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Old 28th July 2013, 11:56 AM   #1
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Default Dynaclone ST70 6F2 Alternative

I've replaced the crappy Chinese 6F2s in my "VAA MKii" Dynaco clone with ECF82s. This works just fine. However, I have only one left. But in my junkbox reside about 100 PCF80s, liberated from scrapped black and white TVs in the seventies, when everybody bought color sets. With a trick I upped the (DC) filament voltage to 8.0 volts (a bit too low for the 9V 0.3A "P" tube) and my goal of getting more than 100Vpp drive voltage for the EL34s was reached.

I've compared the datasheets for ECF82 and PCF80 and apart from minor differences I can't see how they are different in audio circuits. Both tubes are intended for VHF mixer stages, and the PCF80 is also widely used for deflection circuits. I still have to hook up my distortion analyzer but my scope doesn't show any change except for slightly more headroom with the PCF80.

So my question is: what is the difference?
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Old 29th July 2013, 12:09 PM   #2
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Default Dynaclone ST70 6F2 Alternative: PCF80

I've figured it out. With my large-ish collection of PCF80s I've cherry picked a set of almost identical tubes using a test jig. This is the same circuit used in my Dynaclone, see image below. I found there are many ways this tube can be constructed so I first selected visually identical devices. This is a PCF80 with a bright metal shield with notches between the two systems. There are also tubes with a black shield with notches and without notches. There seem to be no tubes with a bright shield without notches. All these tubes had either Philips or Valvo branding on them.

With the jig I measured the plate and screen DC voltage and the output AC voltage on the cathode of the triode (Vpp). The results were all over the place. Not surprising as many of these tubes must have led a hard life in someone's TV. Eventually I selected five tubes that had more or less the same value.

Then I tested the few 6F2/ECF82/PCF82 devices I could locate. They had slightly less gain than the PCF80s but had a lower headroom. My selected PCF80s could produce 94Vpp in my test jig, the other only 86Vpp best case. The Chinese 6F2 even less. Distortion also seems higher, see scope images below.

So that answers it. The PCF80 is a good alternative. I'm considering changing the circuit as the pentode is very finicky to set up properly. But that's a whole new project...
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File Type: jpg IMGP1075.jpg (848.8 KB, 64 views)
File Type: jpg 6f2_no_clip_-5dbu.jpg (487.4 KB, 63 views)
File Type: jpg 6f2_clip_-3dbu.jpg (462.6 KB, 61 views)
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