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Old 25th Jul 2018, 11:04 am   #1
egerton
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Default Bestone TRF - power up and audio stage testing

I've started a new thread as I've moved on to the power up and testing phase on the Bestone 20 TFR I am restoring. The earlier thread here: https://www.vintage-radio.net/forum/...d.php?t=147542 has history of how I got to this point. Learned a lot... thanks guys. Anyway I had established the series heater chain had correct voltages and the rectifier seemed ok - now time to connect HT to the audio stage ONLY.

Got a crackle and hum from speaker when grid touched! We are go! ..but the HT has collapsed somewhat!

Noticed though there was an ongoing random crackling sound - like a noisy component breaking down even when I don't touch the grid. This stops completely when I ground the grid. Is that static build up on grid or noisy valve or component?

Switched off and ponder this test. Previously measured HT (no load) with i/p supply (100w bulb) 121v giving C2 131v, C3 125v.

With HT to audio stage and speaker field coil connected i/p supply (needed 110w bulb) to give 115v a/c supply and C2 91V, C3 86V. Is this enough?

I propose to measure the HT current next. As a check, cathode resistor R3 had 11V across it. Calc. cathode current as 11/650 = 17mA looks reasonable as a quiescent current?

Any comment greatly welcomed, at least I've heard a sound out of the set

Paul
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Old 25th Jul 2018, 12:25 pm   #2
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Default Re: Bestone TRF - power up and audio stage testing

Forgot to say that I have replaced all electrolytics as they were all dodgy. C2 increased from 12 to 16 uF but believe that's ok for the 1D6. Schematic: https://1drv.ms/u/s!AlDn_84okuoJr0yDZUOIlbWixKhu
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Old 25th Jul 2018, 2:04 pm   #3
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Default Re: Bestone TRF - power up and audio stage testing

Around 90v for HT will be OK.
Cracking noise that stops when audio valve grid is grounded will be coming from further towards the aerial end, possibly a capacitor breaking down.
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Old 25th Jul 2018, 2:55 pm   #4
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Default Re: Bestone TRF - power up and audio stage testing

Even with no HT to other stages? Also coupling cap. C5 disc. so just R4 500K grid bias in place. That could be the culprit... or R3 cathode perhaps, but shorting grid to deck clears it so suspect R4. Doesn't seem much volume but will test with audio line signal injection.
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Old 26th Jul 2018, 2:24 pm   #5
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Default Re: Bestone TRF - power up and audio stage testing

Crackling can be from the valve or socket too. If the resistor values read OK I would just go for it and plug the other valves in. The loading from the previous anode may get rid of the noises.
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Old 26th Jul 2018, 2:50 pm   #6
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Default Re: Bestone TRF - power up and audio stage testing

I injected an audio signal into the grid from my iPod. There is not much volume. I am not expecting a lot of output power, but how would I calculate an output power? I would expect more than I'm getting which is akin to "low volume listening". I will press on and connect the other stages as soon as the next batch of caps. arrive - found the old waxies couplers are both shot too, should have anticipated that.
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Old 26th Jul 2018, 6:05 pm   #7
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Default Re: Bestone TRF - power up and audio stage testing

Quote:
I injected an audio signal into the grid from my iPod. There is not much volume. I am not expecting a lot of output power, but how would I calculate an output power?
Usually by substituting the loudspeakers voice coil with a dummy load, eg: a resistor that has a resistance equal to the loudspeakers nominal impedance, a sine wave at say 1kHz would be connected to the grid and the RMS voltage developed across the resistor would be measured, Vsquared/R would give the power, having said that, according to the valve data at the HT voltage you're aiming at the output won't be more than 1 watt, and for that a signal of 15 volts peak would be needed on the grid, which is quite an ask for this receiver.

As an approx. guide for unknown loudspeaker impedance, measure its DC resistance and add 20%.

