I'm currently restoring my ST 70 and PAS 3. I've replaced the quad capacitors in both the amp and pre amp, and the resistors across them had drifted quite a bit. I'm gonna replace them with metal film resistors. I have some with the right value on hand that are spares from another project. So would these resistors be suitable replacements across the quad caps in my amp and pre amp?
For the ST 70:
-6,8k 2w rated for 500v
-22k 2w rated for 500v
For the PAS 3
-10k 1w rated for 350v
-47k 1w rated for 350v
This is the first tube gear I've worked with, only solid state before. I have as of now replaced the coupling capacitors on the driver board of the ST 70, I've replaced the selenium rectifiers with diodes, and replaced all the electrolytic capacitors in both. Also considering replacing the bias resistors in the ST 70, would 10k 1w rated for 350v also be sufficient there? Thanks for any input!
Cheers.
For the ST 70:
-6,8k 2w rated for 500v
-22k 2w rated for 500v
For the PAS 3
-10k 1w rated for 350v
-47k 1w rated for 350v
This is the first tube gear I've worked with, only solid state before. I have as of now replaced the coupling capacitors on the driver board of the ST 70, I've replaced the selenium rectifiers with diodes, and replaced all the electrolytic capacitors in both. Also considering replacing the bias resistors in the ST 70, would 10k 1w rated for 350v also be sufficient there? Thanks for any input!
Cheers.
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There is less than 3ma going through bias network. Any 10k resistor will work. Even 1/4 Watt ones. For me, I would just keep those old resistors in bias section.
Besides you have already mentioned, I suggest you put a NTC Thermistor in series with power cord. It limits inrush current during cold start. I use CL-90 in my st-70. CL-90 Amphenol Advanced Sensors | Mouser
I also replaced the 5AR4 with a solid-state drop-in replacement.
Besides you have already mentioned, I suggest you put a NTC Thermistor in series with power cord. It limits inrush current during cold start. I use CL-90 in my st-70. CL-90 Amphenol Advanced Sensors | Mouser
I also replaced the 5AR4 with a solid-state drop-in replacement.
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(carefully) measure the voltage across these resistors and work out the power and see (how much) it's within the power rating of the resistor. (V^2/R)
Those resistors are all fine.
If you've replaced the selenium rectifier, add a 1k in series with the diode.
This compensates for the lower voltage drop.
Be sure the diode is connected in the correct polarity, with its banded end
connected to the transformer wire.
If you've replaced the selenium rectifier, add a 1k in series with the diode.
This compensates for the lower voltage drop.
Be sure the diode is connected in the correct polarity, with its banded end
connected to the transformer wire.
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I don't think in series 1k is a correct value. I tried before. I measured the voltage of the 1st 50uf cap, 57V with 1k resistor, 71V without. based on the manual, it should be 65v.
With 1k resistor, the range of the bias range is severe limited. I could not adjust the cathode voltage below 1.4v. The target is 1.56 volt based on the manual. I am not comfortable with that. In the end, I just removed the 1k resistor.
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That is the value which Dynaco recommended when replacing the selenium rectifier with a diode.
However, if the AC line voltage runs high, the 1k can be increased to compensate.
If the DC cathode current is too high, you do want a more negative grid bias voltage.
In the USA this is seldom a problem. Sometimes a leaky grid capacitor can cause
excessive cathode current in one of the output tubes. To determine whether this is the case,
remove the coupling capacitors and see if the bias current decreases. If the current decreases,
replace them with less leaky capacitors.
However, if the AC line voltage runs high, the 1k can be increased to compensate.
If the DC cathode current is too high, you do want a more negative grid bias voltage.
In the USA this is seldom a problem. Sometimes a leaky grid capacitor can cause
excessive cathode current in one of the output tubes. To determine whether this is the case,
remove the coupling capacitors and see if the bias current decreases. If the current decreases,
replace them with less leaky capacitors.
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Thanks everyone! I'm gonna install the resistors tonight. I'll probably return with more questions as this journey continues.
Cheers.
Cheers.
Nice to see a neighbor work with dynaco's. Where do you live ? I live in gothenburg and has some spares & tubes for dynaco's.
Hey Peter! I live in Stockholm. I've been visiting your site more than a couple of times actually, you got some nice stuff there. I emailed you last week about buying a couple of RCA 7199 tubes. I was mistakingly under the impressions they should be closely matched. I'm still interested, let me know when you've had the time to check your inventory 🙂
Or new replacement boards are available to use the 6GH8A instead of the 7199.
PC-3B ST-70 MF Resistors - Dynakit Parts
PC-3B ST-70 MF Resistors - Dynakit Parts
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