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Bass Master and compressor
#1
I'd like to build a TUT3 Bass Master with compression.  Would the TUT2 fig 6-54 Simple Transformerless Compressor work for this application?  Following the tonestack ?
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#2
Hi Guys

The Bassmaster only has two gain stages in its preamp and provides a very clean tone. One way to get compression from the preamp is to simply reduce its voltage bt increasing the dropping resistor from the screen node of the supply.

Another way to achieve natural compression is to make the preamp three stages or even four stages. Yes, any of these methods might run into overdrive tones but that is easily controlled. The extra stages add veils of tube character - harmonic build-up - that will fatten the sound and make it a bit rounder as if it were deliberately compressed.

Having compression specifically, as on a dial say, the circuit from TUT2 Fig.6-54 will possibly work for you - depends exactly what you are after.

The general approach can be wrapped around the power amp and linked back to the output tube grids, further to the splitter, or to the splitter input using a jfet or LDR. Some of these methods are shown in TUT4 Figs. 4-28, 4-35, 4-38**, TUT6 &-16, 7-17. Note that ** works on the output stage as a variable current limit and is therefore dependent on how hard the output is driven. This can be made to track Power Scale settings if needed.
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#3
Thanks Kevin,
I reread those chapters for some insight. This will be a dedicated bass amp, built with my son. He uses a compressor pedal to even out the amplitude of his notes. I was wondering where in the signal chain this needs to be.
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#4
Hi sinusoids

I highly favour the multi-stage preamp and/or reduced preamp voltage approach rather than the use of a compression side-chain. However, if you are intent on doing the traditional compressor then the first attempt would be to wrap it around the preamp itself - both stages.

An alternative is to Power Scale the output. With Power Scale and Drive Compensation controls fitted, you can dial in the compression point of the output stage at whatever loudness you wish to play at.

Remember that just because you begin with a specific signal path, the number of ways to achieve that same sound is unlimited. This explicitly means that to attain a compressed tone does not require a compressor.
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#5
Hello Master Sinusoids

At the risk of seeming imprudent, your son will learn excellent finger technique if he is his own compressor. I believe we all start off with less than stellar equipment when we have no skill whatsoever and the challenge to sound like our idols drives us to develop proper attack and string control.

Cheerio
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#6
Very interesting....when using a reduced voltage approach what amount of voltage reduction is good range to try for getting more compression?
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#7
Hi Guys

For the voltage reduction required, think in terms of "a lot" Smile

Bassmaster has Va and Vs of 560V, with the splitter around 500V and the preamp in the 400s. The 22k to the preamp fan-out node would change to 100k or 150k to get the desired result... 220k or more if necessary since there are only four triodes. This gets the preamp down to 200V or less. As voltage is reduced, the current reduces and the change of R required for further change increases.

What you should do is build it as stock to know what the foundation tone is, then change that dropping resistor while the chassis is open on the bench and keep playing. Bass guitars have pretty high output so you might find it does not take too extreme of a voltage drop. Power 'off' for the resistor changes.

If you find that there is a compression point that works for your son and his bass and playing technique, but you also want to have the cleaner headroom at times, you could put the voltage drop on a switch, or for infinite control, Power Scale the preamp over a limited sweep.

To me the Dual-Volime preamp is a waste of tubes. Linking the inputs does not increase the signal strength that much and the cathode follower is a tone killer, as we know from TUT6. So, there is a whole bottle being used in a nonbeneficial way. Of course, you could use that bottle for a half-wave rectifier and gain to implement a compression side-chain although I would still go with extra gain stages instead.

Have fun
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