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Old 18th Jun 2019, 10:59 am   #61
ajgriff
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Default Re: Mullard 5-10 amplifier

80mV doesn't seem much to me especially if you're testing with the volume control set to maximum. All amplifiers hiss to a degree. If you can't hear the hiss at a 'normal' volume setting and when you're a couple of yards away from the speakers I wouldn't worry about it. Just relax and enjoy the music.

Alan
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Old 18th Jun 2019, 11:06 am   #62
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Default Re: Mullard 5-10 amplifier

Hi all,

I've done all voltage readings as per Mullard circuits for audio amplifiers (with Avo 8 mostly on 1000v setting).

EF86
A-140V
S-70V
K-1.7V

ECC83
A-230V
A1-210V
G-50V
G1-50V (the hiss disappears when I take this voltage reading)
K-55V
K1-55V

EL84 1
A-335V
S-340V
K-12V

EL84 2
A-335V
S-340V
K-13V

Could hiss be caused by R6? I will try a replacement shortly.
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Old 18th Jun 2019, 11:07 am   #63
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Default Re: Mullard 5-10 amplifier

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Originally Posted by ajgriff View Post
80mV doesn't seem much to me especially if you're testing with the volume control set to maximum. All amplifiers hiss to a degree. If you can't hear the hiss at a 'normal' volume setting and when you're a couple of yards away from the speakers I wouldn't worry about it. Just relax and enjoy the music.
The hiss is quite loud.
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Old 18th Jun 2019, 11:25 am   #64
G8HQP Dave
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Default Re: Mullard 5-10 amplifier

Hiss which disappears when touched by a probe might be a symptom of oscillation.
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Old 18th Jun 2019, 12:07 pm   #65
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Default Re: Mullard 5-10 amplifier

80mV of hiss is too much. The figure I gave earlier - 75dB below 10W - was for hum and noise (noise includes hiss). It works out to 1.6mV RMS.

The Mullard 5-10 is a hi-fi amp. It should be quieter than, say, surface noise on a good quality 33rpm record, or background noise on a strong, well-tuned FM station heard through a good quality tuner.

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Old 18th Jun 2019, 12:33 pm   #66
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Default Re: Mullard 5-10 amplifier

Theres definitely a problem with hiss, 80mV of hiss is annoyingly loud.

I don't know where to start looking, it's obviously somewhere before the output stage but I'm finding it hard to see on a scope.

The photo is what I'm getting at the speaker. It sometimes gets worse but goes back to this level with a tap of a screwdriver.
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Old 18th Jun 2019, 1:37 pm   #67
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Default Re: Mullard 5-10 amplifier

What happens if you disconnect the feed back resistor and capacitor, R11 and C6, at one end ? In voltage terms that should make the amp's sensitivity 20 times higher and any hum-and-noise 20 times worse. If it doesn't then either the feedback components (the above two plus R3) aren't working properly or the noise is arising outside the fedback loop or, as Dave says, something might be oscillating.

EDIT: You might think that because you can't see any oscillation at the speaker, there isn't any. But it could be that it simply isn't getting through the output transformer, which isn't designed to pass the sorts of extremely high frequencies characteristic of amp instability.

Cheers,

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Old 18th Jun 2019, 2:48 pm   #68
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Default Re: Mullard 5-10 amplifier

Quote:
Originally Posted by Edward Huggins View Post
In Post#45 Terry speaks of the 5-10s "shortcomings". I dont know what shortcomings there really are - maybe the power supply could have used a Choke? Of course the mains and output transformers need to be of very good quality, but the unit was designed to give the best possible sound - esp. the UL version - from readily avaiable components and that could be built by a capable home constructor. I knew of the team that worked on this at Mullard House and their trials were exhaustive.
Hello

To clarify the comment, which looking at it again wasnít that clearÖ

I certainly didnít mean the 5-10 circuit and the good work of the Mullard engineers, of which I have total admiration, but it was more about the nature of Sterns kit per se.

The 5-10 intrigues me so much Iíve something like 10 or more different types, including some really nice home constructed 5-10's

Terry
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Old 18th Jun 2019, 3:24 pm   #69
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Default Re: Mullard 5-10 amplifier

Hi GJ,

I disconnected the components and am getting loud hiss and crackle it does seem to be a lot louder. I'm getting roughly 400 mV. Which is just a bit more than 20 times larger than 80 mV.

It's hard for you to say but can we say that the NFB is okay?
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Old 18th Jun 2019, 3:28 pm   #70
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Default Re: Mullard 5-10 amplifier

20 times 80 is 1600. Are you measuring it with the AVO on an AC range ?

