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 couldn’t get the volume control past 9 o’clock 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 ‘good’ 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 I’m only a solder slinger and I don’t really understand how this circuit works, I was hoping that someone could tell me whether this is a sensible thing to do, or whether I’m 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.
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 couldn’t get the volume control past 9 o’clock 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 ‘good’ 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 I’m only a solder slinger and I don’t really understand how this circuit works, I was hoping that someone could tell me whether this is a sensible thing to do, or whether I’m 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.
Attachments
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.
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.
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
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
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.
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.
Hi,
I’ve 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. I’ll give the amp a test tonight with some tunes.
Cheers
Colin.
I’ve 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. I’ll give the amp a test tonight with some tunes.
Cheers
Colin.
Attachments
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.
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.
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.
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.
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.
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.
Hi Ray,
Just a quick update – I used 2x 39k higher wattage resistors in parallel as plate load resistor to give 19.5kR – not 18k as recommend but probably near enough.
ECC82 valve remains paralleled. Measured voltages on ECC82 were then:-
B+ = 380VDC
Voltage After 3k2 resistor = 340VDC
Anode Voltage = 105V
Voltage over cathode R of 210 ohm = 2.6VDC (equates to ~6mA per section of ECC82)
Using oscilloscope + signal generator, a 2V pk/pk (1Khz sine wave) through ECC82 gave 28V pk/pk output so gain is ~14 – just sufficient to drive EL34 to full volume (bias on EL34 is 25VDC).
I then hooked up the EL34 amp to 12B4 pre-amp and Vinyl front end for listening tests. Firstly, gain was now much more controllable with volume knob at 11-12 o’clock (compared with 8-9 o’clock when using paralleled ECC83). No sign of distortion at my normal listening levels which is not particularly loud as I only have a small room. Next tried CD player as front end – again volume knob was at 11 o’clock so no problems with this source. Of course, downside now is that the gain is too low for my cheapo Creative MP3 player as volume setting is at 4-5 o’clock to match volume levels with CD/vinyl source. So, at the moment I’ll continue to use ECC82 as the driver valve.
I’m also considering changing to a 6CG7/6FQ7 when funding allows. Any suggestions for a suitable operating point for non-paralleled valve? Would the following be a good starting point for B+ of 380VDC?
Plate decoupling resistor: 22k
Plate decoupling capacitor: 22uF
Plate load resistor: 22k
Cathode bias resistor: 470 ohm.
AlexG: Thanks for suggestion of 6N1P as driver. This looks like an interesting alternative although I suspect that it will give me too much gain (spec sheet says x30) for my current system. However, out of curiosity, I might just give this a try
Cheers
Colin.
Just a quick update – I used 2x 39k higher wattage resistors in parallel as plate load resistor to give 19.5kR – not 18k as recommend but probably near enough.
ECC82 valve remains paralleled. Measured voltages on ECC82 were then:-
B+ = 380VDC
Voltage After 3k2 resistor = 340VDC
Anode Voltage = 105V
Voltage over cathode R of 210 ohm = 2.6VDC (equates to ~6mA per section of ECC82)
Using oscilloscope + signal generator, a 2V pk/pk (1Khz sine wave) through ECC82 gave 28V pk/pk output so gain is ~14 – just sufficient to drive EL34 to full volume (bias on EL34 is 25VDC).
I then hooked up the EL34 amp to 12B4 pre-amp and Vinyl front end for listening tests. Firstly, gain was now much more controllable with volume knob at 11-12 o’clock (compared with 8-9 o’clock when using paralleled ECC83). No sign of distortion at my normal listening levels which is not particularly loud as I only have a small room. Next tried CD player as front end – again volume knob was at 11 o’clock so no problems with this source. Of course, downside now is that the gain is too low for my cheapo Creative MP3 player as volume setting is at 4-5 o’clock to match volume levels with CD/vinyl source. So, at the moment I’ll continue to use ECC82 as the driver valve.
I’m also considering changing to a 6CG7/6FQ7 when funding allows. Any suggestions for a suitable operating point for non-paralleled valve? Would the following be a good starting point for B+ of 380VDC?
Plate decoupling resistor: 22k
Plate decoupling capacitor: 22uF
Plate load resistor: 22k
Cathode bias resistor: 470 ohm.
