Vox repeater Tone sucking

[guidoilieff]

Hi. I made this puppy and when I hit it there is a considerable bass loss on any signal.

Im using bc549 transistors and C3 is a 823 instead of a 683.

Do I need to change c4 or c5? Or maybe a resistor? I´m lost here.

[thermionix]

I've never seen "k" used in a capacitor value, does 4k7 mean 4700pf?  If so, I would increase C4, maybe try 10n (10k?) and see where that gets you.

[PRR]

Reading "4K7" as 4.7 K pFd (4700pFd or 4.7nFd), bass falls off at 450Hz.

This may be necessary for the repeat gimmick to work reliably, or it may be a typo.

50nFd (like the output cap) would be a generous (full bass) value. Considering all the gain, you may want to shave bass, 20nFd or 10nFd.

[R.G.]

Yep. As noted, C4 and C5 are the place to start, as  you suspected to begin with.

Let's take a look at how this thing works.
- The input signal is applied to a voltage divider composed of R7, the depth control, transistor T2, and the input impedance through C4. This divides the input signal down to a low level.
- As a side effect, this low value of input impedance, R7 (22K) in series with the parallel combination of the depth control and the impedance through C4 loads down the guitar signal, further reducing signal level, and also losing some treble because of the inductive nature of the guitar's pickups.
- The remaining small signal level is further reduced by the varying collector-to-base resistance of T2, which is modulated by the sawtooth oscillater composed of T1 and that clot of stuff around it. This is where the signal modulation happens.
- The now-varied signal is amplified back up to a more normal guitar level by T3. That's all that T3 is there for, to get the signal level back after the dividing and modulation has reduced it.

With some understanding of what it's doing, it's possible to make some guesses as to what decisions and compromises were made. The bane of audio modulation is control signal feedthrough. The "control signal" in this case is the sawtooth signal from T1 through T2 into the divider. This signal is down at maybe 10Hz, and happens at the input of the amplifier. I suspect that C4 and C5 were deliberately made small to cut out the low frequency control signal feedthrough and the bass loss was thought to be acceptable.

To understand why it was "acceptable" fifty years ago, you need to look at the state of music technology. Back when this was designed, guitar pickups were treble deficient, and it was usually thought that treble boost (that is, bass cut and amplify) was a good thing, so the bass loss wasn't a bad thing. Beside that, electronics of any kind were expensive, so having an affordable effect at all was a big deal. With the advantage of half a century of technology advance, we now think that full spectrum frequency response is the "normal" thing, not something that engineers slaved over. So what was amazing because it could be done at all at a price that a guitarist could afford back in the mid 60s is now viewed as unacceptable.

[guidoilieff]

Thanks to all. It was C4 and C5. I hooked up a big fat 53nfd cap for C4. Now I can hear hum when I dont touch the strings but its fine with me. Maybe Ill roll it off a little bit.



Thanks again

[allesz]

Nice to see the problem was solved.
I jump on the subject since my latest breadboard experiments brought a question: why not put a cap (4,7to 10 Uf?) in parallel with R12? I found that a big input cap frequently increase fuzzyness, wile a bypass(?) cap  on emitters achieve similar results in a cleaner way... but I really would like some input from experts.

[PRR]

> why not put a cap ...in parallel with R12?

Gain as shown is (less than) 10. (Like 3 counting R7 P2 divider.)

Gain with R12 bypassed would be MUCH higher. Like 250! Base impedance falls so we may get under 200 (near 60 with R7 P2).

As this is supposed to be "clean with stuttering gain", but gain of 60 with typical guitar source would wildly OVERLOAD this amp, simple R12 bypass is not wise. (Try it: it may be outrageous fuzz on strong chords.)

Also he is not objecting to the gain, only the bass loss.

> I found that a big input cap frequently increase fuzzyness, wile a bypass cap on emitters achieve similar results in a cleaner way

The two caps are related by transistor hFE. They will generally do similar things although with quite different values. Yes, in some cases one or the other may be "more polite".

[allesz]

Thank you. Nice explanation as usual.
The fact is I refer only on breadboard practice and zero  theory.
I won't start studying...  but I will give a try to this alternatives with more awareness.