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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 21st Feb 2017, 1:18 pm   #1
Clockwork_TV
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Default How to bias Philips EL 3536 valve tape amp?

Hi All

I've got a Philips EL 3536 stereo tape recorder with built-in valve amp. Recently it's started to sound a bit 'harsh'. I've never biased it so wondered if that might be the problem. I've got the service manual and it says to stick a voltmeter into a hole mp1 or mp2 in the side and it should read 70mV when set to stereo (but is that as is or playing?). Adjust capacitors C21 and C121 accordingly for each channel.

Firstly I put the red meter connector into the hole (MP1 or MP2) but what about the gnd? I tried sticking it into the speaker gnd socket. Anyway whatever I did I couldn't get a reading.

Secondly I opened the back to find C21 and it looks a bit odd - not a simple screw hole. Looks like a nut? Is that normal for an adjustable capacitor?

Any thoughts?

Thanks
John
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Old 21st Feb 2017, 1:25 pm   #2
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Default Re: How to bias philips EL 3536 valve tape amp?

Not worked on one of these but for most tape recorders the meter would be an AC millivolt meter capable of an accurate reading at the bias frequency which will be considerably higher than audio frequencies.

Lawrence.
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Old 21st Feb 2017, 1:37 pm   #3
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Default Re: How to bias philips EL 3536 valve tape amp?

Well that's a help already as I thought it'd be DC, doh! I've got a CRO up and running now so could use that?
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Old 21st Feb 2017, 1:50 pm   #4
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Default Re: How to bias philips EL 3536 valve tape amp?

You could use a 'scope, X10 probe for minimum loading and allow for the fact that you'll be looking at peak and not RMS.

I can't comment on those capacitors or much else as I don't have the manual.

Lawrence.
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Old 21st Feb 2017, 2:44 pm   #5
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Default Re: How to bias philips EL 3536 valve tape amp?

This is a bit confusing. Do you mean 'harsh' on playback or when you make recordings?

The 'bias' as such is a high (above audio) frequency signal used and applied ONLY when recording, if that's what you're talking about?
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Old 21st Feb 2017, 3:20 pm   #6
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Default Re: How to bias philips EL 3536 valve tape amp?

I've just looked at a schematic, the bias adjustment capacitors are C22 and C122, not C21 and C121....so far as I can make out.

The "nut" on the capacitor is normal if they are Philips concentric types, turning the nut turns the whole of the variable part of the capacitor's assembly, turn clockwise for increased capacitance (more bias) and anti clockwise for less capacitance (less bias).

Lawrence.

Last edited by ms660; 21st Feb 2017 at 3:50 pm. Reason: Correction
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Old 21st Feb 2017, 4:43 pm   #7
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Default Re: How to bias philips EL 3536 valve tape amp?

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Originally Posted by Techman View Post
This is a bit confusing. Do you mean 'harsh' on playback or when you make recordings?

The 'bias' as such is a high (above audio) frequency signal used and applied ONLY when recording, if that's what you're talking about?
The sound is harsh when playing back.
Hmm looking at the diagram the bias capacitor is at the head.
What I was thinking of was the biasing of the stereo valve power amplifier to the speakers during playback. Does this not need biasing regularly? (Like on a Marshall guitar amp?) Or does this circuit not need that?

So if not biasing what else could make the sound 'harsh'.

Last edited by Clockwork_TV; 21st Feb 2017 at 4:49 pm.
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Old 21st Feb 2017, 4:49 pm   #8
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Default Re: How to bias philips EL 3536 valve tape amp?

Hi,
there are at least two types of biassing, the voltage biassing of the output stage valves, and the amount of high frequency bias used to help the signal be recorded on tape.
if the problem is on playback then it is more likely to be a problem with the playback stages.
One point to note is that some of the DVM's available won't measure high frequencies correctly.
Have any of the capacitors been changed ? Probably some of the electrolytics might need changing by now, and possibly some of the coupling capacitors as well.
regards Peter B
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Old 21st Feb 2017, 4:52 pm   #9
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Default Re: How to bias philips EL 3536 valve tape amp?

Are you sure you want to risk upsetting the bias-settings ?
In my service-manual (Dutch language version) it's shown in one place to adjust bias in mp1 & mp2 (Measure from Ground to one and the other test-point) for 70mV and in another place to adjust for 50mV.
Quite a big variation and which setting is correct, if any ?
You really can't say as it's quite dependant on what tape brand and quality you're using.
Also it's unlikely the bias has changed because of the variable cap's drifting. They're air-trimmers afaik and very stable.
What IS likely is that your output valves (EL84) are showing age.
One valve is used as the bias-oscillator/generator and if that's getting tired it'll have influence on bias.
I suggest to start the recorder in play-back mode, volume down and measure both EL84 cathode voltages.
They should be equal and if they are most likely the valves are good. If not something's not OK.
Also you need both cathode decoupling electrolytic cap's to be OK which I'll bet they're not !
Not after this many years anyway, so they should be replaced in any case.
This is how I'd approach a 'bias-problem' before starting to worry about the actual re-biasing.
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Old 21st Feb 2017, 4:56 pm   #10
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Default Re: How to bias philips EL 3536 valve tape amp?

The 'bias' in a tape recorder is only used during recording, and any adjustment to it won't make any difference at all during playback.

The valve power amplifier in a tape recorder does not require a bias adjustment. It's very different to a high-power guitar amplifier. It's possible that the coupling capacitor to the output valve's grid is leaking, which can upset the output valve's bias, which can result in a harsh, 'Dalek'-type sound if it's really bad.

