Pilot Little Maestro pre war AC/DC 1939 restoration project. - Page 8 - UK Vintage Radio Repair and Restoration Discussion Forum

Radio Scotland · 13th Nov 2021, 7:48 pm

Quote:

Originally Posted by Ed_Dinning

Hi Jay, note that in this setup Vmains does not equal Vcap + Vhtrs. It is actually what is called a vector sum and takes a while to explain and get your head round
Ed

Yeah Ed i was reading about that a few days back, But it's beyond me,,All i know is as previously stated i'm getting a reading of 56 volts from the dropper and a reading of 4.2v across the dial lamp, So i'm falling short somewhere.

Either need more capacitance on the dropper or the bleed resistor value needs altered,

Cheers
jay

G.Castle · 13th Nov 2021, 8:29 pm

Hello Jason.
Read this: https://www.cool386.com/dropper/dropper.html

Capacitor manufacturing can be done very precisely, and knowing that you will probably find your dropper capacitance is lower than you think, they don't give you anything they don't have to! You can pretty much guarantee that a given value component will be on the low side of marked value, complain and the manufacturer will quote tolerances at you, series limiter resistors or parallel bleeder resistors will make little or no difference in a reactive dropper.

If you're just a little under the chain current then just top it up with parallel capacitance.

Simon Gittins · 13th Nov 2021, 8:34 pm

You are getting somewhere now, I think you just need to slightly increase the value of the dropper capacitor.
The dropper calculation page has a useful spreadsheet. For a 75V chain at 0.3A the capacitor value should be 4.19uF without a surge limiter resistor or 4.29uF with a surge limiter of 50 ohms (surge limiter drop set to 15V on the spreadsheet).
As you were reading about 3.8uF for your two 2.2uF capacitors in parallel it seems they are slightly low in value. Do you have another Class X capacitor of about 0.4uF you can put in parallel with them?

Just seen the above, looks like we have the same suggestion.

Radio Scotland · 13th Nov 2021, 8:46 pm

Thanks for the input folks,

My 2 X 2.2uf safety dropper caps are 20% tolerance,

Hence i'm low (3.8uf) Iv'e put an advert in the wanted section for a 0.4uf 300volt safety cap. to bring me up to 4.2uf,

i could buy one, but then the same issue, might be sitting low in value.

Hopefully if i get one on here the person can measure the value for me so i know its suitable.

Scrapped the above idea, I'll just by a new one
jay

Radio Scotland · 14th Nov 2021, 1:15 am

Just to mention.

The voltage at turn on is about 30 volts and steadily climbs up and settles at 56v

G.Castle · 14th Nov 2021, 2:44 am

Quote:

Originally Posted by Radio Scotland

Just to mention.

The voltage at turn on is about 30 volts and steadily climbs up and settles at 56v

It would. Application of ohms law. When the heater chain is cold and the resistance is lower, the reactive dropper is still only trying to force the design current in the circuit, (can be calculated if you want to be pedantic).

Use a polycarbonate motor run capacitors and aim for +5% on the heater voltage.

Radio Scotland · 14th Nov 2021, 11:03 am

Iv'e ordered up a 0.47uf - X2 capacitor, If that doesn't bring the voltage up then the project may need to be shelved, Can't keep throwing money at it, And theres no saying the valves are any good, And a full set which its unlikely to need would be around £100. And that would be madness.

Anyhow the cap will take a few days to arrive so i'll update when its been fitted

Cheers
Jay

FERNSEH · 14th Nov 2021, 12:25 pm

The Little Maestro I serviced two years ago employs a 4microfarad motor start capacitor to supply the 0.3amp heater chain.
Valve line-up in this set is: 6K8G, 6K7G, 6Q7G, 25L6 and 25Z6.
I remember the calcs for the capacitor worked to be 4.4mfd but the 4mfd capacitor works fine.
https://www.vintage-radio.net/forum/...d.php?t=161165

DFWB.

Radio Scotland · 14th Nov 2021, 12:41 pm

Quote:

Originally Posted by FERNSEH

The Little Maestro I serviced two years ago employs a 4microfarad motor start capacitor to supply the 0.3amp heater chain.
Valve line-up in this set is: 6K8G, 6K7G, 6Q7G, 25L6 and 25Z6.
I remember the calcs for the capacitor worked to be 4.4mfd but the 4mfd capacitor works fine.
https://www.vintage-radio.net/forum/...d.php?t=161165

DFWB.

