channel input resistor value straight from humbuckers?

[tapehead ted]

Hi all.
My guitar has no tone controls or other electronics other than a pair of humbuckers, dialed to very low output, a pickup selector switch and a jack. I'm optimizing a tube amp to use with this guitar, and I'm wondering what the theory (or opinion) might be as to an appropriate value of channel input resistor. Right now the amp uses a 68k resistor in each channel. Sounds quite healthy, definitely healthier than it did with all the tone and volume pots, but is it as healthy as can be?

I'm particularly concerned with any impedance matching issues.

Should I be doing anything to address the difference between the output straight of the pickups, and the usual output through the usual pots? The latter, of course, is what the designer expected...

Thanks!
Ted

[firechief]

Sounds like an uncomon set up on the guitar, but thats not going to stop it from sounding good. You should let us know the amp make and model then we can look for the schematic. 68K is very common and going much lower is ill advisied. Also there shoud be another resistor to ground on the input circut like a 1M ohm, Yes?

[Enzo]

A 68k resistor in each "channel"? Or a 68k from each jack both feeding the same channel? Look at a bazillion old Fender amps for examples. In that arrangement, you will find the two jacks have different sensitivity. And the cutout contacts on the two jacks are wired differently. Plug into the high gain jack, and the two 68k are in parallel for a series resistance of 34k. Plug into the low gain jack, and now one 68k is grounded, forming a 2/1 voltage divider, so your signal voltage is 1/2 what it was going into the tube. That is a 6db reduction in level.

If you already have the two jacks thus wired, just leave them. I(f you just have to have only one jack, then use a 33k resistor instead of 68k, and it should sound the same.

[tapehead ted]

Originally Posted by firechief

Sounds like an uncomon set up on the guitar, but thats not going to stop it from sounding good. You should let us know the amp make and model then we can look for the schematic. 68K is very common and going much lower is ill advisied. Also there shoud be another resistor to ground on the input circut like a 1M ohm, Yes?

Yes, there is a big resistor to ground there, 2.2 meg, at each channel jack.

It's a one of a kind amp, not based on any other specific design, but the front end is pretty typical from what I gather- two inputs, V1 is a 12AX7, one half for each channel, and both channels have identical setups, each with it's own input jack, 68k resistor, and 2.2 meg resistor to ground. Same gain in each channel.

However, one channel has a 1meg volume pot (too bright, I think I'll change it to 500k), and one has a step attenuator with a maximum resistance of 412k, and that channel is quite dark. So I'm interested in learning about using different resistor values from channel to channel to brighten up the darker step attenuator channel and darken the volume pot channel. Would now 68k input resistor be a place to do this?

I don't wish lose 6dB at the input, in any case- the output of this guitar, as set up, is quite low but that's where it really shines, with the pole pieces each tweaked just so.

Thanks again.
Ted

[Nigel Goodwin]

Originally Posted by tapehead ted

I'm particularly concerned with any impedance matching issues.

Impedance matching is normally something you DON'T want, the input impedance of the amp should be a minimum of five times the output impedance of the PU.

Putting a lower value resistor across the PU (same effect as lowering the input impedance of the amp) will make the sound more dull and quieter.

[Enzo]

This is a guitar amp, nothing in it is critical. Ever notice you can take just about any guitar amp, and plug just about any guitar into it, and it works fine? You might not like the tone, but tone is a matter of taste. You can go with single coil pickups, or humbuckers, and set the guitar pickup selector to one pickup or two in parallel. All these possibilities, and yet impedance is never an issue.

[Printer2]

First off the 68k resistor is in series with the 2.2M and the triode input so you are loosing 3% of the voltage across the 68k resistor. Doubt you will notice it, the resistor is there to cut down on RF interference. The different pot values should not change the brightness of the channels. Trace out a schematic with part values and post it if you can.

[jjman]

Many people confuse voltage dividing resistors with grid stopping resistors. W/O a schematic it's less than 100% clear which way yours are connected. The common Fender BF/SF arrangement uses one or the other depending on whether you plug into the 1 or 2 input on a given channel. Grid stoppers don't change tone unless they are colossal in value. Voltage dividing resistors attenuate the signal to the tube depending on the ratio of the 2 values. Grid LEAK resistors usual set the input impedance. 3 separate functions that sometimes overlap and change via "switching" contacts on the input jacks. Schemes are much better than words regarding "input resistor" discussions.

[tapehead ted]

Thanks everyone. I'm not currently able to post a schematic, but I'm working on it. This is a much simpler setup than the BF/SF switching inputs- it's more like an early 12AX7 champ.

In any case after looking at all this information, I'm sticking with the 68k or very nearly so.

[Nigel Goodwin]

Originally Posted by tapehead ted

Thanks everyone. I'm not currently able to post a schematic, but I'm working on it. This is a much simpler setup than the BF/SF switching inputs- it's more like an early 12AX7 champ.

In any case after looking at all this information, I'm sticking with the 68k or very nearly so.

Without the schematic, so just guessing based on normal methods - the 68K is completely meaningless - it doesn't affect anything enough to matter.