Champion GM 8

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Champion GM 8

Postby Racing » Sat Jan 07, 2017 6:49 pm

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It´s more rare these days but this was a pleasant surprise! A Swedish made Champion PA amp from the early/mid -40´s.

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Surprise? Why´s that? Well see..Champion GM 8 is a rather low powered and small push/pull amp that uses a pair of ECL-11´s for power. ECL-11´s being a "mix" tube. Triode and tetrode in one bottle.

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A while ago i did a GM 20 which kind of etched. A twin EL-12 amp that certainly put the goods out. This tho...a smaller jobbie.

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A complete half small PA. Haven´t measured the diameter of that speaker but it isn´t overly small at least. Now..take a note of that piece of plywood...

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...which seals off a compartment where the speaker wire is stored. Jack for it tho...a no-no as it´s of the same type used as mains jack back in the day here in Sweden.

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Yep. Adjustable primarys.

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As i first saw the pictures of the amp i thought it was an EL-12 amp. Not so. Didn´t know that it was a GM 8,which traditionally means 8 watts,and thus.. The GM 20 looks basically the same just this one carries fewer pots. That´s the difference them between,and now with amp in hand..this one´s a tad smaller too. ECL-11´s then. A tube released in 1939 and it must have been all the rage as a mix tube and super avant garde at the time. Powerwise...unsure,or was,as i laid eyes on them. Rather large,about the size of EL-12´s approx. So. Checked e-bay and the going rate for them is between 10 and 50 Euros a pop. IOW not all THAT bad,IF they turned out to be da shiznitz. Then again..not exactly mundane glass either is it...

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Onboard we also found a 6J5. A single triode octal based jobbie.

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...and in turn the very fabled EF-12. A tube of guts and glory for various reason. Famous as it powered a number of real high end microphones and mic preamps of the time. A tube i´ve worked with previously...and that one stays no matter.

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...in turn an AZ-4 as rectifier. Overkill you won´t believe. A qualified guess is that the amp sported an AZ-1,which looks the same,and got replaced by this AZ-4 as that was what was at hand. Well. The 4 carries 200mA easy..and with this amp we´ve got no use for that what so ever,and the 1 in turn a 100mA.

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Having measured DC resistence of the speaker i´d say we´ve got a 4-8 Ohm load setup here. Good enough. There is yet another set of taps that were just soldered to a solder support and i guess that´s for either a negative feedback system or some sort of line system. 100V or whatever..

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Speaking of which..this is a little different. The HT centertap does NOT reach ground instead is does so via this big ass resistor which in effect brings the entire amp to be ground lift. Different to say the least. This resistor in turn doubles as cathode resistor for the pair of ECL-11s too. As usual with the old Champion amps it looks like a friggin grenade went off under the hood. The stock "tower",can,e-lyte was dead as a doornail and had been left in place while being replaced within by an axial. This job done ages ago too...

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Kind of says it all doesn´t it?

So what´s the idea here? Phase 1 is to get the thing running. At this stage i had the AZ out of there and just jumpered mains voltage in there and flicked the switch. The PT is alive at least. So idea 1 here is to replace anything capacitor and reference measure the resistors at hand to get the POS running again. Reason for that is that i want to hear them ECL-11´s tell me what they´re made of. If they come up short i´ll just turn this into a dual 6V6 amp,something the transformers will handle with easy.
Have checked datasheet for the ECL-11´s and they are on par Ra-a wise with 6V6´s so..need be..
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Re: Champion GM 8

Postby Racing » Sat Jan 07, 2017 6:59 pm

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Yep! So i got to replace every capacitor and sight alright. OT on the amps outside,it is for various reasons i guess installed within the chassis stock. And?
No.
Just HELL no. As it came alive it oscillated and in general sounded like crap but the worst part was that those 8 watts..was about it. Didn´t even bother measuring output of the thing.
Replaced the stock volume pot too as it was run into the ground but still...no. Just no. This certainly wasn´t anything to call home about.

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Apart the thing came. No idea to hold back. As you can see there´s plenty of primary taps for the PT. Well...out and apart as i wanted to be able to hand the chassis a good wash.

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As i took the octal socket for the 6J5 out it turned out that this was an add on. Amp had been modded in other words.

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This cutout tho IS stock and it is the hole for the socket that keeps the ECL-11´s. Same for EF-12 and EL-12 and what not.

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Here in turn for the AZ rectifier socket. Just about 40mm i diameter.

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So. This is gonna turn into a "20 watter". That simple,which means an IEC jack and a mains fuse for starters. The amp does not sport any form of fuse stock btw. Not even a thermal such on the PT.

