question about adding a 25uf bypass cap in parallel with a 2 uf bypass cap SF Champ

[catnine]

This is build I did of a SF Champ no vribrato. I see the bypass cap on the second gain stage pin 8 of V1 cathode bias is 2 uf on the SF champ . The SF Vibro Champ has a 10 uf bypass cap . So I used alligator clips and two jumper leads to clip the pos side of the 25 uf cap to the Pos side of the 2uf cap . Then jumpered the neg side of the 25uf to the switch and then back to the same 2uf ground before the 46 ohm resister to ground after the 1.5K bias resister.

It seems to be part of the NFB loop but the bypass caps only share the 46ohm resister to ground so I'm not sure if it is in the NFB loop . I heard a more focued bass and some mids . I wired it in the circuit . It works , the only issue is I get a very slight spilt second pop when the 25uf cap is switched into the circuit.

Is the slight pop normal? I was just trying to get a bit more bottom end and this seemed to get me near there. here is the champ schem .

[stormbringer]

The 25uF bypass will set the high pass filter cutoff at about 4Hz, whereas the 2uF cap will set it at about 53 Hz. So in the latter case, you are filtering out a bit of the very lowest signals going through the stage. This will indeed give you a tighter, more focused bass. Try going to 1uF while you are at it.

The pop is normal. I didn't quite picture your switch setup (SPDT?), but you need some path to ground (via a resistor) to keep those caps from discharging instantly when you switch. That would give you a silent switch operation

[catnine]

The 25uF bypass will set the high pass filter cutoff at about 4Hz, whereas the 2uF cap will set it at about 53 Hz. So in the latter case, you are filtering out a bit of the very lowest signals going through the stage. This will indeed give you a tighter, more focused bass. Try going to 1uF while you are at it.

The pop is normal. I didn't quite picture your switch setup (SPDT?), but you need some path to ground (via a resistor) to keep those caps from discharging instantly when you switch. That would give you a silent switch operation

It's a DPDT slide switch thats built into the chassis I got to build this SF champ home brew . I just used one side of the switch a center lug and outter lug so it's basically an off/on switch . I could add a 1 meg resister across the switch lugs to keep the cap charged up and that should stop the pop noise right?

I don't know why I would use a 1 uf cap , there is already a 2uf cap in the circuit stock value so with the switch I can have the 2 uf or the 25uf + to 2 uf in parallel .

I thought the 25 uf cap would not cut highs but just allow the lows to not be cut ie pass all the guitar frequencies.

[Chuck H]

I think it's worth noting that the "on paper" effect of the pass frequency for cathode bypass caps is misleading as it relates to audible perception. With a typical 1.5k cathode/100k plate/12ax7 preamp stage the bypass cap offers an extra 6db of gain. With that consideration it's easy to see that the -3dB point is a much less viable frequency accounting than it would be in, say, a +/- 20dB filter or something. The actual knee frequency is a much more audible measure for bypass caps. IMHE with stages as described above a 1uf boosts treble, a 2.2uf boosts treble and mids, a 3.3uf boosts treble, mids and low mids. Any value larger than that and you won't hear a very gratifying diffrence from the typical 22uf standard "full bypass" value.

To be clear about the switch pop solution... Just place a relatively large value resistor, like 100k, across the switch terminals. This keeps the cap from losing it's voltage relationship with the circuit but the series impedance for the resistor is so comparably large that it won't pass significant AC with the cap switched out. The pop will go away.

[stormbringer]

It's a DPDT slide switch thats built into the chassis I got to build this SF champ home brew . I just used one side of the switch a center lug and outter lug so it's basically an off/on switch . I could add a 1 meg resister across the switch lugs to keep the cap charged up and that should stop the pop noise right?

I don't know why I would use a 1 uf cap , there is already a 2uf cap in the circuit stock value so with the switch I can have the 2 uf or the 25uf + to 2 uf in parallel .

