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Old 3rd September 2015, 01:32 PM   #1
wdecho is offline wdecho  United States
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Default 6EM7 SET Amplifier

This is my first tube build and I would like the members opinion of this design.

6EM7 Single-Ended Triode (SET) Vertical Amplifier

I am going to build a more conventional case and have everything to start except the transformers which should be here today. My first newbie question is, will there be a problem building the power supply section first and testing it without a load as I always do with SS circuits? I understand you need a load before turning on a tube amplifier. I have very efficient horn speakers and have heard so much about the SET that I wanted to at least hear one on my speakers. I recently build an Firstwatt F6 and am enjoying it but I really want to hear a SE SET without spending a lot of money to start with.
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Old 3rd September 2015, 03:10 PM   #2
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Hello wdecho I know you from another forum, I am "thesloth" on that forum. Yes I know how clever of me to use Phish songs for my handles.


I use large resistors to load power supplies. You would want a 50 watt resistor around the 3k value for 86mA. I am looking forward to you getting your transformers from Edcor!

http://www.ebay.com/itm/like/2512993...chn=ps&lpid=82

Last edited by famousmockingbird; 3rd September 2015 at 03:14 PM.
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Old 3rd September 2015, 03:56 PM   #3
wdecho is offline wdecho  United States
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I received my transformers from Edcor and I am impressed with the quality and especially the packaging. It will be a few days before I use them though.

What is your opinion famousmokingbird of testing the power supply before hooking it up to the amplifier circuit?

By the way it only took 2 weeks from the time I ordered until they were shipped.
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Old 3rd September 2015, 03:57 PM   #4
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Power supply voltages will be high with no load. As long as caps are rated high enough, no problem.

Pentode amps can generate high voltage spikes at the output transformer if driven into clipping with no load... much less likely with triodes.
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Old 3rd September 2015, 04:08 PM   #5
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Yes I am very impressed with Edcor too!!! 2 weeks is really good.....the last time I ordered I had to wait 2 months! But I did order like 12 transformers.


Quote:
What is your opinion famousmokingbird of testing the power supply before hooking it up to the amplifier circuit?
I like to test power supplies before using them with a particular circuit. I have a few high powered resistors like the one I linked in my last post that I use. Since you are building a Class A amp the current shouldn't change much during operation so a resistive load is a good approximation of the actual circuit it will be hooked up to, i.e. constant current draw. The current draw will be at it's highest during idle.

I am not a huge fan of the rheostat in the power supply. It's probably best to just calculate the exact value.
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Old 4th September 2015, 09:31 PM   #6
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Quote:
Originally Posted by famousmockingbird View Post
Since you are building a Class A amp the current shouldn't change much during operation so a resistive load is a good approximation of the actual circuit it will be hooked up to, i.e. constant current draw.
Balanced class A draws constant current (if properly balanced). SE does not.

Looking at the load lines at the link in the first post, the output section swings about 60mA of current when fully driven. Not at all constant current draw.

Quote:
The current draw will be at it's highest during idle.
It will be highest when grid approaches 0V, getting close to 75mA from 42mA idle.
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Old 4th September 2015, 09:36 PM   #7
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wdecho: It's a good schematic to get you started building. Pretty good curves on the tubes, simple design and room for improvement should you wish to get deeper.

I'd make the 25F cathode cap bigger, and also the 47nF coupling cap, along with making the 500k resistor something much smaller.
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Old 4th September 2015, 09:47 PM   #8
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MrCurwen, sorry to burst your bubble but my statement is correct. Or maybe you need some new reading glasses Let me reiterate;


Quote:
Originally Posted by famousmockingbird View Post
Class A amp the current shouldn't change much during operation so a resistive load is a good approximation of the actual circuit it will be hooked up to
I never once said SE.

Here it is from someone much smarter than the both of us combined:

Valve Wizard: "If the amplifer runs in class A then its current demand does not change much with drive level, so as far as the power supply is concerned it looks like a constant resistance"



When looking at load lines that is the AC impedance. There is very little DC resistance in the primary of the output transformer so the DC current draw will cause very little drop in voltage.

I stand by what I said in that a resistive load is a perfectly good approximation to test the power supply for this circuit.
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Old 4th September 2015, 10:32 PM   #9
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Quote:
Originally Posted by MrCurwen View Post
.. along with making the 500k resistor something much smaller.
May i ask why? Datasheet limitation or some other consideration? Sorry i am on the phone so i can't check 6EM7 datasheet yet. I thought 5x the plate resistor of the preceeding stage (100k) is a good value to not overload it?
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Old 4th September 2015, 10:50 PM   #10
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Quote:
Originally Posted by ballpencil View Post
May i ask why? Datasheet limitation or some other consideration? Sorry i am on the phone so i can't check 6EM7 datasheet yet. I thought 5x the plate resistor of the preceeding stage (100k) is a good value to not overload it?

Seriously I question why but I didn't really want to ask. I usually shoot for 10x for best results. I believe the output Z of first stage is somewhere around 28k, I wouldn't go lower then 270k. But then you got to go with a bigger coupling cap. Personally I would use 470k (I don't know the reason for the 500k value in original schematic).
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