Champclone, add a gainstage?

[Konny]

Hey Guys,

Iīm about to build a Fender Champ and I want to add some small tweaks.
Basically itīs a Blackface with the typical "tweedifying" mods. (Tonestack switch, NF-switch). I have also improved the filtering etc. but itīs still a Champ

I thought about adding a fenderstyle reverb, such as used in the princeton reverb. To do this Iīll have to add an ECC81 and an ECC83 but with one triode left from the ECC83.

So what to do with that triode? -thought about adding a boost to achieve some overdrive but I need some help with that because Iīm not sure if that works the way I want.
Howīd you do such a boost?

Thanks for your input,
Swen

PS: a schematic of the amp is in the attachment . . . and sorry for my bad german-english

[Rob Mercure]

You don't necessarily need to use two tubes to manifest your reverb - just because Fender used a 12AT7 with both sections paralleled to drive the reverb tranny doesn't mean that you have to. A power pentode/hi mu triode in the same envelope will save your having to add two tubes - something like a 6GW8 will do. The power pentode, or BPT can drive the reverb and the triode serve for the recovery stage. I seriously doubt that you'd notice any difference in the reverb tone.

Rob

[Konny]

Hi Rob,

I already thought about that but you wonīt find a stock-tranny that suits your need in points of impedance. The majority off reverb-trannys have an impedance of 10k on the primary. So I have to go with that ECC81-parallel-thing.

I think Iīll put that reverb volume pot. But Iīm still unsure what to do with the triode Iīve left...

Swen

[Rob Mercure]

Swen,

There is absolutely no difference between an output tranny and a reverb tranny - and many of us have simply used the best tranny we have on hand for a specific application. The input impedance of a Fender type reverb is usually around 8 ohms - a usual speaker impedance. And most small pentodes/BPTs will work well into 10K. Nothing I'm suggesting is original - I "stole" these ideas on this board, or it's predecessor, about 10 years ago. It's your amp but don't get hung up on factory schematics - many of us have cobbled the front end of one manufacturer's amp onto the output section of another's. For myself I've often added capacitively coupled reverbs to what are basically Fender designs but lately I'm trying to "spend down" my stock of small output trannies and low input impedance reverb tanks.

Rob

[chipaudette]

Hi,

What the other guys have been saying might be completely true and very useful to you (why add an extra tube if you don't have to). But, if you do end up adding the extra tube and if you do end up having half a pre-amp tube left over, you can certainly add it to your pre-amp for a little Mesa Boogie Mk 1 style gain. The downside is that your current schematic doesn't have enough volume control pots to make this extra gain usable. I did a similar mod to my Fender Deluxe Reverb Reissue (DRRI) and I found that I really needed to have three volume controls just like the Boogie does.

You need one volume control after the first gain stage, you need the second one after the second gain stage and tone controls (you've got this one), and you need the third control somewhere prior to the power amp section (a master volume).

The reason for the first and second volume knobs is that ,once you kick in the extra gain, you'll need to be able to control how much signal hits the subsequent tubes. You'll find that it doesn't sound right unless you can tweak the signal levels via the knobs. And, you need the master volume because your new high-gain signal is just way too hot and will make your amp unbelievably loud.

Sadly, No, you can't just use your existing volume control to dial back to volume after your signal has cascaded through the first two gain stages. To get a decent sounding pre-amp overdrive, you'll find that this volume control will end up being happiest when set between 5-8. Dialing it back to 1-2 to make your amp quiet results in a sad and unsatisfying sound. Dialing it up to 5-8 gives you that Santana kind of singing that is the point of doing this kind of mod. So, to keep from fully saturating your power amp, you'll need to add the master volume after your third gain stage but prior to your power amp.

So, if you're willing to add the volume pots, here's how I'd do it. First, you'll want add your gain stage prior to your existing circuit (see a Boogie Mk1 schematic). Have it setup just like your current V1a tube but consider using a coupling capacitor of 0.01 instead of 0.02 (go with what sounds better to you. I find that too much bass this early in the circuit sounds bad when at high gain...so I use a 0.01 cap to cut it back. After the coupling cap, add a volume pot to ground. Have the wiper of the volume pot feed into the grid of V1a. I use a DPDT switch to cut all of this new gain stage into and out of the circuit.

Then, I would add a master volume pot in place of your R11 (you can use a 250K pot instead of 1M pot if you want to more closely match your current value for R11). Furthermore, you could make that master volume a pull-switch pot like I did. This will enable you to wire it so that the pot is only active when pulled. That way, when you're not high-gain, you can defeat the master volume and have your old amp back.

