70's Randall came in, did a re-cap on the preamp and replaced a noisy transistor,
had to trace schematic of preamp as no hits,
had to trace schematic of preamp as no hits,
Randall 300 Amp
- Thread starter CJ
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schemo,
amp has a rasp when turned up, must be the clipping diodes across master volume,
this of course is now used all over the place,
their intention was probably not distortion but to save the power amp,
same rare transistors used in some Echoplex versions, (from what i read,)
R11 goes nowhere, and not much diff between Ch1 and 2 save the .002 across the 100K and .01 across fet source resistor,
this amp sounds pretty cool with guitar, although i think it is supposed to be a Bass amp,
amp has a rasp when turned up, must be the clipping diodes across master volume,
this of course is now used all over the place,
their intention was probably not distortion but to save the power amp,
same rare transistors used in some Echoplex versions, (from what i read,)
R11 goes nowhere, and not much diff between Ch1 and 2 save the .002 across the 100K and .01 across fet source resistor,
this amp sounds pretty cool with guitar, although i think it is supposed to be a Bass amp,
Last edited:
CJ
Thank for posting this.
What value is the mid control? I am having a hard time reading it.
Also what tapers are the controls?
The volume control increases the JFET openloop gain when the wiper is cap coupled at the emitter and there is feedback from the 5uf and 2.2k resistor so the JFET needs to be selected if you want to keep the source resistor at 15k
I like to work out by hand or sim some of the older and newer solid state circuits.
The older preamps are often low parts count and can be used for guitar bass effects or DI with a cab sim or a preamp into a modern amp like a small class D.
The older VOX stuff is fun to study. The 3005 Marshall opamp circuit. Old Silvertone solid state stuff. Kay etc.
In the "the tube amp book"? there are some interesting solid state circuits.
Thank for posting this.
What value is the mid control? I am having a hard time reading it.
Also what tapers are the controls?
The volume control increases the JFET openloop gain when the wiper is cap coupled at the emitter and there is feedback from the 5uf and 2.2k resistor so the JFET needs to be selected if you want to keep the source resistor at 15k
I like to work out by hand or sim some of the older and newer solid state circuits.
The older preamps are often low parts count and can be used for guitar bass effects or DI with a cab sim or a preamp into a modern amp like a small class D.
The older VOX stuff is fun to study. The 3005 Marshall opamp circuit. Old Silvertone solid state stuff. Kay etc.
In the "the tube amp book"? there are some interesting solid state circuits.
Marshall did a great 100w transistor amp back in the 70s , I dug one out of the basement at a local music store a few years ago , the output transistors were shorted and hard to find , I emailed Marshall and they gave a modern equivalent part , I fitted them powered it up ,biased it and its worked flawlessly ever since .
It doesnt sound like a tube Marshall but it still has a very pleasing tone ,
It doesnt sound like a tube Marshall but it still has a very pleasing tone ,
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Flatpicker
Well-known member
Yes, those old circuits are interesting. I would be curios to see the power amp section.
Some friends of my sister's were going to toss a Peavey Pacer that appeared to be in decent condition. I ended up taking it, thinking it would be a good platform to convert to a tube amp of some variety, but after reading that these have a pretty good clean sound I decided to overhaul it first, replacing the electrolytics and etc., and give it a try. I took it apart and looks pretty clean on the inside, but that's about as far as I've got. The Pacer circuit uses an interstage drop-down transformer in front of the power transistors, so I'm wondering if it induces harmonics that contribute to the reportedly nice tone. Once restored, I'll compare the primary signal to the secondary signal and see if that's the case.
Some friends of my sister's were going to toss a Peavey Pacer that appeared to be in decent condition. I ended up taking it, thinking it would be a good platform to convert to a tube amp of some variety, but after reading that these have a pretty good clean sound I decided to overhaul it first, replacing the electrolytics and etc., and give it a try. I took it apart and looks pretty clean on the inside, but that's about as far as I've got. The Pacer circuit uses an interstage drop-down transformer in front of the power transistors, so I'm wondering if it induces harmonics that contribute to the reportedly nice tone. Once restored, I'll compare the primary signal to the secondary signal and see if that's the case.
Hi Gus! The middle pot is 2 K.
Do not know the taper, was in a rush but I emailed Randall for the schemo, hopefully the have something, cj
Do not know the taper, was in a rush but I emailed Randall for the schemo, hopefully the have something, cj
