• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

#26 pre amp

It's a beautiful day in London, probably the last of the year and also the first free time ive had in a long time, so alas I am in the workshop trying to get this #26 up and running.

Ive been following Rod's guide setting the regs and everything seems to be in order. I am running filiament bias 10R, so 10V across filiament resistor. Current is about 0.8A with about 1V across filiament.

Problem I am having at the moment is that the trimmer on the reg board is not making any difference to the current supplied. And looking closely it seems that my #26 isnt lighting up at all. I tried the other 26 and that doesn't light up either. And certainly not drawing any HT current as my HT reading is high, as if unloaded.

What steps do you think I should take now? Is it likely that I have 2 bad valves, or maybe it is something else? I am assuming that these things glow right?

The sunshine and pub is calling me outside, but I am keen to get this working... for now.

Charlie
 
Those new with filament bias can connect it up the wrong way without thinking. Make sure your supply negative goes to ground and positive goes to one side of the 26 filament. The other side of the filament goes via the 10 ohm resistor to ground. Your natural tendency is to connect the supply to each side of the filament. Just stating the obvious here! So measure the voltages
a) Across the filament
b) Cathode to ground on the resistor.

If these voltages are correct, the 26 should be working and the problem looks like the HT.
26 glows very faintly, but it should be visible. Also it doesn't get very hot. Can you measure the current the 26 is drawing across the plate choke or anode resistor or whatever by measuring its resistance and using Ohms law?

Andy
 
Hi Andy,

I was measuring no current at all through the valve, so I am assuming my heater wiring was incorrect. Thanks for the tips, I will come back to it later... there are more pressing things to get done first...

Interesting that the trim pot didn't work but the board did, I will go over it again.

Cheers

Charlie
 
I worked fast today to finish the new preamp using the LL2745 that Thomas Mayer send me. The starved filaments at 760mA make the 26 filaments not to glow (use 01a if you are keen to get the nice shine out the valve glass :) )

Improved bass and overall sound compared to the LL1660 version.

26 Preamp Gen2 completed | Bartola Valves

Very happy so far with the results, for now will listen to it.

I'm going out to enjoy this lovely afternoon in London as well!!
Cheers,
Ale
 
Hi Ale,

congrats for the completion of the new preamp!
Glad you like the LL2745!

Thomas

Hi Thomas,
Yes, LL2745 is much better for the 26 clearly. I did not follow your advise in the end :) after some tests ended up wiring up it on 5.6:1 so sacrificed output impedance a bit.
Either way, bass is fuller than before and overall tone is very nice. I need to run this version for a while and then connect back older 26 preamp to contrast properly....
Thanks for the help!
Ale
 
There's quite a lot of talk about not mixing signal ground and filament supply ground, but I guess that is for cathode/battery bias designs? For filament bias should I only ground at the reg filament negative?

Making baby steps with this preamp, but it is coming together. I let the smoke out of a few things yesterday so need to replace those and should be good to continue!

Slowly slowly...
 
Charlie, just don't connect the Filament Raw dc supply to Ground. to be certain: don't connect the transformer secondary, or the rectifier, or caps or any part of the input side of the filament regulator to ground.

Naturally, the filament bias resistor connects to ground. One side of the filament connects directly to ground if you use battery bias or fixed bias - but all of those things are on the output of the regulator.

It's just the input side that must strictly float.
 
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Example (my #26 preamp wiring).
 

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Few results

Got a few results from an initial test.

Firstly THD was quite high - 0.1%, but that settled after a while to 0.07% - seems reasonable.

Noise is quite bad, but that is expected as I still have to work on the PS quite a bit and also the PS is far too close to the preamp. It is currently at about -75dB so room for improvement.

It is however very unstable at low frequencies (<100Hz), this could be down to my PS? I have some SSHV2 boards ready to put in when everything is built - so am hoping this will help. Any ideas why else it may be so unstable?

Also, above 60KHz the signal goes through the roof - havn't seen anything like that before so unsure what the cause is.

10R 50W filament resistor got to about 110 degrees C, which seemed quite high, in fact I think it was burning up a little. So I could put 4 together to make 10R if it becomes an issue.

What is the current trend for operating points? People seem to be very happy with starved filaments...?

Getting there....

Charlie

ps these things are SO microphonic. You guys are nuts! ;)