Champ reverb schematic

[MickeyP73]

Hi everyone, I am planning to build a Champ Reverb. As this will be my first build from scratch, I am a bit hesitant to go forward without more experienced eyes having had a look at the schematic.

[img844]https://lh3.googleusercontent.com/-yQKA ... everb.jpeg[/img]

Most of it I copied from this post http://www.tdpri.com/forum/shock-brothe ... ocean.html and the original poster was succesful with his build. However, there are a few things I am unsure of and I am hoping you guys can help me with:

1. Below the last 1/2 12ax7 in the schematic before the 6v6 there is only a 1.5k resistor. Comparing this with other schematics and how the other tubes are wired up it makes me wonder if it shouldn't be wired parallel with a capacitor, and if so what value?
2. I've put an NFB on/off switch in the schematic, but I am worried about hum. Would it be better to put a switchable resistor in there instead of a simple on/off switch, or perhaps a pot? What values have worked for you the best in this case.
3. I understand using a choke like in a 5E1 could reduce hum, if so, how would I go about incorporating it as the circuit does look a bit different in that area? Or would it be best left as it is now.

Thanks for having a look at this.

[sliberty]

1. The capacitor you are referring to is called a cathode bypass cap. You have them in the first two stages of your schematic. They are not required in order for a tube stage to function, but they add gain and tone shaping. It's not unusual for a stage in an amp to go without one, and I think in this case it is just fine.

2. What makes you think that switchable NFB might hum? Larger Mps often have a pot, referred to as presence. Take a look at some Marshalls for examples.

[xtian]

3. The 400R 10W resistor is serving as your choke. A real choke may have better filtering properties. Single-ended amps often suffer from some hum at idle, so careful grounding and lead dress is especially important, starting with the relative placement of OT vs PT (use the headphone trick).

Welcome to TAG, and let me know if I should come visit Amsterdam to help you out!

[MickeyP73]

Excellent, thanks for the welcome, I didn't expect to get answers so quickly! You guys rock!

That make things much clearer, almost ready to order my parts. Just one additional question.

So I could just swap the 400R 10W resistor for a Hammond 194A 4 H 50 ma choke without any additional modifications?

And of course, if you ever need a personal guide in Amsterdam. Let me know

[sliberty]

I'd suggest adding another filter cap so that the choke is surrounded by filter caps. A 40uF before the choke (that would be new) and a 40uF after it (that one you already have in your circuit diagram) should work out well.

[MickeyP73]

I am totally amazed by the speedy replies. This is great!

Ok, seems like I am set to order parts, I'll put in the extra cap in the circuit together with the choke.

Thanks so much for your help! Don't forget to pm when you get to Amsterdam

[sliberty]

I spent 4 days in Amsterdam back in 2004 with some friends who had been there before. We went straight from the airport to Barney's (both storefronts). Just wonderful. Is Barney's still there?

[MickeyP73]

Would you believe that in the 22 years I've lived here I never heard of the place, even though it's practically around the corner. I had to look it up, and yes, you needn't worry, it is still here.

[pdf64]

So I could just swap the 400R 10W resistor for a Hammond 194A 4 H 50 ma choke without any additional modifications?

A 40uF before the choke (that would be new) and a 40uF after it

The lower resistance of the choke compared to the resistor (400 cf 167) and the CLC cf RC reservoir arrangement may result in the VB+ being higher.
So it may be beneficial to check for and, if necessary, accommodate that.

[MickeyP73]

The lower resistance of the choke compared to the resistor (400 cf 167) and the CLC cf RC reservoir arrangement may result in the VB+ being higher.
So it may be beneficial to check for and, if necessary, accommodate that.

This is where the lack of experience catches up on me. I searched the internet to get a better understanding of this but I seem to be out of my depths here.

Do you suggest to wire something like a 233 Ohms resistor in series with the choke to make up for the lower resistance of the choke? 167 Ohms resistance is mentioned on the datasheet for the choke. 233 and 167 would get me back to 400 Ohms.

I am not to sure what to do with "the CLC cf RC reservoir arrangement". Could you explain or point me to the right information so I can figure this out?

I have the book Designing Valve Preamps for Guitar and Bass, Second Edition coming in the mail to me (still have to wait 10 days), but until then I am mostly dependent on online resources unfortunately.

[sliberty]

You could deal with it that way, or .....

Did you have a target voltages in mind for the various nodes when you drew the circuit diagram? Were you shooting for the same voltages that some specific model of champ normally saw on the plates? If so, you should probably add them to your diagram.

Now, once you know what voltages you were hoping to achieve, my recommendation would be to wait until the amp is built, and see what voltages you measure at the various nodes. Then, the course of action for adjusting those voltages will be more evident.

Remember, the transformers you are using are not "real" champ transformers, and you are adding additional circuitry for the reverb. So at this point, you don't really know exactly what voltages you will end up with (well, I don't know exactly what voltages you will end up with, anyway). So, rather than try to solve a possible voltage problem in advance, I'd suggest waiting to see if you have that problem, and what the specifics are.

Just be prepared to replace or add some resistors in the power supply rail to make those adjustments.

[MickeyP73]

Yes I was aiming for 380 V DC at the first node with the 40/500 capacitor. I don't know if that makes a difference.

Okay, makes sense, I'll build it first and then see if adjustments are required.

[xtian]

CLC is capacitance, inductance, capacitance. Also called Pi filter.

[pdf64]

Yes, I think it would be really difficult to try and calculate what effect the changes may have; best just to be aware that they may have an effect on the VB+, and, when built, to check for / tweak to accommodate it, as required.

[MickeyP73]

the CLC cf RC reservoir arrangement may result in the VB+ being higher.

Ok, I figured out what you meant by this, and I think things should be fine as is (with the additional capacitor), it looks like I have some wiggle room.

One more question on the reverb section if I may.

BF reverbs are notoriously hard to control beyond a setting of 2-3. To tame the reverb, I put a 12au7 in the schematic, but I understand lowering the 1M "dwell" resistor to 820k for instance can work as well. I also understand the 12au7 may colour the tone differently. Even changing the value of the reverb mix pot may be an option.

I want to keep the Fender type tone in the reverb, and I'm not to worried about the cost of the tube vs resistor. So what would be the best option from a tone perspective to tame the reverb?
A. put in a 12au7
B. put in a 820k instead of a 1M as a "Dwell"-resistor.
C. change the value of the Reverb pot (any suggestions what value would work best here).
D. A combination of the above.