AlexG: Thanks for suggestion of 6N1P as driver. This looks like an interesting alternative although I suspect that it will give me too much gain (spec sheet says x30) for my current system. However, out of curiosity, I might just give this a try
Cheers
Colin.
Hi Colin,
For a single triode (either 12AU7 or 6CG7/6FQ7, doesn't matter), I would use the following values:
Plate decoupling resistor: 5.6k
Plate decoupling capacitor: 22uF
Plate load resistor: 39k
Cathode bias resistor: 560 ohm.
That should give plate current of about 5.5 mA and plate voltage of about 130v. The higher plate load resistor of 39k should give a little more gain and lower distortion.
As you know, you can adjust the gain to some extent according to whether or not you use a cathode bias decoupling cap.
For a single triode (either 12AU7 or 6CG7/6FQ7, doesn't matter), I would use the following values:
Plate decoupling resistor: 5.6k
Plate decoupling capacitor: 22uF
Plate load resistor: 39k
Cathode bias resistor: 560 ohm.
That should give plate current of about 5.5 mA and plate voltage of about 130v. The higher plate load resistor of 39k should give a little more gain and lower distortion.
As you know, you can adjust the gain to some extent according to whether or not you use a cathode bias decoupling cap.
Hi Ray,
Thanks for the suggested operating point for 6CG7/6FQ7.
I have one final question: If I change to a 6CG7/6FQ7 driver (in place of ECC82) do you recommend I wire it as a single triode stage or is it better to use it paralleled (as the ECC82 is currently)? I.e. is there a ‘technical’ reason why paralleling the driver tube is better than using the single triode stage or vice versa?
Thanks
Colin.
Thanks for the suggested operating point for 6CG7/6FQ7.
I have one final question: If I change to a 6CG7/6FQ7 driver (in place of ECC82) do you recommend I wire it as a single triode stage or is it better to use it paralleled (as the ECC82 is currently)? I.e. is there a ‘technical’ reason why paralleling the driver tube is better than using the single triode stage or vice versa?
Thanks
Colin.
Hi Colin,
AFAIK, parallelling triodes doubles Gm, halves plate impedance and improves signal/noise ratio but leaves mu unchanged. I think there is nothing to be gained in your application by using two parallel triodes of the type you're discussing (12AU7, 6CG7, 6FQ7). Others may be able to add to this.
AFAIK, parallelling triodes doubles Gm, halves plate impedance and improves signal/noise ratio but leaves mu unchanged. I think there is nothing to be gained in your application by using two parallel triodes of the type you're discussing (12AU7, 6CG7, 6FQ7). Others may be able to add to this.
I am very much a believer in the dictum: "If it ain't broke, don't fix it". If the amp sounded good before, and the only problem is excessive gain, then I'd solve the problem with a 6db resistor pad at the input. (Unless, of course, you think you can improve the sonics with a better small signal triode.)
ECC83s (12AX7) aren't real good, but ECC82s (12AU7) are worse. Most of the 12AU7s appear in RF (oscillators, buffers, drivers, frequency multipliers) and digital (astable/monostable vibrators, Schmidt triggers, RS latches) rather than in audio circuits. Make of that what you will. There have been enough complaints over the sonics of the 12AU7 that I wouldn't want to use one as a gain stage. Cathode followers or SRPPs would be OK. Since 12AU7s appear in so many gee-tah amps (where distortion is not a problem, and even desireable) the type is no less expensive than 6FQ7s/6CG7s (the only difference is that the former type doesn't include the electrostatic shield between triode sections that the latter type does include) so I use the 6FQ7/6CG7/6SN7 for cathode follower grid drivers.
ECC83s (12AX7) aren't real good, but ECC82s (12AU7) are worse. Most of the 12AU7s appear in RF (oscillators, buffers, drivers, frequency multipliers) and digital (astable/monostable vibrators, Schmidt triggers, RS latches) rather than in audio circuits. Make of that what you will. There have been enough complaints over the sonics of the 12AU7 that I wouldn't want to use one as a gain stage. Cathode followers or SRPPs would be OK. Since 12AU7s appear in so many gee-tah amps (where distortion is not a problem, and even desireable) the type is no less expensive than 6FQ7s/6CG7s (the only difference is that the former type doesn't include the electrostatic shield between triode sections that the latter type does include) so I use the 6FQ7/6CG7/6SN7 for cathode follower grid drivers.
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