Comments for page: A New Look

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Posted by Bill November 03, 2017 - 08:25 am
Thatís good to know since I may be using a PT that is less ďrobustĒ than the usual 330-0-330. If I were to use a 300-0-300 PT, my B+ figures will be lower - all things being equal - and I guess I would have slightly less power. But itís reassuring to know that I can still dial in a 0.27VDC bias. I donít have anything sophisticated enough to measure distortion.

Posted by Dave November 02, 2017 - 08:23 pm
Hi Bill -- Assuming that you are maintaining the same output stage mode of operation (UL) with the same screen tap location (25%), the deviation could be relatively large, as large goes. For the discussion at hand, any deviation would almost certainly be in a downward movement, since the 380 volt B+ level is already at the upper limits of what the tubes can safely handle.

With that in mind, the EFB circuit already compensates for about a 35 volt drop in B+ voltage as both amplifiers are driven from quiescent to full power output simultaneously. Dropping it another 35 volts (bringing the static B+ to 310 vdc would not be hard for the EFB circuit to respond to at all, but it would certainly be best to confirm that the .27 vdc level specified in the Dynaco environment would still hold at a 310 volt level as the low distortion operating point. The point is that the EFB circuit will compensate very proportionately over a very wide range of B+ levels, but that doesn't mean that the tubes themselves do, as so a recheck would be recommended. Ultimately, as long as the new operating point remains on the linear portion of the operating curve for the output tubes, then the EFB circuit will compensate very well for any changes made in the B+ operating levels.


Posted by Bill November 02, 2017 - 01:23 pm
How far can one deviate from the original spec of 380VDC B+ before changes are needed to be made to the .27VDC bias current to retain optimum distortion measurements?

Posted by Bill December 31, 2016 - 04:57 pm
Thanks Dave. I meant the driver tube, not the output tube. My bad! 220VDC is a good reading for that location.

I guess I thought the thermistor would prevent the plate from an initial voltage surge, and gradually ramp up and suddenly stop at 220VDC. Asking too much from the poor fellow!

I think a lot of people think likewise as you quite often see people recommending a thermistor in the primary of the PT as a means of slowing the inrush. They probably haven't measured it.

Posted by Bill December 30, 2016 - 05:41 pm
I think the tubes need to be heated (and act as a load) for the thermistor to act on the B+. I assumed everything was working okay until I measured plate voltage on the input tube. No slow turn-on there at all! I wonder how many people are using thermistors and not realizing that they are not acting the way they thought they would. The bias was sloooowly ramping up and I thought that was good enough.

I think I will use separate filament transformers for my next build, or a timer of some sort.

Posted by Dave December 30, 2016 - 04:24 pm
Hi Bill -- The biggest concern I am seeing is that after the turn on surge of 450 vdc at the output tube plates, you indicate that it drops to 220vdc -- which is simply not possible with the output tubes otherwise passing normal current flow. The fuse would blow, or power supply resistors would start smoking! -- Neither of which you are reporting.

Typically, a 450 volt turn on spike is quite normal, where upon then it will drop to about 375-380 vdc under normal biasing conditions. I hope your 220 volt observation is either a misprint, or the product of to scrambled notes!

As for the CL-90 device, understand that it is not a timer, but merely a device with a negative resistive coefficient relative to temperature. Therefore, when the amplifier is first turned on, there is the surge created by the cold heaters which heats the thermistor. But the surge passes before the tubes are warm enough to begin conducting plate current. With the reduced current draw during this period but yet the thermistor warm, the plate voltage is still going to surge to much the same surge level whether the thermistor is in the circuit or not. It's greatest effect then will be on how fast the surge rises to the 450 vdc level, and what the actual operating plate voltage is due the the minimum drop across the device when fully warmed.

As for the surge in the bias voltage, that will be hard to assess since the thermistor alters the rate at which the heaters heat the cathodes of the output tubes. Therefore, the relationship between when the cathodes start conducting versus when the surge in plate voltage is produced is altered as well -- all of which creates the difference in the way you note the bias voltage reacting. Suffice to say however, that since the bias does reach the proper level quickly enough, I doubt that there is anything wrong -- other than what you are indicating the plate voltage drops to when the unit is fully warmed up.

I hope this helps!


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