input resistors

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input resistors

Postby t bone bruce » Wed May 03, 2006 4:56 pm

I'm hoping someone will know..On may amps there is a 1 meg resistor to ground from the tip of the jack. From what I've read this sets the input impedance of the amp. As I'm using a crystal mic on my harp amp is there any benefit in increasing the value of this resistor? I've seen a few suggestions that it can be replaced with a 5.6 meg resistor to improve input impedance. Any views on this?
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RE: input resistors

Postby songdog » Wed May 03, 2006 5:44 pm

It depends on the impedance of your crystal microphone. If it's in the 10-50K range you may not hear any difference between a 1M or 5Mohm input impedance on the amp. I seem to recall that some crystal mics had very high impedances (1-2Mohm) in which case increasing the value to 5.6M might make a difference.
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RE: input resistors

Postby ricochet » Wed May 03, 2006 6:58 pm

BUT, it also sets the DC resistance to ground of the grid of the input tube. (With a crystal or ceramic mic, with magnetic guitar pickups DC goes through the very low resistance of the coils.) Going over 1 Megohm is slightly risky in that gas ions in the tube can make the grid go too positive and increase the plate current. Under the operating conditions of most amps' first tube, the plate resistor's high enough that the tube's not likely to cook, anyway. But you won't increase the amp's sensitivity a whole lot by going up on the resistor. As frequently as harpers put in lower gain tubes in the first stage, it seems a little weird to want to increase the gain this way.

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RE: input resistors

Postby songdog » Wed May 03, 2006 7:50 pm

>BUT, it also sets the DC resistance to ground of the grid of
>the input tube.

Agreed.

He didn't really say he was looking for more gain so I kind of assumed he was thinking a better impedance match might improve the tone (bandwidth). But if he's looking for more gain, I'd suggest adding an inexpensive tube pre to the front end.

Increasing the input impedance can also aid in the reception of radio signals and noise... which isn't a good thing.
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RE: input resistors

Postby t bone bruce » Wed May 03, 2006 8:36 pm

Thanks guys for the prompt responses. I was more after good bandwidth, espcially bass response from the mic,(Hohner Blues Blaster) than increasing the gain. I have a 12AU7 in the preamp tube socket to keep gain down. The amp is a cross between tweed champ and princeton designs ie Champ, but with the princeton tone control. I haven't noticed a significant difference in sound with the 5.6 Meg resistor installed, so I'll probably remove it and replace the 1Meg.
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RE: input resistors

Postby ricochet » Thu May 04, 2006 12:48 am

Hey, I was thinking about this a bit on the way home from choir (with Dan Lange's "Bad, Bad Whiskey" playing.) I'm assuming you have a 12AU7 plugged in where a 12AX7 went. What values are there for the plate resistor, cathode resistor and cathode bypass capacitor? With the lower plate resistance of the 12AU7 the bypass cap is going to have to be something like 5X as big to have the same low frequency rolloff point as it would have with the 12AX7 in there. That's going to be true of both amp stages in that socket. You might want to try bigger cathode caps on those stages if you want a little more beef in the bass. And maybe also a bigger coupling cap after each of those stages. Rule of thumb is that it takes a 10X change in cap values before you can easily hear a difference, going up or down.

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RE: input resistors

Postby t bone bruce » Thu May 04, 2006 4:19 pm

Thanks Rico,
I am running with a 12AU7 in the preamp, Cathode resistors are 1k5, stock value for the champ and princeton design. The first stage has a 25uF 25V cap cathode bias cap. Plate resistors are 100K, and coupling caps are nice big audio-quality 0.1uF's.
I really like the sound I'm getting from the amp, but I was looking for some tweaks to see if I could get it even better. It has a 10" speaker, and the pine cabinet for it is a whopping 22" high, 18" wide and 9" deep, which really adds to the sound. Pretty big for a 5 watt amp!
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RE: input resistors

Postby ricochet » Thu May 04, 2006 4:47 pm

Might consider trying something like 220 uF cathode caps, and leave the rest alone. The caps won't have more than about 1-2V across them.

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