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Seing the 8 watts the transformers,both of them,are severe overkill from a size POW. As stated the OT is bolted within stock,but that´s all gonna change now. B+ under load ended up around the 360VDC mark btw but i suspect that number to rise as well seing the amps new running conditions.

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To make the chassis more roomy i decided to move the AZ rectifier a bit.

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Fuses for each HT tap. In turn a pair for the 6.3V heaters as well.

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This though brought a smile to my face. The ONLY capacitor that was good to go still was this REALLY old Swedish made Rifa. That´s saying something...
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Re: Champion GM 8

Postby Racing » Sat Jan 07, 2017 7:11 pm

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Sat there with a pot of coffee and tried to figure tube layout out. As i moved the OT outside..this is what it came down to. That simple really. Thank god for hole punches btw! So. A piece of sheetmetal was fabbed up and installed. Aluminium of course.

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As you´ve heard me touch upon before,small signal pentodes really are da bomb however they´re real sensitive to vibrations and what not why..i basically toss the book at the install of them. Knowing all to well what they take to work as god intended. Ergo rubber grommet install is No1 on the agenda,but i´ll toss yet a few tricks at it. Point of them being greater bandwidth,attack and in turn amplification rate.
Why bother?
Well. Idea of mine this time out is a single channel three stager aimed at REAL dirty blues work and rocknroll. This time out limited to three knobs up front. Gain,tone and volume out. In other words a PPIMV. Simplicity in short.

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No. The sockets have not ended up where they did by chance and i guess the picture is starting to evolve here.? Them two gigantic holes there needs to be handled tho.

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Uhu. Exactly. A couple of regular ECC-83´s. One for phaseinverter,thus far an LTP in mind,and the second for the basic two stages of preamp. That tube to work in liason with said EF-12 in short.

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Yeah. Three chicken heads all in all..that´s it. I´ll install trimpots within to be able to attenuate to hearts desire tho which in turn means that any guitarist down the road can do the same to cater to his or her needs. So..gain...tone and PPIMV. KISS. I want the impression of the amp to be clean as far as the faceplate. No trix,no glory. Just basic creamy distortion and tone in mind.

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The stock on/off switch certainly got a go and got rid of rust and what not. The faceplate of these things ain´t exactly sexy looking why i plan to clean it out as best as i can and then hand it a few coats of clear. I think that´ll make for a nice contrast vs the rather matte impression for the rest of the amp.

Within...we´re on schedule too. Fuseholders in place,first hit e-lyte in place,the primary taps are soldered back,mains wiring has been laid down..saver diodes and resistors for negative bias voltage out has been installed and so on and so on...
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Re: Champion GM 8

Postby Racing » Sun Jan 08, 2017 11:46 am

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Yep. One preamp/PI board ready to go.

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Taps for bias i´ve provided on the actual rectifier socket (the AZ-4 one). Then a "bias board" was fabbed,and as you can gather the wiring vs the powertube sockets is real temporary just to get the thing going. I wanted to have the readings clear before proceeding...
Per regular color coded as far as what trimpot belongs to which powertube. All good in other words. As you can see the placement of the preamp board leaves them PPIMV capacitors of 100nF that close that all i´ll need to do as that board gets bolted in once and for all is...place the stoppers between each capacitor and pin 5 on each socket.

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That JJ twin can there is a given i suppose but..i´m a tad unclear how to make do them 3 more nodes that are needed. Giving thought to using a trick i´ve done before,and that is use three radials of 450V that i both glue and bolt in as a "pyramid". I know that this can be made rather compact as a solution and see compact is good when working with smaller chassis.

Then. When it comes to the preamp board component values is based on reasonable assumptions. This isn´t exactly the first time out i build one of these three stagers so....got a feeling that it´ll suffice and then some.

Tonecontrol in turn. Looking around as far as options and TBH as of tonight..i´m leaning towards a Garnet setup. Have worked with them before and know them to be effective such,being...shall we call it "semi" active. Basic idea of mine here is that you should be able to run the amp "balls to the wall" and basically just make minor adjustments as far as rocknroll via them three pots up front.

