Building a Push-Pull power amplifier.

[nicholasachurchman]

Hi,

I am looking to build a PP power amp using a number of 6V6’s I have plus a few other valves. Looking around for a suitable circuit I came across this one:

http://www.bonavolta.ch/hobby/en/audio/6v6_2.htm

It’s from the book “Building your own Valve Amplifiers by Rainer Zur Linde. Zur Linde does not give a power supply for this circuit, but I figured it would not be much effort to design one. To aid me in my task I though I would try and get a rough idea of the quicent current to give me some ideas of values.

This is where it all starts to go wrong!

Looking at the circuit Zur Linde does not give much to go on, but he does give the voltage at the top of R9 as 77V. doing a bit of ohm’s law shows the current through this resistor to be 1.13mA. As the voltage at the two anodes to V2 are about the same we can say half this current is going through each, which means R10 is dropping just over 45V. Add this to the 214V at the anode gives a supply of 260V ( or there about )

So from this we can say the current through R19 would be 310-260/10K = 5mA. 1.1mA of this is used by V2, so the rest must be used by V1 i.e 3.9mA

Here where things don’t add up.

As we know the voltage at the top of R3 in the cathode of V1 we can say the current going through it is 3.6mA a little less than calculated above. But if using the lower current we calculate the voltage dropped across R5 we find that 3.6mA * 60K = 265.2V. Deduct that from the 260V feeding V2 and we get –5.2V!!!!

There is then another voltage drop across R4 of 36V meaning the anode of V1 is at –41.2V!!!!

Where am I going wrong? Have I been far too simplistic in my approach, or am I just loosing my marbles? (my wife said I lost my marbles the day I brought something with valves in it) .

Any help or suggestions appreciated.

Regards,

Nick

[Aub]

Nick,
I wouldnt be so complicated. ( being lazy )The 6v6's will give 10 to 15w of audio, so they'll need roughly 30watts of DC input. Add a bit more on for the smaller stages, so use a 300V supply at 120ma or so. Use a 300-0-300 secondary transformer @120ma ( 100ma might do ) . Use a couple of 1N4007 diodes in full wave and use a smoothing choke in between two 32uF 450V electrolytics. You could, of course, opt for a bridge rectifier config, in which case you need something like a single 250V secondary at 150Ma or better. In either case you'll need the heater supply to be 6.3v @ 2 -3 amps.

Hope this helps.

Regards

Aub

[YC-156]

Hi Nick,

If you are loosing your marbles, then so am I. Your calculations and line of reasoning looks spot on as seen from here.

Where you 'fail' is in trusting that the design actually works, or has ever been built for that matter IMHO!

It is utterly madness in design to expect the anode voltage of the EC92 to be realized within a few tenth of a volt, something which would be required if the biasing of the ECC83 should work. If you see a design with tubes, which have the voltages marked to three significant digits: Beware. If you see this in a DC coupled amplifier: Nitwit at work!

This very much looks like yet another of those 'newage' audiophool designs crafted on paper/in a computer with no regard for real world component drift and imperfections. The error made by the 'designer' is that of a classic, newbie tube 'expert': All components are identical and will perfectly mimic the curves in the datasheets.

In my view there are a lot of 'expert' websites out there doing the tube hobby more harm than good in this regard, and some useless designs even make it into books, as you can see. The error in the calculations probably means that the author haven't actually built his jewel of a design, and much less tested and measured the voltages on a live circuit. Same goes for the website owner, who posted the design.

Hope this helps a bit.

Frank N.

[ppppenguin]

I think that Frank has the right answer. Why on earth is there a pot in one anode of the phase splitter? The strangest way I've ever seen of balancing a long tail pair which should be more or less balanced anyway without need for adjustment.

[hotbulb]

Why not try this link Nick:
http://www.du.edu/~etuttle/electron/elect35.htm
A straightforward design with everything explained, a real world amplifier that does what it says on the packet, without straying into the realms of tweaky-freaky audiophoolery

[nicholasachurchman]

Hi,

Thanks for all your comments!

Just to take up a point Frank mentioned. I was in contact with Zur Linde last year concerning another one of his circuits. One question I asked was where he obtained the transformers used in his designs. He informed me that he had them wound by a German company and he even sent me their website address.

So it would seem he built some of the circuits, but perhaps not all of them!

I must admit the DC coupled first stage/phase splitter does worry me a little and the circuit recommended by hotbulb does install me with more confidence.

It's good to see that I have not lost what little sanity I have left and that Mr. Ohm's little equation has not let me down!!!!!

Anyone else got any other good Hi-Fi 6V6 push-pull amps they can recommend?

Thanks for the comments so far – much appreciated!

Regards,

Nick

[mickjjo]

Hi Nick, There is a nice design for a 6V6 PP amplifier on this site:-

http://www2u.biglobe.ne.jp/~tossie/6V6-E.html

Impressive standard of construction too!, As are all the rest of the amp designs here:-

http://www2u.biglobe.ne.jp/~tossie/contents-E.010106.html

Regards, Mick.

[Hunts smoothing bomb]

Nick,
I am currently building a stereo 6v6 pushpull amp (or I will be when the parts turn up in the post)
I have prototyped one channel and tested it through my tannoy monitor golds and I must say it kicks . I've posted my design if you're interested.

Regards
Lee

[ppppenguin]

Looks like a perfectly sensible long tail pair phase splitter driving a push-pull output except for a query about the ECC83 bias.

What's meant to be happening where the 3.3M resistors meet? If the circuit is literally as shown then the grids of the ECC83 will be floating around. If they are meant to be connected to the tap on the cathode resistor that should work but I don't quite understand the point of tapping them most of the way up the cathode resistor. It don't think it will provide -ve feedback because it's common to both cathodes.

[Hunts smoothing bomb]

I don't fully understand the operation but, I took the schematic for the phase splitter from Morgan Jones's book "Valve Amplifiers" where he describes it as a cathode coupled phase splitter and is apparently the design used in the Leak TL12+

I have changed some values though and I can't remember why, The cap should be 100nF on the second half of ECC83 and anode load resistor on the first half should be 91k.

Regards
Lee

[Hunts smoothing bomb]

Sorry!
Misread your post

The 3.3m resistors are connected at the point of the 22k and 1k resistors.

Lee

[Ed_Dinning]

Hi Nick, I can reccommend Morgan Jones - Building Valve Amps it has plenty of circuits in some good tips and plenty of sound constructional advice. The companion Valve Amps is also a good book, plenty of unusual circuits, but with good descriptions to give confidance that the circuit will work when finished. Theres also some good details on transformers. PM me if you want to know about designing and building your own trannies.

Ed

[nicholasachurchman]

Hi Ed,

This is a book I already have and compulsive reading it is too!

The writer gives good solid practical advice and not too much theory that I start getting confused ( This seems to happen a lot since I hit 35 )

Thanks for every ones advice and help, I've just got to decide which design to go for now!

Regards,

Nick