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Old 11th November 2008, 09:16 PM   #1
colinB is offline colinB  United Kingdom
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Default Too much gain - help with changing driver tube.

Hi,
I recently changed the output trafo's on an old triode strapped EL34 SE power amplifier I made ages ago based on the attached design. The only difference from schematic was I used a 68kR in place of the 81kR (I already had it in my small stock of resistors) and the 12uF cap was replaced by 22uF. I used it for some time on test in the garage using a cheapo MP3 player as a source. I was pretty happy with the sound overall.

Finally, I decided to use it in my main system in the house however I quickly realised that I had way too much gain. My set-up was Project 5 T/Table with Ortofon 510Mk2 - VSPS Phono stage -12b4a pre-amp - El34 power amplifier - Fe167 Half Changs. I couldnít get the volume control past 9 oíclock on the pre-amp as it was too loud! So I could put a pot divider on the RCA connectors into the EL34 amp but I thought also about changing the driver valve.

So after some checking around I decided to replace the paralleled ecc83 with ecc82 (still paralleled) primarily as the pin-outs are identical so I would have minimum fuss in changing over, (and I already had a couple of ecc82 to hand.)

So, the first step was simply to replace ecc83 with ecc82 but I also changed the 100kR anode resistor to 50kR leaving everything else the same. I measured ~70V at anode and ~2.7V on 1kR cathode resistor. I tried this set-up and the gain was now much more manageable (~11 -12o'clock on volume knob). However, looking at the spec sheet for ecc82 it appears that a Ďgoodí operating point is ~100V on anode and 4V on cathode (~2mA).

To move to this operating point, with my set-up I calculate that I would need to change the 68kR to 20kR. As Iím only a solder slinger and I donít really understand how this circuit works, I was hoping that someone could tell me whether this is a sensible thing to do, or whether Iím about to do something really dumb. Or should I leave the ecc82 as is at 70V and not worry?

Any help would be appreciated.

Thanks
Colin.
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Old 12th November 2008, 12:49 AM   #2
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Hi,

Firstly, you don't need to parallel the two halves of the ECC82, as even using only one half has a much lower plate impedance than the parallel ECC83. However, for convenience to save having to rewire the tube socket, you can do so.

What you must do is reduce the values of the cathode bias, plate load and plate decoupling resistors. This is because the ECC82 needs to take a lot more current than ECC83. Biasing requirements are also different.

Ideally, you need to calculate from the ECC82 tube curves what operating point to use and select appropriate resistor values.

Alternatively, you can do a bit of trial-and-error testing, based on "reasonable" starting values. I'd take a guess at starting values, assuming you still have both halves of the tube in parallel, as follows:

Cathode bias resistor: 220 ohm (was 1.2k)
Plate load resistor: 18k (was 100k)
Plate decoupling resistor: 10k (was 81k)

If you're using only one side of the ECC82, you should double the resistor values.
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Old 12th November 2008, 05:40 AM   #3
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Colin,
Before doing massive changes grab the side cutters and get rid of the cathode bypass capacitors (3 off shown) on the 12AX7 stage.
That will drop the gain - maybe by enough, maybe not. It will also drop the distortion by the same ammount.

If that does'nt give you enough gain reduction then do what Ray says.

Cheers,
Ian
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Old 12th November 2008, 10:45 PM   #4
colinB is offline colinB  United Kingdom
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Hi guys,

Thanks for the quick responses.

Gingertube: I should have said in the first post that I have only one electrolytic bypass cap on the cathode and I've already tried disconnecting it and I still have too much gain. Thanks for the tip though.

Ray_moth: Thanks for your detailed suggestions. I've had a quick play to-night following your suggestions. I left ECC82 valve paralleled to minimise re-wiring of socketHere's a quick summary of where I'm at

For cathode bias resistor I had no 220R to hand so I paralleled a 270R with the 1kR that was already installed to give ~212R.

