Vibro Champ mods and B+ question

[AnalogPackrat]

I have nice 1977 SF Vibro Champ which I am in the process of cleaning up and modifying (slightly). I've had it since about 1995--great little amp, especially through a full sized cab (something I just tried for the first time last week--holy cow!). Yeah, I know there are amp forae around, and I've read some stuff there, but the brains here seem to have a clearer grasp on technical reality and less mojo mythos.

Here's the Schematic

I've replaced all the cathode bypass electrolytics (all but one were original) with new ones. After doing some math and auditioning several possibilities, I've settled on a 1.5uF film cap on the first stage cathode instead of a much larger electrolytic. This rolls of the bass ever so slightly which sounds better to me--that wimpy little output transformer is probably screaming in agony anyway. I swapped the 0.047uF tone stack cap for 0.033uF and replaced the 250pF ceramic with a silver mica (sorry, Gus, it sounded better to me). Then I slowed the vibrato down by replacing the 0.01uF at the grid of the oscillator with 0.02uF. Finally I'm taking the feedback through a 4k7 directly to the cathode of the second triode. I played around with this a lot--from original to no feedback and all ranges in between. This sounded best to me.

Another mod I found mentioned in several places is to increase the cathode resistor on the 6V6. If you look at it, Fender is really abusing that tube, and not just with raw plate voltage, either. Look at the schematic--nominal cathode voltage is 21V with 470R gives 44mA (minus a couple for screen) which is about 14W of plate dissipation (max spec). But in reality (today), you'll see well over 400V on the plate due to a transformer "upgrade" they did on the 70's as well as 10% higher mains voltage in the US. That results in over 50mA of idle current and 18-20W of plate dissipation :shock:

And that leads to my problem. The quick fix is to increase the cathode R to something more sane. I ended up with 860R (had some nice Beyschlag 910R 5% 2W in the stash) which brings me back down to around 14W again (37mA), but the reduced current draw means the B+ is hovering in the 438-440V range. I haven't replaced the filter cap can--it's the original Mallory 40-20-20 450V unit which seems to work fine and I really don't want it working so close to its rating after 30 years of service. Oh, and I'm running a 5V4 rectifier for slow turn on and higher forward drop already.

I'd like to drop B+ by 20-40V without A) screwing up the tone, 2) changing the PT, III) drilling any new holes. Would adding a 500R-1k 3W+ R before the first filter cap be a bad idea? What about adding a whole new C-R stage in front of the existing B+ filter? There's enough room inside to do this. Another friend suggested a choke, but there is no room to mount one--if you've seen these little chassis, you know what I mean. Is there anything I can do short of replacing the PT or upgrading the filter caps to 500V+ parts?

Thanks,
A P

p.s. I always was a B+ student :razz:

 

[adamasd]

2-5k would be to much, ohms law.

Your power tube is running at 37mA and you got a pair of 12AX7s in there for an extra 8 or so mA so lets just call it an even 50mA. Remember the entire current of the amp will flow through this resistor so you can not forget to add the 12AX7s into the equation. With a 2k resistor you would get V=2000Ωx0.05Amps=100Volt drop in B+.

20V/0.05Amps=400Ω, 470 is nice and common and gives you 23.5Volts drop.

Then figure your needed wattage, VoltageXCurrent

23.5x0.05=1.175watts, so double that for your minimum needed wattage rating.

 

[walter]

Are you putting the 4k7 negative feed back at the same place as the vibrato? just curious. So now you have a surplus B+, you could replace the 10k with something less, but then you may have to retweak the pre.

 

[adamasd]

you could replace the 10k with something less, but then you may have to retweak the pre.

That would not drop the voltage on the first filter caps, which is his goal. In the end 20 volts is not going to make much of a difference in the sound of the preamp. Mains voltage can vary enough to cause that much when the sun goes down and the entire neighborhood turns on all their lights. Here in the winter my mains goes down over 10 volts after the sun goes down and the lights go on/heaters kicking in.

