Clean tube amp design

[FREYES_7]

Hello All,
What aspects should I consider when designing a clean tube amp? Would KT88s help in that matter?

I know reverb deluxe is a great starting point, any other topologies suggestions will be appreciated!

Thanks in advance!

[stratomaster]

Check out the Two Rock John Mayer signature.  One of the hallmarks of that design is a 68k/1M voltage divider -- which is basically a preamp volume control just barely cracked enough to pass signal. 

Another is the Dr Z Stang Ray.  Very different approach with a single pentode.

Be prepared to cut gain via strategic voltage dividers. A lot of the voltage dividers in a Deluxe Reverb are hidden (see: mix stage at PI input, reverb mixing stage, and Trem intensity control) so they aren't as overt as the Two Rock.

None of these examples have KT88s.  Any relatively clean power amp section can power these clean preamp designs. Limit gain into the PI and you'll end up with a clean amp, provided you aren't intentionally overdriving a preamp stage.

[shooter]

what he said^^^^


any amp that keeps the signal un-distorted, Class A, is by definition a clean amp.


most of my builds are just that, I spend lots of quality scope/seg-gen time dialing each stage to minimize signal distortion


most guitarist prefer the distortion, which adds "spice" to their tone/sound, I find all the "spice" can be added pre amp with pedals and talent.

[FREYES_7]

I see, thanks stratomaster and shooter!

My first build is 5F1 like, it start overdriving at 10 using humbuckers in hi input,

I'm thinking on following that line, clean single output amps, while I experiment with a 5E3 for class AB amps.

Would that mean that just by using smaller gaun preamp tubes I would get cleaner tones from the amp?

[tubeswell]

Yes - you surmised correctly - it depends on the biasing of the output tube and the gain of the preamp. As a rule-of-thumb, it only takes 2 x cascaded 12AX7 triodes to push an EL84 grid into clipping - as these are typically biased between 10-15V. A 6V6 grid will eventually also into clipping (a bit later than an EL84) - being typically biased at around 20V.  But 2 cascaded 12AX7 triodes it will not be sufficient to drive a 6L6/KT66 (biased around 30V) - let alone a bigger tube like a KT88/6550 (50V bias) - into clipping.

[HotBluePlates]

What aspects should I consider when designing a clean tube amp? ...

Start at the speaker/output-section, and design working back towards the input.

Bias of the output-section tells the drive signal required for max clean power output (any more just distorts the output-section).

Preamp + phase inverter gain = output-section bias - Peak Pickup Output.
Add a little extra gain for Volume & Tone controls that will make stuff more convenient.


One of the cleanest amps out there is a Standel 25L15.
Its preamp is 12AX7 --> 12AT7 --> 12AU7.  Each stage is less-gain & more able to handle a large signal-input than the stage before.  That means all throughout the preamp, the signal is unlikely to over-drive the stage that follows.

    A triode can swing an Output Signal that is maybe 2/3 of supply volts.
    We can understand the Input Signal size a triode can handle as Output Signal / Gain.
    Say we have 300v supply, so at best 200v Output Signal.

    12AX7 Gain =~ 60 ---> 200v Output Signal / 60 ---> 3.33v Input Signal max before distortion
    12AT7 Gain =~ 34 ---> 200v Output Signal / 34 ---> 5.88v Input Signal max before distortion
    12AU7 Gain =~ 14 ---> 200v Output Signal / 60 ---> 14.29v Input Signal max before distortion

    The lower-gain triode is therefore able to accept a larger input signal, apply a lesser Gain, and avoid bumping into some limit of operation (which would cause distortion).

Eventually, you will need to use load-lines to inspect how the tube is biased.  If you bias any tube near Zero plate current, or near Zero volts grid-to-cathode, the tube will distort after only a small signal is applied.  Biasing the tube halfway between these extremes will allow the tube to accept the largest input signal before distortion.


The 25L15's tone circuit also uses negative feedback for the frequency-control, so anything less than full-up is using some negative feedback which cleans up distortion.

