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 > 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.

Reply
 
Thread Tools
Old 18th Jun 2019, 6:52 pm   #81
snowman_al
Hexode
 
snowman_al's Avatar
 
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
Posts: 305
Default Re: Mullard 5-10 amplifier

I suppose you have replaced the EF86 and then the ECC83?
The EF86 does have a reputation for general noise on some examples...
__________________
Never Leave Well Enough Alone...
snowman_al is offline   Reply With Quote
Old 18th Jun 2019, 9:38 pm   #82
Techman
Nonode
 
Techman's Avatar
 
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 2,333
Default Re: Mullard 5-10 amplifier

I think he said he'd tried about 10 different EF86s.

These old valve pre-amps do have a bit of hiss and noise, but obviously not as much as being experienced here. This is the reason why the Valve power amps are generally very desirable, but the valve pre-amps that go with them are not so desirable to the extent of sometimes being virtually worthless by comparison.
Techman is offline   Reply With Quote
Old 20th Jun 2019, 10:29 am   #83
vampyretim
Hexode
 
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 463
Default Re: Mullard 5-10 amplifier

I've replaced the 1M resistor for a modern low noise type but still have the rustling, hissing sound. I'm going to start replacing anode and screen resistors.
vampyretim is offline   Reply With Quote
Old 20th Jun 2019, 1:46 pm   #84
Radio Wrangler
Dekatron
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 12,124
Default Re: Mullard 5-10 amplifier

Oh, well. It was the most likely candidate. I think you're doing the right thing, it's just a case of going through everything until you hit the culprit.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 20th Jun 2019, 1:51 pm   #85
Techman
Nonode
 
Techman's Avatar
 
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 2,333
Default Re: Mullard 5-10 amplifier

I've just this moment remembered something about these amplifier head units. Correct me if I'm wrong, but doesn't the earthing in the pre-amp head unit rely on all the front panel pots. I seem to remember that there's earth bonding to the tin plated covers of the pots that in turn relies on the crimped round tabs to the actual alloy casting of these pots, and thence the nut and washer on the spindle connection to the chassis. I once had a problem a few years ago where there was either noise or hum (or both) caused by either relaxed connection or oxidation between the crimp tab of the pot casing and the actual pot body. Going round them all with a sharp screwdriver blade to dig in and confirm a connection between tab and body will confirm (with the unit running of course). Crimp them up with long nose pliers or replace them if this doesn't work - remember what you've learned about earth returns, so you can't just run another bypass earth. It may not be what I've just said, but worth checking out just in case.
Techman is offline   Reply With Quote
Old 20th Jun 2019, 5:26 pm   #86
Valvepower
Heptode
 
Valvepower's Avatar
 
Join Date: Aug 2008
Location: Southend-On-Sea, Essex, UK.
Posts: 810
Default Re: Mullard 5-10 amplifier

Hello,

Just a thought….

I’ve had instances where both the contacts and phenolic base material in valve bases have caused noise in high impedance circuits. From memory this caused a crackling sound where the contacts were dirty. I also suspected the base material had adsorbed moisture over the years thus causing tracking effects between the pins.

Regards
Terry
Valvepower is offline   Reply With Quote
Old 21st Jun 2019, 6:17 pm   #87
buggies
Hexode
 
buggies's Avatar
 
Join Date: Jan 2009
Location: West Lothian, UK.
Posts: 339
Default Re: Mullard 5-10 amplifier

I had trouble locating a continuous crackle (not exactly a hiss) which turned out to be a silver mica in the EF86 anode load C1 position breaking down.
__________________
George
buggies is offline   Reply With Quote
Old 24th Jun 2019, 11:24 am   #88
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 4,118
Default Re: Mullard 5-10 amplifier

The HT voltages are all a bit high. Do you have the mains setting on the highest voltage? The plate + Screen dissipation is 15.5W.

I would expect the hiss to disappear when you put the meter on the ECC83 grid that is not connected to the EF86 as the meter resistance will mess up the bias conditions.