I thought the 25 uf cap would not cut highs but just allow the lows to not be cut ie pass all the guitar frequencies.

Ah, I read your post too quickly and thought you were starting with the 25uF and switching to 2uF. In which case I would suggest to try going lower. But that's irrelevant in your case.

[stormbringer]

I think it's worth noting that the "on paper" effect of the pass frequency for cathode bypass caps is misleading as it relates to audible perception. With a typical 1.5k cathode/100k plate/12ax7 preamp stage the bypass cap offers an extra 6db of gain. With that consideration it's easy to see that the -3dB point is a much less viable frequency accounting than it would be in, say, a +/- 20dB filter or something. The actual knee frequency is a much more audible measure for bypass caps. IMHE with stages as described above a 1uf boosts treble, a 2.2uf boosts treble and mids, a 3.3uf boosts treble, mids and low mids. Any value larger than that and you won't hear a very gratifying diffrence from the typical 22uf standard "full bypass" value.

To be clear about the switch pop solution... Just place a relatively large value resistor, like 100k, across the switch terminals. This keeps the cap from losing it's voltage relationship with the circuit but the series impedance for the resistor is so comparably large that it won't pass significant AC with the cap switched out. The pop will go away.

Good point. So in a typical Fender circuit, might it be useful to replace a 22uF 100V cap with a 3u3F or 4u7F non-polarized cap?

[Chuck H]

Good point. So in a typical Fender circuit, might it be useful to replace a 22uF 100V cap with a 3u3F or 4u7F non-polarized cap?

Sure. And I have. I will say that for clean tones the larger full bypass value of 22uf does seem to affect the tone a little. But mostly the way the amp "feels". Probably because all the lower frequencies, that you can't hear, are still using the amps available current and so affecting how it reproduces frequencies in the audible spectrum. But that's just a theory.

[catnine]

I really didn't think the added 25 uf bypass cap was going to make much difference yet by adding it the the 2uf that was already there in parallel it did change the tone enough to solder the cap in with the switch . I couldn't hear much difference just using alligator clips to hold the 25uf bypass cap in place but once I connected it to the switch with jumpers then I could hear the difference being able to switch it in and out . It did bring out the bass and some middle that was not there before. I can't explain it and I have been thinking about this and looking for info on this . It may be one way fender used a smaller bypass cap in the second gain stage to cut just enough lows as not to tax the cheap stock 8 " speaker they used on most all SF champs . The stock speaker flabbed out real easy . now replace the stock speaker with a weber sig 8S alnico and retain the 2 uf bypass cap and the bottom end held in there , but use a 25 watt 10" speaker and for some reason the 25uf does well , at least better than with the 2uf cap .

[Chuck H]

No rules. Guitar amps are instruments. Just like guitars, picks (or fingers), pedals, cabinets, etc. WRT electric guitars and their amplification, it doesn't usually have anything to do with reference. It's more about signal processing!!! Whatever gets you there is good. Two paths to this end are typical. One is the artist that can find something musical about whatever they use. The other is the artist that manipulates the gear to recreate the sounds in their head. I, as well as most of us here, am in the latter. Most famous guitar players are the former. But not all! The best experiences for me are in practicing a little of both. Results may vary. Tweak and rock on.

[catnine]

No rules. Guitar amps are instruments. Just like guitars, picks (or fingers), pedals, cabinets, etc. WRT electric guitars and their amplification, it doesn't usually have anything to do with reference. It's more about signal processing!!! Whatever gets you there is good. Two paths to this end are typical. One is the artist that can find something musical about whatever they use. The other is the artist that manipulates the gear to recreate the sounds in their head. I, as well as most of us here, am in the latter. Most famous guitar players are the former. But not all! The best experiences for me are in practicing a little of both. Results may vary. Tweak and rock on.

