Help Me Wreck My Champ

[bcmatt]

So here is the thing. I have a clone of a Black-Faced VibroChamp that I built from a kit. It sounds decent and I have it in a great Cabinet that holds a good 12" speaker. http://bp0.blogger.com/_4u9l5q2yFBA/SCpoxkw6QRI/AAAAAAAAAZY/Qu9huKLHhcE/s1600-h/champ.jpg
For several reasons, I would like to sacrifice this amp for some experimentation. I've wanted to taste a bit of the Trainwreck styled amps and may try to build a clone of a Liverpool or Express at a later date. What I'm wondering is if I could get something voiced around that level of gain within this little Single ended 6V6 chassis, using the transformers that are in it and the 2 12ax7s. http://bp1.blogger.com/_4u9l5q2yFBA/SCpo90w6QSI/AAAAAAAAAZg/e7ckg1tZwhM/s1600-h/VibroChampCeriatone.jpg
Does anyone have any advice on turning this into a little SE Wreck of some sort?

There is a marshalised Champ on SEWATT called "the beast" that uses a single 12ax7 preamp tube, and a SS rectifier instead of the 5Y3. However, I'm looking for something not quite that High Gain, and more gradual like the Trainwreck clips I've heard. I'm assuming that the beast is too all of a sudden and in your face with distortion because there aren't many gain stages, making it much less articulate than the wrecks.
Here is an example of the Ceriatone Version of a TW Express (layout):
http://bp1.blogger.com/_4u9l5q2yFBA/SCpu60w6QTI/AAAAAAAAAZo/78RHwC75Uw8/s1600-h/Twreck-Xpress-Ceriatone.jpg
Of course, I am missing 1 tube socket for that 3rd 12ax7 to get the whole preamp in. (I plan to sacrifice my tremolo circuit for an added gain stage). I wouldn't need the phase inverter due to this being SE, but does that help enough? What can I realistically due to voice this champ more towards a Trainwreck? Is it worth trying?
Here is my blog post with all the pics on it, in case these pics didn't work:
http://yeomansinstruments.blogspot.com/2008/05/how-to-wreck-champ.html

[bagpipe]

Hey bcmatt. Sorry, I dont have anything to offer on the technical amp questions you asked. But I did wander over and read your blog. Great stuff, that Matchless DC-30 clone looks fantastic in the cabinet you guys constructed. I bet it sounds great too.

[bcmatt]

Hey bcmatt. Sorry, I dont have anything to offer on the technical amp questions you asked. But I did wander over and read your blog. Great stuff, that Matchless DC-30 clone looks fantastic in the cabinet you guys constructed. I bet it sounds great too.

Hey, thanks Bagpipe. I do love that DC-30. It is by far, my most versatile amp. The cleans are gorgeous, and the saturation is great too if I can handle that kind of volume, so it's good that it takes pedals real well too.

[sliberty]

I build an amp called a MiniWreck, and one of its configurations is with a SE 6v6 output stage, In order to get anywhere near the voicing of a TW, you will need 2 preamp tubes (the Express uses 1 1/2 stages of 12ax7 before the PI), and your Champ probably has only 1 12ax7, So you will need to find a place to install one more. Even then the level of gain that you can achieve without a PI is not the same as what you can achieve with a PI in a push pull amp. The voicing can be close, but the "feel" won't be there fully.

[bcmatt]

I build an amp called a MiniWreck, and one of its configurations is with a SE 6v6 output stage, In order to get anywhere near the voicing of a TW, you will need 2 preamp tubes (the Express uses 1 1/2 stages of 12ax7 before the PI), and your Champ probably has only 1 12ax7, So you will need to find a place to install one more. Even then the level of gain that you can achieve without a PI is not the same as what you can achieve with a PI in a push pull amp. The voicing can be close, but the "feel" won't be there fully.

Yess!! So it is possible! I know an SE version will lack some of the real Express feel, but, I think I would like to figure something out. Since it is a Vibro champ right now, that means that I already have 2 12ax7s hooked up. (one is currently used for the tremolo)

[sliberty]

Looking at an AA764 schematic, I'd say that it looks promising. The PT voltage is about right (315V vs. 300V for an Express), and the extra pots from the trem will allow you to add in the missing mid and presence pots. I wouldn't try to reuse the circuit board at all - just rip it out and build an all new one. Do you have room for a 10" speaker in there (with a new bafle)? If not, I'd conside moving the chassis to a head cabinet.

[bcmatt]

Looking at an AA764 schematic, I'd say that it looks promising. The PT voltage is about right (315V vs. 300V for an Express), and the extra pots from the trem will allow you to add in the missing mid and presence pots. I wouldn't try to reuse the circuit board at all - just rip it out and build an all new one. Do you have room for a 10" speaker in there (with a new bafle)? If not, I'd conside moving the chassis to a head cabinet.

Yes! This is good news! I currently have a 12" speaker in it actually. It's a homemade cab built by my father, with a strip of Purpleheart wood and using marine vinyl. If it seems better with different speaker combinations, I can make a new head cab instead for it.

