Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on April 18, 2018, 02:11:03 pm
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Hi guys, This rare gem was mention on my local forum, so I was compelled to do a clone.
Fortunately the 6GW8 valve is still kinda available, but getting expensive.
A local guy has one in his shed and put together a detailed description of a restoration with great pics.
https://ivanrichards.com.au/2012/03/01/a-very-rare-marshall-jmp/
From his pics I'm able to copy the circuit board layout.
The Marshall schematic is a bit hard to read but I was able to nut it out.
There is also some variations of the circuit when comparing my schematic to other amps that I was able to find on the net.
(https://i.imgur.com/feELdtQ.jpg)
Hope to get it to sound something like this.
The one thing that caught my eye is the plate resistors that feed V1a and V1b.
The Marshall schematic shows a 220k for V1a and 100k V1b on another amp they are both 100k, so I could imagine this one would have a much cleaner sound?
http://www.chambonino.com/work/work11g.jpg
Working backwards, cab is being built similar to the Marshall.
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I like the simplicity of the tone stack, I might experiment :laugh:
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Hey shooter, Just waiting on parts.
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Just waiting on parts.
:laugh: I'm waiting on money, so I can wait on parts
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I think there may be an error on the tone controls; as shown, they look to be just a series volume control, albeit one that gets more muffled as it's turned up.
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This schematic says this is a 10 watt amp.. what are the power tubes? EL 86? That's what it looks like.. http://el34world.com/charts/Schematics/files/marshall/Marshall_jmp_1930_10w.pdf (http://el34world.com/charts/Schematics/files/marshall/Marshall_jmp_1930_10w.pdf)
Maybe this isn't the same amp.
Pretty simple amp and it sounds great.
Also I think this PT is pretty low voltage ,, do you know what a suggested PT would be?
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Ahh the tubes are ECL86 here is the schematic http://www.bolimbo.dk/guitarer/forstaerkere/marshalltype/Popular/ukPopular.htm (http://www.bolimbo.dk/guitarer/forstaerkere/marshalltype/Popular/ukPopular.htm)
a thread here said the PCL86 was a substitute but the heaters are 13 volt.. guess you could run them on 12 volts (series)
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If you want to build one just get a Hammond AO-44 for a donor.
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TIMBOooooooo
IMO, the amp sounds a little thin in the video.
Not surprising seeing how the last coupling cap before the PI is 500p :w2:
I'd have to try a larger value just to hear the difference, maybe on a switch...paralleling two different types might even be beneficial :dontknow:
Always enjoy seeing what you're up to.
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TIMBOooooooo
IMO, the amp sounds a little thin in the video.
Not surprising seeing how the last coupling cap before the PI is 500p :w2:
I'd have to try a larger value just to hear the difference, maybe on a switch...paralleling two different types might even be beneficial :dontknow:
Always enjoy seeing what you're up to.
about 300Hz -3dB with 500pF. 2.2nF would be a better choice. i'd start with 1nF, then 1.5nF, then maybe up to 3.3nF. probably settle on 2.2nF
matchless type switchable coupling C selector might be just what the dr. ordered... :icon_biggrin:
--pete
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If you want to build one just get a Hammond AO-44 for a donor.
dang steve! i just thinking the same thing and trying to remember where i put them, but now suffering from CRS.
--pete
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Thanks for the input guys.
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Hi guys, parts in.
(https://i.imgur.com/hLxsJCA.jpg)
(https://i.imgur.com/X49oPY2.jpg)
(https://i.imgur.com/XgKrU18.jpg)
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Ahh the tubes are ECL86 here is the schematic http://www.bolimbo.dk/guitarer/forstaerkere/marshalltype/Popular/ukPopular.htm (http://www.bolimbo.dk/guitarer/forstaerkere/marshalltype/Popular/ukPopular.htm)
The tone controls in that circuit would work properly.
I would make it single channel, feed the input straight to the input stage and move the vol/tone before the LTP.
The 500pF may be helping to keep thump from the trem oscillator flapping the speakers around too much.
