Sunn Sentura 2

[Mars Amp Repair]

Has anyone had experience with this amp & ground loop issues. I have a nasty hum with the volume in any position. It's more noticeable with vol at 0but is just masked by the preamp hiss at other levels.

I've had to resolder all of the rivetted ground terminals on the terminal strips as well as the grounds on the Electrolytic can. Not too big a surprize there.

In addition, I also had to remove a ground wire that goes from the ground at the can to one of the tone controls which eventually ends up in preamp being grounded on a term strip. Looks like they created their own ground loop.

The hum is at an acceptable level now, but wonder if anyone else has had any experience like this on this amp & had to do any special ground mods.

Also, I'm getting about 39watts out of the amp & wonder if anyone knows what the power is supposed to be. Kt88's. thanx, glen

[Enzo]

Scope the bias supply too. Loss of filtration on that can cause plenty hum. ANd make sure the LV rail for the trem/reverb is clean too. The trem bulb could even inject some hum if the rail is hummy.

The volume control is right at the first stage, so the whole amp is between it and the speakers. DO the EQ controls affect the tone of the hum? if not the hum comes in after them. How about a hummy 6AN8? Pull the thing - the PI - and listen. Any hum now is pure power tubes.

Isolate the problem.

Power? That amp uses their 60 watt OT and 60 watt PT. That's a clue.

[Mars Amp Repair]

Ooops for got to mention that the hum is there with the preamp & the reverb tubes removed.
the hum is coming from the output of the tone stack (yes even with the aforementioned tubes removed).

The unit has 2 ground wires that meet & ground the tone stack. One ground wire comes from the ground at the filter can (which I've hard soldered the chassis) & the other makes its way to a ground on a terminal strip near the preamp (which has also been hard soldered to the chassis).
I found if I remove the one ground wire that makes its way back to the preamp ground, I can get the hum to an acceptable level.

Sometimes it's just difficult for me to accept that a solid metal chassis can have a difference of potentials from one end to the other.

As far as the low power is concerned, I picked up another 6AN8 (PI) to see if it's causing the power out issue. Won't hurt to have one around. I have an Eico 666 tube tester, but it has an issue with the merit test...the meter just vibrates like there is bad filtering but there are hardly any filters in the thing. I also have an old Sencore Mighty Mite with issues, too...I'll have to start another thread for those items...thanx for your input Enzo....glen
glen

[Enzo]

Solid metal chassis have resistance, just like wire does. Half an ohm may not sound like much, but it is enough to cause a voltage drop. And if that is ripple current, it will add that signal to the music signal.

The reverb tube is drive only, so it shouldn't add hum much. The recovery is that transistor, hence the warning about the LV rail for it. The recovered reverb is off the collector of the transistor, so even at zero on the control, it still is subject to ANY ripple on that supply rail. Ground the end of the 620k resistor farthest from the 6AN8. That tells us if the reverb is involved.

Tone stack as the source? Ground pin 8, the grid, of the 6AN8. Kills the hum or not? If it does, then the hum is coming from the stack or earlier. If not, the tube sounds bad. Pulling the 6AN8 makes the hum go away?

How about the wire dress to those controls? The treble wiper wire gets next to a heater wire, and there you are. Are the pot bodies well grounded to the chassis at their bushings? Are the rear covers of the pots grounded? An ungrounded pot cover can act like an antenna.

[Mars Amp Repair]

Yeah, The hum was from the gound loop created by the ground loop created between the ground to the tone control & then to another ground point. mostly resolved by removing one of those paths.

This turns out to be the Sentura 1 with a 7199 driver tube & 6CA7 power tubes. It appears the Sentura 2 has a 6AN8 driver tube. Same type of pentode/triode style, just different pinouts.

I'm still having issues with low power out, but I'm not actually certain what tubes the Sentura 1 originally took. The power tubes have 100v p-p driving the grids, so I don't think the PI is at fault. Seems like it's gotta be pretty simple issue.
I'll have to put a pair of 6L6's in it to see how it does. It does miserably with EL34's (like 25watts). A bit better with KT88's which are what the Sentura 2 was designed for. I'm thinking it's an issue with mismatch impedance between the wrong tubes & the O.T. Any ideas?...g

[Enzo]

Mismatch? See the end of this post.

