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Old 12th April 2008, 06:33 PM   #1
nitrate is offline nitrate  United Kingdom
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Default I'm going in.....

Ok, i'm going in head first, into the world of tubes. I thought its about time i tried building an amp out of thermionic valves as i've tried everything else. I started my amp building passion building classD amps and i must say i'm rarther impressed by the sound of those beasts. After that i tried a very basic ClassA design, it used very few components and it sounded really good ( my best sound to date ). The only drawback of that amp was it was so basic the bias voltage was set by a simple preset and the overall output power was only 12 watts. I then spent ages developing a push pull classA with all the trimmings, capable of 50W in A then up to 200 A/B. It was ultra low in measured distortion and noise, based on diff input with large amounts of NFB and massive open loop gain. It was supposed to be my master peice but i was always dissapointed with the sound. I'm convinced that the diff input and masses of negative feedback were ruining the sound. As a result i've now shelved the project and i'm going to try out these mysterious peices of glass. I must confess my basic instincts tell me a hybrid amp with valve front end and mosfet follower for output is gonna sound rubbish, especially since i like my amps totally transparent with no coloring. I'm hoping to be pleasently supprised.

Anyway i've never used a valve before, i know the general theory of operation but i have a few questions. I was gonna experiment with an old valve i found rattling around in my farthers draw but i managed to break it before even powering it up LOL.

I don't like high voltages much so will a valve, an ECC83 for example work at voltages as low as 60V?

If they will what effect on the linearity will this low voltage operation have?

Are there any problems in using an ECC83 for a current source?

How do valves REALLY sound? can i get clarity, transparency and detail from them as i've seen a few ppl spend ages building stunning amps spending loads of money and time on them just to say they sound 'not as good as i'd like', or 'i'll grow to love the sound'.

My master plan is to build a dual stage single ended classA input out of two ECC83 triodes ( one half for CC and the other for volt gain ) and simply run that into a heavily biased classA/B FET voltage follower for the output. This should get rid of the output transformer, and the supply transformer can be a simple 0-30-0-30 tordial. With this i can get a good +-40V for the fet output stage, and use a doubler across the 80v to give me 160v for the valve anode HT.6.3 heater voltage from a 7805 and a few diodes under its reference. If the amp sounds any good i'll refine the design and use it. If it sounds really bad from the start then i'll scrap it and go back to full solid state. Does any body see any unforseen trouble ahead with my idea on this hybrid amp?

Regards
Leigh
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Old 12th April 2008, 07:11 PM   #2
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Leigh,

Forget about the 12AX7/ECC83. If you want voltage gain at a reasonably low B+ voltage, look towards the 6DJ8/ECC88. Go to TDSL and peruse the data sheets.

IMO, a 60 V. B+ rail is wishful thinking, but under 150 V. is not.

Truly low voltage operation is possible, IF you restrict yourself to a voltage follower (buffer) I/P stage fed with the 2 VRMS a "standard" CDP yields. The automotive service intended 6GM8/ECC86 will do nicely, under those restrictions.
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Old 12th April 2008, 10:03 PM   #3
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Default Re: I'm going in.....

Quote:
Originally posted by nitrate
How do valves REALLY sound? can i get clarity, transparency and detail from the
The most linear devices yet made by man are triodes, so with careful tube selection, operating point and careful execution they are hard to beat.

dave
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Old 12th April 2008, 10:08 PM   #4
EC8010 is offline EC8010  United Kingdom
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Just like transistors, you can make valves sound any way you like. Forget 60V for an ECC83 - it needs 200V across it for linearity. I'll assume you want the clarity of valves, rather than the classic rose-coloured vision. In that case, they simply sound more natural than transistors. And, as P10 points out, triodes rule.
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Old 12th April 2008, 10:45 PM   #5
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Just a thought from a rookie...

If you want to try out tubes I think that you will get a clearer picture if you leave out the solid state stuff and go all tube with the possible exception of the rectifiers.

I am sure that the good folks here can steer you toward an appropriate first project if you give them some details of your speakers and sources.

I am currently working on my first project which is a guitar amp but next in line is a preamp/crossover with matching single ended amp. With my particular set up SE is the logical choice but your situation might be different.

mike
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Old 12th April 2008, 11:18 PM   #6
nitrate is offline nitrate  United Kingdom
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O.K, ECC83 may not be the best valve to start my journery with due to its non linearity at low voltage. I'll have to investigate more on this. My setup demands quite high power due to inefficeint home made speakers so i need the mosfet output end, but i've learnt that distortion figures mean nothing in terms of musicality, so i'm willing to give tubes a chance. I cant comment more until my tubes arrive, i just hope i'll be shocked at the quality of sound.

Here's hoping,

Leigh
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Old 12th April 2008, 11:42 PM   #7
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Of course you are going to only experience part of the tube sound, but it's a good start. A full tube setup will sound different again, as will the different topologies. A push-pull tube output stage sounds different to a single ended, and different tubes each have their own characters. I'm sure you'll get into it as you progress.
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Old 13th April 2008, 12:43 AM   #8
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John Broskie discussed the 6GM8 as an alternative line stage tube some time in the recent past. If I understand correctly, it'll work with good results down to B+ voltages of approximately 24V.

Of course, if you're interested in a power stage too, you'll have to consider something else.

--k
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Old 13th April 2008, 10:38 AM   #9
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Hi,

A super linear valve at low voltages is the ECC88

Cheers!
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Old 14th April 2008, 05:13 AM   #10
Klimon is offline Klimon  Belgium
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IIRC 6gm8 (=ecc86) is actually a ecc88 with specially treated anodes to give better performance at very low voltages, like the 24v stated supra.

Simon
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