Sunn Musical Equipment > DIY

Repairing Sunn Tube Amps

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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:

--- Quote from: Watson on July 31, 2020, 01:35:58 pm ---Did you ever find a suitable substitute for the SM-1 in the trem circuit?

--- End quote ---
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.

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