Who's wrong: I or the schematic? Vintage battery amp tone stack

[Halkbi]

I'm looking at building something based on the preamp of an old battery amp, but I can't wrap my head around it's tonestack.


(47n and 100K going to ground)

This is right after the first (and single) transistor stage. Pure and simple 1M filter blend knob followed by a 100k volume pot. The 470pF cap along with the 100k pot makes for a high pass filter set at 3.3kHz, right? No surprises there. However if I'm not mistaken the other end of the pot with the 220k followed by a 47n to ground should create a low pass filter set at only 15Hz. That makes absolutely no sense at all. I'm thinking that the schematic is wrong and that the 220k is actually a limiter resistor that connects to the 47n which goes to the pot that then effectively blends between two high pass filters set at 3.3k and 30Hz respectively. That also matches the behaviour of the amp as I remember it; no scoop thing going on, just a variable bass level which basically set the amount of distortion when cranked. I don't have access to said amp, but the gutshots I've found online seem to confirm the 220k and the 470pF as well as pot values.

Do you think I'm right in my assumptions or did I miss something really basic here? Just started dusting off my 10 years old knowledge in audio electronics.

[antonis]

Hi & Welcome..

Is it possible to post the complete schematic..??
(and caps as caps, if you plz..)

[anotherjim]

I see a scoop. The blend pot gets high treble at the top and bass (most treble removed) at the bottom. When it's blended, the mids are going to be at a reduced level compared to highs and lows.

[Fancy Lime]

Frequency response aside, a 1M tone pot before a 100k volume pot in the configuration shown mean a  big loss of signal in all except the most extreme tone pot settings and a probably annoying amount of interaction between the two controls. Does that match the behavior of the amp? If the tone was 100k and the volume 1M, that would make much more sense to me.

Andy

[antonis]

I think we must see previous stage..
OP stated it's a single transistor stage without any other info..
(probably is a CE amp one but it does mater its output impedance..)

[iainpunk]

that's a bridge T filter, with variable high end.
as Andy stated, it seems weird that there is a higher resistance pot going in to a lower value pot... i expect them to be mixed up in the drawing.

if you want to calculate the bridge T filter, use this: http://sim.okawa-denshi.jp/en/TwinTCRkeisan.htm
for the unused components, (in your case C2 and R3) use 999G ohm and 100 farad, those numbers are so high that you can mathematically ignore them.

for small little battery amps, with sh!tty speakers, not having some huge mid scoop makes the thing sound like ''chainsaw death metal'' due to the characteristics of small speakers.

cheers, Iain

[Rob Strand]

The fact the 100k pot is so much lower than the 1M pot means the circuit behaves quite differently at different settings of the 1M pot.

The 3.3kHz is when the 1M is at one extreme.    On the other extreme the 100k loads loads down the 220k path.  You end-up with a voltage divider and the impedance see by the cap is 1/(1/100k + 1/220k) = 68.8k.    That shifts the low-pass cut-off up a bit to 49Hz.  It's a fairly aggressive cut-off.    The frequency placements on this things is often more to do with the notch frequency that results.  If the low pass and high pass frequencies are too close together you don't get notch.     

With the pot in the center,
- the high-pass side will see about 500k in series with (100k in parallel with 500k)  = 583k, putting the high-pass at 580Hz.
- the low-pass side will see  about 220k in parallel with (500k + 100k) = 160k, putting the low-pass cut-off at  21.2Hz.
The geometric center, which is where the notch ends up, will be sqrt(21.2*580) = 110Hz.

Which is pretty darn low.   I haven't checked by things could change at different settings.

It's probably best to plug-in the values into one of those on-line tone-control simulators.

If you think about a Fender-ish tone stack it's like the resistors got bigger but the caps stayed the same and that's push all the frequencies down.

[Halkbi]

Alright, here's the full thing. Had access to one of these in my teens:



This fed into a 5.5" speaker and sounded very british 70's, anything but mid scooped. I would often bring this amp while gigging, sometimes miking it and sometimes utilizing it's line output to use it as a dirt box along with whatever amp that was at hand at the venue. The tone control ranged from thick and honky Marshall-like tones to pristine bright and clean that would turn even the muddiest humbucker into a funk machine (especially on the brilliant input). While the magic of the overdrive likely lies within the TSA810S chip, I'm intrigued to see how the preamp section would perform on it's own pushing a tube amp (or whatever distortion stage put after it). The thing definitely had character and I'm curious to see what part the preamp played in it. Worst case scenario is ending up with a decent variable frequency booster.

Listening back to some old recordings I did with it, it honestly doesn't sound like its cutting much highs at all. Pick attack comes across perfectly and there's plenty of presence despite the fact that I always kept the tone control on the extreme bass setting. I guess I will have to just build it and see what the outcome is. But there is definitely frequency content above 150Hz   

Here's the gut shot I found which confirm that pot values are right. Here we can also see four of the five 220ks in the along with the 470p in the preamp section (the fifth 220K is cropped out on the lower left side but visible in other photos I've seen):

[Rob Strand]

Here's the gut shot I found which confirm that pot values are right. Here we can also see four of the five 220ks in the along with the 470p in the preamp section (the fifth 220K is cropped out on the lower left side but visible in other photos I've seen):

These things are what they are.   Sometimes this stuff will do your head in asking why it's like that.

I guess the question is if you made it more conventional would it sound better or worse.

[PRR]

IMHO: the "Treb" is really a reverse volume control, turning-down lows-mids as you rotate to more treble.

It is of course a $3 experiment which would probably "work" as transistor and Treb pot etc in front of a standard amp.

[anotherjim]

I think it could be a drawing error. 100k and 1M values are swapped.

[Rob Strand]

I think it could be a drawing error. 100k and 1M values are swapped.

From what I couldn't work out from the pic Halkbi posted the 100k pot is next to the input sockets.  On the amp that pot is actually the Volume control - unless it got moved over time/models!

It wouldn't be hard to confirm the circuit.    I think Halkbi already confirmed the parts on the PCB matched the schematic.