Help with a 68 Sunn Sentura II

[shiner555]

Hi Guys,

I'm becoming stumped while trying to repair an old Sunn head. I have an all original 68 Sunn Sentura II I'm looking at

Prowess Amplifiers - Sunn - Schematics - Sentura 2

It was brought to me with a complaint of a loud hum, as well as a popping/crackling in the reverb. I was able to verify that all of the original NOS tubes were still good, however the lydic's inside were suspect. So I replaced the big can cap, as well as all of the other high voltage lydic's. No other cap changes in the EQ or otherwise. The amp still hum's. A few things I've been able to find out in-relation to the hum.

1) Hum is directly related to the volume of V1, with the volume at 0 the amp is very quiet.
2) If I pull V1 with the volume turned up to hear the loud hum, the hum then disappears. Hum is not related to the tube, I've swapped out several other 12AX7's.
3) I've disconnected the reverb circuit as well as the tremolo, so that V1 is only feeding the 68N8, hum is still there.
4) I've replaced the 100K plate resistors, no change in hum. I also checked out the .01uF cap, and there is no change when replaced. In the V1 circuit.
5) I've also totally disconnected the input jacks to pin 7 of V1 and the hum is still there.

I'm getting kinda stumped. Any ideas on where I should look?

BTW, when looking with a scope at the power supply, I can see the hum on the A point 485-500V area as a funky waveform, however I do not see the hum on the other output points of the power supply....thus further head scratching.

Many thanks!

[52 Bill]

5) I've also totally disconnected the input jacks to pin 7 of V1 and the hum is still there.

The input grid needs a ground reference, which is the 150k resistor. Check it and the ground connection.

If you ground pin 7 does the hum stop?

[shiner555]

Hi Bill,

Good point! I'll try that tonight and see what I can find out.

Question, based on the fact that the amp is quiet with V1 removed or at low volume, do you think that the 68N8 circuit is ok?? Or hard to say at this point.....all voltages check out in that area as well.

[shiner555]

Hi Bill,

I just tried grounding pin 7 and there was no change, except for removal of a loud 60Hz hum from pin 7 being un-grounded. The higher pitched hum is still there and get's worse when you turn up the volume. Pull V1 and the amp is quiet, or at least I thought so until tonight. When looking at the output I noticed a very slight oscillation that I didn't notice before because my scope wasn't turned in enough. Sure enough you can hear a very quiet ticking/oscillation when listening to the speakers. If you pull the 68N8, the ticking is gone. Note the ticking is there without V1 in, put V1 back in and put the volume to zero and the ticking is still there....this is with the trem vol at zero and the circuit disconnected, as far as I can tell.

Now I'm even more confused. ;P

[shiner555]

Ok!! Finally made some solid headway. I was able to determine that the issue has nothing to do with V1 or the power supply caps I replaced. The hum and the clicking/popping sounds are all coming from the tremolo circuit. If I cut the power to the trem circuit, point D on the schematic, all of the problems go away, amp sounds great! I did re-cap the trem circuit, so I wonder if something I did there was wrong, or if one of the transistors has gone bad. The trem actually works and sounds good......but not with the hum.

Does anyone know if the SM-1 can cause the hum even if it is still functioning?

[52 Bill]

If I cut the power to the trem circuit, point D on the schematic, all of the problems go away, amp sounds great! I did re-cap the trem circuit, so I wonder if something I did there was wrong, or if one of the transistors has gone bad.

Did you recap the low voltage power supplies? There are two, one negative for the power amp bias and one positive for the tremolo. The only direct connection between the tremolo circuit and the signal circuit is through the power supply.

Does anyone know if the SM-1 can cause the hum even if it is still functioning?

It is just an opto-isolator, with a light bulb and a LDR. If the wires going to the LDR picked up the noise, the problem would not go away when you disconnected the power supply.

[shiner555]

Did you recap the low voltage power supplies? There are two, one negative for the power amp bias and one positive for the tremolo. The only direct connection between the tremolo circuit and the signal circuit is through the power supply.

Hi Bill,

Yes, I re-capped the bias supply, as well as the trem "D" supply, and the trem PCB (low voltage caps).


It is just an opto-isolator, with a light bulb and a LDR. If the wires going to the LDR picked up the noise, the problem would not go away when you disconnected the power supply.