 

[AnalogPackrat]

Adam--my bad, I meant 1K...will edit. Major brain fart. Total current is about 45mA at the moment. I would really like to get the B+ down to 400-410V.

Walter--yes, taking the NFB to the same node as the vibro modulation goes. Vibro still works fine. Now that I think about it, I suppose that may be why they did that funky divider on the cathode where the feedback connects--wait, the regular champ has it that way, too. Hmmm.

Yeah, I had also thought about increasing the 10k, but wanted to deal with the first stage voltage reduction first. I've got 396V after the 10k right now. Crazy.

In the mean time I think I'll rework the heater wiring which is done the sloppy way with one side of the 6.3V grounded with all the return current flowing in the chassis. And of course you can't twist the other side with the chassis for hum reduction, either. :roll: I'll do the 2x100R fake center tap thing. I read somewhere that it's best to reference it to the 6V6 cathode--does that make sense?

Oh, and would a 1-2k grid stopper on the 6V6 make any difference?

A P

 

[adamasd]

I would really like to get the B+ down to 400-410V.

You got the math now, you are set. Just remember to calculate the wattage being cooked off in the resistor. Also you will want to rebias the power tube after you drop the B+.

I've got 396V after the 10k right now. Crazy.

12AX7s are cheap and can take a fair amount of abuse. If it sounds good go with it, having to spend $10 once a year to replace a dead 12AX7 is not going to hurt you much, and most likely they will last quite abit longer then a year as long as you have a good 12AX7 in there.

I'll do the 2x100R fake center tap thing. I read somewhere that it's best to reference it to the 6V6 cathode--does that make sense?

It is not always best, but in this amp it will not hurt. In bigger amps you need to pay attention to the max heater-cathode voltage, in these Champs its a non issue. Just mount the 100 ohm resistors right on the power tube socket if there are no other parts in the way.

Oh, and would a 1-2k grid stopper on the 6V6 make any difference?

It could, the grid stopper resistor makes a highpass with the input capacitance of the tube to filter out things like thatl AM radio station down the street and keeps HF parasitic oscillations at bay. They also limit grid current to reduce the chances of blocking distortion. So if you like blocking distortion or want to play like Niel Young keep them out of there unless you have probably with AM radio in your amp or HF oscillations. If you add one in you want to mount it right on the socket to take full advantage of its effects.

 

[walter]

Oh my! I'm back from the bar and playing with high voltage. I am building an amp based on the champ but with a ss rectifier and an EL84 so this is all good info for me. The cathode voltage can be used to bias the heater AC, it keeps the AC from crossing zero. I doubt if any benefit is noticed. I recently worked on a silvertone that took dc from a power tube cathode resistor(s) for two pre-amp tube heaters. It seemed like an engineering feat, but not necessary. I may need to add a grid stopper to my amp, but why would you want to? I had to look that one up, and contrary to what I have read, I am sure a grid stopper does indeed affect gain. for example on the input, Input 2 has a grid stopper of 68k, Input 1 is 34k, and Input 1 is more gain than 2. So don't add resistance to the signal if you don't have to. Achoke! Ahchooo! Ahem! A Choke will not drop much voltage, so a resistor is the way to go. Adam has shown how to figger the value, good ol' math. I suggest using a new 500v can cap, go with the 470 ohm cathode resistor, or a readily available 600 ohm, and be done with it.

 

[Gus]

You could try a buck transformer at the line side of the amp. Look for vintage ac power at http://www.geofex.com/

Or a variac.

Is the fil voltage high with todays wall voltage?

Caps are a taste issue.

I wire the center of the fil supply to the output cathode resistor.

Small amps to a 4x12 can be cool

 

[AnalogPackrat]

Thanks for the extra info, guys. For the record, I am quite familiar with ohms law and basic electronics...just made a silly mistake. I guess I should have phrased my questions more clearly. Is there any downside (ther than heat/power wastage) to adding a resistor before the first B+ filter cap? Is there any advantage (or disadvantage) to putting a 10-20uF filter cap here to make the PSU cap input again?

