ETI 477 resurrected but with problems.

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They are upgrades in almost any area. VAS in eti477 takes enough current to make transistors without correcting diode/resistor very hot, with diode they are just warm. Nevertheless Sanyos are just better.

cheers,


Thanks Janusz,
So just to be clear.
I can just directly substitute the original BF's with your quoted 2sa1209 and 2sc2911 in the VAS ( Q5, Q6, Q7, Q8 ??) with no other mods ?


You also mentioned an "anti clipping" diode ?
Is this the 18v zener that has been discussed, fitted between the collectors of Q7(BF470) and Q5 (BF469) ?
 
Thanks Janusz,
So just to be clear.
I can just directly substitute the original BF's with your quoted 2sa1209 and 2sc2911 in the VAS ( Q5, Q6, Q7, Q8 ??) with no other mods ?


You also mentioned an "anti clipping" diode ?
Is this the 18v zener that has been discussed, fitted between the collectors of Q7(BF470) and Q5 (BF469) ?

Yes, you just drop 2sa1209/sc2911 instead of BFs. No need to change anything.

No, the 18V zener diode balances currents and reduces VAS operational temperatures.

Anti-clipping diode is D16 BAS21 in the asc file I have attached. Original eti477 did not have it and it is not essential. I tried it only in simulation.

Simulation also shows improved distortion figures if q5-q6 are ksa992 while q3-q4 and q7-q8 are ksa1142/ksc2682 but simulation results may be misleading as results depend on transistor model parameters and algorithms. Reality is usually at least somewhat different.

Simulation with ksa/ksc transistors gives extremely good results so I do not believe reality would be equally good. At 2kHz and with 8 ohm load and anti-clipping bas21 in thd is 0.000029%. Seems too good to be true.

cheers,
PS put VAS transistors on bigger heatsinks, the original ones are too lousy. Especially if you did not use the 18V zener.
 
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Yes, that's how the extra greencaps supplied in kits were noted, back in the days of the ETI 5000 (the stereo amplifier that featured the ETI 477 boards). As suggested by others, the sound quality as originally presented, wasn't as good as David Tilbrook's later versions like AEM 6500 which in more recent times, suzyj nicely recreated in SMT for us at diyAudio to marvel at.
 
Some of these old amps can be on the edge of oscillation.
A couple of years back I bought in a Maplin 1980's 225WRMS power amp with some blown and missing parts. I replaced the parts with brand new parts and the amp started oscillating badly. I had to increase the VAS capacitor to stop it oscillating.
I can only guess the new transistors had wider bandwidth or more gain or both hence the problem.
In regards to your amp I would start by replacing all the electrolytics.
I would then set DC offset and bias current.
Connect a load and look for oscillation.
If it oscillates then you will need to change VAS capacitor to a slightly larger value and try again.
 
Yes, you just drop 2sa1209/sc2911 instead of BFs. No need to change anything.


The 2sa1209/2sc2911 seem to quite difficult to get.
None of the big electronics distributors ( RS, mouser, digi key, farnells ) even list them.


fea-bay has some stupidly expensive ones in the UK, and the china ones are of unknown quality, could even be fakes.



Jaycar still have the BF's listed, so might just need to stick with these?
 
The lag comp cap is always straddling base and collector on the VAS - voltage amplifier - which drives the bias generator of the output stage.

Usually it's a fast resistor, often npn, and chosen to have lowish collector current (up to 200mA rated no higher) and low input capacitance, less than 5pF. This is often a video transistor, getting hard to come by now, originally used in TV sweep circuits that drive the screen.

HD
 
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ta 477 kit

see attached
 

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On Semi's KSC3503/KSA1381 have been the go-to CRT driver transistors here for many years. If SMD is OK, BF720-722 are Nexperia and Infineon's SOT223 versions of the original Philips and Siemens BF469/470. Their wide collector tabs provide for some essential cooling but perhaps not enough for ETI477, where the VAS normally runs very hot.
 
On Semi's KSC3503/KSA1381 have been the go-to CRT driver transistors here for many years. If SMD is OK, BF720-722 are Nexperia and Infineon's SOT223 versions of the original Philips and Siemens BF469/470. Their wide collector tabs provide for some essential cooling but perhaps not enough for ETI477, where the VAS normally runs very hot.


Thanks for the info Ian.
Would these be direct "drop in" replacements for the BF's ??


Also, I notice that the 2SC3505 and the KSC3505 are noted on the data sheets as the same.
Is this correct or is there actually a difference ??
 
see attached


Thanks Arthur, I can see a few component changes in your schematic.


A couple of quick questions please.


The position of R16 and C5 are reversed and the voltage of C5 reduced from 25v to 16v in your schematic. Is there a reason for this ?


C6 (10pF) has been increased to 12pF in your circuit. (NFB)



Cap C14 on the base/collector of Q6 is increased from 220pF to 470pF in your circuit. Is this to stop oscillations ?


Gate drive resistors increased to 470 ohm
Gate clamp zeners increased from 12v to 13v .
Diodes changed to 4148's

C3 on the input leg has been changed from 330pF to 470pF.

C10 and C11 increased from 47nF to 68nF.
Values at output zobel(?) changed



I would be very appreciative if you could take the time to explain to me the reason why these values have been changed.
 
I built six of the Tilbrook amps in 1982 for a 3 way active unit. The design was published in Germany in elrad magazine in 81. It took me quite a while to cure a wild oscillation of each and every amp - R29 in the kits didn´t look like but was wire wound. Replacement solved the issue once and for all.

Regarding replacement of the BF´s, there obviously is NOS of a GDR/CSSR-equivalent for the BF 470 (Tesla KF 470) available here for peanuts:

https://www.pollin.de/p/pnp-transistor-tesla-kf470-131455
 
I also built the Elrad PowerMosfet from 1981 and solved the oscillating , with 5 x 1 Ohm resistors in parallel, later when dig out again with 0.22 Ohm Metal band resistors (MPC-71) for the source resistors of each MOSFET. It's still running and remaind me at the former HITACHI HMA-7500.
 
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