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In Reply to: Re: Ella in Super Triode posted by Kev T on April 16, 2005 at 11:07:45:
decided to wire the amp correctly this time!! I really mustn't mess with electronics when I'm tired.
Replaced the 1K with 82K, wired 2 1M resistors in series and used 0.1uF polypropylene caps. All ordinary bits out of my box. Checked the outputs, yep Ella is swinging 25 volts again, bias hadn't changed, still 60mA current flowing. This I don't understand as the resistance in the grid line has increased 82 times yet the bias is the same. Somebody please explain in words of one syllable.
Bass is now back and definitely better defined, just as deep and if anyhting more tuneful. THere is more info in the midband, hearing things I've not noticed before, not hugely improved but a definite stayer. Now to buy some decent resistors and caps to do the job properly.
If any of you didn't notice; this Ella is swinging 25 volts on the 4 ohm tap into a 4ohm load @1KHz, thats 78W, plenty to drive my Maggies.
Now can someone look at the maths and work out what feedback is applying with these resistances and is it optimised?
Hi Kev T,
Like you I hadn't tried triode mode as I need the power to drive my speakers (B&W 802s3) but this change sounds interesting. Would you be able to post a step by step guide to making this mod, starting from the standard NFB pentode Ella?
Replacing the 1K is easy but I'm not sure which pins get used for the other connections etc nor how/where to measure it to check it's OK. Does the normal NFB need to be disconnected too?
I disconnected the NFB completely to start with. Reason being that this mod puts NFB around the output stage where it's needed, so I figured global NFB would be too much.
THen I removed the 1k resistor from the connection between the PCB (mine is a p2p Ella) and pin5 on the valve and replaced it with an 82K resistor.
Then I connected 2x 1M resistors and a 0.1uF cap in series with each other and between pin5 and pin3 on the valve base. The reason I used 2 resistors is simply because 1M is the biggest value I have and I figure 2M is close enough to 2M2. Same goes for the cap, I have some 0.1uF caps and as they are only for DC blocking I figured the value was OK.
The only measurement I took was with my dvm on the bias which hadn't changed and with my 'scope which shows that the amp is now clipping in the driver stage before the output stage, so I'll look into more drive in the phase splitter/drivers.
Getting more used to the sound now and I figure the improvements are in the bass and mid. Definitely keeping it modded.
Many thanks for that Kev. Next time I see Brian I'll be buying some bits off him to try it myself.
Did you also consider adding the 100pF cap in parallel with the 82K (as in the note at the end of the original document) or isn't that required in the Ella?
bias remains the same because no current flows through this additional series resistance in the grid circuit. the 1k is a preventative, "stopper" resistor for HF oscillation.
I don't know about your power figures.. seems too high. If you are really getting 25Vrms on 4ohm load, I would think the output would be audibly distorted.
the feedback can be calculated, its just a voltage divider. I think the idea was to apply the same amount of feedback that would otherwise be had from local feedback inherent in a triode. (~10dB?)
I struggle to get my head around valves being a voltage device with minimal current flow :(
25V pp just before clipping into 4ohms is measured on my 'scope, I agree if the amp was pulling that on music I would hear clipping distortion on loud passages. 25Vpp is 17Vrms, power is 78W into 4 ohms unless I've calculated wrong.
What sets the amount of feedback round this valve, and how to calculate it please?
Do you think better quality resistors and cap will be of benefit here. At the moment it's a messy lashup but definitely an improvement.
Don't confuse peak-to-peak with peak.
Remember that (for a sine wave)
Vpp = 2xVp
Vrms = Vp / sqrt(2)
Wrms = (Vrms)^2 / load
25Vpp = 12.5Vp
12Vp = 8.8Vrms
8.8Vrms will give ~19Wrms into 4 ohms
Remember it's all about how it sounds to you.
I'm very interested in the STC mod. When you made this mod, how did it affect the gain of the output stage? It sounds to me like you are running out of drive from the phase splitter.
First time I did it I wired it wrong.
When I wired it correctly the gain increased from standard, i.e. I use less volume on the volume control now.
When it was standard I looked at clipping in the drive and power stages and noticed that the power stage clipped marginally before the drive stage. I haven't checked it with this STC connection.
I'm driving Magneplanar MMg's, 83dB/W/M to loud levels with this amp. The speakers are an easy resistive drive, but with that sensitivity I'm surprised the 19W can convert to such a high spl. I must by a meter to see what it goes to but in my room, 14'x10'the results are excellent.
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