No rules. Guitar amps are instruments. Just like guitars, picks (or fingers), pedals, cabinets, etc. WRT electric guitars and their amplification, it doesn't usually have anything to do with reference. It's more about signal processing!!! Whatever gets you there is good. Two paths to this end are typical. One is the artist that can find something musical about whatever they use. The other is the artist that manipulates the gear to recreate the sounds in their head. I, as well as most of us here, am in the latter. Most famous guitar players are the former. But not all! The best experiences for me are in practicing a little of both. Results may vary. Tweak and rock on.

I am not much of a tweaker really . I don't know that much about electronics other than trial and error . I would have prefered to keep my champ build stock and that was what my goal was to build an amp to replace the 73 SF Champ I had to sell .

There is no way to duplicate an original champ and each one I had was a bit different . I had a 1980 model and a 1968 model at the same time yet one sounded better than the other . They were both stock and sounded fine to me and I didn't have an amp since the mid 70's until I got the used 1980 SF Champ I got from a friend cheap in 1986. THen when I finally had some money in 1994 I got a tweed fender bronco thinking it would be great , they called it tube emulation but it lacked something (tubes) then I got the 1994 tweed Pro Jr when they first came out yet most of the time I used the Champs since living in an apt I could get a good sound without them being cranked up even with the stock speaker.

I'm 64 and have been playing since 1963 and had silvertone amps and then fender bandmasters 1968 and then a 1967 fender super reverb since the 68 bandmaster was something I got because the other members in my band back then had more money and traded their silvertone 2 twelve piggy back amps in to bandmasters and a bassman but the bandmaster didn't have reverb and was to clean for my liking.

At the time way back then I didn't know a thing about amps just how they sounded to me. One day I had this insane idea that I needed a large amp and since I was always in some music shop saw the 1994 tween fender blues deluxe , well now we're talking 40 watts and 6L6's and reverb so I traded all of the 4 small amps I had for the deluxe plus a small amount of cash , Well I went by ad's and music store promotions not thinking a 40 watt amp was far more than I needed so I sold it in 2003 and got a 77 music master bass amp and recapped it and played it and kept it then shortly after I got the 73 SF Champ and also recapped it and then knew about speakers because by that time I by then I had the internet and found a forum dealing with fender amps and learned what I needed to do to an old amp to keep it going . This lead me to weber speakers and to other speakers and then to build my own amps .

My point in all of this is . In order to recreate the 73 champ I would need the old tranny's and old tone caps and the reading on everyu resister and voltage in that amp . So I came close but not close enough in this build I have . I did get replacement Champ trannys and all the same types of caps and resisters and the amp is close and this is what brought me to the tweaking simply because the 73 champ even with a weber sig 8s alnico had a better bottom than mine does with the ten , Perhaps If I built the champ the same way using same chassis and built the cab just like the 73 had and used a weber sig8s alnico it would be closer but who can say . it's juat what I did and had to work with at the time . I still feel that a better 10" speaker voiced more into the bass frequencies would make more difference than any mod I can come up with . And one day funds permitting I will seek out that speaker so far it looks like another weber alnico or a jensen MOD 10/50 might do the job. A larger OT may help yet if the speaker is not up to it then what point is a larger more expensive OT going to help.

[jazbo8]

I think you raise a good point (as others have in the past as well) - in order to do a true clone of the original, it is very difficult using modern components - especially when it comes to transformers and speakers, that's why the original parts are so sought after.

[catnine]

I think it's worth noting that the "on paper" effect of the pass frequency for cathode bypass caps is misleading as it relates to audible perception. With a typical 1.5k cathode/100k plate/12ax7 preamp stage the bypass cap offers an extra 6db of gain. With that consideration it's easy to see that the -3dB point is a much less viable frequency accounting than it would be in, say, a +/- 20dB filter or something. The actual knee frequency is a much more audible measure for bypass caps. IMHE with stages as described above a 1uf boosts treble, a 2.2uf boosts treble and mids, a 3.3uf boosts treble, mids and low mids. Any value larger than that and you won't hear a very gratifying diffrence from the typical 22uf standard "full bypass" value.

