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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 10th Oct 2017, 9:55 pm   #1
filmprojectors
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Default How do I reduce valve amp HF response

Hi. Please could I have some advice on how to reduce high end hiss on my old cinema amp?
It uses one EF86, two ECC83, two 6L6 valves. It has treble and bass controls, but I have to turn the bass nearly full on, and the treble off to make it useable. Even then there's a lot of high end hiss being picked up from the inputs, film/tape or CD.
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Old 10th Oct 2017, 11:14 pm   #2
kalee20
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Default Re: How do I reduce valve amp HF response

Do you have a circuit diagram?

It's possible that the hiss is coming from a faulty resistor - or possibly valve.

With a pair of 6L6 valves, it's a reasonably powerful amplifier. I'm assuming that one ECC83 will be a phase inverter, and probably preceded by the other ECC83 working with the tone controls, and as voltage amplifier. Then the EF86 as low-noise front-end.

Is it optical or magnetic sound? Photocells can be noisy, and if left in circuit all the time (maybe just exciter lamps switched off to mute the film sound) could cause the problem.
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Old 11th Oct 2017, 6:36 am   #3
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Default Re: How do I reduce valve amp HF response

Probably not helped by having a high gain input, converting the EF86 to triode connected is a salve used used sometimes.

As kalee says check your resistors, old carbon comp R's suffer from noise, Johnson noise specifically. Check for cracks, loose leads, R's gone high in value.

Andy.
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Old 11th Oct 2017, 9:34 am   #4
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Default Re: How do I reduce valve amp HF response

I run a pair of old cinema amps as a HiFi. They are an all octal valve line up.
They originally had a bit of bass cut to stop the bass getting through the shared party walls to the shop owners who lived above there shops.
They just used a lower value cathode cap in the preamp stage to cut it. This is an easy mod.
Have you tried a signal generator and counter so check the frequency response?
The hiss may well be old resistors but do make sure it is not some other kind of pick up as the preamps could have quite a bit of gain if they were old optical sound track ones.
Mine bring up the 30 ohm outputs of MP3 players to full power and make everything shake.
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Old 11th Oct 2017, 7:20 pm   #5
filmprojectors
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Default Re: How do I reduce valve amp HF response

I don't have a circuit diagram for this 1960's French amplifier
I have replaced all the caps and most of the resistors
may be in the past some of the values of the resistors or caps have been changed and so will look for a similar circuit to copy
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Old 11th Oct 2017, 11:26 pm   #6
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Default Re: How do I reduce valve amp HF response

I had to reverse engineer mine to see what was going on.
Repeatedly turning the chassis over to see where the wires to the controls went was quite tiring after a while.
It gains an extra stage by using a single triode phase splitter.
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Old 12th Oct 2017, 2:14 am   #7
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Default Re: How do I reduce valve amp HF response

Quote:
Originally Posted by Refugee View Post
I run a pair of old cinema amps as a HiFi. They are an all octal valve line up.
They originally had a bit of bass cut to stop the bass getting through the shared party walls to the shop owners who lived above there shops.
They just used a lower value cathode cap in the preamp stage to cut it. This is an easy mod.
I would caution against this method of lowering the bass response, that is when the capacitor cathode cap value is reduced on the very front end tube, typically an EF86 or 5879, EF37A etc. Especially for a high gain amp. It results in hum injection into the cathode (when the heaters are supplied by the usual 6.3V AC). The varying amount of coupling between the heater & cathode, even if not frank leakage, is such that different tube specimens will result in different amounts of hum injection.

I once tested 20 NOS 5879's that all produced a different amount with a reduced or absent cathode bypass cap. This effect is not usually audible if the cathode bypass caps are reduced further along the amplifier chain as the gain is too low. Or it is not an issue if the heaters are DC powered.
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Old 12th Oct 2017, 2:56 am   #8
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Default Re: How do I reduce valve amp HF response

Hum at 50hz can happen in that way.
As things are now there is a little bit of 100hz hum due to a tangle of earth wires that were in the amps as found. I have made some improvements but to fully cure it will require an earth blitz.
As my amps were for cinema use they would have run at high volume most of the time to overcome all the acoustic damping from seating and viewers. This would have drowned out all low level hum up to a point.
Once I have done an earth blitz on one mono block the others will be easy.
I also have room for a hum-dinger where the exciter pot used to be for the optical sensor for the sound track.
There is a bass pot that effectively cuts treble in the feedback loop to the cathode of the preamp. It may well have to be a hum-dinger in the end,.
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Old 12th Oct 2017, 4:15 am   #9
Argus25
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Default Re: How do I reduce valve amp HF response

Refugee,

The amps I had the front end hum problem with absent cathode bypass caps had two more stages of amplification than the one you posted above. The input sensitivity was a few mV and they had plenty of gain even for a Shure V15 magnetic phono cartridge. But the hiss was very low as I recall. I think (might be wrong) that the 5879 is better in this respect than an EF86, might be worth looking into.
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Old 12th Oct 2017, 12:29 pm   #10
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Default Re: How do I reduce valve amp HF response

EF86, pins 2 and 7 should be grounded, but 5879 pins 2 and 6 should be grounded, to reduce noise and hum.
Am I correct in thinking that hiss could be "thermal noise"?
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Old 12th Oct 2017, 12:46 pm   #11
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Default Re: How do I reduce valve amp HF response

That sounds like quite a posh amplifier with a full two valve RIAA preamp on it.
Without a preamp I can hear the signal from a mag pickup but without any EQ.
I am making a preamp for it.
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