How about a scope picture of the EF86 anode. Try the probe on x10 and x1 as the extra probe capacitance might stop any instability.

Can you also confirm that setting the volume control of the power amp to zero has no effect on the hiss as this shorts the EF86 grid to earth.

Without a pre-amp connected, I would expect the hiss to be so low that you need to put your ear against the tweeter to hear it.
PJL is offline   Reply With Quote
Old 24th Jun 2019, 11:39 am   #89
Techman
Nonode
 
Techman's Avatar
 
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 2,333
Default Re: Mullard 5-10 amplifier

Quote:
Originally Posted by PJL View Post
Can you also confirm that setting the volume control of the power amp to zero has no effect on the hiss as this shorts the EF86 grid to earth.
I guess you mean the pre-amp as the power amp doesn't have a control, and as far as I know the one under discussion has a separate pre-amp head unit with two valves and all the controls on its front panel. I'll go and look the circuit out a bit later.
Techman is offline   Reply With Quote
Old 24th Jun 2019, 11:54 am   #90
Techman
Nonode
 
Techman's Avatar
 
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 2,333
Default Re: Mullard 5-10 amplifier

Just looked at the circuit and the volume control which is a 250K pot is right on the output of the head unit and would actually ground the grid of the first stage (EF86) of the power amp, but it looks like there may be a resistor stopping it doing so.
Techman is offline   Reply With Quote
Old 24th Jun 2019, 12:01 pm   #91
Techman
Nonode
 
Techman's Avatar
 
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 2,333
Default Re: Mullard 5-10 amplifier

Quick snap of the circuit -
Attached Thumbnails
Click image for larger version

Name:	P1120900.JPG
Views:	103
Size:	164.7 KB
ID:	185603  
Techman is offline   Reply With Quote
Old 24th Jun 2019, 5:01 pm   #92
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 4,118
Default Re: Mullard 5-10 amplifier

Your right, the Mullard circuit puts the volume/tone control in with the power amp but this is not how this one is done. My excuse is the Beam Echo I have do include a power amp input volume control.

However, from what I understand, the hiss is from the power amplifier.
PJL is offline   Reply With Quote
Old 8th Jul 2019, 2:43 pm   #93
vampyretim
Hexode
 
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 463
Default Re: Mullard 5-10 amplifier

Hi,

I've replaced quite a few resistors (R4 120k, R5 470k, R2 1.8k, R3 100 ohm) but the hiss is still there. This is the EF86 anode on X1 probe setting x10 is flat and you can't really see anything.

I don't want to replace components for the sake of it and would prefer to be methodical about it. I'm losing patience though.

Many thanks.
Attached Thumbnails
Click image for larger version

Name:	IMG_2531.jpg
Views:	41
Size:	64.9 KB
ID:	186366  
vampyretim is offline   Reply With Quote
Old 8th Jul 2019, 3:45 pm   #94
Boulevardier
Hexode
 
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 254
Default Re: Mullard 5-10 amplifier

Quote:
Originally Posted by Techman View Post
Just looked at the circuit and the volume control which is a 250K pot is right on the output of the head unit and would actually ground the grid of the first stage (EF86) of the power amp, but it looks like there may be a resistor stopping it doing so.
Yes, that resistor lifting the pot from ground puzzles me too...
Boulevardier is offline   Reply With Quote
Old 8th Jul 2019, 3:51 pm   #95
vidjoman
Nonode
 
Join Date: Jun 2013
Location: East Sussex, UK.
Posts: 2,110
Default Re: Mullard 5-10 amplifier

Quote:
Originally Posted by Boulevardier View Post
Quote:
Originally Posted by Techman View Post
Just looked at the circuit and the volume control which is a 250K pot is right on the output of the head unit and would actually ground the grid of the first stage (EF86) of the power amp, but it looks like there may be a resistor stopping it doing so.
Yes, that resistor lifting the pot from ground puzzles me too...
I have a feeling that the resistors Ra & Rb after the pot are to be set according to which power amp you have connected - 5-10 or 5-20. Think it was something to do with getting the correct input sensitivity. Long time (about 55 or more years) since I read the Mullard book.
vidjoman is offline   Reply With Quote
Old 8th Jul 2019, 4:02 pm   #96
Boulevardier
Hexode
 