[bcmatt]

So, this is what I have come up with so far for the layout:
http://bp3.blogger.com/_4u9l5q2yFBA/SC-9UEw6QVI/AAAAAAAAAZ4/EG2lHcdejYI/s1600-h/Lil%27+ChooChoo.JPG It is basically just me trying to replace the vibrochamp preamp with the Express preamp. However, You will notice that there are 3 loose wires and probably some mistakes. I am quite unsure of what exactly I should be doing with the "Presence" components, B+ connections, and the second half of the 2nd 12ax7s connections in general.
Anyone have any thoughts? I'd like to have a clear plan before I actually start butchering my champ.

http://bp3.blogger.com/_4u9l5q2yFBA/SC-9UEw6QVI/AAAAAAAAAZ4/EG2lHcdejYI/s1600-h/Lil%27+ChooChoo.JPG

[sliberty]

I haven't analyzed your layout, but here are some tips:

1. V2b is simply not used if you are replicating the Express preamp. That doesn't mean you can't use it though. Some use it to generate a little more gain to compensate for the lack of a PI. I didn't find it needed however.

2. Presence is negative feedback. it is a connection from the OT to a preamp stage (often the cathode of the last preamp stage). Look at the schematics for various Fender SE amps, and you'll get the idea.

3. You'll need either 3 or 4 nodes on the B+ rail. First for the plate of the 6V6, second for the screen, third (and fourth) for the preamp stages. Look at the Express schematic for the target preamp voltages, but keep your 6V6 plate and screen voltages in the range that they already since the power stage is very differerent from that of an Express.

I'd recommed drawing a schematic before drawing a layout. The schematic is much easier to analyzefrom a signal flow and circuit perspective. The layout is just the way you implement the schematic, and should be considered last. Just my two cents.

Steve

[bcmatt]

I'd recommed drawing a schematic before drawing a layout. The schematic is much easier to analyze from a signal flow and circuit perspective. The layout is just the way you implement the schematic, and should be considered last. Just my two cents.

Steve

Thanks, I'm realizing I should do that now first as well.
Thanks for the tips. I'll look at all this again in a few days.
Cheers.
Matt

[bcmatt]

Ok, I'm finally back; taking a few more kicks at my proposed project. I hacked away at the schematics and came up with this:
http://bp3.blogger.com/_4u9l5q2yFBA/SEcsB2DnlcI/AAAAAAAAAaI/Qe-FkgHw52A/s1600-h/SE+Wreck.GIF
http://bp3.blogger.com/_4u9l5q2yFBA/SEcsB2DnlcI/AAAAAAAAAaI/Qe-FkgHw52A/s1600-h/SE+Wreck.GIF

I don't know if this would work, but hopefully someone can tell me. You can see I ditched the presence control. Also, the component values coming out of the last 12ax7 cathode are the champ values and not the express values (even though the style of drawing might lead you to believe otherwise). Is this what I would want? Also, these B+ connections out of the first two 12ax7 plates; can they be connected like that (before the filter caps with those 100k resistors)?

Also, I'm guessing that I need to introduce myyself to the world of actually considering voltages. Is there a way to predict them in the design stage, or do you just tweak and measure them?

[bcmatt]

How would I know if I need to add another filter cap?

[mhammer]

Something doesn't look right. There is a 2700R negative feedback resistor shown from the speaker side of the output transformer, that leads back to a 47K resistor to ground. If you were looking for an amp that was great for bass, maybe. But that path works by taking post-transformer content and applying it as feedback to reduce bite. The more feedback is applied the more bite-reduction. If you think of the feedback resistor and the ground resistor as a kind of volume pot or voltage divider, the smaller the ground resistor is, compared to the feedback resistor, the LESS treble-taming negative feedback is applied because it is mostly bled off to ground. A 2.7K/47K pair will conserve most of the negative feedback - indeed more than any amp I've ever seen - and result in a serious shutdown of harmonic content.

In a Blackface Champ ( http://www.ampwares.com/ffg/schem/champ_aa764_schem.gif ) that pair is shown as 2700R/47R. I think that's what you should be aiming for. If you look at Fender amps that have Presence controls, like the Tweed bandmaster ( http://www.ampwares.com/ffg/schem/bandmaster_5e7_schem.gif ) or later-issue Pro ( http://www.ampwares.com/ffg/schem/pro_6g5-a_schem.gif ), you can see that the Presence control is essentially like the tone control on a guitar. That is, it is simply a variable resistance in series with a cap to ground to "bleed off" treble content in the feedback path. The gist is that the less negative feedback is applied (especially in the upper frequencies) the less suppression of harmonics there will be, and the more ragged and biting a tone you get.

I have one of these tweed Princetons ( http://www.ampwares.com/ffg/schem/princeton_5f2-a_schem.gif ) and have monkeyed with the 22k feedback resistor from the outpt transformer. Lifting it gives you icepick-through-the-head tone. Ideally, I shuld probably install something like the variable treble-bleed found in the schematics linked to above.

