Tube Amp Archive

Have you looked at how much gain / signal level gets dropped when passing thru a passive tone stack when even just adjusted to normal settings? Yes but not really sure how this links in with how many stages are distorting. In an SLO the tonestack is at the end of the preamp, once all the distortion...

I've started going down the rabbit hole of developing overdrive circuits for pedals, in particular pedals that emulate valve amps. Something I've noticed looking at other designs is they don't always match the same number of distorting stages as the amp but still do a good job emulating them. Why is...

What attenuator are you using? And how much are you attenuating? And is the amp built to the spec in the Express files? What transformers did you use? I run mine through a 2x12 G12M cabinet and have no trouble running everything at 12 o'clock with the bright switch in the 500pf position. Used to use...

All sorted. Made the bias hotter so it’s bang on 100% and used a smaller preamp coupling cap value.

Thanks Steve, it's a great site! Lots of useful information that I was missing these last few weeks when doing another build.

I’ve built a hybrid amp with an 18w Lite2b preamp going into a Vox AC15 power amp. If I run the tone and volume on 10 I find the sounds gets quite congested and not good for rhythm. If I back off the tone a bit it starts to clear up. Is this normal on this preamp?

What is the bias of the output valves? And what is their plate and screen voltages? When you tested for output did you increase the input signal until just before clipping? And have you tried sending the signal into the grid of V2a to bypass the preamp stage and controls? And what if you do push the...

Are both 6L6s drawing current?

Thanks for all the suggestions but it's rebuilt now and I'm pretty pleased with it. I had already tried increasing the EL84 grid stoppers to 10K, adding an anode bypass capacitor, paralleling 0.68uF capacitor with the filter caps, disconnecting the cut control, chopsticking around the amp and physic...

Came to the conclusion the amp is just too small to be high gain. So I’ve rebuilt it into a single stage preamp with a one knob tone control, same as the Marshall 18W Lite2B and now it sounds fantastic. Plenty of gain for my needs.

I haven't yet, I just tried a cap across the plate resistor of the first stage. I haven't bothered with the PI cap as the cut control only kills the oscillation when it's killing all treble essentially. So for the PI cap to work it'll have to do the same and would render the amp useless. I've just f...

Done a whole bunch more testing and getting confusing results. - With nothing plugged into the input jack if I turn the volume up above halfway I start reading a sine wave on the speaker jack. Quite a large sine wave at that. I can trace this back to the first gain stage. As I vary the volume the fr...

With the changes I made

Well I've been measuring the amp with a scope and it oscillates without anything plugged into the input jack. I can measure the oscillation (Seems to start at 18khz but also appears below 6Khz) from the first stage all the way to the output jack. Removing any of the valves individually stops it and ...

It mostly occurs with the amp volume above halfway. It stops if the guitar volume is on 0, if I have it on 1 or 2 it will be quiet and if I strum the guitar sound is clean with a load of fuzz on top. I can mitigate it a bit if I sit really far back from the amp, but as soon as the amp volume is abov...