Restoring/modding a 1976 Champ speaker recommendation?

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HH1978
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Restoring/modding a 1976 Champ speaker recommendation?

Post by HH1978 » Wed Oct 03, 2018 2:22 pm

Hello,

I've started restoring the first amp I bought, back in the early 90's : a 1976 SF champ.

This amp started to smoke, literally, years ago, and has been sitting in my cellar since then. Time to give it a second life!

I didn't know anything about amps when I bought it, I was 13 or 14 I think. I never checked the inside, and If I had done it, there was no internet to help me at the time anyway.

It appears that the amp has been messed with quite a bit. The baffle has been cut to accomodate a 10" speaker, first in the center, then they probably realised that the basket was hitting the tubes, and they relocated the speaker on the side. The speaker itself is the worst I've ever heard (a Mckenzie) and has the wrong impedance.

Since the amp was already in a sorry condition regarding originality, and I already have another SF VC, I decided to try the G. Weber "tweedifying mods", which are easily reversible anyway. I also changed the cap can, other electrolytics. I also changed the power tube socket, which was obviously where the smoke came from.

I tried the amp with the speaker from another amp, and I'm very pleased with the sound. Yet, I have a few things I'd like to have your opinions on :

1) The original cap can was 40-20-20. The new one is 20-20-20-20. I made a few searchs, and there seem to be a agrement that 40 on the first filter stage is not really crucial with a 5Y3GT. Is it correct?

2) The amp sounds much louder than I remember, and much louder than my VC (which has been serviced). I wouldn't go past 3 or 4 without getting troubles with the neighbors. Is this explainable by the combination of modifying the circuit/using a 10" speaker vs a 8"? Or does it indicate something wrong?

3) Which speaker would you recommend out of those 3 (they are the only 4 ohms available here), given the amp is now meant to sound closer to a tweed Champ than a SF? I don't want to try to get the 10"baffle out and replace it with a 8", at least for now. My options are :

WGS Veteran 10

Jupiter 10SC

Jensen P10R


They're about the same price (WGS a little cheaper).

Thanks in advance for you advices!

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Re: Restoring/modding a 1976 Champ speaker recommendation?

Post by oid » Thu Oct 04, 2018 1:59 am

HH1978 wrote:
Wed Oct 03, 2018 2:22 pm
1) The original cap can was 40-20-20. The new one is 20-20-20-20. I made a few searchs, and there seem to be a agrement that 40 on the first filter stage is not really crucial with a 5Y3GT. Is it correct?
It will cause no problems, but you have four 20 uf caps in there, wire two in parallel (just tie two positives together) and you have a 40 for the first stage and will get slightly quieter operation Fender did the SF champ with both 20-20-20 and 40-20-20, probably just used what they got a good deal on, The 40 on the first stage will give better filtering, so slightly lower noise.
HH1978 wrote:
Wed Oct 03, 2018 2:22 pm
2) The amp sounds much louder than I remember, and much louder than my VC (which has been serviced). I wouldn't go past 3 or 4 without getting troubles with the neighbors. Is this explainable by the combination of modifying the circuit/using a 10" speaker vs a 8"? Or does it indicate something wrong?
Unlikely it is a sign of something wrong. Could be your memory, could be the impedance mismatch, could just be you had sickly tubes. The smoke coming out at the power tube was likely caused by two pins arcing, which the combination of the impedance mismatch and a tube near death could cause. So a sick tube is my guess.
HH1978 wrote:
Wed Oct 03, 2018 2:22 pm
3) Which speaker would you recommend out of those 3 (they are the only 4 ohms available here), given the amp is now meant to sound closer to a tweed Champ than a SF? I don't want to try to get the 10"baffle out and replace it with a 8", at least for now. My options are :
If I was going for the old dirty champ sound I would go for the Jensen, if I wanted more versatility I would go with the WGS. Alnico magnets impart a quality to your dynamics that can be hard to work around, especially when driven hard, it can be quite unbearable for those who dislike that quality. Ceramic tends to be more versatile, plays with pedals better, less likely to sound muddy or indistinct when driven hard. I have not played the Jupiter.

For my tastes I prefer alnico for clean sounds and ceramic for distorted most of the time. With a 10" speaker you are not going to get much of a tweed champ sound. I would probably stick the Jensen in, especially if I already had a VC sitting about which, if stock, has a ceramic.
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Re: Restoring/modding a 1976 Champ speaker recommendation?

