Bush VHF61 distortion at lowest volume setting - UK Vintage Radio Repair and Restoration Discussion Forum

unitelex · 27th Oct 2007, 9:57 am

My VHF61 gets a clear undistorted audio for BBC stations (reasonable modulation levels !) except when the volume control is at or close to the minimum setting.

Suspect it is something to do with the AGC?

Any clues ?

PJL · 27th Oct 2007, 11:14 am

Have you replaced all the capacitors including the little hunts ones?

If so, try it on an external speaker first....Peter

unitelex · 27th Oct 2007, 3:17 pm

Quote:

Originally Posted by PJL

Have you replaced all the capacitors including the little hunts ones?

If so, try it on an external speaker first....Peter

Hi PJL, yes, should have mentioned, all paper/mettalised paper caps have been replaced.
Also replaced the EL84 output pentode and the EABC80. The new EL84 made some improvement, but the distortion is still there.

Good suggestion about the speaker, just tried and external on, but alas, same distortion...

Leon Crampin · 27th Oct 2007, 3:20 pm

To enlarge on PJL's post a little, the external centralising "spider" on Celestian 'speakers of this vintage was glued to the frame with animal glue which by now, usually lets go. This causes the 'speaker voice coil to go off centre and rub on the magnet poles - giving a scratchy sound. Due to the stiction caused by this, it's a lot worse at low volumes and may not be too bad at higher sound levels.

The centering device can be fixed quite easily with PVA glue. Apply DC to the 'speaker to move the cone outwards and apply the glue with a cocktail stick. Reverse the power supply and fix the ring temporarily with 4 small croc clips. Check that the 'speaker is properly centred (switch off power supply), then leave for half a day with the power supply energised to preload the joint.

This fix works well for all post-war Celestian 'speakers but go easy on the DC - don't move the cone by more than about 3mm.

Leon.

unitelex · 27th Oct 2007, 3:27 pm

Quote:

Originally Posted by Leon Crampin

To enlarge on PJL's post a little, the external centralising "spider" on Celestian 'speakers of this vintage was glued to the frame with animal glue which by now, usually lets go. This causes the 'speaker voice coil to go off centre and rub on the magnet poles - giving a scratchy sound. Due to the stiction caused by this, it's a lot worse at low volumes and may not be too bad at higher sound levels.

The centering device can be fixed quite easily with PVA glue. Apply DC to the 'speaker to move the cone outwards and apply the glue with a cocktail stick. Reverse the power supply and fix the ring temporarily with 4 small croc clips. Check that the 'speaker is properly centred (switch off power supply), then leave for half a day with the power supply energised to preload the joint.

This fix works well for all post-war Celestian 'speakers but go easy on the DC - don't move the cone by more than about 3mm.

Leon.

Thanks Leon,
Some neat ideas there for speaker repair, will take a note of that for the future. The current problem however is not the speaker, I've tried a modern speaker externally - same distortion present unfortunately

Leon Crampin · 27th Oct 2007, 3:32 pm

Just missed your last post - so it's not the 'speaker.

The EL84 is not my favourite valve and is prone to grid current. The coupling capacitor must be above reproach and the cathode bias resistor checked - these often go high.

Run the set with a DVM set to DCV between EL84 g1 and chassis, volume at minimum. A good valve will give no more than about +50mV indicating a low value of grid current. Many will be worse than that - the value increasing wildly as the valve heats up. If you have less than 100mV after half an hour, it's probably OK.

The 6CH6 is a far better output valve. To use in a VHF61, shunt the cathode bias resistor to give a total of 100 Ohms. The theoretical shunt value is 225 Ohms, but you can take into account the inevitable error in the original bias resistor when you make your calculation. Rewire the base connections so that g2 is on pin 8 and pin 9 is earthed - this is dead easy to do on a VHF61.

You will need a large pot bead as a spacer so the valve retainer will fit - the 6CH6 is a bit shorter.

Beam tetrodes in my view are superior to pentodes in every way and I have yet to see even a high mileage 6CH6 with grid current. They're very cheap, too.