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Hi guys, spotted a mistake the .68uf cap should be .068uf.
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Hi guys, circuit board done.
(https://i.imgur.com/XF9WPqS.jpg)
(https://i.imgur.com/3JNFvsQ.jpg)
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Hi guys, The cab is partly built and the chassis been made....
(https://i.imgur.com/9GHpw0f.jpg)
(https://i.imgur.com/qiWyRRl.jpg)
So the fun begins :icon_biggrin:
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Nice work, looking fwd to your evaluation;
did you have to resort to steel because our aluminum tariffs :icon_biggrin:
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Hey shooter, As much as steel is a pain to work with, I can use a lighter gauge and it is a bit cheaper.
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Updated board layout.
(https://i.imgur.com/BUn3HES.jpg)
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Hi guys, With the heater wiring sorted, things are moving along nicely.
Another schematic update.
(https://i.imgur.com/TCOteUT.jpg)
I have added a switchable rectifier, this worked well in another amp.
Chassis wiring just about complete.
(https://i.imgur.com/MJfENX2.jpg)
(https://i.imgur.com/rp2ARZn.jpg)
(https://i.imgur.com/xpqLM1O.jpg)
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Hi guys, Circuit board is in and wiring checked.
(https://i.imgur.com/qC22wVu.jpg)
(https://i.imgur.com/yJKZDqd.jpg)
(https://i.imgur.com/JRsliSu.jpg)
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very nice!
--pete
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Thanks pete, We have sound.
And it is living up to it's reputation of noise and drive. :drool
First up, the noise I'm mainly getting is from the fluro above my bench, so I'm hoping that will subside once in the cab.
It does have a low volume hum that does not increase with volume, maybe from heater wiring.
Sound is great, but I think some voltage dividers may tame some of the gain.
Humbuckers really push the issue, awesome!!!
Reverb needs some tweaking.
I have seen it connected to the other half of the phase inverter???
Otherwise the two mixing resistors 100k/330k may need to be adjusted.
Possible a dwell pot (set and forget) mounted in side chassis.
Issues....
Low level hum.
I'm thinking to elevate heaters.
Can the heater Artifical CT be connected to just one 6GW8 cathode, is 8v DC be enough??
Reverb pot scratchy.
Thought it might be the .01u coupling cap leaky.
Replaced with another, still scratchy????
Probably need to address these issues before I proceed any further.
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Thanks pete, We have sound.
And it is living up to it's reputation of noise and drive. :drool
First up, the noise I'm mainly getting is from the fluro above my bench, so I'm hoping that will subside once in the cab.
It does have a low volume hum that does not increase with volume, maybe from heater wiring.
Sound is great, but I think some voltage dividers may tame some of the gain.
Humbuckers really push the issue, awesome!!!
Reverb needs some tweaking.
I have seen it connected to the other half of the phase inverter???
Otherwise the two mixing resistors 100k/330k may need to be adjusted.
Possible a dwell pot (set and forget) mounted in side chassis.
Issues....
Low level hum.
I'm thinking to elevate heaters.
Can the heater Artifical CT be connected to just one 6GW8 cathode, is 8v DC be enough??
Reverb pot scratchy.
Thought it might be the .01u coupling cap leaky.
Replaced with another, still scratchy????
Probably need to address these issues before I proceed any further.
In the schematic it looks like the reverb pot is seeing DC off of V1 plate (through 100K then 330K resistors)
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about 300Hz -3dB with 500pF. 2.2nF would be a better choice. i'd start with 1nF, then 1.5nF, then maybe up to 3.3nF. probably settle on 2.2nF
matchless type switchable coupling C selector might be just what the dr. ordered... :icon_biggrin:
--pete
[/quote]
Thanks pete, I replaced the 500p with a 560p as suggested by another guy that did a clone, but it seems that the 500p or 560p it needed.
I replaced it with a .01 and this caused a heap of thumping and speed increased, so it put it back to 560p.
