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Thinking about building a valve amp...

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Avalanche

Tue Aug 31 2010, 03:44PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Many years ago I tore apart a huge old music centre, and saved the valves, thinking they might be useful one day. They've sat in my drawer for probably about 12-13 years now. A couple of months ago after restoring a 1951 GEC valve radio, I became interested in finally trying to understand valves properly, with the hope of building something with those in the drawer.

Anyway, after reading some old books and searching around the net, I was quite inspired by Uzzor's valve amp here -

and was thinking of building something similar, however I'm going to go for 2 channels.

The values I've managed to unearth are:

1 x EZ81 (full wave rectifier, good for 100mA)
4 x ECL83 (triode-pentode, with the pentode rated at 30mA, the triode 2mA)
2 x PCF802 (triode-pentode, 3.5mA/6mA actually television valves)

What I want to build is a 2 channel stereo amp (I have 2 almost identical 1950s extension speakers) but make it bridgeable for using as a guitar amp also. Just about ready to get started, I'll be ordering a few bits later.

Just wanted to post my ideas up here before I get stuck in, just in case I'm about to do something stupid! I'm thinking of keeping it simple and building a class A power amp, using 2 of the ECL83 tubes. I'm not too worried about THD, as long as the harmonics are 'nice'. This will mean using the triode as a preamp, and the pentode as the power output stage - so that's only 1 valve per channel! Of course I'll also build a power supply with the EZ81. To bridge this design, there's various ways I could bridge it at the output transformers, saving the need to shift one of the channels by 180*. I'll have 2 ECL83's left over, and I'm not considering using the TV tubes at all. I'll probably get stuck in within the next few days, but I'd appreciate any thoughts if anyone has anything to add! It'll probably help if I post a schematic, but the design is fairly simple - just 2 common cathode stages, with automatic grid bias (cathode resistors) and AF transformers to impedance match the speakers to the pentodes.

Tubes

radiotech

Tue Aug 31 2010, 06:25PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Interesting tubes there. you might consider a 'paraphase' circuit whereby you have the option of driving the pentode from either the cathode or the anode of its triode. This will invert the phase leaving you to connect the output transformer secondaries in series..

The PCF802s dont have the same heater voltage as the rest.

Tubes aside, Iron is the issue. have you the transformers.

Avoid the temptation of using silicon to kick out the EZ81.

If you do decide to go class AB, adjusting the screen voltage seperately of the output tubes will get you minimum IM distortion
if you have a way to measure it.

Proud Mary

Tue Aug 31 2010, 06:48PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I see that with the pentode section of ECL83 in Class A, and Va 200V to give Pout 2W5, then Ra is 7K5, so your transformer turns ratio needed to match 8R will be 31:1 to the nearest whole turn, but 30:1 will be the choice in practice.

At the lower Va 170V for Pout 2W2 then Ra falls to 5K5, giving a 26:1 turns ratio to match 8R. This means that you could use a 240/9V mains transformer, so long as you take out the laminations and reload them to leave a small air gap to prevent saturation by the DC component of Ia.

Nicko

Tue Aug 31 2010, 09:41PM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

Go to the DIYaudio tubes forum - hugely active & very helpful people there who do nothing but build tube amps...

Cheers

Avalanche

Wed Sept 01 2010, 03:21PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Thanks for the replies folks, I've gone ahead and ordered a few bits

As for the transformers, firstly I'm glad Proud Mary mentioned the use of ordinary mains transformers, because that's what I wanted to do but didn't want to mention it

I had considered trying to do something with those high voltage PA transformers as the cores will already be gapped, but I gave up on that one. I've bought some slightly over-rated transformers with the optimistic view to not having to do anything to the core; lets see if they will stand the small DC component first with no gap. I've gone for 230:6V transformers as they are a reasonable match to 5ohms (my antique extension speakers are 4ohms, my other speakers are 6ohms). As for my HT transformer, I am going to brave it and rewind a 230:115V isolation transformer for 250:0:250 which will give me a 200VDC rail with the EZ81 (going by the core size it looks to be about 150VA).

I've decided to fit all the valves to the case, then if I do ok building the class A, I can have a go at making it into a class AB. And if I never get around to doing that, then at least I won't lose the 2 spare valves!!

When I have made some proper progress I'll start a project thread

DIY audio forum looks interesting, I've had it on my favourites bar for a while but haven't joined up yet. Some serious building going on there!

radiotech

Wed Sept 01 2010, 03:54PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

If you include the extra sockets and you are rewinding transformers consider wiring the valve sockets to terminal boards instead of connecting parts directly. This will allow you to change parts and circuits easily.
The schematic of your amplifier is one thing. The layout of the wiring is something else, especially where grounds are concerned. Also reducing magnetic coupling from the mains to the output transformer by orienting the cores will reduce hum. Consider a centertapped 6.3 volt winding for heaters, grounded to reduce hum.

Avalanche

Wed Sept 01 2010, 09:02PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

I'll do the center tapped 6.3V winding when I rewind the transformer again tomorrow

Yes unfortunately this afternoon, I spent a few hours re-winding the transformer with a 500V, center tapped secondary. 1534 turns. As I was winding, I was becoming concerned with the voltages between the turns at the end of the spool - but I carried on anyway. I actually thought I'd got away with it, got a nice solid 494V output, but later on the transformer broke down with corona, and eventually went short circuit altogether when I started prodding it

I had a small load on too to keep the voltage down.

Going to have to re-think this one... from an insulation point of view. I completely overlooked how important it would be with the voltages involved!

half way...

Proud Mary

Wed Sept 01 2010, 10:40PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

You've made a good choice with ECL83 for a first project, because the Mullard data sheet helpfully gives you the values of the cathode and grid resistors, so you won't need the brains of an Ayatollah to get all the circuit values right!

radiotech

Thu Sept 02 2010, 01:16AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Proud Mary Wrote "you won't need the brains of an Ayatollah to get all the circuit values right! "

The only thing valves care about is that the capacitors coupling plates to grids dont leak.

Proud Mary

Thu Sept 02 2010, 11:32AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I see that Mullard give Dtot 10.5% for ECL83, and though you say you don't mind a bit of distortion, it's as well to remember that triode-output pentodes in radios and televisions very commonly used a simple negative feedback loop from the output transformer secondary to the triode cathode line both to reduce distortion and unwanted input/output coupling consequences - "motorboating," gain peaking, hound-like baying, and air-raid siren effects - to which the casually configured triode pentode is prone.

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