Leon.

unitelex · 27th Oct 2007, 3:54 pm

OK -thanks for the ideas - will check the Cathode resistor and grid voltage now...

unitelex · 27th Oct 2007, 4:02 pm

OK, at minimum volume,

G1 has 144mV
Cathode has 6.7V

Output valve is an Edicron K5LH5

Should I try a resistor to chassis on G1 to cancel the leakage current or is my only option to change the valve ?

Thanks

unitelex · 27th Oct 2007, 4:05 pm

Quote:

Originally Posted by unitelex

OK, at minimum volume,

G1 has 144mV
Cathode has 6.7V

Output valve is an Edicron K5LH5

Should I try a resistor to chassis on G1 to cancel the leakage current or is my only option to change the valve ?

Thanks

It had been running for about an hour when I measured this - so fully warmed up.

Leon Crampin · 27th Oct 2007, 4:15 pm

I would regard this value of grid voltage to be too high. If your cathode resistor is to spec. the anode and g2 current is 37mA which is OK but this value of grid current will cause partial rectification of the input signal at low levels - hence the distortion. You perhaps ought to do a final check on the value of the EL84 g1 to chassis resistor before you condem the valve but these resistors don't often cause trouble and can be miles off value with little ill effect.

Out of interest, let your set cool right down and re run the test. Often gas current increases as the valve heats up - sometimes alarmingly.

I would try a better EL84 if you can find a good one (I have loads of gassy ones) - or you could try my mod...

Leon.

unitelex · 27th Oct 2007, 4:21 pm

Just measured G1 again and it was 70mV ??

Disconnected the grid coupling capacitor altogether and checked G1 again
now 50mV

Reconnected coupling capacitor, G1 now measures 50mV

Perhaps the valve socket need cleaning...

unitelex · 27th Oct 2007, 4:44 pm

Grid voltage is now 44mV, and the distortion at lowest volume is much less apparent.

Can't say for sure whether the improvement was due to my probing of the valve socket pins (dodgy contact on the cathode?) or my disturbing of the coupling cap.

Anyway, I've replaced the cap that was there (should have been good as it was a modern 400V polyester) with a new one.

Will leave it to fully warm up again and see if the grid voltage stays in check...

I'll get a beam tetrode anyway and try your mod...

Thanks for you help Leon.

Leon Crampin · 27th Oct 2007, 5:34 pm

When you switch your set off the electrode structure in the EL84 cools - in particular the control grid. When you switch it back on again, the grid current will be lower and may stay lower for some time - such is the nature of the problem. Positive feedback effects often take some time to stabilise, and often don't settle twice the same way.

The EL84 has copper grid rods and heat radiator plates at the top welded to the g1 rods in an attempt to keep the grid cool. Why Philips (in particular) continued to make these inefficient valves where energy is expended by electrons bashing themselves into non-aligned grids is anyone's guess. I suppose the pentode was a Philips patent and the beam tetrode an RCA patent - these Companies can be a pig-headed lot. (I used to work for RCA).

Best of luck with your VHF61 - very good sets. I have two, one with an EL84 (a good one) and the other with a 6CH6. The set with the beam tetrode sounds better.

Leon.

unitelex · 27th Oct 2007, 5:51 pm

OK, Although it seems better for now, I'm convinced enough to try the Beam Tetrode.

Thanks Leon

PJL · 27th Oct 2007, 7:23 pm

It sounds like there was a bit of gas in the valve. As long as there is sufficient gettering this will normally burn off with a little use. This is a common problem even mentioned in Langfords book from the 50's...

Unlike power cords as some would suggest, this is an example where electronics really does need time to be broken-in ...Peter

unitelex · 28th Oct 2007, 8:44 pm

Quote:

Originally Posted by Leon Crampin

Best of luck with your VHF61 - very good sets. I have two, one with an EL84 (a good one) and the other with a 6CH6. The set with the beam tetrode sounds better.
Leon.

I got a couple of Brimar 6CH6's from George at the Manchester Swapmeet today. Just tried your suggeted mod Leon. It certainly works and there is no low volume distortion noticeable

Would I be right in noticing slightly more pronounced treble?

Thanks,
Chris

Leon Crampin · 28th Oct 2007, 9:03 pm

You could be right, Chris, but the 'speaker in the VHF61 is not world class - even with the suspension in good order.