Post by HH1978 » Thu Oct 04, 2018 11:08 am

Thanks for your detailed and helpful answer!

I'll wire the two terminals in parallel. I was hesitant to do it because I read on another forum that it 40uF could stress the 5Y3GT, but as the amp was wired that way before, I don't see why I wouldn't do it.

The low volume before was probably a mix of impedance mismatch and very poor speaker quality/condition. Actually, I tried with the old 6v6GT, which still works, and the amp got as loud as with the new one. Yet, it's really loud. Volume on 3 through input 2 is as loud if not louder than volume on 6 through input 1 on the VC, but cleaner and warmer on the champ. Speaker size/efficiency is probably a good part of the explanation.

I think you have a good point in saying that I won't get close to a tweed champ with a "10 speaker. So I think I'll try to remove the baffle and cut a new one. Actually, I have a Jensen P8R in the VC, so I'll probably go for ceramic, and be able to swap speakers between the two amps if I want to.

There's also a little more options, prices low to high :

Jensen Mod

WGS G8C

Jensen C8R

Jupiter 8SC

Eminence 820H

The Eminence being twice the price of the Jupiter, I don't think I'll go for it, unless it is really fantastic.

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Re: Restoring/modding a 1976 Champ speaker recommendation?

Post by sookwinder » Thu Oct 04, 2018 9:21 pm

The cathode resistor for the 6V6 was under rated from the very start of the BF champ and VC builds in the early 60s all the way through to the SF models. The design calls fro a 470ohm cathode (2 watt) resistor... you actually need something between 680 - 900 ohms (depending upon how "hot" you want the design to be) What happens after about 20 years of someone playing the Champ or VC at 10/10 the 470ohm res burns out smokes and eventually falls apart. In between that occurring you have the danger of fire and/or damaging other parts of the amp.
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Re: Restoring/modding a 1976 Champ speaker recommendation?

Post by oid » Fri Oct 05, 2018 12:31 am

sookwinder wrote:
Thu Oct 04, 2018 9:21 pm
The cathode resistor for the 6V6 was under rated from the very start of the BF champ and VC builds in the early 60s all the way through to the SF models. The design calls fro a 470ohm cathode (2 watt) resistor... you actually need something between 680 - 900 ohms (depending upon how "hot" you want the design to be) What happens after about 20 years of someone playing the Champ or VC at 10/10 the 470ohm res burns out smokes and eventually falls apart. In between that occurring you have the danger of fire and/or damaging other parts of the amp.
The SF/BF Champs are biased up at 13 watts plate dissipation, so one watt over what the data sheet calls the max plate dissipation of the 6V6GT, 12 Watts. This is more or less in line with the Fenders of the era and max dissipation is not actually a limit, it is just how hot you can run the tube and still get advertised life. The published curves in those data sheets show operating characteristics above max dissipation for a reason, sometimes the shorter lifespan is worth the trade off.

The cathode resistor is undersized in wattage and not even a power resistor, it is actually a plain old carbon 1 watt resistor and it is dissipating 3/4 of a watt 100% of the time, impressive how long these lasted. General rule of thumb is double your wattage and go up to the next standard value, so a proper 2 watt power resistor is plenty and will last many decades, I generally use 5 watters, modern 2 watters have thin little short through hole leads, 5 watter leads are thick enough that they will stay where you put them in a point to point amp and are generally long enough to reach from the socket to the ground. Still shorter than the old resistors though, what can you do.

A 680 Ohm cathode resistor would put the plate dissipation at 9 watts, or 75% of max, this is right were the manufactures recommended you design to for best trade off between lifespan and acceptable performance. Switching out the 470 for this would lose you a small amount of output and gain a few % increase in power tube lifespan, assuming the same tube, even at 75% you will get the odd short lived tube. The biggest difference in sound will be in the overdrive characteristics. If you played this amp regularly for the rest of your life, you would probably save the cost of a power tube or two with the decreased bias. You would not want to go much beyond 680 if you want it to sound like a champ.
HH1978 wrote:
Thu Oct 04, 2018 11:08 am
I'll wire the two terminals in parallel. I was hesitant to do it because I read on another forum that it 40uF could stress the 5Y3GT, but as the amp was wired that way before, I don't see why I wouldn't do it.
40uF will increase the stress on the 5Y3, but it is still within spec. When you power on an amp there is a rush of current and the rectifier has to supply this, the greater the capacitance in that first stage the greater the current. 20uF will get you longer life out of the rectifier but a tad more noise.
There's also a little more options, prices low to high :
The Mod8 is one of my own most hated speakers, it has good bass for an 8, but that is about it, just sounds kinda dead to my ears. The rest are all perfectly good, once again have not tried the Jupiter, it would be more or less of a toss up between them for me. The Celestion Eight-15 is my favorite of the current crop of 8" speakers, has that nice clear and detailed Celestion high end, really helps tame the tendency of SE amps to get muddy and really shows off the single ended distortion, great cleans. I would say it would be the exact opposite of the P8 in your VC, so perhaps good contrast. Someday I am going to make a 4x8 of these, get my bass back but maintain that sound, love it.