Was reading your resto thread the other night. Thats the bit thats baffling me. Folk get away with 4uf motor run caps, I'm very close to 4uf but 20v short. Must be that small bit of missing capacitance thats making all the difference

Station X · 14th Nov 2021, 12:56 pm

It's all down to tolerance in the capacitor values. Components made nowadays are not specifically designed for use in valved radio receivers, although they are used for that purpose. They are definitely not designed for use as droppers in valved radio receivers, but once again they can be used for that purpose.

With the extra 0.47uF in circuit the voltage will increase. So long as it's within 5% of the specified figure you'll be fine.

Radio Scotland · 14th Nov 2021, 12:59 pm

Quote:

Originally Posted by Station X

It's all down to tolerance in the capacitor values. Components made nowadays are not specifically designed for use in valved radio receivers, although they are used for that purpose. They are definitely not designed for use as droppers in valved radio receivers, but once again they can be used for that purpose.

With the extra 0.47uF in circuit the voltage will increase. So long as it's within 5% of the specified figure you'll be fine.

Fingers crossed station X

Radio Tech · 14th Nov 2021, 1:25 pm

Hello Jay

I admire the vast work you have done on this set so far, but you may have found it easier to use axial electrolytic capacitors in your post #122 last picture, this is not a criticism but just my thoughts.

Ken

Sideband · 14th Nov 2021, 1:28 pm

I had a similar problem with my Pilot Model 10 about 12 years ago when I modified it to capacitor dropper. In that case I used a capacitor from a florescent lighting unit which was slightly low from its marked value. When I measured the voltage across a couple of the valve heaters I was getting about 10.5 across the 12v heaters and around 32V across the 35V heaters. I remember putting a couple of 0.1uF caps pinched from a couple of old power supplies in parallel across the 2.2uF to bring it up to.....2.2uF! That seemed to cure the slightly low voltage and it's been OK ever since. However after all this time, I might just check to see if there has been any change in the total capacitance although probably not enough to make a huge difference. The capacitance is likely to reduce over time anyway.

Station X · 14th Nov 2021, 1:33 pm

Quote:

Originally Posted by Radio Tech

Hello Jay

I admire the vast work you have done on this set so far, but you may have found it easier to use axial electrolytic capacitors in your post #122 last picture, this is not a criticism but just my thoughts.

Ken

You can't use electrolytic capacitors in a capacitive dropper as they're polarised and unsuitable for use with AC.

Personally I'd use motor run caps.

Simon Gittins · 14th Nov 2021, 1:40 pm

Post #122 last picture shows the electrolytic HT capacitors though.

Radio Scotland · 14th Nov 2021, 2:07 pm

Quote:

Originally Posted by Radio Tech

Hello Jay

I admire the vast work you have done on this set so far, but you may have found it easier to use axial electrolytic capacitors in your post #122 last picture, this is not a criticism but just my thoughts.

Ken

Hi ken, No all comments are welcome, I like to hear from the guys that really know their stuff. I'm just a hobbyist and love all things radio related, And working on this is a real labour of love, And has been a real learning curve.

Yeah they're not ideal, But i'm trying to keep the cost as low as possible.

Cheers
Jay

Radio Scotland · 14th Nov 2021, 2:10 pm

Quote:

Originally Posted by Simon Gittins

Post #122 last picture shows the electrolytic HT capacitors though.

Aye simon ken is referring to the HT Smoothing caps being radials and not the original axial type

Jay

Radio Scotland · 14th Nov 2021, 2:13 pm

Quote:

Originally Posted by Sideband

I had a similar problem with my Pilot Model 10 about 12 years ago when I modified it to capacitor dropper. In that case I used a capacitor from a florescent lighting unit which was slightly low from its marked value. When I measured the voltage across a couple of the valve heaters I was getting about 10.5 across the 12v heaters and around 32V across the 35V heaters. I remember putting a couple of 0.1uF caps pinched from a couple of old power supplies in parallel across the 2.2uF to bring it up to.....2.2uF! That seemed to cure the slightly low voltage and it's been OK ever since. However after all this time, I might just check to see if there has been any change in the total capacitance although probably not enough to make a huge difference. The capacitance is likely to reduce over time anyway.

