Repairing Sunn Tube Amps

[patlaw]

Restoring my own Sentura II was a great experience. This will sound strange, but if anyone has a Sentura II or similar tube head that you want restored, I'll do the labor for free. Just pay for parts and shipping. Warning. I'm slow, but I'm true to form. If I can get the original part, I will.

Here is the thread on my own restoration, which went perfectly. I'm in North Carolina. Send me a private message. References available.

[Watson]

Did you ever find a suitable substitute for the SM-1 in the trem circuit?

[patlaw]

Did you ever find a suitable substitute for the SM-1 in the trem circuit?

Yes and no. I did a lot of research as to which LDRs would best simulate the Sunn bug and bought a variety of parts to try. However, I never got around to trying it. If you're stuck and need one, I'd be willing to continue the project if you'll be the guinea pig. My tremolo works, and I'm gun shy about removing the bug from the circuit. If we could get the parameters of the SM-1, it would be much easier to duplicate the circuit.

The key parameters are voltage versus resistance. With 1 volt across the bulb, what is the resistance? With 5 volts across the bulb, what is the resistance? What is the maximum voltage across the bulb? What is the resistance at the maximum voltage? If I had an amp with a non-working tremolo, I'd be willing to try to recreate a workable bug. Again, though, the SM-1 seems fragile, so I'm hesitant to take mine out. If worse came to worse, I'd create one with a microcontroller that allowed me to program in the resistance at a particular voltage, but I don't think the parameters are that critical.

[Watson]

In early 2018, I asked Conrad for any information he might have on the trem bug used in the early SUNN amps.  His reply was:

We custom made the photo cell devices from purchased components.  They were each hand tuned for maximum swing.  I have no idea what the part numbers were for the incandescent bulb and photo resistor.  No LED's back in the day.

Here's what I found this evening looking for more info.  I went ahead and ordered 2 of the cells.  Not sure if I need a new bulb yet.  Sounds like a 6V grain of Wheat bulb will work if I should need to replace it to.

Here is information about the photocell from another forum post:

The Silonex NSL-5160 photoconductive cell has the same or similar properties as the custom cell in Sunn amps.
The bulb used with the cell is probably a 6v grain of wheat bulb.

https://www.alliedelec.com/luna-optoelectronics-nsl-5160/70136785/

http://music-electronics-forum.com/t40984/

Sunn Sentura Tremolo Fix - Dead CDS cell
I've been trying to resurect a Sunn Sentura I. The amp is a real science project. While it has a rectifier tube & socket the power transformer is missing the 2 yellow leads to supply 5 VDC to the rectifier. I fabricated a solid state rectifier tube which works fine plugged into the original socket. After cleaning up the wiring & the soldering everything worked except the solid state tremolo. The oscillator was working powering the original lamp. No volume change. The CDS cell in Sunn's version of the Fender bug was dead. As I have said in previous posts, I spent many years working as camera repairman. I got some surplus cells from junk cameras from a friend & found that they were the wrong sensitivity for this circuit. The internet lead me to Allied Electronics who actually sells CDS cells made by Silonex. I spent considerable time with the spec sheets finding a suitable replacement cell. It is: Silonex NSL-5160 The cell fits inside the original housing, the tremolo is now strong. I did replace the 6 electrolytics in the circuit after I confirmed the bad cell.
The cell is the black object in the photo. I used Pliobond covered with black paint to glue the cell back into the housing & to secure it to the PCB as it originally was.
Here is a photo of the Reverb / Tremolo PCB used in this amp.

 

[patlaw]

Looking at the circuit, the bulb has to be illuminated for the amp to work. When the bulb is lit, it shines on the photocell, which causes the resistance of the photocell to decrease. When it decreases to a certain point, the signal is coupled between the plate of the first section of the 12AX7 and the grid of the second section of the tube. The following measurements would be very useful to find a replacement photocell:

Resistance of the photocell with the bulb illuminated.
Voltage across the bulb with the tremolo off. Is it 4.5 volts?

