Sound City 120R - What the...?

[Mark Black]

I've got this long-simmering back-burner project I'm trying to finalize - it's a Sound City 120R head with reverb. 7 12Axx preamp tubes & 6 EL34 power tubes.

Replaced all the dried-up filter caps, retensioned all the tube sockets, cleaned controls, etc.

The OT had been previously replaced, with a Dagnall C2668 which apparently is a 100W transformer used in some Marshalls. More on this later...

The weirdness lies with the power tube hookup (well actually there is quite a bit of weirdness in there but I'm mostly concerned with the power section right now). Three of the power tube sockets (on one side of the OT primary) have the cathodes (pin-8) grounded through 68 ohm resistors and the other three are grounded through 22 ohm resistors. G3 (Pin-1) is direct-grounded on all. This appears to be correct as per what few schematics I have found online. I was doing voltage drop measurements over the cathode resistors to determine current & dissipation and found (of course) huge discrepancies between the two sides, which led to my discovery of the different values (the resistors are physically very similar and wouldn't you know the markings are turned away from view).

So what could be the reason for 68 ohm on one side and 22 ohm on the other?

The two sides DO have separate bias controls...

On the subject of the replacement OT I'm leaning toward telling the customer to use only 4 power tubes since that appears to be correct for the transformer design. What do you think?

Thanks guys -

Mark

[Satamax]

Hi Mark!

Well, thoses SC's are a beast of their own! IIRC i've seen referenc eof thoses cath resistors being different on the two sides, but never seen one having them. You might have seen this http://home.earthlink.net/~iktoblikto/sc_home.htm and this http://vintageamps.com/plexiboard/viewforum.php?f=9 on teh plexi palace, i'm sure we've discussed the cathode res before. Do a search. Myself, i'd try to remove them, but if you're in the US it might ba e bad ides since you have often bias probs over there. If you want, i have a 120 OT, not too keen on selling it, but you never know what might happen

[Satamax]

Huh, something which came back to mind, someone sugested that the two different cath res were may be for equilibrating the fact that the PI had the same resistors on both sides.

[Mark Black]

Wow that was quick!

I kind of hoped I might hear from you on this since I have seen you post on Sound City before.

These really are a different critter aren't they...

Many things I originally thought were amateur hack mods turn out to be factory stock - like the half-wave rectifier DC heater circuit for some of the preamp tubes and the whole reverb circuit. Thankfully someone recently posted a lot of great pictures along with an ebay ad when selling a 120R, so I got a lot of clues that are not obvious in the schematics. http://cgi.ebay.com/Sound-City-MK-4-...QQcmdZViewItem

Thanks for the links - I'll do a more comprehensive search on the Plexi-Palace site when I have time - so far too many results not pertinent.

Thanks -

Mark

[Mark Black]

Hey Max -

I guess you authored at least one of the L120 schematics with mods posted at Schematic Heaven and Plexi-Palace (I'm currently waiting for my registration email there).

Can you elaborate on the mods at V4? Reason I ask is the one I'm working on has a problem with symmetry at the phase inverter, and I was wondering if a similar problem led you to those mods, or if it was a quest for tone kind of thing.

Seems with this one there is too much positive voltage on the pin-2 grid of the PI so that side flattens out early. A quick clip-in of a 150K over the existing 1M cathode resistor of V4A (simulating your 100K on the mods) seems to make things about perfect, even without the other V4 mods, so I'm really curious now.

Thanks -

Mark

[Satamax]

Hi Mark.

Well, most of the schemos on schemoheaven, i had compiled from courteous guys over here, and sent them to the guy few years back, (50, 120 and 100) The schemo with mods onschemoheaven is wrong, i did a big mistake on the output stage grid resistors. I asked him to remove, but he didn't so far.

