Seduction caps and tube life

[Paul Joppa]

This arose in the general forum but these comments are specific to the Seduction so I started a new thread. (original thread is now in the Seduction  forum as well http://www.bottlehead.com/smf/index.php/topic,2529.0.html- Doc B.)

The capacitors in the RIAA network (0.033uF, 0.01uF, and 0.0012uF) are 5% and should only be replaced with equal or better precision. The interstage and output caps (0.1uF and 0.47uF) do not need to be precise and can be replaced with 10% precision. The output cap (0.47uF) can be replaced with a larger value such as 1.0uF (used in Eros); above 1.0uF you will see diminishing returns.

The tube life *should* be quite long. Of course, not all tubes are created equal, YMMV, and all that. Aside from manufactured quality, the biggest known determinants of tube life are plate and heater voltages. In Seduction, the plate voltage is low enough to be no problem. You can check the heater voltage, which is designed to be 6.0 +/- 0.3v. This is tighter and more precise than the specified 6.3 +/- 0.6v for the tube. I chose this target to help keep the tube quiet, and to extend its life. Below 5.7v, the space charge is small and there is more possibility of cathode poisoning; above 6.3v the heater wears out more quickly and there is a greater possibility of grid emission.

If the voltage is outside the target, there are two ways to adjust it, use a different tube or change the 1.2 ohm resistor in the power supply. Here is a table of resistor values - you may need to experiment but these should be good starting points:

	Tube type	6922	6H23Pi	7308	6DJ8
	Current, mA	300	310	335	365
Power Line Voltage
110		0.68 ohms	0.68	0.47	0.47
115		0.91	0.91	0.91	0.68
120		1.20	1.20	1.20	0.91
125		1.80	1.50	1.50	1.20
130		2.20	1.80	1.80	1.50

[Grainger49]

Paul,

Excellent information!  I'm probably part of the reason you posted.

Quite possibly, and very likely, I have replaced my tubes more often than necessary.  I'm always looking to roll tubes. 

[Pfenning]

Thanks to both of you guys. This is exactly the help I needed.

Pfenning

[Dr. Toobz]

Is there any benefit to going up to 1uF for the output coupling caps, other than being able to drive a little bit more cable? I can't imagine that the difference in frequency response/bass rolloff would be that significant. I ask because I have some 1uF film caps of various makes laying around, and I've wondered if there is any benefit to swapping these in for the stock "orange drops" since things already sound fine as-is.

[Paul Joppa]

If you are driving a preamp with a low input impedance - say, less than 20K - then the larger cap can make an audible difference. I specify 50K minimum load for full performance, but we have tried it at 15K with quite acceptable performance. I would definitely not go below 10K though.

In certain circumstances, it is possible that it will produce slightly less hum. But note that, in other circumstances and with some caps, it could go the other way.

[Laudanum]

I asked a similiar question before my build.  I got the green light from Paul so I used 1.0uF in my Seduction build (Obbligato golds).  It made a bit of difference (bass) when I built my Cornet 10 years ago but I cant say it made a difference with Seduction as I never tried the .47 first.   They are in different systems anyway.  Cornet feeds a hybrid integrated of which Im not sure of the imput impedance.  Seduction feeds a non extended version FPIII.