On all those amps of the era, Sunn used the 7199 to drive the EL34s, and the 6AN8 for the KT88s. That way they made essentially the same amp in two power versions.

The two Senturas are pretty much identical. Different power tubes and PI tube, but the circuits are about the same. The Sentura 2 with the KT88s used the 60 watt power and output transformers. The Sentura 1 with the EL34s used the 40 watt power and output transformers. The Sentura 2 has slightly higher B+, and ther are four B+ nodes to only three in the Sentura 1.

What kind of 7199 is in there? The SOvteks are notorious for humming.

The Sentura 1 uses EL34, and the 3-4OT. That has a 16 ohm output for the main speaker jack. Pluging anything into the EXT speaker jack switches things over to the 8 ohm tap. Are you using a 16 ohm load?

The Sentura 2 starts with 8 ohms and steps down to 4 ohms.

Both OTs have all three taps though, so you could change that:
Black - common
Yellow - 16 ohms
Oeange - 8 ohms
Brown - 4 ohms

[Amp Kat]

Also take note to that PI can be wired up different so if the model you have is layed out for the 6AN8 you'll have to change the wiring over from the 7199 on pins 2 & 9 which is what I had to do on the Solarus. However the hum was subject to a number of things including the negative feedback which is strange but you may try bypassing that feedback loop which is really part of the PI circuit. You will also get hum if the reverb transistor is bad or making bad contact or even if that reverb pot is in need of cleaning are simple turned up. The resistors on that reverb/tremelo bd can crack on may need replacing. Another thing to check is the voltages on the schematic should be somewhat close to spec as the old caps and resistor drifts can change a good bit and may throw things out of whack. The one I did had all blown up trannys in it and I converted it to a 6V6 amp and it sounds great and hum free unless you turn the reverb pot up when no tank is connected. IMO the only weak link in this amp is that reverb/tremelo circuit that the parts are hard to find. Enzo told me he hasn't seen a replacement for that SM-1 module so if it's bad you may be SOL.

[loudthud]

I've worked on several bass amps in this series but never a guitar amp with tremolo and reverb. I have to say up front that I don't think I've ever brought one of these amps up to it's full potential.

The can cap is usually fried so I build a G-10 eyelet board with series connected radial lead caps to replace the can cap. I install the board where the reverb-trem board would go. These amps have lots of grounding issues that I try to address by separating grounds for the caps, re-routing the PT center tap directly to the first cap and the preamp and driver cap grounds to their respective circuits. In addition, I build a voltage divider between the standby switch and the bias supply terminal strip that is used to bias the heater supply up to about 50V or so. This gets rid of the 60HZ component of the noise but not the 120HZ buzz component. I've never been completely satisfied with the results, but short of completely re-doing the grounds, isolating the input and speaker jacks etc, I'm out of ideas. Best of luck to you in this regard !

The output seems to be limited to about 55 watts at the onset of clipping with 6550/GZ34's that are available these days. I've never tried using high dollar NOS types or currently available KT88's.

Schematic link:
http://sunn.ampage.org/site/schematics/sentura2.gif

A couple of issues I've had to deal with:

1) Power or standby switch worn out.
2) Output impedance switch/jack excessive resistance (install toggle sw).
3) Bias of the 6AN8 pentode not correct ( check the screen and plate resistors and adjust the 680 ohm cathode resistor if necessary).
4) The (in)convenience outlet and fuse holder for 3 wire conversion.
5) Corrosion around input and speaker jacks.

[Amp Kat]

FWIW I did everything Loudthud mentioned and that Power switch is also another hard to find item. I never did find a replacement that would fit that size or mount right. The schematic you posted is the exact same as the 100S with the 6AN8 tube. The Solarus schematic is a little different with the 7199 tube. I did eliminate the negative feedback and it sounded better without it.

[Mars Amp Repair]

Well, the power issue ended up being something dumb...5U4 in place of 5AR4. Hate when that happens.

Hum was minimized with the grounding done in my last post. As I mentioned it occurred with both preamp & reverb driver tubes removed. Only eliminated by grounding out the input to the pentode stage of the 7199 or of course removing the 7199.

I see below where some fiddling with the NFB was a cure for hum...I tend to think it is really just symptomatic of the grounding issues, but hey if it fixed that problem, can't argue.

glen