The sound being 'harsh' is difficult to diagnose. Perhaps you could record how it sounds using a smartphone's video recording facility and post the result on YouTube? That might help us understand what's going on. Different faults have characteristic sounds that some people on here might recognise.

That said, I think the most likely explanation is a faulty speaker. Has dirt collected in it, causing rattles? Is the cone torn or damaged? Or is the voice coil rubbing on the magnet? That has a characteristic harsh sound. You can check this by gently pushing on the cone with your fingers, being careful to apply even pressure around the centre. It should move freely without making any sounds. If it makes a light scraping noise, it's faulty.

Chris
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Old 22nd Feb 2017, 1:48 pm   #11
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Default Re: How to bias philips EL 3536 valve tape amp?

Thanks for the feedback
This is purely about playback -

So biasing isn't an option.
It's not the speakers because I think it sounds the same through my hifi amp via the 5 pin din output (I'll have to check again).

When I say 'harsh' it's not that obvious (I think if I recorded it on my phone it wouldn't come across) but it is noticeable compared to before, maybe a touch distorted - kind of sounds like the input is a little too loud. I've seen this mentioned before about valve amps sounding 'harsh' which is why a say this.

So reading above it could be the valves getting old or leaky caps. I'll try investigating at weekend
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Old 22nd Feb 2017, 1:56 pm   #12
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Default Re: How to bias philips EL 3536 valve tape amp?

Quote:
Originally Posted by Clockwork_TV View Post
Thanks for the feedback
This is purely about playback -

So biasing isn't an option.

So reading above it could be the valves getting old or leaky caps. I'll try investigating at weekend
It could be that leaking coupling capacitors are causing the valves to be biased incorrectly. The easiest way to check for this is to measure the voltage between chassis and pin 2 of the EL84 valve holders. The voltage should be zero.

Looking from under the chassis, valve holder pins number clockwise from the gap.
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Old 23rd Feb 2017, 5:39 pm   #13
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Default Re: How to bias philips EL 3536 valve tape amp?

If the distortion is audible via the auxiliary output, it's unlikely to be due to a fault in the power output stage.
Since you have a scope, why not record a tone (square wave if you can manage it, sine wave if not) and play it back while monitoring with the scope. What you see might pin down the problem more tightly.
Is the harshness only on new recordings, or on those which previouly sounded OK?
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Old 24th Feb 2017, 6:54 pm   #14
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Default Re: How to bias philips EL 3536 valve tape amp?

I only play prerecorded tapes on this so nothing new. I'll see what I can do with a wave. Cheers
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Old 25th Feb 2017, 8:02 am   #15
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Default Re: How to bias philips EL 3536 valve tape amp?

Quote:
Originally Posted by cmjones01 View Post
The 'bias' in a tape recorder is only used during recording, and any adjustment to it won't make any difference at all during playback.

Chris
Is that strictly true, would an incorrect recording bias not affect the quality of the recording when it was played back?

Mike
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Old 25th Feb 2017, 8:27 am   #16
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Default Re: How to bias Philips EL 3536 valve tape amp?

That's why I asked the OP whether the problem was only with recent recordings.
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Old 26th Feb 2017, 4:37 am   #17
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Default Re: How to bias Philips EL 3536 valve tape amp?

If bias were set too high on the EL 3536, playback would be somewhat
toward the bass sound, lacking highs. These would have undergone
self erasure during recording.

On the other hand if bias was too low, in the extreme , i.e.
the erase head could not properly remove a prior recording,
the sound would be annoyingly harsh.

The TP1 and TP2 bias adjustment in the Canadian manual (1962)
EL-3536/ EL 3536A is 50 mV.

The bias trimmers may be easily pranged, if the sealant has vitrified
after 55 years.

Since the query is playback, TP1 and TP2 are the points needed to adjust
L1 and L101, if the tests of the frequency characteristics diverge from
the playback and record table values at various frequencies. The latter
values are had by killing the bias oscillator during record by shorting out
one section of the bias oscillator transformer during record.

An odd cause of harshness, from those days was certain tapes squeaking
as they passed the play head.
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Old 6th Mar 2017, 1:54 pm   #18
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Default Re: How to bias Philips EL 3536 valve tape amp?

Hi - so I got my little signal generator 1khz working and applied it to the mic input and could hear it when I set the machine to PA. I could control the volume with the mic I/P too. Is it safe to put the CRO leads across the speaker cables with the speakers still attached to see the o/p signal and compare signals?

I also have another of these machines where only one channel plays back.
As well as checking voltages with a DVM I could perhaps use the CRO to see where the signal is getting through to. However I would need something to cut out the DC voltage to the CRO any suggestions?

Cheers
John
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Old 6th Mar 2017, 2:04 pm   #19
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Default Re: How to bias Philips EL 3536 valve tape amp?

According to the schematic one side of the loudspeaker(s) is grounded so no problem, 'scope probes earth lead to chassis and probe on the speaker connection.

To check waveforms at other circuit points then keep the probes earth lead connected to the chassis, switch the 'scopes Y input to AC, center the trace with the Y shift and probe the circuit.

Lawrence.
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Old 6th Mar 2017, 3:37 pm   #20
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Default Re: How to bias Philips EL 3536 valve tape amp?

Cheers for that - just to double check the CRO can deal with cutting out voltages up to around 150V DC?
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