Stereo valve amp not working.

[dinkydi]

I think that the crux of the matter is in post #76: "The volume is at maximum and I control the volume locally...", that is, the amp is probably being run flat out (with above normal amplitude levels, perhaps regularly into overdrive) without any significant load.

The headphone level is being set only AFTER the amplifier. To make matters extreme there is at least a 100k-to-50 voltage divider together with "a volume-control switch" between the amp and headphones.

Unfortunately, to be kind, this is not a well thought out configuration and is likely to result in valve and transformer damage as others have explained. This valve amplifier is NOT suitable for headphone use and should always have the speakers attached when switched on.

The excellent headphones that you are using are so sensitive they do not need an amplifier of any consequence to make them perform. They might work well BEFORE the amplifier.

Peter

[maninashed]

Hi Don, from your measurements it would appear the valves are being under-run, but you are running the amp flat out into what is virtually NO LOAD. You have a severe mis-match on the output, this will cause high voltage spikes as the output transformer tries to dissipate the energy you are pushing in. As I stated in an earlier post, this amp needs a good pair of speakers connected and the volume does not need to be at max. Please set it up with some speakers attached and see how the amp performs under 'normal' conditions. You may be lucky and it may be ok but I would not use those headphones in the way you are doing, If you wish to use headphones, invest in a small headphone amp. Let's see how its performing with a properly matched load.
Bill

[Station X]

I take you point Peter. We also read:-

Note this output is not original on my amplifier, I later added this thanks to Piet from Jukebox-Revival.
He first advised 470 Ohms resistors for my headset (this was a Synq headset with 32 Ohms).

Later on I replaced the resistors from 470 Ohms to 100 kOhms, because the output volume was to high even when the volume was low.
Note I use my computer as media center and the volume is transmitted trough HDMI to my TV, from my TV to the amplifier by shielded analog cables.

It appears that the "modification" for the headphone connection was devised by an employee of Juke Box Revival the suppliers of the amplifier kit. I would have thought he'd have devised something bettter than a 470R resistor in series with the headphones. Perhaps there's more to this modification than meets the eye? Maybe it's connected to the input stage and intended for use with the amp being operated into a dummy load?

I also note that the original 470R resistors have been replaced with 100K, so if they're in series with headphones connected to the LS socket, the mismatch is now even worse.

[Herald1360]

Originally Posted by Station X

Perhaps there's more to this modification than meets the eye? Maybe it's connected to the input stage and intended for use with the amp being operated into a dummy load?

Where said dummy load should be something like 8 to 10ohm resistors rated for the maximum output the amplifier can deliver!

[Don_Zalmrol]

I don't the think my output transformers are busted. When I play music trough my speakers (2 Artsound AS30 max 100W and 8 Ohms) the music sounds perfect and even after a while (30min or so) it's still playing perfect (no screeching sounds, strange noises, …)

I also think the original 470 Ohms (now 100K Ohms) resistors are a dummy load.
On the devised schematic I've gotten from Piet, I see that the resistors are (like Graham says) in series with the output transformers (left & right) on the 8 Ohms output and the GND.

It might be very plausible that my amp is not suited for headphone usage and now after 2 years is showing the strain of the headphone usage. If the amp is really not suited to usage with a headset, I will look at a new valve amp for headsets.

Also my speakers are always connected even when the headphones are connected.
My main speakers are 2 20year old Kenwood LSK-700 speakers (max 160Watt, 8 Ohms).

I will then order 2x new 470R/1W metallic resistors, and will not set my amp to max. volume.

Late on today, I will adjust the anode voltages to 25mV and post the results of P8 and P5 back to you guys when I have my cable clamps.

[Station X]

So you have 8 ohm speakers connected and in parallel with them 50 ohm earpieces in series with 100K resistors? I don't see how you can keep the volume from the speakers down.

Your amp is rather wasted with headphones and you'd do better to connect them directly to the audio source.

[Don_Zalmrol]

yes, the jack input uses a switching system.
Headphone jack in => "switches off" the speakers => only the headset plays the music
Headphone jack out => "switches on" the speakers => only the speakers play the music

So in other words my amp is overkill for the setup that I am using (for headsets). And this overkill is the results of the recent valve #2 malfunction?

So if I where to look for an amp kit for headsets, this is a far better one for my setup.
http://jukebox-revival.eu/index.php/...ersterker.html

Or if you guys have other suggestions?

[Station X]

So in other words my amp is overkill for the setup that I am using (for headsets). And this overkill is the results of the recent valve #2 malfunction?

In my opinion yes.

As has been suggested, connect up some speakers, set the anode/screen currents to 25mA and play some music for a few hours whilst observing the valves. If they start overheating check the anode/screen currents and bias voltages again.

[Sean Williams]

I have a question.....

At no point have I seen a picture of a glowing anode, just glowing heaters, and classic ion bombardment.

Is the OP looking at the anode structure, or confusing this with the heater?

[maninashed]

Don, yes it is over-kill for headphone use, you can still use them but you would need a dummy load, of say 10ohms rated at 25 watts, and they will get HOT! So best advice is as already stated, either use your headphones direct from your computer etc and keep your amp for when you want to play it through the speakers. Just make sure the amp operates without fault when run through the speakers. Set ALL valves to use say 25ma cathode current and see howthat goes, you can push them a lot harder, but lets just get things working ok first.
I think we may be getting somewhere chaps, Seasons Greetings to everyone.
Bill

To Sean, I think there has been some confusion over the difference between the normal ion bombardment blue glow and a gassy valve being over-run. Anyone used to dealing with EL34s will have seen a slight blue glow seen when the lights are low(almost in the dark) its especially noticeable when HT voltage is very high for example in the back of a 100w Marshall in good fettle! looks fantastic. 807s have a fantastic blue glow also.

