EF86 Preamp - UK Vintage Radio Repair and Restoration Discussion Forum

EF80TVVALVE · 28th Sep 2020, 3:06 pm

Hi all

I'm just working through the pre-amp for my AEI power amp previously mentioned in another thread. I've gone through it and replaced any out of spec resistors and the capacitors, leaving the mica caps as these all look to be okay.

I'm currently having a problem where there looks to be a loss/leakage of signal between the output of the first stage and the input of the second stage. I have attached a photo of the front controls, I can scope a signal at the junction of the 470pF Mica where it joins the 0.1uf (going to the circuit board) and the 68k resistor. The control on the left is the bass and the one in the middle is the treble.

At the output of the 470pF joining the treble pot is where I seem to lose a lot of the signal. I bridged a Croc lead across and joined the output of the first stage to the output of the treble control (centre contact) and was met with a nice waveform and good signal level from the output. With the amplifier working normally I'm barely getting anything out of the treble pot, say around 20mv with a 500mv input signal.

There is some signal coming through the output, for example if I connect a CD player to the radio input I do get some volume from the power amp but at full volume it's only at a moderate listening level and it isn't driving the amp as much as it should be. I imagine with the CD player plugged directly in with a pot there would be a lot more volume available.

Anyone got any suggestions on where to look next?

EF80TVVALVE · 28th Sep 2020, 7:28 pm

Having a closer look I've drawn out a rough schematic of the equalisation controls between the two stages. I suppose the difficulty of not having a schematic is if anything isn't quite right then it's sometimes hard to work out what.

I've used the mullard pre amplifier schematic as a guide to look through this circuit with consideration that there will be some differences, notably the biggest is that the eq controls on the mullard are on the output stage rather than between the two.

Other differences I can see are mainly with the bass control where on my amplifier there is a 0.02uF across the potentiometer winding rather than one each side joining to the wiper, along with an extra 68k from the wiper to the end of the pot joining to earth through a 6k8.

I'm wondering if this 0.02 across the pot could cause some signal leakage to ground, enough to reduce the output and whether it should be fitted from the wiper to the earth side of the pot.

If anyone has a schematic for the original then it would be a great help, I believe it is the FKIPB

Herald1360 · 28th Sep 2020, 7:36 pm

That loss sounds a bit excessive, but don't forget that a passive tone control like this can only provide "boost" by virtue of having a "straight through" loss of around 20dB when the controls are set for a flat response.

Herald1360 · 28th Sep 2020, 8:38 pm

Simulating the circuit as below (100k resistor shorted and 1meg load on the stack) suggests a "flat" loss around 23dB. "0mV out for 500mV in is around -27dB so not a million miles out?

EF80TVVALVE · 28th Sep 2020, 10:26 pm

Hi Chris,

Thanks for going through the trouble of doing that for me. So really it looks like this part of the circuit is working as it should be.

There are still some original resistors around the EF86 which may affect their functionality, I'll investigate more tomorrow and hopefully get more readings, voltages etc. Maybe the first stage isn't amplifying as much as it should do to give enough gain through the controls and to the second stage.

EF80TVVALVE · 29th Sep 2020, 1:57 pm

Some investigating today has revealed a problem with the first stage voltages. The anode voltage of the first EF86 measures 423vDC obviously way over what it should be. G2 is also fed from this HT rail via a 390k resistor but I only have approx 130mV on the G2

I've drawn out the circuitry supplying the HT to the first stage. The HT from the amplifier comes in at 434vDC and is then fed to a seperate first stage HT rail via a 47k resistor. The anode is then supplied by this via another 47k resistor. I checked the anode voltage of the second stage which is 126.7v which is closer (I imagine) to what it should be.

Just wondering if the 390k has gone o/c or high resistance and could cause this?

Herald1360 · 29th Sep 2020, 7:56 pm

Is there a (leaky) decoupling capacitor from 390k/g2 to 0V?

Sideband · 29th Sep 2020, 8:37 pm

The anode load for that first EF86 seems far too low. Are you sure it shouldn't be a 470K? EF86 are high impedance valves and run at a very low anode current. Most pre-amps I've seen using these valves use anything from 470K to 1 meg for the anode load.

