Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Platefire on August 21, 2023, 10:31:06 pm
-
This is a 1975 Peavey Pacer Combo and Chassis I converted. I was checking it out today and it seems to be working like it should. I was looking at the schematic that I made back then and noticed after V3a .1 coupling cap there is a 47K to ground. Looking at my schematic and Leo's, that resistor to ground shouldn't be there. I would have to pull the chassis to verify if it's actually there, but pretty sure it is if it's on the schematic. Not sure what negative effect it would have on the circuit. I do want it to sound like a vintage DR AB763 since I never nabbed a real vintage one. So I welcome you to look over my shoulder at this Sch and if you see anything out of order, please let me know.
I did a little mod to the NFB to add a switchable 50K pot to be able to cut off the NFB completely or either dial in from 0 to 50K additional resistance on top of the 820 Ohm NFB resistor. Is this thing wired right, I can't tell hardly any difference in the sound switching it off and adjusting the pot?
OK, I have attached original Fender Sch plus my personal Sch for easy comparison. Thanks, Platefire
-
That 47K simulates the missing 50K INTENSITY pot. If you remove it the signal level will increase significantly and the amp won't sound like a real DR. Hoffman uses a 47K. I used a 100K master volume pot to accomplish the same thing....
https://el34world.com/Hoffman/files/Hoffman_AB763_1.pdf
http://sluckeyamps.com/tdr/tdr.pdf
If you set the NFB pot to zero ohms, then flip the switch on and off, you should be able to hear a difference.
-
OK, Thanks! I remember that now. Probably need to make a note on my sch. to remind myself. That 100K MV sounds pretty tempting because my DR is really loud. If you turn it up to get the "feel" then it's too loud in most cases.
On the NFB, you said turn the pot down to "0" ohms, then switch it off/on and compare the difference. Presently the switch is at the completely counter clockwise position at the full 50K position before is switches off. Pictures attached to how I got it labeled and wired. If it's wired
correctly probably the reason I can't hear the difference is the volume needs to be cranked up a bit to hear it. I'm playing quieter lately because of my hearing issues :dontknow:
-
On the NFB, you said turn the pot down to "0" ohms, then switch it off/on and compare the difference. Presently the switch is at the completely counter clockwise position at the full 50K position before is switches off. Pictures attached to how I got it labeled and wired.
That explains why you can't hear a difference. When the pot is at 50K you have ALMOST no NFB. Then the switch clicks off and you surely have ABSOLUTELY no NFB. Not much difference between ALMOST no NFB and ABSOLUTELY no NFB. You may as well not even have a switch on the pot. :icon_biggrin:
-
Yeah but it sure gives you such a warm and fuzzy when you hear that loud "CLICK"
Thanks for having a look at it sluckey! Don't be surprised when Hartly Peavey starts re-issuing the Pacer with this circuit :l2: