2022 Monkeymatic NDS #4

[xtian]

I built this Hot Rod Deluxe conversion using the PCB Lou and I designed, clean channel only. Since then, I loaned the amp to a local promotor, and it has been the star of a dozen blues jams. Recently I played it at my own gig, and thought it sounded great, but too bassy even with the Bass control fully CCW. (BTW, it has an Altec Lansing ER12.)

Where's the most likely place to trim bass? One of the highlighted coupling caps?

HRD-NDS #4 schematic.png

IMG_1462.jpg

[martin manning]

PI input cap would be easiest.

[sluckey]

Looks like you have already trimmed the bass back in all the usual places. Consider a brighter speaker.

[xtian]

Looks like you have already trimmed the bass back in all the usual places. Consider a brighter speaker.

This speaker is the beez kneez. I'm willing to tweak the circuit!

[pdf64]

I vote for C44 at the V3 phase inverter input too. Black panel Fenders use 1nF there with open back cabs, 500pF with closed back.
But also consider trying a bigger bright cap, eg 220 - 470pF.

[xtian]

I haven't opened it up yet. But, any takers on changing the V1B or V1A cathode bypass cap to 0.68uF?

[didit]

I haven't opened it up yet. But, any takers on changing the V1B or V1A cathode bypass cap to 0.68uF?

Experiment at V1b, try no bypass just to hear what your getting in to, and then less than 5uF but maybe more than 0.7uF. Run a model of your tone stack and see what’s possible to tweak inside there.

While we’re in it, what purpose does C47 provide and does it flatten treble and therefore fatten bass?

Best ..

[xtian]

This TSC was helpful: https://www.ampbooks.com/mobile/amplifi ... alculator/

For various bypass caps, here's the calculated drop in volume at 82Hz (low E):

25uF no drop
4u7 -0.41dB (this was the bypass cap before change)
1uF -3.94dB
0.68u -5.15dB
0.1u -7.15dB

I found 1uF bypass cap sounds best to me.

[didit]

I found 1uF bypass cap sounds best to me.

Cool. In past, my only round of experimentation settled on 1.5uF.

Still curious about C47?

..

[TUBEDUDE]

C24, C34.

[xtian]

Still curious about C47?

Treble snubber.

[didit]

Still curious about C47?

Treble snubber.

That was my assumption Aaron. And so does lifting it out help improve treble <> bass range balance or does it simply make treble to shrill?

Inveterately curious.

..

[pdf64]

This TSC was helpful: https://www.ampbooks.com/mobile/amplifi ... alculator/

For various bypass caps, here's the calculated drop in volume at 82Hz (low E):
..,

Bear in mind that due to the tone stack, Rg here will vary according frequency and tone control settings. eg at low mid settings, it’ll be down to around R5, 47k.

Additionally, with partial bypass, the input stage’s output impedance will vary according to frequency, a bit lower in the bypassed frequency range, a bit higher in the unbypassed range. So that’ll complicate the resulting responses.

So the ear test is probably more valid than focusing much on what you get out of simple calculators.

[xtian]

Still curious about C47?

Treble snubber.

That was my assumption Aaron. And so does lifting it out help improve treble <> bass range balance or does it simply make treble to shrill?

Inveterately curious.

..

It's a pretty high frequency low-pass filter, right? I think it's added to snub "fizz". Removing it won't affect bass or tone balance.

[pdf64]

…
It's a pretty high frequency low-pass filter, right? I think it's added to snub "fizz". Removing it won't affect bass or tone balance.

Yes, I think so, it’s probably primarily there to help stability, it should have no LPF effect in the audio range, -3dB may be up above 20kHz.