Nice thing about 8s, they are cheap and one does not have to feel so guilty about buying and trying.
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Re: Restoring/modding a 1976 Champ speaker recommendation?

Post by sookwinder » Fri Oct 05, 2018 5:30 am

Just for information ... not for accusation.

If you look at the changing circuit drawings put out by Fender from the tweed champ (the fore runner of the BF Champ) to the earliest BF Champs and VC in the early 60s, my understanding of amp design Leo forgot to update the 6V6 cathode res from what he was using with the tweed champs. Irrespective of whether the Rc is not rated from a power perspective, IMO 470 ohms is too low...

5E1 ran a B+ voltage of about 320V with the 470 Ohm Rc
5F1 ran a B+ voltage of about 360V with the 470 Ohm Rc
AA764 B+ voltage 355V 470 Ohm Rc
AB764 B+ voltage 420V 470 Ohm Rc

So the B+ has gone up about 100V without any consideration of changing the Rc from 470 Ohms to something a little safer, so 560 or 680 ohms.
To me this is an oversight. Yes you can say "the sound" of the Champ or VC is with the 470 ohm Rc, but the sound with the volume up full is crap (I have 4 vintage VC of different eras so I do know what they sound like). If you want an great OD sound, then full volume is too far.
However everyone back in the day who played them probably turned them up full just to see if the can get a little bit louder ... all they got was shit sounding OD.

Also the amount of B+ filtering increases dramatically (most likely as capacitors, and cap-cans get cheaper)
Tweed CHAMP 5F1 : 16uF / 8uF / 8uF
CHAMP / VC AA764 : 20uF/20uF/20uF
CHAMP / VC / BRONCO AB764 : 40uF/20uF/20uF

yeah the 5Y3 , like all valve rectifiers, has limits on the first filtering caps values, but I have never had any issues with 40uF on the first node.
But I have seen too many old VC and Champs run red plate when there is 470 Rc.

Out of the two "design issues" I would argue the too low Rc is more an issue than the 40uF on the first filtering node.

Just my 2.5 cents worth ! (I increased the Rc value to lower it down from 5 cents)

this is a pic of my 68 VC with 20/20/20/20 and the first two section wired up in parallel to get the 40uF.

Image
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Re: Restoring/modding a 1976 Champ speaker recommendation?

Post by oid » Fri Oct 05, 2018 3:37 pm

sookwinder wrote:
Fri Oct 05, 2018 5:30 am
If you look at the changing circuit drawings put out by Fender from the tweed champ (the fore runner of the BF Champ) to the earliest BF Champs and VC in the early 60s
If you want to know the reason for the voltage increase you need the go back before the 5E1, any of the prior models will do, circuit is more or less the same, 5C1 is the easiest schematic to find. You can ignore the cathode resistor, 470 became a standard value and made the 500 ohm of the A though D slightly more expensive, Leo was a consummate penny pincher and knew well enough that the 30 ohms made little difference.

I can go into more detail after work if you do not figure it out.

20uF is actually the limit for the 5Y3, but time has proven it a tough little tube and the decreased life span acceptable. I have not seen a 40 in any of the champs or broncos, schematics also say 20, would not surprise me one bit if Leo threw them in when he got a good deal.

This is giving me the urge to make an amp, got a few designs drawn up I have yet to build, may need to do something about that. Fun stuff!
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Re: Restoring/modding a 1976 Champ speaker recommendation?

Post by sookwinder » Fri Oct 05, 2018 6:31 pm

just for info for everyone.... those that like trainspotting... and my father did literally trainspot in the 1940s in the Middlesbrough area!

