Low HT triode operation - is it OK?
 

Years ago I built a Class A stereo amp to the original Linsley Hood design to use in my workshop. It’s been great, and for a simple design I’m very impressed at the results, and the 10 Watt output per channel is adequate.

However, the CD/ hard drive player I use does not have quite enough output to drive the Class A to full volume, which sometimes I like to crank up. I’ve measured that my Class A requires approximately 1.5 - 2.0 VRMS, to achieve its full output. My CD seems happy to provide a maximum of 1 V, which is not quite enough when I need full grunt!

Yes, I could use a standard pre amp unit, but I would rather not. Too much gain. I could make up a simple common emitter stage and use that, but as I had a few 6J5 single triode octal valves knocking around I thought it would be fun to use a pair of these😜

I’ve settled on the circuit below. This works really well, provides enough gain, and seems to be pretty linear. I arrived at the resistor values by experimenting, rather than try and work out a load line. The 30 volt HT is already available within the amp. The heater voltage being dropped from the 30 volt rail, too ( yep, more heat)

So, after all this waffle my question is, is this low HT voltage in any way detrimental to the longevity of the valve ?

If you’ve got this far without dropping off.......thank you!


SimonT.

Re: Low HT triode operation - is it OK?
 

I would say a low HT is not detremental to a valve. It is the current flow and anode dissapation which have far more effect (or too high an HT). If the current flow is very minute (due to low HT) then the condition of cathode poisoning may be relevant, but in this case the current flow is within reasonable limits (in my opinion).

Re: Low HT triode operation - is it OK?
 

Looks fine to me, cathode is obviously passing current, just.

Re: Low HT triode operation - is it OK?
 

Well the hybrid valve RF / Transistor AF car radios did just fine and without apparent cathode poisoning which has always surprised me given that HT was only 12V. I enjoy messing about with these radios and many have the original (mains valve derived) valves still in good working order.
Maybe poisoning is more of a risk where no HT current goes via the anode such as AM/FM radios using ECC85 and where on AM only the heater draws current.

Nice pre-amp you have there,adaptable to many dual triodes as too.

Re: Low HT triode operation - is it OK?
 

Various low power aircraft valve electronics used 28 volts HT without problems.

Off grid homes sometimes used valve radio sets with the HT from a 32 volt lighting plant, without problems.

Re: Low HT triode operation - is it OK?
 

Thank you all for your responses. I had in the back of my mind the plight of those ECC85’s in AM / FM radios which had the HT removed when switched to AM, causing the valve to eventually stop working, as VT FUSE has mentioned.

Yes, the Anode current is very minimal, but I simply don’t want any more gain than what I’ve managed to achieve.


I’m happy now to leave it as it is.



Cheers. SimonT.

Re: Low HT triode operation - is it OK?
 

I've run valve front end's reet low, in the micro amp range and have run ECC83's at 12v with no problem's.

Noticed the cathode is un-bypassed, you'd get better results with a cap or use a LED.

Your frequency roll off for the 2.2u and 47k is a bit low @ 1.5hz, might be better @ 15hz or above.

Just observation's not criticism's, Andy.

Re: Low HT triode operation - is it OK?
 

Thanks Andy.


I used 1Hz as the roll off point, so 2u2 was near enough! Why 1Hz? Well, I can remember an old electronics engineer telling me many years ago, ‘ if ya can’t go DC, go in at one’. He was a very smart old boy, but rather eccentric. It’s just stuck!


The cathode I left un bypassed simply because of the gain, and I did not want to lower the anode current further.

What improvement would my roll off starting at 15Hz give me? Also, what benefit would the led give in the cathode?

As said -no criticism, just interested now....😜.


SimonT.

Re: Low HT triode operation - is it OK?
 

Andy,
I have run triodes like 'SN7's, 'J5's etc with as low as 6V HT on the plate(As an experiment decades ago) and had them working just fine. Of course, if you are concerned, you might try the 12V plate tubes found in car radios. They are available, and relatively cheap.
I had built a triode AM receiver and wanted to use it in my truck that had a 6V pos. gnd. power setup.

Re: Low HT triode operation - is it OK?
 

A higher cut off point should make the amp more stable at low frequency's ( if you have them), that is if your using NFB, we don't really hear or our speakers etc don't go down to DC anyway, 25hz might be a better roll off point.