Walter--after reading Randall Aiken's tech notes, I'm under the impression that properly sized grid stoppers don't affect gain. If you put them after the grid leak R, that is. Otherwise they form a voltage divider and reduce the input accordingly. The reason I asked about it is that I seem to get the classic "farting out" (blocking distortion) sound when I crank up the amp and play surf riffs (ala Dick Dale) on the low E string. I know it's not the speaker bottoming out (using a 12" with a big magnet in a large 2x12 cab) from the massive 6W output :razz: I'm going to try a couple of different values and see if it helps (after my wife finally wakes up).

Gus--thanks for that bucking transformer idea. I found it over at Geofex--never heard of it before. I could actually mount something like that in a hammond chassis and bolt it into the bottom of the cabinet. Might be the best solution of all. Let me see if I have a suitable transformer in my horde. Line voltage today is 119.5VAC, filament supply is at 6.5VAC

Adam--yeah, current production 12AX7's are cheap (like JJs), but NOS RCAs aren't. I also like to use 12AY7 and 5751 sometimes. I'd rather run the tubes a little closer to their spec and make them last. Yeah, Neil definitely likes the blocking distortion tone on his extended one note solos, doesn't he? :razz:

Cheers,
A P

 

[walter]

Of course, the voltage divider, I too have heard about ohms law. That's what I get for drunk posting. About the farting out, a thirty year old filter cap may be the cause. What is the heater voltage?

 

[adamasd]

Is there any downside (ther than heat/power wastage) to adding a resistor before the first B+ filter cap?

No mater where you lower the voltage you are going to generate heat and waste power, its what you do when you lower voltage. Weather you use the GEO transformer method, a resistor or a zener on the center tap you are wasting power. Just use an oversized part and be happy. 1.2watts is nothing for a 5 watt resistor to cook off, it will probably only get warm.

Adam--yeah, current production 12AX7's are cheap (like JJs), but NOS RCAs aren't. I also like to use 12AY7 and 5751 sometimes.

Its less then $10 a year with with good old tubes. I will bet you a new set of good NOS tubes that any decent fresh NOS tubes will get at least 5 years in that amp, probably a fair amount more.

filament supply is at 6.5VAC

Anything within half a volt is more then in spec for the tubes. While mains voltage was lower back then it was no more constant then it is now. The tube will die because of other reasons long before that extra 0.2volts causes any problems. I would go with the resistor and throw in an extra cap in front of it to take advantage of the cleaner B+ and quieter amp. Tubes are not 0.1% tolerance items, probably closer to 20% tolerance, we needn't worry so much about voltage being abit high.

I had to look that one up, and contrary to what I have read, I am sure a grid stopper does indeed affect gain.

It does, but you can choose the value of it to minimize that to the point it is a non issue. Our ears are stupid when it comes to amplitude, the difference between it in and out of circuit is never going to be noticed. Its like worrying because your 100kresistors are actually 99k, sure we can measure the difference but thats the only way we know its there.

A Choke will not drop much voltage,

I assume his friend meant to add a choke as a choke input filter, which will drop the voltage enough for his needs along while adding some regulation and better smoothing of the B+. The problem is (besides no room for a choke) at power up a choke input filter can have voltage peaks close to double what goes in it, so he would need to upgrade the caps, which is what he is trying to avoid.

Neil definitely likes the blocking distortion tone on his extended one note solos, doesn't he?

Who doesn't?

 

[AnalogPackrat]

I'm back from my latest round of mods. I started out messing with the 6V6 grid stopper. Went with an aggressive 15k first, but didn't like it. The amp seemed lifeless--no touch sensitivity. I swapped in a 3k9 and that did the trick. No blocking distortion but still plenty of tone and touch. One down.

