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Author Topic: Bassman to JCM Conversion Project  (Read 788 times)

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Offline gc

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Bassman to JCM Conversion Project
« on: October 09, 2017, 07:52:55 PM »
Just can't stop this guano.  Good for the soul.  Here are some pics on Imgur of the overall effort: 

https://imgur.com/a/u3a99

I had an old Fender Bassman 100 clone that was sitting around broken & waiting for attention.  Was going to 'restore' it but decided instead to rebuild it as a JCM 800.  I kept the 100w/4 EL34 tube configuration of the clone but have it running on 2 tubes for 50W. 

Using schematics & layouts from Marshall & Mark Huss, replaced the old eyelet board with a new Marshall 2203(100W)/2204(50W) layout, stuffed it full of components, rebuilt the capacitor 'doghouse' with values to match, fought power & ground issues & bad tubes & came away with a decent sounding amp.  Tranny's, choke & chassis about all that's left of the original.  Still cleaning up lead dress but it's pretty much all it's going to be.

Sweetwater had Celestion Seventy-80 refurb 12's for @ $30 USD ea, so I grabbed two & stuck them in an old cab.  Sound ok...see if I can break them in a bit. 

I never was much good at posting sound clips...but once again, I'll try to put something together & post it.

Regards,

Gordon

Offline Joom

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Re: Bassman to JCM Conversion Project
« Reply #1 on: October 09, 2017, 09:03:21 PM »
Nice pics. GC.  And looking forward to the sound clips...  :)
A computer once beat me at chess, but as it turns out it was no match for me at kickboxing...

Offline neilmac

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Re: Bassman to JCM Conversion Project
« Reply #2 on: October 10, 2017, 02:01:10 AM »
Looks the part....
Looking forward to the sound  ;D
"If it looks good, you'll see it.If it sounds good you'll hear it. If it's marketed right you'll buy it. But if it's real, you'll feel it"

Offline Jahva

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Re: Bassman to JCM Conversion Project
« Reply #3 on: October 10, 2017, 12:42:59 PM »
Oh that is an awesome idea and project...
Love the JCM 800 solid staple of Rock with Gibson humbuckers or the single coil Strats.
Good Luck with it GC.
Fight on brother!  ~:)
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Offline frankenxtein

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Re: Bassman to JCM Conversion Project
« Reply #4 on: October 14, 2017, 03:27:52 AM »
Very nice  :) Do the Zeners get very hot?
Okay ... lets do this one more time ... take 43 http://www.soundclick.com/bands/default.cfm?bandID=1277143

Offline gc

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Re: Bassman to JCM Conversion Project
« Reply #5 on: October 14, 2017, 03:38:56 PM »
FX...yup, they get a bit hot.  Enough to give me pause & ask around a few places to hear others experiences with this method. 
Had a Bassman '100' clone from the 60'-70-2 sitting around, broken & decided to fix it.  Existing insides were a mess of broken/reuse parts & aged components.  Decided to reuse iron & grab what I had available for rework.  JCM 800 it is.  Had to put it together somewhat upside down & backwards & adapt the capacitor doghouse-can setup to fit.  Lead dress suffered accordingly but works ok.  Interesting 'make-do- project.

121VAC line voltage here.  PT tests out at 405-0-405.  That drops off 501+VAC at the rectifier bridge, 488VDC on the drop side (loaded with tubes).  W/O tubes I was reading over 500VDC at the B2+ cap (rated 500VDC).  Fixed bias w/o PT CT (pulls bias voltage off one PT HT secondary leg).  Zeners dragged it down to acceptable levels.  Price of power dissipation?    @116-120 deg F by my infrared rectal barometer, tubes in, sitting at idle.  Still in the shake down mode, so I'll see where this goes as far as longevity.  Works better than a dropping resistor.

Not used this method on this large an amp.  Not sure of how stable or trustworthy this will be.   Trying to get a handle on the amps 'tone'.  One really haws to lean on these to get 'that sound'.  And this amp doesn't even think about getting into it's groove until ear bleed levels..

Offline frankenxtein

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Re: Bassman to JCM Conversion Project
« Reply #6 on: October 16, 2017, 12:59:51 PM »
120.F doesn't seem that hot  8) Is this a old PT ?

Bucking a little of the primary goes a long way on the HT. From what I've read 6v is enough on the heaters so there's a juggling contest with newer transformers. Half winding on my 12vct bucking transformer gives me 115-116v. Take into account the bucking transformer has a 115v primary  :P 121-123v here on the outlet depending season and time of day. Really messes with my OCD when I check voltages in a amp  AM then do it in the PM.

 https://www.tubesandmore.com/products/transformer-hammond-filament-63-vct-4-open-style
Okay ... lets do this one more time ... take 43 http://www.soundclick.com/bands/default.cfm?bandID=1277143

Offline gc

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Re: Bassman to JCM Conversion Project
« Reply #7 on: October 16, 2017, 02:33:44 PM »
Yup...this is from circa 1960-70.  PT & OT are huge iron, built when men were men & sheep were scared.  And I'm really trying to get this to work & remain reliable using what's there.  Zener bridge is weak link & have yet to find a truly solid solution.  Zener works, but this is a rather large amp.  Still tinkering with the maths & Senior Ohms to see what science says.  But so far so good as far as holding up.  Bucking xformers, Variacs, dropping resistors, Mosfet ckts, even converting to a tube rect are considerations. 

Simple solution is get a new, correct voltage/rated PT.  Don't want to spend the $$$.  I played thru this for a bit yesterday at some volume & managed to see the temp get to 151 deg F.  Still not bad.  Still shaking down this sucker.  Never met a Zener that I trusted.

Tell you one thing though...even with the leads extended from the 'cap doghouse' and the overall compromise Marshall to Fender layout,this sucker is very quiet in the buzz & hiss dept.  Still trying to dial in the tone & gain.  Sensitive little thing, this.