I tried two methods of disconnection last night. One, killing the main power feed into the D supply. Hum is gone. I then only disconnected one of the leg's of the LDR that feeds into the 1K resistor. Basically killing the bias voltage to the 2N2012, hum is totally gone once again. This is all leading me to suspect the LDR or the circuit involved in it. Again, I'm confused because the LDR works....just don't know if it can hum that bad or not and still work. What I didn't have time to do last night was to disconnect the LDR and check for ripple on the "D" supply.....I'll try that tonight.

Thanks!

[52 Bill]

Again, I'm confused because the LDR works....just don't know if it can hum that bad or not and still work. What I didn't have time to do last night was to disconnect the LDR and check for ripple on the "D" supply.....I'll try that tonight.

The photo-resistor is still in circuit when the power supply is cut off. Anything is possible, so maybe the photo-resistor is failing in some way so that it is making noise only when there is a little bit of light falling on its surface. And it is silent when there is no light hitting it.

A good test would be to remove the LDR from the audio circuit with the power supply still connected. That way you can tell if the photo-resistor is adding the noise to the audio circuit or not. Start by reconnecting the D power supply and then just add a jumper to short across the photo-resistor. Does the noise come back?

My bet is on a power supply problem or the 2N2102 lamp driver transistor.

Edit:
Wait a minute, are you sending a signal through the amp when the D supply is shut off and the hum goes away?

[shiner555]

Hi Bill,

Yes, great idea. I'm going to try what you suggested as well, that is shorting the power side of the LDR and see if the hum is still there. When I disconnected the D supply, I only had a input cable installed and I was tapping on the end of it to make some noise. Amp was dead quite, with no hum, but loudly producing signal once I touched the end of the input cable.

I just found out that the customer of mine has two more of these amps, one a 68 and the other a 69, that have never been serviced. If I have to, I'll pull the LDR from one of em and swap them out. I'm also starting to think that I may have also gotten one of the low voltage caps in the wrong orientation when performing the cap job in the trem circuit......that could cause a hum.

Thanks again for your help!

[shiner555]

Well.....another interesting night on the bench. Turns out I was chasing a rabbit down a hole with the tremolo. I actually sent a signal the amp tonight while killing the "d" power supply, and yes the hum was gone but the signal also dropped out quite a bit. I then disconnected the trem audio lines and ran a jumper from the .01uF cap to the end of the vol pot, hum is back and louder than ever. I then connected and disconnected the "d" supply and there was no change at all in the hum. I've been chasing the wrong thing.....the trem circuit is working just fine.

Only when I ran a new ground wire all the way from where the power xfmrs are joint grounded did the hum go away. Looks like I have had a ground loop in the V1 grounding scheme. I had read in another thread about how the grounding of these amps was suspect....I also noticed that none of the heater wires are twisted for RF rejection.

Do you happen to have any suggestions to look at for grounding in general?

[soundmasterg]

Check to make sure all the ground connections are good. A lot of times on vintage amps they ground through the ground terminal on a terminal strip, and they might rivet it to the chassis...and corrosion gets in there and you lose the ground. Check resistance on all grounds...should be as close to 0 as possible.

Greg

[52 Bill]

Greg is right about the ground connections, check them all very carefully.

The trem problem was why I had asked if you were sending a signal through the amp when cutting the power to the circuit. When the trem is turned off, the light bulb has to be turned on to lower the resistance of the photo resistor. When you cut the power to the circuit you turned off the light making the photo resistor go to a high resistance, thus reducing the hum level.

The schematic is full of errors that I have been noticing. The "D" node is shown as the low voltage positive supply, but the preamp tube plates are shown as connecteing to the D node. And the ground connection is missing from the preamp tubes cathode circuit and volume control.

[patlaw]

Inquiring minds want to know. Did the problem get fixed?

[TimmyP1955]

I suspect that the 260V node was supposed to be labeled D, and the 16.5V labeled E, along with what it feeds.

From the grid of the 6AN8 onward, I believe this is a clone of a Dynaco MKII or MKIII.

[patlaw]

From the grid of the 6AN8 onward, I believe this is a clone of a Dynaco MKII or MKIII.

MKIII