Sound City B120 MKIV restoration

[tayste_2000]

You just let me know when you want to sell that Matamp Slave to pay for all this work

[Mr. Foxen]

Sold the Laney, pretty much covering it. Fair few amps to hit the market before the Matamp comes up. Plus I've got Al Cisneros ahead of you on the list of people that want to buy it.

[tayste_2000]

Ah well I'm sure I'll acquire one, one way or another

[umph]

[quote name='tayste_2000' post='1289091' date='Jul 1 2011, 02:41 PM']Ah well I'm sure I'll acquire one, one way or another[/quote]

haha good luck with that quest! think olis 300 might be a one off.

[Mr. Foxen]

[quote name='umph' post='1289318' date='Jul 1 2011, 05:35 PM']haha good luck with that quest! think olis 300 might be a one off.[/quote]

I always suspected there's a red and a yellow ones to go with, due to its blatant dub rig origin. Matamp factory trip is pending to collect a mates GT200 and 8x10, gonna see if I can poke about in the records.

Plus part of the condition I got it on was it is used for Doom.

[tayste_2000]

Ok we'll see

[Mr. Foxen]

I just cautiously turned it on with no valves in it to see if it seemed about right. Haven't done any poking in it with a meter because I wanna make sure someone reliable is around in the house, my lodger is here, but she is actually too stupid for me to place the slightest risk to my life in her hands. Plus I would rather do it with Umph available online so I can pester him. Thanks also to Subthumper for the fuses. It is fine so far, no exploding.

[Pow_22]

Gonna have to book mine into a techs. It has developed a load of intermittant crackling. Sounds like when you turn a switch that needs cleaning but the crackling is all the time (even when you dont turn a knob). Out of interest if you or you know anyone selling a unloaded one (i.e just the wooden box) id be very interested, mine is very battered and at some point someone has painted it black (presumably to hide the nicks and stuff)

[escholl]

[quote name='Pow_22' post='1290735' date='Jul 3 2011, 08:03 AM']Gonna have to book mine into a techs. It has developed a load of intermittant crackling. Sounds like when you turn a switch that needs cleaning but the crackling is all the time (even when you dont turn a knob).[/quote]
Sounds to me like it's probably just a dirty valve socket, or perhaps a valve on its way out. Should be an easy fix.

[Pow_22]

Hope so mate. With 6 EL34's a full set isnt cheap. To be honest i did open it up briefly when i got it and by the looks of it that was the first time it was ever opened. A good 40 years (ish) worth of dust n grime. So at the very least a service is needed. Just emailed dave Lund, has very good recommendations on here and he is very local. Fingers crossed its something simple.

[Mr. Foxen]

Just get in there and clean it yourself. I already put a bunch of how to on here. The sleeve can be reupholstered fairly readily, either DIY or find a local upholsterer.

[umph]

[quote name='Pow_22' post='1290773' date='Jul 3 2011, 09:13 AM']Hope so mate. With 6 EL34's a full set isnt cheap. To be honest i did open it up briefly when i got it and by the looks of it that was the first time it was ever opened. A good 40 years (ish) worth of dust n grime. So at the very least a service is needed. Just emailed dave Lund, has very good recommendations on here and he is very local. Fingers crossed its something simple.[/quote]

+1 on giving it a good clean. does it sound like frying eggs?

[Mr. Foxen]

Moment of truth. Valves in, the tape is where I had them all tested and wrote a note on them, RCD in place.





Totally works, woo. Issues are that the power switch doesn't turn off (I was turning it on from the wall, in case of exploding), and there is still some noise from the pots an hiss, occasional crackle, but loads quieter than the last one I had. Some new resistors in the pre should sort that. New switch is fairly important too.

Edited July 3, 2011 by Mr. Foxen

[Phil Starr]

Well Done!

[Mr. Foxen]

Having a little play with this now the switch is replaced, an some resistors replaced to clean up some hiss. Noticed a new problem: the speakers are moving in and out really slow, too slow to make noise, like 5hz or something. Any clues?

[3below]

[quote name='Mr. Foxen' post='1298304' date='Jul 9 2011, 04:13 PM']Having a little play with this now the switch is replaced, an some resistors replaced to clean up some hiss. Noticed a new problem: the speakers are moving in and out really slow, too slow to make noise, like 5hz or something. Any clues?[/quote]


My first guess would be some ac ripple 'artefact' due to poor filtering that is getting into signal circuit. I use the phrase artefact since you would expect ripple at 100 Hz or mains hum at 50 Hz.

This sounds like 'motorboating', possible cures are changing plate resistors, screen resistors, and decoupling capacitors. however before diving in you can try and isolate the input stage it happens at by removing preamp tubes in turn.

At this point it is time for the oscilloscope and start looking at the waveforms at various points in the circuit. It might just be a poor solder joint on a resistor or capacitor rather than a failed component.


Your noise reduction reports encourage me to get started on mine, and the summer hols are only 4 days away.

