Power Supply in Home Brew TRF - UK Vintage Radio Repair and Restoration Discussion Forum

Neil Purling · 26th Mar 2013, 1:30 pm

Here is what there is at present:
Centre-tapped 250V HT & the usual seperate LT windings.
A 6C4P/6Z4 rectifier (depends on whether you use the Reflektor or Shuguang item). The rectifier is a EZ90 with pin differences.
The 'can' is a triple-section 32+32+20uf.
The present scheme is arranged thus:
Rectifier cathode-32uf-10K-32uf-HT1-20uf-HT2.
HT1 is the supply to the o/p stage & HT2 the HT for the small-signal Russian ECF80 clone.
There is only a wire link between the 32uf smoother & the 20uf section & ought there to be a smaller value of resistor?
The output stage has less hum & I want even less ripple in the HT to the anodes within the ECF80.

G6Tanuki · 26th Mar 2013, 2:14 pm

When I built these sorts of receivers back in the 1970s - typically using something olike an EF91 pentode as the regen detector and an ECL82 as the audio - I tended to use a 32+32uF with something like a 5K 5-watt wirewound resistor between the sections.

The output-transformer primary being fed direct from the 32uF section that took the rectified-AC feed from the rectifier cathode (typically a 6X4).

The rest of the regenerative stuff, the first audio-triode and the pentode section of the output-valve's screen-grid came off the junction of the 5K resistor and the second half of the 32uF.

I generally found attention to good RF screening and decoupling (lots of 0.1uF capacitors dotted around) was more important for smooth and stable operation than DC-smoothing.
Remember, electrolytic capacitors don't work well at RF! Paralleling each of the smoothing sections with a 0.1uF 500VW polyester or mylar capacitor can help. Also consider putting low-valur (1000pF) 1Kv-rated capacitors in parallel with each half of the rectifier - this can help if you have "modulation hum" [a hum that appears when you tune in a station but goes away when the radio isn't tuned to anything].

--G6Tanuki

G8HQP Dave · 26th Mar 2013, 3:07 pm

The 10k feeding the output stage probably ought to be a bit smaller, so you lose less voltage. A second resistor between HT1 and HT2 would reduce ripple to the RF section.

Neil Purling · 26th Mar 2013, 4:58 pm

Any idea as to the value of the 2nd resistor?
I will try 1K & I swop the 10K to 4.7K.

26th Mar 2013, 5:46 pm

I would go... Rectifier 32u 2k2 32u Output HT 22k 20u Detector HT and a few 0.1s as sugested.

26th Mar 2013, 1:30 pm	#1
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,085	Power Supply in Home Brew TRF Here is what there is at present: Centre-tapped 250V HT & the usual seperate LT windings. A 6C4P/6Z4 rectifier (depends on whether you use the Reflektor or Shuguang item). The rectifier is a EZ90 with pin differences. The 'can' is a triple-section 32+32+20uf. The present scheme is arranged thus: Rectifier cathode-32uf-10K-32uf-HT1-20uf-HT2. HT1 is the supply to the o/p stage & HT2 the HT for the small-signal Russian ECF80 clone. There is only a wire link between the 32uf smoother & the 20uf section & ought there to be a smaller value of resistor? The output stage has less hum & I want even less ripple in the HT to the anodes within the ECF80.

26th Mar 2013, 2:14 pm	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,995	Re: Power Supply in Home Brew TRF When I built these sorts of receivers back in the 1970s - typically using something olike an EF91 pentode as the regen detector and an ECL82 as the audio - I tended to use a 32+32uF with something like a 5K 5-watt wirewound resistor between the sections. The output-transformer primary being fed direct from the 32uF section that took the rectified-AC feed from the rectifier cathode (typically a 6X4). The rest of the regenerative stuff, the first audio-triode and the pentode section of the output-valve's screen-grid came off the junction of the 5K resistor and the second half of the 32uF. I generally found attention to good RF screening and decoupling (lots of 0.1uF capacitors dotted around) was more important for smooth and stable operation than DC-smoothing. Remember, electrolytic capacitors don't work well at RF! Paralleling each of the smoothing sections with a 0.1uF 500VW polyester or mylar capacitor can help. Also consider putting low-valur (1000pF) 1Kv-rated capacitors in parallel with each half of the rectifier - this can help if you have "modulation hum" [a hum that appears when you tune in a station but goes away when the radio isn't tuned to anything]. --G6Tanuki

26th Mar 2013, 3:07 pm	#3
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Power Supply in Home Brew TRF The 10k feeding the output stage probably ought to be a bit smaller, so you lose less voltage. A second resistor between HT1 and HT2 would reduce ripple to the RF section.

26th Mar 2013, 4:58 pm	#4
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,085	Re: Power Supply in Home Brew TRF Any idea as to the value of the 2nd resistor? I will try 1K & I swop the 10K to 4.7K.

26th Mar 2013, 5:46 pm	#5
Guest Guest Posts: n/a	Re: Power Supply in Home Brew TRF I would go... Rectifier 32u 2k2 32u Output HT 22k 20u Detector HT and a few 0.1s as sugested.