PW Texan Amp – Audio Nostalgia, Capacitors and Upgrades

[Boulevardier]

Originally Posted by MelJon66

The cost of repair jobs escalates out of control very quickly if you have to keep buying one piece of seldom used items that are not worth buying in bulk.

Thanks again.
Mel

That's one of the huge benefits of having this forum - members often have large supplies of infrequently used stuff, and can provide very small amounts to those who only need that at very little cost to either party. I sometimes buy more than I need for small jobs, both because I can't buy a smaller quantity and because I know that what I have left over might be of use to someone else.
Mike

[MelJon66]

Hi All,

I have done some work but I am held up waiting for parts. I am considering fitting emitter (output transistor collector) resistors to improve stability. It appears to be quite possible to do a neat job using compact 5 watt vertical ceramic resistors with a lead pitch of only 11mm. Track cutting will be minimal and virtually invisible. The question is what value to fit?

Paul Kemble's website shows that the later PE Rondo amplifier circuit is identical to the Texan and uses 0.5 ohm value, apparently for maximum stability. Other designs use values as low as 0.1 ohm for minimum distortion. Since stability is my goal is there any point in considering anything less that 0.5 ohms? Also, must these be non inductive types?

Any thoughts?

Cheers
Mel

[mhennessy]

0.5 ohms is fine. Almost certainly no point going much lower to reduce distortion there given the rest of the design.

[MelJon66]

Originally Posted by mhennessy

0.5 ohms is fine.

Thanks,

Is there any other straightforward modification I can make that will reduce the chances of the Texan self destructing?

Cheers
Mel

[MelJon66]

Hi All,

The work on the Texan is complete and remarkably I made no mistakes and it still works! The cracked film capacitors have been replaced by new polyester caps and the tantalum DC blocking caps (C13 / C113) have been replaced by small film types. There was JUST enough room to fit Wima MKS2 10uf 50V caps which are very compact. I have left the tantalums (C3 / C16) in position as there seems little risk of failure at these voltages. I have also done some track cutting and inserted 0.47 ohm emitter (collector) resistors after the power transistors and I made quite a neat job of it. Attenuator resistors (10k in series/1k to ground) are now fitted after the input sockets to bring down the sensitivity for CD input.

Overall the Texan is now more user friendly and hopefully a bit more trustworthy, although I will continue to use it only with an external speaker protection box just to be sure. At normal listening levels the volume pot is now in the halfway position rather than just above minimum. The sound is notably better with the pot in the new position.

While waiting for parts I did some reading on why the position of the bias pot on the BC side of the VBE multiplier seems to cause forum members concern. I now understand the potential risk but I am a bit reassured that this Powertran version has a resistor in parallel with the pot so in the event that the pot goes O/C the resistor sets a limit on how high the bias current will rise. I have left this as is for now but might revisit next time I have the amp apart. That might be quite soon as I have just discovered that the balance control operates with the channels reversed when headphones are in use.

I have also done some homework to understand why fitting better op amps might not be wise in an amp of this vintage designed around slow op amps. I now have at least some understanding of the situation and have decided to leave the 741/748’s in. There seems to be a trend these days for “op amp rolling” and they seem to do this without much consideration at all. I now realise that these people are invariably doing this with more modern circuits. As someone said in an earlier post it wouldn’t be a Texan if the op amps were changed anyway.

So, I am happy with the improvements made and hopefully the Texan will continue to give good service in my study where it won’t be put under too much pressure. I also listen (by rotation) to a Rotel RA-412, Hitachi HA-250 and Pioneer SX-434 all of similar vintage and it will be interesting to hear which, if any, has the edge now. The Texan was always held back by its user unfriendliness, but the worm has turned! The handful of 741/748 op amps in the Texan are supposedly noisy but I can’t hear any noise at all. When you are deaf to any frequency above about 9kHz it levels the playing field a lot and vintage gear sounds fine. But I still enjoy the music as much as ever and that’s all that counts.

Thanks to all for your thought provoking input.

Cheers
Mel

[stevehertz]

Well and very interesting thread.