If anyone is still looking for a SLP-98

In my setup I’ve found I like the old NOS Sylvania 6SN7 chrome top tubes except for the balanced input position tubes.  Because my DAC tends to be a bit lean for my taste I really like the Sylvania 6F8G tubes (which need to be used in an adapter).  These tubes sweeten things up nicely yet don’t loose much in the way of dynamics.

The SLP-98P F1 is a very nice little preamp.  The jaguar red is especially beautiful I always thought. My favorite color on the Cary equipment. 
Cary Audio only used the Jensen oil caps as an upgrade (early oils were Audio 1’s), otherwise it was polypropylene. Eventually moving into the Mondorf’s.  The Jensen oil caps are a bit more laid-back and tame compared to polypropylene’s, which are typically more detailed and airy.  It’s all going to depend on the rest of your system and your taste.  Also polypropylene caps last longer than oil. Oil caps last about 5 to 10 years and may start leaking or dry up.  The leaky issues are more on amps with higher voltages than on the preamps though.  
Hexfred’s I thought we’re great upgrades on these. Upgrading from the standard diodes to the hexfred’s you’ll notice more detail and faster bass. The SLP-98 has always had solid state rectification, which does not mean it’s a hybrid. Hybrid has to do with the actual analog circuit not the power supply.  
The SLP-98P F1 can still be taken a lot further in sound quality with some power supply upgrades and even better coupling caps.  Installing some large10uf polypropylene bypass caps on all the PS electrolytic caps makes a big improvement.  We always used Solan as they are a good size and fit well.  The F1 should have Kimber Kable in it but maybe not.  Upgrading to a good quality wire makes a big difference inside.  The only thing I didn’t like about the Kimber was it’s not shielded and consequently, picked up information from the other inputs wiring if a source was still on. Now days, I’d look at some good shielded OCC wire. 
As for tubes, I always thought JJ was crap. Sorry but just my opinion.  EH are Russian and tend to be a more on the hard/bittle bright side for my taste. Old 6SN7 Sylvania’s are fantastic for detail or RCA for more smooth body.  Also Tungsol, etc. 

There’s not a ton of room inside of the SLP 98P F1.  It’s pretty jammed in there already so there’s not a lot of room for huge upgrades.  Adding the Solen 10uf Polypropylene’s caps will help a lot. 4 in the preamp and a couple in the PS.  And the PS I used to always at a couple of higher quality .22uf propylenes to bypass the Solen’s.  Really makes the power supply cleaner and improves not only imaging and that blacker background but bass improves  

Somebody else mentioned they had an SLP 05. It’s super important to match the gain of the tubes to get correct channel balance.  I used to do this on the bench with a scope and meter but it can be done with just a meter and a signal generator or CD player with a 1K sign wave CD into the single-ended inputs.  There are trim pots inside the unit, which has to be opened up and turned upside down to get to the pots that are mounted on the pc board.  Those trim pots adjust the volume to the rear negative phase tubes.  First make sure the two positive phase tubes are putting out equal gain by measuring the output with the volume control turned all the way up.  Once they’re equal then you adjust the negative phase trim pot to match the volume of the positive phase tube that’s in the front tube socket. You do this for the left side and then the right side until you have all four phases of equal output voltage.  DONT TOUCH THESE TUBES NOW.  LEAVE THEM IN PLACE. Now you move on to the balanced inputs. if your DAC has balanced outputs it’s perfect.  You can play the 1K sign wave into the balanced inputs and swap tubes until you have equal output voltage on all four phases.  Yes there’s a bit to this.  And this is why most people send it in to have it done but you can’t do that every time you want to swap tubes around so hopefully this will help somebody.  When ordering tubes it’s important to ask for tubes that are matched for output/gain.  This will make everything much easier.  Especially for those two front line level positive phase tubes and the two balanced input tubes.  BUT every time you change the line stage tubes (not the balanced input tubes) the negative phase trim pots will always need to be adjusted to match the positive phase tubes, otherwise the negative phase may be louder than the positive phase or way too quiet and not canceling out noise correctly and the volume into your balanced amplifier won’t have the same channel balance.

