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Old 21st February 2010, 09:19 PM   #1
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Join Date: Feb 2010
Default mps6566 transistor substitution

HI all,
I'm attempting to rebuild a pair of the 80 watt tiger power amps (SWTPC) from 38 years ago. The amp uses a pair of MPS6566 transistors to form a differential configuration in the input stages. Would anyone if there's a better choice for the MPS6566 transistor? There is an NTE123AP listed but I couldn't find any mention of any noise values.
Is noise in this sort of application important? Perhaps someone would know of a replacement. The amp's split power supply voltage is +/- 40 VDC and the 2N5210 looked OK but the VCBO is 50 VDC. Is that rating high enough?
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Old 21st February 2010, 10:01 PM   #2
jaycee is online now jaycee  United Kingdom
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Join Date: Apr 2004
Location: Norwich, UK
To be honest any modern device will be far better than what was originally specified! MPSA06/56 should work well.
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Old 22nd February 2010, 12:59 AM   #3
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thanks for the tip, it looks like a good sub.
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Old 22nd February 2010, 03:58 AM   #4
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Location: Canandaigua, NY USA
No idea why Dan Meyer chose the MPS6566, as I'm pretty sure the rest of the common MPS series was in existence at the time. I still run a scratch built Tiger amp. Usually the last thing to go is the 6566, as failures tend to take out the outputs and the weird drivers with the heatsinks attached. Those are near impossible to get now. The original outputs were MJ802 and MJ4502 (I think) and there was a suitable sub made by TI as well. I found to get the best distortion performance, matched output pairs were desirable, though nearly impossible to find. The N and P channel devices rarely seemed to match. I think I have quite a few of the bias diodes if anybody needs 'em. The amp is construction sensitive and will oscillate if the output network isn't right. The choice of ground points for HF signals is fussy. I seem to remember that running this amp at full power and 20kHz guarantees destruction. If I were building a Tiger today, I'd keep the output configuration, but incorporate a better front end using the methods of Doug Self in his amplifier book.

I may be barking up the wrong tree, but at least I'm barking!
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Old 22nd February 2010, 05:13 AM   #5
jaycee is online now jaycee  United Kingdom
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Join Date: Apr 2004
Location: Norwich, UK
Tiger amps were way before my time (and not in the UK anyway) but from all I have read about them, a quote from William Blake (or Tangerine Dream!) springs to mind....

"Tiger, tiger, burning bright..."
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Old 23rd February 2010, 06:15 PM   #6
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Default tiger 80 watt amps

Hi guys,
thanks for replying re the MPS6566 substitutions. They are for a Tiger power amp. I built 4 - 80 watt and 6-60 watt tiger amps in '72 to 75 and they all worked great. At the time they provided real cheap clean reliable power. My friends wanted them too so I built another 4 for them, never had a problem with any of them.
I blew the very first one after I connected it to a new speaker system. Here's how; I turned it on and couldn't hear anything, no hum or hiss, and decided to introduce some feedback into it by placing a finger on an output collector and the another finger on the input terminal. A tremendous screech and then silence, taking out the woofer and both outputs transistors. Got new output transistors and woofer and everything worked great, no feedback this time though. Never had a problem with matched output pairs and never heard anything about matching them either at the time; 35+ years ago. Just took the parts the supplier offered, installed them and carried on.
I wanted to know why the amp blew, I'd done the same thing with 6L6's tube jobs all the time. Here's what I found: somewhere above 30 khz the base storage times on the output transistors prevented them from turning off quickly enough resulting in producing a rail to rail short.
I still use the 60 watt tigers daily. Five years ago, one of them blew a PS cap so I replaced them both. I read a post somewhere about subbing the trim pots with 10 turn pots so I did that at the same time. With the bias set as specified the sound was a lot cleaner. That was the only time I opened them up, they're still working flawlessly.
Heat sinks always ran cool to the touch, you could leave a finger on them all the time. The temp feedback diodes from the heat sink may have been improved regarding the mechanical joint. The diode was in a clip which was bolted to the heat sink - too many thermal joints and time lag methinks
On sunday night I read the posts about other users experience with the tigers. I decided to rebuild mine after some unnamed bozo got ahold of the original 80 watt amps and destroyed them no ventilation, boosting cars, blacksmith anvil, if there was a path to destruction he'd naturally find it. There was a hole burnt through one of the boards so big you could put your finger through it - right under the 1/2 watt R15 R16 resistors.

Here' s what I found to make improvements, would anyone care to add comments:

-replace the power supply filter caps with 8000 to 10000 uf units, install an integrated bridge rectifier at the same time, might need a varistor in the line circuit because I remember the lights used to blink when I turned the amps on, get a varistor and the bridge out of an old PC power supply

-place 22 uf power supply bypass caps on the PC boards to filter each ps rail to ground

-use a quality 100 ohm pot in parallel with a 100 ohm resistors for R21 to set the bias, this resistor is to provide continuity in case the pot goes open circuit

-upgrade R12, 14 15 and 16 to 2 watts instead of 1/2 watt resistors

- sub the MPS6566 transistors with MPSA06 transisors

-there is a thermal snap disc stat on the heat sinks too, supposed to break the line side circuit. It would really be good if that was replaced by a manual reset type, but I don't know if that's available, on bigger sizes it is.

Grounding is another story. As I recall, the shell on the RCA input connector had to be really tight on the chassis in order for there to be zero DC offset at the speaker terminals. I guess the bias depended on the bond connection. The original design has a ground lug between the chassis and the underside of the connector shell nut - had to be really tight to the aluminum chassis. The nut has a fine thread, it's really thin, and located in a poor access, so it's dubious if it got tightened properly. There's got to be a better way to ground and bond than that sort of joint.

Would anyone care to offer opinions and advice about keeping the operating frequency below 30 khz to protect the output transistors? If it's a worthwhile consideration now is a good time to work it in. I'm thinking about trying a double sided board in Eagle pcb editor, would anyone care to comment on the board design? I ain't an engine ear and I do not bite.

If anyone is interested I've got all the old notes and construction instructions check lists and all. I don't know whether it's OK to display them, perhaps someone could advise about uploading them, like basically is it OK to upload them or not. I got all sorts of stuff from SWTPC, a dozen tiger amps, function generator, dual power supply, and fet preamp which worked but extremely noisy and distorted, and everything worked.

And in reply to the'Tiger Tiger burning bright' set, all I can say is hardy har har. Remember, this is the year of the tiger and these tigers still ROAR
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