Vox Pathfinder mods

[Rob Denton]

Just used some spare change to buy a 15 Watt Vox Pathfinder. Great amp for the price. Certainly got my money's worth. I wanted an amp that I could unabashedly ravage without worrying about the consequences. Cheapest and easiest mod was to slap a 3/4" piece of pine on the back to close it up and give it some warm and compressed-sounding bass. Swapped the 4558s out for 5532s but that didn't help the noise floor problem. I want to clean up the final output and smooth the transition from hot -to-not; or balls-to-the-wall vs nothing-at-all, or simply round off the curve before clipping. Yes, I know this isn't a tube amp. Any ideas?

[Robert M. Martinelli]

Hi Rob,
I own a Pathfinder 15R, and I agree it's great value for the money, still, I couldn't keep myself from performing some mods.

I replaced the original op-amps with 5532s ( and it did help in lowering the noise, are you sure your 5532s are not crappy chinese fakes? ) and added DIP sockets for a quick replacement, but as a matter of fact I did the major mods on the PS and PA sections.

I replaced the original PT with a toroidal PT, with a slightly higher voltage and a better current rating, then I changed the rectifier to a 4 Amp bridge, this allowed me to increase the filter capacitors' value ( to improve the dynamic range ). I also added voltage regulators for the preamp section ( to avoid early clipping on the op-amps due to voltage drops when the amp is driven hard ).

I substituted the output chip - seems to me it was a TDA 2030 - with a TDA2040 and changed the original 8" speaker, putting in a 10" Celestion I salvaged from a wrecked Cambridge 30.

Hope this helps

Best regards

Bob

[Rob Denton]

Thanks Bob,

THe 5532s I used were Texas Instruments. I couldn't get the hand picked "A" series, that might make a bit of a difference. I didn't put in sockets as I thought that might add to the problem. Frankly, I thought the new Op Amps cleaned things up a little but not a lot. I have some TL072s on order and some OP227s I'll try those as well.

I'll definitely try your PS and PT tips next. I've also been sniffing around for better power amp chips, the one in there looks like it's had a questionable background. I've been looking at things in the LM series (LM3875, 47XX). It really sounds to me like the noise comes from the final amp stage.

The VRs on the IC's is a great idea to smooth things out. I'll let you know how it goes.

[Jaybird]

I had a Pathfinder that went up in smoke, so I remade it into a 5F1 in a Vox cab. It sounds great and still has that nice Voxish look.

[Rob Denton]

Nice Pic

Impressive logo. I think I might end up doing something similar combining the Vox with a Blackheart Killer Ant.

[Robert M. Martinelli]

Thanks Bob,

THe 5532s I used were Texas Instruments. I couldn't get the hand picked "A" series, that might make a bit of a difference. I didn't put in sockets as I thought that might add to the problem. Frankly, I thought the new Op Amps cleaned things up a little but not a lot. I have some TL072s on order and some OP227s I'll try those as well.

I'll definitely try your PS and PT tips next. I've also been sniffing around for better power amp chips, the one in there looks like it's had a questionable background. I've been looking at things in the LM series (LM3875, 47XX). It really sounds to me like the noise comes from the final amp stage.

The VRs on the IC's is a great idea to smooth things out. I'll let you know how it goes.

Hi Again Rob,
The 5532s I used were indeed "5532AN" - as to the power amp section the LM3875 and 3876 are probably too much for the cabinet and speaker, you would also need much bigger a transformer, as the 3876 is rated at 56 W....
I would suggest you to go ahead with a TDA2050 which is rated at 28W and is pin to pin compatible with the original.
Let me know how the mods work
Best regards
Bob

[Rob Denton]

Oooops

Hi Bob,

Can you think of any reason why simply switching the 2030ah for a 2050v would cause the fuse to blow and the transformer to rumble?

[Robert M. Martinelli]

Hi Rob,
well, as I told you in my first post, I upgraded the PT ( because the 2040 and the 2050 obviously need more current ). The 2030 yields 15W RMS, the 2040 22 and the 2050 28 W, so I think it's normal for the fuse to blow and for the PT to "complain" and (g)rumble.

Once you decide whether to install a 2040 or a 2050 you' ll have to choose a new PT ( I installed a toroidal one ) according to their voltage/current demand.
I would go with a 40W PT for the 2040 and a 50W PT for the 2050. I would replace the speaker too, as the original is not capable to cope with the output power.

Hope this helps
Best regards
Bob

[Rob Denton]

Dumbo!

Well, Bob, I obviously need to go back to a re-learn some stuff that I thought I already knew. Thanks for the guidance and suggestions. I've decided to close the amp up before I do any real damage and open my textbooks for some basic refreshers.

