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...

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.

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.

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

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.

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

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.

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.

Re: Mullard 5-10 amplifier
 

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.

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.

Re: Mullard 5-10 amplifier
 

Quick snap of the circuit -

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.

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.

Re: Mullard 5-10 amplifier
 

Yes, that resistor lifting the pot from ground puzzles me too...

Re: Mullard 5-10 amplifier
 

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.

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

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

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

Re: Mullard 5-10 amplifier
 

Thanks GJ,

That's very helpful!

I will have another go at it tomorrow.

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