10th Apr 2020, 11:34 am | #81 | |
Tetrode
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Re: Pilot U650
Quote:
Want to try to get the pick up working first so i know the back end is ok then tackle the front end / potential alignment etc but with the speaker being 3000 ohms instead of 1400 ohms not sure hoe this impacts things |
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10th Apr 2020, 11:37 am | #82 | |
Tetrode
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Re: Pilot U650
Quote:
Perhaps in the meantime i should try connecting ipod to my audio preamp then preamp to radio as the power amp? |
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10th Apr 2020, 11:44 am | #83 | ||
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Re: Pilot U650
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10th Apr 2020, 12:32 pm | #84 | |||
Tetrode
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Re: Pilot U650
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10th Apr 2020, 12:37 pm | #85 | |
Dekatron
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Re: Pilot U650
Quote:
Some may argue that the isolation cap is redundant, but I don't like the idea of connecting an expensive smartphone directly into the gram socket of a valve radio even if - as may be the case - there's an isolation cap in the set. Hope that helps.
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10th Apr 2020, 1:03 pm | #86 | |
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Re: Pilot U650
Quote:
With 3k rather then 1.4k in series with the HT, the HT is going to be low. What effect this will have on the anode and screen voltages of the output valve I wouldn't like to say. You could try temporarily substituting a 1200R wire wound resistor for the energising coil. The speaker won't work of course, but you could then check the voltage across R25 to see how the anode and screen currents have changed. Have you tried a simple buzz test? Touching the centre tag of the volume control with the metal blade of a screwdriver should give a loud buzz from the speaker if all is in order.
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10th Apr 2020, 3:36 pm | #87 | |
Tetrode
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Re: Pilot U650
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10th Apr 2020, 9:57 pm | #88 |
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Re: Pilot U650
I'd say you have a problem with the output stage. The fact the energising/smoothing winding has a DC resistance of more than twice the specified value can't be helping much. Try substituting it with a resistor and see what the voltage readings on V5 look like.
Other possibilities are low emission or low gm in V5, mismatching of the speaker to the output valve and problems with the speaker itself. Extract from circuit attached. L33 is the energising winding.
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11th Apr 2020, 6:13 am | #89 |
Tetrode
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Re: Pilot U650
Thanks Graham, have ordered a 1.5K wirewound to substitute as a dummy load will then report back
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11th Apr 2020, 6:56 am | #90 |
Tetrode
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Re: Pilot U650
ok, just noticed something looking at your snip of the circuit Graham, should the heater feed "a" be grounded to the centre tap ground point before proceeding to the valve sockets?
Mine is not, the 2 leads go straight to V5 from the transformer then parallel off from there. |
11th Apr 2020, 8:05 am | #91 |
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Re: Pilot U650
It's usual for one side of the heaters to be grounded.
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11th Apr 2020, 9:06 am | #92 |
Tetrode
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Re: Pilot U650
Thanks Graham, will run a ground to the pin. Should the incoming -ve power lead be grounded via a safety cap perhaps? Currently it just feeds into the transformer primary.
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11th Apr 2020, 9:25 am | #93 |
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Re: Pilot U650
Positive and Negative have no meaning as far as an AC mains supply is concerned. They change over 50 times a second. The correct terms are live and neutral.
The neutral shouldn't be grounded via a capacitor or otherwise. As this set has a mains transformer you could earth the chassis via a three core mains lead if you wished.
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11th Apr 2020, 9:25 am | #94 | |
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Re: Pilot U650
Quote:
As an example, in the case of the Marconi 559 here, a similar radio from a similar era, the circuit diagram shows much the same thing- yet in practice, the heater wiring is thick twisted pair from the transformer tags linking all valve sockets and kept away from chassis- except for a single link between one side and chassis at the mixer valve socket. This is to keep hum currents out of the chassis, and out of the negative side of the HT rail- which is exactly the problem that would occur if the circuit diagram was interpreted over-literally. An excellent case of the difference between theory and practice is the Mullard 5-20 amplifier- as well as the circuit diagram, they also published a recommended wiring arrangement scheme. Over the years, innumerable queries have arisen along the lines "I built it exactly to the circuit diagram, yet it hums horribly/hoots/motorboats/makes tweeters smoke...." |
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11th Apr 2020, 11:44 am | #95 |
Tetrode
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Re: Pilot U650
Thanks StationX & Turretslug, thats helpful. Should all grounds be routed to the same point - the centre tap of the transformer perhaps or are several ok? I have 8 currently.
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11th Apr 2020, 1:23 pm | #96 |
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Re: Pilot U650
If the grounds are as original, even if they're not optimal (and some sets are far from the ideal as built) the set should be fine. As soon as you start modifying you need to understand exactly how any common mode ground return paths might affect things or you may just make things worse.
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11th Apr 2020, 1:35 pm | #97 | |
Tetrode
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Re: Pilot U650
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12th Apr 2020, 10:34 am | #98 |
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Re: Pilot U650
I regard a circuit diagram as being as being akin to the "map" of the London Underground. It's designed to be easy to understand, but bears no resemblance to the geographical layout.
It's possible that the heater rail has been grounded at a point remote from the mains transformer. A quick resistance check would show whether it's grounded or not.
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12th Apr 2020, 11:23 am | #99 |
Tetrode
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Re: Pilot U650
Thanks Graham, resistance confirmed not grounded - but it is now between pin 6 on V2 to chassis about 30mm away which was the simplest way to do it.
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12th Apr 2020, 12:33 pm | #100 |
Dekatron
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Re: Pilot U650
A resistance check is a wise move- you wouldn't want to connect one side of the heater supply to chassis and then find that the other side was also connected after all, say by an inconspicuous wire link hidden under other things. Heater windings can supply a lot of current and might not necessarily blow a fuse very quickly.
Making the heater supply-to-chassis connection adjacent to V2 sounds like a good way to ensure that the local oscillator signal is less likely to leak along the heater chain to other parts of the circuit- the sort of thing that can cause whistles or spurious responses. Even in the mid-'30s, a lot had already been learnt about how to make radios work well, and it's often interesting to take note of what was done back then and take it on board. |