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Old 26th Jul 2018, 6:38 pm   #8
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Default Re: Bestone TRF - power up and audio stage testing

I connected the HT up to the RF and detector valves and powered up. I was pleasantly surprised that I could tune in to no less than 5 stations on MW - after I connected the external long wire aerial! Volume control is somewhat strange but perhaps typical of a TRF? Everything seems to happens in the last 75% of the pots travel... suddenly starts detecting and volume goes from min to max. so quite sensitive. LW doesn't work though so still work to do. When the waveband switch is in the open position the set 'motorboats' if i turn the volume down it stops but no stations heard. Something is wrong with the coil switching and circuit arrangement for LW. But overall a pleasing result and I can't help wondering how long since this set has been receiving a station seeing as it had a 2 pin UK plug on and a resistive cable dropper! Paul
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Old 26th Jul 2018, 8:11 pm   #9
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Default Re: Bestone TRF - power up and audio stage testing

Quote:
Originally Posted by ms660 View Post
Usually by substituting the loudspeakers voice coil with a dummy load, eg: a resistor that has a resistance equal to the loudspeakers nominal impedance, a sine wave at say 1kHz would be connected to the grid and the RMS voltage developed across the resistor would be measured, Vsquared/R would give the power, having said that, according to the valve data at the HT voltage you're aiming at the output won't be more than 1 watt, and for that a signal of 15 volts peak would be needed on the grid, which is quite an ask for this receiver.
Thank you for the info. With the set now working I can report the volume is not too bad, so the detector stage must be driving the audio stage harder than my iPod at max. Good to know how to measure the power.
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Old 26th Jul 2018, 8:51 pm   #10
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Default Re: Bestone TRF - power up and audio stage testing

I think the iPod will be completely the wrong impedance (far too low). This set uses an anode bend detector and they have a fairly high output at a high impedance...just right for driving the output valve directly.
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Old 28th Jul 2018, 7:33 pm   #11
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Default Re: Bestone TRF - power up and audio stage testing

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Originally Posted by ms660 View Post
The way C9 is connected is a mystery to me, is it possible to photo the 6D6 valve socket showing the connections so we're absolutely sure that the suppressor grid is floating in DC terms as shown in your schematic. The wave change arrangements still look wrong if it's a MW and LW receiver.
Out of interest, I tried disconnecting C9 to see what happens - and the set still works but the output level drops by a large amount, so C9 is doing something good whatever it is....! I should add that it's not floating; the supressor is connected to cathode as in the corrected schematic. I'm still amazed how well this ancient little thing works on MW bringing in 5 station loud and clear. Just been listening to smooth radio no problem at all.

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Old 28th Jul 2018, 7:38 pm   #12
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Default Re: Bestone TRF - power up and audio stage testing

It will drop, traditionally C9 would be a screen grid bypass capacitor.

EDIT: Ah, I see the schematics been changed again.

Lawrence.

Last edited by ms660; 28th Jul 2018 at 7:45 pm.
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Old 28th Jul 2018, 9:33 pm   #13
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Default Re: Bestone TRF - power up and audio stage testing

So C9 is coupling the cathode and suppressor of the RF stage to the screen of the detector. I now know that anode bend detectors work with a large negative grid bias. Can anyone explain how that works in this circuit? Is the suppressor "suppressing" the grid to this affect?

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Old 28th Jul 2018, 11:15 pm   #14
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Default Re: Bestone TRF - power up and audio stage testing

The restricted screen grid (g2) current is keeping it in the non-linear operating area.
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Old 28th Jul 2018, 11:22 pm   #15
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Default Re: Bestone TRF - power up and audio stage testing

The -ve grid bias voltage is developed across the cathode resistor, in other words when DC current flows from the cathode to the anode it has to flow through the cathode resistor, that's the resistor that's connected between the cathode and HT -ve (chassis)) the voltage developed across that resistor by the current flowing through it makes the cathode +ve with respect to HT -ve (chassis) the grid is connected to HT -ve (chassis) via a resistor therefore the grid is -ve with respect to the cathode, the grid resistor does not drop any of this bias voltage because under normal operating conditions no grid current flows because the grid is now -ve with respect to the cathode.