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 254
Default Re: Mullard 5-10 amplifier

I may have missed the answer to this, but could the OP confirm that the hiss is present with the 5-10's coaxial input socket grounded at the socket itself. If he's just zeroing the pre-amp's volume control to ground the input, then the hiss could be coming in from the pre-amp. That bit of the pre-amp's output circuitry isn't the same as the Mullard 2-valve pre-amp circuit.

Mike
Boulevardier is offline   Reply With Quote
Old 8th Jul 2019, 4:04 pm   #97
GrimJosef
Nonode
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 2,532
Default Re: Mullard 5-10 amplifier

The methodical approach involves locating the source of the hiss and only starting to replace components when you know which part of the circuit has the hiss source in it. This is much harder to do if you have the negative feedback connected because the feedback does what it says on the tin - it takes any fault (hiss, hum, distortion etc) which is present at the output and it feeds it back to the input, from where it travels through the whole of the rest of the amp. So if you make measurements anywhere you will find hiss because the feedback circuit has put it there.

So the first thing to do is to disconnect the feedback and leave it disconnected (when you did this before you heard a lot more noise - that's to be expected).

1. Power the amp down. Disconnect one end of C8 and one end of C9 (it doesn't really matter which ends). Power the amp back up. Now the output should be much quieter since only the output valves are connected to the speaker. If the noise hasn't fallen much then that means it is coming from the output stage somewhere.

2. If the output stage on its own is not the source of the noise then power the amp down. Reconnect C8 and C9. If you have all new resistors in the R2, R3, R4, R5 positions then take the EF86 out and solder two resistors in its place - a 100k in the cathode to anode position and a 470k in the cathode to screen grid position. If you want to do this using the valve socket pins as attachment points then the 100k should go from pin 6 to pin 3 and the 470k from pin 1 to pin 3. If I've done the sums right then these resistors should replicate, near enough, the DC currents which were flowing through the EF86 and should therefore set the DC operating point of the ECC83 correctly. With the EF86 still not in its socket power the amp back up. If the noise is now much less then it must have been coming from the EF86 valve, or from something before it in the circuit. If it is not much less then it is coming from one or more of the passive components in the EF86 circuit, or from something in the ECC83 circuit.

Cheers,

GJ
__________________
http://www.ampregen.com

Last edited by GrimJosef; 8th Jul 2019 at 4:10 pm.
GrimJosef is offline   Reply With Quote
Old 8th Jul 2019, 5:42 pm   #98
Boulevardier
Hexode
 
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 254
Default Re: Mullard 5-10 amplifier

"That bit of the pre-amp's output circuitry isn't the same as the Mullard 2-valve pre-amp circuit."

Sorry, my mistake, it is the same (I hadn't looked further into the output attenuation options given in the Mullard text). Still it might be worth checking the resistors in that pre-amp output attenuator if the OP isn't grounding the PA's input at its co-ax socket.

Mike
Boulevardier is offline   Reply With Quote
Old 8th Jul 2019, 6:44 pm   #99
vampyretim
Hexode
 
Join Date: Aug 2011
Location: Bristol, UK.
Posts: 463
Default Re: Mullard 5-10 amplifier

Thanks GJ,

That's very helpful!

I will have another go at it tomorrow.
vampyretim is offline   Reply With Quote
Old 8th Jul 2019, 7:27 pm   #100
GrimJosef
Nonode
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 2,532
Default Re: Mullard 5-10 amplifier

You're welcome. While I was writing Mike (Boulevardier) at post #96 requested a quick check just to make sure the hiss really is coming from the amp and not from the pre-amp. If you're not certain that it's from the amp this check could save you a lot of time and fuss.

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 5:55 am.


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 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2019, Paul Stenning.