So. I fired the thing up,powertubes only,and as it turns out the gut feeling of mine seems to have been on the money. B+ was all in at 400VDC@22mA per 6V6 tube and..couldn´t ask for more. Shoved an AZ-1 in there...and for the second time every i´ve now run into a rectifier tube that is at EOL. Net result was ~56VDC. So..that one in the bin and in the next one went. Just shy of 390VDC... As to which type i´ll settle on i guess practical tests will tell. Either an AZ-1 or an AZ-4
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Re: Champion GM 8

Postby Racing » Mon Jan 09, 2017 2:52 pm

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For various reasons progress is slow,but..there´s at least progress. Yep. The whole thing is up and running and in summary...voltages are about where i want them to be. That simple really.
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Re: Champion GM 8

Postby Racing » Wed Jan 11, 2017 3:33 am

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So shoot me,but there´s something really pleasing about a mere three dials. Yep. Up and running and doing its thing. Seing the width of output out of that EF-12 that brings that the amp can be dialed from mild to wild. Make no mistake. Then again...being a rocker at heart i´ll leave trying to dial Fender clean to others.
(Ironic but..the two 12AX7s are Fender marked 7025s).

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"Three dials,no waitin´!" Or?

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Someone voiced that the chassis looks rather dirty within. Agreed,it´s just that..that´s the way old Champions look within. It IS clean,i assure you. That said..entire setup is half blunt and to the point in my opinion. The two small blue trimpots on the board,which are gain pots,truly makes for a difference. Amp is very well behaved,no wonder seing that it´s put together by the book. As you increase gain out of the EF-12 there´s hiss due of course but that´s it.

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As those reading my posts are familiar with i normally do the mainstay of tweaking through and old 212 made for car bass use containing a pair of the REAL ill fabled Celestion 50´s that reside under my bench. Brings that when i finally hook amps up through the 312 Max box of mine an entirely different creature evolves.
I reason as such that what sounds decent through that 212 under the bench will sound downright fantastic through anything else.

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The stock speaker is an old AlNiCo jobbie of approx 10"...or..well in millimeters then. Stock there´s a rather coarse mesh up front that basically eats up half the area. Out that mesh came and out the speaker did too. Curtains. Speaker was damaged from moist/water. Not all that surprising maybe...

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Stay with me here. The stock speaker got replaced by a generic 12" Celestion i had laying around.
So. This is NOT a combo. More so a "package" that makes the entire thing rather easy to haul around. Put the amp in itself on a scale,which pegged at a mere 7kg.
Basic idea here is IOW that you tout the thing around as a package,open it up and take the amp out and then place the amp either intop of the speaker enclosure or..wherever it fits. Of course there´s the option to head what comes out of the amp into a 212 or 412 of choice too.

Is all that glitters gold? No. Of course not. I came to tweak the thing a bit and all said and done the tweak that made for the most difference was when i upped rail voltages from the PI forwards approx 35VDC. Then the thing really came to life.
I further need to look into the use of capacitors across the stages cathodes. I need to bring gain (amplification) down a tad as there´s pointless amounts of the stuff,which just clusters tone IME.
When you DO dial the thing for tone tho...it sounds REALLY good. It really does.

At wide open throttle B+ dips a mere 30VDC approx. So..the PT and rectifier tube certainly can stand the abuse.

So what´s the bottom line here? In short it´s that the player dials the trimpots within on his bench...for whatever genre he wants to tailor for,and then go at it.
Control the amp via the volume pots on his guitar.
That simple.

Happy? Yeah. Happy. This was a different take and one that turned out to float. No doubt.
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Re: Champion GM 8

Postby NoiseNoiseNoise » Wed Jan 11, 2017 5:15 am

Have you measured power output? I got lost as to if you kept the original power stage or went all 6V6 on it.
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Re: Champion GM 8

Postby Racing » Wed Jan 11, 2017 5:25 pm

6V6 alright.
That and 23 watts.
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Re: Champion GM 8

Postby NoiseNoiseNoise » Thu Jan 12, 2017 2:27 am

Racing wrote:6V6 alright.
That and 23 watts.

Quite a bit more than the stocker, then :D Shame, really: 8 Watts of Tube power could be quite useful for studio/home use.
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Re: Champion GM 8

Postby Racing » Thu Jan 12, 2017 11:21 am

https://www.youtube.com/watch?v=Rm1VzKttrxw&feature=youtu.be

Real poor recording but i hope it hands you guys an idea at least. Remember that this is just ONE of its faces. We set gainlevels,and structure,by playing around with trimpots within.
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Re: Champion GM 8

Postby NoiseNoiseNoise » Thu Jan 12, 2017 1:26 pm

Sound pretty sweet clean AND driven. I envy your talent ;)
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Re: Champion GM 8

Postby Racing » Fri Jan 13, 2017 2:22 am

Thank you. :)
Much of its behavior can be attributed to that small signal pentode,the EF-12. As i´ve touched upon before the span of such a tube is MUCH wider than for a triode. Hence...
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