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Old 7th May 2003, 08:31 PM   #11
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Hi Frank,

Your comments (and EC8010s) make sense and as it is working nicely without - I'm happy to give up the idea until I've thought through much more what I hope to acheive.

EC8010,

Have you told us what your main amplifier is? Sorry to ask again if you have... I guess you need a few watts to drive the Jordan and as 2A3 would match for the treble it might be a 300B PP amp?

ciao

James
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Old 7th May 2003, 11:24 PM   #12
EC8010 is offline EC8010  United Kingdom
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Niet 300B. The ribbon is allegedly very sensitive (haven't measured it yet), and 1.5W would match the 40W of the main amplifier. However, I was prompted to think about the effect of active crossovers on peak-to-mean ratios, and decided that 6W would be better. There is no way I would pay the outrageous prices for 300B.

All will be revealed in due course...

It has almost as many transformers as yours, it's just that they're in the power supply.
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Old 9th May 2003, 12:16 PM   #13
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Default ...patience, patience...

EC8010,

I'm very interested to see what you are using and what you decide for your tweeters...

I'll try not to be too impatient!

ciao

James
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Old 27th May 2003, 04:31 PM   #14
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Howdy,

I just ordered 20 6N6P's today, and I've also been experimenting with C3m pentode drivers for my GM70's, and will try some EL84's next. Excellent, so good in fact that I'm not tempted to buy another set of ITs to do a say, 6H30-IT-2A3-IT-GM70 like the Super Aurora.

Something to try too, directly applicable to your design is a C3g trioded. Mu is about 40, and in a quick experiment sounded good here. They're also nowhere near as popular as the C3m/C3o so are quite cheap, and have a nice internal mesh contruction under the metal can.
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Old 27th May 2003, 05:03 PM   #15
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Hi Brett,

I have been playing with C3m in both pentode and triode mode recently too. They are really very good! But don't tell anyone.... Thanks for the C3g tip.

I'm having a loctal period at present - just knocked up a loctal guitar amp with C3m as input and output valves and 1LE3s inbetween... it's interesting quite different from my ECL82 version... It makes a good bass practice amp as well


ciao

James
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Old 18th June 2003, 12:31 AM   #16
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James,

Did you ever construct this amp? About a year ago I designed a similar one using PP 6C45's to drive a single GM-70. The differences are I intended it to be used with balanced output from a CD player with 2V rms sensitivity and the IT had a slight step down so the Gm-70's could be biased to run well into A2 near full power. I only got as far as the power supply which was conventional and supplied 950VDC B+. Lacking money for quality transformers I began a similar design using Amperex 7308's PP driving a 6A3 again with 2V sensitivity and stepdown IT. It is nearly finished using mostly FAA surplus iron and Lundahl OPT. I will report on the results in the near future.

John
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Old 20th June 2003, 11:56 AM   #17
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Hi John,

I have made this amp. It was almost exactly as drawn but the power supply doesn't have the chokes in the 0V line.

I've played around with 845 op points and found that I like the low voltage points a lot between 550 and 700 volts and that low current works just as well as high current i.e. 50mA versus 100mA.

The 845 seems easy to drive and the 6C45 PP is nice and clean. My phase splitter Input transformer is inexpensive and has some in band resonances that are tamed by the 22uF capacitor between CT and cathodes. The Cn neutralising caps help with IT resonances - again a result of using a cheap IT. I believe proper iron would eliminate the need for them - but you never know... there is always rf rubbish being picked up and these help the valves pass them cleanly to the IT where they die... Many thanks to all the advice and pointers I got from Forum members on these issues.

The bipolar psu for the 845 is great. I have got a brush contact shock from the B+ line and I was very glad it was at 380V rather than 760V - NOTE: my other hand was behind my back and all loose clothing was tied down - so my reaction to the shock took me clear of the amplifier

It's in pieces now as I'm working up an all DH PP amp using 845s and this psu... This one is fun as its a three stage amp with DHPs in the middle and the first two stages direct coupled with IT to the OPT. It's also readily adaptable to all 845s....Silly but fun six 845s per channel!!! Once I'm happy it will work I'll show the design...

Your experiments sound fun too. I've got a 6B4G PP that is fabulous. I use 6N6Ps as drivers. It's in pieces waiting for new (better) iron

I'm intigued by the GM70. But the sockets seem difficult to get and the 845 is cheaper for what seem good ones. The GM70 curves look very very nice though. I want to try 813 triode connected too... Gary Dahl is right "sooooo many valves, so little time..."

Listening through an ECL82 SE at present and loving it! But the PPs have better PRAT!

ciao

James
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Old 20th June 2003, 01:33 PM   #18
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James
how does the input Cn ( neutralising capacitance) work?also how do I calculate the value for it? In your case the 6c45. From the diagram it look like the Cn is connected from grid of the first 6c45 to the anode of grid of the second 6c45 via the Cn and vice versa for the second tube.
For the capacitor value between ct of input transformer to ground u utilized 22uf. How did you come about to get this value? test and check for lower distortion? Also how does this cap tamed band resonances?
thanks for the help
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Old 20th June 2003, 03:10 PM   #19
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Nick,

The neutralising capacitors are there to help overcome the input capacitance of the triode by providing extra current drive at ac from a suitable in-phase point of the circuit. So it aims to 'neutralise' the input capacitance. In PP it's straight-forward to provide a suitable added current flow (positive feedback really). The value is found by experiement (according to all the text books) as there are too many variables to make calculation reliable. Start with the equivalent input capacitance and work away from that value... I wasn't after raising the frequency response of the amplifier (the original use) but wanted to avoid slew rate distortion. Since the pick off point also has the effect of the transformer imprinted on the signal it might help to offset some OPT resonance effects - pure serendipidy if it does - and in my case with cheap Xfmrs I think it does help.

The 22uF was arrived at by trial and error. Origonally I had the 22uF between IT CT and cathodes and 2uF between input IT and cathodes - I then swopped them and it sounded much better....

There has been conjecture in these pages that this can only be so if the 22uF is working against a resonance effect inthe input transformer. Checking with a scope looking at the 6C45 anode showed a resonance at abot 4.5Khz (fairly broad and about 4.5dB peak) that dissappeared with the 22 uF. I have not measure the IT and then modelled what is happening to understand this properly. I assume it is peculiar to my cheap transformers and topology but it works for me.

ciao

James
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Old 20th June 2003, 06:29 PM   #20
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james
can this be done for se. From your explanation, the tubes draws some positive feedback from another tube. IN se how can we utilize this technique. Draw signal from cathode. Signal phase is similar but amplitude is lower, can this work. As i am planning to built a 6c45 se pre-amp transformer coupled soon. And input capacitance is my main worry.
For trial and error for Cn. Start with the input cap of the tube and then move up and down the values. How do we know that we are going in the right direction how do we know that we are doing the right thing. I don't think the scope can tell except use ears to tell? When you state move away from the initial cap value, means go for bigger size or smaller size cap.
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