This is the early Vibro Champ schematic 20uF/20uF/20uF circuit number AA764
Image

This is a later iteration of the Vibro Champ schematic 20uF/20uF/20uF still with circuit number AA764 but with the "field fix" for getting rid of some noise by the addition of the 320pF cap on the 6V6
Image

This is an even later iteration of the Vibro Champ schematic but now with 40uF/20uF/20uF and the circuit number changed to AB764, with the "field fix" for getting rid of some noise by the addition of the 320pF cap on the 6V6. It also references export models as well
Image

This is the first BRONCO circuit, the same as the first VC circuit and also using the same reference number AA764 with 20uF/20uF/20uF
Image

This is a later BRONCO schematic but now with 40uF/20uF/20uF and the circuit number changed to AB764, with the "field fix" for getting rid of some noise by the addition of the 320pF cap on the 6V6. It also references export models.
Image
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Re: Restoring/modding a 1976 Champ speaker recommendation?

Post by oid » Fri Oct 05, 2018 7:21 pm

sookwinder wrote:
Fri Oct 05, 2018 6:31 pm
just for info for everyone.... those that like trainspotting...
haha, I double checked that before posting even, guess I checked the AA schems twice. Thanks for pointing it out.
Logic gates based on billiard-ball computer designs have also been made to operate using live soldier crabs of the species Mictyris guinotae in place of the billiard balls.

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Re: Restoring/modding a 1976 Champ speaker recommendation?

Post by oid » Sat Oct 06, 2018 1:49 am

A mystery solved, seems most of my AB schematics are the AA with incorrect file names, should have read the title on the top of schematics, apparently when I sorted and renamed all my schematics so they would sort nicely on the computer I messed things up. Guess I need to browse through them all now and find out how bad of a mess I made.

I lucked out with my calculations here since both SF and BF Champs are AA764 circuits. So still 14 watts plate dissipation for HH1978s amp, which is fine for many of the modern tubes and we even have a few 6V6s on the market these days with a higher dissipation. The JJ, Groove Tubes, and Electro Harmonix are all rated at 14 watts, so will do just fine in this amp as long as speaker impedance is proper. And most any of the NOS 6V6s will be just fine as well. Curious as to how the Tung-Sol reissue compares to the old, the originals really could take a good deal of abuse.

I do not think I ever came across an AB764 VC/Bronco in the wild, perhaps seen or played, never worked on one though. Now your suggestion of 900 ohm cathode resistor makes sense, 900 puts the AB764s right at 70%, which is quite conservative for a SE amp, but probably a good idea on these with a B+ of 100 Volts over max, either that or an EH 6V6 with its 475 Volt plates.

Thanks for sorting me out on this.

Oh, on Leo's forgotten cathode resistor, I always heard it was on the prototype he forgot it in, liked the sound and worked with it, adapted it to the Deluxes. The (non AB) Champs have 14 watts dissipation, the Deluxes have 14 watts during peak current, do not think that is a coincidence.
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Re: Restoring/modding a 1976 Champ speaker recommendation?

Post by HH1978 » Sat Oct 06, 2018 5:57 am

Thanks so much to you two for all the infos!

Regarding the cathode resistor, I didn’t change it, but the other mods I made are supposed to bring the voltages lower, closer to a tweed champ. I’m not home right now, and talking about the modifications just from memory could lead to mistakes. But I’ll post the details when I have the amp in front of me.

Thanks again!

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Re: Restoring/modding a 1976 Champ speaker recommendation?

Post by Paul-T » Tue Oct 30, 2018 1:01 pm

Any update on this?

I have a 54 champ and a P10R... but today the fella who was making a groovy custom extension cab for the latter told me he couldn't deliver. Wish I could give a user report. ... but I do reckon alnico is the way to go. There are wise comments earlier, with which I agree, but alnico Celestions sounded great in my old AC30, cranked up big-time.
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Re: Restoring/modding a 1976 Champ speaker recommendation?

Post by HH1978 » Fri Nov 16, 2018 6:27 am

Sorry,

Due to some family issues I’ve been away from the forum, and could not go very further in speaker experimentations either.

I tried a p10r from another amp, but the sound wasn’t very convincing. Not sure if it’s the speaker or the mods. Usually I like Jensen Alnico with blackface circuits, but here I’d be tempted to try something a little warmer sounding.

I also tried the modded amp with a load box and cab sim, and I found the amp has too much gain for my liking, so I’m considering reverting it to original bf/sf circuit.

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