Without a working cell, these numbers can be determined experimentally. Connect a 100K pot across the terminals of the photocell. Adjust the pot to find the point at which the coupling between the first and second sections of the tube is "normal". That's the target resistance of the replacement photocell when the bulb or LED is illuminating it. Second, connect the LED or lamp across the 1.5K resistor. Play with the Depth control to determine whether the photocell's off resistance is high enough to provide sufficient attenuation of the signal to have a good tremolo effect while making sure that the voltage across the bulb/LED does not exceed its normal operating parameters.

Based on the schematic, it appears that the bulb is a 6 volt bulb that is operated at 4.5 volts to cause it to last longer. That's just a guess, though. The circuit can absolutely be duplicated. The replacement may not be perfect, but it will be good. It may even be better sounding than the original. I have 2N2102s and 2N2923s in my parts inventory. If it weren't for the three-section 25K pots, I'd probably build one of these just for fun. In reality, the oscillator can be replaced with a 555-type timer or a dozen other circuits.

[Watson]

Doesn't seem to be a need to experiment/calculate anything at this point.  Based on the Music Electronics forum thread by member Drewline, that work was already done and produced a working circuit using the Silonex NSL-5160 cell from Allied Electronic.

"The internet lead me to Allied Electronics who actually sells CDS cells made by Silonex. I spent considerable time with the spec sheets finding a suitable replacement cell. It is: Silonex NSL-5160. The cell fits inside the original housing, the tremolo is now strong."

I ordered a couple of them and will go forward using it.  If I need a bulb, I'll use the grain of wheat 6V as mentioned in the same thread. I'll post pictures when I get back to restoring my Sentura II and get it working.

Heres the thread from music-electronics-forum.com: https://music-electronics-forum.com/forum/amplification/guitar-amps/sunn/42045-sunn-sentura-tremolo-fix-dead-cds-cell?t=40984

[patlaw]

Understood and agreed, but based on Conrad's comment, there may be a selection process to get one that sounds right. I refurbish a piece of broadcast equipment called the Dorrough DAP 310 Discriminate Audio Processor. They were manufactured starting about 47 years ago. They used a J112/VCR2N JFET in the compressor cards. While the JFET is technically still available, they have been improved over their 47 year-old ancestors, and they don't work in the circuit. The specs are so loose on them that they meet the specs on the tech sheet, but they don't meet the operating parameters of the circuit. They have to be hand selected, and the yield of working JFETs is low to zero. When the problem first arose in the eighties or nineties, the manufacturer was able to get newer units to work by tweaking the circuit. Things went downhill after there. JFETs are garbage devices. They work perfectly for some purposes but not others. They should not have been used in this circuit (although the units were extremely successful, worked very well, and are still highly sought after today.)

A CdS cell is also not a precision device. While off resistances are predictable, on resistances can vary quite a bit. If the on resistance isn't low enough, the amp will not have enough gain with tremolo depth at zero. I don't know what happens if it's too low. Maybe the tremolo has a ticking or thumping noise. So, while you may have to tinker with the circuit a little, you should be able to make it work. If you do, I recommend getting a super-bright white LED, and trying one of those in the circuit. If it works as well or better, you should never have to replace the "bulb" again. You will also have to play with the light source to CdS coupling and the ambient light leakage.

I see from your other thread that the CdS cell is missing in your amp. I'm excited to hear the results of your project. A lot of people are struggling with bad bugs. A YouTube video or even a post about your success in repairing your unit would be appreciated by a lot of people, I believe. Maybe you can build a few and offer them for sale. By the way, I know that you already know what I'm posting, but I offer this information for the benefit of someone who may not have your level of expertise.

[Watson]

Yep, it is just a starting point.  2 photocells were ordered anticipating there is likely to be a variance in their sensitivity.  Also trying a white LED as a light source is a great idea as that would be a permanent solution.

Progress will be posted to this thread as it progresses.

https://forum.sunnstillshines.online/index.php?topic=10657.0

Thanks for the good information and suggestions!