Interesting Loft Find

Hosted by Marshall Founding Father G3SDW. Discuss JTM45, JTM50, JTM100, Bluesbreaker, Super Lead, Super Bass, 200W, 100W, 50W, 20W, Plexi Panels, Metal Panels, 4x12 straight, 4x12 slant, Pinstripe, Basketweave, Checkerboard, Park, Kitchen-Marshall and Narb

Interesting Loft Find

Postby Tauntonhobbit on Mon Mar 06, 2017 3:27 pm

I thought there might be some interest in this one - no-one as yet has commented on the gut shots, so I thought I'd try over here?

http://basschat.co.uk/topic/301613-anyo ... msearch__1

Cheers!

:)
Tauntonhobbit
Court Jester
 
Posts: 52
Joined: Tue Jul 22, 2014 8:25 pm

Re: Interesting Loft Find

Postby LD50 on Mon Mar 06, 2017 4:33 pm

It's a 1965 JTM45

The rectifier mod is quite common an need care to undo

It looks relatively intact and unmolested, hust needs a clean, service and a try at cap reforming, a new GZ34 and some GEC KT66s, oh and a logo ) cant tell from pics if it is early script or late gold block

Nice loft find, quite like working on these myself.
All bleeding stops.......eventually.
LD50
Prince
 
Posts: 4195
Joined: Tue Jun 12, 2007 12:27 pm

Re: Interesting Loft Find

Postby BygoneTones on Mon Mar 06, 2017 4:41 pm

That is an awesome and rare amp to just "find". Looks like an early to mid 1965 gold block logo amp like these:

Image

Image

Very cool and very valuable. But make sure you get it looked at by a good tech who knows and cares about vintage amps.
BygoneTones
Certified
 
Posts: 146
Joined: Tue Aug 27, 2013 3:53 pm

Re: Interesting Loft Find

Postby Alnico on Mon Mar 06, 2017 9:33 pm

Tell them under no circumstances should they send it to Marshall for "servicing".
Alnico
Knight
 
Posts: 1198
Joined: Thu Nov 03, 2005 6:57 pm

Re: Interesting Loft Find

Postby G3SDW on Thu Mar 09, 2017 5:26 pm

BygoneTones wrote:That is an awesome and rare amp to just "find". Looks like an early to mid 1965 gold block logo amp like these:

Image

Image

Very cool and very valuable. But make sure you get it looked at by a good tech who knows and cares about vintage amps.


Uxbridge Blues Festival 1965, Dudley Craven was there in that picture

Regards

Ken Underwood
G3SDW
Marshall Founding Father
 
Posts: 428
Joined: Fri Aug 28, 2009 8:26 pm
Location: Kirkby in Ashfield, Nottingham, UK

Re: Interesting Loft Find

Postby candletears7 on Thu Mar 30, 2017 1:01 am

Oh my God. Incredible find.
candletears7
Peasant
 
Posts: 6
Joined: Fri Jun 07, 2013 1:11 am

Re: Interesting Loft Find

Postby Ned B on Thu Mar 30, 2017 3:25 am

Here's #3008 Bass. It also has the 560 pf across the feedback resistor.
Image
Image
Ned B
Baron
 
Posts: 2632
Joined: Fri Jun 15, 2001 7:01 am
Location: Chicago

Re: Interesting Loft Find

Postby spangled99 on Wed Apr 05, 2017 3:43 pm

Same as mine - on the Marstran site - serial 3013, a little earlier (than this one, which is 3054)

http://www.marstran.com/65%203013.htm

Looking at the amp in the previous post, you can see where they both had the rather rough cutout to accomodate the Radio Spares OT. This happened for a while.
spangled99
Baron
 
Posts: 2625
Joined: Tue Mar 02, 2004 8:01 am
Location: UK

Re: Interesting Loft Find

Postby LD50 on Wed Apr 05, 2017 5:24 pm

I believe a forum member (at least was a member) is now in possession of said amp. 8)
All bleeding stops.......eventually.
LD50
Prince
 
Posts: 4195
Joined: Tue Jun 12, 2007 12:27 pm

Re: Interesting Loft Find

Postby bobster on Thu Apr 06, 2017 8:49 pm

Hi , i have a slightly later script bass model. Can i ask the more experienced here what was the the thinking behind the 560pf cap over the feedback resistor?
What would this do to the overall sound?
thanks !
b
beelzebub.....be boppa loobub
bobster
Knight
 
Posts: 1216
Joined: Wed Sep 01, 2004 7:01 am
Location: scotland with a moose loose aboot this hoose

Re: Interesting Loft Find

Postby LD50 on Thu Apr 06, 2017 9:13 pm

Simple bypass cap, presumably to tighten up the bass, I confess I would have to look at th Duncan calculator to see where the shelving point is. Preumably a reverse effect to what might happpen in a preamp due to it being in the NFB loop.
All bleeding stops.......eventually.
LD50
Prince
 
Posts: 4195
Joined: Tue Jun 12, 2007 12:27 pm

Re: Interesting Loft Find

Postby Vortexion on Thu Apr 06, 2017 9:30 pm

bobster wrote:what was the the thinking behind the 560pf cap over the feedback resistor?
What would this do to the overall sound?