Sorry for the delay in getting back to you on this immattewj. Too much going on in my life lately. Getting ready for a knee replacement soon also.
Just because you’ve pretty much matched the channels voltage wise, doesn’t necessarily mean you’re gonna get perfect dynamics since you have such a difference in gain of the first tubes on balanced input tubes. If there’s a big difference here on the input and then you’re trying to make up the gain afterward, in the line section it could change the sound in ways of the dynamics or image.
Also the important thing is to have equal output on all four phases of the signal. Meaning the left positive and negative phases and right positive and negative phases need to all be equal.
on your question as far as opening up the bottom, yes, that has to be done and they can be turned upside down on top of some foam or rubber. The tubes don’t get super hot so we never really had a problem with it, melting, some good packing foam. No cage is needed.
In a home environment you can simply do what you’re doing using a test signal in to the preamp and using a voltmeter to measure the AC output with the volume all the way up.
You should always start with the preamp set to a single ended input. Your DAC or CD players usually outputs about 2V into the preamp.
Now with signal into the RCA input you measure the RCA single ended output. Step 1: Now swap tubes 1 and 2 with other tubes until you have equal output at the R&L RCA outputs.
Step 2: Continue with the signal into the RCA single ended inputs. Now with your AC voltmeter measure the balanced outputs. Pin two of the XLR is the positive phase and pin three is the negative phase. If these are not equal the potentiometer inside needs to be adjusted until the rear tube (neg phase) matches the front tube.
Tube 1 is the left positive phase tube, tube 4 is the negative phase.
tube 2 is the right positive phase, tube 5 is the negative phase.

The potentiometer inside, adjusts tube 4 and the other one adjusts tube 5 gain. The potentials are aligned with the channels.
Once you have all four phases equal on the XLR output jacks using the single input then you’re ready to move onto the balance input tubes.
*Do not touch these four tubes now. Move on to the balanced input tubes.

Now switch the selector switch to a balanced input and plug your DAC into that input. While playing a test on measure all four phases of the output at the XLR jacks. All four phases should be equal again, if not swap tubes 3 & 6 with each other or with other tubes until you match the channels.

Tubes 7 and 8 are just for the headphones. If you are not using headphones, you can leave these two tubes out. That will actually provide less current draw on the regulators for the filament supply.

For people who have an SLP-05 and are only using it single ended, you’re missing out and you are only using tubes 1 and 2. Nothing else. But you do have to have four tubes in the preamp, them being 1, 2, 4 & 5. Tubes 3 & 6 can be left out if you’re only using RCA single ended in and single ended out. if you leave tubes 4 and 5 out, you will not get any filament supply to tubes 1 and 2 as it uses a 12V supply with tubes in series.

Hope this helps! Let me know if you have any other questions.

Ahhhh yes.  The is a nub on the connector that fits into the notch on the chassis’s jacks.  Always have to make sure you get that in the groove. 
If it’s working you may have lucked out.  It was probably shorting the transformer and so got hot.  Lucky it didn’t fry it.  
About the only real failure I see on these is the bridge rectifier in the power supply.  Typically though the SLP-98’s just work for years.  

It’s OK billpete. It happens to the best of us. Even as a technician it happened a couple times to me. Among other things! I’m retired and a few years out from my tech days. First my back went, then my neck went-had a fusion, now my knee is gone. Sucks getting old! It’s a good thing your body doesn’t fall apart till your old, young people couldn’t handle this.

As a tech the worst thing that ever happened to me I think was being shocked by a CAD-805 amp that was wired wrong. An assembler wired the main B+ directly to a speaker post so that every time I grabbed hold of the chassis to turn it over and I grabbed the speaker post I got shocked by 1000 volts even with it off. It took a second time getting shocked, which turned my finger white and burnt a deep hole, to realize where I was being shocked from. Believe me 1000VDC will make your muscles spasm good. And that’s with the amp off and unplugged. Those caps hold a charge for a while.

