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Old 28th November 2011, 01:04 AM   #211
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JMF, it has already been mentioned that dropping the rail voltage helps the amp to survive... you did it because you did not have the higher voltage transistor complementary devices.

The higher power drivers transistors also help since it is unclear which fails first, the outputs or the drivers - it has been said it is the drivers, thus the baker clamp idea...

I have tried the miller comp cap, it failed to make the higher voltage stock amp "stable".

Next time I pull them out I am going to do the Baker clamp idea, and see if that is sufficient to make it not let out the magic smoke or not. Then the next step would be to see if it has any parasitics, deal with that and finding an appropriate higher SOA driver devices, along with the other passive parts that need a corrected implementation.

Maybe someone else will have the time and get to this before I can.

_-_-bear

PS. Conrad, the lead length thing implies a very high freq parasitic...
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Old 28th November 2011, 04:15 AM   #212
JMFahey is offline JMFahey  Argentina
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True.
I also think the TIPs were somewhat slower, either because of lower Ft or higher capacitance, loading "somebody else". Dunno.
As I said, I was beginning, not that much theory under my belt, happily the combination worked, and was quite robust.
Guitar amps live in a dangerous world.
As a side note, Dan Meyer was one of my personal idols (as was John Simonton, the PAIA guy and a few others), I built most of what they published, generally substituting parts with "close enough" ones (which sometimes were more robust than originals).
Oh well, old times.
Although, 40 years later, what Im doing is conceptually not that far away, my bread and butter earner is still a Lin type power amp with TIP142/147 outputs and +/-40V rails.
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Old 28th November 2011, 04:29 AM   #213
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OK, here's my output inductor. I machined the bobbin out of Delrin. It has a 1" diameter by 0.375" wide winding window. I can't see exactly, but it looks like ten turns on the thing. The wire is AWG 16 solid magnet wire with single build insulation. You can't see it, but the other 10 ohm resistor is soldered directly across the back side of the inductor. For whatever reason I never had good luck with inductors wound on resistor bodies, regardless of resistor type. I think the form factor was just ineffective. This inductor measures 3.25 microhenries with a Q of 1.6 (@ 1kHz) and the DCR is about 13 milliohms, though I could be slightly off on that. (measured without the resistor on the back side) No doubt my construction is a bit wide open, but this amp was a "mule" that was taken apart and reassembled many times. I built it for development, not for compactness. Oddly, I built my first Tiger for compactness. It was impossible to service and had all sorts of oscillation problems. Had I known what I know now, I probably could have fixed it by changing the output inductor and a few other tweaks. That was many years ago and I ended up trashing the entire chassis and starting over. Too soon old, too late smart. Or at least not quite so dumb.
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Last edited by Conrad Hoffman; 28th November 2011 at 04:34 AM.
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Old 28th November 2011, 05:05 PM   #214
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Conrad,

Are you saying that moving the output inductor OFF the board causes the amp to become stable??

The improvement in the inductor itself noted, but is that a requirement for stability?

Is it one or both of the above - AND - if it is the second then what exactly is causing the problem using the stock inductor??

_-_-bear
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Old 28th November 2011, 05:18 PM   #215
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Hi Bear,

The original Tiger inductor was never on the board, but was wired from some junction to the output connector or something- you'd have to look at the links way back at the beginning to see exactly. IMO, the improvement is the high-than-normal inductor value more than anything. I seem to remember that when Tigers have problems it occurs at something like 6 MHz. The inductor shown should maintain good behavior way beyond that. Also, FWIW, I did some very fussy amps some time back and the output inductor construction is more important than sometimes thought. Any core at all, even resistor wires, increases the THD over what you get with an air core inductor on an inert form. I don't know how important damping really is, considering wire resistance, but much amplifier damping is lost to resistance in the output wiring, including the inductor. The inductor shown gives good measurements at the amp terminals, if nowhere else!
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Old 2nd December 2011, 03:40 AM   #216
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OK, I must eat my words on Tiger reliability! They do fail. The last couple times I powered the thing up, it was running hot. Measuring the bias on one channel showed several hundred mA, and the bias pot was only allowing it to be set high or near zero. After 31 years the Clarostat wire wound pot I had used for the bias adjustment failed. Actually the lubricant just hardened up and prevented contact. Photo of the guts below. I could have cleaned it, but decided to just replace the pair with new pots.

Now, you have to get up pretty early in the morning to blow up my stuff- I always install a safety resistor across my bias pots, so if they go open the amp won't be damaged by excess current. I had done so in this case and it paid off. Admittedly 31 years later, but it paid off.

