Biasing output valves.

[I like valves.]

As some of you know,I have two AVC amps, both EL34 models but the first one I bought came fitted with KT66's and biased accordingly. I duly bought another because it was cheap and I happen to like them. I decided to fit that with KT66's also and whilst waiting for two new sets of valves to arrive thought I'd chuck in a spare set I had lying around as a starter. Checking the specs for the KT66 I find the anode dissipation to be the same as the EL34 at around 25 watts. To me that makes all the maths come out the same so I should just be able to put the KT's in and leave it at that or am I missing something?

 

[I like valves.]

As no one else had any ideas I decided to measure the voltages present on the amp that already had KT66's fitted. These came out at -61 volts on the control grid and .15 volts across the 10 ohm cathode resistor.A KT66 has a maximumanode dissipation of 25 watts and I'm going to run them at 70% of maximum dissipation in orderto get some reasonable valve life, soafter calculation I would expect to see .39 volts across the cathode resistor. I put the spare set of KT's in the other amp and biased the valves to .15 volts across the cathode resistor, ie the same as theoriginal amp. Control grid voltages were slightly different due I suppose to these valves being a different make. Anyway it sounds ok and after running for a few hours the valves show no signs of overheating so I've just altered the biasing slightly to give .2 volts across the cathode resistor. I shall keep checking for overheating and if everything's ok I'll increase the bias again and keep going until I see signs of overheating. If I've done the maths right I should be able to get to .39 volts without any sign of distress to the output valves. Si.

 

[i_should_coco]

Kt66 and El34 are similar enough, sdo you shouldn't have too many problems. They will in all likelihood sound quite different, though.

 

[I like valves.]

You're right, they do sound different.I prefer the slightly easier sound of the KT66. The thing that threw me was that when I bought the first amp already fitted with KT's, I just took it home and played it. When I came to check the voltages they were nothing like the figures he had written down in the manual. So I did a search and found some info on biasing and as the the KT66 has roughly the same dissipation as an EL34 thenit figures thatthe maths shouldcome out similar. I doubt my own ability sometimes, but I've bought the amps and modified both of them so I've got to get on with it. I don't think Lim would be overjoyed to see these two amps when I've finished with them. He'd probably have a nervous breakdown. Si.

 

[I like valves.]

I ran the amp for an hour tonight just to get everything warmed up nicely and I've now adjusted the bias to give .3 volts across the cathode resistors. After adjustment I returned the cd to it's previous track and turned up the volume and the improvement was immediately obvious. The valves are not overheating and have been running at the new bias setting for well over an hour now so I'm going to leave it at that for the moment. At this setting the output valves are probably runningat around 55-60% of max anode dissipation.This amp will go into my system tomorrow and the other amp, the one that came fitted with the KT66's will be on my workbench to get the alterations necessary to make it the same as the other. This one is badged Model two Dark, the other, Model three Silver. The Dark model has some extra resistors and a capacitor added to the output valve bases. Apart from that both are the same so I shall be removing these tomorrow and see what effect it has. Si.

 

[i_should_coco]

Any idea what these extra bits do?

 

[I like valves.]

No idea. But they aren't there on the later amp and everything else is the same. I've checked every component. I'd post some pics buy I don't know how to reduce them. By the way, the matched quads have arrived and the first set is in and biased and sound great.I'm going to put the amp into my system tonight and start work on the other amp( the original that came with the KT's already installed ). Once it's hooked up to my TDL's then I'll be able to make a proper comparison. Si.

 

[jonjin]

I like valves. wrote:

I'd post some pics buy I don't know how to reduce them. Si.

Interesting thread... I wish I knew more about electrics...

Anyways, to resize a picture, most photo editors will have a resize image option. For example in Microsoft Photo Editor, you can click under Image>Resize. Then use a web-based picture hosting page like www.photobucket.com to upload your pictures there. Photobucket also gives you an automatic link under the picture which you can cut and paste onto the posts...

hth...

JJ

 

[I like valves.]

jonjin wrote:

Anyways, to resize a picture, most photo editors will have a resize image option. For example in Microsoft Photo Editor, you can click under Image>Resize. Then use a web-based picture hosting page like http://www.photobucket.com to upload your pictures there. Photobucket also gives you an automatic link under the picture which you can cut and paste onto the posts...

hth...

JJ

You may not believe this JJ but I haven't got a bloody clue how to do anything like that. I'm a nuts and bolts engineer who's had to learn the hard way.I bought a pc three years ago because I thought this was the way to go.I now realise that learning to speak Japanese is probably easier. Why can'tmy pc simply have a button that says press this to resize your image so it will fit this? Or am I missing something.Why does everything have to be so complicated. Maybe it's like this to keep loads of people in work writing loads of complicated computer programs. It took me a fortnight to work out how to put an attachment to an email! Si.

 

[Tel]

Si

Just get a photbucket account.

Uploading pictures is exactly the same as putting an attachment on an email. To save on bandwidth the free version of photobucket will automatically resize the pics if they are over a certain size (I think 1MB).

Posting thenm on here is as easy as cutting and pasting a link. If the pic is too big on the forum you can reduce the size in photobucket and it automatically changes on the forum without any further changes to your post.

And if you want a really simple free picture editor download a prog called Irfanview

 

[I like valves.]

Thanks for that Tel, I have the link for photobucket ( cheers JJ ) and I'll have a mess about with it later on. It says at the bottom of the reply that the file size should not exceed 500kb.Is that each photo or the whole attachment? Si.

 

[I like valves.]

Ok, I now have two AVC EL34 amps both fitted with matched quads KT66's and I'm using one amp to drive the left speaker, one to drive the right. Each amp uses one channel to drive the trebleand the other to drive the bass. I now enjoy a superbly spacious sound with excellent bass. I had to modify my TLC1 pre to give two left and two right outputs because I still wanted both amps to retain their stereo capabilities.There is still some work to do but it's definitely been worth the effort so far! Si.