Food for thought.

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Food for thought.

Posted by FenderLover on February 29, 2012 at 22:35:23

In Reply to: Thx. I too asked for those data. posted by jeffreybehr on February 29, 2012 at 21:52:04:

If what you are describing is similar to the Fender Bassman input. This is what I gather about the resistor network and it's effect on the grid of that first 12AX7.

" Those resistors are not simply put on the control grid for signal level attenuation purposes; rather, they also act as a very high frequency low-pass filter in conjunction with the input capacitance of the triode (which is a sum of the grid-to-cathode capacitance and the Miller capacitance).

This low-pass filter does a couple of things: (1) it helps prevent high frequency parasitic oscillation in the tube itself; (2) it helps prevent radio frequencies from getting into the input stage, where they can be rectified and lowpass filtered (AM detection) and become audible at the amplifier output; and (3) it can limit grid current when the tube is driven into the positive grid region.

You will notice on two-input amps, such as Fenders, when you plug into the low-level input, the 1 MEG grid resistor is shorted out and the two 68K resistors act as an attenuator to cut the input signal in half. The downside of this is that the input impedance drops from approximately 1MEG down to approximately 136K, which is a rather heavy load for a high impedance guitar pickup.

If the resistor is connected in series with the input jack and before the 1MEG grid resistor, there is a small attenuation of the input signal (0.94 times). If the low-level input attenuation is not necessary, the resistor is best placed after the 1MEG grid resistor and should be soldered directly to the grid pin instead of back at the input jack. This will be the best position for RF and parasitic oscillation attenuation.

Bottom line: leave it on there. "

Hope this helps,
Randall Aiken

http://www.webphix.com/schematic%20heaven/www.schematicheave (Open in New Window)

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Follow Ups Full Thread

Topic - Amplifier Modifications - Hawk28 06:34:14 02/29/12 (25)

RE: Amplifier Modifications - giantnutz 20:26:12 03/09/12 (2)

RE: Amplifier Modifications - jsm71 07:07:29 03/10/12 (1)

RE: Amplifier Modifications - giantnutz 19:41:00 03/11/12 (0)

RE: Amplifier Modifications - giantnutz 07:35:20 03/03/12 (0)

RE: Amplifier Modifications - Alex M 01:50:01 03/02/12 (0)

Yes, I had that need. - jsm71 14:07:38 02/29/12 (0)

RE: Amplifier Modifications - Hawk28 11:41:01 02/29/12 (5)

RE: Amplifier Modifications - wheezer 12:18:13 03/02/12 (0)

Thx. I too asked for those data. - jeffreybehr 21:52:04 02/29/12 (3)

Check the impedance ratings at frequency extremes - jsm71 13:54:28 03/02/12 (0)

RE: Thx. I too asked for those data. - Hawk28 05:34:18 03/01/12 (0)

Food for thought. - FenderLover 22:35:23 02/29/12 (0)

RE: Amplifier Modifications - Palustris 10:51:07 02/29/12 (0)

'Loading' a preamp with a poweramp, input impedance, and the load resistor - jeffreybehr 10:33:05 02/29/12 (5)

Goldpoint shows how to accomplish this... - wheezer 12:12:10 03/02/12 (0)

RE: 'Loading' a preamp with a poweramp, input impedance, and the load resistor - Palustris 11:11:58 02/29/12 (3)

Perhaps I should have written 'poweramp system' instead of just 'poweramp'. - jeffreybehr 21:46:17 02/29/12 (0)

no,,,, that makes perfect sense - bwb 14:29:39 02/29/12 (1)

RE: no,,,, that makes perfect sense - Hawk28 16:24:28 02/29/12 (0)

It is usually set by a single resistor from input to ground, very easy to change /nt - bwb 07:20:31 02/29/12 (5)

RE: It is usually set by a single resistor from input to ground, very easy to change /nt - ahendler 09:29:38 02/29/12 (1)

RE: It is usually set by a single resistor from input to ground, very easy to change /nt - Palustris 10:42:12 02/29/12 (0)

RE: It is usually set by a single resistor from input to ground, very easy to change /nt - Hawk28 09:15:38 02/29/12 (2)

RE: It is usually set by a single resistor from input to ground, very easy to change /nt - Karma16 03:26:24 03/02/12 (1)

RE: It is usually set by a single resistor from input to ground, very easy to change /nt - Hawk28 05:39:35 03/02/12 (0)

Follow Ups

Food for thought. - FenderLover 22:35:23 02/29/12 (0)

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