Reducing trainwreck clone gain?

[JimmyR]

Hi gurus

I have built a 'wreck clone which I like very much but it is a little too gainy. What I would love to be able to do is get more use from the vol pot, so the area between barely on and tons of gain is more like 0-5 rather than 0-2!

Currently the mods I have made to stock are:

- removed bright switch (Does anyone really need any more treble??)
- changed slope resistor from 100K to 33K - fatter
- changed the .0022 coupling cap between V1 and V2 to .0047 - like this one!
- added choke - BIG improvement IMHO.

The changes so far are getting me where I want to be. I don't care if it's not an authentic Trainwreck, I just like what I'm hearing and want to pursue it. The clean sounds are pretty tasty - they just don't last long on the dial and I'd like to explore what else I can get. I'm not a hi-gain player.

Will increasing the value of the cathode resistors help? As it is I have 1K5/25uF on the first stage, 2K7/25uF on the second and 10K on the third.

TIA!

[allynmey]

Jimmy, replace the 150K load resistor with a 56K resistor in series with a 100K linear pot. Dial it to the gain you want, shut off the amp and measure the total series resistance and replace it with a resistor that equals the value. Easy!

[Jackie Treehorn]

Change the tail/cathode resistors on the phase inverter from 10k/470 to 39k/1k. I would also reduce the feedback resistor to 68k since you would be reducing phase inverter gain.

There's still a ton of gain, but better cleans and a more useful range on the volume knob - more bass, less noise, etc. etc.

[JimmyR]

Thanks guys - I'll try your suggestions!

[rhinson]

you could also do a split load plate resistor set up on the second stage (ala komet and several vintage amps) ---ie.--instead of a single 100k plate resistor make it a 82k and a 22k in series and take the signal off the top of the 82k. will tame the amp a good bit and reduce the high end as well. you could make it switchable (ala komet) and this might let you reinstall the bright switch for some real chimey cleans with that engaged. rh

[JimmyR]

Hey Rhinson - that sounds like something I would like. Which plate resistor are you refering to? How do you wire it?

Things I have found so far: I changed the PI cathodes from 470/10K to 1K and 47K - didn't have a 39K resistor on hand! This increased the lows so much I could replace the .0022 coupling cap I had changed to .0047 and replace the slope resistor to 100K. I haven't changed the NFB resistor on the presence pot yet - it's sounding pretty good so far.

Next I swapped to 6V6s biased @ 27-ish mA - the lowest I can get without swapping out the bias resistor. Now it's sounding even better. Kinda warmer and chewier with not much reduction in volume. I am amazed at how well the 6V6s handle low-end in this circuit. I'm using JJs and suspect that they are only just handling the 410-420 plate voltage. No red-plating or anything though.

With a 335 it sounds like a tight, LOUD tweed Deluxe which can only be a good thing in my book. But I'm still finding all the action takes place between zero and 1 1/2 on the dial. Some fabulous tones there though. Great stuff! I just wish I could extend the clean-to-dirty zone a bit more - maybe a change of pot?

Thanks for the ideas so far!

[PRNDL]

>>all the action takes place between zero and 1 1/2
>> I just wish I could extend the clean-to-dirty zone a bit more

The Trainwreck has three preamp gain stages followed by a LTP-PI, which gives this a huge amount of preamp gain. For comparison, the Matchless Spitfire / AC15 has only one preamp gain stage, and it gets serious rock distortion at full volume.

It is possible to add a master volume after the second gain or phase inverter.

Removing the cathode capacitors (22u and 220u) will also reduce the gain, but may also increase preamp noise (or not).

I'm planning on putting in a bright/normal/bold switch into my latest amp design (Similar design, but only two gain stages). Bright adds the bright cap (for humbuckers). Bold adds a .68µF cathode cap across the first preamp stage. Normally the amp is a tad dark, which is fine for single coils.

>> V6s biased @ 27-ish mA with 410-420V
>> the lowest I can get without swapping out the bias resistor.

That's about as high as you can get, which means that your amp is biased hot, which isn't helping with clean headroom.

[JimmyR]

Thanks - I'll sort it.

[TheAmpNerd]

Or, take the simple route and start pluggin in 12AU7s and 12AY7s
until you find the right mix.

[Geetarpicker]

Or, take the simple route and start pluggin in 12AU7s and 12AY7s
until you find the right mix.

I did that trick on one particular cut on my CD where I needed a clean tele country tone. A 12AY7 in V1 and V2 will make a world of difference.