I'm glad the 6CH6 mod is working well - are you able to re-measure the grid voltage (cold and hot) to check my theory?

I would stick a label on the chassis or mains transformer stack to indicate that the output valve type has changed -an EL84 in there now would not be good.

I have a huge bag of pot beads which are perfect as spacers if you want to fit the retainer. Send me a pm and I'll post you one.

Leon.

Tim · 28th Oct 2007, 9:41 pm

Nice one Leon. I'll remember that one as I have a quantity of CV2127s in the loft. I might even try it on my VHF 61 (which is currently missing it's EL84. ISTR replacing the speaker on my VHF61 some 20 years ago, but can't remember if it went o/c or distorted.

27th Oct 2007, 9:57 am	#1
unitelex Hexode Join Date: May 2005 Location: Greater Manchester, UK. Posts: 496	Bush VHF61 distortion at lowest volume setting My VHF61 gets a clear undistorted audio for BBC stations (reasonable modulation levels !) except when the volume control is at or close to the minimum setting. Suspect it is something to do with the AGC? Any clues ?

27th Oct 2007, 11:14 am	#2
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Bush VHF61 distortion at lowest volume setting Have you replaced all the capacitors including the little hunts ones? If so, try it on an external speaker first....Peter

27th Oct 2007, 3:20 pm	#4
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: Bush VHF61 distortion at lowest volume setting To enlarge on PJL's post a little, the external centralising "spider" on Celestian 'speakers of this vintage was glued to the frame with animal glue which by now, usually lets go. This causes the 'speaker voice coil to go off centre and rub on the magnet poles - giving a scratchy sound. Due to the stiction caused by this, it's a lot worse at low volumes and may not be too bad at higher sound levels. The centering device can be fixed quite easily with PVA glue. Apply DC to the 'speaker to move the cone outwards and apply the glue with a cocktail stick. Reverse the power supply and fix the ring temporarily with 4 small croc clips. Check that the 'speaker is properly centred (switch off power supply), then leave for half a day with the power supply energised to preload the joint. This fix works well for all post-war Celestian 'speakers but go easy on the DC - don't move the cone by more than about 3mm. Leon.

27th Oct 2007, 3:32 pm	#6
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: Bush VHF61 distortion at lowest volume setting Just missed your last post - so it's not the 'speaker. The EL84 is not my favourite valve and is prone to grid current. The coupling capacitor must be above reproach and the cathode bias resistor checked - these often go high. Run the set with a DVM set to DCV between EL84 g1 and chassis, volume at minimum. A good valve will give no more than about +50mV indicating a low value of grid current. Many will be worse than that - the value increasing wildly as the valve heats up. If you have less than 100mV after half an hour, it's probably OK. The 6CH6 is a far better output valve. To use in a VHF61, shunt the cathode bias resistor to give a total of 100 Ohms. The theoretical shunt value is 225 Ohms, but you can take into account the inevitable error in the original bias resistor when you make your calculation. Rewire the base connections so that g2 is on pin 8 and pin 9 is earthed - this is dead easy to do on a VHF61. You will need a large pot bead as a spacer so the valve retainer will fit - the 6CH6 is a bit shorter. Beam tetrodes in my view are superior to pentodes in every way and I have yet to see even a high mileage 6CH6 with grid current. They're very cheap, too. Leon. Last edited by Leon Crampin; 27th Oct 2007 at 3:40 pm.

27th Oct 2007, 3:54 pm	#7
unitelex Hexode Join Date: May 2005 Location: Greater Manchester, UK. Posts: 496	Re: Bush VHF61 distortion at lowest volume setting OK -thanks for the ideas - will check the Cathode resistor and grid voltage now...

