Vibro Champ capacitor questions

[echuta13]

The Vibro Champ came out in 64 so technically yes. I think the mods they made came in much later though (when CBS started cost-cutting and the wiring got more sloppy).

[Chuck H]

Very good then. I've never seen a pre CBS schematic for the VC. But I didn't really pay much attention. The ones I worked on were both silvers and had pretty straight forward wiring. It's such a simple amp. I don't recall anything grossley CBS about them. Not like any of the late era silver xxxxx Reverb amps.

[stokes]

I believe the Vibro is one of the only amps CBS didnt bastardize.Vito,of course your voltages are going to be high.The original was designed when wall voltages were 110 vac.Being over 120vac nowadays would explain what you see.Your ripple voltage readings look fine.The preamp filter is mainly for decoupling purposes not ripple filtering,do not increase the value here,it will adversely affect the tone.As I explained earlier,grounding the preamp filter in the same place as the main and screen supplies is not good.Get a single 20uf and replace the can connection.Make sure you ground this new cap down by the other preamp grounds,that 120 cycle hum should be gone.Also find your heater supply ground reference,either a CT on the winding or 2-100 ohm resistors,one on each side of the heater supply and lift the ground and connect it to the cathode of the power tube,pin 8.DO NOT replace that cathode resistor on pin 8 of your power tube with a 2 watt,you need a 5 watter here.It gets hot,thats normal,a 2 watt here will fry.I wouldnt try lowering voltages with the RC network you describe.There is nothing wrong with running the voltages you see.everything in your amp is designed with a +/- 5% to 20% tolerance.There are many of these amps out there still working fine on todays wall voltages.I have quieted many Champs over the years exactly as I have described here.

[Chuck H]

+1
But I would suggest sticking with a modern production 6V6. NOS types can't take the high voltages but the newer ones hold up fine IMHE. I blew up a NOS 6V6 (RCA) on the last one I worked on while putting it through the crank and wail test. 423Vp! It just couldn't take it. The EH I put in held up fine and sounded good.

[stokes]

I'd have to think you had a bad tube there,Chuck.The old stock tubes hold up fine,in my experience.I've got a couple amps I built,both have 6V6's running close to 400 v's biased at 80% and been running fine for two years.Look at the schem for a BF Deluxe,it had 415v's on the plates.I just had a Brown Deluxe in for a cap job,400v's and still has the original RCA's and they sound/perform great.It was absolutely the cleanest looking '62 vintage amp I ever saw,obviously the original owner didnt use it much,but the guy that bought it to me said he's been using it regularly for the last year and a half.I've serviced many BF Deluxes and I only use NOS tubes,wont put those current productions in an amp and havent had anybody come back cause the power tubes blew up.

[Tom Phillips]

And then there's the Jim Kelly amps running almost 500V on the plates of the 6V6s.

[Chuck H]

I'd have to think you had a bad tube there,Chuck.The old stock tubes hold up fine,in my experience.I've got a couple amps I built,both have 6V6's running close to 400 v's biased at 80% and been running fine for two years.Look at the schem for a BF Deluxe,it had 415v's on the plates.I just had a Brown Deluxe in for a cap job,400v's and still has the original RCA's and they sound/perform great.It was absolutely the cleanest looking '62 vintage amp I ever saw,obviously the original owner didnt use it much,but the guy that bought it to me said he's been using it regularly for the last year and a half.I've serviced many BF Deluxes and I only use NOS tubes,wont put those current productions in an amp and havent had anybody come back cause the power tubes blew up.

What about the crank and wail test? Do you really think it's not an issue? You can put any tube you want into in an amp that never get's turned up past "2". If I send an amp off my bench it needs to take any reasonable and common abuse thrown at it. NOS 6V6's haven't proved reliable in the crank and wail test for me. A bad tube is a possibility for certain. But without more data, what I know is that the NOS tube blew up and the EH didn't. Do as you like. Results may vary. I don't directly disagree with anyones assesment of tone or it's criteria. But I'm not inclined to burn anymore NOS 6V6's in curcuits over 400Vp when I know that current production tubes have a good rep for both tone and ruggedness.

[stokes]

You have to do what works for you,of course.My sons amp is basically a 5E3 clone,with beefier PT and OT and some other mods to the PI for head room.He plays in 3 different bands,uses it 3-5 times a week between gigs and rehearsals,its dimed almost constantly.The one I use gets far less use,of course,but 3 times a month in a "band" situation when it is cranked to the max.It has a preamp Ken Fisher drew up for me back in the late '80's and a basic Deluxe type output section.It has a single 10" speaker,so to be heard over a drum and bass,it has to be cranked.My experiences are directly opposite yours,I've had far better results with old stock tubes and seen current productions die a far earlier death.Of course the cost these days makes many old stock tubes out of reach to most,I dont know that I would be such a big proponent of old stock if I had to pay these prices.I've spent over 30 years amassing a stock of tubes,both preamp and power tubes.I have a dozen or so amps all full of NOS tubes and enough spares that my sons should have enough to last,so I guess I am a bit biased toward the old stock.