BTW, I also looked at some notes from the last time I fooled with it, and I had reduced the feedback cap to 50 pF and increased the LF feedback cap to 1000 uF.
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Old 16th September 2012, 04:27 PM   #217
redjr is offline redjr  United States
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New to this thread...

I know this is a rather old thread, but thought there was enough recent activity to warrant a post. Wondering if anyone has a working Tiger (of any variety) that they're not using anymore and it's been sitting in the basement or closet for years, if not decades, that they would be willing to sell (or donate, )?

In the mid-70's I attempted to build one of the first Tigers to appear in either Popular Electronics, or Radio-Electronics. Can't remember the exact model. I tried doing it from scratch, but using the SWTP design. However it died a quick death upon power up. I was a complete novice at the time (still am ) without the skill to troubleshoot and fix. Being a newly wed and in graduate school I was also on very limited funds.

Fast forward 35 yrs and I now have plenty of time to try and relive some of those early days of kit building that I simply could not afford at the time. Which brought me to.... 'I wondered whatever happened to SWTP' and their audio kit line. It's been refreshing to find the few threads here at DIYAudio, and some of you guys who have followed more closely the issues that plagued the early Tigers.

I don't really want to restore, or rebuild a non-functioning Tiger, but would like to find one to add to my ever growing vintage collection. If it happens to be in good shape and operational that would be a bonus.

If you have one you don't letting go of for cheap please leave me a PM.

Rick
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Old 16th September 2012, 08:42 PM   #218
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I'd gladly give you mine for the cost of shipping BUT I have the overwhelming need to find the time to repair one and put a baker clamp in to see if that prevents it from self immolation or not!

_-_-bear

PS. it's on my bucket list...
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Old 16th September 2012, 08:51 PM   #219
redjr is offline redjr  United States
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Quote:
Originally Posted by bear View Post
I'd gladly give you mine for the cost of shipping BUT I have the overwhelming need to find the time to repair one and put a baker clamp in to see if that prevents it from self immolation or not!

_-_-bear

PS. it's on my bucket list...
Hey bear - If you change your mind drop me a PM. I'd gladly pay shipping. In the mean time I'll keep looking, and hopefully with a few that read this thread something may shake loose.

I've noticed there's - what appears to be a very clean looking - set of Tiger .01 on ebay, but for a price. Almost brand new looking! I don't want one that bad yet.

Rick
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Old 14th October 2012, 03:32 AM   #220
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Default SWTP Universal MKII Selling. . . . maybe

Hello, I registered and made a post or two awhile ago and happened upon this thread. I was an avid SWTP fan back in the early 70s when I was in an Air Force tech school in Texas in 1970 and built everything from a function/signal generator (still have it and use it!), some other audio kits like a couple of the "color organ" light boxes that flashed lights in tempo and frequency to music, and in 1972, two Universal Tiger MKII amps. I still have most, if not all, of the original documentation, a couple of the little mailing catalogs that they sent to me almost every month, and the UT MKII's along with some original spare parts.

I have been an electronics tech since then and worked in several field service and in house repair jobs over the years. I last fired up the UT MKIIs a few years ago and they worked and measured fine. They have been in storage mostly since 1975 when I got my Phase Linear equipment, but I used them for several years even then in another room occasionally. I'm sure they could use some check up and such to verify filter caps are ok, crossover pots set right, etc. I no longer have a working scope and can't easily check or adjust crossover on it and have so much other audio equipment I am thinking of selling them on Ebay or if someone may want them here.

They are installed in a custom made solid walnut cabinet with plenty of venting and even an additional "muffin" fan that I really never needed to use. The cabinet is complete with an aluminum face plate and two large illuminated VU meters I purchased from Lafayette Electronics back in the day. I used to drive the UT MKII pair with a Dynaco PAT-4 and FM-5 Tuner which I also still have in matching solid walnut custom cabinets that looked great with the UT MKII. When I get a chance between projects I will post some pictures after I locate all the documentation I have in one of several file cabinets that are stuffed with years of electronics owners manuals.

I'm just writing this to to give a heads up to anyone who might be interested in a purchase down the road. The units were never abused and I even have a pair of test chart results from when McIntosh Clinics were held in various HiFi shops around the country. As I remember, the UT MKIIs held there own and surprised the Mac technicians! If I sell these units I expect I would get a good price for them since I also have one or two sets of MJ output transistors and other drive components I bought as "spares". . . . I was always a bit OCD about having spares!

I'll WILL post pictures when I have a bit of time to dig things out of storage and if I get any interest.
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