modding a S.E.X.

[punkaroo]

Hello all

I recently purchased a beautiful and highly upgraded S.E.X. amp. It was built by the Doc himself and upgraded later.

It has a C4S board, stepped attenuator, MUSIC caps, and everything's been coated in a layer of Anti-Vibration Magic fluid.

Plus 5 sets of tubes, it's way over $1000 in upgrades. But I'm a super finicky listener, and I adore my Grado HF-2s. I feel like the 6DN7 tubes could use a touch more gain. If I increase the volume too much, then the mids come off as simply being too loud. If I put it down a touch, then it's just a tad too quiet. I seem to listen at around 11 or 12 o'clock.

I'm not technical in the least, but I've found a modding place in Toronto that I could possibly bring my S.E.X. amp to. Could I mod the S.E.X. to take different tubes? Would this be difficult? Would it be worth it? I love tube rolling and would love your suggestions!

Thanks everyone

 

[JC]

Do you know if it has the stock output transformers/chokes, or have they been upgraded to the MQ iron specifically designed for the task? 

The modification to switch to the 6EA7 will get you some more power, and it is a simple mod, but my understanding from reading here is that the MQ iron is really necessary to put the extra Watts to good use.  I don't know how these mods might effect Voltage gain, if at all, but I am reasonably sure that they would have some effect on the tonal balance.

These sound like some very cool amps; please post some pics in the gallery, if you can.

 

[punkaroo]

I'm not sure if the output transformers/chokes are stock. How can I check? Should I take pics of the internals? Everything's covered in AVM fluid, so everything's blue! I'll post pics later.

Thanks!

 

[JC]

Well, that might be a little tricky, if everything is coated, but the stock outputs were Speco 70.7 Volt line matching transformers, so they have lots of extra leads that will have either been clipped off short, or probably taped off and bundled up with wire ties if the instructions were followed to the letter on that point.   So, if you see extra leads coming out of the output transformers that don't go anywhere, you probably have the stock iron.

Looking forward to the pics, when you can!

 

[Doc B]

I don't want to burst bubbles here, but I bought a bottle of liquid latex used for making molds of antique radio knobs several years ago that looked EXACTLY like the AVM stuff, even the same blue color. It was about $13 a quart, not $95 for a little bottle. I'm not saying the stuff doesn't work, but people can save a lot of money by doing some homework. It's also great for making painted on bikinis.

 

[punkaroo]

Hi Doc!

I'm glad *I* didn't spent the $95 on the AVM stuff! Well, I guess I did in buying the amp.

Here's the original pic from the ad:

Does that help any?

Thanks for your patience, everyone!

 

[JC]

LOL!  I'll have to take your word for that last part, I reckon!

Bruce Rozenblit wrote one time of going down to the hardware store/home center and picking up a bunch of household items to try out for "tweaks".  His rationale was that for ~$50, he had materials enough for a bunch of experiments, and if he found something he liked he  usually gained enough knowledge to hunt for the "audiophile-approved" equivalent later, IIRC.

This got me started with using slices of 3/4" automotive heater hose as "tube dampers" for microphonic miniature 9-pins, and I never looked back!

 

[Grainger49]

I bet a picture of the top of the amp will help decide if you have the MQ upgrade iron or the stock iron.  As a second thought, you can look at the SEX page to see if the transformers on top look like yours:

SEX Amp Product Page

 

[Paul Joppa]

You have the upgrade iron. The transformers do not have visible solder tabs, and the lamination stack is larger than the stock unit.

 

[punkaroo]

So...in plain English...what does this mean?

Here's the top:

 

[Grainger49]

Those are better, beefier than stock transformers.  The stock transformers pictured at the top of this page are smaller and have Yellow insulation on the outside:

SEX Amp Product Page


So, you have an upgrade.  I can't verify that they are the MQ iron but they are not stock.  I think since PJ says, "the upgrade iron," it is the very nice MQ iron.  

BTW, I like the knob, very retro.