Heck I use the bright switch all the time on my original '89 Express, but then I make sure to get the volume UP on the dial (1/2 or more) so that it's not totally leaning on the bright cap for the tone. I actually prefer the tone of an Express with the bright switch on but with plenty of loading on the front with a long cord to roll off the highest harmonics. Lately I just solder a small cap inside the cord plug, sometimes as much as a 2500pf cap even on a 20' cord that already reads 700pf. That works well with a strat, but probably too dark for a LP.

Once you get the gain of the amp down somewhat you might find a use for the bright switch after all.

www.myspace.com/glenkuykendall (http://www.myspace.com/glenkuykendall)

[phsyconoodler]

Wy not change the volume pot in your guitar?Put in one with a different taper so it doesn't come on in a rush.

[AdmiralB]

The Trainwreck has three preamp gain stages followed by a LTP-PI, which gives this a huge amount of preamp gain.

I wish this myth would go away. The Express/Komet has comparitively LOW gain. Look at it - it's a BF Fender Bassman topology. All the gain developed by the first triode is thrown out driving (inefficiently via plate) the tone stack. The second triode recovers, and the third is a clipper circuit with a voltage gain of only about 7.

It has a lot of DISTORTION available. NOT a lot of gain.

[JimmyR]

Good point. That is what puzzles me - I am in no way an amp tech, but I'm good at building! I can't see where all the distortion is coming from. Why is this circuit so distorted? Excuse my ignorance!

[JimmyR]

BTW I AM running a mixture of 12at7s and 12ay7s in the preamp. 12ax7s are just way too distorted for me! But don't get me wrong - I love the sopund of the amp - just wish the graduation between good and evil was wider.

[AdmiralB]

The third triode is a clipper - it's biased REALLY cold, so that it is easily driven into cutoff, which is a type of clipping. Sort of the opposite of overdrive.

It's like the second stage in a Marshall 2203/2204, but it generates more distortion here because the signal level is (or at least can be) higher when it's hit.

[JimmyR]

So that could be my answer right there?

[AdmiralB]

Yeah, if you want to reduce it, you can put a voltage divider in front of it (the Komet does this), or maybe a trimpot. If you reduce the value of the cathode resistor, you'll reduce the clipping effect but you'll also increase the gain of that stage, hitting the PI harder.

[JimmyR]

Hey Admiral - by the time you replied I had already put in a 2K7 where the 10K goes to bias the second tube. I see what you mean! LOUD!!! And noisy. But the distortion wasn't as nice. So the 10K goes back in.

Thanks to everyone! I have tried some of everything that has been mentioned and even if I don't have the vol pot where I want it I do have a better sounding amp now. It sounds great. The cleans I am getting are beautiful and the neck pickup on my 335 sounds amazing - kinda Fender BF-meets-plexi + Matchless - whatever, it's a beautiful sound. I really like the 6V6s in this amp.

Thanks guys!

[Mike T]

I just wish I could extend the clean-to-dirty zone a bit more - maybe a change of pot?



If you lower the value of the pot the taper will will appear more gradual. I'm not sure what other affect it would have though.....

[Old Tele man]

...remove the preamp tube bypass capacitor...or make it incrementally smaller in value = less gain.

[Clutch21286]

Jimmy, Let us know the final changes you made after you have completed your mods.

FWIW-I have been down a similiar road as you but returned it to stock in the end.

[JimmyR]

Hey Clutch - this is where I'm at now. As you have found the stock values are pretty good. So the only areas where I'm not stock right now are:

- 10Henry choke instead of power resistor - best change I've made IMHO!
- No bright switch
- PS resistors from 10K/470 to 47K/1K
- 6V6 tubes biased at around 24mA

I may try a 250K or 500K vol pot next. But I am truly amazed at how fat but tight 6V6 tubes can sound in this amp. I didn't realise that 6V6s could produce so much chunk!

[AdmiralB]

Seriously - put a 250K pot in front of the third triode. Top to output from second triode, wiper to third triode's grid.

[Roe]

I just use two ecc81s

[Clutch21286]

Another option is to replace the first 1meg resistor into the PI.(right after the .1 input cap) with a 1 meg pot. If you wire it as a variable resistor you can dump some of the signal and not hit the PI so hard. I used this mod and it worked great and was transparent tonewise.

Hint: mount the 1meg pot in the extra speaker out jack. That way you can try it out and not have to drill any holes!

Good luck.

[Realfi]

BTW I AM running a mixture of 12at7s and 12ay7s in the preamp. 12ax7s are just way too distorted for me! But don't get me wrong - I love the sopund of the amp - just wish the graduation between good and evil was wider.

Aren't 12AU7's lower gain than 12AT7's and 12AY7's JR?