The trem is a bit fickle, had to try a few AX7s to get it to work, but it makes the valve micro phonic. :dontknow:
SILVERGUN, I measured 33v DC at the wiper of the reverb pot so I changed the .01 to .033 pot still scratchy.
I took a voltage reading at the junction of the 330k and the top of the pot and there was no DC.
?????
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That tremolo circuit is almost identical to the 18W trem which is well known to be finicky. See schematic which shows my band aid fix for my 'ticking' trem...
http://sluckeyamps.com/18w/18w.pdf
Also, notice that the 100K/220K plate resistor positions are reversed between the two schematics. Might be a factor. :dontknow:
To get rid of the DC voltage on the reverb pot you need to ADD a coupling cap between V1-1 and point RV. Or, you could add a coupling cap between the lower point RV1 and the upper point RV1. The voltage SG speaks about is coming from V1-1.
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That's a beautifully done build Timbo.. :icon_biggrin:
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Hi guys, I was able solve a few issues.
Scratchy reverb pot.
A .01u cap was placed between the 330k mix resistor and the pot lug, this has remove the DC and the pot is now clear.
Micro phonic V1.
I putting this down to the valve just operating on the low plate voltages.
The 220k/100k resistors that are connected to v1 plates could be placed in a number of ways and from the photo's on Ivan's site suggests that the original Marshall circuit is how it should be, but finding that other amps may be wired differently as per some other photo's of the circuit board.
I mucked around with this part of the circuit for sometime and had no real joy with sound or trem function.
Sluckey offered this as a solution http://sluckeyamps.com/18w/18w.pdf
This worked a treat with a strong trem and NO micro phonic valve.
One other issue I was having was the reverb didn't have much boing, but what was there I felt was sufficient, on inspection of the tank I found a piece of packing foam still wedge under the springs, removing piece of foam resolved this issue. :frowny
(https://i.imgur.com/GtWmTF0.jpg)
(https://i.imgur.com/KvXeQkt.jpg)
I liken the hum to something like the background hum that you hear from a PA system.
I have grounded the 2x16uf filter caps to the ground buss that runs along the back of the pots. The low level hum is still there.
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Nicely done, congratulations!
I bought one of those when I was a student, around 1982 I guess, for £45. It was the 2x10 version. Sounded great up loud, quite raw but in a good way, but was noisy, trem didn't work.
I sold it a year or two later for the same amount to buy a solid state amp. :dontknow: Would love to have it back!
There was an article on them last year in The Guitar Magazine. It goes into the noise issue and also how to convert them to an 18W. They used the same chassis, with one of the valve base cutouts blanked off. There's an online version of it here, might be of interest for you:
http://www.theguitarmagazine.com/workshop/diy-workshop-marshall-1930-popular/ (http://www.theguitarmagazine.com/workshop/diy-workshop-marshall-1930-popular/)
The modified amp from the article has been up for sale recently for £2,000 !
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Thanks JB, I had read that article as part of my research and the circuit board photo that shows the 220k/100k plate resistors connection.
Going by the description my hum level is probably much lower than he describes.
And the non working trem (although it did work at times) I'll put it down to the plate voltage too low to get it to start.
Also JB, I bought that magazine and built my version of the Denis Cornell's "Stinger Amp"
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just an afterthought. you could have used another 6GW8 for the reverb driver and recovery.
that circuit is bound to be noisy with all that shaping network in front of the first stage. perhaps a mu-metal shield around those components would help.
--pete
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Hey pete, Getting some more done to the cab, so I can get a better idea of the noise level.
(https://i.imgur.com/HoS6gOF.jpg)
(https://i.imgur.com/RgtJLpE.jpg)
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OK guys, Looking for ideas.........
With humbuckers plugged in I have to turn the guitar volume to "5" as I am getting what sounds like blocking distortion.
Is this possible off one triode gain stage.
OR is it a result of the low plate voltage on V1a.
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Hi guys, I think I've got this one sorted.
(https://i.imgur.com/CJDkgnB.jpg)
I'm thinking the insertion of the reverb mixing resistors were causing some problems.
