Early SS input impedance (Kustom)

[Rhodesplyr]

I just finished refurbishing a 1970 Kustom K200B-1 head, and I noticed on the schematic that its BJT input impedance appears to be at most around 50k, probably somewhat less. I guess this before JFETs became popular for SS input stages.

I know that the general standard for tube guitar amps is a 1M input impedance, though I've seen some amps as low as 330k.

How did Kustom get away with such a low impedance for guitar/bass, given that this amp actually sounds pretty good to me, though a big mid-rangey. It definitely responds differently from a tube amp, but not in a bad way.

Kustom PC105 preamp_and_boost.PDF

[pdf64]

Yes, the input impedance will be ~52k at the most; it will likely reduce as the amp's volume control setting is turned louder.
That will act to damp down the treble resonance of the the guitar pickup connected to it, assuming a direct connection, eg no pedals, similar to turning the instrument's tone control down a bit.
It may also act to steepen the taper of the guitar's volume control.
These effects are unlikely to cause a problem as such, but a higher input impedance would probably be preferable.

[Enzo]

Every part in the circuit has an effect on the whole. Keep this in context. A lower impedance might dampen the signal a little bit, but that is a small portion of the overall circuit. For example, if you have to turn the treble control up from 6 to 6.2, is that really anything to concern over?

[R.G.]

Some people like the effect of wiping some treble off their guitar signal.

And some people with old or battered ears can't tell. Treble gets wiped off the ears first from loud sounds.

[Mike Sulzer]

I just finished refurbishing a 1970 Kustom K200B-1 head, and I noticed on the schematic that its BJT input impedance appears to be at most around 50k, probably somewhat less. I guess this before JFETs became popular for SS input stages.

I know that the general standard for tube guitar amps is a 1M input impedance, though I've seen some amps as low as 330k.

How did Kustom get away with such a low impedance for guitar/bass, given that this amp actually sounds pretty good to me, though a big mid-rangey. It definitely responds differently from a tube amp, but not in a bad way.

This is actually a significant effect, one that is not so easy to compensate for exactly. One way to think of it is to compare it with the tone control on your guitar. Remember that as you turn down the tone control from 10, you get a similar effect; that is, the capacitor plays a role only when you get to the lower numbers, and the guitar tone control just loads the pickup with a lower value resistance as you turn down from 10. So this input impedance is like turning down the tone control most of the way, but before the really bassy effect from the capacitor becomes important.

[loudthud]

A Fuzz Face has a pretty low input impedance, it didn't stop Jimi.

[Mike Sulzer]

A Fuzz Face has a pretty low input impedance, it didn't stop Jimi.

Exactly. If you use a lot of distortion, you want to throw away high frequencies before the distorting circuits. One way is to load the guitar with a lower than normal impedance.

[R.G.]

One of the miasma of new pedals that slosh through the market caused me a lot of chuckling. It was called a "brown tone" or "browning pedal" or some such - can't remember exactly. It was a metal box with pretty graphics, and inside were two jacks and ... a pot. The pot was set to load down the signal wire that went between the two jacks. There may or may not have been a "bypass" switch that broke the connection between the pot and the signal wire for times when you wanted a "treble boost".

[Enzo]

In another industry, a guy was marketing a module to do some mod to a system. There were a half dozen wires you connected to various points on the main circuit board. Something like a green and a blue to points A and B, Yellow and white to points X and Y, and a red and black up to a switch on the panel. The module itself was maybe half the size of a cigarette pack, and potted in black epoxy, so no one could steal his circuit. It's purpose was to speed up a Pac Man game to double or triple speed. It was a "turbo" button on the panel.

Well, we knew you could achieve this effect by cutting two traces and running jumper wires from A to x and from B to Y, or something like that. So we chipped one of the things open. Inside was no circuitry at all, just wires of one color soldered to wires of another color. SO all the wires amounted to nothing more than the jumpers we knew about, and the panel switch covered the cut trace.

[Leo_Gnardo]

"brown tone" or "browning pedal" or some such - can't remember exactly. It was a metal box with pretty graphics, and inside were two jacks and ... a pot. The pot was set to load down the signal wire that went between the two jacks.

I've made a couple of custom tone browners, handy for studio recording when you need a microamp or similar to run a long line from the control room, but want to load the guitar with the same resistance it sees plugged into an amp. Or two. Or three... I put a 10K resistor at the bottom of a 1 meg pot, so you'd still have some signal all the way down. Complaint without the tone browner was "it's too bright!" because MXR Microamp has 5.6M input loading.

They were delighted to know they never needed to change the battery!