input jack loading question

[rp]

I know I should post this on the AX84 forum but I got tired of the very slow registration process. And, I'm liking it here.

I'm building AX84-P1 (earlier lower gain High Octane, I think) into a dead ss Marshall Lead 12 combo as a freebee for a friend. I chose the AX84-P1 for the only reason that it was all I could find that let me keep the Marshall faceplate stock, no plugged holes, no electrical taped-out lettering. Only bugger is that the Marshall has High/Low inputs and the AX84-P1 has just one in w/ a 1 Meg load and no series resistor. I'm sure this was well thought out and I don't want to mess with the input gain/voicing by just putting a typical 1M-68K-68K set up.

Any advice how to do a high-low set up without changing the high input gain from the design?

Coming out sweet so far. Sure hope this design sounds all right, I got an unemployed buddy to dish out $200 for the parts.

BTW be extra careful unsoldering those square light up Marshall switches, I knew to go easy on the thing it just eeked with cheap, but it seized right up anyway. Ain't no Carling.

Thanks

[M Fowler]

It looks like the way you have it now is correct, one jack with 1m and other without both wired to V1 entry pin 7.

[labb]

I do believe that the 68K input resistor that you speak of is mostly just to keep out external radio interference. The folks on AX84 have left it out in the later versions of the P1. It is on the schematic for the older versions and is mounted right on the pre amp tube. Some times you can get away with leaving this resistor out, sometimes not..Last one that I built with out it when I fired it up I was getting Texas Flood out of the amp..Dang it sounded good..Ended up putting in a 10K resistor on the input, problem solved. You want to have some fun with the P1 then put in a line out mod..Run the line out into the input of a bigger amp..Anyway, if you want to do the 1M, 68k, 68K go ahead..Don't think that it will hurt anything.

[rp]

It looks like the way you have it now is correct, one jack with 1m and other without both wired to V1 entry pin 7.

So I don't need any load resistor for the low input? I didn't know this, I've never seen it. I thought you need something to ground on any input, like the 68K to ground you get when you plug into the low jack on a Fender etc. And I knew the series resistors were for taming RF noise but I've never seen a circuit with no series resistor except really ancient ones. I figured you couldn't get away without them and was curious why the AX84P1 left them out. Why did they leave them out?

So if I understand: you don't have to have a load resistor at an input and the series resistor's effects on gain are marginal, when at 34k or 68K anyway, and it's the builder's/designer's option to leave series resistors out if they don't think RF will be a problem?

Hmm I never played around here, I hate resoldering jacks too may times, especially the plastic Reans that get ugly quick.

[M Fowler]

Here are two examples for you to look at the way the input jacks hi and low are wired. These both use the 68k grid blocker.

You may not need the grid blockers on V1 pin 7 (which can be anything from 15k to 68k or even more some times). But you should not need them, if you do start small and increase if needed.

[PCollen]

YOu can mount the two 68K resistors on a Terminal Strip mounted to the chassis, wiring the junction of the two resistors to the V1 input grid, and the tail of each resistor back to your input jacks. You may have to re-wire the input jacks to ensure proper switching when the cable is plugged in to each . See attachement. A picture is worth a thousand words (although I have added a few).

[rp]

You want to have some fun with the P1 then put in a line out mod..Run the line out into the input of a bigger amp..

labb

I was thinking of doing this, my friend does varied gigs and it might be a useful feature for him. Any favorite resistor combination for the line out on this? Ever try the line out with a pot? What's so special about the P1 this way - does it make a crazy overdrive pedal?

[labb]

I think that it is best to us a pot. I will have to get back into my files to see exactly what I used. I know it was a knock off of the Firezog/Herzog circuit. Pretty simple. The pot allows you to adjust the output level..The effect approaches the Garnet/Herzog sound..ie: Randy Bauchman's American Woman. Course that is based on how hard you are driving the P1.

[labb]

Do a search for Firezog and you will find the schematic. I think that Doug Hammond is who put it out on the Web. I followed the schematic and used the 1M line out level pot and the 150K series resistor..Don't think that there is any magic to the values.

[rp]

Thanks for the info, I found the Firezog schematic easy enough. I also came across this archived on an other forum by someone maned Web Surf which leads me to more questions:

The problems with that (Firezog) schematic are :

A) The possibility to output 10-30 Volts out on the line out. This is way too high.
B) The impedance of the line out will be very high ( 150 - 350 K). It should be 5-47 K Ohms

Please see earlier thread last week where I suggested a different DIY line out for a Champ, which overcomes these deficiencies.

http://acapella.harmony-central.com/sho ... t=1492902

So now I'm unsure about the Firezog, any comments?

A few more questions on the line out thing, is the firezog designed more as an other gain stage to distort into the following amp than a PA out? I'd rather have a clean line out for going into a house amp, but the ability to do both would be nice. I guess if you turn the firezog down clean is what you get, any confirmation?

Also can you really just put a big resistor on the jack switch and bypass the speaker? Is this safe playing cranked for hours, even with a resistor sunk to the chassis? If going into a PA will it have the same sound (or decent sound) as with the reactive speaker, or is the Firezog in the schematic intended solely for using a small amp as a booster?

[labb]

Don't know about that post..My Son has been playing the P1 with the line out option for about 5 years now..I do think that he does not unplug the speaker when he is using the line out. Do a search for "Kalamzoo Amp Field Guide" go to Mods and take a look at their schematic..Pretty much the same with different values of the resistors.