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

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Audio (record players, hi-fi etc)

Notices

Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

Closed Thread
 
Thread Tools
Old 29th Apr 2019, 3:40 pm   #41
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

Quote:
Originally Posted by Radio Wrangler View Post
I'd go for testing the living daylights out of both transformers to look for any clues to differences.
Definitely!

Start with the easy things - DC resistance of all windings (even that can be tricky to measure with autoranging DMM's).

Then leakage inductances, secondaries open-circuit, but one anode shorted to HT+ (centre tap), two readings per transformer. 10kHz is probably a good frequency to measure at

Primary inductance is rather indeterminate for iron-cored transformers - you can get 2:1 variation in a figure easily just by using a different measuring level. So stick the whole primary across the mains, and measure magnetising current for each transformer.

Whatever the transformer issue is, there has to be a mechanism for oscillation to take place. To me, the feedback MUST be via the screen-grid taps, so that's another load of measurements to take!

One thing not answered - does it still oscillate with no load applied? Because if Yes, the loudspeaker secondary (and coupling thereto) can be ignored in all of this, saving yet more measurements!
kalee20 is online now  
Old 29th Apr 2019, 4:19 pm   #42
Valvepower
Octode
 
Valvepower's Avatar
 
Join Date: Aug 2008
Location: Rayleigh near Southend-On-Sea, Essex, UK.
Posts: 1,852
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

Quote:
Originally Posted by PJL View Post
Ageing or overheating could change leakage capacitance. I also wonder what effect a shorted turn would have. I still suggest you try measuring the attributes of the transformers to see if there are any differences.
Hello,

Interesting saying that as I was thinking the same thing with aging and overheating.

Back in the very, very distant past in the guitar amp days we used to get the occasional amplifier where the amp just sounded flat and lifeless – after valves and components have been checked we’d turn to the output transformer – we’d replace the transformer and the amp came back to life – this was back when we had replacement transformers, plus we had a shelf of typical output transformers we could substitute the suspected faulty one with.

We had limited test gear so doing any in depth analysis was out the question.

Ron and myself put these transformers down to extended use into totally incorrect loads and this use “cooked” the transformer – so to speak. This is very, very un-scientific, but it seemed to ring true for some transformers.

Regards
Terry
Valvepower is offline  
Old 29th Apr 2019, 6:03 pm   #43
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

I'm afraid I'm well off my patch when it comes to proper transformer measurements, and not really equipped for it I fear. My primitive first stabs show the two transformers to be pretty similar.

Using a cheapie commercial inductance meter (Tenma 72-8155 https://cpc.farnell.com/tenma/72-815...lcr/dp/IN05113) and shorting the ends of the secondaries together gave leakage inductances of 9.9mH (good trans) and 8.6mH (bad trans) for the full primaries. Putting a 22nF cap and a 1k resistor in series with the full primary and varying the audio drive frequency across the whole lot while monitoring the signal across the resistor gave a resonant frequency around 10.8kHz (good trans) which again indicates 9.9mH for the primary inductance. The agreement with the Tenma was reassuringly good.

Trying to measure the primary inductance with the secondary open-circuit was more challenging. A few web pages warn of the confusing effects of stray winding capacitances. The Tenma meter took ages (and ages !) to settle, eventually indicating a little over 11H for each transformer. The resonant frequency technique, still using the 22nF cap but now with a 6k8 resistor, also took a long time to settle each time I changed the audio frequency (any ideas what's going on there ?) but the resonances seemed to peak around 210-230Hz in both transformers. They were broad and the peak frequency seemed to vary with the choice of series resistor, and waiting time, and maybe voltage. But the results corresponded to 22-26H. Now the agreement with the Tenma is not good.

Attached is a pic of the bad transformer with the shrouds removed. The external dimensions of the laminations are 4.5" x 3.75" and the stack of them is 1.5" thick. The windings are wax-impregnated. Does 10-20H sound about right (somehow I thought it might be more) ?

