1969 Sentura II Circuit Questions

[patlaw]

Are the pots linear or audio taper? I assume they're linear. Do the pots from Tubes and More fit?

What should I do with the accessory AC outlet on the back? Just disconnect it and leave it in place? I could put a three-wire receptacle there, but I'd have to use a knock out punch on the chassis to make a bigger hole, which I really don't want to do.

What's the consensus on the "death" cap? Take it out? It actually makes a difference in the level of buzzing in the amp.

Are the knobs the same as Fender's? I need a new set of knobs for mine.

[Ryan Phelps]

Look on the back of the existing pots to see if they are linear or audio taper (or check the schematic). Standard (not metric) pots will fit properly.
Leave the existing AC outlet. If it is polarized (one slot is larger than the other), be sure it is wired correctly (larger slot is neutral, smaller slot is hot). Otherwise, I'd eliminate it from the circuit.
Remove the death cap and take the polarity switch out of the circuit when you have a 3-prong AC cord installed.
The knobs are not the same as Fender. Sunn knobs were made by Rogan and are numbered 0 - 10 (Fender are numbered 1 - 10). You'll find them on ebay sometimes. One of our members had some made a few years ago....he might have some left. Post under Classified.

[patlaw]

What is written on the back of the pot to tell whether it's linear or audio taper? There are no indications on either schematic that I have.

Thanks for the information on the knobs. Hopefully I can find some.

EDIT: It appears that the knobs sources have dried up. George is not active on the forum any longer, and Dave Kiptka does not show up as a seller on eBay. Maybe Dave's name is misspelled.

[Soundmasterg]

What is written on the back of the pot to tell whether it's linear or audio taper? There are no indications on either schematic that I have.

Thanks for the information on the knobs. Hopefully I can find some.

EDIT: It appears that the knobs sources have dried up. George is not active on the forum any longer, and Dave Kiptka does not show up as a seller on eBay. Maybe Dave's name is misspelled.

There should be some numbers on the side of the pot....indicating the value....you may also see an A or B...A indicates a log pot and B indicates linear in most cases, though read the below quote from wikipedia....

A letter code may be used to identify which taper is used, but the letter code definitions are not standardized. Newer potentiometers will usually be marked with an 'A' for logarithmic taper or a 'B' for linear taper. Older potentiometers may be marked with an 'A' for linear taper, a 'C' for logarithmic taper or an 'F' for anti-logarithmic taper. The code used also varies between different manufacturers.

Remove the death cap as Ryan noted. it makes a difference in the buzzing of the amp because you are still using a 2 wire plug which is not grounded.

On my old 200S I took out the accessory outlet and the existing cord and cut out the chassis a bit to install a standard IEC plug that I could then plug a standard power cable into. I did this because I think having an attached power cord on a head without something to wrap it around is dumb and an IEC plug makes more sense. I also did this because I didn't have the proper D shaped punch to enlarge the hole for the proper size 3 wire power cord. You can drill a new hole with a stepped drill bit and it will work, but the round hole doesn't hold the cord that well and a hard yank can sometimes pull it out. Also the proper D shaped punch is like $500 for a Greenlee.

If you rewire the amp for proper 3 wire operation, the death cap should go and the ground switch should be disconnected. I used the ground switch in my amp as a ground lift switch and rewired the rest of the amp so all the grounds were isolated from the chassis except through the ground switch. it is a LOT of work and I don't recommend it unless someone really wants to do it or needs to use the amp in a situation where that would be useful. If you just do the standard setup and change to a 3 wire and disconnect the ground switch and get rid of the death cap, then you should be ok....however the proper way of connecting the wires in a 3 wire can't be done on the Sunn because then the light in the power switch will not work anymore. So just rewire it up to 3 wire and connect as it is now to the light on the power switch and the amp will work fine.

The proper way to do a 3 wire conversion is to have the hot (black wire in the US) wired to the fuse, then the switch, then one power transformer lead. The neutral (white wire in the US) wire should go direct to the other power transformer lead. The ground (green wire in the US) wire should go directly to it's own bolt on the chassis at least an inch away from any other grounds and with some slack so if the cord gets pulled out it will be the last thing that stays connected. But as I said you can't do it this way in the Sunn unless you want to lose the power light function, so you have to stick with one side going to the switch and the other going to the fuse. It will work and is safe but is not as good as the other way.

Greg

[patlaw]

Thanks, Greg. That's very helpful. Modifying the chassis doesn't keep the unit stock, but like refretting guitars, some things are just necessary.

Unfortunately I cannot find any indicators on the pots as to their tapers. They do have a 1966 date code. I'm fairly sure the volume pot is audio taper, but I have no clue on the tone pots. I would have thought that it would be a readily answered question. Maybe I'll do it the old way and measure the wiper resistance at 25%, 50%, 75%, and 100%.

The 270pF cap in the treble circuit looks like its leads might have been stressed a little. It's strung pretty far from the terminal strip to the pot. I'll probably replace it.

[Soundmasterg]

Thanks, Greg. That's very helpful. Modifying the chassis doesn't keep the unit stock, but like refretting guitars, some things are just necessary.

Unfortunately I cannot find any indicators on the pots as to their tapers. They do have a 1966 date code. I'm fairly sure the volume pot is audio taper, but I have no clue on the tone pots. I would have thought that it would be a readily answered question. Maybe I'll do it the old way and measure the wiper resistance at 25%, 50%, 75%, and 100%.

The 270pF cap in the treble circuit looks like its leads might have been stressed a little. It's strung pretty far from the terminal strip to the pot. I'll probably replace it.

I can ask Conrad if he remembers what type of pots they used back then if you want? Might take a week or so though as I have finals this next week and a lot to do between now and then. I am sure the volume pot is a log, but the others....not sure. Fender usually uses log pots but Marshall often used linear pots. Who knows what Sunn used....

Greg

[patlaw]

I did some measurements with the pots in circuit. It appears that the volume is log and the others are linear. The volume pot is fine. The treble is the only one misbehaving, but it still works. I just treated with Deoxit, so it may be fine, now. The 270pF may get replaced with a silver mica, just because. There may be some other flakiness in the tone circuit, but one thing at a time.

Conrad must surely get tired of Sunn questions.

[Soundmasterg]

I did some measurements with the pots in circuit. It appears that the volume is log and the others are linear. The volume pot is fine. The treble is the only one misbehaving, but it still works. I just treated with Deoxit, so it may be fine, now. The 270pF may get replaced with a silver mica, just because. There may be some other flakiness in the tone circuit, but one thing at a time.

Conrad must surely get tired of Sunn questions.

It wouldn't surprise me if the other pots were linear....Conrad thought Leo was misguided to use audio pots in some of his amps there.

You know I am not sure if he gets tired of the Sunn stuff.....he is certainly proud of what he accomplished and rightfully so, and it has allowed him to do a lot of things since that he wouldn't have been able to since his name wouldn't be known like it is now.

Greg

[patlaw]

I finally got the amp fully restored and back into service. It sounds as good as it did the day it was new. It's been so long since I've used it that I'd forgotten how rich the lows are on this amp. As a bonus, it's quiet enough for recording. The only issue I see or hear is the thumping noise when the tremelo circuit is engaged. Since I don't use it, it's no big deal. We'll have to give it a 50th birthday party in 2019.