I’ve been retired for 4 years now.  Worked there for almost 11 years.  I designed the ultimate upgrade for the 05 after tinkering at home for a year on my own.  Then for myself I hand wired a completely from scratch custom 2-chassis unit at home with a remote control resistor ladder volume control and input selector with display.  All teflon sockets, pure silver Kimber for hard wiring and silver solder, Mundorf’s top of the line EVO SilverGoldOil coupling caps, a completely from scratch larger power supply with a slightly higher voltage transformer, 10Hy choke, a few large electrolytic can caps and over 500uf’s of Polypropylene caps in the PS.  I installed a PS isolation circuit between the balanced input circuit PS and the other tubes that is a capacitance multiplier circuit.  
Did the last part this last year and was a wonderful improvement to the imaging. I have a hard time leaving well enough alone even now days.  
Then there’s my custom amps.  But that another story. 

A little history on the “F1” version of the SLP-98. That was originally asked for by Upscale Audio. They wanted an upgraded version of the SLP-98 with the HexFred diodes, the grayhill switch and I believe Jensen oil caps. The last few years I was there, we started doing extra things like added 10uf polypropylene capacitor’s across all the electrolytic power supply caps, higher quality coupling caps, etc. in addition to the F1. It really is a very nice preamp!

As I remember “SLP” stood for Sweet Little Preamp and “98” was the year it came out. 

You are right on both questions.
Yes, when the power switch is ON on the PS it is only applying power to the relay board inside the PS. This powers a small 12V transformer that powers a relay. The relay is then activated by the switch on the main Preamp when it is set to ON.
The switch on the preamp connects a ground wire back to the relay in the PS which turning on the power to the main transformer. That transformer provides the high voltage and the filament supply.
The reason for all this is to keep all AC voltages out of the actual Preamp chassis. The Preamp switch triggers the relay by connecting it to ground.
Am I explaining that well?

P.S.  The meters are connected to the DC voltage coming from the rectifier tube like you say.  They are not perfectly accurate but are a great reference point to keep an eye on.  Once warmed up they should always read pretty much the same.  Baring some AC voltage variations from the house wiring.  Sometimes people freak because the current is reading a little low.  You may also notice  then that the voltage is a little higher that day. 
Where the meters are useful is if you suddenly notice a higher current draw.  This would mean there is a short somewhere.  Start by changing the rectifier tube.  If a new tube doesn’t correct the high current draw then you either have a 6SN7 tube shorting or a bad PS capacitor.  If you pull out all the 6SN7 tubes and turn it on and it still drawing high current with a good rectifier tube then you know you have a shorted capacitor somewhere.  

Greetings immatthewj.  If you’re only seeing one diode rectifier, it’s probably the negative bias supply rectifier.  I do put HexFreds in the bias supplies of my personal amplifiers.  Had a hard time telling if it really helped there though, but figured it couldn’t hurt.  Because like the old saying goes, everything matters.  I do know adding more filtering to the bias supply definitely makes a cleaner sound quality.

The V12 amps should have four strings of three diodes totaling 12 diodes for the high voltage supply.  There is such a high current draw (large caps) that there are three in series for what should be one diode in order to keep them from blowing out from the turn on surge.  I remember doing a few amplifiers (not just V12’s) with HexFred’s, but that gets very expensive as you’re installing 12 so most people didn’t want to do it. In fact, I think on a pair of CAD-805 amps we installed 16, four sets of 4 just to make sure the amp never came back with an issue.  Now days, there are higher current HexFred’s available that can be used but you still want a couple in series.  I’ve had to buy some of these because I use so many large value capacitors in my mono-block PS’s.  My amps have two amp chassis’s and two power supply chassis’s. 
It’s not that you’re drawing a lot of current tube wise. It’s the turn-on surge to charge the capacitors.  The more capacitors, the bigger, the surge, and the more the rectifiers see a short initially at turn on.  

There may have been some V12’s with only four diodes.  Two strings of two in series but mostly it should be the later that I explained.  
My dual-mono amps I built have essentially the V12 input circuit but using a 6SN7 instead of the 6922 and are fully balanced.  The output is essentially the SLAM-100 but with everything taken to the maximum.  I have large cap banks and 10Hy chokes on the B+ and running 550VDC to four KT-150 tubes.  Then another  20Hy choke before the regulator circuit for the input tubes. Lots of Polypropylene al through the PS. I get a very nice 124 watts per channel output.  These drive my Eminent Technology LFT-8b speakers that I’ve completely rebuilt the crossovers.  I use a couple Rythmik F12 subs for the low end driven by the preamp with balanced cabling.  Going fully balanced made a big improvement in imaging for me.