The 2u2 being a bit big, could cause issue's with the next stage, maybe not so in your application. there's the voltage rating too, 2u2's not being generally available in high voltage's, but again it only has to block 20v. If you can find one of those low ESR orange jobbies, they'd be perfect.

A cathode bypass as you know increases gain, but reduces THD, an LED biases the valve, you don't need a bypass cap, but it will increase gain. diodes or a zener do the job too.

Am probably over thinking things here Simon, I think what you've got is fine, just cogitating.

Andy.

Re: Low HT triode operation - is it OK?
 

Thanks for that, Andy.

When I get chance, I’ll have a play around with some other values. I’ve only done a scope and ear assessment so far. I’ve got an old distortion anylyser & audio spectrum anylyser under the bench , so might have to dig these out!


Thanks. SimonT

Re: Low HT triode operation - is it OK?
 

A record that is warped or drilled off-centre can produce an infrasonic component in the cartridge output that can saturate iron-ware or make loudspeaker cones flop about if the intervening amp can reach too far down in frequency!

Re: Low HT triode operation - is it OK?
 

Doesn't a cathode bypass increase THD? It removes a form of NFB from the stage.

Re: Low HT triode operation - is it OK?
 

Must admit, that’s what I thought, but I’m theory rusty ! There’s more NFB, but less gain!

Re: Low HT triode operation - is it OK?
 

He’s using it to increase the gain of a CD player which dont or certainly shouldn’t have the problems mentioned.

Re: Low HT triode operation - is it OK?
 

Oops- my mistake.

Re: Low HT triode operation - is it OK?
 

"Doesn't a cathode bypass increase THD?" Yes it does Chris, but once you've increased the IP to get the same OP as a bypassed cathode, THD is more, therefore on a like for like comparison a bypassed K "wins".

However like anything in life and electronics it's swings and roundabout's; lose gain, improve bandwidth/THD possibly etc.

I did a load of tests, comparing bypassed, diode etc. Bypassed or diode comes out best, also the bypass cap can be used to alter/improve LF roll off.

Saw this yesterday Simon - http://www.valvewizard.co.uk/Triodes...s_Blencowe.pdf thought it might interest you.

If you want a low gain stage you really can't beat a triode gain stage with local FB, flat from near DC to 25khz and above, low THD, ideal for this application. See - http://www.valvewizard.co.uk/localfeedback.html There are lot's of thing's you can do to tweak a CTGS, CCS instead of R as anode load etc, worth having a play just for the craic.

Andy

Re: Low HT triode operation - is it OK?
 

Hi Andy.

Thanks for those links, certainly some good info there to allow more experimentation! It’s the simplicity of the circuits that interest me. This is what attracted me to the original 1969 version of the Linsley Hood Class A amp I cobbled together here- https://www.vintage-radio.net/forum/...ad.php?t=80152.

I still use this amp in the workshop, and this triode gain stage is now temporarily feeding its input.

I’m now thinking of knocking up a bigger version, incorporating the triode gain stage and a fan to help heat the workshop, too😜


Cheers. SimonT.

Re: Low HT triode operation - is it OK?
 

Been meaning to build a Class A amp for a while, I have some very big heatsinks of a welder and a few vintage 3055's, does yours sound well?

I've run my big valve amp in class A, but it gets stupid hot and the OPT isn't really suited.

Here's another interesting front end - http://www.angelfire.com/electronic/...ain_Block.html for your edification and perusal.

Andy.

Re: Low HT triode operation - is it OK?
 

It’s been a while, but I thought I would update this post.


I had a play around with various circuit variants, but I’ve settled on the circuit attached. This proves to be the best and most linear that I can achieve with a 30 Volt DC HT.
Both valves were tested beforehand to ascertain that they were still within their design limits.

It serves its purpose very well, and is pretty flat up to about 40 kHz. I found a salvaged switch mode power supply in my junk box that provided +18, and -18 Volt at 1 amp, so I decided to use this for the power supply, with the valves heaters wired in series through a dropping resistor of 18 Ohms between one 18 Volt rail and the mid point. The feed for the valves anodes was taken between the -18 and +18 Volt rails.

There is something strangely satisfying to me to play music fed from a modern hard drive player through valves developed just prior to WW2!


SimonT.