I did a repair on a Vox Pathfinder 15R last weekend where there was no reverb. Replacing the 4558DD fixed the problem. The op-amp out to the reverb unit was working but the op-amp input from the reverb had died.

This may be a common issue. Some of the other amps I've looked at use a 4560 op-amp pair instead of the 4558.

I managed to get my hands on a "fixer-upper" Pathfinder 15R with a couple of issues that required some resoldering and an op-amp swap (yes! An op-amp swap that actually fixed a problem!).

I was going to go germanium-clipping-diode on the Pathfinder but then thought that I was going to try the famous LED mod instead. The latter being an obvious misnomer since there are no LEDs post-mod.

I ran the amp through an external speaker with an attenuator. So I was able to max the gain while maintaining the volume at room temperature.

Anyway, I was surprised at the end result of the mod. The sound was pretty amazing: a very nice crunch with no fuzz/fizz. Wow!

The problem, I realized, was that you could only really get this nice sound if the gain controls were somewhat past 2 o'clock (with my Tele). Which made for a very loud amp if I were not using the attenuator.

So in typical Bluesky6 fashion, I changed the volume circuit:
1. Swapped out the linear taper volume pot for a similar value audio pot
2. Swapped out the 4.7K series resistor to the volume pot for a 470K. I know: it's high, but it was one value that gave me room volume with the volume pot set at 9 o'clock while the gain pot was at max
3. For good measure, I put in a 4.7nF tone bypass cap. I may yet swap that out for a slightly larger value

Once I had done this, I had reduced the main signal so much with respect to the reverb return signal that the latter basically overpowered the main signal. So I swapped one of the reverb resistors from 4.7K to 100K (after experimentation with different values).

The end result is a pretty simple amp with a very nice tone at reasonable volume.

The attached pic shows the changes I made. Apologies for the crude mouse-drawn tone bypass cap.

I didn't realize it but it looks like same Ebay seller who sold smackoj the "100W amp" PCB also sells the FutureKit 100W amp.

That same op-amp driving power transistor design is used in a kit by a US-based seller (xkitz). This kit is based around the TIP41/TIP42 pair and is advertised at "30W".

Although I may not agree with PRR's views on discrete audio amps, I would agree that the best way to achieve 100W in a home-built would be something using a couple of chipamps in parallel mode or better still, one of those Class D amplifiers that are also advertised on Ebay. The Vox VT80+ uses one of those, for example. I am also pretty sure that the ZT 200W shoebox amp uses the same approach.

For laughs, the attached picture is that of a transistor-based 100W stereo kit on my Ebay watchlist.

Notice the size of the heatsinks...

PRR sounds rather arrogant/ignorant with the following: "The days of making an audio POWER amplifier under 70 Watts out of separate parts are long-gone, and gladly. Make a necklace out of that PCB and buy a LM3886 kit off an asian eBay supplier. IMHO."

There may be a thriving industry around chipamps, gainclones and the like, but there are hobbyists out there interested in building audio amps with discrete components.

Google for John Linsley Hood or Class A.

There is a Thai manufacturer of electronics kits called FutureKit. They have a 100W kit (FK666). You can find a reseller on Ebay. Schematic found here: http://www.funnykit.co.kr/bemarket/shin/menual/fk666.pdf

I don't have that kit but have their 50W one built around a 2N3055/2N2955 pair. Just because when I was a wee lad, I used to dream about 2N3055-based power amps... and that's also for that reason I'm keeping the kit a kit ...

Quote from: Roly on May 20, 2014, 05:51:57 AM
Aside from agreeing with Enzo's precision lawnmower, the Frontman is the last circuit I would suggest as a first build project. 

^^^ agree x10000

The Fender SS pre-amp circuits are really VERY complex compared to some others that I've seen.

I would humbly suggest the Vox Pathfinder 10 circuit as a good starting point for a DIY build. It has a very simple PRE->GAIN->TONE approach with just 3 op-amps. The same design approach (including clipping diodes) are used in the Pathfinder 15R, SS Cambridge Reverb, AC15VR (with transistors) etc.