 

[JC]

Can't say 100% from the pictures, but it sure seems that you have the upgraded MQ transformers and chokes.  Since the transformers (top side) haven't been coated, you will probably find some nomenclature on them that will say for sure.  One thing is certain from the picture: the output transformers are not the stock ones.

So, to get back to your original question, you may indeed want to modify your amp to use the 6EA7.  IIRC, this will require replacing one cap and one resistor on each channel in order to change the bias to an appropriate level for the different tube.  So, not a terribly elaborate modification; but, you will need the construction manual.

The manual is in the form of an Adobe file on a CD-ROM.  If you didn't get it when you bought the amp, perhaps the seller still has it.  If not, you will want to contact Bottlehead for the info.

If you decide you like the different tube, then, of course, you are done.  If you want to change back to the original 6DN7, you will have to change the resistors back to the original value to return the bias to the appropriate level; the caps won't need to be changed back.

Nice looking amp!

 

[Doc B]

Yeow! Suffice it to say that if anyone ever sends us an amp for service that has been completely coated like that underneath, it will be going right back. That would be a nightmare to work on. It is a shame that folks don't take some time to figure out more or less how damping works and use a little logic in the choice of materials and points of application. Slathering the stuff wholesale over solder terminals and identifying numbers on parts is going to create a lot of extra work when it's time to fix or upgrade anything. That is indeed the MQ upgrade iron.

 

[punkaroo]

Hi Doc,

I asked the seller about the AVM fluid posing a problem when modding. He told me it'll scrap off, as it's just paint.

I certainly wouldn't expect you or anyone at Bottlehead to repair it if it ever needed it - I can accept that. I'll have to search around for outside help. I must admit - this was an impulse buy! I'd been reading a lot about this amp, and when I saw an upgraded one for sale, I jumped on it! I guess I didn't put much thought into the fact that the AVM might hinder any upgrading I want to do with it :-/

I apologize for being so ignorant on the topic. One day I'd like to be a little more knowledgeable on the technical side of this hobby - for now, I'll just enjoy it!

 

[Doc B]

As long as it doesn't affect function there's nothing wrong with the stuff being on there - assuming it doesn't get stinky or burnt on the hot parts. If it is slathered on things like transistor heat sinks or resistors that get hot I would scrape it off. It's going to make dissipating heat more difficult. Mostly I just meant I wouldn't want to have to do all that scraping to identify parts or make any changes. If it's a DIY mod one can just take the time necessary. But if someone else was working on the amp on an hourly basis it would really run the time up on the job.

 

[punkaroo]

Thanks Doc. I wonder if I should just hold off on tube rolling until I can either:

A) Feel comfortable enough to do it myself, or
B) Find a local DIY-er to do it for me

I wonder if rubbing alcohol would take the AVM stuff off? And since I'm an idiot, where are the transistor heat sinks that I might try to scrape this off myself?

Thanks again!

(I think I'm going to have to have a look at that CD-ROM I got with it. I'm stuck at home, post operation...so I have a lot of time to kill for the next few weeks!)

 

[Doc B]

If the AVM stuff is latex as I suspect, it should just peel off.

 

[punkaroo]

If the AVM stuff is latex as I suspect, it should just peel off.

LOL! That's awesome.

I wish I'd known a bit more about this stuff before jumping on the amp. Seems like a pain.

Live and learn I guess!

 

[2wo]

No wonder you hurt you leg ...John

 

[jrebman]

Doc,

The AVM appears to be something more than latex, though I suspect the latex is the matrix in which the other stuff is mixed.  I have some AVM and it's somewhat gritty/chalky (depending on thickness of coat) to the touch, and tends to chip or flake off, not peel off.  The texture reminds me very much of Silent Running 1000 -- the vibration damping compound used in engine rooms on power boats.  Anybody have access to a GC or spectrograph?

I can say that this stuff does work as advertised on signal tubes, but coating the whole underside of an amp seems a bit extreme to me.

-- Jim