EF80TVVALVE · 29th Sep 2020, 9:44 pm

I've had a look tonight and put a meter across the 390k and couldn't get a reading, I pulled it out and checked again and it was O/C. Replaced it with another and it is now working fine, I'm getting about 6.5v on the output before clipping so enough power there now to supply the power amp. I do wonder if the old capacitor was short and caused the 390k to burn out or whether it just hasn't aged well.

Sideband, as far as resistance is concerned this has crossed my mind when fault finding although I am positively certain that a 47k was fitted there, in fact I have a bag of all the old components and there are no 470k removed from the amp, the second stage also has a 47k fitted. The anode now has 240v DC on the first stage which is within the operating limits of the EF86. I'll still do some digging around and see if I can pull up a schematic but I'm beginning to wonder if any even exist as they seem to be hens teeth.

Herald1360 · 29th Sep 2020, 10:36 pm

47K is a bit low but it'll just reduce the gain and lower the source impedance feeding the tone control stage. Some Philips suggested values for 400V HT are Ra 100k, Rg2 390k, Rk 1k, rg1 330k.

https://frank.pocnet.net/sheets/010/e/EF86.pdf

There are other higher values suggested for lower current operation.

The Mullard 3-3 operates its EF86 under near starvation conditions in an unusual directly coupled circuit with a 1M Ra and screen supply from the cathode of the output valve but this wasn't a typical application.

http://www.r-type.org/articles/art-003h.htm

EF80TVVALVE · 30th Sep 2020, 12:51 pm

Yes I do think that it is a bit low, but I'm certain that is what was fitted originally. Thinking about it, this unit dates to the earlier days of HiFi so it wouldn't be surprising to see a few unusual things here and there, I think each company wanted to be a bit different from the others and this is often the result.

It works well now but it makes you wonder whether to go down the route of modifying the circuit or keeping it as BTH made it in the first place. Personally for authenticity I'd much prefer to keep it as it was designed, even if it's not quite as it should be.

28th Sep 2020, 3:06 pm	#1
EF80TVVALVE Octode Join Date: Feb 2010 Location: Leicester, UK. Posts: 1,433	EF86 Preamp Hi all I'm just working through the pre-amp for my AEI power amp previously mentioned in another thread. I've gone through it and replaced any out of spec resistors and the capacitors, leaving the mica caps as these all look to be okay. I'm currently having a problem where there looks to be a loss/leakage of signal between the output of the first stage and the input of the second stage. I have attached a photo of the front controls, I can scope a signal at the junction of the 470pF Mica where it joins the 0.1uf (going to the circuit board) and the 68k resistor. The control on the left is the bass and the one in the middle is the treble. At the output of the 470pF joining the treble pot is where I seem to lose a lot of the signal. I bridged a Croc lead across and joined the output of the first stage to the output of the treble control (centre contact) and was met with a nice waveform and good signal level from the output. With the amplifier working normally I'm barely getting anything out of the treble pot, say around 20mv with a 500mv input signal. There is some signal coming through the output, for example if I connect a CD player to the radio input I do get some volume from the power amp but at full volume it's only at a moderate listening level and it isn't driving the amp as much as it should be. I imagine with the CD player plugged directly in with a pot there would be a lot more volume available. Anyone got any suggestions on where to look next? Attached Thumbnails

28th Sep 2020, 7:28 pm	#2
EF80TVVALVE Octode Join Date: Feb 2010 Location: Leicester, UK. Posts: 1,433	Re: EF86 Preamp Having a closer look I've drawn out a rough schematic of the equalisation controls between the two stages. I suppose the difficulty of not having a schematic is if anything isn't quite right then it's sometimes hard to work out what. I've used the mullard pre amplifier schematic as a guide to look through this circuit with consideration that there will be some differences, notably the biggest is that the eq controls on the mullard are on the output stage rather than between the two. Other differences I can see are mainly with the bass control where on my amplifier there is a 0.02uF across the potentiometer winding rather than one each side joining to the wiper, along with an extra 68k from the wiper to the end of the pot joining to earth through a 6k8. I'm wondering if this 0.02 across the pot could cause some signal leakage to ground, enough to reduce the output and whether it should be fitted from the wiper to the earth side of the pot. If anyone has a schematic for the original then it would be a great help, I believe it is the FKIPB Attached Thumbnails

28th Sep 2020, 7:36 pm	#3
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: EF86 Preamp That loss sounds a bit excessive, but don't forget that a passive tone control like this can only provide "boost" by virtue of having a "straight through" loss of around 20dB when the controls are set for a flat response. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