That's it.

Don't forget that parasitic oscillation (uncontrolled squeeling sounds) often comes with high-gain setups. Be careful with your wiring and consider using shielded cable for wires servicing your new high-gain stage.

Chip

[chipaudette]

Hi Again,

Looking at your schematic, why do you have a cap at C2?

I note that you have a Fender Blackface style tone stack just after it (C3, 4, 5). If you use properly rated caps for C3, 4, and 5, they will properly block the DC voltage on your V1a anode so that you don't need a stand-alone coupling cap (the presumed reason for your inclusion of C2). I've never seen a setup like this (though I haven't really looked for it either). To my knowledge, Fender blackfaces don't have your C2. My blackface re-issue certainly doesn't.

Furthermore, I'm worried that C2 might actually be really messing up the sound of your amp. Because you've got C2 in series with your tone stack, the tone-stack capacitors will interact with C2 thereby changing all of their effective values. This means that the frequency cutoffs within your tone stack will be all wrong. The biggest risk is that the tone stack's natural mid cut could be way too high or way too low in frequency. If you like the sound, cool. But I would at least try it in the more standard configuration (ie, no C2) before deciding.

Does anyone else see what I'm seeing? Is what I'm seeing true?

My recommendation would be to check to see if your C3-C5 caps are rated to take 400-500V (whatever your supply voltage is to V1a) and then to remove C2.

Chip

[Bruce / Mission Amps]

Hi Again,

Looking at your schematic, why do you have a cap at C2?

...I'm worried that C2 might actually be really messing up the sound of your amp. Because you've got C2 in series with your tone stack, the tone-stack capacitors will interact with C2 thereby changing all of their effective values. This means that the frequency cutoffs within your tone stack will be all wrong. The biggest risk is that the tone stack's natural mid cut could be way too high or way too low in frequency. If you like the sound, cool. But I would at least try it in the more standard configuration (ie, no C2) before deciding...
My recommendation would be to check to see if your C3-C5 caps are rated to take 400-500V (whatever your supply voltage is to V1a) and then to remove C2.
Chip

Well, generally speaking , I agree, it isn't need.
C2 at 22nF is >"OK"< , it actually sounds fine most of the time... but it does reduce the over all bass response a small amount.
That can be a good thing in a small bore amp like this when it is cranked up high.
However, upping the C2 uF value to something like .1uF (100nF) or .22uF (220nF) will make it pretty much audibly invisible with respect to the effect on the tone stack and allow the use of smaller voltage valued caps in the tone stack if you want.

[Konny]

Hey guys,

thanks for your replies. That coupling cap indeed is a mistake. Happened while drawing the schematic, been there - done this, you know

Iīm still insecure what to do with the reverb, I think I will try both versions and then go for the best sounding.

So, you think an additional recovery stage after the reverb isnīt neccessary? You often see this in Fender amps e.g. the Princeton.
Question is if the signal gets too low?

Swen

[Steve A.]

Hey guys,

thanks for your replies. That coupling cap indeed is a mistake. Happened while drawing the schematic, been there - done this, you know

Iīm still insecure what to do with the reverb, I think I will try both versions and then go for the best sounding.

So, you think an additional recovery stage after the reverb isnīt neccessary? You often see this in Fender amps e.g. the Princeton.
Question is if the signal gets too low?

Swen

Hmmm... the AA1164 Princeton Reverb uses a recovery stage after the reverb. FWIW the Epiphone Galaxie 10 amp uses an extra cap ahead of the tone stack caps (which are rated for 630vdc)- I yanked it out of there because a common complaint is that the amp doesn't have enough bass. On a higher gain amp it can be slick to switch in the extra cap to thin out the bass a little bit.

After reading about them here for years I *finally* got a Holy Grail pedal. They work fairly well- much better than the solid state reverb you often find in modern amps. And they get nice and dirty, just like a real Fender reverb.

Steve Ahola

[Konny]

Hi Steve,

I know, this recovery stage is common on fender amps and thatīs what makes me wonder if I need it to.

The holy grail indeed is a good sounding thing but to me it makes to much noise, the hiss is unbearable to my ears
In general, I donīt like having too many pedals on the floor.

Swen

[Bruce / Mission Amps]

Steve go check out the Line 6 "Verbzilla" pedal.
I think it is the best reverb pedal I've ever heard... when using the '63 Spring' position.

http://line6.com/tonecore/verbzilla.html

I tell everyone with a tweed Fender or tweed clone to forget about wasting time trying to build reverb into it and just use this pedal for tweed with reverb!
Same should go for any amp with no reverb.