UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Closed Thread
 
Thread Tools
Old 21st Jan 2006, 10:31 pm   #1
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,843
Default Motorboating

I am part-way through the process of building my TRF radio and as I said I have completed the AF side first.
I switch on and after the HT has come up the thing is motorboating.
I think that is the right term. Even with no input it sounds like a boat on a lake.
The amplifier is a ECL80. I applied a signal from my record turn table to the grid of the triode section, via its coupling capacitor (0.01uf) and the volume would rightly be described as loud. However the motorboating doesn't go away when I apply the input signal and it distorts the audio of course so Status Quo sounds like a Dalek convention.

This is my first valve powered project. I have probably done something really silly but tell my why the amplifier would do this thing and i'll go look for the likely culprit.
Neil Purling is offline  
Old 21st Jan 2006, 10:37 pm   #2
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 4,978
Default Re: Motorboating

Hi Neil, look at your layouts to see that the inputs and outputs are not close together. Look at how you make common connections to chassis (star point earthing is a good idea.
Try the effect of a 1uF paper cap of suitable voltage across the main e-caps. Check what actual value the grid leak resistors are.

HTH Ed
Ed_Dinning is offline  
Old 21st Jan 2006, 10:56 pm   #3
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,843
Default Re: Motorboating

Star point earthing? Do tell me what that is.
How much distance should I have between input & output? I assume it is a matter of seperating the wires if any are in close proximity. Would using shielded cable help?
As to the HT supply: Please look at the attached image. This is what I have done.

So where would I put the 1uf capacitor. I would use a polypropylene type, you can't get a paper capacitor these days.
Attached Thumbnails
Click image for larger version

Name:	TRF Power Supply.gif
Views:	234
Size:	15.7 KB
ID:	2543  
Neil Purling is offline  
Old 21st Jan 2006, 11:50 pm   #4
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,843
Default Re: Motorboating

By the way I am drawing HT to the anode of the pentode section with a wire to the first electrolytic.
In a previous thread on TRF's it was suggested so as to save undue stress on the 1K smoothing resistor.
Do you think this has made any difference?
I wonder of Hunts Smoothing Bomb, Heatercathodeshort and Adibrook had problems relating from the wiring under the chassis?
Neil Purling is offline  
Old 22nd Jan 2006, 12:17 am   #5
yestertech
Octode
 
yestertech's Avatar
 
Join Date: Dec 2002
Location: Coulsdon, Surrey, UK.
Posts: 1,466
Default Re: Motorboating

Hi Neil, the additional cap would go across the 47 mF, after the 1K smoothing resistor. The effect you hear is caused by stray coupling, either in the HT -ve ( chassis ) wiring, or in the HT +ve connections.
It is usually exacerbated by high gain circuitry and can be cured by careful layout and perhaps additional HT decoupling.
If the anode load resistor for the ECL80 triode section is returned directly to the same HT point as the pentode section ( a & g2 ) , try inserting an additional 10k resistor between them, and adding a 4.7mF to ground at the junction. This usually works.
Also, check that the earthing points you have used are good low resitance paths. If using tags, don't forget to insert a star washer between tag and chassis to improve contact, and bolt down tight.
If the chassis is coated, scrape away some coating before doing this.
Lastly, you may find the effect is made worse by excessive frequency response. Try inserting say, a 4k7 resistor between the coupling cap to the ECL80 pentode section grid and its grid leak. This will roll off the frequency response to prevent unwanted hi frequencies interfering with correct operation.
One , or a combination of the above is usually guaranteed to cure the problem.
ANDY
yestertech is offline  
Old 22nd Jan 2006, 7:40 am   #6
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,843
Default Re: Motorboating

Here's the amplifier stage.
The 1 Meg triode grid resistor: I have a 1 Meg pot to serve as a volume control. Would you reccomend any extra resistances. I was going to connect the top of the pot to pin 2, with the wiper to thejunction of the 220P & 150R cathode resistors.
Any other changes anyone would like to suggest, in the interests of stability?
Attached Thumbnails
Click image for larger version

Name:	1_5 watt amplifier.gif
Views:	451
Size:	13.2 KB
ID:	2544  
Neil Purling is offline  
Old 22nd Jan 2006, 10:56 am   #7
Heatercathodeshort
Dekatron
 
Heatercathodeshort's Avatar
 
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 6,265
Default Re: Motorboating

Hello Neil,
Interesting that you have motorboating with your ECL80 circuit. The ECL80 with its common cathode tends to be a bit unstable unless everything is in place. I spent a long while recently with the same problem but it was a Ferguson 988T television that was the problem. The chassis was out of the cabinet and I was using the bench speaker and output transformer as a monitor. After many frustrating attempts to stabilize the output stage I discovered it was the output transformers incorrect matching that was the cause of the problem.....Substituting the original cured the problem. Your circuit should work well and the previous suggestions regarding a common earth point and layout should cure your problem. The little ECL80 can pack a punch for one so small. If its any consolation I remember building a TRF set when I was about 10. I put the anode coil UNDER the chassis and the grid coil ON TOP of it and then puzzled for hours as to why the signals were SO weak..! The cog then fell in and I realized you had to wind the two coils on the SAME former....It comes with practise! Regards. JOHN.

