you have your preamp jiving the way you want you can then control the volume. To keep that tone consistent at any volume you'll want ample power to stay clean at higher outputs. That means a burly power amp with good dynamics. You can't do that if you're fudging with the screens to make them sag. Further, if the preamp is designed to null dynamics the power amp would seldom be pushed into a demand situation that will induce sag anyway. So I think making the preamp compressed AND making the power amp prone to compression becomes moot and counter productive. Why not build a compressor into the preamp and set up the power amp to accurately represent that tone at a greater range of outputs? If the compressor is made adjustable you'll then have an amp that will do clean and compressed OR clean and tight depending on the compression setting. A do-all clean amp that can make a range of tones in a range of volumes for any situation or venue. This just makes sense to me. If the amp is designed with only SOME compression in the preamp and SOME compression in the power amp it will suffer from "sweet spots" in the volume settings where it sounds best and lack versatility at any volume. That seems like a shot in the foot for a clean only amp.

[bob p]

Expanding on Chuck's idea a bit: in a studio application compressors like the infamous Meek designs are line level signal processing devices. By keeping your focus on line level signals the task of building something useable into the signal processing chain of your amp becomes much easier. I'm inclined to agree that involving the power amp in the circuit just complicates the design.

BTW, there's a really good article on Wikipedia about the various ways to compress dynamic range. If you haven't seen it already, then it might be worth looking at:

Dynamic range compression - Wikipedia, the free encyclopedia