Mod 102 amp; Is the preamp clipping because of too high of signal going through?

[SpiderWars]

Looking back on this one of the first things I'd do is remove the cathode bypass cap on the second stage. It's a straight up Fender preamp but with a boost and turbo switch that feeds straight into the EL84 grid. The EL84 is being run at a pretty low voltage and it's getting slammed too hard.

 

[Shadsunscrash]

Looking back on this one of the first things I'd do is remove the cathode bypass cap on the second stage. It's a straight up Fender preamp but with a boost and turbo switch that feeds straight into the EL84 grid. The EL84 is being run at a pretty low voltage and it's getting slammed too hard.

I absolutely agree, i configured my P.T. to get a little more output to raise the voltages throughout the amp. I also got rid of that turbo switch as it was just completely at odds with every other mod that had already been done. In my case i added a 6V6 to my build so i can flick a switch to have the 6V6 or the EL84 so I wanted to keep that second stage bypass cap on v1B so i could still drive the 6V6 enough. And to counter the excess signal level hitting the EL84 i added back in the NFB loop on a switch as well as a switch to swap the first and second stage bypass caps and resistors to Marshall values. Im certain there were better ways to go about all this but in the end i only kept the mods i really felt improved the versatility of the amp and i still enjoy them after a year or two of use.

Thanks for having a look at the circuit for the other guy, it helped me and will likely help more first time builders when they inevitably come across the same issues.

 

[MadAsAHatter]

This was also my first tube amp build (id been restoring them prior but wanted to do a complete build and im always broke so this fit the bill lol).

So first ill say you arent alone i had the same issue. Only thing is i built mine with an extra tube socket for a 6V6 biased to function as it would in a stock champ and put each power tube on a cathode lift switch so you can select on or the other at will. And in the 6V6 configuration its much better but can still get a bit wooley if turned all the way up with humbuckers but in a endearing way not a, man this is shit way. I also nixed the "turbo" mod which is just basically an overly complicated RAW mod. In this amp with its current design you already have way too much signal and as someone else said EL84's are much easier to drive by the previous stages so its easier to overdo it.

What i did for the volume push pull is make it a high/low input control which i really like especially since we already had to much signal, not too little, very novice design choice indeed (as glpg80 so eloquently put lol). Another thing i did is add back the NFB loop on a switch and thats were a lot of the excess is coming from i believe. Without that negative feedback taming things the amp gets out of control real quick then when you add an eq defeat (the "turbo" mod) things get real stupid real quick.

Last thing i did was to add a switch to the back panel that swaps the cathode bypass caps and resistors on each preamp tube to the Marshall values. That cuts a lot of the low end thats coming through and makes the gain character much tighter. This amp was designed supposedly by a guy that is no longer working at Amplified Parts/MOD kits, at least thats what they told me was the reason they couldnt answer my more technical queries. This kit was literally a matter of some guy finding mods and changing the circuit with no regard to how it would actually sound with those mods interacting with one another. He or she clearly didnt know what they were actually doing. But its really cheap and still worth it for the right person.

But i honestly, and i really mean it, love this amp now that ive got it playing nicer with itself. And as others have mentioned already there are much better ways of taming this circuits shortcomings than ive even tried so i would give them a go cause its completely possible to get this amp sounding fantastic. I also should mention i did some changes to get my voltages up a bit higher as well and it probably wasnt necessary but it makes me feel better since they are closer to an actual champs voltages and still within safe margins that havent negatively impacted tube life, if fact im running the plate dissipation on the lower side for a class A cathode biasd amp because it sounded just as good like that so i figured i might as well go easy on the tubes.

Anyways sorry for reviving a months old thread. Im on my 2nd amp build (a Bassman clone thats also starting its life as a kit build cause thats the only option i can afford despite being capable of sourcing my own parts and building from scratch lol) and in getting prepped for that build i ended up seeing this post and thought id share my experience with it.

Thanks for the extra info. I like the idea of adding an extra tube socket for a 6V6 option. My Ampmaker N5X kit was designed similarly, but can use EL34's or 6L6's as well. Did you have to use different transformers than what came with the MOD kit or were they suitable for a 6V6?

