Separate names with a comma.
Discussion in 'Let's Talk Vintage' started by Flano_mark, Jun 29, 2018.
Let me know when you're coming through Chicago. We do get warm weather for part of the year!
Back at this once again!
The O/P TX and Choke are just back from Marstran now so ill start the referb over the next few months. Here are a few more pictures of the cab just for further info. If anyone has any info or opinions it would be much appreciated. There are a number of peculiarties on the cab, and I think its been subject to quite a bit of modification over the years; Handle, Grille cloth, possibly some tolex and maybe the baffle might have been replaced at some point. Oddly, the handle (which is long gone) was drilled out on a 4 bolt pattern (which is non-Marshall as far as I know?) and it looks like the original two hole pattern was drilled out but has been filled underneath at some point. However, there are no holes for this two bolt pattern above, yet the tolex looks original on top. It is quite strange.
Also, the grille cloth is left too long behind the baffle. It looks like early RS white cloth, however I cant say.
Unfortunately the baffle will need to be replaced, as the laminates are coming apart.
And heres a few more pics. One thing to note - sadly the amp was kept in a damp garden shed beside the sea for a number of years and this has taken its toll on the timberwork and other bits. It looks to have also shrank the gold rope piping somewhat. The plastic piping is still the right lenght, but the inner rope has contracted half an inch or so...
Nice amp! I wish you the best in getting it going again.
Ive been chatting to Ken Underwood about these old combos and he has been very kind to impart some information about the ealy days at Marshall which I found very interesting! I had asked Ken if he could shine some light on the early run of these amps - Please see below message which I've added here with Kens permission:
We;ll Mark i will do what i can in as much that i was on the side lines at the time that this amp was made, which must have been at the Silverdale Road factory, late 64, in Hayes Middlesex, my home town.
All the classic signs of Dudley Craven`s work, his trade m,ark was the red and black twisted pair of the heater wiring.
I made yards and yards of this myself and the difference between the very early amps was i used to use a hand brace at one end with the other shut in a bench vice and just turn the brace until the wires were twisted nice and tight, later amps were fairly loose, as you can see i am sure, i am sure you know why this was done that way.
Another sign that it is an early one is the mains connector, not like the IEC type that is used today.
So you can see that you have a classic amp dating back some 55 years.
Wonderful to hear memories of how much effort the two Kens and Dudley put into their work, some history made in those early days...
@ Ken - you will get that pint of Guinness yet Many thanks
Two quesitons for the circuit experts! I've come across two strange minor oddities to the amps circuit which I'm hoping some of you may have seen in the past. I've included some pictures here for detail.
The first is an oddly placed 32kOhm resistor between ground and V1 input mixing resistors, its not on any Marshall schematics Ive seen, nor have I noticed it in any JTM45 board pictures from other amps. It looks to be original (rather than a mod or afterthought) and is the same composition of resistor as most of the rest of the amp. You can see this in the first two pictures below.
Secondly i've noted pin 3 of the bass pot is not connected to anything - again you can see this in the 3rd picture. It looks like this terminal was never used, and there is no residual solder on it from a disconnection/lead removal etc. Am I missing something simple here? The schematic shows a standard tone stack arrangement i.e. all bass pot terminals used.
I've never seen a resistor placed that before (between the mixers and ground). Also, it looks like it's a 1.5 meg rather than 32K ohm.
The Bass pot is normally setup that way with the one solder lug unused.
Since it is a high value resistor Could it be put as "ground loop" eliminator originally?
Might be useful to see if there is a 1Meg to ground on each of the input jacks like normal. Someone may have wanted a pair of inputs of channel 1 to be the high impedance ones and channel 2 (minus the resistor to be low impedance - maybe for bass??). As you know that normally the top two inputs are the high impedance inputs, one per channel.
As Tazin says the Bass pot never has all lugs used (take a peek at the schem).
From the limited pics the rest looks pretty straight and original (I see a new cc 1watt to one of the trem pots.
Can the baffle be saved by brushing wood glue into the gaps in laminations and clamping it up tight for a couple of days?
That would seem like an odd place for a ground loop elimanator. I was thinking more in line of what neikeel mentioned regarding the channels...
Thanks for the reply's folks.
Tanzin - you are indeed correct, its 1.5 MOhm. When I had a meter on it it was reading 33kOhm to ground, but this is of course the parallel path to ground through the 68k mix resistors - My bad.
An odd one indeed. I'm inclined to leave it in place and see if it makes any difference to the sound when I get the amp back to life, if nothing else just to see what its effect is.
Neikeel - there are the usual 1M resistors mounted to the bottom of the jacks as normal, so it seems that this additional resistor ( i.e the 1.5M resistor) was installed independently of all else by the look of it.
Unfortunately the original baffle was in very bad shape. The laminates had separated and warped entirely across it, and the plys were that weak that the driver star nuts were pulling through in places. I had intended to try and re-glue the baffle plys by brushing wood glue in as per your suggestions, however I feel it was just too far gone to get any kind of strength back into it. I had to do this for some small sections of the cabinet, and it has worked well, however I just ended up building a new baffle and remounting the original kit, piping, cloth etc. I've kept the original one though, and haven't given up on it just yet.
Otherwise the referb is going well - I'm at a stage now whereby the cab is structurally solid, baffle is remade, dressed and installed and ive every component in the chassis bench tested. I'm hoping to get away with replacement of just the two main filter caps, and if possible I may even try and keep them. However, ill only know exactly what can be kept when I start to get everything energized again, its been a very long time since the amp has been live - possibly before 1970. As I'm sure many on here know, its a slow process with plenty of hiccups. Life also gets in the way sometimes too!
Ill upload some progress pics over the next week or so. If anyone has any tips or input please chime in!
PS - I see now what the crack is with the Bass pot, I couldn't quite make head nor tail of the schematic as it shows a link to both sides inclusive of the wiper. Makes sense now!
So after many hours work and a few late nights, I'm glad to say the amp is back to life and sounding amazing! I've been able to keep just about everything original in the chassis, with the exception of V1 bypas cap, the output tubes screen grid resistors and the input AC snubber cap. I reformed the filtercaps by running them up in 5 VDC steps while keeping an eye on the leakage current - I was surprised they held, and they have been working in the amp now under load for a number of hours. I've even had a look at the B+ ripple with a scope, and incredibly enough they all seem to be good with the circuit showing little ripple.
The amp was running way hot with the existing 58 k on the lower side of the bias voltage divider, giving a bias voltage of 54 VDC. I've brought this up to 60k for a bias voltage of around 56VDC - the amp is running better for it however still a little hotter than I would like at approx 19 watts per tube. Getting around 420 VDC plate voltage on a disapation current of 43.5 mA. I'm going to install a bias trim pot as I want to be able to fine tune the bias.
The speakers were a total PITA to repair, and the gaps had shifted in two of them - I managed to remove the cones, recentre and reglue to get them back to life, but as you can imagine this was a right difficult job. All 4 are NIB cones, so I wanted to try my best and get them repaired.
I've been playing the amp now for a number of hours over the past week, and without exagerating, I can say now that I really understand what all the fuss is about with these amps - it sounds like few others I've heard. Its tone is thick, woody, and has that GZ34 sag that you instantly notice when you dig in at high volumes. The sound is very full and almost blossoms from the amp - hard to describe! Also, very Bassman like! I've not had it anywhere near fully cranked, and wont be doing so for a while - I would like the speakers to free up a bit before I drive it too hard. On the speakers, I'm not sure if they will take the full output of the amp cranked, but so far they really sound great, and i've been quite surprised by them. I still cant say if they are original to the amp, however they seem to be of a similar age so who knows?
One thing I am concerned about is the heat of the amp when its mounted in the cab - it runs very hot, and due to the nature of the output tubes/ Output TX arrangement (i.e. vertical mounting) most of this heat is unfortunatly being concentrated on the output TX itself. I tested the temperature of the TX after an hour of use and its up around 65/70 degrees C - more than I would like. When the chassis is out in free air it cools properly, and the TX is not nearly as warm, its only when it is in the cab - I can see now why Marshall went to bigger vents on the S2 combos. Has anyone encountered this issue? I'm planning on running the amp with a small fan just to be on the safe side (I dont want another rewind!) and will lower the bias marginally also to reduce heat somewhat.
On this issue, it seems that this was a bit of design flaw on the older JTM45 chassis' in that the KT66's are in really close proximity to the output TX - a lot of the pictures i've seen of similar vintage jtm45s with Drake transformers show the same, in some cases the tube bottle is touching the impedance selector. This was the case on my chassis with TAD KT66s (same dimensions as original GEC's) - the impedance selector mounted on the end bell was tight up against the tube. I ended up moving the output TX forward by about 6mm to alieviate this. Not good to have to do it, but the engineer in me couldn't leave it - the selector had been burning away in the amp originally because of this unfortunately. Also, the output tube sockets were not drilled out exactly in line so this didnt help either - one was forward by approx 2.5 mm. Not an exact science back in the day!
With the amount of time I've spent working on the amp at this stage I'm convinced it is something special - everything from the cab right though to the chassis seems to be bespoke (like they all were back then). Its a rare old beast no doubt, and to me it seems like it was one of the first of its kind. I really would love to know its full story, it has seen plenty of road at this stage!
Pics to follow in next post. Also, I'm bringing it out to a friends studio towards the end of next month so will have a few demo videos up then!
As usual, any thoughts much appreciated!
Fantastic job. A truly sympathetic restoration keeping signs of its work and its history.
Edit: forgot to ask how did you resolve the floating 1M on the input grids?
Lets hear it !!
Thanks guys - sounds clips on the way soon! I need to get into a studio with this one!
@tomsvintage how do you find the heat buid up in your BB Tom? Is it hard on the output TX?
@neikeel thanks for your kind words. I tried to keep the alternations to a minimum, and didnt want to affect the look of the amp at all - I've just done enough to get it back to life. Its a players amp, and I didnt want to dress that up. As for the 1M resistor, I left it in circuit and found it made that channel quite a bit brighter than normal, and made the top end more present. However it wasnt a huge difference to normal, so I've taken it out and reverted to the stock circuit. An odd one...
Other than the amps noted in this thread and the series 1 BB's on amp archives does anyone know of any more pictures of the S1 4x10 floating about? I've been looking about lot but struggling to find any more then what I've mentioned.