Well, about the voltage divider changed to 100K instead of 1 meg, that's Ken Gilbert's idea, as well as lowering the cathode resistor, he felt the cathode follower wasn't giving enough curent to the phase inverter, so lower the resistors went. Thought he got the idea from hiwatts i bet. (ken, sorry, you're still the best ) But basicaly, the differential PI is partly biassed by the cathode follower (check hiwatt's schemos on mark huss's site) I'd suspect that you have a bad burgundy resistor somewhere. Thoses 1 meg, if the burgundy type are real shite. Anyway, without assuming anything, if the voltage is too big on V4G2, check your voltages on the cathode of that side (both are connected anyway), and on the cath of teh cathode follower. If something is severly off, that should be the cause.

Here's one with some mistake, but which is workable. http://home.earthlink.net/~iktoblikto/maxscmod.jpg

Bye.

Max.

[HTH]

I actually like the L100 model much more and have converted a L120 over - it's alot of work but the tonal benefits are worth it.

It leaves you with an ECC83 free - hmm, what to do with that eh?

[Enzo]

Tremolo....









Kidding...

[Mark Black]

he felt the cathode follower wasn't giving enough curent to the phase inverter, so lower the resistors went.

That's kind of the gut instinct I had too, especially in light of the rather large cathode resistor value. I will do a thorough check of surrounding resistors & other components though, as it seems the amp should have operated correctly when it was built. Or then again, judging by the build quality of this particular unit maybe that is too much to assume. I don't want to do a mod as a band-aid for a problem component for sure.

I actually like the L100 model much more and have converted a L120 over - it's alot of work but the tonal benefits are worth it.

At this point I'm just trying to get this thing completely operational in purely stock form so the customer can decide whether he likes it or thinks it sucks. Then maybe we will discuss mods.

Tremolo...

Hah!

Funny thing is even with all 7 preamp tubes in use there is still an empty socket hole in the chassis...

[HTH]

Funny thing is even with all 7 preamp tubes in use there is still an empty socket hole in the chassis...

it's almost like they made these amps with modding in mind

the stock L120 has a fair bit of hiss and the EQ sounds wierd to me. having gone through modding one into a L100 I'd say it's well worth the effort. I'd like to get another on the cheap to turn into a DR103

[drewl]

Never got around to modding the L120 I had.
I got rid of any hum or hiss and had a crossline MV in it just so I could use it at rehearsal.
I used the "reverb out" jack to install a footswitch to control the reverb and the PI gain switch.

[Mark Black]

Good stuff guys - thanks!

I forgot to say thank you earlier to all who have replied, so - Thank You!

I do like the reverb-out mod to f/s function - that had crossed my mind too. Almost wouldn't even consider it much of a mod since I don't know what good the existing circuit is or why they kept it. I think I'll suggest that to the customer as a good usability feature.

Thanks again -

Mark

[HTH]

ok, a back from the dead thread, but I think its good to keep info about an amp like this in one place for easy reference.

I've had an L120 in for service, was motorboating and had really bad hiss and hum. All the filter caps have been replaced, I've DC-referenced the heaters via a pair of 100ohm resistors (actual CT taped up and unused). Also replaced the bias cap, well worth doing on an amp of this age.

What you don't always see on these amps is a choke in place of the 100ohm resistor in the first filter stage - I like the feel this gives the amp, used a Hammond 159S (4H, 65ohms, 225mA).

Its also worthwhile laying the OT on it's side too, this made a fairly notable reduction to the hum.

Busy going through some of the other things that Max posted earlier in this thread like the PI mods, will report back on how I feel they work out.

Regarding the active EQ section, this is REALLY noisy - are the mods Max posted to tame this hiss?

[drewl]

I think the mods are to reduce the hiss.

And change those 1meg resistors all over the place, the stock ones are very noisy.
I think I changed out a bunch of those and most of the plate resistors which reduced the hiss alot.

In an amp with an "active" EQ or tone controls, each control can amplify frequencies as opposed to just cutting them, so you have more amp sections in this amp which contributes to the hiss.
Also makes them tricky to "dial in", but once you get the right sound, they can sound fantastic.

[HTH]

Yep, they are pretty sensitive to dial in - the EQ can go from just right, to not right in a small change of one of the EQ pots. They do sound very nice when set right though, I agree - more for clean, slight crunch though (unless you use a fuzz box, which they happen to love).

Thanks for the advice too, will try swapping out the 1M resistors and plate loads.