200S Sunn

[Helmholtz]

This one has 3 adjustments -
V/DIV 5mv - 20v
SEC/DIV = 0.1ms - 50ms
Trigger = auto/ norm / Sing

If that "Sing" is a sine symbol it means line frequency triggering and should be used here.
Set vertical to 5mV/div with a 10x probe.
Set horizontal to 5ms/div.

[Valvehead]

If that "Sing" is a sine symbol it means line frequency triggering and should be used here.
Set vertical to 5mV/div with a 10x probe.
Set horizontal to 5ms/div.

Well.....heres what i have.. ac and ground settings.. probe is 1x directly on speaker jack , amp idling , no input

Tell me what Im doing wrong here.......

[Helmholtz]

Well.....heres what i have.. ac and ground settings.. probe is 1x directly on speaker jack , amp idling , no input
Tell me what Im doing wrong here.......

Make sure to connect the ground clip of the probe to amp ground.
In pictures 2 and 3 the signal is too large for the screen, so reduce input sensitivity by setting to a higher V/div (e.g. 50mV/div).
Are you using a 10x probe?

[Valvehead]

1x probe ...not sure why my pics are sideways....

[Valvehead]

The first thing you need to know about how to operate an oscilloscope with a 10X probe is how to adjust "Probe Compensation". This adjusts the probe to compensate for the input capacitance of the oscilloscope. There will be a small adjustment, either in the probe or in a small box at the end of the cable where the cable connects to the scope. You will need a source of 1KHz square-waves. Most scopes have a test point on the front of the scope where you connect the probe, then adjust the small adjustment screw so the scope displays nice square waves. If you scope has no such signal, you will need an external generator. If you search on YouTube, you can probably find a video.

To look at mains frequency hum, set the horizontal sweep rate to 5 to 10 mS per division. The Vertical deflection factor will depend on the amplitude of the signal you are looking at, simply adjust the Volts per division for a display between half and full height on the screen.

Thanks--i have 1x /10x probe and a sig gen

[Helmholtz]

1x probe ..

Ok, fundamental frequency is 120Hz.
Waveform doesn't look like power supply ripple, though.
I suspect a bad (oxidized?) ground connection..

[glebert]

1x probe ...not sure why my pics are sideways....

Your scope shot convinces me that the 100S I am working on has the same problem as yours (although maybe a different spot in the circuit). This scope capture is right after the first gain stage on the 100S.

[Helmholtz]

Your scope shot convinces me that the 100S I am working on has the same problem as yours (although maybe a different spot in the circuit). This scope capture is right after the first gain stage on the 100S.

Your signal has an additional 60Hz component.

[Mars Amp Repair]

Hello Folks,
My two cents:
I had issues with the 200S and the elevated B+ as a result that this amp was designed for an ac input of 115V and of course we all know nowadays, it can be between 120 & 125 at times.
An end user installed one of those SS rects & it actually fried the cap-can. No surprise there and which been mitigated by the prev tech on Valveheads Sunn. The cust wanted to try both and possibly switch back and forth.
I also was going to suggest the cust could use the SS rect given the PS was upgraded, but found that I also was going to have to modify the bias Ckt to allow the switching between the 2 diff rects. Fortunately, I convinced the cust to just use the tube, as he wasn't willing to pay any more for all that futzing around.
So, I installed 2ea 47uf radial caps in series, which came in a bit lower than the 30uf in the original Sunn 200s, but did not adversely affect the operation in any way. The original was 30UF. I used radial caps as I can get brands like Nichicon that are top-notch as well as they just fit better on only one terminal strip as oppose to two opposed strips.
The output wattage at clipping was as designated for the amp and no addl hum was evident. I suppose the series equivalent of 50uv would have been ok, but didn't really want to take the chance. There are 62uf radial caps available, but they're designed for flatscreen tv's and are rather long & skinny.
I have had experience that the JJ 5AR4's are just kinda crap. The Sovtek and Tungsol brand seem to hold up very well under situations like this one. Looking at the pix on CE, it appears the innards have similar scalloping of the plates and may be both made by Sovtek.
I instructed this customer to NOT use the SS sub that the amp came with as it just pushed the B+ way higher than the amp was designed for even with the upgraded series filters that will handle the higher voltage. I was thinking of the insulation in the output xformer being compromised and such.
As for the hum, the scope pic almost looks more like a ground loop issue, although it doesn't necessarily sound like one, which typically has a Buzz along with the hum.
I'll leave it to the rest of you here.
Glen

[loudthud]

One thing that hasn't been looked at on this 200S amp is where and how those new caps in the power supply are grounded. With those little humps in the scope pics, that looks like rectifier charging spikes are getting onto the B+.

[Valvehead]

One thing that hasn't been looked at on this 200S amp is where and how those new caps in the power supply are grounded. With those little humps in the scope pics, that looks like rectifier charging spikes are getting onto the B+.

Looking thru the amp and that picture you posted , my amp is grounded way different. Almost as if he did kind of a "star ground" thing. theres even a ground from the input jacks running across the amp over past those filter caps. No ground at the filters or bias supply. I need to trace it out Maybe i should move the ground points --like the picture ??

Do you know where the input jacks ground went ?

[The Dude]

...... No ground at the filters or bias supply.....

Where, then, does ground come from for the supplies?

[Valvehead]

Where, then, does ground come from for the supplies?

Ok...i went thru the grounds...this doesnt seem right...?


ALL the power supply caps / bias supply / pwr tube cathodes / PT ct , go to......the input jack ground. Ground 1. all the way across the amp with 1 wire. All those meet up at terminal strips near the PT . The strips are not grounded .

ground 2 is only the ac cord and filament ct , near the PT

ground 3 and 4 ( Over by V1 ) only have the pots and V1 cathode grounds + that 680 ohm/ 1w (top right of scem ) . These are 2 terminal strips that ARE grounded .

[loudthud]

One thing that hasn't been looked at on this 200S amp is where and how those new caps in the power supply are grounded. With those little humps in the scope pics, that looks like rectifier charging spikes are getting onto the B+.

An important addendum to this question is; where does the red wire with the yellow stripe ( high Voltage center tap ) ground ?

Grounding the caps to the input jacks is just about the worst idea ever !

[The Dude]

An important addendum to this question is; where does the red wire with the yellow stripe ( high Voltage center tap ) ground ?

Grounding the caps to the input jacks is just about the worst idea ever !

Yes, THAT!!!!!!