Cheers,

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Old 18th Jun 2019, 3:42 pm   #71
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Default Re: Mullard 5-10 amplifier

Something is wrong with those voltage readings in post #63. The ECC83 grids are direct coupled to the EF86 anode, so the voltages should be the same, especially for the first half triode. The Mullard book suggests 61V, whereas the voltages posted were 140V for the EF86 Va and 50V for the ECC83 grids.

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Old 18th Jun 2019, 3:44 pm   #72
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Default Re: Mullard 5-10 amplifier

Quote:
Originally Posted by vampyretim View Post
I'm getting roughly 400 mV. Which is just a bit more than 20 times larger than 80 mV.
5 times surely? 5 X 80 = 400
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Old 18th Jun 2019, 4:19 pm   #73
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Default Re: Mullard 5-10 amplifier

Quote:
Originally Posted by ronbryan View Post
Something is wrong with those voltage readings in post #63. The ECC83 grids are direct coupled to the EF86 anode, so the voltages should be the same, especially for the first half triode. The Mullard book suggests 61V, whereas the voltages posted were 140V for the EF86 Va and 50V for the ECC83 grids.
Hi,

My mistake the voltage of the EF86 anode is the same as the grids of the ECC83.

My mistake too on the 20x noise calculation.

I'm getting the image in the photo on the second ECC83 grid.
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Old 18th Jun 2019, 4:35 pm   #74
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Default Re: Mullard 5-10 amplifier

Just been studying the oscilloscope noise image and wondering about the 80mV figure. The peak to peak value of most of the noise signal seems to be about 80mV. However, the variable sensitivity control appears to be turned to maximum ie, at least 2.5 times the set value. Presumably the x5 magnifier is not activated (knob pulled out) and a x1 probe is being used. Also I'm not quite sure how the RMS value of the noise signal can be evaluated and compared with GJ's 1.6mV calculation. Just thinking aloud really.

I'd still be interested to know whether or not the hiss is audible at anything like normal volume settings and listening distance. I accept that this may be of no interest to anyone else.

Alan
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Old 18th Jun 2019, 4:43 pm   #75
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Default Re: Mullard 5-10 amplifier

The scope picture back in post 67 is interesting. The noise looks a bit spikey, fairly infrequent extra-large narrow spikes sticking out from more normal noise. Suspicious!

Any bad connections, dry joints or carbon composition resistors?

These would fit with the screwdriver tapping sensitivity.
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Old 18th Jun 2019, 4:49 pm   #76
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Default Re: Mullard 5-10 amplifier

The 5-10 has quite high gain from the input grid to the LTP anodes, and one of the anodes will be the same polarity as the input. Big caps or poor layout can provide enough capacitive feedback to tip things over the edge. In a 5-20 I built I had to insert a small screen to prevent this.
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Old 18th Jun 2019, 5:44 pm   #77
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Default Re: Mullard 5-10 amplifier

Hi all,

Hiss is very audible, the noise is definitely coming from the 5-10 and not the preamp and is unaffected by volume. I'm using the 1x setting on my probe.

The 1M resistor connecting the ECC83 grids is a carbon composite type and I'm seeing the spiky noise on one side of that resistor only, it might be unrelated.

I have fixed a few solder joins today and there might be a few more that need redoing.
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Old 18th Jun 2019, 6:02 pm   #78
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Default Re: Mullard 5-10 amplifier

Maybe it's time to try a good low noise resistor in place of that 1 meg carbon comp? They are amongst the noisiest of resistors from new, and they have habits of going high over time and also turning into carbon microphones.

It fits a couple of the symptoms, so would be a suitable place to start the hunt.

David
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Old 18th Jun 2019, 6:47 pm   #79
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Default Re: Mullard 5-10 amplifier

Quote:
Originally Posted by GrimJosef View Post
The hiss usually arises in the earliest stage of the amplifier i.e. the EF86 and its circuitry. Some EF86s are noisier than others - all you can do is work your way through a few, listening for the quietest one. Another possible source is the anode and screen grid resistors. Mullard specified that these should be 'high-stability, cracked carbon' types for precisely this reason. These days we would call them 'carbon film'. Modern metal film ones are even quieter, but, as always, it's important to make sure that they are not running too close to their voltage limits.
GJ has already mentioned the carbon composition resistors being a possible cause of noise around the EF86. Suggest you replace these with modern carbon film types.
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Old 18th Jun 2019, 6:50 pm   #80
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Default Re: Mullard 5-10 amplifier

Actually if high stability cracked carbon types were used originally, as Mullard specified, then they might well still be OK. They are quite durable. Unlike the carbon compositions, which were less good to start with and do degrade with time.

cheers,

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