Also converting the 220k/100k plate resistors back to the original schematic is working much better and finding the right valve to do the job is needed.
The 5k trim is needed to find the sweat spot to make the trem function.
On the schematic I have shown the grounding, this also has helped with some of the hum.
The reverb is huge.
And connecting it as shown on the schematic has worked well.
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Hi guys, To manage the reverb I added a set and forget trim pot, this has helped with the amount of reverb.
Still trying to reduce the amount of fizz/hum although it's not much but enough to be annoying. :cussing:
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Wondering why you would ground the transformers to the preamp ? Although , maybe I'm not seeing the pictures correctly .
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Hi guys, Doing some hard thinking to get this one sorted.
Adding another valve seems to be the only option.
(https://i.imgur.com/tjUgr4M.jpg)
With some info from sluckey's 18w
http://sluckeyamps.com/18w/18w.pdf
The 6av6 should do the job nicely, as the smaller heater draw will be less stress on the PT.
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Hi guys, This build is really :BangHead:
I have tried so many mods and still the hum persists.
The hum does not increase when the volume is turned up.
It decreases when the 12ax7s/6AV6 are removed but not totally.
It increases when the SS rectifier is engaged.
It does not change when wire are moved around.
(https://i.imgur.com/1QhSpRJ.jpg)
:help:
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Does hum change when you turn reverb up or down?
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Take out V4 & 5 and see if it's partly at least inductive coupling into the output transformer.
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I have isolated the hum to the 6gw8s, replaced with other gw8s and no change.
The reverb circuit does increase the hum but it does not increase the volume of the hum when turned up.
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Try another OT.
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Try another OT
I think Sluckey's idea is reasonable one to try.
Timbo, you've got years and years of success with your amp building and implementing good designs. So, if you're still having problems with hum, I don't think it's a wiring or layout problem given the your high level of skill and expertise. You would've probably resolved anything like that at this point.
And given it seems to be isolated with the tube and changing tubes not making much difference, the OT sure seems like a possible suspect?
with respect, Tubenit
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Thanks T.
I have tested the OT and it seems to be OK.
This is what I am using.
(https://i.imgur.com/7amJglw.jpg)
From other amps using the gw8s suggest the plate load to be 10k.
From the test I've done the plate load works out to be about 8.7k with an 8ohm speaker on tap 2&5
Also on tap 1&4 it's about 13.7k
Could a heavier load be better??
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Try another OT.
I have a PT that hums, every time I use it I have an excessive amount of hum, I figured it out on the 3rd proto-type build :think1:
everything works fine, voltages good, no help with orientation, it's been relegated to BB PT
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Thanks guys, For the moment I have ruled out the OT.....
Doing some probing and grounding the signal @ "RV" on the schematic in reply 38, kills both the guitar sound and the hum.
Disconnecting the .001u to the reverb input and turning the reverb pot to 0 has removed the hum and both channels are hum free and working well.
Replaced the .001u with another also tried a 500p but the hum remains.
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:cussing:
WELL.................
WTF>>>>>>>>>
Found it.
Simply connected the reverb driver transformer to node"B" lush strong reverb and completely hum free.
Now this is upsetting...
Was the original Marshall popular circuit OK????
Seems I'll have to build another to find out. :l2: :laugh: :icon_biggrin: :help: :sad2:
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That's called a character building exercise! Soundclips?
Congrats!
Jim
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Thanks mate.
Might be waiting awhile, need to build a new cab.
The one I built for this one is home to the 6V6 build I did.
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Hi guys, The Marshall Popular MKII is now on the bench.
My local supplier has a bunch of 14GW8s on hand and so they have sourced a transformer with a 14.5v tap and 6.3v for the preamp valves.
(https://i.imgur.com/GlfbJP3.jpg)
(https://i.imgur.com/MDZzV3P.jpg)
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Hi guys, New board done.
I changed the value of a couple of the capacitors as this is all I have.
I don't think it will change the tone/sound that much.
(https://i.imgur.com/T3HnuAC.jpg)
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Hi guys, new board is in and wired up.
