pa100 reverb conversion question

01redcobra · 03-30-2009, 03:29 AM

I've been working on converting a Fender PA100 reverb to a guitar amp. I have rewired channels 1 & 2 to a two stage Fender style preamp with a Fender tone control circuit with t-b-m controls. So far I'm happy with the sound of the channel, but I am perplexed as to how to rewire the reverb to work on it. I have attached a portion of the original Pa100 schematic that shows the push-pull switch for the reverb on the volume pot. What really has me puzzled is the input jacks on the original amp are like non I've ever seen before. I'm using a switchcraft shorting jack on the channel now. Can anyone help me with getting the reverb to work on this channel? Thanks in advance.

loudthud · 03-31-2009, 03:48 AM

Those funny jacks short the signal of each channel to ground when the channel isn't being used (no input plugged in). The 270K that goes to V4B is the reverb send. The switch just turned the reverb on and off. Hook that 270K to the output of your preamp and you should have reverb. The Reverb Master is the only control.

01redcobra · 03-31-2009, 06:18 AM

Just one more question. The other 270K that goes to V5A in the original circuit, what do I do there. Right now I have one input with 68K and 1meg resistor(Fender) running to v1a and then thru T-M-B pots to a 1 meg pot then to V1B and out of that to another 1meg pot to V5A.

loudthud · 03-31-2009, 06:18 PM

You could just leave the resistor where it is connected to nothing, remove it, or connect the free end to ground. When I mod an old amp, I like to use all the available knobs and then stop. With the PA100 you could build other different sounding preamps. Have fun!

01redcobra · 03-31-2009, 07:49 PM

Thanks loudthud,
I'm still considering making channel 3 a cleaner Fender channel, but still looking for ideas and leaving channel 4 alone.
If I run that resistor to ground or leave it out altogether, what would be the difference in the sound?

loudthud · 03-31-2009, 09:59 PM

Grounding it will give you slightly less signal to the reverb pan but you probably won't notice. The one looming problem is that there is no control of how much signal is sent to the reverb driver so you may end up with a situation where the reverb signal gets distorted even when the channel is operating clean. In that case, you could increase the 270K or install some other means to reduce the signal.

zapped · 04-21-2009, 06:53 PM

Hi, this is my first post on this forum, I'd like to wish you luck on your conversion of your PA 100. I got one for free some years ago and have used it as a parts source to build a Twin Reverb. It has been fun and rewarding but it is getting expensive. It was my first build and i was quite suprised that it worked. I'm presently trying to convert to a bias vary tremolo, that is not working yet. This looks like a great forum and i shall keep checking in.... Ed

03-31-2009, 03:48 AM	#2
loudthud Supporting Member Join Date: May 2006 Location: Texas USA Posts: 915	Those funny jacks short the signal of each channel to ground when the channel isn't being used (no input plugged in). The 270K that goes to V4B is the reverb send. The switch just turned the reverb on and off. Hook that 270K to the output of your preamp and you should have reverb. The Reverb Master is the only control. __________________ WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

03-31-2009, 06:18 PM	#4
loudthud Supporting Member Join Date: May 2006 Location: Texas USA Posts: 915	You could just leave the resistor where it is connected to nothing, remove it, or connect the free end to ground. When I mod an old amp, I like to use all the available knobs and then stop. With the PA100 you could build other different sounding preamps. Have fun! __________________ WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

03-31-2009, 09:59 PM	#6
loudthud Supporting Member Join Date: May 2006 Location: Texas USA Posts: 915	Grounding it will give you slightly less signal to the reverb pan but you probably won't notice. The one looming problem is that there is no control of how much signal is sent to the reverb driver so you may end up with a situation where the reverb signal gets distorted even when the channel is operating clean. In that case, you could increase the 270K or install some other means to reduce the signal. __________________ WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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03-31-2009, 06:18 AM	#3
01redcobra Member Join Date: Mar 2007 Location: Eastern PA Posts: 60	Just one more question. The other 270K that goes to V5A in the original circuit, what do I do there. Right now I have one input with 68K and 1meg resistor(Fender) running to v1a and then thru T-M-B pots to a 1 meg pot then to V1B and out of that to another 1meg pot to V5A.

03-31-2009, 07:49 PM	#5
01redcobra Member Join Date: Mar 2007 Location: Eastern PA Posts: 60	Thanks loudthud, I'm still considering making channel 3 a cleaner Fender channel, but still looking for ideas and leaving channel 4 alone. If I run that resistor to ground or leave it out altogether, what would be the difference in the sound?

04-21-2009, 06:53 PM	#7
zapped Junior Member Join Date: Apr 2009 Posts: 1	Hi, this is my first post on this forum, I'd like to wish you luck on your conversion of your PA 100. I got one for free some years ago and have used it as a parts source to build a Twin Reverb. It has been fun and rewarding but it is getting expensive. It was my first build and i was quite suprised that it worked. I'm presently trying to convert to a bias vary tremolo, that is not working yet. This looks like a great forum and i shall keep checking in.... Ed

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