Mini 2/3w amp. 12ax7 + 12au7. Too much bass when gain increases

[nonost]

Hi. I'm building a mini amp. All credits to Thomasa! Here the schematic without the power supply section:



MAX1771 for the power supply (around 200v) and two tubes. One 12ax7 (two preamp stages) and a 12au7 (one preamp stage and one power amp stage).

The 12ax7 triodes are biased quite to the left whereas the 12au7 are right on center (although the preamp one is at very low current).

I built it with same values per schematic. I only had to replace de grid stopper of the output stage for a voltage divider, since the amp was making noise/oscillating a bit when vol&gain pots were cranked. So now the signal is halved two times, one before each 12au7 triode.

I have a problem regarding the overdrive quality. It sounds really really nice with the gain pot at 12:00. Beyond that point it starts to add to much bass. I'm using a HSS strat with low output singles and a bridge PAF. Using the PAF at max gain I get almost Fuzz sounds when picking hard the EAD strings. I've tried so many things:

- Each stage section with its grid leakage & grid stoppers resistors
- Treble peeker caps at the voltage dividers (both of them)
- Center the second stage bias
- 1uf cathode bias for every section but the first one
- Other tone controls sections

I'm almost out of ideas... I can't put 4n7 caps instead of the 22nf cause don't have right now these big voltage ratings. But nothing more in mind.

What do you think? I'm kind of lost right now

[mozz]

The 100nf caps aren't helping. 

[nonost]

 The are not 100nf. I forgot to include the partlist which has some updates values by the author.

First couple cap is 4.7nf
Second one is 22nf
Third one is 22nf

The cap at the voltage divider before the third stage is 100nf, but I put there other values. Such as 470pF.

[anotherjim]

The tone stack arrangement looks awkward. The 1M pot feeding it means the source impedance varies quite a bit according to that pot. It would be more usual to have the 1M gain pot after the tone pot wiper. Then the tone stack values can be selected to suit the known 100k plate resistor as the source impedance. 100k source is I think too high for the combination of loading following it but you don't have too much wiggle room to alter that. It might be a little better to also scale the filter values for a 250k tone pot.

[nonost]

Hi Jim. I tried another tone control. Right now I'm using this:



From the Princetone. It works better (more gain I think), but it's still really bassy.

[antonis]

IMHO, you need a Cathode follower between 1st stage and Tonestack, with Gain pot moved right after Tone pot wiper (as Jim already pointed out)..

[nonost]

Ey Antonis! I removed the tone control and it's not better. The Princetone tone control I posted works better than the lack of it.

[PRR]

Are you using a guitar speaker? Or non-guitar?

There's NO bass cutting in this plan. Some Fenders were like this but used stiff speakers in open cabinets so did not have big bass.

Change cathode caps to 1uFd (Marshall used 0.68u) and coupling caps to 3nFd (0.003uFd). Change one at a time, try, stop when it gets gut-less.

[antonis]

The Princetone tone control I posted works better than the lack of it.

With Princetone Volume pot fully CW, it acts as a single LPF..

[nonost]

Hi Paul. I'm using a semi open 2x12 cabinet. I changed the wiring so only one speaker is working (8ohms) since I don't know if it would be a problem to use a 16ohms load. The OT is a 125A one, secondary is 8ohms.

I've found some 3n3 and 4n7 caps and replaced the 22nf coupling ones. I've also changed to 1uf the cathode caps... I guess it just needs a bass cut. Right a the input?

At the input I'm using a 10k instead of a 68k, and added a tiny 220pf to ground.

[PRR]

....a bass cut. Right a the input?

Change cathode caps to 1uFd (Marshall used 0.68u) and coupling caps to 3nFd (0.003uFd). Change one at a time, try, stop when it gets gut-less.