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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 19 Jun 2020, 22:12 
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houston99 wrote:
HI Gregg,

I read your project with great interest as I am in the process of building a SE EL34 amplifier. A few design questions I would like to clarify with you. Hope you can point me to the right direction.

1. The bias current for 12AT7/ECC83 seems to be low. It looks like you prefer 470K anode resistor, so the bias current has to be lower than 1 mA. Most designs I found on the Internet biased it hotter. I currently have a triode strapped 6EJ7 runs at 8 mA bias. I just wonder should I ditch it and move to low current 12AT7?

2. The cathode diodes for 12AT7 are 2X 1N4007. The voltage drop should be similar to an IR LED. I currently use an IR LED for my 6EJ7. What is your opinion on the sound of 1N4007 vs IR LED? Somehow I had the impression that LED is less noisy than diodes and is preferred. But I am not sure is it important here.

3. For EL34 output stage, you said that you prefer ES UL. What does it mean by "ES", a diode connected? How does it compare against the 22V Zenner that recommended by RH amplifier? Is it important to have a lower UL voltage against the plate?

4. For the CCS, you choose a 5 watt 15 ohm resistor. I assume you like the sound of it. Otherwise, it is an overkill for 1.25 V low current application.

5. One of my assumptions that people in general do not prefer triode connection due to lack of power. I somehow believe that a single stage triode driver just do not have the gain to drive triode with ease. This might be particularly the case for a low biased 12AT7. Have you tried other driver configurations in your amp with triode output? If so, what did you find out?

6. For UL output, have you tried to incorporate Kegger Schade feedback design? George from TubeLab apparently does not like the sound of Schade feedback in his design. He prefers CFB as in your design. In your case, it can be done by adding a 200k resistor between two plates. If you have tried, I would appreciate some comments.

7. For pentode operation, have you tried RH Universal design? If it works well, I don't need output transformers with a UL tap. In RH Universal, 12AT7 has an un-bypassed cathode resistor, a Schade feedback resistor between two plates, and no CFB at EL34.

8. You mentioned that you tried 6L6 in the circuit. I have both EL34 and 6L6. 6L6 apparently has lower gain and may not sound as loud in the same circuit. What is your observation?

Sorry for a long list of questions. This thread is exactly what I was looking for. Hopefully you don't mind to share your thoughts.

Bing


Another Houston. About time.

My EL34 amp is one of the best sounding amps I have made. Suncalc tweaked a more uinversal amp for me to make it more EL34 specific. It worked. In fact it worked so well a friend heard It the same week I made it and bought it.

http://diyaudioprojects.com/Forum/viewt ... hilit=EL34

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PostPosted: 20 Jun 2020, 06:00 
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Anode resistor on the driver tube is 390K not 470K.

And switchable to 150K in different configurations. :)

Cathode diodes are not 2 X 1n4007 :)
If you look at the blue box one is a 1n4007 the other is a 1n5818

I don't like LED bias not a great fan of the sound and at low current levels you have to use a feeder resistor to push the LED into a more linear region of operation.

The ES is a diode connected in the U/L circuit, its important to look at the RH ideas and then realise there is a direct correlation with voltage on the grid vs sound. Think about triode strapped which used anode voltage, U/L uses 25% between main B+ and anode voltage. The Zener situation creates a voltage drop across the Zener that moves the grid voltage towards triode operation without the use of the winding tap, however there is a marked difference with operation point output power vs drivability of the output tube and pentode operation which has a sound all of its own.

The ES U/L "sounds Like" triode but with more power and better drivability of the output tube. So triode is harder to drive than pentode and from a sound point of view requires you to move or redesign the driver operation. All this is very subjective and a YMMV situation.

The resistor in the CCS has an effect on sound and the type of CCS VS discrete circuit. NB the type of regulator also has an effect and the LM317 can be snubbed with a ferrite bead to change the conditions of operation.

Regarding Schade feedback, I find the Schade feedback is very circuit dependent maybe even output tube dependant NB it seems to work well on el84 based circuits but to me seems pants with EL34 or 6L6 etc. CFB has an effect of improving the linearity of the output tube without effecting the parts of the circuit ie driver. The CFB idea works in a similar way to tietery windings on the old circuits.

You mentioned that you tried 6L6 in the circuit. I have both EL34 and 6L6. 6L6 apparently has lower gain and may not sound as loud in the same circuit. What is your observation?

I run 6L6/EL34/KT66/5881 etc in this circuit the point is:
The switch on the driver changes the drive and to some extent the gain of the driver, NB the diodes are not quite the same as a resistor in the cathode and have a marked effect on the drive ability. The switch also changes the anode resistor as U/L or triode/pentode are selected (wonder why). The driver tube can be selected 12at7/12au7/12ax7/5751/7025 ect. So the gain or loudness of the different tubes is dependant on the wiring configuration ie an EL34 in triode vs a 6L6 or KT66 in pentode configuration is not the same as all in pentode or all in U/L or pentode.