Next I rewired the heaters which was a bit of a chore. Unfortunately that did virtually nothing to reduce hum. At least it looks nice now. Fender tended to do heaters last and kept them high in the chassis unlike what most builders do these days. I ran them high and tucked them under the back lip of the chassis.

Next up was the B+ change. I started by re-reading Roger's tweedish Champ thread (thanks for reminding me about that--old man forgot). Since I don't have a suitable bucking transformer in my stash, not a decent little box to put one in, I decided to try adding another C-R filter stage like Roger did. Unfortunately I don't have a lot of 450V+ caps in my stash. I ended up using a 10uF @450V Panasonic EB (105C). These are made for "high ripple long life" but they aren't very big. I mounted a 3 pin tag strip to one of the PT mounting screws with the cap on one side and a 1k2 5W sand resistor on the other. Rerouted the B+ wiring and gave it try.

Wow--the hum/buzz is almost all gone. Now I can hear the hiss at full volume. So I swapped out the first stage 100k plate resistor for a carbon film. Not much difference. Maybe its the 1M on the input jack (which is a right bitch to get at with an iron). I may mess with that today.

Now I have 396V on B+ and about 300V on the preamp supply. Much better. Of course now my 6V6 is idling with 27V on the cathode which is about 31-32mA and 11.6W dissipation. I'll probably decrease the cathode R to somewhere in the 600-700R range to get it back in the 13-14W area.

That little Panasonic cap worries me a bit. I'll probably order something else in my next Mouser or Digikey buy to replace it. I can't fit an Atom in there. Anyone have a recommendation for a radial high voltage cap? And if I wanted to replace the existing multi-cap can, where might I find one?

Cheers,
A P

 

[Gus]

Not radial nichicon VX that I bought from
http://www.angela.com/
measured OK for value and the ESR was OK(using a sencore LC102)

 

[pstamler]

[quote author="AnalogPackrat"]Wow--the hum/buzz is almost all gone. Now I can hear the hiss at full volume. So I swapped out the first stage 100k plate resistor for a carbon film. Not much difference. Maybe its the 1M on the input jack (which is a right bitch to get at with an iron). I may mess with that today.[/quote]

The 1M is probably not the problem unless it's drastically defective; it's being shunted by the impedance of the guitar pickups. A more likely culprit is the pair of 68k resistors, which (even at their best) generate significant thermal noise. Try replacing these with metal films, and if that doesn't help enough, try changing them to someething like 10k (change back if you start getting radio stations). Oh, and try a metal film on the 100k plate resistor too.

Peace,
Paul

 

[walter]

CE distribution has a good selection of can caps, you could get a quad can cap so it looks original. Have you scoped the filter section? Another way to identify a bad filter cap: connect a new cap to ground, and touch the positive lead to each filter cap, when the hum goes away, thats the bad cap. That's the hillbilly way. For the resistors on the input jacks, remove the jacks to work on them. But put them back in before soldering the new resistors so the leads are in the proper place.

 

[AnalogPackrat]

I replaced the 1M--it seems to be a little quieter. I'll try the 68k's next--thanks, Paul. Gus, those VX parts look pretty similar to the Pana EB. I guess I'll stick with the EB for now.

Walter--thanks for that CE link. They have everything--even some cool geekware! But the claim to only sell to businesses. Clicking the "consumer" link takes me to Antique Electronic. I guess I'll buy from them.

I don't have a 680R 2W+ on hand, so I paralleled a 2k 1W with the existing 860R 2W which gets me in the 600R range. Now I'm running about 13.8W total dissipation with about 39mA idle current. The B+ is down to 375 or so which is fine by me.

I ran it for about an hour playing my '64 Rick 375 and newer Rick 650C. Sounds really nice--much more versatile than before. Breakup is smoother and the blocking distortion is gone. After it cools down I'll swap out those 68ks on the input and button it back up. I think I'm done with this one. On to the Champ-12 and then the SF Deluxe Reverb :grin:

Thanks to everyone for the advice and assistance.

:guinness:
A P