[bcmatt]

Something doesn't look right. There is a 2700R negative feedback resistor shown from the speaker side of the output transformer, that leads back to a 47K resistor to ground. If you were looking for an amp that was great for bass, maybe. But that path works by taking post-transformer content and applying it as feedback to reduce bite. The more feedback is applied the more bite-reduction. If you think of the feedback resistor and the ground resistor as a kind of volume pot or voltage divider, the smaller the ground resistor is, compared to the feedback resistor, the LESS treble-taming negative feedback is applied because it is mostly bled off to ground. A 2.7K/47K pair will conserve most of the negative feedback - indeed more than any amp I've ever seen - and result in a serious shutdown of harmonic content.

In a Blackface Champ ( http://www.ampwares.com/ffg/schem/champ_aa764_schem.gif ) that pair is shown as 2700R/47R. I think that's what you should be aiming for. If you look at Fender amps that have Presence controls, like the Tweed bandmaster ( http://www.ampwares.com/ffg/schem/bandmaster_5e7_schem.gif ) or later-issue Pro ( http://www.ampwares.com/ffg/schem/pro_6g5-a_schem.gif ), you can see that the Presence control is essentially like the tone control on a guitar. That is, it is simply a variable resistance in series with a cap to ground to "bleed off" treble content in the feedback path. The gist is that the less negative feedback is applied (especially in the upper frequencies) the less suppression of harmonics there will be, and the more ragged and biting a tone you get.

I have one of these tweed Princetons ( http://www.ampwares.com/ffg/schem/princeton_5f2-a_schem.gif ) and have monkeyed with the 22k feedback resistor from the outpt transformer. Lifting it gives you icepick-through-the-head tone. Ideally, I shuld probably install something like the variable treble-bleed found in the schematics linked to above.

Thank You MHammer!!!!
You have helped me properly understand negative feedback, as well as the possibilities of using a presence control. I had started to give up on a presence knob, thinking that they only work well with push-pull tube set-ups with a phase-inverter. I will try to add one back into the design. Thanks you for showing me those schematics.
Also, good catch on the 47R resister typo. I was copying the BF design on that part and for some dumb reason I put a K instead of an R and ended up multiplying the value by 1000 - not cool.
Thanks for the lesson. I'll apply my new learning to the schematic and repost.

[bcmatt]

Well, it was suggested that I should use my free triode for an extra gain stage to make up for the gain that the phase inverter would have added. Also, someone suggested that negative feedback might not be necessary or favorable in an SE amp. So, this is my latest result:
http://bp2.blogger.com/_4u9l5q2yFBA/SEi-3nqK5uI/AAAAAAAAAag/Y9WB074i6mM/s1600-h/4+stage+SE+Wreck.GIF
Am I getting closer to something workable?

[bcmatt]

I ran across this schematic, and it might actually be exactly the style of amp I am looking for. I've heard that it is an extremely versatile amp that an extremely gifted amp builder/designer/tweaker uses as his main rig.
The question is whether it works well with my transformers.

http://bp0.blogger.com/_4u9l5q2yFBA/SFE-kx8WNgI/AAAAAAAAAaw/t69nJRcDhw0/s1600-h/Latest.jpg

Interesting how the tone stack is after that last gain stage.

[bcmatt]

Ok, I need tips on my new layout...
I am modding my Ceriatone Vibro Champ into this schematic:
http://bp0.blogger.com/_4u9l5q2yFBA/SFE-kx8WNgI/AAAAAAAAAaw/t69nJRcDhw0/s1600/Latest.jpg
http://bp0.blogger.com/_4u9l5q2yFBA/SFE-kx8WNgI/AAAAAAAAAaw/t69nJRcDhw0/s1600/Latest.jpg

Here is my new layout that I tweaked in Paint over the last few days:
http://bp3.blogger.com/_4u9l5q2yFBA/SFLtaq_NlvI/AAAAAAAAAa4/j8Bbxdh9o5g/s1600/Classy.JPG
http://bp3.blogger.com/_4u9l5q2yFBA/SFLtaq_NlvI/AAAAAAAAAa4/j8Bbxdh9o5g/s1600/Classy.JPG

Any thoughts on what I am trying to do? Am I complete idiot? Apparently the Ceriatone OT is supposed to handle up to 15 watts. However, I have no idea what voltage this new schematic is supposed to run at. My PT provides 320V. So, I don't know if the transformers fit the schematic properly. I just know it is meant for the same tube compliment.

Am I ok to leave off the Standby switch in this way because I plan to keep the 5Y3 rectifier?
Also, any idea where that fluorescent green wire is supposed to connect to on my PT? The schematic shows it connecting to some tap between the 6.3V heater taps but only describes it as green/yellow.

[bcmatt]

Well, I finished making the layout from the schematic, and then ordered my new lectro-bits that I need to make this conversion this morning. Ima gonna do it. This is my layout; it's a bit messy, but it should work out better in real life. I think I'll put the hum balance pot in the back (replacing one of the speaker jacks).
http://bp2.blogger.com/_4u9l5q2yFBA/SGPNQ8nmdXI/AAAAAAAAAbQ/NJkTJNzndBA/s1600-h/Classy.JPG

http://bp2.blogger.com/_4u9l5q2yFBA/SGPNQ8nmdXI/AAAAAAAAAbQ/NJkTJNzndBA/s1600-h/Classy.JPG