"Unmodding" an ARC SP3 Preamplifier

[gadget73]

No. It’s broken. I did not think that “fixing” it for a subjective test was a good use of time or client resources.

this is the sort of thing I'll do to waste my own time with, but I agree its not good for a customer job. In this case I probably wouldn't have wasted my own time with it if it wasn't already working though. It just seems like such a bad idea right out of the gate.

 

[Blue Shadow]

There were a number of bypass caps in Tom Clone’s SP-3A-1, same model as this one, but they were orange chiclets mounted on the bottom of the board. Removed when the unit got a refresh.

This one, phono two disconnected so phono 1 goes directly to phono board, not through a switch. Same with the main outputs, direct to the RCA jacks, not through the tone control switch used to select the output from tubes 7&8.

Don’t know the sonics of the Electronic Controls brand output coupling caps, I used Clarity CMR of the main output. Tom liked em but we never got around to changing the tone control section coupling caps.

You mentioned Curio’s upgrades and I was expecting to see some PS changes, a board mounted vertically at the first input end of the PS board. What I see is a pile of extra caps billowing off the main board, but those caps do address both the PS and audio boards.

Be nice to get more info on the yellow coupling caps to figure out what they are specifically. And why two per channel, to make correct μF? And are they bypassed by a pair of 715Ps?

Just looking at this one pic, making it right should make it look a lot better.

 

[TriodeLuvr]

according to Curcio Audio's recommendations, which included changing the phono RIAA curve to "correct the severe inaccuracy in the original design;" removing the phono stage cathode follower which is allegedly "responsible for serious veiling";

I love my cathode followers. They're the most transparent stages in all of tubedom. I would trash all those recommendations and restore it to factory. Then test the RIAA curve and other specs for yourself. Do you know if ARC is able to supply components like switches and pots, in case any of them are worn out? And hopefully for less than the cost of those cap adaptors. Argghhh.

Jack

 

[ConradH]

I don't know what the difference is between this and a SP-3A-1, but the latter was extremely highly regarded, and one could make the case that it doesn't need any mods at all, just "balance & blueprint".

 

[Blue Shadow]

I don't know what the difference is between this and a SP-3A-1

This is that SP-3A-1 unit. You can see just enough of the (probably Avery) paper, rounded-corner rectangular sticker under the caps in front of the transformer. It says A-1, the way ARC identified many of these.

They also had a black ink stamp A-1 in a rectangular box maybe 5mm x10mm and a black box with white A-1 on it. And of course the full name on the back of the unit. This took some time so they probably used one of the other three methods. I expect the paper sticker to be used for authorized upgrades at dealer/service centers as it could be provided in the kit of parts. You see the sticker in various orientations.

The two different ink type markings might have been factory for upgrades or production as they would have the stamp to do the black ink marking. I need to see more of the black box white writing to conjecture as to its use.

But there are at least 4 methods of marking an A-1 model out there. Here are two of them. I’ll owe you the other two.

My SP-3C is a factory modified A-1 as can be seen in the intro thread I did for it. It was converted this century, maybe -06 to 2014, not sure. I need to ask ARC. A rebuild they still do to these preamps from the 1970s.

 

[Blue Shadow]

Do you now if ARC is able to supply components like switches and pots, in case any of them are worn out? And hopefully for less than the cost of those cap adaptors. Argghhh.

They are out of the stepped replacement volume control, an Alps Blue Velvet and the smooth one is so costly, it hurts to ask about it and some other controls.

I replaced my volume with ARC parts late last century when prices were a bit less than half what they are today (inflation calculator) and they got me for just under $40 for a normal, stepped Alps volume control and two caps to be replaced with the control. Blue Velvets were about ten back then, I guess. So look at triple digits for those parts today. Someone spoke to this not long ago here on ak. Oh here it is, ARC SP-3 Volume Control Issue

 

[BuzzK]

They are out of the stepped replacement volume control, an Alps Blue Velvet and the smooth one is so costly, it hurts to ask about it and some other controls.