[Mr. Foxen]

This is really low frequency, like you can see the cones slowly moving, rather than audible frequency. Plugging into the reverb return doesn't stop it.

[3below]

The symptoms are described here [url="http://www.scribd.com/doc/55820085/73/Stability"]Valve Amps[/url] page 410.

The problem is not new [url="http://vacuumtubebrasil.profusehost.net/Hi%20Fi%20amplifier%20instability.pdf"]Radio and Television News 1955[/url]

The encouraging answer is found here [url="http://www.geofex.com/ampdbug/mtrboat.htm"]Ampdbug on Geofex[/url].

Whether we consider that a Sound City 120 "has ever worked correctly once" is another matter

Hope this helps.

Edited July 9, 2011 by 3below

[Mr. Foxen]

All of the electrolytic capacitors were replaced at the start.

[3below]

[quote name='Mr. Foxen' post='1298684' date='Jul 10 2011, 12:40 AM']All of the electrolytic capacitors were replaced at the start.[/quote]

I remember that you mentioned that earlier, a good decision.

Satamax's advice on Vintageamps about lead dress is well worth exploring. To minimise coupling you want wires crossing at right angles or if they must run (semi) parallel get as much distance between them as possible.

Oscillsocope and looking at the B+ waveforms from the power supply filtering is where I would start, then looking for any 'ripple' on the signal chain. If you lived nearer I would borrow a 'scope from school and help out on this one.

The 'brute force' method would be to unsolder one end of each resistor in the power supply section (one at a time). check that the resistor is in spec, then resolder. Check the 100R and 6K8 resistors that connect through standby to output transformer centre tap.

If these check out OK you will have to plough through the pre amp sections for resistors out of spec (or poor joint). This is where the 'scope will speed matters up no end by helping locate the stage where the problem starts at.

Keep posting the progress.

[Mr. Foxen]

Would out of spec resistors in the pre cause the oscillation? That's the problem rather than noise, the noise is ok currently.

[3below]

[quote name='Mr. Foxen' post='1298890' date='Jul 10 2011, 11:58 AM']Would out of spec resistors in the pre cause the oscillation? That's the problem rather than noise, the noise is ok currently.[/quote]

The theory suggests so, courtesy of Merlin, [url="http://www.freewebs.com/valvewizard1/smoothing.html"]Valve Amp Power Supplies[/url], it is a good book, highly informative and readable.

The RC network is a [url="http://en.wikipedia.org/wiki/Low-pass_filter"]low pass filter[/url] since you want to pass d.c. (0 hz) or nearly d.c. only.

From a pragmatic angle, you have replaced all electrolytics and still get the motorboating / LFO. This really only leaves three options- either faulty resistor(s), decoupling capacitors need to be larger or amp has inherent design flaw.

Once you know that the power supply resistors are OK, then you might be able to find the 'faulty' pre amp stage by measuring the plate voltages (HT) on each pre amp stage and looking for a significant discrepancy - the soundcity site LB120 schematic has the reference voltages for these points.


I do remember that using a choke instead of the 100R resistor between plate- and screen supplies has been suggested [url="http://audiopaja.ws/gpage3.html#Sound%20City"]here[/url].

Edited July 10, 2011 by 3below

[Mr. Foxen]

[quote name='3below' post='1299055' date='Jul 10 2011, 04:03 PM']decoupling capacitors need to be larger or amp has inherent design flaw.[/quote]

Are these the 0.047uf, 400v mustards roughly between the pre and power sections? It was suggested I replace these.

[Mr. Foxen]

Opened it up, replaced some more resistors in the preamp bit because they had arrived, sprayed the volume pot with the crackle some more, the spray came out brown, so must be doing something, and stirred the wires round the phase splitter some. Fired it up again, no more moving cones, played a little, still fine, then I turned some knobs and post a crackly bit, it started again. It changed speed whilst I was fiddling with the mid knob, but I can't replicate that. The really loud crackle from the volume pot I know is duff really set it off.

Have pulled pre valves till I find which one doesn't stop the oscillation, and it is the middle one.

[3below]

[quote name='Mr. Foxen' post='1308404' date='Jul 18 2011, 08:47 PM']Opened it up, replaced some more resistors in the preamp bit because they had arrived, sprayed the volume pot with the crackle some more, the spray came out brown, so must be doing something, and stirred the wires round the phase splitter some. Fired it up again, no more moving cones, played a little, still fine, then I turned some knobs and post a crackly bit, it started again. It changed speed whilst I was fiddling with the mid knob, but I can't replicate that. The really loud crackle from the volume pot I know is duff really set it off.

Have pulled pre valves till I find which one doesn't stop the oscillation, and it is the middle one.[/quote]


Promising progress, when you say the middle one, V3, V4 or V5 phase splitter. Won't hurt to replace the 0.047uf, 400v mustards between the pre and power sections as you mentioned earlier.

On the schematic there are several non electrolytics leading into V4, some have values on schematic some do not. If it is V4, replace these. The fact that you have stopped the oscillation and can identify where it originates means you can stop it (theoretically ).