1974 Fender Vibro-Champ - Page 1
Forum section: electronics / repair

author: Old Printer
author_rank: Frequent Contributor
timestamp: July 10, 2018, 03:17:59 am
content: I am the proud original owner of this 100% original amp. It still has the original RCA tubes and the 40+ year old bypass capsIt has had very little use over the years and pretty much none in the last ten. I did recall it sounding poorly back then and when I had a shot at a Mustang modeling amp for $100 I grabbed it and have been using that, what little I play anymore. Over the last few years I have become addicted to electronics, acquired a couple old TEK scopes (2225 & 475) & a Analog Discovery. Lately I have been studying repair & maintenance of the Champ style amps and dug mine out over the weekend. The output tube, a 6V6 was red-plating badly and running very hot. I fed it a nice clean sine wave and by the time it reached the speaker output it looked and sounded pretty rough. A check of the 470 ohm bias resistor showed a reading of about 3.6 ohms. It has a 25uF 25V bypass electrolytic and as soon as that was removed the resistor read about 515 ohms. I replaced the cap with a 33 uF 50V because that is what I had. Fired up the amp and the redplating/overheating is much more in the normal range. I need to change the tubes and go through the circuit and replace the rest of the eligible caps.During all this I see that at some point I had removed the ground pin on the power cord, wanting to play somewhere that did not have a grounded outlet no doubtDuring my testing the temporary chassis earth ground clip fell off with a spark. I find the chassis showing 122VAC @ 55mA between it and mains earth. My best guess at the moment is another cap leaking to ground. Maybe the "Death" cap?? It is still there. I ran out of time today so if anyone has an opinion on where this stray voltage is coming from I would love to hear it. Thanks

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: July 10, 2018, 11:53:57 am
content: from the looks of the schematic, ungrounded operation is a definite no-no for that amp because of the 0.047uF/ 600V grounding cap. it is also likely that the xformer isn't even specced for double or reinforced isolation, as the designer was (safety-wise) relying on the ground connection. so, again, grounded operation only. just my 0.02you could try one thing, though: just flip the ac plug in the wall outlet by 180°. is the leakage still there?

author: Shock
author_rank: Super Contributor
timestamp: July 10, 2018, 12:58:45 pm
content: It's up to you how hot you want to bias the tube. A resistor at 515 ohms indicates it's just in tolerance but could be drifting up due to age.Since you can expect the plate voltage to be higher (caused by modern mains input voltage) you need to consider if an adjustment to the bias is required along with replacing that cap. In other words a tube might not fix it, so check out the recommended current or power in the datasheet first.Any untouched caps that age are well past their use by date so I would suggest if you want the amp in best possible playing condition to check and replace all components before you look to tubes. Cleaning tube sockets, lubricating pots and other preventative maintenance is also recommended.

author: Old Printer
author_rank: Frequent Contributor
timestamp: July 10, 2018, 01:11:12 pm
content: Quote from: Le_Bassiste on July 10, 2018, 11:53:57 amGood thought, I will try that tonight. I am going to have to cut the factory plug head off and replace it. I have been grounding it with a separate wire while working on it. I hate to do it because of the value placed on these things being as close to original as possible, but in my youth (excuse for being stupid) I cut the ground pin off. The capacitors need changing as do the tubes and this thing is not quite pristine enough to put in a museum, so I might as well get it playable, though I will save the old parts I change.

author: Old Printer
author_rank: Frequent Contributor
timestamp: July 10, 2018, 01:21:19 pm
content: Quote from: Old Printer on July 10, 2018, 01:11:12 pmAgreed on all points. Thanks. Fortunately this is one of the most popular small amps ever made and there is a lot of detailed troubleshooting and rebuild info available online. I bought this amp because my main amp, a 1972 Vibrolux Reverb, was in the shop for a warranty repair. I still have that amp as well and it is also all original and in need of the same level of work as the Champ. I have not turned it on in 20 years, so I will recap it and invest in a Variac and before doing so. I remember it being pretty noisy back then so the tubes in it may well have redplated too.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: July 10, 2018, 03:46:48 pm
content: ooh, didn't mean to suggest to spoil the original looks by snipping the plug offyou could just swap mains live and neutral lines inside the amp chassis for that leakage test. is your variac of the isolating variety?Shock has a good point concerning the increased mains voltages. you may want to check heater voltages as well to confirm.i've attached the schematics i found in the "Tube Amp Book Vol. 3" by Aspen Pittman (highly recommended!). that schematic also contains some export voltage setting informations, and of course the info that the death cap is not on export versions, go figure.