Sounds promising sideband, Mistake i made was buying 20% tolerance caps, And it backfired on me

It shall return from the grave or put me in it

Jay

Radio Scotland · 14th Nov 2021, 5:08 pm

Few odd jobs today whilst waiting on the dropper cap.

Volume on/off switch stripped and cleaned, on/off was a bit flakey, But after a good dose of switch cleaner it seems to be behaving. Volume tracking ok, but original calls for 250k, This is 650k. Don't think it's original, Chunky robust piece of gear.

Valve shield fell off, Re-stuck with epoxy putty, Grid caps and clips cleaned. Time for

now.

Next project will be an easier one, Like restoring the titanic.

Cheers
Jay

FERNSEH · 14th Nov 2021, 5:24 pm

Checked the AC voltages in my Little Maestro, Mains voltage today is 238V.
The voltage at the top of the heater chain is 65V, should 70volts so the valves are slightly under-run. The voltage measured across the heater dropper capacitor is 228V.
Nevertheless, the set is working perfectly and is receiving all five of the local medium waveband stations with no aerial. With 10 feet of wire the set is very lively.
The 4 microfarad RS Components capacitor has the stock number 388-7686.
Correction to the valve line-up: the output valve is a 25A6G and the rectifier an Osram U31.
Checked the value of an unused new stock RS capacitor and the figure is 3.94 microfarads.

DFWB.

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13th Nov 2021, 8:29 pm	#142
G.Castle Heptode Join Date: Dec 2020 Location: Swaffham, Norfolk, UK. Posts: 587	Re: Pilot Little Maestro restoration project. Hello Jason. Read this: https://www.cool386.com/dropper/dropper.html Capacitor manufacturing can be done very precisely, and knowing that you will probably find your dropper capacitance is lower than you think, they don't give you anything they don't have to! You can pretty much guarantee that a given value component will be on the low side of marked value, complain and the manufacturer will quote tolerances at you, series limiter resistors or parallel bleeder resistors will make little or no difference in a reactive dropper. If you're just a little under the chain current then just top it up with parallel capacitance.

13th Nov 2021, 8:34 pm	#143
Simon Gittins Heptode Join Date: Jan 2006 Location: Cornwall, UK. Posts: 506	Re: Pilot Little Maestro restoration project. You are getting somewhere now, I think you just need to slightly increase the value of the dropper capacitor. The dropper calculation page has a useful spreadsheet. For a 75V chain at 0.3A the capacitor value should be 4.19uF without a surge limiter resistor or 4.29uF with a surge limiter of 50 ohms (surge limiter drop set to 15V on the spreadsheet). As you were reading about 3.8uF for your two 2.2uF capacitors in parallel it seems they are slightly low in value. Do you have another Class X capacitor of about 0.4uF you can put in parallel with them? Just seen the above, looks like we have the same suggestion. Last edited by Simon Gittins; 13th Nov 2021 at 8:35 pm. Reason: Addition.

13th Nov 2021, 8:46 pm	#144
Radio Scotland Heptode Join Date: Jul 2020 Location: Larkhall, South Lanarkshire, Scotland. Posts: 671	Re: Pilot Little Maestro restoration project. Thanks for the input folks, My 2 X 2.2uf safety dropper caps are 20% tolerance, Hence i'm low (3.8uf) Iv'e put an advert in the wanted section for a 0.4uf 300volt safety cap. to bring me up to 4.2uf, i could buy one, but then the same issue, might be sitting low in value. Hopefully if i get one on here the person can measure the value for me so i know its suitable. Scrapped the above idea, I'll just by a new one jay __________________ Help!! I've been electrocuted by a glass thing that glows Last edited by Radio Scotland; 13th Nov 2021 at 9:12 pm. Reason: Extra info

14th Nov 2021, 1:15 am	#145
Radio Scotland Heptode Join Date: Jul 2020 Location: Larkhall, South Lanarkshire, Scotland. Posts: 671	Re: Pilot Little Maestro restoration project. Just to mention. The voltage at turn on is about 30 volts and steadily climbs up and settles at 56v __________________ Help!! I've been electrocuted by a glass thing that glows

14th Nov 2021, 11:03 am	#147
Radio Scotland Heptode Join Date: Jul 2020 Location: Larkhall, South Lanarkshire, Scotland. Posts: 671	Re: Pilot Little Maestro restoration project. Iv'e ordered up a 0.47uf - X2 capacitor, If that doesn't bring the voltage up then the project may need to be shelved, Can't keep throwing money at it, And theres no saying the valves are any good, And a full set which its unlikely to need would be around £100. And that would be madness. Anyhow the cap will take a few days to arrive so i'll update when its been fitted Cheers Jay __________________ Help!! I've been electrocuted by a glass thing that glows Last edited by Radio Scotland; 14th Nov 2021 at 11:33 am.

