200S Sunn

[Helmholtz]

Depends on the scope model, many don't attenuate DC in AC coupled mode as the 1Meg is AC coupled from the input, but other scopes DC couple the 1Meg even in AC coupled mode.

Interesting.
I wasn't aware of such differences between scopes.
Both my Tek scopes (453 & 2430A) don't support DC attenuation.
But I just found that my new Siglent SDS 1202X-E does.

But 10x probes have voltage limits themselves (often only 400V DC +AC peak), which won't change with the scope used.

[soundmasterg]

Yes, that amount of filtering is higher than the 5AR4 spec recommends. The 200$ operates the rectifier tubes at the very top of what a NO$ tube can withstand, most if not all of the tubes manufactured today will fail under those conditions.

It is unlikely that this change will cure the Hum. Those big caps may have been installed as an attempt to cure the hum, without finding the actual cause. Verify that the center tap of the output transformer is connected AFTER the choke like the schematic depicts.

Two 100uF caps in series result in a 50uF cap at twice the voltage. Vintage 5AR4's/GZ34's could handle 60uF in the first stage, though I wouldn't try that with modern 5AR4's/GZ34's for long. When I recommended using series connected caps on the Sunn still shines forum, I recommended if people use larger series connected caps like these to use a solid state rectifier instead of a tube one. Most players will like that setup better for bass anyway as it is a little tighter. I probably would have used 220k resistors in parallel with each cap instead of 470k, but the larger resistors would likely work also without problems. I don't think the fault is here unless some of the caps or resistors are bad, but it seems it wasn't blowing the fuse with the solid state rectifier in place, so maybe the tube rectifier failed.

Seems like there is still something going on in that amp that hasn't been solved yet.

Greg

[Valvehead]

Two 100uF caps in series result in a 50uF cap at twice the voltage. Vintage 5AR4's/GZ34's could handle 60uF in the first stage, though I wouldn't try that with modern 5AR4's/GZ34's for long. When I recommended using series connected caps on the Sunn still shines forum, I recommended if people use larger series connected caps like these to use a solid state rectifier instead of a tube one. Most players will like that setup better for bass anyway as it is a little tighter. I probably would have used 220k resistors in parallel with each cap instead of 470k, but the larger resistors would likely work also without problems. I don't think the fault is here unless some of the caps or resistors are bad, but it seems it wasn't blowing the fuse with the solid state rectifier in place, so maybe the tube rectifier failed.

Seems like there is still something going on in that amp that hasn't been solved yet.

Greg

Yep, and I cant find it....

[Valvehead]

Ok... pulled V1 - hum

removed the b+ feed to v1 ...hum

removed the .0022 and .0075 after v1. ....hum

ground Treble wiper ...louder hum with buzz.

remove treble wiper wire = no hum.


So if v1 is out of the equasion.........this has to be in the filter cap area ??????? V2 has been replaced . Please read my previous posts

yes I've moved around every wire for lead dress

The caps are 1 year old but...you see my drawing....

I'm pulling my hair out.

[loudthud]

ground Treble wiper ...louder hum with buzz.

remove treble wiper wire = no hum.

Does the Voltage on pins 1 and 3 of the 6AN8 change when you do these steps ?

[Helmholtz]

Could still be power amp oscillation. The increased current draw often shows as ripple hum.
To test disconnect the NFB.

[Valvehead]

Could still be power amp oscillation. The increased current draw often shows as ripple hum.
To test disconnect the NFB.

removed wire from speaker jack (yellow 16) no change in hum

[Valvehead]

Does the Voltage on pins 1 and 3 of the 6AN8 change when you do these steps ?

ground treble wiper = pin1 goes UP 18v . pin 3 - drops 15v

remove wiper wire , pin1 goes down 2v , pin 3 - drops 2v

[g1]

removed wire from speaker jack (yellow 16) no change in hum

If yellow goes to the speaker jack, either it has been rewired or the schematic from the 1st post is not a match for this amp.

[Valvehead]

If yellow goes to the speaker jack, either it has been rewired or the schematic from the 1st post is not a match for this amp.

Yep ! It was wired wrong. I just corrected it and.....same hum .

[Helmholtz]

Please post a scope pic of the hum at the output.

[g1]

Yep ! It was wired wrong. I just corrected it and.....same hum .

For sure the NFB loop got disconnected as a test?

[Valvehead]

Helmholtz , please see post 38. How should I set the scope ?

[Valvehead]

G1... yes just double checked

[loudthud]

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.