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Old 17th June 2012, 08:54 PM   #1
tuhkam is offline tuhkam  Estonia
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Default EL84 SE advice for a beginner

Hi all!

I'm quite a noob with tubes (built some chipamps, headphone amps and distortion pedals in the past), but wanted to try a tube amp also.

I'm currently testing my EL84 SE triode mode amplifier (6P14P to be exact), to get the most out of it before etching a pcb.
The OPT's are not ideal, they are 4,5k/4 ohm (Grundig from 50-60's), but they were really cheap from ebay. The power transformer is giving out 245V AC, this is lowered significantly by my "choke" (a winding of some soviet iron).

The amplifier is working ok and is even a pleasure to listen to.
I was wondering if anybody could give some advice to improve the amplifier further?

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Old 17th June 2012, 10:12 PM   #2
7N7 is offline 7N7  United Kingdom
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Quote:
Originally Posted by tuhkam View Post
Hi all!

I'm quite a noob with tubes (built some chipamps, headphone amps and distortion pedals in the past), but wanted to try a tube amp also.

I'm currently testing my EL84 SE triode mode amplifier (6P14P to be exact), to get the most out of it before etching a pcb.
The OPT's are not ideal, they are 4,5k/4 ohm (Grundig from 50-60's), but they were really cheap from ebay. The power transformer is giving out 245V AC, this is lowered significantly by my "choke" (a winding of some soviet iron).

The amplifier is working ok and is even a pleasure to listen to.
I was wondering if anybody could give some advice to improve the amplifier further?

Click the image to open in full size.
First, well done; it's good that you are enjoying the sound of your amplifier.

As for improvements, well you should increase the available HT; EL84 would be quite happy at 285-300V; you could aim for 35-40 mA at this voltage so as to operate the valve efficiently and at a good operating point.

Next I would bin the 12AU7; 12AU7 is not a good valve; satisfactory in old TV sets, and guitar amplifiers and that's it. Depending on the space you have, Ideally the best alternative would be the octal 6SN7 or the Loctal 7N7 (!) which is electrically the same. However these would require much higher supply voltage - about 400V is ideal with 47k load resistor. If you prefer to remain with lower voltages, then you could try ECC88 or the Russian 6N1P which is happy running at lower voltages. However 140V HT is really too low for any of the popular valves - including 12AU7!. You are running only 2mA through the 12AU7 and it needs to run quite a lot more than that. A higher supply voltage would mean that you could a) use a larger value of anode load and b) run more current through the valve.

Best of luck

Paul
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Old 18th June 2012, 07:15 AM   #3
tuhkam is offline tuhkam  Estonia
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Thanks for the reply Paul,

I have some 6N1P available (circuit needs to be adjusted to the pinout), I think I should try it. My power transformer gives 245V AC, to increase the voltage available after recification the best option seems to drop the choke and perhaps add another CR filter stage to make up for the noise increase.

Is this the right direction?
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Old 18th June 2012, 07:37 AM   #4
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Quote:
Originally Posted by tuhkam View Post
Thanks for the reply Paul,

I have some 6N1P available (circuit needs to be adjusted to the pinout), I think I should try it. My power transformer gives 245V AC, to increase the voltage available after recification the best option seems to drop the choke and perhaps add another CR filter stage to make up for the noise increase.

Is this the right direction?
Your voltage is lower because of choke input. Change your PS to CL(RC) and you'll get some 250+ V if your power transformer has 230-250 secondary. See my schematic in post #22 at 1-stage (killer, literally) amp made of junk. I mostly listen to it in triode with input directly to EL84 (yes, it is in the 50mW range but I still have to turn the volume down with speakers 1m away)
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Old 18th June 2012, 07:42 AM   #5
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What would happen if a cap was placed in front of the choke I.e make a CLC filter?
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Old 18th June 2012, 07:52 AM   #6
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Originally Posted by djgibson51 View Post
What would happen if a cap was placed in front of the choke I.e make a CLC filter?
If there is now 185V at VA point in the schematic then moving the 220u cap in front of the choke would result in some 290V in the same place (assuming EL84s are biased at 40mA each)
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Old 18th June 2012, 08:40 AM   #7
7N7 is offline 7N7  United Kingdom
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Quote:
Originally Posted by tuhkam View Post
Thanks for the reply Paul,

I have some 6N1P available (circuit needs to be adjusted to the pinout), I think I should try it. My power transformer gives 245V AC, to increase the voltage available after recification the best option seems to drop the choke and perhaps add another CR filter stage to make up for the noise increase.

Is this the right direction?
Yes that's good. When you get your higher voltage available, you will need to de-couple the supply from the output stage; a resistor and capacitor are the usual routes to this. You could run the 6N1p at about 80V meaning you could use a 47k or perhaps 39k anode load resistor depending on the HT you have available. I am sure this arrangement will sound much better.

Please ask if this is not clear.

Paul
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Old 18th June 2012, 10:13 AM   #8
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Do a search for the RH84 amplifier schematic. Many many happy users. I'm planning to build that one too.
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Old 18th June 2012, 10:24 AM   #9
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Try modelling your power supply in Duncan power supplies simulator


Download

Loads of fun.

RH84 is a good amp and uses the 12at7 ( ECC81 ) which is pin compatible to the 12au7 ( ECC82 ) Loads of Mullard 12at7's on Ebay for about 1.

Link to the RH84 for comparison, note the feedback.

RH 84 - Tube Audio ...... RH DESIGN

Your output transformers are within 10% of the 5k Ra normally used so should be fine. I've heard good things about Grundig OT's.

Capacitor C5 looks a bit high at 2.2uf, where did you get that value from?

HTH Bill
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Last edited by Soonerorlater; 18th June 2012 at 10:48 AM.
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Old 18th June 2012, 07:09 PM   #10
tuhkam is offline tuhkam  Estonia
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Thanks everyone!

7N7, what do you mean by de-coupling the supply from output stage? I thought I'm ok with a cap between 12AU7 and EL84? Could you post a link to some example.

Soonerorlater, I started with 47nF as C5, as there was not much bass I continued with 470nF, yesterday I changed it to 2.2uF, but it has no improvement over the 470nF, seems to be as good as it gets. I mean it's not bad at all and I realize the OPTs have there limits as well.
As a side note, I also tried some blue rectangular 470nF caps that sounded very bad, later I discovered the X2 marking on them

I have lurked around RH84 before, but as this is my first real tube project I would like to get it running optimally in SE triode with no feedback first, but I will seriously consider a feedbacked pentode mode as a switchable option. The purists might object, but I like a lot of switches on most of my devices

I have looked at 6N1P. Do I understand correctly, that it is ALMOST pin compatible with 12AU7, main difference being that the heaters of two halves are in series (not center tapped by pin 9 like 12AU7) and consume more current?
What to do with pin 9 of 6N1P?
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