author: Old Printer
author_rank: Frequent Contributor
timestamp: July 10, 2018, 07:21:00 pm
content: Le_Bassiste, Thank you for the schematic, I have a few versions, but not that one. It is very sharp. There were a number of versions, and from 1974 of course mine is a CBS Silverface. This looks like it could be a match, I will have a look when I get home.

author: Old Printer
author_rank: Frequent Contributor
timestamp: July 11, 2018, 01:59:10 am
content: Quote from: Le_Bassiste on July 10, 2018, 11:53:57 amWell you were correct, I turned the plug around and the stray 122VAC (55 mah) disappeared. Unfortunately that orientation would not be possible if the ground pin were there. This is going on in my attached garage that the previous owner had converted into a "livable" room. Since that was against the zoning code at the time, there could not have been any permits issued, including electrical. First thing I need to do is find out if the hot and neutral were flipped by some "weekend" electrician. The whole room only has one circuit, and that was added after the rest of the house was built. It is run in PVC conduit up under the soffit, all the way around the house from the main breaker panel. If they were flipped I would think there would be more than 55 mah there, but maybe not. My math is weak and my AC math is nonexistent. Can someone give me an idea of how much 120VAC current can pass through a good (or bad) .047uF "death" cap? Thanks

author: boffin
author_rank: Unknown
timestamp: July 11, 2018, 03:07:39 am
content: Quote from: Old Printer on July 11, 2018, 01:59:10 amThe equivilent R for 0.047uF @ 60Hz is 1 / (2 pi f c)or 1 / (2 * 3.14 * 60 * 0.000000047) or about 56kand 56k @ 120V should pass (V/R) about 2mAFrom that circuit, I'd remove the death cap (0.047), and before powering anything on, measure resistance between Live and chassis and Neutral and chassis, and make sure you don't have a leak in the transformer (or a nick in the power cord).

author: Old Printer
author_rank: Frequent Contributor
timestamp: July 11, 2018, 12:46:08 pm
content: Thanks Boffin, I will check that out. It does generate a noticeable little spark, and I originally discovered it by getting between the chassis and the grounded outside bnc ring on my signal generator. It wasn't a serious shock, and I have had plenty, but it was strong enough to get my attention as I wasn't expecting anything there. I guess that's why it's called the death cap. I was too focused on the known high voltage areas.

author: TimFox
author_rank: Super Contributor
timestamp: July 11, 2018, 07:06:30 pm
content: To check the outlet wiring, look for the wider flat slot, which in the US is neutral. The voltage between neutral and the third (round) prong (ground) should be only a few volts, while the narrow slot (line) should have approximately the full voltage to the third prong.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 12, 2018, 02:00:08 am
content: Quote from: Old Printer on July 10, 2018, 01:11:12 pmI would strongly recommend replacing the power cord with a 3 pin one.Having worked over many years with equipment of considerable age, I am not a great believer in the "showroom condition" school of thought with vintage equipment.In Broadcasting, 20 years old is "young", with 30 or more years old being not uncommon.Over such lifetimes, equipment inevitably has components replaced with more modern types.I feel that this reflects the long history of the devices, & the work done by various Techs over the years.It is always interesting to run across a repair you carried out many years ago.The memories come flooding back---- how you obtained equivalent "bits & pieces", adapted others & "got the show back on the road", who you worked with at the time, & so on.This amplifier will have better output power & distortion figures than a common AM radio audio output stage,due to the better output transformer & higher HT for the former, & the negative feedback loop for the latter,but not by much,----- my guess for power is about 3-4 watts.