Another guitarpreamp design using an oddball tube

[Alf]

Right , thank you for a very extensive reply.
I have used multiple transformers before in an amp so I know a little about that although it was quite a number of years ago.
This preamp has been built on old old radio chassis because when I 've finished experimenting I will make a nice layout for it and look for a better platform to build it on.

Years ago I bought five big pieces of Garolyte and eyelets as well as turrets which will finally be used then.

I tried SS switching before but failed so I used relays then.

The procedures to prevent popping sound are known to me but thank you all the same.

[Alf]

This is an ongoing project. I changed it a bit but are not satisfied with the amount of bass I'm getting.
Can anybody tell me if can use the general tonecontrol components of an 12ax7 preamp, like I did?
I use it now with a Boss Ge-7 eq behind it which is much better imho. But there must be a way to get a better basscontent without it.

Tia , Alf

[Enzo]

Do not assume the tone control is where all the tone comes from. You could work with the coupling caps between stages, larger caps might pass more bottom. But sometimes a lack of bottom is too much top. You have that big 470pf brightness cap across the first 470k, what happens if you remove it? And in your tone stack, what if you reduced the 470pf treble cap to 250pf?

Just random thoughts from my foggy mind.

[Alf]

Thanks for the advice Enzo, I already changed some couplingcaps and had the combi of 470 k and 470pf for the Marshall crunch which works well if I turn the 1M pot up.
I'll first try to lift the 470 Pf cap.

Alf

[Enzo]

They were just idle thoughts, maybe a different value bright cap.

GO over to Duncan amps and download his really cool tonestack calculator, which is a neat bit of software that lets you wantch in real time the freq response of various tone stacks as you set the controls, but it also lets you alter the component values to see how that changes things.

[Chuck H]

Well, your bright switch only has two settings.?. Super bright and super dull. Maybe reduce both cap values for a more subtle change and, perhaps, two more useful tones. Also, is there too much bass at all gain settings? Or do you actually find the extra bass useful for cleaner tones? You may be able to make the knob settings more aesthetically pleasing using a more severe log taper for the bass pot. If you genuinely think there's too much bass overall you could reduce the bass pot value as well. One of my designs uses a 250k/10% log pot for the bass control. Downloading Duncan TSC is a good idea as well. If you find yourself going to the slope resistor for LF/HF balance adjustments TSC will clue you in quickly (and without math ) as to shifts in the frequency knees when changing that value.

[Alf]

I should have phrased it differently Chuck . I have too little bass.

[Chuck H]

Ah! Hmmmm... The design doesn't look like that would be a problem. If you're sure it's all wired as per the schem and there are no mistakes in component values I don't have a suggestion.

[Alf]

I'll check the components again , especially the cap values.

[Alf]

I'll check the components again , especially the cap values.

Calculates the total capacitance of two capacitors in series and parallel.

I just noticed that puttting two caps in series 22N and 470 Pf gives a total of 460 pf so that explains the lack in bass I suppose . Am I right?

I made the 470 p cap switchable and that makes a noticeable difference.


Alf

[Alf]

So I realized that putting a 22n cap and 470 pf in series gives a total of 460 p and that explains, I think, the lack in bass. I mean in between the 1st and 2nd stage of the first tube . Am I right?

[Chuck H]

Well that 470k resistor in parallel with the 470p cap is only dropping about a third of what's coming from the 22n independent of being in series with the smaller cap. So I don't think that's it. But it would certainly make the darker if you lifted that 470p cap. You could also reduce the value of the bright cap on the volume control. But...

If the amp seems strangely lacking in bass I still suspect that there may be a different problem than voicing as per the schematic. The schematic doesn't look like it would a "thin" sounding amp at all. Possibly bright. But not lacking in bass. Enzo's point about the overall frequency balance is very valid, but there's a difference between a bright amp and an amp with no bass. Looking at your design I would expect you to be able to crank up the bass and get farty distortion. An indication that plenty of bass is getting through.

[Alf]

Well that 470k resistor in parallel with the 470p cap is only dropping about a third of what's coming from the 22n independent of being in series with the smaller cap.

Thanks Chuck, could you explain why you said a third of what's coming.....


How do I calculate that ?

And the amp doesn't fart out, it sounds just fine.

I made the cathode resistor of V1a bigger to 680 ohm and that's much better imo.

By the way in my first post it says ecc85 = 6aq8 - it's wrong !
It must be 6aq5.

[Chuck H]

The root of the circuit between the first and second gain stages is a 22n cap followed by a 470k resistor with a 1M volume control. The 470k/1M circuit forms approximately a 2:1 voltage divider where about a third of the signal goes to 0V. Since the 470p cap across the 470k resistor isn't part of that division none of it's pass frequencies are loaded to 0V. So the frequencies passed by the 470p cap are about 1/3 greater in signal voltage at the second stage input grid. This acts independent of the voltage division of the volume control itself. Now...

The volume control also has a bypass cap. The 1000p "bright" cap. So if the volume control is, say, at a 50/50 resistance setting you now have the same phenomenon happening. Any signal from the 22n cap at the input of the volume pot, which is already cut by a third remember, gets cut again. This time by half because the voltage divider ratio of the volume circuit is 1:1 at a 50/50 resistance setting (maybe 6 or 7 on the volume control). So you now have roughly 1/6th of the signal from the 22n cap going into the 2nd stage grid AND the HF frequencies passed by the 1000p cap across the volume control are not divided. The 470p across the 470k resistor earlier in the circuit is in series with the 1000p across the volume control, pretty much giving a lot of HF frequency a free pass, while the LF through the circuit for much of the volume controls range is greatly loaded. Enzo may be right about you hearing an imbalance rather than a lack of LF.

Of course you have to have some loading of the signal or the volume control wouldn't function. But 1000pf is a very high value for this circuit. I don't think it would sound right having NO cap there, but a smaller value might be better for you. A lot smaller. Like 100p.

Now, if what you have is actually a lack of bass rather than a preponderance of treble it could be that part of the circuit is incorrect and the 22n cap is not passing through the 470k resistor in the circuit. Try lifting one end of the 470p cap. If your signal disappears then you have a problem. If your signal just becomes less bright then you don't.

[Justin Thomas]

Off Topic: just saw the new Sig, Chuck! Thanks!

Justin