I put ~120V pk-pk at 50Hz across the primary winding and measured the voltages on the tappings of the transformers. As precisely as the automated measurements in my 8-bit HP scope (16-trace average) could tell the centre-taps were at exactly half voltage and the screen taps were at 50% and 49.2% of their respective half-windings (bad transformer) and at 50% and 50% (good transformer). Steven Spicer says, in Firsts In High Fidelity, that the tappings were specified to be at 49%+/-2%. He also says that the anode-anode impedance was specified at 5k2. Using measurements of the secondary voltages and working out the turns ratios from them I found 5k2 to be the reflected impedance for the 4ohm and 16ohm secondary tappings. But putting an 8ohm load onto the 8ohm tapping would give just 4k4 anode-anode. The load would need to be 9.4ohms to get 5k2. This is anomalous, but it was identical for both transformers.

I then put 7.5mA DC through the primary and measured the voltages across the sections. In the good transformer the resulting resistances were 9.6ohms, 12.4ohms, 11.5ohms and 14.4ohms. In the bad one they were 9.4ohms, 12.1ohms, 11.3ohms and 14.3ohms. The overall resistances (end-to-end) were 47.8ohms and 47.1ohms respectively, which compare well with Steven's figure of 45+/-10ohms. It sems the wire usd to wind the bad transfiormer might have been very slightly thicker than that used for the good.

The last measurement I managed today was the capacitance between the centre taps of the primary and secondary windings, again measured with the Tenma meter. It was 1.82nF in the good transformer and 1.75nF in the bad one.

To be honest I can't see any difference between these transformers on the basis of these measurements, nor can I see any indication of gross faults (large internal shorts etc). However it may be worth noting that all the measurements were carried out with relatively low voltages and currents and no large DC currents (typically 90mA under normal operation) flowing in the primary.

Cheers,

GJ
Attached Thumbnails
Click image for larger version

Name:	Leak TL50 Plus output transformer small.jpg
Views:	122
Size:	53.4 KB
ID:	182109  
__________________
http://www.ampregen.com

Last edited by GrimJosef; 29th Apr 2019 at 6:15 pm.
GrimJosef is offline  
Old 29th Apr 2019, 6:12 pm   #44
G8HQP Dave
Rest in Peace
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

Could it be the slight imbalance in the screen taps on the bad transformer? I would be surprised if such a small problem caused instability, but imbalance can convert common-mode currents to differential-mode voltages.
G8HQP Dave is offline  
Old 29th Apr 2019, 6:31 pm   #45
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

To be honest it's hard to be sure whether that difference is real. I was measuring with an 8-bit scope and exciting the primary with 50Hz from a synthetic (UPS) mains supply since making precision measurements using the real mains is a good deal tougher (even then, I was monitoring the excitation voltage with the other scope probe).

Cheers,

GJ
__________________
http://www.ampregen.com

Last edited by GrimJosef; 29th Apr 2019 at 6:40 pm.
GrimJosef is offline  
Old 29th Apr 2019, 7:10 pm   #46
John_BS
Octode
 
John_BS's Avatar
 
Join Date: Dec 2008
Location: Wincanton, Somerset, UK.
Posts: 1,757
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

GJ: My gut reaction, having read all your measurement results, is that the problem is related to a fault with the electrostatic screening in the "problem" transformer. I believe high quality audio transformers are normally wound with the windings sectioned and interleaved to reduce leakage inductance, so the screen will not be one simple layer.
John
John_BS is offline  
Old 29th Apr 2019, 8:48 pm   #47
bikerhifinut
Octode
 
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,993
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

For what it's worth GJ, I think Majestic in Poole have the winding info to rewind/manufacture Leak output transformers, they certainly can make you a set of QuadII transformers as my memory serves and I think they do a service for replacement trafos for various classic amps. Sowter may be worth a punt too, if your customer has deep pockets.
I have no experience of Majestics output jobbies, I did have issues with a set of mains trafos that I had custom wound though, but the problems i encountered wouldnt apply to output trafos.
I am fairly sure Mike B has some knowledge of leak output trafos too, may be worth an email to Mike?
Regards.

