JTM45 Build

Reeltarded · Post by **Reeltarded** » Sat Jun 20, 2015 12:49 am

I do not like JJ preamp tubes. They are more inconsistent than any others I can think of and EH ax7s are gritty as dirt, often too much for a 4 holer. Middy and trashy.. If you have trouble figuring that out try some cheap unmarked Chinese junk. You might be surprised heh.

I was.

norburybrook · Post by **norburybrook** » Sat Jun 20, 2015 9:15 am

Littlewyan wrote:Thanks! Didn't expect that kind of response! I had a lot of fun building this amp. I will try and do a clip for you guys at some point, haven't got any proper recording equipment but I'll see what I can do!

I think the tone of the amp is off as I've got JJ ECC83S Valves in V2 and V3 which are very dull sounding compared to original Mullard 12AX7s! I've got a JJ ECC803S in V1 which can probably stay, tomorrow I'll try my EH12AX7s in the amp and my JJ ECC83MG. At the moment I'm having to run the bass on 0 and the rest of the controls on 10!

It's quite normal on a JTM45/bassman to have to turn the bass right down

they don't call it a BASSMAN for nothing

The top end should be nice and chimey though in the high treble channel.

MC

norburybrook · Post by **norburybrook** » Sat Jun 20, 2015 9:19 am

Reeltarded wrote:I do not like JJ preamp tubes. They are more inconsistent than any others I can think of and EH ax7s are gritty as dirt, often too much for a 4 holer. Middy and trashy.. If you have trouble figuring that out try some cheap unmarked Chinese junk. You might be surprised heh.

I was.

I've found the opposite to be true of JJ's , they've been very consistent for myself and a friend who's built about 20 amps or so over the last few years commercially.

Ah...the vagaries of tubes

Marcus

statorvane · Post by **statorvane** » Sat Jun 20, 2015 9:39 am

Ah...the vagaries of tubes

... and the ears.

Littlewyan · Post by **Littlewyan** » Sat Jun 20, 2015 12:50 pm

OK so I've done a bit of testing. Messed around with the EQ a lot and found that at the moment it sounds best with everything on 10 except for the Bass and Normal controls, they need to be on 0. I worked out that the JJ ECC803S in V1 was making the amp sound harsh as hell and as soon as I put a JJ ECC83MG in there it sounded so much smoother.

Will continue testing. Not sure if I have any chinese valves but I do recall my old 1987xl having chinese valves in it and that thing rocked!

rp · Post by rp » Sat Jun 20, 2015 1:10 pm

Looks damn fine in and out, parts and build. Love the 2-tone! It can't help but sound good, leave it on when you're home and burn in the filter caps. Try to find some ANOS tubes for it - A couple mullards or brimar 12ax7s and a good used set of USA 5881s should knock the edge off and sweeten it up.

Littlewyan · Post by **Littlewyan** » Sat Jun 20, 2015 1:17 pm

Well after playing around with it some more I think I like it with the middle on 5. Tried my EH12AX7s in V2 and V3 and it sounds much better. Another thing I noticed with it is if I hit an open E really hard the sound dies down a bit and comes back. Is this due to the low 32uF reservoir capacitor?

Thanks for all of the kind comments by the way, much appreciated

ToneMerc · Post by **ToneMerc** » Sat Jun 20, 2015 1:24 pm

Littlewyan wrote:The amp is complete!

Started up first time without any issues so am chuffed with that. It sounds ok at the moment, may need some tweaking as I can't get a tone out of it that I really like and it doesn't seem to have enough gain. Going to do some valve swapping tomorrow as I know what these amps can do so I'm confident I can get my tone out of it .

One thing I am concerned about is I accidentally melted part of the 0.1uF Cap in the Phase Inverter circuit. Was holding my iron on a turret inbetween the 0.022uF Cap and 0.1uF Cap and slipped . I don't know if its affected the function of the cap or not, I did measure it and it measured 0.1uF so I think its ok.

Going to order a 0.05uF Capacitor to go on the hot side of the standby switch and may order a new 0.1uF Cap at the same time.

Great looking build!

TM

Littlewyan · Post by **Littlewyan** » Sat Jun 20, 2015 2:01 pm

Ah I think there might be some oscillation going on. I found that if I turn the treble down to 8 then I no longer get the issue of the sound dying out when hitting a bit chord. I know these Marshalls are prone to some oscillation with the presence and treble up high. I can shorten the lead for the presence control a bit, will see if that helps.

joeboo88 · Post by **joeboo88** » Sat Jun 20, 2015 2:13 pm

Beeeeuuutifal job Ryan!
I hope you dial it in the way you like it and let us know your settings. I also have my bass on 2 and that's it. If you figure how too get more gain let us know. I noticed that you have a pot for PPMIV? I did that and found the overdrive I get with the 'MV turned down, sounds ugly. So I just turn up the MV up all the way.
Don't stick that amp in a wooden cabinet it looks too good

rp · Post by rp » Sat Jun 20, 2015 2:27 pm

Another thing I noticed with it is if I hit an open E really hard the sound dies down a bit and comes back. Is this due to the low 32uF reservoir capacitor?

That doesn't sound right. These amps (+5F6A) may be primitive but are full of life and rearing to go. Flawless tone machines unless it's just not your thing. They are not high gain and prone to oscillation so it should be a minor bug just have to find it. Maybe a wire in the wrong place. That dropping out could be a cold solder joint or a wanky socket pin not making a good grip to.

Littlewyan · Post by **Littlewyan** » Sat Jun 20, 2015 2:38 pm

Just tested the amp again and as soon as I turn the treble up above 5 and start hitting open chords it sounds like blocking distortion. It only seems to be the treble control that does it and it doesn't happen when using the Normal channel.

rp · Post by rp » Sat Jun 20, 2015 2:49 pm

Littlewyan wrote:Just tested the amp again and as soon as I turn the treble up above 5 and start hitting open chords it sounds like blocking distortion. It only seems to be the treble control that does it and it doesn't happen when using the Normal channel.

This and your other tone dissatisfaction sounds like an oscillation. Bummer when it's all so beautifully wired up. If the amp is out of the cab and it's just sitting idling or played softly and either of the trannies is oddly hot or gets hot too quickly that's a sign too.

Littlewyan · Post by **Littlewyan** » Sat Jun 20, 2015 2:53 pm

I'll start shortening wires. I don't want to resort to shielded wire everywhere as it looks nasty in these old amps and I know they can work without it!!

Strange thing is the amp still cleans up ok. Very odd. The transformers don't get hot that quick but I did notice the rectifier gets hot very quickly. It sounds just like blocking distortion its very strange. Especially as the treble control would be the last thing that causes blocking distortion surely!

Reeltarded · Post by **Reeltarded** » Sat Jun 20, 2015 3:56 pm

Parallel signal wires from different stages.. split that out.

It should be very forgiving. What value is the PI fizz cap? 47p original, 100p suggested.

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