28th Dec 2016, 10:05 pm | #181 |
Octode
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Re: My first valve radio project - Philips 462A
Use 10k resistors in series with your phone outputs otherwise you will overheat the phone's (or other devices) output stage.
You would not connect the two outputs of a stereo amplifier together and then use one speaker would you? The same applies. |
28th Dec 2016, 10:08 pm | #182 | |
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Re: My first valve radio project - Philips 462A
Quote:
At the moment I'm connected my iphone via the 3.5mm jack but ultimately I want to use a bluetooth device. I have used a bluetooth device on a radiogram to get wireless audio playing through the radio input in it (to complement the existing grams and radio functions). So its the output of the bluetooth device that I need to consider. I have plugged in my BT device to the mains and sent audio to it from my phone. It has a 3.5 mm jack coming from it so I tried to measure some voltage across the ground and audio part of the jack - can't detect anything. So either its very low signal voltage - or I'm just not doing it right... So I don't know about voltage but from my experiences last time I know that when I switch off the power to the bluetooth is goes very high impedance and caused lots of buzz from the radiogram - so I connected 47k resistors from each channel input to ground. https://www.vintage-radio.net/forum/...d.php?t=132236 |
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28th Dec 2016, 10:17 pm | #183 |
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Re: My first valve radio project - Philips 462A
I doubt if you will get a voltage reading from the signal output of your device, the signal voltage will be low and will be AC, ordinary meters measuring AC won't normally measure audio frequencies very well.
Don't forget the voltage checks mentioned earlier. Lawrence. |
28th Dec 2016, 10:45 pm | #184 |
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Re: My first valve radio project - Philips 462A
Hi Nicola, I am sure 2.2k ohm resistors will be sufficient. Just add one resistor in series with each signal (left & right) then combine them together.
Did I see that you had removed the wiring to S16 and R12 (trader) to isolate the point you connected to from the radio circuit. That should stop any stray unwanted volts in the radio from damaging your phone. Without the resistors you have shorted one channel output onto the other channel output, this is not good practice. C18 (trader) will give protection from stray DC voltages that may happen in the valve radio, so it may be worthwhile changing it for a known good one of high voltage, just in case a fault develops in V2 triode. (unlikely but just to be safe) Mike Last edited by crackle; 28th Dec 2016 at 10:57 pm. |
28th Dec 2016, 11:50 pm | #185 | |
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Re: My first valve radio project - Philips 462A
Quote:
V3 pentode: Anode (pin2) = 260V Screen grid (g2; pin4) = 240V Control grid (g1; pin3) = -3.68V V2 triode: Anode (a.t; pin3) = 15.1V Control grid (g1.t; pin4) = 60mV The control grid for the V2 triode is very small but not negative as you suggested it should be. Is this cause for concern? |
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28th Dec 2016, 11:55 pm | #186 | |
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Re: My first valve radio project - Philips 462A
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I only have 47K Ohms to hand so used those; obviously a bit higher than Peters suggestion of 10K Ohms (and a lot higher than 2.2k Ohms from Mike). I made the assumption that something is better than nothing. I can of cause change these to lower values if I order some in. What is the potential issue (if any) of using a resister that is too high? Will I lose any signal? |
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28th Dec 2016, 11:56 pm | #187 |
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Re: My first valve radio project - Philips 462A
By the way, I have now fitted my bluetooth device to the set rather than my phone - working well (but with the same volume control issue of course).