[Chuck H]

It has a preamp Ken Fisher drew up for me back in the late '80's

Analyzing Ken's circuits always reveals some intrinsic genius for me. I'd love to see what he drew up for you, of that's not too forward.?.

On the NOS thing... I have been happier with the tone of NOS tubes than new production to be sure. I just haven't had luck running 6V6's biased into class A at 423Vp. I know that new production tubes can handle the abuse because I've seen it and it's been discussed here. Lots of posts from experienced repair and builders that indicate new production 6V6's are good at higher plate voltages. That's also been my personal experience. At lower Vp the NOS tubes should take plenty of abuse. And probably last longer than new production tubes. It's just in high Vp and class A bias conditions that I've had problems.

[stokes]

Sorry,I cant do thast,Chuck.Its nothing too drastic,pretty close to the Liverpool "copies" that are all over the 'net.What really made his amps special were the trannies.He had them custom wired and I'm not sure the guy who is carrying on with the Traiwreck amps even has it.From what I understand the guy was a close family friend who worked with him for years.I think he may be from his days at Ampeg.There are rumors I've heard that it is Marshall,but I dont think that is the case.

[Chuck H]

I'm down. The new Wrecks sound very good IMHO. But not as interesting or intuitive as the originals. By interesting I mean that the originals seem to have a built in touch wah effect because of how the power amp responds. The crossover distortion (or "swirl") is never nasty sounding. It's super cool and none of the clones do it as well. By intuitive I mean that quality some amps have to grab a note and sustain it or bloom one way or another dependent on attack, etc. KF amps do this VERY well. But this is only based on research, not personal experience.

I agree that the transformers are a big part of the TW mojo. Looking at the changes and reading misc. stories it seems that Ken went to some trouble over the matter as well. But I also see things in the circuits, as simple as they are, that are really elegant. Like employing a cold clipping stage that also trims the gain feeding the PI. That may have been a happy accident. Maybe not. Looking at the tonestack it's obvious that it's voiced for the right curves with an average guitar and all the controls set half way. A little tweak for whatever guitar you plug in is there if you need it. But as an actual tonestack it doesn't offer much. Still, it voices the amp as he wanted it with the controls set nominal, allows for some player control and pads the gain structure just right. So is that two happy accidents? Or is it possible that there is actually a lot of thought, trial and error and intrinsic understanding that went into such a simple circuit.?. I've never known of any other amp builder, tech or tweak that can intentionally induce "swirl" in a desireable and predictable way. Some people question Kens actual technical prowess. But I think there were a lot of things Ken knew (and others don't) that he never shared for one reason or another.

[stokes]

Yeah,I was quite fortunate to have met and had many conversations with him,met him thru a mutual friend,and he lived pretty close to me.Although he was very approachable and would talk at great lengths about amps,he really didnt give up a lot about what was going on in his own amps.When he did he was very vague and talked over my head,but he did seem to be more of a tweaker than an engineer.It was before he really became well known and his amps took off.I was introduced to him as "Ken,a guy that used to be a big shot at Ampeg and is building his own amps now".The prices have really become a mind blower.There is a studio in NYC I've been to that has the biggest collection of Trainwrecks,its called Ultra Sound,they have a website and the picture of the roomful of TW's is to drool over.30k for an amp is insane,tho.The guy that inroduced me to him was the owner of a studio I worked for and was a big gear head,spent a fortune on amps and guitars.He bought 2 amps back then from him and they were unbelievable,very Voxish,but with that incredible smooth distortion.I think at that time he paid around 3g's for them,which was steep for an amp in the '80's.What I noticed about the overdrive sound was that it seemed to be an underpowered OT about to saturate but not quite there almost as if it was going to go into an oscillation.I remember asking him about that and he just grinned and changed the subject,not an abrupt change,he went into talking about the power section,but really got away from it,could tell he didnt want to give anything concrete away.It was almost like he pretended to not know what he had,like it was an accident,but he was able to repeat it,so he knew what he was doing.we lost a good one when he passed.Lost contact with him in '90,guess he got overburdened,changed his number,and when my friend stopped going to see him after he had the amps a while I really couldnt bring myself to just show up at his door,not like I was going to buy an amp for 5 grand.

[Chuck H]

Yupper. There's something to be said for living the life rather than reading the books. And with the nature of amps changing as it is I doubt the qualities that Ken learned to exploit will be exhaulted. It's everyones loss.

