D & G - UK Vintage Radio Repair and Restoration Discussion Forum

boiss · 17th Oct 2006, 6:37 pm

Finally David has tamed Goliath and all on 26V Ht. Unfortunately I couldn't get the original EC88 to modulate the 813 with any sort of reasonable linearity. In the end I used a triode strapped EF80, an EF 91 would have been better but I didnt have one and couldn't find one locally. The Ht transformer is a 3VA 18V toroid and the heater transformer is an 80VA 9V jobby. While working on it, the primary went O/C had to dig in and find roughly where the break was and loop it, I now get about 10V out of it. The heater voltage for the 813 is 8.5V stabilised. Current input Ia is about 0.75mA, power output is 0.8mW into 50 Ohms and modulation is about 70% max without noticeable distortion. The O/P is very clean with no detectable harmonics on my ICF7600 unlike my previous attempt with the PL36. I'm not very happy with the LP filter but couldn't find any software that copes with different I/P and O/P impedances. If anyone knows where I can find some "Free" software that can give me some values for the filter, please let me know.
Cheers,
Mike.
Must start on my 813 amp now that this is running, How about mains Toroids for O/P transformers, (PP)

adibrook · 18th Oct 2006, 9:54 pm

Wow. Looks cool. I like the Shassis.

May i ask why such a huge valve is used for such a seemingly low power role?

boiss · 19th Oct 2006, 9:17 am

Just for the fun of doing it, apart from the fact that I've always liked running valves at low voltages. ie space charge mode etc. Here I tried it on the modulator but It wasn't very effective, putting more than a couple of volts positive on the control grid meant that the control grid current dragged down the Ht supply. The EF80 isn't suitable for this type of operation IMO.
Anyway it looks good.
Mike.

17th Oct 2006, 6:37 pm	#1
boiss Pentode Join Date: Jun 2006 Location: Malaga, Spain. Posts: 235	D & G Finally David has tamed Goliath and all on 26V Ht. Unfortunately I couldn't get the original EC88 to modulate the 813 with any sort of reasonable linearity. In the end I used a triode strapped EF80, an EF 91 would have been better but I didnt have one and couldn't find one locally. The Ht transformer is a 3VA 18V toroid and the heater transformer is an 80VA 9V jobby. While working on it, the primary went O/C had to dig in and find roughly where the break was and loop it, I now get about 10V out of it. The heater voltage for the 813 is 8.5V stabilised. Current input Ia is about 0.75mA, power output is 0.8mW into 50 Ohms and modulation is about 70% max without noticeable distortion. The O/P is very clean with no detectable harmonics on my ICF7600 unlike my previous attempt with the PL36. I'm not very happy with the LP filter but couldn't find any software that copes with different I/P and O/P impedances. If anyone knows where I can find some "Free" software that can give me some values for the filter, please let me know. Cheers, Mike. Must start on my 813 amp now that this is running, How about mains Toroids for O/P transformers, (PP) Attached Thumbnails

18th Oct 2006, 9:54 pm	#2
adibrook Retired Dormant Member Join Date: Dec 2004 Location: Weymouth, Dorset, UK. Posts: 377	Re: D & G Wow. Looks cool. I like the Shassis. May i ask why such a huge valve is used for such a seemingly low power role?

19th Oct 2006, 9:17 am	#3
boiss Pentode Join Date: Jun 2006 Location: Malaga, Spain. Posts: 235	Re: D & G Just for the fun of doing it, apart from the fact that I've always liked running valves at low voltages. ie space charge mode etc. Here I tried it on the modulator but It wasn't very effective, putting more than a couple of volts positive on the control grid meant that the control grid current dragged down the Ht supply. The EF80 isn't suitable for this type of operation IMO. Anyway it looks good. Mike.