[Robert M. Martinelli]

Well Rob,
I wouldn't be too hard on yourself...
from time to time we all make mistakes, and often that's the only way to learn, I'm not immune from making mistakes either; time ago I wrote a post with good advice but unfortunately wrong values ( because I was trying to do more than one thing at a time and I didn't put much attention on what I was doing ) and misled a member...nothing serious, but it gave me a chance to think very thoroughly about it...

Should you decide to go on with the mods on your Pathfinder I' ll gladly try to help.

Best regards

Bob

[Cney The MayStar]

Hey guys,

I know this is a somewhat of an older post, but googling for a nice mod of my pathfinder came up with actually only this post.

i got mine almost 10 years ago while wanting to start playing guitar. and now that ive moved came across it again and felt it had to have more to it.
ive already modded my squier fat strat via guitarnuts.com, great mods on there. you should check it out. put shielding in there, fender 4 wire humbucker with series/parallel push pull pod. and other capacitors.
sounds awesome now.

my question is if you could give me a somewhat more of a noobie discription of what and how to change and more importantly, specs of the parts needed.
this because im dutch and there could be some diferences in techtalk.
don't want to be lost in translation .

second of all. ive noticed that the cirquitry where the headphone out is located has more possibilities. i think that vox uses the same board there for other amps. i'd like to add a send/return or line out for recording. something like that. and and maybe also add a jack for the speaker out.

i hope you guys could help me out.
im on a learning curve here, so feel free to mail me.

[thirstyghost]

Update on Vox P15R mods (op amps)

I just wanted to update this thread for those that get a Pathfinder. I have a 9168 model made in Korea (not Vietnam) and the op amps are not all 4558's like in the schematic I found online. It's a different circuit. The online schematic shows 6 of the 4558's in use, but the amp I have uses 4, but only the one for the Reverb was a 4558. I replaced that with a 5532 and there was no noticeable benefit, especially regarding the buzz that so many report with this amp (reverb and master vol).

The others I recall were a 072D for the gain/volume in combination with some FETs that I can see, a CA30805 for the tremelo depth and a 062D for the tremelo speed.

So if you get a modern Vox P15, save yourself the trouble of trying to upgrade these opamps unless you really have time on your hands. For lay folks like me, it wasn't hard to get a low power solder iron and grounding wriststrap to do the job. But it seemed like a lot of trouble for nothing, other than a good chance to spray the pots, since I bought mine used.

These P15R's have that Vox sound, but some people can't seem to get past the noise that's worse in some rooms than others. Cool amp, but can be noisy compared to most small amps, especially if you want to record with them.

I may try placing some foil along the back top panel to see if some shielding will help. Hopefully won't make it worse with a loop.

[Robert M. Martinelli]

.....I replaced that with a 5532 and there was no noticeable benefit, especially regarding the buzz that so many report with this amp (reverb and master vol).....

.....So if you get a modern Vox P15, save yourself the trouble of trying to upgrade these opamps unless you really have time on your hands.....

I may try placing some foil along the back top panel to see if some shielding will help. Hopefully won't make it worse with a loop.

Hi,
If the problem is hum/buzz related, then you just can't expect swapping the op-amps to be a solution.

5532s have a lower En ( equivalent input noise voltage ) so they improve the overall "noise figure" of the amp; it's worth noting though, that we're talking about white/thermal/shot noise and NOT about buzz/hum/ripple, which is caused by other factors ( poor voltage regulation/poor shielding/poor decoupling/ground loops etc. ) and thus needs other actions to be taken.

I started modifying my PF by replacing the output IC with a TDA 2050, then I changed the PT and threw in a ( bigger ) toroidal one, ( fixing it to the chassis by means of a nylon bolt and nut ) I also threw in voltage regulators, put in bigger filter caps and a bigger rectifier bridge to better cope with the TDA2050 current demand. I also replaced all the 4558s with 5532s for the above reasons, and, last, but not least, I glued a "cut-to-measure" brass foil on the amp's back cover, to completely shield the circuitry from external noise sources.

The result is a hiss/hum/buzz free 28 W RMS Pathfinder with a tighter low end and a better dynamic response ( increased headroom ). The next step will be to replace the speaker, since the original one just can't cope with the increased output power. ( I'm thinking about throwing in a 10" in place of the original 8" ).

Cheers

Bob

[jayrope]

URL for schematics PF15R?

Hi all, i am a new member here, and owner of PF15R since today.

Your thread is very informative, thank you for this, could you point me to a download link for schematics for the PF15R eventually?

Can't find this on the Vox web site.

Thanx very much in advance, and greetings from Berlin

jrp

[Robert M. Martinelli]

Hi,
check this:

http://music-electronics-forum.com/t9222/

One of the links is about the PF15.

Cheers

Bob