There's probably a little more tweaking I could do, but for the most part I have everything under control in the amp. It can still get a little farty using hot humbuckers, having it fully cranked, and "Turbo" engaged with a 12AX7. I've been running it with a 12AU7 in the preamp. I like the way it overdrives better. There's more range on the volume before it goes full bore saturation.

 

[Shadsunscrash]

Thanks for the extra info. I like the idea of adding an extra tube socket for a 6V6 option. My Ampmaker N5X kit was designed similarly, but can use EL34's or 6L6's as well. Did you have to use different transformers than what came with the MOD kit or were they suitable for a 6V6?

There's probably a little more tweaking I could do, but for the most part I have everything under control in the amp. It can still get a little farty using hot humbuckers, having it fully cranked, and "Turbo" engaged with a 12AX7. I've been running it with a 12AU7 in the preamp. I like the way it overdrives better. There's more range on the volume before it goes full bore saturation.

Yes the 269ex was luckily capable of running the 6V6 as well so it was a fairly straight forward mod (at least it was once i worked out the values and what extra components id need to ad) and i highly recommend it if you ever decide you want to do something similar. Just let me know and ill detail the process in a reply. As for my previous comment about changing the bass cap as per the recommendation in a previous post on this thread i slept on it and had some time today to run the amp wide open and get a better feel for it with the other bass cap value. And after doing so i think the stock tone stack cap values are still the best configuration so i swapped them back and confirmed that stock values are indeed better even if the bass pot response seems less effective.

I should add that adding back the NFB loop to the circuit really helps tame the amp as well (to do that you just copy the negative feedback loop from the "Champ AA764" schematic since the "Champ Amp" schematic feedback loop wouldnt work with this circuit). I just couldnt get along with the turbo switch lol, it was just too much signal hitting the EL84, but i will say it was much better with the 6V6 so i can see why some guys recommend that on Champ builds that are part for part clones of a Fender Champ. But since you are running the 12au7 I imagine that makes the turbo switch much more usable. The mid boost mod and the bright mod on the other hand i love, especially when i have the NFB loop engaged.

As for using a 12au7 in the preamp I can absolutely see why you prefer that. Ive run several NOS 12at7's and 12au7's in mine on occasion and i actually really like the 12au7 a lot in this circuit for the same reason as you (more room on the volume control before you start to brake up) but i ended up going back to the 12ax7 because of the volume loss with the 12at7 and 12au7. But ill still put in the 12au7 every now and then when i need the amp a bit quieter. I recently put together a really simple 15 watt attenuator to use with the champ and its been great for getting the amp to the sweet spot without being excessively loud late at night. With just a few mods it really is a great little recording/practice amp.

 

[CPGearHead]

This amp just doesn't get old! I built the 102+ kit and found most of the same issues, very over-driven and bassy. I actually found this thread before I purchased and built it. To all who contributed, thank you, I found it well worth the time to read.

I had an extra demand on my amp, a friend recently gifted be an '06 Fender J Bass. After some strings and a setup, I found it very fun to play! So I wanted this amp to handle bedroom recording levels without heavy distortion. I think I found a unique combination that not only plays the guitar beautifully, but plays a mean bass too. Here are the changes to the displayed schematic on page 1:

V1 Grid resistor: From 33K to 43K
Bass Cap: From 0.1uF to 0.047uF
Treble Cap: From 0.047uF to 0.022uF (cap changes right on the thread's discussion)
Added 10K, 1/2 W (from kit) from the Bright Switch pin 5 to the volume pot's wiper.
Changed V2 Grid Leak resistor from 220K to 470K.
Changed V2 Grid Resistor from 5.6K to 10K.
Added an additional 680K c. grid leak resistor from c. grid of V2 (pin 2) to ground. Note: this effectively puts the original 220K in parallel with the 680K plus the 10K, which is 278K, very close to original. The new c. grid leak now has a direct tie to the grid's capacitance. Real nice tone effects!
Changed V2 cathode resistor from 150 to 120, 5W. Left the 47uF bypass cap, works well with the bass.
Changed V2 grid from 470 to 680, 1W

I had to lower the 10K resistor to 7.5K in the main power volt chain (B1 to B2) to get the amp to the voltage values on the schematic. That helped in a bit more head room. Mine was about 15VDC lower than listed.