Andy.
bikerhifinut is offline  
Old 29th Apr 2019, 9:23 pm   #48
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

One thing Kalee20 said back up-thread a bit was that perhaps you didn't have a 92kHz oscillation as such, but possibly a higher frequency oscillation that was squegging at 92kHz.

If that was the case it may be a matter of stray capacitances in the transformers. High-frequency capacitances.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 29th Apr 2019, 9:52 pm   #49
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

I can't rule out an RF component to the oscillation. There is certainly much higher frequency structure on the scope waveform sometimes. But this seems to come and go as consecutive scope frames are captured. This is the major failing of digital scopes of course - the loss of higher-frequency info at low sampling rates. My HP3561A audio spectrum analyser shows a strong, pretty stable high-Q peak at 92kHz though and the HP8903A analyser also indicates 92kHz with a corresponding amplitude. I tried a few scope frames at 100ns/div and while there was structure there on a slowly-varying (92kHz) background its amplitude seemed quite small.

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline  
Old 29th Apr 2019, 10:00 pm   #50
John_BS
Octode
 
John_BS's Avatar
 
Join Date: Dec 2008
Location: Wincanton, Somerset, UK.
Posts: 1,757
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

Re post 46 above; if possible measure some capacitances: primary / screen(s) / secondary. Also examine physically as I'd expect screen bonding to happen across the layers at the end of the windings.

John
PS if you have a meter with a guard terminal, connect to core.
John_BS is offline  
Old 29th Apr 2019, 10:28 pm   #51
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

another measurement would be the AC current when you apply your 120V AC to the primary.
PJL is offline  
Old 30th Apr 2019, 9:54 am   #52
Valvepower
Octode
 
Valvepower's Avatar
 
Join Date: Aug 2008
Location: Rayleigh near Southend-On-Sea, Essex, UK.
Posts: 1,852
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

Hello,

I was reading the Steve Spicer book in bed last night (as one does) thinking about GJ’s problem and I wonder if there is a clue in the fact Leak changed the design of the transformer in the early in the life of the TL12/25+ amplifiers, but the design of the TL50+ stayed the same throughout it’s life.

From what I can remember of the Gilson & Leaky UL article in WW the earlier design of Leak transformer was more prone to instability, whereas the latter design was less so – I may be wrong though.

The question is, was the TL50 output transformer constructed like the early TL12/25+ transformer…?

This amplifier could have left the factory right on the edge – so to speak – and was marginally stable with no perceivable oscillation – well as could be detected back in the late 1950’s early 1960’s with fairly test basic bench test gear. And after some hard use, as normally found with this amplifier, the transformer “changed” properties – for want of a better phrase – and the amplifier is now hooting at HF.

Ps. I've always struggled to get the quoted THD figures for the TL50 – from memory I don't think I ever have

Regards
Terry

Last edited by Valvepower; 30th Apr 2019 at 10:11 am. Reason: triping errot
Valvepower is offline  
Old 30th Apr 2019, 1:14 pm   #53
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

Quote:
Originally Posted by Radio Wrangler View Post
One thing Kalee20 said back up-thread a bit was that perhaps you didn't have a 92kHz oscillation as such, but possibly a higher frequency oscillation that was squegging at 92kHz.
Not this Kalee20, someone else deserves the credit for that thought! It's a good suggestion. scoping the waveforms will give a clue - squegging is likely to show a relaxation-oscillation sort of waveform rather than a sinewave.