Quote from: TC5 on May 19, 2014, 07:55:28 PM
Now the weak point of this amp is the noise floor, which is higher than I would like

I had a Champion 110 which is the predecessor of the 25R and I agree that it's a noisy beast. I did a bunch of work to try to figure out where the noise comes from and narrowed it down to the circuits before the tone control. I finally gave up and sold off the beast.

On the other hand, I had really good results with a Fender Reverb Amp (predecessor of the 15R).  The judicious use of aluminum foil around the reverb unit, grounding wire from PCB to chassis etc resulted in near silent operation. Since I'm totally non-scientific, I can't say which exact change tipped the noise switch. Please look at my "NAD" thread for details.

Quote from: TC5 on May 19, 2014, 07:55:28 PM
So I've been pondering the idea of having a point to point version made of this amp using all top shelf components (clean circuit only, no hi gain section, aux in or headphones out). I would then put this circuit in a new chassis and cab.

It can be done, but it will be (a) tedious and (b) you will probably end up with a higher noise floor.

The plus point I can see with a fresh ground-up DIY approach is that you can encase the whole thing in a shielded enclosure which "may" help with the noise.

Here's a schematic of the mod.

I thought it was easier to hand-draw and scan than to learn LTSpice or some other "modern" tool.   xP

GE means Germanium diode.

The one thing I wanted to try out was to "improve" the OD channel. Like the Vox Pathfinder 10, I felt that the gain control was not able to provide a good range of crunch sounds. You basically went from silence to fuzz and fizz.

So the first thing I tried doing today was the same GE diode mod that I did with the Pathfinder: putting a GE diode in series with each of the overdrive LEDs. This did not help at all. I think because the Marshall used the diodes in the overdrive op-amp feedback circuit whereas the Pathfinder used the diodes in the signal path. Increasing the diode clipping voltage on the Pathfinder by adding in more diodes meant that the gain control had more play before you hit the fuzz.

Finally, I realized that it was effectively the gain of the op-amp that was the key factor in reducing the fuzz effect. Duh.

I changed R8, which is the input resistor to the op-amp and whose value contributes to the gain. The factory value is 560 ohms and I changed it to 5.6K.

The end result was a better range of crunch/overdrive sounds. Depending on the contour settings, I could go up to 12 o'clock on the gain before I hit the fuzz zone. And there was still sufficient play on the gain knob for heavy distortion and fuzz.

Pic shows the 3 changes that I made:
1. R49 for the volume on the OD channel
2. R8
3. Op-amp swap. I had a spare RC4559P so I ended up with that.

EDIT: I reviewed what I had done and realized that I'd most likely tested the original GE diode mod with the Contour set to 0. This would have meant fuzz all the way. Ugh! Looks like I'll have to tear the amp apart again next weekend.

The Celestion Eight 15 arrived today.

Pics to show differences compared to the stock Marshall speaker.

The Celestion has a larger cone surface area, shallower basket, larger diameter magnet, and a taller magnet "tower" (what's the right word for this?).

I used 7mm socket to undo the nuts and hand-tightened them on the new speaker. The latter was a somewhat tricky fit: if the bolts had been a little skewed, it would have been impossible to fit in the new speaker.

I'm going to do more work on the amp itself before putting everything together again and doing a full sound test.

I just managed to get back to the amp last night. Weekend was no-go because we had a whole day blackout in the neighborhood on Saturday (great to be living in the greatest country on earth...).

Swapped out the power rectifier caps. It helps with the sparkle/pop noise when shutting the amp down but not entirely. I may need to try the cap across primary on the PT next.

I also swapped out the volume pot for a 10K audio taper in lieu of the 5K and replaced R49 with a 22K. This immediately helped with usable volumes for playing in company.

After some staring at the schematics, it finally dawned on me that the speaker simulated out is really speaker simulated (unlike the Pathfinder 10) using U5A and U5B. If that is the case, a speaker swap may "interfere" with the intended design of the amp. In any case, a Celestion 8 Fifteen is on order and I'll try it out and report.

