SCA35 photob 850

Dynaco SCA-35

A New Look at an Old Friend

Think the popular mods to your Dynaco SCA-35/ST-35 achieve it’s best performance? Think again. Here’s a new mod that really does improve performance, and has the documentation to prove it. See the article here:

A New Look At An Old Friend thumb

Dynaco SCA-35/ST-35 mod article (1.4MB)

Table of SCA-35 performance, before and after the mod

Power increases about 30% at low frequencies where it’s needed. Distortion at 1kHz is reduced 5X !

EFB Modification for Dynaco SCA-35
Dynaco SCA-35 mod connections at the "can" capacitor

EFB™ circuit at the heart of the Dynaco SCA-35 mod.

Complete instructions for an easy mod!

 

Reader Comments


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!

Dave

Posted by Bill December 30, 2016 - 03:17 pm
I use a thermistor (CL-90) on the A/C primary before the fuse. It's housed in small box with a couple of switches to run the ST-35 with or without the thermistor. With the thermistor engaged, I observe the following:

1) A/C gets ramped up slowly to about 2-3 volts shy of normal wall voltage.

2) output bias gets ramped up very slowly to about 0.270VDC. I use the EFB.

3) Plate voltage on the output tube starts up at 450VDC and drops to 220VDC (normal) after about 5-10 seconds.

It seems that the thermistor is doing its job with the bias but not the High Voltage on the plates. When I bypass the thermistor with the other switch (MBB), the voltage at the plate remains the same, but the voltage at the bias suddenly spikes to 0.300VDC and drops to 0.270VDC after a second.

When I don't engage the thermistor:

1) Plate voltage starts out at 450VDC and drops to 220VDC - just like with thermistor engaged.

2) Bias ramps up quickly but does not exceed 0.270VDC.

Dave, any idea what's going on here?

Posted by Bill October 12, 2016 - 07:01 pm
Great advise Dave. Thanks.

Posted by Dave October 12, 2016 - 01:27 pm
Hi Bill -- Of the caps you are considering, the 5 uF @ 35 vdc is the most ideal -- but all of them should work fine.

The capacitor's function is to help maintain stable operation of the regulator -- just as virtually all 3 terminal regulators require in most applications. It needs to be big enough to accomplish that task, without being so big as to limit response time to changes in B+ voltage. It would be very hard to distinguish any performance difference between the specified value, and that of any of your caps.

One other point to consider however is the rated working voltage of the cap you use. The cap normally sees about 15 vdc across it, making a cap rated for 25 volts ideal, and one rated for 35 volts perfectly acceptable. But as you go to caps rated for increasingly higher operating voltages, the capacitor -- over time -- won't hold its form as well as one that is more appropriately rated for the voltage it actually sees in service. Granted, it can take years for issues with form to occur -- but they will occur. Just something to consider.

Dave


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