Quote:
Originally Posted by GrimJosef View Post
Trying to measure the primary inductance with the secondary open-circuit was more challenging. ... The Tenma meter took ages (and ages !) to settle, eventually indicating a little over 11H for each transformer. The resonant frequency technique, still using the 22nF cap but now with a 6k8 resistor, also took a long time to settle each time I changed the audio frequency (any ideas what's going on there ?) but the resonances seemed to peak around 210-230Hz in both transformers. They were broad and the peak frequency seemed to vary with the choice of series resistor, and waiting time, and maybe voltage. But the results corresponded to 22-26H. Now the agreement with the Tenma is not good.
I'm not surprised. Iron is pretty non-linear in its magnetising characteristics (which is why tape recorders need bias oscillators). It's like trying to measure Young's modulus of elasticity for chewing gum. The value you get depends on recent history, measuring levels, frequencies. Luckily it doesn't usually matter as long as inductance is high enough. It's why I suggested measuring current drawn with AC mains applied to the whole primary, at least you are hitting the transformer with voltages likely to be seen in practice.

Quote:
Originally Posted by GrimJosef View Post
Does 10-20H sound about right (somehow I thought it might be more)?
I would, too - 5.2k anode-anode load and 10H gives a LF cut-off frequency of 83Hz, not exactly hi-fi. I'd expect nearer 20Hz (40H). And add that NFB is wrapped around the whole thing, for stability you'd want even more inductance. But as suggested, measure current drawn from the mains, with O/C secondary, and calculate effective inductance from that.

Quote:
Originally Posted by GrimJosef View Post
To be honest I can't see any difference between these transformers on the basis of these measurements, nor can I see any indication of gross faults (large internal shorts etc). However it may be worth noting that all the measurements were carried out with relatively low voltages and currents and no large DC currents (typically 90mA under normal operation) flowing in the primary.
I can't, either.

A large internal short should show up by hitting with AC mains for a while. It won't hurt a good transformer (let's face it, at full power you'd expect to see 500V a-a)

DC currents flowing ought not to make any difference other than generating a bit of heating.

I'm still curious about the actual oscillation mechanism! You say that disconnecting the grid capacitors doesn't stop it oscillating, but sticking a probe on the grids does. That may because the probe is picking up, capacitively, some of the oscillation and applying it out-of-phase hence neutralising the circuit. What happens if you hard-wire the grids to deck?

And what happens if you strap the anodes?

And does it still oscillate with O/C load (not the nicest thing to do of course, so if you can contrive to reduce HT voltage, peak voltages are less likely to be damaging).

As measurements on the transformer have failed to give a clue, it's got to be back to the circuit as a whole. Once the mechanism for oscillation is understood, more subtle measurements on the transformer might pinpoint the difference.
kalee20 is online now  
Old 30th Apr 2019, 4:16 pm   #54
crackle
Rest in Peace
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 4,100
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

Hi GJ
I am well out of my depth on this, but did you check the connection to the OP trans screen.

Mike
crackle is offline  
Old 30th Apr 2019, 6:57 pm   #55
Marconi_MPT4
Heptode
 
Marconi_MPT4's Avatar
 
Join Date: Mar 2011
Location: Dorset, UK.
Posts: 519
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

Hi,
Interesting problem, and in the past have found that placing a damping (stopper) resistor in the anode circuit as close as possible to the pin would prevent some types of parasitic oscillation with beam tetrodes. Something in the order of 22 to 100 ohms. Entirely possible to use ferrite beads slipped over the anode wire.

Another I have seen but not tried is to wire a capacitor and resistor in series between anode and g2 for each valve. Typical values might be 1k and 1nF.

Cheers
Rich
__________________
To an optimist a glass is half full; a pessimist half empty; an engineer twice as big as need be!
Marconi_MPT4 is offline  
Old 30th Apr 2019, 8:09 pm   #56
G8HQP Dave
Rest in Peace
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

I had to use an anode-g2 snubber on my 'nearly 5-20' with a Sowter transformer.

A KT88 is a TT21 transmitter valve without a top cap so it is quite capable of singing at RF. Squegging at 92kHz is possible, as I suggested, although this frequency ought to change with circuit changes. If it seems very stable, being either present at 92kHz or missing altogether, then squegging is less likely as an explanation.
G8HQP Dave is offline  
Old 30th Apr 2019, 10:03 pm   #57
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

Sorry for the recent silence on this. I was committed to another job today. But I should get back to it tomorrow afternoon, all being well.