[Vito]

...Vito,of course your voltages are going to be high.The original was designed when wall voltages were 110 vac.Being over 120vac nowadays would explain what you see.Your ripple voltage readings look fine.The preamp filter is mainly for decoupling purposes not ripple filtering,do not increase the value here,it will adversely affect the tone.As I explained earlier,grounding the preamp filter in the same place as the main and screen supplies is not good.Get a single 20uf and replace the can connection.Make sure you ground this new cap down by the other preamp grounds,that 120 cycle hum should be gone.Also find your heater supply ground reference,either a CT on the winding or 2-100 ohm resistors,one on each side of the heater supply and lift the ground and connect it to the cathode of the power tube,pin 8.DO NOT replace that cathode resistor on pin 8 of your power tube with a 2 watt,you need a 5 watter here.It gets hot,thats normal,a 2 watt here will fry.I wouldnt try lowering voltages with the RC network you describe.There is nothing wrong with running the voltages you see.everything in your amp is designed with a +/- 5% to 20% tolerance.There are many of these amps out there still working fine on todays wall voltages.I have quieted many Champs over the years exactly as I have described here.

stokes:

Thanks for your thorough reply. Alas, I didn't see it before I completed the work I proposed to do. Apparently there's something wrong with the notifications in the forum. I have consistently selected instant notification via email for replies to this thread since I started it. I have never received any notifications, so I never know when anyone posts anything new since my last post.

Anyhow, I did install the RC filter as proposed, except that the resistances that worked best turned out to be different from those I initially calculated. In the quietest configuration, the output (pin 8) of the rectifier connects to the first cap section through a 500Ω, 10W power resistor, and from there it connects to the second cap section through a 400Ω, 10W power resistor. The photo (below) shows how I mounted the resistors on extra long machine screws I used when I installed the clamp for the new filter cap can. They're mounted inside clamps I made from galvanized perforated pipe strap. The 500Ω resistor gets warm, but not too hot to touch. The 400Ω resistor barely warms up at all.

The 120Hz hum is essentially gone. I have to put my ear on the grille cloth in a very quiet room to hear it, and even then it's barely audible with the volume on "1". When I turn up the volume it's buried by the normal hiss and noise in the audio circuit, which isn't objectionable either. The hum is completely lost when I plug in a guitar and start playing.

The voltage at the output transformer primary on the filter cap side is +357VDC. The schematic shows it as +355VDC, so that's close enough for rock 'n' roll, as they say. The power tube is running cooler now. The amp still sounds great.

I did replace the 470Ω cathode resistor on pin 8 of the power tube with a 2 watt, 1% resistor before I saw your warning. It seems to be running OK, but it's no problem to stick a 5 watt resistor in there. The original was a 1 watt resistor, and obviously that wasn't beefy enough to handle the current.

I do not doubt that any of the methods you so kindly described have been successful for you in quieting the amps you've worked on, and I might yet apply those mods to my amp. I like the idea of a dedicated path for the tube heater circuit (i.e., one that doesn't use chassis ground as part of the circuit). It also makes sense to use a separate 20µF cap grounded near the preamp tube. Perhaps those mods will remove the remaining barely audible vestiges of hum.

Another thing I did was to install a three-wire power cord, with the ground wire connected to chassis via a spade lug on one of the power transformer mounting bolts. I also replaced some of the resistors with 1 watt or 2 watt 1% resistors. Here's a thumbnail that links to a photo of the finished job, with all new capacitors:



I intend to change the power cord wiring. The photo shows the hot (black) wire going through the fuse to the the power transformer primary, with the neutral (white) wire going to the power switch, and then to other side of the power transformer primary. Instead, I'll connect the hot wire from the fuse block to the switch, and then to the PT primary, with the neutral connected directly to the other side of the PT primary. I removed the "death cap", which (in the original factory wiring) was a .047µF 600V capacitor between the fuseblock and the chassis. I can't see any reason for it with a properly grounded chassis, except maybe to make certain the fuse (and probably a circuit breaker) blows when the death cap shorts...which apparently it has been known to do.

I'm very happy with the outcome of this job. The amp is quiet, and sounds terrific. Thanks again to all who provided guidance.

[stokes]

The schem calls for a 2 watt cathode resistor on that 6V6.It is adequate,but I always prefer 5 watt for added safety margin and heat disipation.I am trying to wrap my head around this R/C network you are describing.What it seems to me is that you added a series resistor of 500ohms between the rect and the first cap,and then lowered the resistor value to the screen tap to 400 ohms,from the stock 1K?Do I have that right?I have seen adding series resistance like your 500ohm added between a SS rect to try to simulate "rectifier sag".Not sure in your case if it has any benefits except to lower the voltage to that first node,and thereby down the line.But why then reduce the 1K to 400 ohm bringing the voltage back up to the screen and beyond?But,hey,if it works for ya,go with it.Changing the ac cord config as you describe with the switch and fuse isnt going to quiet anything,it is a prudent safety thing,tho.I cant see in the picture,but is your filament string a single wire arrangement,with one heater pin connected to ground,or is it the twisted pair?If the single wire arrangement,change it to the twisted pair,that alone will quiet things then try my suggestion with the heater ground reference connected to pin 8 of your power tube and you should see a lot of that hiss,etc when you raise the volume disappear.