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-   -   50 triode amp (https://www.diyaudio.com/forums/tubes-valves/342283-50-triode-amp.html)

disco 17th September 2019 08:49 PM

9 Attachment(s)
Will investigate tomorrow. Yes, Philips was competitive in those days. It invented ways to arrive at noval products at lower prices. Lot of plastics involved unfortunately ;)

I keep coming back to that decoupling to signal ground as it's much more agreable in comparison to a big electrolyte. Will have to build me a 1KV PS soon :)

Quote:

Originally Posted by jhstewart9 (https://www.diyaudio.com/forums/tubes-valves/342283-50-triode-amp-post5917415.html#post5917415)
The poor response to the 30 Hz square wave might be due to the interstage transformer. Put the scope on the 300B grid, that will tell you.

Some sagging after the interstage but no deformation (left pic). Resistive load to the OPT (second pic) and 8R speaker (right pic). Deformation has strong dependence with output level.
Exiting the interstage a sinus up to 57Vrms seems neat, beyond its top flattens out. So, I'll have to shift the 10 operating point a little while it's not cleanly driving the final in to grid current. I had to cure an interesting 3uS ripple while at the interstage, a shielded interconnect from 26 to 10 was sufficient.

The 120mA quiescent current (Vak= 412V) hits a sweet spot (to my ears). Less current adds a "brown filter" to the music. The tonal balance is fine now. What's next... feedback?

Quote:

Originally Posted by jhstewart9 (https://www.diyaudio.com/forums/tubes-valves/342283-50-triode-amp-post5917415.html#post5917415)
The poor response to the 30 Hz square wave might be due to the interstage transformer. Put the scope on the 300B grid, that will tell you.

Some sagging after the interstage but no deformation (left pic). Resistive load to the OPT (second pic) and 8R speaker (right pic). Deformation has strong dependence with output level.

jhstewart9 5th October 2019 01:16 AM

Did the One KV PS move forward your Expiration Date? Haven't heard from you for a while.:D

disco 5th October 2019 09:56 AM

No worries, I'm still not electrocuted. But I did play around with loadlines and found the #26 is a hastle free tube once you ground the PS :D
LF is still not up to expectations with another loudspeaker. My OT was 17 years on the shelf, I suspected it could suffer from magnetostriction as found by co-builders who did report so on new outputs coming alive after no less than 200 hours. So, I ran the amp at 35Hz (half power) for two hours, if there's a difference I can't tell. It has played 20CD's meanwhile and perhaps this is too short to let the stress out of the parts. For instance, in the power supply I stacked two NOS elektrolytes from the 1980s, after reforming the aluminiumoxide at nominal working voltage. When the amp is playing its 15 watt output power, the music signal is shunted by this PS cap above the OT. To what extent is this cap taxed with AC?

disco 8th October 2019 02:17 PM

4 Attachment(s)
Replaced the #10 with #45 for a driver, resulting in less gain but a "fatter" sound. Not yet arrived at that "authoritative" organ notes but lows cary more weight now while control is excellent.

Perhaps the 45 is a better fit for the interstage, its primary self inductance is not specified. Added a grid leak resistor too, might be superfluous because of the direct coupling.

I had to lower the original operating point of the 300BXLS because of running in to OT saturation. It is a 40W specced unit, 95mA would be the target to pursue.


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