(https://i.imgur.com/o6HblHO.jpg)
(https://i.imgur.com/GLIzbuP.jpg)
(https://i.imgur.com/kLK4Va8.jpg)
(https://i.imgur.com/ZOoC48Q.jpg)
Updated schematic.
As you can see the OT is Ultra Linear, for no real reason other than this is the OT I had on hand.
So it's worth a try.
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it's worth a try.
I like using UL, cathode biased, seems to create a secret sauce :icon_biggrin:
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Thanks shooter, good to know. :icon_biggrin:
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Hi guys, The Popular II has kicked off well.
The reverb circuit was not connected as I was wanting to see what the original circuit noise level was like.
As stated before there is a background hum of an acceptable level, some buzz is present, but I'm hoping that will reduce once inside the cab.
Reverb circuit was connected, this did not add any additional hum.
Reverb is lush and plenty of it.
(https://i.imgur.com/haqGCdW.jpg)
(https://i.imgur.com/H46o5qf.jpg)
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Hi guys, A mate was kind enough drop in to have a bash.
I think it does the Marshall thing well and does sound similar to the video clip in the first post.
Noise wise, it is more than acceptable at about half volume but is much nosier at full volume.
If this and was sold as a "cheap" practise amp half volume is more than enough volume for bedroom practise.
Tone pots have two rolls, tone of course with a good range between dark and light.
When turned to the bass sound is muddy and dull, turned to treble sound is bright and gainy. Me like it.
Roll the guitar back and cleans are great with a bit of hair. ( P90's in a LP JR)
Guitar at max and it's Marshall all the way.
Very happy with the end result.
(https://i.imgur.com/IWphYgX.jpg)
(https://i.imgur.com/9CExSoF.jpg)
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Wow! That looks super nice! Thanks for sharing the photos and info.
With respect, Tubenit
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nice work! :-)
--pete
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:cussing:
WELL.................
WTF>>>>>>>>>
Found it.
Simply connected the reverb driver transformer to node"B" lush strong reverb and completely hum free.
Seems the B+ at node A wasn't clean enough to feed the verb driver stage?
The output stage being PP has noise cancellation built in, but the SE verb driver stage doesn't.
Man, nice work sticking with it! :bravo1:
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Hi guys, Thought I'd have another go at getting the amp a bit quieter.
A few ideas have popped up over the last year or so...
(https://i.imgur.com/5aij9PH.jpg)
I have added a voltage divider to simulate a volume pot to tame some of the ratty fizzy noise that is generated from the preamp.
I think what I have done may have helped.
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Guy's this one needs to be finished.....
Some time injected into this has some good results..
(https://i.imgur.com/o7U7kgr.jpg)
I returned the circuit to the what I believed was the original Marshall circuit. (reverb disconnected)
The trem was a problem.
Finding the sweet spot of the 5k trimpot had the speaker flapping all over the place, but the trem was working.
I had played around with a voltage divider placed before the PI, and this somehow fixed the pumping speaker.
The sweet spot of the trim is about centre of the pot rotation.
I have since adjusted the voltage divider and it does not appear to affect the overall sound of the amp
I have replaced some of the coupling caps to be closer to the original circuit.
Fizz/hum has been reduced by adding a cap to ground at the junction of the 220k/100k plate resistors
There is a very faint background noise, I can live with.....
(https://i.imgur.com/60s7FkJ.jpg)
(https://i.imgur.com/OcYeDzJ.jpg)
(https://i.imgur.com/nxlyB6P.jpg)
A new cab built with an old Celestion to give it a Marshall flavour
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Cab completed
(https://i.imgur.com/UfIgyJV.jpg)
(https://i.imgur.com/xaPpXkM.jpg)
(https://i.imgur.com/Pcifgqz.jpg)
(https://i.imgur.com/GGr231Y.jpg)
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Looks great !! Now put some skull knobs !!
(https://images.reverb.com/image/upload/s--VRsa5t5m--/f_auto,t_large/v1479272027/q0udtysijioxrq9qijle.jpg)
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The cab looks great!