[dinkydi]

Hi Graham, The 470R (or similar resistance) output interface for headphones is quite common (Sansui often use 220R, for example) but it is employed in amps that can handle operation without speakers (for example, most solid-state designs). It is not appropriate for a transformer-coupled valve amp, if the speaker is also disconnected.

Whether the series resistor is 220, 470, or 100Kohms it makes no material difference in the sense that the impedance is so high that the amp in all cases is effectively operating without a load, when the speakers are disconnected.

I acknowledge that if the signal level is always kept low when using headphones via the modification then the user may get away with it, but it is an accident waiting to happen.

However the poster's scenario is quite extreme. As I noted, the amp is operated at maximum with headphones and the speakers disconnected as a matter of course! It is the worst combination of circumstances and only a matter of time before breakdown occurs.

Peter

PS This was intended as a response to post #83 but I'm going to need to increase my typing speed to keep up with you all!

[Station X]

According to post #85 the speakers are always connected even when headphones are used. Whether plugging in headphones disconnects the speakers internal to the amp I don't know.

[Don_Zalmrol]

Originally Posted by Sean Williams

I have a question.....

At no point have I seen a picture of a glowing anode, just glowing heaters, and classic ion bombardment.

Is the OP looking at the anode structure, or confusing this with the heater?

Is the anode not the above the the valve footer or is this the heater?



@Graham, Bill & Peter.

I use the amp always with the headset, almost never with the speakers. But the speakers are always connected to the amplifier (like Graham acknowledges and what I wrote in post #85).
The input jack has an "automatic" access switching system that makes or brakes a connection (like a relay). When I plug the headset in, it breaks the connection to the speakers and parallel to that connection is the 100K Ohm resistors soldered upon as a "dummy-load".

So do I need to change the resistors back to 470R or not?

If indeed my amp is not suited for a headset, I will remove the connections from & to the jack input and restore them to the original state.

I can always use this amplifier for something else with the Kenwood speakers.
But I do want to power my headset trough a valve amplifier, is the EL84/12AX7 kit suitable for this task or is it again overkill?

I will however change the voltages to 25mV to change the current to P8. And see and report what happens, as soon when I have those cable clamps.

Thanks!

[Station X]

No matter how good your amplifier is, what you get out of it will never be better than what you put into it. So if your audio sources are capable of driving headphones directly you might as well use them that way.

If you must have a valve amp, chose a low power one specially designed for use with headphones.

Enjoy using you present amp with the output fed to speakers.

[Herald1360]

Originally Posted by Don_Zalmrol

So if I where to look for an amp kit for headsets, this is a far better one for my setup.
http://jukebox-revival.eu/index.php/...ersterker.html

Or if you guys have other suggestions?

Even that amp is total overkill for headphone use. I would think that little more than a couple of 12AU7 valves would be needed for a headphone driver- particularly sensitive 'phones.

Something like these, perhaps:

http://gilmore2.chem.northwestern.ed.../ciuff_prj.htm

As others have said, if you can drive the headphones direct from the source, putting any further amplification in between will only alter the sound from the original. Of course, if the alteration provides a sound more to your liking, then by all means go for it.

[maninashed]

Even if the speakers are connected, it seems to me its wired so the speakers will disconnect when you plug in the headphones. Otherwise, with the amp on full volume you would be rattling the windows! Even the el84 amp is way too powerful for use with your headphones. A small solid state one will be fine and can be purchased quite cheaply, if you are insisting that you need one.
Bill

[dinkydi]

Hi Graham, Thanks for pointing that out. I believe that the speakers are connected to the switch which is an integral part of the phono socket (headphone socket). They are always connected to the circuit, but the switch turns them off when headphones are plugged in.

I think this interpretation is confirmed by the description in post #87.

Peter

PS My typing speed still hasn't improved - this is in response to #92

[Sean Williams]

The anodes on the EL34 are the outer metal plates - these run the whole length of the electrode structure, and are the part you are looking through to see the purple/blue glow.

The bit you can see glowing at the top and bottom of the valve would be the heater assembly.....

[Don_Zalmrol]

@Sean Williams:
Then it would be the heater plates I'm looking at. Valve socket #2 seems to be a bit brighter then the other valve sockets… (like I showed in my other pictures of the valves, it might be normal)

@Herald1360:
Thx for the link, the problem is I've never made my own PCB. So if there is an existing kit with housing and so on available I welcome the link(s)

But I see that this amp delivers 6 Watts, and the one from Jukebox-Revival delivers 12 Watts so this is not such a big overkill or is it?

And I liking listening to music, tv trough my valve amp. The sound is much more different then the on amplified from my TV.

@dinkydi:
I will look up the revised schematic for use with the headphone jack, but I need to find it and scan it. I will post here in the topic

[dinkydi]

Hi Don (response to #99), Put your amp aside and only use it with speakers.

You obviously like the sound of clipped or compressed music. This preference is quite common. You should be looking at an audio signal processor to drive your headphones.

Many types of analog processors were used in recording studios that I think you would have great fun with. They often appear on the auction sites. They may already have a headphone socket, or you could fit your modification. Look for compressors where you can vary the settings.

Peter