Working on a Blues Junior

Lyle Caldwell · 07-25-2011, 12:36 PM

Trying to make a friend's old Blues Junior into a good sounding amp. I'll post more later when I finish, as I don't mind "giving away" mods for inexpensive amps like this.

But for now, jeebus. I don't think there's a single rule of good design Fender didn't entirely negate here. Do they have a random circuit generator or something?

Got a lot of stupid to undo...

hipofutura · 07-25-2011, 12:59 PM

These amps severely suffer from an undersized cab and dog crap speaker. Definitely want to consider insulating it. I had one for a brief period and swapped several speakers. My favorite was an old JBL D120, but it all depends on the voicing one's after.

Lyle Caldwell · 07-25-2011, 05:16 PM

They suffer from worse than that. Take a look at the schematic:

http://ampwares.com/schematics/blues_jr.pdf

Look at the resistor in parallel with lugs 2 and 3 of the volume pot and then look at the resistor ahead of the master volume and then look at the value of the master volume pot.

See why this is such a weird weird thing Fender did? Meanwhile there's an unused triode...

Lyle Caldwell · 07-26-2011, 11:45 AM

OK, rather than try to fix the stock amp I just redid it.

Preamp is now the Bright side of a 5F6-A with the added cascaded gain stage from the JCM800 (this extra gain is completely bypassable - I repurposed the "FAT" switch), so the preamp goes from Clean Bassman to Marshallesque, though I shaped the "Marshall gain" for this specific amp (220K plate bypassed by a 1000pf cap for example).

Reverb is now pre-MV, and the MV is bootstrapped to the PI.

PI and output section are essentially JTM-45, though with the EL84s.

New Switchcraft input and speaker jacks.

Stock speaker was just nasty. Bright and nasal and boomy. I had a slightly used Vox/Wharfdale cheap clone of a G12H30 sitting in my closet for a few years, so I tried it in this. Great match. Best speaker ever? Of course not, but it sounds very good in this amp and the price is perfect (who doesn't like free?).

I beefed up the output tube screens from 100R to 1K but now the highs aren't quite where I want them. Sure, I could bring those back down to 470R or lower, but EL84s are hardworking little dudes and it's good to make their lives easier. I'll decrease the cathode bypass cap at one of the preamp stages and see if that brings the balance back where I want it.

Lurko · 07-26-2011, 11:57 AM

50k M/V? Ummmm.....okay.

I'll give them a break on the unused triode. That amp is not supposed to be a gain machine and it would just add noise into the circuit. (Although I guess they could've at least paralleled the two triodes in that tube.)

Lyle Caldwell · 07-26-2011, 12:05 PM

But that's the thing! They massively ramp up the gain, then they bring it way way back down with those voltage dividers. 130K in parallel with lugs 2 and 3 of the volume pot, then the 430K resistor before the 50K MV.

Insanely bad gain staging. They could have made a better sounding version of what they were aiming for with just two 12AX7s (one for preamp, one for PI).

Lyle Caldwell · 07-26-2011, 02:37 PM

OK, finished with the exception that I need to get two new chicken head knobs as I replaced two of the stock pots (bass and MV).

Added a presence control:

The stock Fender/Groove Tube EL84s hummed terribly. I put in a pair of Sovteks I pulled from an amp months ago, hum gone now. My friend should definitely replace all the tubes with something better though. The amp is worth it now.

Lurko · 07-26-2011, 04:19 PM

Okay, you know that in 2032 so dude is going to be on an Internet (or whatever replaces the Internet) forum going, "man, some IDIOT drilled a hole in the chassis; totally botched the resale value of this Fender classic!"

Lyle Caldwell · 07-26-2011, 04:35 PM

Nah, this is an ultra rare highly sought after Psionic Audio modded amp, back when people could afford them.

hipofutura · 07-26-2011, 05:26 PM

Quote:

Originally Posted by Lyle Caldwell

Nah, this is an ultra rare highly sought after Psionic Audio modded amp, back when people could afford them.

I remember, back in 2011 I think it was, when Caldwell was this up and coming amp designer. Who would have guessed?! In those days a guy named Dumble, or some such thing, was all the rage. You could buy a caldwell amp for next to nothing. Why, if I had kept the Psionic pedal, I could retire on it now.

07-25-2011, 12:36 PM	#1 (permalink)
Lyle Caldwell Senior Member Join Date: Dec 2010 Posts: 1,917 Thanks: 7 Thanked 186 Times in 41 Posts	Working on a Blues Junior Trying to make a friend's old Blues Junior into a good sounding amp. I'll post more later when I finish, as I don't mind "giving away" mods for inexpensive amps like this. But for now, jeebus. I don't think there's a single rule of good design Fender didn't entirely negate here. Do they have a random circuit generator or something? Got a lot of stupid to undo... __________________ Lyle Caldwell Psionic Audio High quality audio effects, professional amplifier services, and custom audio solutions.