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29th Dec 2016, 12:32 am | #188 | |
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Re: My first valve radio project - Philips 462A
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In theory, with a headphone output designed to work into typical 32Ω phones, the mixing resistors need not be very high - I often use 470Ω when working into a mono line input, so that the source impedance is 235Ω and the load on each output around 1kΩ (i.e. the path from one output through both resistors back into the other output, which we assume has minimal source impedance). Higher values such as those suggested above will work well too provided the input impedance of the mono destination is an order of magnitude higher still. |
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29th Dec 2016, 12:34 am | #189 | |
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Re: My first valve radio project - Philips 462A
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No significant change, although I think there is now some weak audio heard at the lower vol pot positions rather than at all suddenly coming on at the highest pot positions only. Only a slight improvement, with most of the useable volume coming at the end of the turn of the dial. Why is volume a log pot rather than a linear. Surely a log pot will be prone to giving all the volume at the top end? I'm still having the strange issue with tone control - it seems to act like a volume control. |
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29th Dec 2016, 1:08 am | #190 | |
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Re: My first valve radio project - Philips 462A
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29th Dec 2016, 8:23 am | #191 |
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Re: My first valve radio project - Philips 462A
Hi
C23 (trader) is the capacitor associated with the tone control. When the tone control wiper is at the bottom (see trader circuit) C23 is in effect connected across the grid of the output valve and chassis. The effect of the capacitor will be to create a path for the higher frequencies to ground thus removing some of the high frequencies, it is a treble cut control. The value .0039uF seems high to me, it may actually be cutting out the high frequencies and also encroaching onto the middle frequencies. If this is a wax type capacitor I would change it, either for a new 0.0033uF capacitor (3.3nF), or you could also try a 1nF rather than the 3.3nF, it would then tend to cut less of the "middle" frequencies and make things sound louder and clearer. If 1nF makes the tone control have too little effect, then try something in the middle, say 2.2nF capacitor. I have often seen the tone control capacitor in my radios as a small value such as a few hundred pF, rather than a few thousands of pF's. Mike Last edited by crackle; 29th Dec 2016 at 8:34 am. |
29th Dec 2016, 10:14 am | #192 |
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Re: My first valve radio project - Philips 462A
Hi Nicola, it may need an oscilloscope to measure the voltage out of your I pad, see if you can borrow one.
An AC millivoltmeter would do, but with a scope you can "see" the actual signal and be sure it is not something else. Ed |
29th Dec 2016, 10:35 am | #193 |
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Re: My first valve radio project - Philips 462A
I have just come across this thread having repaired the same model before Christmas. Just one point I would make is that in mine the wiring was all rubber insulated and all the rubber was crumbling. If yours also has rubber insulation I would poke around to make sure it is ok.
In my example the mains transformer had shorted turns and the output one was open circuit! |
29th Dec 2016, 11:08 am | #194 | ||
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Re: My first valve radio project - Philips 462A
Quote:
First thing I would check are R6 (2,2M) and R7 (470k) providing C17 and C18 aren't electrically leaking those two resistors can be measured in circuit with the valve cold. If those resistors check out ok then check the anode load resistor R14 (100k) and it's decoupling resistor R13 (100k) those can also be measured in circuit providing C22 isn't electrically leaking and the valve is cold. Lawrence. |
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29th Dec 2016, 11:26 am | #195 | |
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Re: My first valve radio project - Philips 462A
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Can you also confirm that my thinking around working voltage is OK and that it is safe to leave the 50V cap in permanently? |
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29th Dec 2016, 11:33 am | #196 |
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Re: My first valve radio project - Philips 462A
As this is no longer a thread about a vintage radio but an audio amplifier shouldn't a new thread in the vintage audio be started. It would have been interesting to know if the radio worked. The anode voltage of the triode section of V2 should be around 40 Volts so the valve could be conducting too heavily. As V1 will never be used again this could be removed, and tried in V2 position.
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29th Dec 2016, 12:14 pm | #197 |
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Re: My first valve radio project - Philips 462A
50v cap for C23 should be fine, no real harm can come if it goes faulty anyway, other than loss of audio if it fails shorted.
Have you changed all the wax paper capacitors, in the AF part of the receiver (e.g. C22 & C17 trader) . Philips sometimes used paper caps covered in black pitch which also need to be changed. Mike |
29th Dec 2016, 12:31 pm | #198 | |
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Re: My first valve radio project - Philips 462A
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29th Dec 2016, 12:39 pm | #199 | |
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Re: My first valve radio project - Philips 462A
Quote:
C17 (the V2 triode GB decoupler) has already been changed but I will change C22 now also (the V2 triode HT decoupler) as you suggest. C10 is the AVC line decoupler so I'll change that too. C24 is the fixed tone corrector - this sounds relevant to tone control so I will replace this also. C3 and C4 are band pass couplers so I won't bother changing those as they aren't going to be used now. |
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29th Dec 2016, 12:48 pm | #200 | |
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Re: My first valve radio project - Philips 462A
Quote:
I get the same reading regardless of which ECH21 is in the V2 position so we can rule out a valve fault. |
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