28th Sep 2020, 8:38 pm	#4
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: EF86 Preamp Simulating the circuit as below (100k resistor shorted and 1meg load on the stack) suggests a "flat" loss around 23dB. "0mV out for 500mV in is around -27dB so not a million miles out? Attached Thumbnails __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

28th Sep 2020, 10:26 pm	#5
EF80TVVALVE Octode Join Date: Feb 2010 Location: Leicester, UK. Posts: 1,433	Re: EF86 Preamp Hi Chris, Thanks for going through the trouble of doing that for me. So really it looks like this part of the circuit is working as it should be. There are still some original resistors around the EF86 which may affect their functionality, I'll investigate more tomorrow and hopefully get more readings, voltages etc. Maybe the first stage isn't amplifying as much as it should do to give enough gain through the controls and to the second stage.

29th Sep 2020, 1:57 pm	#6
EF80TVVALVE Octode Join Date: Feb 2010 Location: Leicester, UK. Posts: 1,433	Re: EF86 Preamp Some investigating today has revealed a problem with the first stage voltages. The anode voltage of the first EF86 measures 423vDC obviously way over what it should be. G2 is also fed from this HT rail via a 390k resistor but I only have approx 130mV on the G2 I've drawn out the circuitry supplying the HT to the first stage. The HT from the amplifier comes in at 434vDC and is then fed to a seperate first stage HT rail via a 47k resistor. The anode is then supplied by this via another 47k resistor. I checked the anode voltage of the second stage which is 126.7v which is closer (I imagine) to what it should be. Just wondering if the 390k has gone o/c or high resistance and could cause this? Attached Thumbnails Last edited by EF80TVVALVE; 29th Sep 2020 at 2:05 pm.

29th Sep 2020, 7:56 pm	#7
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: EF86 Preamp Is there a (leaky) decoupling capacitor from 390k/g2 to 0V? __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

29th Sep 2020, 8:37 pm	#8
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,571	Re: EF86 Preamp The anode load for that first EF86 seems far too low. Are you sure it shouldn't be a 470K? EF86 are high impedance valves and run at a very low anode current. Most pre-amps I've seen using these valves use anything from 470K to 1 meg for the anode load. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

29th Sep 2020, 9:44 pm	#9
EF80TVVALVE Octode Join Date: Feb 2010 Location: Leicester, UK. Posts: 1,433	Re: EF86 Preamp I've had a look tonight and put a meter across the 390k and couldn't get a reading, I pulled it out and checked again and it was O/C. Replaced it with another and it is now working fine, I'm getting about 6.5v on the output before clipping so enough power there now to supply the power amp. I do wonder if the old capacitor was short and caused the 390k to burn out or whether it just hasn't aged well. Sideband, as far as resistance is concerned this has crossed my mind when fault finding although I am positively certain that a 47k was fitted there, in fact I have a bag of all the old components and there are no 470k removed from the amp, the second stage also has a 47k fitted. The anode now has 240v DC on the first stage which is within the operating limits of the EF86. I'll still do some digging around and see if I can pull up a schematic but I'm beginning to wonder if any even exist as they seem to be hens teeth.

29th Sep 2020, 10:36 pm	#10
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: EF86 Preamp 47K is a bit low but it'll just reduce the gain and lower the source impedance feeding the tone control stage. Some Philips suggested values for 400V HT are Ra 100k, Rg2 390k, Rk 1k, rg1 330k. https://frank.pocnet.net/sheets/010/e/EF86.pdf There are other higher values suggested for lower current operation. The Mullard 3-3 operates its EF86 under near starvation conditions in an unusual directly coupled circuit with a 1M Ra and screen supply from the cathode of the output valve but this wasn't a typical application. http://www.r-type.org/articles/art-003h.htm __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

30th Sep 2020, 12:51 pm	#11
EF80TVVALVE Octode Join Date: Feb 2010 Location: Leicester, UK. Posts: 1,433	Re: EF86 Preamp Yes I do think that it is a bit low, but I'm certain that is what was fitted originally. Thinking about it, this unit dates to the earlier days of HiFi so it wouldn't be surprising to see a few unusual things here and there, I think each company wanted to be a bit different from the others and this is often the result. It works well now but it makes you wonder whether to go down the route of modifying the circuit or keeping it as BTH made it in the first place. Personally for authenticity I'd much prefer to keep it as it was designed, even if it's not quite as it should be.