[FREYES_7]

Yes - you surmised correctly - it depends on the biasing of the output tube and the gain of the preamp. As a rule-of-thumb, it only takes 2 x cascaded 12AX7 triodes to push an EL84 grid into clipping - as these are typically biased between 10-15V. A 6V6 grid will eventually also into clipping (a bit later than an EL84) - being typically biased at around 20V.  But 2 cascaded 12AX7 triodes it will not be sufficient to drive a 6L6/KT66 (biased around 30V) - let alone a bigger tube like a KT88/6550 (50V bias) - into clipping.

Nice! thanks for the knowledge! I will test with less gainy preamp tubes, would this make the amp sound quieter? or just cleaner at the same volume?

[FREYES_7]

What aspects should I consider when designing a clean tube amp? ...

Start at the speaker/output-section, and design working back towards the input.

Bias of the output-section tells the drive signal required for max clean power output (any more just distorts the output-section).

Preamp + phase inverter gain = output-section bias - Peak Pickup Output.
Add a little extra gain for Volume & Tone controls that will make stuff more convenient.


One of the cleanest amps out there is a Standel 25L15.
Its preamp is 12AX7 --> 12AT7 --> 12AU7.  Each stage is less-gain & more able to handle a large signal-input than the stage before.  That means all throughout the preamp, the signal is unlikely to over-drive the stage that follows.

    A triode can swing an Output Signal that is maybe 2/3 of supply volts.
    We can understand the Input Signal size a triode can handle as Output Signal / Gain.
    Say we have 300v supply, so at best 200v Output Signal.

    12AX7 Gain =~ 60 ---> 200v Output Signal / 60 ---> 3.33v Input Signal max before distortion
    12AT7 Gain =~ 34 ---> 200v Output Signal / 34 ---> 5.88v Input Signal max before distortion
    12AU7 Gain =~ 14 ---> 200v Output Signal / 60 ---> 14.29v Input Signal max before distortion

    The lower-gain triode is therefore able to accept a larger input signal, apply a lesser Gain, and avoid bumping into some limit of operation (which would cause distortion).

Eventually, you will need to use load-lines to inspect how the tube is biased.  If you bias any tube near Zero plate current, or near Zero volts grid-to-cathode, the tube will distort after only a small signal is applied.  Biasing the tube halfway between these extremes will allow the tube to accept the largest input signal before distortion.


The 25L15's tone circuit also uses negative feedback for the frequency-control, so anything less than full-up is using some negative feedback which cleans up distortion.

Nicee, thanks for the clear explanation! Sure biasing in the middle is key.  Is the Standel 25L15 using both triodes in each tube? It would hurt my wallet if it weren't haha but maybe I could use them to have a doble channel clean amp, with different tone stacks

[Merlin]

Any amp is clean if you turn down the gain far enough that none of the stages clip. That's basically all there is to it. Less gain = cleaner. A clean 100W amp is gonna be louder than a clean 5W amp, but loudness is a separate issue.

My first build is 5F1 like, it start overdriving at 10 using humbuckers in hi input,
Would that mean that just by using smaller gain preamp tubes I would get cleaner tones from the amp?

Or just plug your guitar into the lo input...

[labb]

just for info: the last amp i built I took an SEL Pre Amp schematic from the old AX84 site and married it to the power section of Ken Fisher's Trainwreck Express power section using EL34's tubes. Amp turned out to be really clean untill it was really cranked.. This from the guy playing it.

[HotBluePlates]

...  Is the Standel 25L15 using both triodes in each tube? It would hurt my wallet if it weren't haha but maybe I could use them to have a doble channel clean amp, with different tone stacks

All of the 12AT7 is used, all of the 12AU7 is used.

A channel only uses 1 side of the 12AX7, but there are actually 2 channels (the 2nd channel is fed from a hidden jack & uses a hidden Volume control).

When I built one, I used a 7-pin socket instead of the 12AX7 that could take a 6AV6 or 6AT6.  I didn't think a 2nd channel with an extra Volume (but shared tone controls) would ever get used.