07-25-2011, 12:59 PM	#2 (permalink)
hipofutura Senior Member Join Date: Feb 2009 Location: Maine Posts: 6,995 Thanks: 42 Thanked 346 Times in 72 Posts	Re: Working on a Blues Junior These amps severely suffer from an undersized cab and dog crap speaker. Definitely want to consider insulating it. I had one for a brief period and swapped several speakers. My favorite was an old JBL D120, but it all depends on the voicing one's after. __________________ Don Unencumbered by Knowledge Trapped inside this old man's body, is the mind of an old man www.dcsysinc.com www.seatrakinc.com

07-25-2011, 05:16 PM	#3 (permalink)
Lyle Caldwell Senior Member Join Date: Dec 2010 Posts: 1,917 Thanks: 7 Thanked 186 Times in 41 Posts	Re: Working on a Blues Junior They suffer from worse than that. Take a look at the schematic: http://ampwares.com/schematics/blues_jr.pdf Look at the resistor in parallel with lugs 2 and 3 of the volume pot and then look at the resistor ahead of the master volume and then look at the value of the master volume pot. See why this is such a weird weird thing Fender did? Meanwhile there's an unused triode... __________________ Lyle Caldwell Psionic Audio High quality audio effects, professional amplifier services, and custom audio solutions.

07-26-2011, 11:45 AM	#4 (permalink)
Lyle Caldwell Senior Member Join Date: Dec 2010 Posts: 1,917 Thanks: 7 Thanked 186 Times in 41 Posts	Re: Working on a Blues Junior OK, rather than try to fix the stock amp I just redid it. Preamp is now the Bright side of a 5F6-A with the added cascaded gain stage from the JCM800 (this extra gain is completely bypassable - I repurposed the "FAT" switch), so the preamp goes from Clean Bassman to Marshallesque, though I shaped the "Marshall gain" for this specific amp (220K plate bypassed by a 1000pf cap for example). Reverb is now pre-MV, and the MV is bootstrapped to the PI. PI and output section are essentially JTM-45, though with the EL84s. New Switchcraft input and speaker jacks. Stock speaker was just nasty. Bright and nasal and boomy. I had a slightly used Vox/Wharfdale cheap clone of a G12H30 sitting in my closet for a few years, so I tried it in this. Great match. Best speaker ever? Of course not, but it sounds very good in this amp and the price is perfect (who doesn't like free?). I beefed up the output tube screens from 100R to 1K but now the highs aren't quite where I want them. Sure, I could bring those back down to 470R or lower, but EL84s are hardworking little dudes and it's good to make their lives easier. I'll decrease the cathode bypass cap at one of the preamp stages and see if that brings the balance back where I want it. __________________ Lyle Caldwell Psionic Audio High quality audio effects, professional amplifier services, and custom audio solutions. Last edited by Lyle Caldwell; 07-26-2011 at 02:51 PM.

07-26-2011, 11:57 AM	#5 (permalink)
Lurko Senior Member Join Date: Jan 2011 Posts: 5,405 Thanks: 5 Thanked 188 Times in 59 Posts	Re: Working on a Blues Junior 50k M/V? Ummmm.....okay. I'll give them a break on the unused triode. That amp is not supposed to be a gain machine and it would just add noise into the circuit. (Although I guess they could've at least paralleled the two triodes in that tube.) __________________ Ich bin ein NERD.

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07-26-2011, 12:05 PM	#6 (permalink)
Lyle Caldwell Senior Member Join Date: Dec 2010 Posts: 1,917 Thanks: 7 Thanked 186 Times in 41 Posts	Re: Working on a Blues Junior But that's the thing! They massively ramp up the gain, then they bring it way way back down with those voltage dividers. 130K in parallel with lugs 2 and 3 of the volume pot, then the 430K resistor before the 50K MV. Insanely bad gain staging. They could have made a better sounding version of what they were aiming for with just two 12AX7s (one for preamp, one for PI). __________________ Lyle Caldwell Psionic Audio High quality audio effects, professional amplifier services, and custom audio solutions.

07-26-2011, 02:37 PM	#7 (permalink)
Lyle Caldwell Senior Member Join Date: Dec 2010 Posts: 1,917 Thanks: 7 Thanked 186 Times in 41 Posts	Re: Working on a Blues Junior OK, finished with the exception that I need to get two new chicken head knobs as I replaced two of the stock pots (bass and MV). Added a presence control: The stock Fender/Groove Tube EL84s hummed terribly. I put in a pair of Sovteks I pulled from an amp months ago, hum gone now. My friend should definitely replace all the tubes with something better though. The amp is worth it now. __________________ Lyle Caldwell Psionic Audio High quality audio effects, professional amplifier services, and custom audio solutions.

07-26-2011, 04:19 PM	#8 (permalink)
Lurko Senior Member Join Date: Jan 2011 Posts: 5,405 Thanks: 5 Thanked 188 Times in 59 Posts	Re: Working on a Blues Junior Okay, you know that in 2032 so dude is going to be on an Internet (or whatever replaces the Internet) forum going, "man, some IDIOT drilled a hole in the chassis; totally botched the resale value of this Fender classic!" __________________ Ich bin ein NERD.

07-26-2011, 04:35 PM	#9 (permalink)
Lyle Caldwell Senior Member Join Date: Dec 2010 Posts: 1,917 Thanks: 7 Thanked 186 Times in 41 Posts	Re: Working on a Blues Junior Nah, this is an ultra rare highly sought after Psionic Audio modded amp, back when people could afford them. rocknhorse1 and Lurko like this. __________________ Lyle Caldwell Psionic Audio High quality audio effects, professional amplifier services, and custom audio solutions.

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