Last edited by Heatercathodeshort; 22nd Jan 2006 at 11:02 am.
Heatercathodeshort is offline  
Old 22nd Jan 2006, 11:54 am   #8
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 4,863
Default Re: Motorboating

Quote:
Originally Posted by Neil Purling
By the way I am drawing HT to the anode of the pentode section with a wire to the first electrolytic.
In a previous thread on TRF's it was suggested so as to save undue stress on the 1K smoothing resistor.
Do you think this has made any difference?
Hi Neil. Initially I would not do this. Concentrate on getting the circuit working as it is drawn. You can always play around with it later. Usually when the supply to the pentode is taken from the reservoir, this is taken to a tap on the primary of the output transformer which serves to cancel out the higher level of hum that would otherwise be heard. This is only an ECL80..a relatively low power valve so if you use a 5 watt resistor for smoothing, it should run only slightly warm.


Rich.
__________________
'And, in the end, the love you take is equal to the love you make'
The Beatles.
Sideband is offline  
Old 22nd Jan 2006, 12:25 pm   #9
mickjjo
Rest in Peace
 
mickjjo's Avatar
 
Join Date: Jul 2004
Location: Dartford, Kent, UK.
Posts: 1,662
Default Re: Motorboating

Hi Neil, Most of the ECL80 circuits I have seen use a higher value of cathode bypass condenser, so it may be worth trying 470- 1000 uf in place of 47 uf. Also many designs use a resistor between the 2 anodes for a measure of negative feedback to help stability, something between 330k- 1meg should help. Finally try the effect of connecting one side of the speaker to chassis, as leaving the transformer secondary "floating" can cause instability in some cases..

Regards, Mick.
mickjjo is offline  
Old 22nd Jan 2006, 12:44 pm   #10
Aerodyne
Octode
 
Aerodyne's Avatar
 
Join Date: Jun 2004
Location: Hampton Vale, Peterborough, UK.
Posts: 1,629
Default Re: Motorboating

Failing all else, you could try fitting grid 'stopper' resistors in series with the control grids - about 1k in value might help. I also feel that, contrary to other posts on this topic, you could try dispensing with the cathode bypass electrolytic completely as this would provide negative feedback and so should help stability - at the cost of some output power.
In the past I too have had problems with instability with projects using the ECL80 and I put these down to the common cathode.
Good luck!
-Tony
Aerodyne is offline  
Old 22nd Jan 2006, 6:56 pm   #11
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,843
Default Re: Motorboating

I have not been able to decouple the amplifier stages because its difficult to get high voltage non-polarised caps locally.
Maplins!
I want non-polarised electrolytics, yes?
I note that putting a 1000uf capacitor as cathode bypass slows the motorboating down, but even 2000ufwon't stop it.
Going the other way, removing all cathode bypass capacitors makes the thing into some sort of siren!

Nowt else I can do untill I get the capacitors.
Watch this space.
Next one I use a seperate triode & pentode!
Neil Purling is offline  
Old 23rd Jan 2006, 10:47 am   #12
Aerodyne
Octode
 
Aerodyne's Avatar
 
Join Date: Jun 2004
Location: Hampton Vale, Peterborough, UK.
Posts: 1,629
Default Re: Motorboating

I don't understand your requirement for non-polarised electrolytics.
However, it seems to me that you have built a neat little oscillator, the coupling from one anode to the other grid being possibly via the HT+ line. Ensure that the 3k3 part anode load is the correct value and that the 22uF decoupler is in place. Assuming they are, try increasing the 3k3 value. Try parallelling this cap with a lower value paper type, say 0.1uF. At this point, its down to guesswork for all of us.
-Tony
Aerodyne is offline  
Old 23rd Jan 2006, 11:41 am   #13
Sean Williams
Dekatron
 
Sean Williams's Avatar
 
Join Date: Feb 2005
Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW
Posts: 3,165
Default Re: Motorboating

Quote:
Originally Posted by Neil Purling
Here's the amplifier stage.
The 1 Meg triode grid resistor: I have a 1 Meg pot to serve as a volume control. Would you reccomend any extra resistances. I was going to connect the top of the pot to pin 2, with the wiper to thejunction of the 220P & 150R cathode resistors.
Any other changes anyone would like to suggest, in the interests of stability?

Something does not look right here.

I would have expected the 1M resistor to return to ground on the grid of the triode, and the 330k resistor on the grid of the pentode to be lower in value - about 47k.....

Well, thats what I can see anyway, I have an ECL80 in a direct conversion RX here somewhere, I will have a look later on and try to see how that is set up....