I replaced my volume with ARC parts late last century when prices were a bit less than half what they are today (inflation calculator) and they got me for just under $40 for a normal, stepped Alps volume control and two caps to be replaced with the control. Blue Velvets were about ten back then, I guess. So look at triple digits for those parts today. Someone spoke to this not long ago here on ak. Oh here it is, ARC SP-3 Volume Control Issue

This is great info. The unit has a 500K control, and I can't find a 500K ALPS replacement. I wonder why ARC chose a 100K pot when 250K ALPS pots are readily available?

 

[TriodeLuvr]

This is great info. The unit has a 500K control, and I can't find a 500K ALPS replacement. I wonder why ARC chose a 100K pot when 250K ALPS pots are readily available?

Reverb has the 500K Blue Velvet log pot.

ALPS 500K

EDIT: Sorry, I just realized the above part is being sold out of Hong Kong. It's not a known reputable dealer. Forget I mentioned it.

FWIW, I bought a 250K from Angela some time back, and I'm reasonably certain it was counterfeit.

Defective 250K ALPS

I tossed it and bought a 250K from Parts Connexion. That one works great, and I'm planning to buy a few more.

My main preamp uses a 100K Blue Velvet, but I don't remember now where I bought it.

None of these pots have detents, but I personally have never found that feature desirable in a volume control. The Balance control in my main preamp has a center detent that I really like, and I wish the tone controls did too. They can be switched out though, so not overly important.

Jack

 

[gadget73]

detents tend to be overly course, same reason I don't love stepped controls unless they have a whole bunch of positions. I will accept it for dual mono controls just for ease of balancing things but I really don't love that setup to begin with.

they are really nice for balance pots though.

 

[BuzzK]

Reverb has the 500K Blue Velvet log pot.

ALPS 500K

EDIT: Sorry, I just realized the above part is being sold out of Hong Kong. It's not a known reputable dealer. Forget I mentioned it.

FWIW, I bought a 250K from Angela some time back, and I'm reasonably certain it was counterfeit.

Defective 250K ALPS

I tossed it and bought a 250K from Parts Connexion. That one works great, and I'm planning to buy a few more.

My main preamp uses a 100K Blue Velvet, but I don't remember now where I bought it.

None of these pots have detents, but I personally have never found that feature desirable in a volume control. The Balance control in my main preamp has a center detent that I really like, and I wish the tone controls did too. They can be switched out though, so not overly important.

Jack

I have a 500K "ALPS" with terminals instead of leads. It doesn't have the "feel" of a genuine part, and I don't know if ALPS actually made one like it. All of the detented ALPS I bought were crap and failed quickly and I'm certain they were fakes.

I agree about the Balance center point detent.

 

[BuzzK]

There were a number of bypass caps in Tom Clone’s SP-3A-1, same model as this one, but they were orange chiclets mounted on the bottom of the board. Removed when the unit got a refresh.

This one, phono two disconnected so phono 1 goes directly to phono board, not through a switch. Same with the main outputs, direct to the RCA jacks, not through the tone control switch used to select the output from tubes 7&8.

Don’t know the sonics of the Electronic Controls brand output coupling caps, I used Clarity CMR of the main output. Tom liked em but we never got around to changing the tone control section coupling caps.

You mentioned Curio’s upgrades and I was expecting to see some PS changes, a board mounted vertically at the first input end of the PS board. What I see is a pile of extra caps billowing off the main board, but those caps do address both the PS and audio boards.

Be nice to get more info on the yellow coupling caps to figure out what they are specifically. And why two per channel, to make correct μF? And are they bypassed by a pair of 715Ps?

Just looking at this one pic, making it right should make it look a lot better.

I'd bypass the PS cap with 0.1 MFD to extend HF response -- it's unclear if you replaced the "orange chiclets."

You must have much better ears that I do -- I'm not sure I can discern capacitor brands / types by listening.

Whoever modded this pre didn't do all of Curcio's recommendations -- hence no vertical PCBs.