14th Nov 2021, 12:25 pm	#148
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Pilot Little Maestro restoration project. The Little Maestro I serviced two years ago employs a 4microfarad motor start capacitor to supply the 0.3amp heater chain. Valve line-up in this set is: 6K8G, 6K7G, 6Q7G, 25L6 and 25Z6. I remember the calcs for the capacitor worked to be 4.4mfd but the 4mfd capacitor works fine. https://www.vintage-radio.net/forum/...d.php?t=161165 DFWB.

14th Nov 2021, 12:56 pm	#150
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Pilot Little Maestro restoration project. It's all down to tolerance in the capacitor values. Components made nowadays are not specifically designed for use in valved radio receivers, although they are used for that purpose. They are definitely not designed for use as droppers in valved radio receivers, but once again they can be used for that purpose. With the extra 0.47uF in circuit the voltage will increase. So long as it's within 5% of the specified figure you'll be fine. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

14th Nov 2021, 1:25 pm	#152
Radio Tech Nonode Join Date: Dec 2012 Location: Bocking, near Braintree, Essex, UK. Posts: 2,071	Re: Pilot Little Maestro restoration project. Hello Jay I admire the vast work you have done on this set so far, but you may have found it easier to use axial electrolytic capacitors in your post #122 last picture, this is not a criticism but just my thoughts. Ken

14th Nov 2021, 1:28 pm	#153
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,578	Re: Pilot Little Maestro restoration project. I had a similar problem with my Pilot Model 10 about 12 years ago when I modified it to capacitor dropper. In that case I used a capacitor from a florescent lighting unit which was slightly low from its marked value. When I measured the voltage across a couple of the valve heaters I was getting about 10.5 across the 12v heaters and around 32V across the 35V heaters. I remember putting a couple of 0.1uF caps pinched from a couple of old power supplies in parallel across the 2.2uF to bring it up to.....2.2uF! That seemed to cure the slightly low voltage and it's been OK ever since. However after all this time, I might just check to see if there has been any change in the total capacitance although probably not enough to make a huge difference. The capacitance is likely to reduce over time anyway. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

14th Nov 2021, 1:40 pm	#155
Simon Gittins Heptode Join Date: Jan 2006 Location: Cornwall, UK. Posts: 506	Re: Pilot Little Maestro restoration project. Post #122 last picture shows the electrolytic HT capacitors though.

14th Nov 2021, 5:08 pm	#159
Radio Scotland Heptode Join Date: Jul 2020 Location: Larkhall, South Lanarkshire, Scotland. Posts: 671	Re: Pilot Little Maestro restoration project. Few odd jobs today whilst waiting on the dropper cap. Volume on/off switch stripped and cleaned, on/off was a bit flakey, But after a good dose of switch cleaner it seems to be behaving. Volume tracking ok, but original calls for 250k, This is 650k. Don't think it's original, Chunky robust piece of gear. Valve shield fell off, Re-stuck with epoxy putty, Grid caps and clips cleaned. Time for now. Next project will be an easier one, Like restoring the titanic. Cheers Jay Attached Thumbnails __________________ Help!! I've been electrocuted by a glass thing that glows Last edited by Radio Scotland; 14th Nov 2021 at 5:15 pm.

14th Nov 2021, 5:24 pm	#160
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Pilot Little Maestro restoration project. Checked the AC voltages in my Little Maestro, Mains voltage today is 238V. The voltage at the top of the heater chain is 65V, should 70volts so the valves are slightly under-run. The voltage measured across the heater dropper capacitor is 228V. Nevertheless, the set is working perfectly and is receiving all five of the local medium waveband stations with no aerial. With 10 feet of wire the set is very lively. The 4 microfarad RS Components capacitor has the stock number 388-7686. Correction to the valve line-up: the output valve is a 25A6G and the rectifier an Osram U31. Checked the value of an unused new stock RS capacitor and the figure is 3.94 microfarads. DFWB.