805 Power stage.

[Cressy Snr]

I just get a headache thinking about the theory,have tried reading MJ but it goes over my head,just like it did when i was an apprentice at Marconi in the 60s.That's why i left and joined the motor trade !!

Hi Philip have a look at this website http://www.valvewizard.co.uk/

It's biased towards guitar amps but the guy has a very friendly and easy to understand style of writing. He goes into the all the circuit types needed to build a valve amp eg triode gain stages, pentode stages, output stages both PP and SE.

If I had known about this site when I started, it wouldn't have taken me about half a dozen amps before I learned how to design power stages properly. Every one of them I did was woefully under-run until I read his article on loadlines for SE power stages. He explains how to do them far more clearly than MJ.

Even then, after taking my latest SE amp to Nicks place, after he had measured it I was still under running (not enough HT) The penny finally dropped about a week ago, and the amp sounds absolutely brilliant after I set it up properly by changing the mains TX for a higher voltage one.

I suppose what I am saying is that it takes a lot of experimenting, making mistakes and help from such as Nick, Steve S and Paul plus copious reading up before one can finally be confident that a design will perform as one thought it would.

That I finally realised the other day why most of my amps other than the Rocky EL34 were often underpowered and shouty, is more of a tribute to my own dogged perseverance in trying get a handle on power stages than any inherent ability on my part. However now I have a good idea what I am doing I can approach the design of the PX25 breadboard free from doubts. That's with about 8 amps under my belt.

What I would say Philip is to have a go at designing something like a KT66 triode connected SE amp, posting the schematic and asking for comments, advice etc. Also peruse the website I have linked to.

The feeling you get when something you have designed yourself starts making music is hard to describe.

Steve

[pre65]

Hi Steve/Nick-thanks for the info.

I eventually found what i thought was what i wanted (not sure now !!)-a circuit diagram for an 805 driven by a 300b.

Perhaps Nick could say if this one looks safe ?

[Darren]

The B+ looks to be around 1000V

Up to you if you think that's safe for you, no valve amp B+ is safe if you touch it

[Nick]

In the sense that its got adjustable bias, and doesn't have a strange DC feedback path I don't understand, yes its safe.

Its a similar shape to my 211, cathode follower to output valve.

It would need dismantling your 300b amp though. I used a el84 instead of the 300b in this circuit, can't see any big reason to use a 300b there (other than bling).

[pre65]

Hi-i'm having a listen to ALL my different amps today !

Been using the ECL82 monoblocks recently while 832A is out for earthing mods.300b is in situ now and sounds good.

Andrew I wants to hear Amity with the Pre3 so i might get a chance to listen to that before Owston,see how it compares with 832A.

The reason behind the need for a more powerful amp (20W ish) was to give KLS3 a fair bash.The Quads don't do bad (15W) but they are a bit "woolly" compared to my other amps and i wanted to use the PT15 amp as parts,so that really meant SE.

Might be easier to sell the KLS3 and stick with what i have.

[pre65]

e.

It would need dismantling your 300b amp though. I used a el84 instead of the 300b in this circuit, can't see any big reason to use a 300b there (other than bling).

I seem to remember Paul saying that an 805 driven by 300b was a good way to go !
There was discussion some time ago (WD forum ?) about how the "sound" of the driver valve influenced the final "sound" and i think 300b driven by 300b was suggested as a good thing (or was it 2A3 driven by 2A3 ?).

As i have neither the skill or experience to experiment with all these ideas i have to be guided by what others have done.

[Nick]

Oh, I agree that the driver valve has a big impact.

I seem to remember Paul saying that an 805 driven by 300b was a good way to go !

I don't know, I haven't tried to drive a 805 so can't comment, but I wonder if Paul meant using a 300b as a cathode follower when he said that?

[Paul Barker]

That circuit is frightening, I wouldn't dream of building it.

I would do something much simpler for the direct coupled cathode follower stage biasing than what they have done which I don't yet understand nor do I care to..

I think worrying about which vales to use is like complaining about the colour of the bung needed to plug a hole in a damn.

With such an output stage I would keep it elegantly simple like for instance choke load a cathode follower with a dc resistance in the choke determining the positive bias of the output valve.
Once you have decided to use a cathode follower you have left the territory in which I recommend a 300b as a driver.

You might find it sounds better than a idht cathode follower but unlikely, as yet I haeven't attained satisfaction using a 211 as a cathode follower.

It is common to use a kt88 cathode follower in such applications.

[SimonC]

The reason behind the need for a more powerful amp (20W ish) was to give KLS3 a fair bash.

Hi Phil,

I have gone down a different route for giving my KLS3's some grief, plus learning about designing amps instead of just building them.

I have chosen to go single ended 6c33, driven by a c3g. Should be around 15W flat out, but after hearing how loud my KLS3's went with the 8W of Nick's 300b (built in 3 days from left overs!) amp I may even back it off a bit,

One of my main reasons for going 6c33 instead of 805 etc is it's relatively low voltage (200V B+) and cheap so it doen't matter if I melt a couple of them. The disadvantage of them is that they kick out enough heat to BBQ on. (It also helped that Andrew I had a couple of OPT's and some c3g's going.)

At the moment its had power on it, but I've got some sorting out to do with the c3g bias before I can hook it up to any speakers. Should get that done tonight. If I was going to Owston I would throw it in the car so that you could have a listen. Maybe next time (or move to Yorkshire...)

Without wanting to put you off, or lecture you in any way, one thing I would do is to have a sit down with Nick, Paul etc. at Owston and listen to their words of wisdom on working with 1kV+ power supplies. If it doesn't scare you then don't get involved in big volts.

Simon C

[edit: typo correction]

[pre65]

Hi Simon-one of the first amps i had the "hots" for was the "Panzer" from the book "Best of Audion" but as no one (yet !) has managed to get a lasting result from the 6C33 i have not persued that idea any further.
Even Nicks version seems to get no mentions these days.

As said before,i am (was) trying to use the PT15 amp bits for my next project but it is playing now and sounds OK.The 300b either has an emphasis on mid/upper vocals or all the other amps (so far) have something missing (the former i suspect) and the 832A bass does not seem so dominant compared with ECL82 and 300b (both SE) so it's been an interesting comparison.

I did once have a "silly ?" thought of PT15 push pull,but then its no more silly than most of my other suggestions recently !!

I would certainly like to hear your 6C33 amp at some time,pity your not going next week.

Do they allow "Essex boys" in Yorkshire ?

[Nick]

Even Nicks version seems to get no mentions these days.

They still exist, I haven't got them here at the moment as I leant them to someone.

They were ok, very good at some things, they seemed to be a very good match with Martins TLs, but I still prefer DHTs.

Looking forward to hearing Simon's amp.

[Darren]

You can run an 833 with no B+,

Is that safe enough?

[Nick]

Is that safe enough?

Try dropping a spanner across the filament supply

[SimonC]

Is that safe enough?

Try dropping a spanner across the filament supply

Or a wedding ring...at least the stump would be pretty well cauterized, should heal nicely in time.

[Darren]

Funny