There's been some talk of screens (in the sense of interwinding screens, I assume). I haven't seen signs of there being any. All of the wires leaving the windings appear to be connected either to primary or secondary sections. There is no external terminal labelled S as there commonly is on mains transformers. Is the photo in post #43 detailed enough to show that there are or aren't such screens ?

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline  
Old 30th Apr 2019, 11:31 pm   #58
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

Quote:
Originally Posted by kalee20 View Post
I'm not surprised. Iron is pretty non-linear in its magnetising characteristics (which is why tape recorders need bias oscillators). It's like trying to measure Young's modulus of elasticity for chewing gum. The value you get depends on recent history, measuring levels, frequencies. Luckily it doesn't usually matter as long as inductance is high enough. It's why I suggested measuring current drawn with AC mains applied to the whole primary, at least you are hitting the transformer with voltages likely to be seen in practice.

Quote:
Originally Posted by GrimJosef View Post
Does 10-20H sound about right (somehow I thought it might be more)?
I would, too - 5.2k anode-anode load and 10H gives a LF cut-off frequency of 83Hz, not exactly hi-fi. I'd expect nearer 20Hz (40H). And add that NFB is wrapped around the whole thing, for stability you'd want even more inductance. But as suggested, measure current drawn from the mains, with O/C secondary, and calculate effective inductance from that.
AC current is the one measurement that has not been done yet.

Not sure I agree with your cut-off calculation (but I am often wrong!) as the winding impedance will be in parallel with the load and the valve is not linear so you would need to look at the load line to work out the impact.
PJL is offline  
Old 1st May 2019, 6:13 am   #59
joebog1
Banned
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

When I test output transformers I use AC coupling from my AC generator ( mains if your brave ) through a BIG capacitor, say 1uF. I then hang off the primary also a couple of AA batteries DC coupled, so that I can "magnetise" the core. Using your CRO connected to output windings, vary the DC applied to the primary through a 100 or 220 ohm WW pot.
Obsrve the output waveform. Take into account any already existing distortion from your mains. ( even if synthesized as you say). If nothing "major" is noted, try tetsing BOTH transformers similarly, but inverting one CRO input to get a +/- additive trace, and compare both transformers.

The very small DC bias you can apply can upset some "very expensive" transformer specs..
Take a music waveform!! its the most complex imaginable, especially if its full orchestral sound.

The small DC bias can upset an unstable transformer by "bending" the waveform.
You are obviously very thorough in your approach, so an extra ten minutes to apply a DC bias is not hard or time consuming to do. Changing the polarity of the DC bias can change your results as well.

AS an aside, since Leak made transformers there have been any number of advances in transformers. Iron, Insulation, interleaving methods, interleaving insulation ( although paper remains the best in my experience and my choice)
even interleaving of the iron itself. Stacking three laminations in one step I always thought was "cheap skate" engineering, BUT in some cases it increases inductance at high frequencies and reduces leakage inductance. ( my measurements, although I cant prove it)

I wish you well and await your results

Joe
joebog1 is offline  
Old 1st May 2019, 9:43 am   #60
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: Cause of ultrasonic oscillation in a push-pull output stage ?

Quote:
Originally Posted by PJL View Post
Not sure I agree with your cut-off calculation (but I am often wrong!) as the winding impedance will be in parallel with the load and the valve is not linear so you would need to look at the load line to work out the impact.
You're not wrong. The calculation I gave was fairly basic, it is valid if the valves are infinite-impedance current generators and you can then consider the load in parallel with the transformer's self-inductance, and nothing else.

The fact that the valves are operating in UL ('partial triode') to an extent nullifies this and I ignored it. However, the answer coming so way off what I'd expect for a hi-fi amplifier does tie in with GrimJosef's thoughts, that he'd expect rather more inductance.
kalee20 is online now  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 2:51 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 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.