I'm going in.....

[nitrate]

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

 

[Eli Duttman]

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.

 

[planet10]

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

 

[EC8010]

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.

 

[mashaffer]

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

 

[nitrate]

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

 

[hotbottle]

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.

 

[koolatron]

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

 

[Geek]

Hi,

A super linear valve at low voltages is the ECC88

Cheers!

 

[Klimon]

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

 

[nitrate]

Hmm, would a normal ecc88 work good and show good linearity at around 40V anode?? if not do they still manufacture ecc86 and if so were?

Leigh

 

[Geek]

Hmm, would a normal ecc88 work good and show good linearity at around 40V anode?? if not do they still manufacture ecc86 and if so were?

Leigh

NOS floats around for cheap

http://thetubestore.com/nos-6gm8.html

More info:
http://tdsl.duncanamps.com/show.php?des=ECC86


Cheers!

 

[Miles Prower]

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'.

Hollow state sounds really good. You can easily get clarity, transparancy and detail. I certainly have managed this without too much trouble. Get the VTs with the best linearity (i.e. 6SN7, 6SL7, 6J5, 6FQ7, 6SF5) and develop a loadline with minimal estimated THD. Make sure to have adequate headroom, and go light on the gNFB.

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?

Class AB MOSFETs aren't real good. The crossover behaviour is terrible, and "complementary" N-Channel / P-Channel devices are a good deal less complimentary than NPN / PNP complimentary pairs. If you're gonna go hollow state, then go hollow state all the way. No need to be afraid of OPTs, as there are some quite good ones out there, plus linear device + nonlinear OPT is still a good deal more linear than SS. Get a copy of the Radiotron Designers Handbook and take a good look at that before doing anything. Also, Pete Millet's Site has lots of freebies from the most basic to most advanced. Also, Steve Bench's Site is a must see.

Don't forget to go light on the gNFB.

 

[nitrate]

Thanx for that input Miles. With any look i'll use no global negative feedback. Just rely on a bit of local degeneration were design allows. I'm still gonnna go with a FET backside. I have some IRFP260N's lying idle, left over from a motor controller project. Not the fastest i know, but they wern't cheap so if i can make use of them i'll give them a whirl and see how they sound in SE classA.

Regards
Leigh

 

[SpreadSpectrum]

Leigh,

I too am fairly new to tubes and audio amp design in general.

A few years ago, just after graduating from college, I was helping my mother-in-law clean out her attic and came across an old tube amplifier, a Fisher X-101-B. I tuned it up and plugged in some speakers and tested it out. I was amazed at how much better it sounded than my solid-state amp. My solid state amp has much lower distortion and much more power, but it does not sound as good. I don't know if the tube amp is coloring the sound in some way, but whatever it is it just sounds good.

Don't be afraid of high voltages, just be safe around them. I am afraid that you will try some weird low-voltage tube that doesn't sound that good and not be satisfied with the result.

On my current project, I have similar goals- minimal feedback and minimal coloration of the sound. I am looking at 6SN7s with active plate loads(transistor cascode). Pull up a 6SN7 data sheet and draw a horizontal load line at 8mA and you'll see why.

I've done some experiments with the ECC83 and I don't like it.

Morgan Jones' Valve Amplifiers is an excellent reference as is the Radiotron Design Handbook. However, start with the Jones book, since it is not 50 years old.

-Heath

 

[nitrate]

.....Still waiting for my ecc83 from rapid. I took the opertunity whilst i wait to order some Electro Harmonix 6922 tubes from the net. The hope is they will be okay and quite linear being run at around 60V per triode. These will prob get here before the Rapid tubes. Anybody used the new Electro Harmonix tubes? Are they any good? Have i made the right choice for 60V operation or have i wasted money LOL??

Leigh

 

[nitrate]

O.k,

My 6922's have arrived. I wasted no time in trying one out and playing at a very basic level until i feel comfortable with these strange objects. I first tried a very basic se classA gain stage, setting the idle current to 5mA. At the first power up all was well. I was supprised at how easy this seemed to be. I had 10X gain from a single stage with minimal noise and wide bandwidth. I was running at 70V supply, 6.8K anode resistor, 330R cathode resistor. Excellent i thought. Now to improve PSU rejection and boost gain i would replace the anode resistor with the other triode in the envelope set up as a constant current source. So i tried this and everythng seemed well except gain didnt change. It stayed at X10. Also, the CCS dosnt seem to be constant. Am i doing somthing wrong here?? The way i did the CCS is to connect its anode to the positive rail and then connect its cathode through a 330R resistor to the anode of the lower amplifiying triode. The upper triodes grid is also connected to the lower anode. When the suply voltage varies so does current. Whats wrong here?? It should work. Do you think i'm running the supply too low at 70V??

Anybody shed any light on this??
Regards
Leigh

 

[Nordic]

I have had lots of fun with EC88s (6dj8/6922)in all kinds of things from buffers to headphone amps... The nicest was useing a small 2x24V PCB transformer and a greinacher doubler topology on the PCB, providing about 119VDC to use it as a buffer/preamp.
Capacitors for PSU was from a dead PC PSU.

Will look if I can find or draw from pcb my voltage doubler...you'd get a good voltage from that.... seeing what I got from just 2 24V secondaries in series, doubled..

Also headphone amps using 12 or 24V supplies with a 9V battery used for biasing... very versatile little beasties.. unfortunatly some time has pased since my tests and my memory fickle...
I do rember that the 119V one was most stable and reliable...and according to the datasheet, more linear...the little ones would change sound or drift bias in the course of listeing to a CD...


Ordering some tubes and a transformer today to build a guitar amp... so I have been brushing up a little over the weekend..Scored a 100VA 200VAC + 12V @2A tranny for just over $10...

Good luck

Here is one you could try... with the doubler...

or maybe for some power...

 

[Nordic]

http://www.headphoneamp.co.kr/bbs/zboard.php?id=diy_sijosae&no=207

Some fun circuits ... in korean.. use google translate... but the schematics are universal

 

[DigitalJunkie]

If you just want to get a taste for tube sound (and once you do,you're likely be hooked!!) don't worry TOO much about power output..
My first tube amp is a PP 6V6 dealie,and it makes ~8Wper chan. before any visible clipping/nasties on the scope. My speakers aren't very efficient either(old homebrews, I'd guess ~85db efficiency?),BUT,I'm in a fairly small room,so the 6V6 amp can get plenty loud in here.

So,I'd say (just my opinion),Go all tube,skip the SS part for now,and don't get too hung up on power(you can build a bigger amp later!) Just make sure you have enough power for your speakers.
On that note,I also have a pair of SE 6BQ5/EL84 mono blocks.They sound really nice,but there's just not enough power for my speakers.So,they've mostly been collecting dust for the last several years.

Something in the PP 6BQ5/EL84,6V6,EL36,etc. area should do you well for starting. If you are a SE purist and don't like PP,you're gonna need a fairly hefty tube to get a decent amount of power in SE. EL34,6550,KTxx might be a good start.