For Plate load R I had no 18kR so I paralleled a 22k with the 100k that was already connected to give ~18kR.

Plate decoupling resistor of was replaced with 10 kR.

These are the DC voltages I measured:

B+ = 380-400V depends on time of day as mains fluctuates alot.
Voltage prior to plate load resistor = 290-295V
Anode Voltage = 95-98V
Voltage at Cathode Resistor = 2.3V (Ia = ~11mA).

These values look sensible based on Ohms Law. However, I don't really understand loadlines etc so my question is 'Is this a good operating point for the ECC82 valve?' or is more tweaking required? Any comments?

Thanks again
Colin.
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Old 13th November 2008, 09:02 AM   #5
colinB is offline colinB  United Kingdom
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Hi,

Iíve had a quick look at load lines for the ECC82 this morning. I attached a modified load line diagram showing my new operating position (95V, 5.5mA) for the paralleled ECC82. From my limited knowledge it looks like the valve should be able to swing ~60V pk to pk before grid current arises. From memory, the EL34 is biased at ~25V in this circuit so I should be ok. Iíll give the amp a test tonight with some tunes.

Cheers
Colin.
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Old 13th November 2008, 10:17 AM   #6
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Hi Colin,

Your plate voltage seems low. I think you will find a better operating point if you reduce the decoupling resistor to about 3.3k. You might then need to increase the decouplig capacitor to 32uF.
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Old 13th November 2008, 08:25 PM   #7
colinB is offline colinB  United Kingdom
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Hi Ray,

Made the changes you recommended in your last post i.e. changed decoupling cap to ~33uF/450VDC (22uF+10uF in parallel) and decoupling resistor 10kR + 4k7R in parallel (~3k2R).

ECC82 is still paralleled.

Voltages measured as follows:

B+ = 400VDC
Voltage after decoupling resistor (3k2R) = 358VDC.
Voltage at Anode = 115VDC.
Voltage at cathode = 2.9VDC.
Ik = 2.9/210 = 13.8mA (i.e ~7mA persection of ECC82).

So overall small increase in anode voltage (~20V) from previously. Also I'm pretty much out of suitable power resistors now as I only have mainly 2W to hand and the voltage drop over 18kR resistor is now ~250V so I really need a 5W type there.
I can order more but It would be better to do this when the operating point has been decided.

So, what now? Is this a good operating position for the ECC82 or are more trimming changes required?

BTW thanks for all the advice over the last 2 days - much appreciated

Cheers
Colin.


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Old 14th November 2008, 06:16 AM   #8
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Hi Colin,

That operating point seems as though it should be OK. Have you done a listening test?

People will probably tell you that there are better options than an ECC82, which is not a particularly linear tube. The 6CG7 and 6FQ7 are better, but you would need to rewire the base because of different pin-out. Someone might also suggest the ECC81 (12AT7), but its amplification factor may too high for your purposes.

Anyway, you're probably better sticking with the ECC82 until you get the result you want, then you can worry about reducing distortion if you find you need to.
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Old 14th November 2008, 08:45 AM   #9
colinB is offline colinB  United Kingdom
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Hi Ray,

I've not carried out any listening tests yet as the resistors I'm using will burn up if the amp is left on for any significant length of time. I need to order some higher rated ones particularly the 18k ones.

I'll stick with the ECC82 for now even though it is probably not the best option here - after all I already had these in stock from a previous aborted project so no financial outlay was involved.

Thanks for the tips about using 6CG7 and 6FQ7 - might be something for the future.

I'll let you know how the listening tests go when I've installed higher rated resistors.

Thanks

Colin.
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Old 14th November 2008, 01:22 PM   #10
alexg is offline alexg  Philippines
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6n1p will be a great driver for the EL34 in SE configuration. 47K plate resistor and 1K cathode resistor, bypassed with about 100uf. This will work with your current B+ of 380 to 400VDC.
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