To be clear about the switch pop solution... Just place a relatively large value resistor, like 100k, across the switch terminals. This keeps the cap from losing it's voltage relationship with the circuit but the series impedance for the resistor is so comparably large that it won't pass significant AC with the cap switched out. The pop will go away.

One this added resister . say I use a 100k across the switch and the switch is off so the added 25uf cap is not in circuit and the 2uf is always in circuit won't the added 100k add to the cathode bias 1.5K and alter the bias of the 12ax7 cathode? Would the bias resister then be 1.6K? Or will the added 100K not alter the bias enough to notice or make a difference. That's basically what you said , right? What if I went higher to say a 1 meg or larger resister? I assume if I switch out the NFB loop which I sometimes do all else remains the same .

Bottom line . If I don't add the resister will switching is the added 25uf cap dischanged hurt anything because the pop is not loud at all. I just don't want to slam the bias while charging the cap and I don't want to remove the original 2uf bypass cap in case I want it stock again.

[Enzo]

With a resistor across the switch, and the switch open, that resistor is in series with the cap. No DC flows through it. It won;t upset any bias voltage therefore.

But just for science, pretend there was no cap, and you put a 100k in parallel with the 1.5k. Calculate what the new total resistance would be. Go ahead, I'll wait...


I got about 1.48k - 20 ohms lower. That is a hair more than a 1% difference. So even without the cap, that parallel 100k won;t do much. But it will stop your switch from popping.

A larger resistor like 1meg or 470k? Go ahead. I usually use 1meg resistors for pop suppression in these cases.

The pop won;t hurt a thing, the currents through this circuit are tiny. It just sounds bad to have it pop.

[catnine]

With a resistor across the switch, and the switch open, that resistor is in series with the cap. No DC flows through it. It won;t upset any bias voltage therefore.

But just for science, pretend there was no cap, and you put a 100k in parallel with the 1.5k. Calculate what the new total resistance would be. Go ahead, I'll wait...


I got about 1.48k - 20 ohms lower. That is a hair more than a 1% difference. So even without the cap, that parallel 100k won;t do much. But it will stop your switch from popping.

A larger resistor like 1meg or 470k? Go ahead. I usually use 1meg resistors for pop suppression in these cases.

The pop won;t hurt a thing, the currents through this circuit are tiny. It just sounds bad to have it pop.

Thanks Enzo . My mistake , I wasn't thinking the resister would be in parallel and since the cap is so would be the resister . I just wasn't thinking clear. I had to draw it out to understand it , now I see there is no way DC can pass through a cap just like a coupling cap blocks DC. I try to do these things in my head which is not the way. I knew how to wire the cap in without drawing it out .

[catnine]

I think you raise a good point (as others have in the past as well) - in order to do a true clone of the original, it is very difficult using modern components - especially when it comes to transformers and speakers, that's why the original parts are so sought after.

I agree . You can come close but I don't think the PT's and OT's are made quite the same . I have seen some made in the USA yet they are expensive but not the same thing. The speakers too none one makes the old fender speakers like Jensens and the like. Yes prehaps Leo was buying in bulk yet like anything else they used to build things better way back then .

If I had my 73 SF Champ to compare to this build then I might fine the balance but I can't recall how it sounded and I had three SF Champs one from 1980 and another 1968 and the 1973 but even the 68 and 80 sounded different and I had them at the same time. The only difference I saw was the older one had a 2 wire plug .

I still feel a 10" speaker that is focused more on the bottom end would help yet I don't have the funds to go buying speakers and to find out the new one didn't change things much. I keep thinking of the jensen MOD 10/50 because at least it is described as a more focued low end and good mid and smooth highs were the C10R only talks about highs . But then it all depends on what amp is driving it.