 

[Lavane]

detents tend to be overly course, same reason I don't love stepped controls unless they have a whole bunch of positions. I will accept it for dual mono controls just for ease of balancing things but I really don't love that setup to begin with.

they are really nice for balance pots though.

Same here. The small volume control I bought from Tubes4hifi back when I bought a ST-70 kit has plenty of steps compared to many others I've had.

 

[Lavane]

Reverb has the 500K Blue Velvet log pot.

ALPS 500K

EDIT: Sorry, I just realized the above part is being sold out of Hong Kong. It's not a known reputable dealer. Forget I mentioned it.

FWIW, I bought a 250K from Angela some time back, and I'm reasonably certain it was counterfeit.

Defective 250K ALPS

I tossed it and bought a 250K from Parts Connexion. That one works great, and I'm planning to buy a few more.

My main preamp uses a 100K Blue Velvet, but I don't remember now where I bought it.

None of these pots have detents, but I personally have never found that feature desirable in a volume control. The Balance control in my main preamp has a center detent that I really like, and I wish the tone controls did too. They can be switched out though, so not overly important.

Jack

There is a thread someone posted some years back with pictures of Blue Velvet pot labels showing the subtle differences between the real ones and rhe fakes. Most seen for sale these days are most likely fakes.

 

[Blue Shadow]

You must have much better ears that I do -- I'm not sure I can discern capacitor brands / types by listening

Original caps vs nice, new Clarity cap, you would hear the difference. There wasn’t much cap engineering directed at sound quality in the late 60s, early 70s and there are a number of quality caps today. ARC has even changed brands over the years for sound quality reasons.

 

[ConradH]

Some years ago I needed a dual Alps Blue Velvet pot that was unobtainable. I ended up buying a half a dozen fakes, then selecting the one with the best matching at the bottom end, as that's where they tend to be bad. Worked OK but I only got one usable one out of the half dozen.

 

[Lavane]

Some years ago I needed a dual Alps Blue Velvet pot that was unobtainable. I ended up buying a half a dozen fakes, then selecting the one with the best matching at the bottom end, as that's where they tend to be bad. Worked OK but I only got one usable one out of the half dozen.

The fakes aren't very well made inside at all. I've used a few myself. One became scratchy after a few months, and another I bought just spun all the wau around brand new. I got refunded for that one. I only have a couple still in use that so far have held up. So Q.C. is all over the place.

 

[Blue Shadow]

detents tend to be overly course, same reason I don't love stepped controls unless they have a whole bunch of positions

My SP-A has the ARC supplied stepped Blue Velvet. I love it because the control is smooth and it has that big knob controlling it. 30-steps.

My SP-C has the non-stepped Blue Velvet and its missing the tactile feel that exudes quality in the big knob, stepped attenuator.

Maybe next gig I’ll have em set up for evaluation.

 

[Blue Shadow]

If you need a volume control…I’ve got Goldpoints in my Foreplay Bottlehead and Tom had a Black Beauty in his SP-3A-1 I restored.

Goldpoint level controls should fit but they are a bit more expensive at $165 for 24-step and $246 for 47-step. They are almost top rated units. @tubelectron selected a Khozmo 48-step unit. Similar in price to the Goldpoint in € plus shipping, tariff and such.

Either should be Alps Black Beauty equals or better and this long discontinued Alps is superior to the Blue Velvet which has only a handful of values left in production.

 

[Lavane]

My main preamp has a stepped volume control and a seperate gain control for each channel as well. They have fixed resistors. The gain controls are nice if you need better control of the volume when using hotter sources like a CDP.
21 4o 24 stepped attenuators just don't have enough steps. There is always too much or not enough volume when using them. The more steps the better.

 

[arts]

@tubelectron selected a Khozmo 48-step unit. Similar in price to the Goldpoint in € plus shipping, tariff and such.

I use the Khozmo 48-step 100K series attenuator in my own SP-3A-1. A wonderful component, but pricy. It is NOT a drop-in mechanical replacement.