The function of a suppressor grid is to repel any electrons that have bounced off the anode (called secondary emission) If the suppressor grid wasn't there those electrons would flow to the screen grid if the screen grid was at a high enough potential, and that, in effect, would shunt the anode load, if the load was a tuned circuit it would reduce it's Q in other words it's selectivity, this was a problem with some hexode mixer valves used in receivers when the screen grid voltage was fed by a single series resistor because when the -ve AGC voltage kicked in the screen voltage would rise due to the reducing screen current and thus attract the electrons that were the result of secondary emission, to help to reduce the screen voltage from increasing with an increasing AGC voltage the screen grid would often be supplied via a potential divider who's total current draw was much greater than the screen grid current and thus had a stabilizing effect on the screen grid voltage. A method of helping to stabilize the mixer screen grid voltage in hexode mixers without a two resistor potential divider was to supply the anode voltage for the oscillator and the screen grid voltage via a common resistor, the oscillator valves internal impedance in effect forming the bottom resistor in a potential divider.

The other downside of the early valves with no suppressor grid was that there was a kink in the Ia/Va characteristic curves, an area of negative resistance if you like where by an increase in anode voltage produced a decrease in anode current and not an increase. Later on kinkless/beam tetrodes were developed that ironed out the kink.

The above are just brief basic explanations which hopefully might help a bit, there are plenty of good articles/books etc on valve basics on the web, on of the best online ones is Inside the Vacuum Tube by J.F. Rider, worth keeping on file:

https://archive.org/stream/InsideTheVacuumTube#page/n1

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Old 28th Jul 2018, 11:39 pm   #16
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Default Re: Bestone TRF - power up and audio stage testing

Thanks for the detailed explanation, I get the gist of it but definitely need to read up on valve theory, so thanks for the link also.

the grid is connected to HT -ve (chassis) via a resistor

Regarding this bit, my grid has no resistor but is connected to a tuned circuit so is connected to chassis via the coil L2b.

https://1drv.ms/u/s!AlDn_84okuoJr0yDZUOIlbWixKhu
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Old 29th Jul 2018, 12:10 am   #17
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Default Re: Bestone TRF - power up and audio stage testing

Same difference as it were, the grid is at HT -ve (chassis) potential which is -ve with respect to the cathode, all cathode bias circuits work that way eg: there must be an external DC path between the grid and cathode. Some other methods of obtaining the bias voltage include a battery or a -ve supply from within the receiver by using the voltage developed across a resistor in the HT -ve rail as the source or by grid leak.

Lawrence.

Last edited by ms660; 29th Jul 2018 at 12:29 am. Reason: clarification
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Old 30th Jul 2018, 9:52 am   #18
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Default Re: Bestone TRF - power up and audio stage testing

There are 2 types of early detectors:
Leaky grid - The cathode is wired to chassis and the grid wired to chassis via a grid leak resistor with capacitor coupling for the AC signal. This uses the grid to rectify the incoming AC signal. Larger RF signals will push the average grid voltage lower causing a change in average anode volts. Their main disadvantage is their low input impedance which affects the Q of the tuned circuit feeding it.

Anode bend - A correctly biased valve amplifies the input on the grid linearly, however, if the anode current is reduced to near cut-off, the transfer characteristics become non-linear. By biasing the valve in the non-linear area, the +ve half cycles of the input will be amplified more than the -ve and will contribute more to the average anode current. If the amplitude of the RF signal changes there will be therefore be a shift in the average anode current. If this were a triode, the bias is set to the non-linear area solely using a very high cathode resistor but as a pentode the anode current can also be controlled from the screen grid hence the 2M screen feed.

Both create quite a bit of distortion and I assume the bias and screen feed resistor values for a pentode would have been chosen by trial and error.

Last edited by PJL; 30th Jul 2018 at 9:57 am.
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Old 4th Aug 2018, 5:13 pm   #19
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Default Re: Bestone TRF - power up and audio stage testing

As reported previously, this set works well on MW but did not appear to work on LW, when switched to LW it low frequency oscillates sounds like a motorboat unless the volume is reduced right down. After a lot of playing around I found that wrapping my finger around the tuning coil stopped the motorboating and I could then hear some some stations on LW most notably BBC radio 4. I'm not sure what that means. Probably I am changing the inductance / capacitance coupling of the coils by touching them, so it suggests to me that a capacitor or inductor is well out of spec, causing the set to oscillate of LW only. Does anybody concur?

Regards - Paul
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Old 4th Aug 2018, 9:32 pm   #20
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Default Re: Bestone TRF - power up and audio stage testing

Your hand is probably reducing the gain so preventing it from oscillating. Is it unstable over the entire waveband? I would look at the waveband switch and check the LW aerial coil continuity.
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