So its a horses for courses situation in this case the output tube has CFB but the bias is set with CCS where the CCS is at a point so that the different output tubes can be plugged in but a bias point is set to accommodate all tubes to prevent red plating or over bias. So its a compromise situation for convenience. NB the bias voltage is not run through the output transformer to reduce core saturation.

Regards
M. Gregg

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PostPosted: 20 Jun 2020, 12:04 
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Hi,

I also should point out that the two multi position switches are actually on the same shaft.
Ie the switch is a four gang 3 way rotary switch with silver contacts. (break before make with silver plated contacts).

so as you select Triode/Ultra linear or pentode the driver tube anode resistor changes.
But the amp must be switched off before operation.

So two poles are for the left channel and two for the right channel.

Regards
M. Gregg


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PostPosted: 21 Jun 2020, 04:34 
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Hi,

just to expand on ES ultra linear its an idea that I tried out and its interesting you haven't seen it you might find the flag on this page interesting.
So within this project I tried many different ideas this was explained on the first page quote:
"NB Electron stream is copy write protected even if you think its pants or not (not by me)"

I have no connection in any way.

http://www.oestex.com/tubes/oes.html

There are some interesting ideas in the link things like power tubes pulling down the driver supply for the driver tube during transient.
I have tried many of the ideas from the RH amps the ES ideas and loads of others including "real sound" ideas so its been an interesting journey but its a strange journey when you are willing to try anything out of curiosity even if you think it goes against my engineering ideas. But I found it interesting none the less :D

Regards
M. Gregg

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PostPosted: 21 Jun 2020, 05:51 
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Just as a refection,

I don't agree with everything from all design philosophy's but who does? :D

But its always interesting to take a look, and when someone is trying to further ideas in this case:
( It follows then that a useful object of amplifier design would be to eliminate the Screen-grid as a variable from the system.
This proposition is very well explained by Renato D. Tancinco of the Philippines, in his 1961 US Patent 3153766 )

My philosophy is a bit like Bruce Lee take what is useful and don't look at the finger pointing to the stars or you will miss all that heavenly glory.. :D

The problem I find in things like Zener's in grid supplies is by in effect trying to run a tube in reduced grid voltage while in pentode is that pentode can sound very bright and without a stabilised supply on the grid using a Zener gives the reduced voltage but if the supply isn't smooth you get the pentode brightness "Lift" as sometimes seen in the RH design (Schade).

So this has been an evolving project in a fun sense but there hasn't been much that hasn't been wired and tried. Its now used for films music and anything else and tends to be constantly in operation "on". Which is bit strange for me..

NB its interesting to hear the effect of feeder resistors on LED bias<<the effect is interesting (not always good) :)

Regards
M. Gregg

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PostPosted: 27 Jun 2020, 04:45 
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Just a small addition about components.

This is very subjective some people are interested others don't bother with it.

The switchable 150K and the 390K both 1 watt in the anode of the driver tube are tantalum magnetic and there is a marked difference in clarity compared to something like the Kiwame 2 watt. I didn't realise it until I tried a swap over for fun just recently.

There is also a marked difference between Silmic 2 and kaisei I also tried some black gate I had and some Cerafine in the cathode bypass position.
Its interesting to note that the Kaisei sounded quite weak compared to the silmic 2 which sounds warmer but there is no doubt that the kaisei are clearer and warn up after an initial time period.

The wire on the input to the first tube also makes a big difference.
As does the wire type on the cathode of the driver tube.

Its interesting to try 22K carbon composite on the input resistor.
For fun I have tried:
Charcroft, Kiwamie, AMRG, I tried every version of tantalum as well and its interesting when playing about I decided to replace the tantalum resistor lead out wire with silver both solid core and stranded both are different.
The 1 meg on the input ground can be changed to a 2.2meg which adds some warmth to the sound a bit like "old school" and is the value used on the PYE Mozart.
NB I tried every version of resistor I could in the 1 meg and 22K input to the driver tube. AMRG sound quite good in this position but I prefer the old carbon film piher in the 1 meg position these used to be sold by RS in the distant past. Carbon film are VERY variable don't get caught up in the carbon Takman idea they are very noisy. YMMV

Anyway its not for every one but I found it interesting, some people don't like to look at components from an objective point of view.
I'll leave it at that just trying to cover everything I could think of that was asked in the:
Another Houston. About time question :D

Regards
M. Gregg

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