27th Oct 2007, 4:02 pm	#8
unitelex Hexode Join Date: May 2005 Location: Greater Manchester, UK. Posts: 496	Re: Bush VHF61 distortion at lowest volume setting OK, at minimum volume, G1 has 144mV Cathode has 6.7V Output valve is an Edicron K5LH5 Should I try a resistor to chassis on G1 to cancel the leakage current or is my only option to change the valve ? Thanks

27th Oct 2007, 4:15 pm	#10
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: Bush VHF61 distortion at lowest volume setting I would regard this value of grid voltage to be too high. If your cathode resistor is to spec. the anode and g2 current is 37mA which is OK but this value of grid current will cause partial rectification of the input signal at low levels - hence the distortion. You perhaps ought to do a final check on the value of the EL84 g1 to chassis resistor before you condem the valve but these resistors don't often cause trouble and can be miles off value with little ill effect. Out of interest, let your set cool right down and re run the test. Often gas current increases as the valve heats up - sometimes alarmingly. I would try a better EL84 if you can find a good one (I have loads of gassy ones) - or you could try my mod... Leon.

27th Oct 2007, 4:21 pm	#11
unitelex Hexode Join Date: May 2005 Location: Greater Manchester, UK. Posts: 496	Re: Bush VHF61 distortion at lowest volume setting Just measured G1 again and it was 70mV ?? Disconnected the grid coupling capacitor altogether and checked G1 again now 50mV Reconnected coupling capacitor, G1 now measures 50mV Perhaps the valve socket need cleaning...

27th Oct 2007, 4:44 pm	#12
unitelex Hexode Join Date: May 2005 Location: Greater Manchester, UK. Posts: 496	Re: Bush VHF61 distortion at lowest volume setting Grid voltage is now 44mV, and the distortion at lowest volume is much less apparent. Can't say for sure whether the improvement was due to my probing of the valve socket pins (dodgy contact on the cathode?) or my disturbing of the coupling cap. Anyway, I've replaced the cap that was there (should have been good as it was a modern 400V polyester) with a new one. Will leave it to fully warm up again and see if the grid voltage stays in check... I'll get a beam tetrode anyway and try your mod... Thanks for you help Leon.

27th Oct 2007, 5:34 pm	#13
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: Bush VHF61 distortion at lowest volume setting When you switch your set off the electrode structure in the EL84 cools - in particular the control grid. When you switch it back on again, the grid current will be lower and may stay lower for some time - such is the nature of the problem. Positive feedback effects often take some time to stabilise, and often don't settle twice the same way. The EL84 has copper grid rods and heat radiator plates at the top welded to the g1 rods in an attempt to keep the grid cool. Why Philips (in particular) continued to make these inefficient valves where energy is expended by electrons bashing themselves into non-aligned grids is anyone's guess. I suppose the pentode was a Philips patent and the beam tetrode an RCA patent - these Companies can be a pig-headed lot. (I used to work for RCA). Best of luck with your VHF61 - very good sets. I have two, one with an EL84 (a good one) and the other with a 6CH6. The set with the beam tetrode sounds better. Leon.

27th Oct 2007, 5:51 pm	#14
unitelex Hexode Join Date: May 2005 Location: Greater Manchester, UK. Posts: 496	Re: Bush VHF61 distortion at lowest volume setting OK, Although it seems better for now, I'm convinced enough to try the Beam Tetrode. Thanks Leon

27th Oct 2007, 7:23 pm	#15
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Bush VHF61 distortion at lowest volume setting It sounds like there was a bit of gas in the valve. As long as there is sufficient gettering this will normally burn off with a little use. This is a common problem even mentioned in Langfords book from the 50's... Unlike power cords as some would suggest, this is an example where electronics really does need time to be broken-in ...Peter

28th Oct 2007, 9:03 pm	#17
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: Bush VHF61 distortion at lowest volume setting You could be right, Chris, but the 'speaker in the VHF61 is not world class - even with the suspension in good order. I'm glad the 6CH6 mod is working well - are you able to re-measure the grid voltage (cold and hot) to check my theory? I would stick a label on the chassis or mains transformer stack to indicate that the output valve type has changed -an EL84 in there now would not be good. I have a huge bag of pot beads which are perfect as spacers if you want to fit the retainer. Send me a pm and I'll post you one. Leon.

28th Oct 2007, 9:41 pm	#18
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Re: Bush VHF61 distortion at lowest volume setting Nice one Leon. I'll remember that one as I have a quantity of CV2127s in the loft. I might even try it on my VHF 61 (which is currently missing it's EL84. ISTR replacing the speaker on my VHF61 some 20 years ago, but can't remember if it went o/c or distorted. __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly."