Firing up an old valve amp of unknown provenance

[DrNomis_44]

Hey Simon, how are things going with getting your Sound City amp fixed?

[Simon Barden]

I took it to an amp tech and it got transformed in two stages (one would have been cheaper!).

First stage was getting it working as it was. New input transformer, re-cap and new valves. New original transformers are unobtainable, so it was either trying to find a used original at the same cost as the amp, or a custom wound transformer at a similar price or adapting a different transformer. My tech went down the last route, fitting a Marshall transformer and deriving one of the voltage supplies (the original had its own winding for this) via a resistor circuit, as Marshall do. He also replaced some dodgy pots and fitted an IEC socket to replace the captive power lead.

So I took home a working, but very noisy amp. They were known to be noisy amps, mainly due to the active tone controls.

He said that there was a circuit mod on the web, done by the original amp designer, that changed the EQ to passive, along with a few other tweaks that turned it into a very similar amp to a Hiwatt (as the designer had worked for Hiwatt). After living with it for a week, I decided the hiss was too much (and the active EQ a bit weird sounding), so it went back for more mods.

He spent a lot of time trying to make it as quiet as possible. He would have liked to have fitted a choke, but there wasn't room on the chassis. It is a very compact head, probably 2/3 the length of a full-sized Marshall, so not a lot of room inside. The awful hiss has gone, but there's a fair bit of hum still. But that gets lost when you play. It is now a very loud, great sounding amp with that Hiwatt character, making it very easy to get those Dave Gilmour clean tones. Used with a speaker attenuator, or just turned up very loud it makes a great drive sound, very different from a more crunchy Marshall sound. It does have a lot of bass, and I have to keep the bass control at minimum (I rotated the knob so that minimum was at 12 o'clock so it doesn't look strange).

I now know a lot more about valve amp circuits than I did, so I plan to see if I can make it quieter still once I've finished reading a couple of books on the subject. Also see what type of taper the volume pots have, as it goes from quiet to very loud with a few degrees of rotation, and maybe a high pass filter to cut out excessive bass and reduce the 50Hz noise.

The two-stage approach to the repair made it an expensive project, more than it would sell for, but I do now have a working, quite rare, 70s amp.

[DrNomis_44]

I took it to an amp tech and it got transformed in two stages (one would have been cheaper!).

First stage was getting it working as it was. New input transformer, re-cap and new valves. New original transformers are unobtainable, so it was either trying to find a used original at the same cost as the amp, or a custom wound transformer at a similar price or adapting a different transformer. My tech went down the last route, fitting a Marshall transformer and deriving one of the voltage supplies (the original had its own winding for this) via a resistor circuit, as Marshall do. He also replaced some dodgy pots and fitted an IEC socket to replace the captive power lead.

So I took home a working, but very noisy amp. They were known to be noisy amps, mainly due to the active tone controls.

He said that there was a circuit mod on the web, done by the original amp designer, that changed the EQ to passive, along with a few other tweaks that turned it into a very similar amp to a Hiwatt (as the designer had worked for Hiwatt). After living with it for a week, I decided the hiss was too much (and the active EQ a bit weird sounding), so it went back for more mods.

He spent a lot of time trying to make it as quiet as possible. He would have liked to have fitted a choke, but there wasn't room on the chassis. It is a very compact head, probably 2/3 the length of a full-sized Marshall, so not a lot of room inside. The awful hiss has gone, but there's a fair bit of hum still. But that gets lost when you play. It is now a very loud, great sounding amp with that Hiwatt character, making it very easy to get those Dave Gilmour clean tones. Used with a speaker attenuator, or just turned up very loud it makes a great drive sound, very different from a more crunchy Marshall sound. It does have a lot of bass, and I have to keep the bass control at minimum (I rotated the knob so that minimum was at 12 o'clock so it doesn't look strange).

I now know a lot more about valve amp circuits than I did, so I plan to see if I can make it quieter still once I've finished reading a couple of books on the subject. Also see what type of taper the volume pots have, as it goes from quiet to very loud with a few degrees of rotation, and maybe a high pass filter to cut out excessive bass and reduce the 50Hz noise.

The two-stage approach to the repair made it an expensive project, more than it would sell for, but I do now have a working, quite rare, 70s amp.

Glad that it is fixed and working, my Marshall MA100C has been working perfect ever since a local Darwin tech worked on it to stop it blowing it's HT fuse all the time, turns out the amp's power transformer was designed to work on a 230V AC mains supply, so, when I plugged it into our Australian 240V AC mains supply it was putting out too much HT, I think I measured about 680V DC (yikes!!!!) of HT when I was working on it (glad I'm still here to tell the tale), the tech said that he thought that the 680V DC of HT was a bit too much for four EL 34 tubes, and really was supposed to be more like 450V, he theorized that the excessive HT caused one of the EL34s to red-plate, and an arc-over between two pins of one of the octal sockets, leaving some carbonizing, so he fitted a power resistor in the wiring to the power transformer's primary to bring the HT down to a more sane/sensible level, he also replaced the original four EL34 valves with four new Groove Tube EL34s, been great ever since.


Funny thing is, I remember reading in a Valve Data book, that it said that two EL34 valves should be able to take up to 800V DC of HT on their anodes, my guess is that earlier versions of the EL34 may have been more robust than what's currently available, the Valve Data book was a pretty old one from I think the early 60's or so if my memory isn't too rusty.

[Simon Barden]

Ther3 really shouldn't be an issue with 230v and 240v. The UK is nominally 230V now, but unless you are supplied by a recently replaced substation, we'll still get 240v or more, not 230v. And CE marked equipment (like the Marshall should be) is designed to work on 230v +10%/-6%. So should work OK from between 216v up to 253v.

680V sounds weird, as 450vDC at 230v should give a maximum of 469v DC at 240V. The main smoothing caps are rated at 500VDC, so they would have all blown up at 680v. 680v sounds more like the peak AC voltage you'd get before any rectification and smoothing. The schematics I found on line didn't give any test voltages, which isn't very helpful.

[DrNomis_44]

Ther3 really shouldn't be an issue with 230v and 240v. The UK is nominally 230V now, but unless you are supplied by a recently replaced substation, we'll still get 240v or more, not 230v. And CE marked equipment (like the Marshall should be) is designed to work on 230v +10%/-6%. So should work OK from between 216v up to 253v.

680V sounds weird, as 450vDC at 230v should give a maximum of 469v DC at 240V. The main smoothing caps are rated at 500VDC, so they would have all blown up at 680v. 680v sounds more like the peak AC voltage you'd get before any rectification and smoothing. The schematics I found on line didn't give any test voltages, which isn't very helpful.

I agree, I'm not sure exactly why I was measuring 680V DC HT, maybe I made an error, or something, good job I was careful enough to ensure I didn't get a potentially fatal electric shock, anyway, the tech managed to fix my Marshall MA100C and it hasn't blown the main HT fuse yet.....touchwood.


Here's a link to a page on the Freestompboxes.org Forum, where the tech, blackbunny, explains exactly what he did to fix my Marshall MA100C amp (scroll down to the bottom of the page):

https://www.freestompboxes.org/viewt...2202&start=120