Next, I'll try out Phil's suggestions.

Quote from: g1 on May 02, 2014, 01:42:08 PM
  Well, the only good news is I tried to buy the domain name through the godaddy link and it said the "domain is not available".

There's usually a grace period after which the domain name goes up on auction.

Quote from: tonyharker on May 02, 2014, 04:28:51 AM
I see now that their domain name expired on 30/04/2014 and is pending renewal or deletion!!!

I saw that yesterday while trying to respond to a thread.

The site should have been on auto-renewal. That's what I do with my important domain names (actually singular nowadays).

The problem is that of email notifications. A lot of domain name owners don't pay attention to the email reminders to renew or the latter is dumped to gmail's overly excited spam folder.

Let's hope it gets back up soon.

Quote from: phatt on April 30, 2014, 07:29:21 PM
I had another quick glance at the Princeton as well as the MG15 and my immediate thought for MG15,,,(assuming I'm looking at correct schematic?

*Ed Whoopsy I meant C8 Not C10*
C10 is way too small at 10pF go much bigger. try 1nF or even higher to get rid of diode hash. some call this harshness sand in your dirt channel.
....

I thought that was a typo but thought you were referring to C1

Quote from: phatt on April 30, 2014, 07:29:21 PM

....

Also the two input circuits you mentioned are very different. The Princeton input has a high pass filter built into the very first stage which Fender use in a lot of there models.
....

Is that the circuit around U1A on the Princeton schematic? I noticed that this same (values may vary) design is used in the clean input stages in the FM65R, Fender Reverb aka 15R and Champion 110. My op-amp theory is too rusty/limited to help me understand the "dual" input implementation that Fender is using here.

Quote from: Roly on April 29, 2014, 10:26:19 AM


{be careful of redcircits and runoffgrove, several of their circuits have problems.}

Ah yes. The 80-20 rule of any "free" intellectual property: they get you 80% of the way there and you have to spend your own blood sweat tears and knowhow to complete the rest of the 20%. Speaking as a veteran of the open source software world.

It will be great if you can highlight dubious circuits you know of. Some of the runoffgroove stuff look very interesting but I would be severely challenged if I had to debug any one of them.

Quote from: DrGonz78 on April 29, 2014, 04:03:11 AM
Well I have done a mod a Fender Princeton 112+ that did a really good job. It involves of course changing the pot to an audio taper, but that did not really get it done.

I have a Fender FM65R which is a direct descendent of that Princeton and what I did was to build a "master volume" control/attenuator between the pre-out and the power in: put a 100K resistor in series with a 1.5K (after some experimentation), pre-out to one end of the 100K, ground to one end of the 1.5K and power in to the junction of the 2 resistors. This was very effective and didn't need me to pull the amp apart. And it remains virgin to my prying hands to this day...

Quote from: DrGonz78 on April 29, 2014, 04:03:11 AM
So I looked at the input of the amp and tried to increase resistance of the input signal just before the first op amp. This would R1 and R2 on the Fender amp and looking at the schematic for your MG15CDR it looks very similar of course. R1 is 10k and you could raise that to 20-30k and/or R2 (1meg) can be brought down to 750ohms/500ohms. The idea is that at the input of the amp R1 is resisting the guitar signal just a bit more. R2 then lets more of the signal bleed into the ground, thus reducing the signal a bit more. Both of these approaches have made more differences than just changing out the volume pot from reverse log to log.

In your case I get that you just want to increase resistance using higher value pot. However, my example is to show that there are other ways to get similar results.

Thanks. This is a good idea. Or perhaps just change the gain of the first input stage. I think the RC networks on the input also act as a high-pass/low-pass filter (if I understand an analysis of the Vox Pathfinder 15 circuit correctly), so any change in resistor value will have to be accompanied by capacitor changes or they will affect the freq response of the amp (to Phil's point). I'll experiment.