On tubes, what can I say. It's like talking anout someone else's girl sometimes... I found the JJ's to have raised noise floors and were excessively harsh, not rolling into distortion smoothly. I also found the 12AX7 was just too hot. After trying a 12AU7, I ended up with the mid-gain 12AY7 and changed the tube to Electro-Harmonix and found that vastly superior! Especially the EL84, which is a Sovtek tube.

This is a keeper, it plays great through my 4-ohm 12" Katana50Ex speaker. I'm going to change out the output transformer to a 4 and 8-ohm outputs and have two speaker jacks. To anyone thinking about this amp, you won't regret it!

Edit: Bass plays great in Turbo mode, set bass/treble to taste. That's why I left the 0.047uF in there...

 

[Music&Chaos]

This amp just doesn't get old! I built the 102+ kit and found most of the same issues, very over-driven and bassy. I actually found this thread before I purchased and built it. To all who contributed, thank you, I found it well worth the time to read.

I had an extra demand on my amp, a friend recently gifted be an '06 Fender J Bass. After some strings and a setup, I found it very fun to play! So I wanted this amp to handle bedroom recording levels without heavy distortion. I think I found a unique combination that not only plays the guitar beautifully, but plays a mean bass too. Here are the changes to the displayed schematic on page 1:

V1 Grid resistor: From 33K to 43K
Bass Cap: From 0.1uF to 0.047uF
Treble Cap: From 0.047uF to 0.022uF (cap changes right on the thread's discussion)
Added 10K, 1/2 W (from kit) from the Bright Switch pin 5 to the volume pot's wiper.
Changed V2 Grid Leak resistor from 220K to 470K.
Changed V2 Grid Resistor from 5.6K to 10K.
Added an additional 680K c. grid leak resistor from c. grid of V2 (pin 2) to ground. Note: this effectively puts the original 220K in parallel with the 680K plus the 10K, which is 278K, very close to original. The new c. grid leak now has a direct tie to the grid's capacitance. Real nice tone effects!
Changed V2 cathode resistor from 150 to 120, 5W. Left the 47uF bypass cap, works well with the bass.
Changed V2 grid from 470 to 680, 1W

I had to lower the 10K resistor to 7.5K in the main power volt chain (B1 to B2) to get the amp to the voltage values on the schematic. That helped in a bit more head room. Mine was about 15VDC lower than listed.

On tubes, what can I say. It's like talking anout someone else's girl sometimes... I found the JJ's to have raised noise floors and were excessively harsh, not rolling into distortion smoothly. I also found the 12AX7 was just too hot. After trying a 12AU7, I ended up with the mid-gain 12AY7 and changed the tube to Electro-Harmonix and found that vastly superior! Especially the EL84, which is a Sovtek tube.

This is a keeper, it plays great through my 4-ohm 12" Katana50Ex speaker. I'm going to change out the output transformer to a 4 and 8-ohm outputs and have two speaker jacks. To anyone thinking about this amp, you won't regret it!

Edit: Bass plays great in Turbo mode, set bass/treble to taste. That's why I left the 0.047uF in there...

"I actually found this thread before I purchased and built it. To all who contributed, thank you, I found it well worth the time to read."

Isn't that cool?? There is a lot of good stuff to be found on here. Welcome to RT!

 

[CPGearHead]

"I actually found this thread before I purchased and built it. To all who contributed, thank you, I found it well worth the time to read."

Isn't that cool?? There is a lot of good stuff to be found on here. Welcome to RT!

Looking forward to digging in the site to see what jewels we have in here.. I wanted to add a quick note about my build, I didn't have to even think about NFB in the amp after I attenuated the signal. It's just a brilliant little amp running open loop! But, I'm going to have to check that out in the near future...