AC30 standalone preamp build complete + question

[fossilshark]

Hi all, I finished building the standalone preamp based on the vox AC30 with top boost, I have dubbed it the TB420. It honestly sounds so great im really happy with how it came out besides this minor flaw:

With the master volume below halfway it sounds amazing, tons of headroom and tone options with the top boost, when the master volume goes past halfway however the output becomes really distorted in a flubby unpleasant way.

I threw a 470k resistor in right before the master volume and that made the sound really really nice but the output level is very low, really have to crank the power amp to get it loud.

Is my only option to just mess around with that resistor to limit the level going to the output section?

Any suggestions on this build are appreciated!

[separateness]

I could be mistaken but that output stage is a departure from the source material, I believe? It seems that you are overdriving it thoroughly, or overdriving the input (phase inverter?) of the power amp unpleasantly or both. This would probably sound pretty good in an amp that was voiced for it. I understand you probably want to buffer the output, but if you are going to keep that output stage I would advise setting the common cathode stage up for clean, linear gain and then attenuating the output of the cathode follower to the level the receiving PI is wont to see (if necessary). Also if you want that output CF to work best as a regular old transparent buffer I think I would advise making it AC coupled.

[kagliostro]

Ciao fossilshark

Instead of adding a 470K before the Master Volume, try to swap the two 220K mixing resistors for two higher value resistors

To have the max control on this try, use a pair of 500K pot connected as rheostat in place of the 220K resistors, rotating the shaft if you find a good spot measure the value of the rheostats and put on the circuit two resistors with a similar value

Franco

[pdf64]

How about trying a CF after the master volume, with no potential divider on its output. Why’s the common cathode gain stage needed?

[fossilshark]

The distortion is question is not pleasant at all, this is the same output stage I used on my Wendigo design from previous posts (stolen off the soldano x88r schematic) wich sounds very nice and not at all like the distortion I am describing. With the signal going to the output stage attenuated and the top boost gain cranked it actually has some insanely nice crunch to it.

As for modding the output stage, I don't really know where to start with redesigning it. I've been reading valvewizard alot lately but my theory knowledge is hindering me alot in situations like this.

I am going to try kagliostros suggestion and see if i can find a sweet spot, but if it still doesn't output enough level for me then I will have to really hit the books.

Any suggestions on books or something I should read on designing gain stages from scratch?

[separateness]

That nice crunch is probably because you are attenuating heavily everything except for the upper range, which is what amps voiced for high gain tend to do (usually through judiciously chosen coupling and cathode bypass caps). I think pdf64's suggestion of just having an (needless to say AC coupled) cathode follower, unattenuated after the MV is a very good experiment.  If it turns out you need more gain then you can worry about designing a clean common cathode stage to put between the MV and the CF later. I kind of doubt you will, though.

If you know how to draw loadlines, what they mean etc, the Universal Loadline Calculator is very, very useful.  It coupled with Valvewizards page and his excel loadline plotter (in his downloads area) are quite useful. Design and Construction of Tube Guitar Amplifiers was the first book I got on the subject and it was very informative for me.  Valve Wizard's Preamp book is a treasure trove as well. I don't really think you will need more gain than what you have but it is fun to know how anyway.

[shooter]

to actually "understand" what's going on you need more senses than ears.  A scope is the best bang per dollar.
2 examples of what I seen, what changes I made to fix the "ugly" distortion.

[pdf64]

The distortion is question is not pleasant at all, this is the same output stage I used on my Wendigo design from previous posts (stolen off the soldano x88r schematic) wich sounds very nice and not at all like the distortion I am describing…

Audio clips would be really helpful in communicating what you’re trying to describe.
Links to the wendigo and x88r schematics would be helpful too.

[kagliostro]

About the previous try (to swap the mixing resistors with other higher value) I forgot to say that may be the good spot is reached with a different value for each channel, not necessarily the value is to be the same on both resistors

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One other option I'll try is to use this configuration on your CF, it also adds an output level control to adapt the signal level to different Power Sections (or effect units)



(always borrowed from Mr. Merlin)

Franco