Sentura II Restoration - Tremolo Bug/Roach Replacement Photocell

[Watson]

I have a Sentura-II I am going to be restoring and one of the issues is a tremolo circuit that is missing the photocell. Nothing I've been able to find lists anything specific about the original components used, notably the photocell and the light bulb.

In early 2018, I emailed Conrad asking 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.

The other night I began looking for information again and with Google's help I found a post on the Music Electronics Forum discussing what was found to be a suitable replacement for the original photocell. Here is information about the photocell from that forum post: http://music-electronics-forum.com/t40984/

The Silonex NSL-5160 photo-conductive 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.

Here is a link to the photocell on the Allied Electronics website: https://www.alliedelec.com/luna-optoelectronics-nsl-5160/70136785/

I've ordered a couple of the photocells and will use one in the process of restoring my Sentura-II.  I'll post pictures as I make progress in the restoration. I've restored the 2x15 cab using original grill cloth (from Conrad) and original Tolex (from member Ducatidoc) and will post pictures of it as well.

[Watson]

This Sentura II was a parts project that cost $200. It will be a fun restoration.  It is missing one tranny and the other is damaged.  Tube sockets are either cracked, chipped or missing and it is missing tubes.  The electronics are present minus the bug.  It was well used as the knobs had long ago lost most of their white numbering.  The chassis will be striped down and cleaned up and we'll go from there. 1 ohm resistors, a second bias pot and top side test points for the tube biases will also be added since it will be gutted anyway.

Replacement parts in hand for things missing or damaged are:

1. a 28-2360 PT
2. a 28-1360 OT transformer.  I also have a new Triode A431-S OT (replacement for the 28-1360) and will probably use it as it has more speaker impedance options.
3. a set of original Rogan knobs in good condition.
4. an NOS SUNN amp head grill with the SUNN logo badge. It was on a shelf in the back room of a music store out east since the late 60's.
5. enough original Sunn Tolex left to recover the head
6. NOS Sunn corners from Conrad
7. NOS GE 6550A output tubes
8. NOS GE 6AN8 tube
9. Mullard 12AU7 and 12AX7 tubes

I'll begin this project soon and will update the post as it progresses.

[patlaw]

Do you already have a schematic showing how you are going to wire the second bias pot? If so, could you share it?

This project will be fun. My Sentura II is a great amp.

[Watson]

I haven't drawn anything up yet but had in mind to just replicate the existing bias pot circuit that sits between the 100K resistors coming off pins 5 of the output tubes. Each would then have their own 10K pot, 18K resistor and 50uF cap and both pots would tie to the 1K resistor.

I don't think I'll expose probe points as I always use my Compu-Bias for tube biasing but if you wanted to do so, I guess you would want to put a 1 ohm resistor between pins 1 & 8 on the output tubes, have a common black  probe point to ground and then separate red probe points to pin 8 of the output tubes. Would probably be most convenient to place these probe points on the back of the chassis.

[Watson]

I won't get to if for a bit yet, but here is what I think makes sense for the bias pots and probe points although I doubt I'll put the probe points in.  It's what I'll start with and see if I've wonked it up or not.

I started with SUNN's schematic so this should just lay over the original.

[patlaw]

What schematic capture program did you use to draw this schematic?

[Watson]

Just Photoshop.

[patlaw]

The wiring for the input circuit and photocell are different from the schematic I have. Is that intentional?

[Watson]

There are a multiple versions of these early SUNN amp schematics.  I was just interested in the output section for drawing the mod I want to make for the dual bias pots.  The one you posted is a later circuit design and probably more accurate.  The one I used for the output section showed 6550 tubes rather than KT88 so it is an older schematic and also listed 100S, Sceptre and Solarus although the output section doesn't differ. 

I went through service bulletins I have and here is what I think is some more good information on the vibrato related components. These are Fender service bulletins but the information is also applicable as SUNN used similar or the same technology for the photo-optic bug/roach as I was told by Conrad.

I hope these are helpful for anyone interested in the tremolo circuit (actually a vibrato circuit).  FWIW, a Tremolo modulates volume (like turning the volume control up and down quickly) whereas Vibrato shifts the frequency of the notes being played (like bending a string) so despite being labeled Tremolo on the amp's face plate, it is really a Vibrato circuit.

Service Bulletin 5 (Tech Notes-Defective_Photocells.pdf) addresses Vibrato failures and among other things mentions that the Vibrato bug assembly consists of and LDR and a NE2 Neon lamp.   

Service Bulletin 9 (Tech Notes-Vibrato_Ticking_Fix.pdf) addresses Vibrato ticking and corrective action.

Service Bulletin ? (Tech Notes-Tube_Amp_Topics.pdf) is a general review document of many tube amp issues discussed in previous Service Bulletins. Page 3 contains more information on the Vibrato LDR/Lamp bug and also a little bit on the design of the phase shift oscillator used in the design.  Again, the same basic design used by SUNN with the triple pot circuit.

[Watson]

Had a few minutes and added a few more changes to the Sentura II schematic.  Just updated it to reflect the 3-prong wiring, updated the input jack wiring and added the mid-boost switch (3PST) in addition to the dual bias pots and probe points.

The 2nd attachment is the "corrected" Sentura II schematic I've been working from. 

I think I have the Sluckey bias mod changes correct now in the schematic (http://sluckeyamps.com/sunn/sunn.htm).  If anyone notices any errors please point them out.  Thanks in advance.