Sound City 120r

[TIMBO]

Hi guys, I was asked to have a look at this monster.
It stop working with no signs of anything having a melt down.
All Ecaps are original with the usual signs that there pack'n it in.
This schem is close http://el34world.com/charts/Schematics/files/soundcity/soundcity_100b.pdf
To give me an idea of the B+ rail all seems to match the above schem.
Checking the values of the dropping resistors it looks like the 6.8k/2w is OPEN and as there is no other chard/discoloured resistor in the chain, i'll take it that this is the reason it stop working.
The 2A HT fuse is intact.

The Ecaps measure all over the place and I think for the small cost to replace them would be a good place to start to rectify the problem.

There has been a MV added and at this stage is only a minor add on.
When replacing the open 6.8k resistor is it a good idea to replace it with a similar wattage as I see this as a sort of INLINE fuse. Thanks
None of the schems in the library have the 120r with a reverb circuit, so if any one has a schem would be great.

[DummyLoad]

the 6.8k/2w is OPEN and as there is no other chard/discoloured resistor in the chain

is the 6.8K charred?

70V drop across 6.8k, so dissipation is around 3/4 watt. use a 2 watt wire-wound part.


i'd give it a cap job too...

--pete

[sluckey]

Finally, you have an amp I've actually heard of! 

I don't consider that 6.8K as an inline fuse. I'd try to discover why it's open. Likely suspects would be downstream filter cap(s). Then I'd replace all filter caps.

[TIMBO]

Thanks guys, The heater circuit has a rectifier for a DC supply and the 10000uf cap is oozing guts badly. I'm sure this is not helping also.

[TIMBO]

Some pics

[TIMBO]

Hi guys, All Ecaps and dead resistor replaced.
Before I kick start it, any thing I may need to do???
Also a BIAS setting would be great. Thanks

[plexi50]

Yeah that one Erie radial cap looks shot. If your finished with what you have done then flip the switch and watch your light limiter.
Set bias and listen for any other hum that may get louder if you have any other power supply caps that have not been changed.

[TIMBO]

Thanks guys, I took some voltages and all look good, about 10v above what is marked on the sound city schem.
the voltages on the EL34 are much higher than the schem.
With the bias set at -40v on each of the bias pot I get

PIN 3 V6,7,8     - 467v
PIN 4               - 456v
PIN 8               - 2.9v (across 22r resistor)
PIN 3 V9,10,11 - 467v
PIN 4               - 456v
PIN 8               -  0.9v
The plate and screen voltages vary but are around the above voltages.
BUT as you can see the cathode are high and miles apart.

[TIMBO]

Hi guys, this sounds like a similar problem. http://music-electronics-forum.com/t21722/

[TIMBO]

Hi guys, its been brought to my attention that the sense resistor are different.There is 22r on one side and 68r on the other, so could this be giving a false reading on the bias setting. If so why would they have different resistors here. thanks

[kagliostro]

TIMBO give a look to this tread, may be there is something of your interest

http://forums.vintageamps.com/viewtopic.php?f=9&t=74093

Ciao

Franco

[PRR]

> PIN 8  - 2.9v (across 22r resistor)
> PIN 8  -  0.9v

You now find a 68r resistor.

Let's pretend you got the 68 and 22 sides mixed-up.

2.9V across 68r is 42.6mA
0.9V across 22r is 40.9mA

I say this is "perfect". 4% different is fabulous matching for any two tubes.

[gldtp99]

I still say gut these miserable active preamp Sound City 120's and build Hiwatt (or other) circuits into them----- same as I said in that linked post from 2008----- I still believe it.
Did you notice how long they were F'ing around with their SC120 and the fact that it never seemed to work properly even at the end of the post ?
One of my SC 120 to Hiwatt conversions came in for service last week----- it had taken a fall off the speaker stack during a show----- two input jacks broken and a Xicon 470 ohm/10 watt cement resistor had broken the lead---- I replaced the jacks and power resistor and the amp was back to its old self----- the owner plays it in four different local bands and he says it is the best bass amp on the planet----- I'm not sure it's the best bass amp on the planet but the fact that the owner likes it so much makes me feel good.
These amps, with Hiwatt circuit rebuilds, sound great and usually go and go and go without problems................until somebody knocks over their bass stack while in operation--    ...................gldtp99

PS--- I have a 120R chassis that's inop and missing the reverb tank and other parts------ There is zero chance I will ever try and get it running again as a SC 120R, I have a great sounding Fender SF Twin Reverb----- But one day I'll rebuild it as an EL34 x 6 amp of some type--- the iron and chassis are good.

PSS--- In the last two SC 120/Hiwatt conversions I've done I used 100 ohm/5 watt cathode resistors on all six output tubes----- I have no idea why the factory used the 22 ohm/68 ohm cathode resistors on some SC 120's----- with the six 100 ohm cathode resistors the amp puts out plenty of power, sounds great, and it makes it a bit easier for the somewhat underpowered PT bias winding to bias the EL34's in a safe range---- and no voltage doubler is needed in the bias circuit---- Simple, Safe, Reliable

[TIMBO]

Thanks PRR, Yeah got a bit of a surprise to find that half the power tubes have a 22r and the other has a 68r.
Now that I know this thinks start to make a bit of sense, sorta.
With the bias set on both sides at -35v
Plate-cathode 460v
1.2/22=.054 - 460x.054=24.8w
1.1/22=.05   - 460x.05  =23w
1.2/22=.054 - 460x.054=24.8w

3.9/68=.057 - 460x.057=26w
3.8/68=.055 - 460x.055=25.3w
3.5/68=.051 - 460x.051=23.4w

After seeing this I trimmed the bias down to get a better balance.
With the bias set at -45v
Plate-cathode 467
1.1/22=.05  - 467x.05=23.3w
.9/22  =.04  - 467x.04=18.6w
1.1/22=.045 - 467x.045=20.9w

With bias set at -43v
3.1/68=.045 - 467x.045=20.9w
3.0/68=.044 - 467x.044=20.5w
2.8/68=.041 - 467x.041=19.1w

I'm a bit out of my depth with this stuff and I'm not able to make a call on the health of these tubes and how much of a mismatch can be thrown into the mix to make a reliable amp.
I have suggested to the owner that a new set is required as the tubes are old and to rule out any problems that may arise. Thanks

[TIMBO]

Thanks 99, I'll keep that on my records for any future work on this amp.

[PRR]

I don't see a problem with a 23.3w to 18.6w spread.

"Matching" is never vital for gitar work (only for low THD numbers); and three on each side has powerful averaging effects.

+/-12% is WAY better spread than you need.

The amp was surely intended to sound as-designed with Pdiss in the 20W range.

I think you are done here.

[TIMBO]

Thanks PRR, I have returned the amp to the owner and discussed the option of new power tubes, but they are not in his budget at the moment.
This is not a gigging amp, just was his first amp he bought about 10y ago and the tubes would be that old.
Who knows whether they were matched to start with.
Thanks for help.