OK, here goes. Sorry if this is a bit over-wordy.

It's called a "phase compensation" capacitor. When we play through an amplifier we tend to assume that the sound is coming out of the speakers at exactly the same instant we're playing the notes, right? In fact, the process of amplification - getting all those electrons to jiggle about and eventually drive a speaker - introduces a very tiny delay.

Now, think about that for a minute, and you'll see that if the amp uses a negative feedback loop, such a delay could become an issue. Throughout most of the audio frequency range it isn't an issue at all, because the amount of delay is miniscule compared to the length of the waveform, but as we get into the high frequencies, the delay effectively puts the NFB out of sync with the original signal. The general effect of this is that, at these higher frequencies, the negative feedback is less effective than it should be at doing what we want it to do (which is mainly to make the amplifier reproduce in a controlled, even fashion and to increase speaker damping). Consequently, the high frequency end of things can end up a bit loose, peaky and uncontrolled.

So how do we get things back in sync at these high frequencies? Well, the short answer is that we can't, because nobody's yet invented a time machine that will undo the delay! :mrgreen: ...but what we can do is to increase the amount of high frequency information being fed back, so that the NFB effect becomes more pronounced at these frequencies. Designers of valve Hi-Fi amplifiers go to some lengths to optimise this effect, introducing sometimes complex, finely tuned capacitor/resistor filter networks into the NFB loop - but hey, this is an instrument amplifier folks, so let's be thankful that the manufacturer thought it worth spending a few pennies on a 560pF cap. :wink:

In case you were wondering how they choose the right value of cap, it's usually done by injecting a square wave signal into the amp and looking at the shape of the waveform that comes out of the speaker output. They then try out different values until they find one that makes the square wave look as smooth/accurate as possible.

So, to put all of the above in a nutshell, the cap is there to tame any tendency to unruly top end or instability - not things that a bass player is likely to welcome.

Hope that made sense! :fingersx:
Vortexion
Newt
 
Posts: 1251
Joined: Thu Jan 10, 2008 4:28 pm
Location: Norwich, UK

Re: Interesting Loft Find

Postby LD50 on Fri Apr 07, 2017 6:29 am

Vortexion wrote:OK, here goes. Sorry if this is a bit over-wordy.

It's called a "phase compensation" capacitor. When we play through an amplifier we tend to assume that the sound is coming out of the speakers at exactly the same instant we're playing the notes, right? In fact, the process of amplification - getting all those electrons to jiggle about and eventually drive a speaker - introduces a very tiny delay.

Now, think about that for a minute, and you'll see that if the amp uses a negative feedback loop, such a delay could become an issue. Throughout most of the audio frequency range it isn't an issue at all, because the amount of delay is miniscule compared to the length of the waveform, but as we get into the high frequencies, the delay effectively puts the NFB out of sync with the original signal. The general effect of this is that, at these higher frequencies, the negative feedback is less effective than it should be at doing what we want it to do (which is mainly to make the amplifier reproduce in a controlled, even fashion and to increase speaker damping). Consequently, the high frequency end of things can end up a bit loose, peaky and uncontrolled.

So how do we get things back in sync at these high frequencies? Well, the short answer is that we can't, because nobody's yet invented a time machine that will undo the delay! :mrgreen: ...but what we can do is to increase the amount of high frequency information being fed back, so that the NFB effect becomes more pronounced at these frequencies. Designers of valve Hi-Fi amplifiers go to some lengths to optimise this effect, introducing sometimes complex, finely tuned capacitor/resistor filter networks into the NFB loop - but hey, this is an instrument amplifier folks, so let's be thankful that the manufacturer thought it worth spending a few pennies on a 560pF cap. :wink:

In case you were wondering how they choose the right value of cap, it's usually done by injecting a square wave signal into the amp and looking at the shape of the waveform that comes out of the speaker output. They then try out different values until they find one that makes the square wave look as smooth/accurate as possible.

So, to put all of the above in a nutshell, the cap is there to tame any tendency to unruly top end or instability - not things that a bass player is likely to welcome.

Hope that made sense! :fingersx:


Thank you, elegantly expresses the issue.
All bleeding stops.......eventually.
LD50
Prince
 
Posts: 4195
Joined: Tue Jun 12, 2007 12:27 pm

Re: Interesting Loft Find

Postby G3SDW on Fri Apr 07, 2017 11:54 am

I am sure that if Dudley was still with us then he would not have been able to explain it any better, as it was him that would have been responsible for this addition.

Thanks for the explanation which is not easy to put in simple terms.

Regards

Ken Underwood
G3SDW
Marshall Founding Father
 
Posts: 428
Joined: Fri Aug 28, 2009 8:26 pm
Location: Kirkby in Ashfield, Nottingham, UK

Re: Interesting Loft Find

Postby bobster on Fri Apr 07, 2017 3:03 pm

Great stuff guys , many thanks for the detail ;)
b
beelzebub.....be boppa loobub
bobster
Knight
 
Posts: 1216
Joined: Wed Sep 01, 2004 7:01 am
Location: scotland with a moose loose aboot this hoose

Next

Return to Marshall

Who is online

Users browsing this forum: No registered users and 4 guests