Cheers
Sean
__________________
Engineers make things work and have spare bits when finished
Sean Williams is offline  
Old 23rd Jan 2006, 12:05 pm   #14
paulsherwin
Moderator
 
paulsherwin's Avatar
 
Join Date: Jun 2003
Location: Oxford, UK
Posts: 16,725
Default Re: Motorboating

Have a look at the circuit for the 'Baronette' on this page: http://vintageradio.me.uk/radconnav/valvetrf/

That uses an ECL80 audio stage but doesn't have the odd feedback arrangement. Try moving the 1M resistor to ground. THe Baronette circuit also places the volume control between the triode and pentode of the ECL80 which will reduce instability.

Good luck, Paul
paulsherwin is online now  
Old 23rd Jan 2006, 12:14 pm   #15
radioman
Hexode
 
radioman's Avatar
 
Join Date: Apr 2003
Location: Ayr, Ayrshire, UK.
Posts: 386
Default Re: Motorboating

Hi Neil,
Regarding the 1M volume control, I think you should connect this with the wiper connected to the triode grid - possibly via a stopper resistor. The bottom (minimum) side should go to the junction of the two cathode resistors with the top (maximum) end of the pot connected to the input coupling capacitor.
I can't see anything wrong with a value of 330k for the pentode grid resistor though
Another thing, are the wires from your power supply on the long side ?
If they are, it might help if the smoothing cap was positioned nearer the amplifier circuit.

Hope this helps,
Andy
radioman is offline  
Old 23rd Jan 2006, 8:18 pm   #16
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,843
Default Re: Motorboating

I have followed the reccomendations by member 'radioman'.
Look at the attached image. It is a modified version of the circuit I first posted.
I have managed to stabalise the amplifier by putting a resistor 'X in. The existing pentode grid resistor was left where it was. The thing doesn't motorboat now. The value is presently 2.2 Meg. I think further experimentation is warranted.

However I have been using the output from my turntable as a test signal.
What is the rated output of a Sonotone 9TA.
I would like to know if any of the TRF builders on the forum have measured the peak-to-peak signal output of the detector with a scope. I wondered how it compares to my test signal?
Attached Thumbnails
Click image for larger version

Name:	1_5 watt amplifier.gif
Views:	164
Size:	14.8 KB
ID:	2560  
Neil Purling is offline  
Old 23rd Jan 2006, 9:25 pm   #17
quantum
Heptode
 
quantum's Avatar
 
Join Date: Mar 2003
Location: Birmingham, West Midlands, UK.
Posts: 785
Default Re: Motorboating

If you've managed to get stability with an ECL80 with a cathode bypass cap of only 47uf, you've done very well indeed - normally, because of the different bias required by the triode and pentode parts of the valve, to ensure stability you need a much higher value. If you look at circuits which employ ECL80s as audio amps (such as in TV receivers and gram amps such as the Champion Rev-ler no. 805 or Radio Rev-ler no. 820) values such as 250-500uf are common. When I've used an ECL80 in the past I've employed grid bias for the triode part of the ECL80, so the bias resistor is 330 ohms and the triode grid is biased by a 10 meg resistor connected to the top of this resistor (not the ground side), and I've always used a high value cathode bias cap, 470 - 1000uf or whatever I have handy. The triode grid then connects through a small cap (0.005uf) to a the wiper of the volume control.

I once cheated with an ECL80 amp - I was making a low powered stereo set up, and I used the triode of one valve driving the pentode of the other, with 330 ohm resistors and low value bypass caps, without any trouble of instability, as the two sections of the amp were not in the same valve envelope!
quantum is offline  
Old 23rd Jan 2006, 11:14 pm   #18
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,843
Default Re: Motorboating

I have two 1000uf 15v capacitors I tried as cathode bypass in the original circuit.
It slowed the motorboating but didn't eliminate it. I would like as much gain as possible, but not so the damn thing starts it's antics.
Is the biasing wrong in the diagram I provided? I stabalised the thing by cheating with the 2 meg resistor ('X').
Could the motorboating problem be because there is too much gain?
Neil Purling is offline  
Old 23rd Jan 2006, 11:18 pm   #19
paulsherwin
Moderator
 
paulsherwin's Avatar
 
Join Date: Jun 2003
Location: Oxford, UK
Posts: 16,725
Default Re: Motorboating

Neil, why don't you try moving the volume control so it's between the triode and pentode, as in the 'Baronette' circuit? This will allow you to control the gain through the triode/pentode combination rather than having it running at maximum gain all the time.

Good luck, Paul
paulsherwin is online now  
Old 23rd Jan 2006, 11:29 pm   #20
Neil Purling
Octode
 
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 1,843
Default Re: Motorboating

I started with the not unreasonable assumption that the circuit (culled from Isoplethics Web site) would be a usable amplifier.
I did try and move the volume control to between the triode and pentode sections as suggested. That didn't seem to make much difference.
The volume control pot I have is a 1 Meg, same value as the pentode grid resistor in the Baronette circuit. Not suitable...?
Neil Purling is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 4:28 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Copyright ©2002 - 2018, Paul Stenning.