DIY Audio Projects Forum

Hi All,

I was wondering if people can help with a few questions, I've searched all over google for this but there seems to be limited info. I'll try and keep it short and simple

I decided to put together a tube amp, the valves are EF95/6AK5 (GE Jan 5654W, Mullard M8100 etc) running in triode mode, an op amp is then used to amplify the tube output for the headphones.
I've tried a lot of different values of components and I think I have found the sweet spot.

Info about the amp:
100VDC provided by DC booster, input voltage is 12V, oscillator runs at 75KHz using UC3843C IC, it sounds clean to me and scope shows very little ripple.
EF95 tubes running at 85V plate voltage (depending on tubes), anode resistor (3K - R7 & R8) is dropping 15V.
Cathode resistor (270 - R2 & R5) is dropping about 1.35V.

Q1
Based on ohms law, the current drawn by each tube is 5mA (1.35/270 & 15/3000), this sounds about right. Now I've followed various differnt methods for calculating the anode and cathode resistors, none of these calculations come close to producing the desired voltages.
Is this normal, I ask this because I was trying to set the bias to 1.5V but but settled on 1.35V as different tubes bias at different voltages using the same resistor.

Q2
Does it matter if I set the bias to 1V, 1.35V or 1.5V.

Q3
I'm running the plate coupling caps (C8 & C10) at 85VDC, they are rated for 100VDC and are made by TDK/EPCOS and are PET/Polyester. Will this be a problem, the voltage is stable.

Q4
I decided to add cathode bypass capacitors of 100uF (C5 & C6), is that too high or does it not matter. The audio out sounds fine to me.

Q5
People say the Russian tubes 6J1P-EV are compatible with EF95 tubes, I tried them with the current resistor values, the plate runs at 95V and the bias is less than 1V. I take it they aren't quite compatible.

Q6
I've noticed with some "matched" tubes the bias voltage is different by 0.5V, I take it the sellershop did not match the tubes?

Sorry about the incorrectly drawn tubes in the attachment, the pins are correct though.

Apologies if the questions seem stupid, any help appreciated, thanks

You're questions don't seem stupid to me at all. I'll give it a try.

A1: I made a loadline for 3K and (like you) came closer to 1.5 V at Va = 85 V than to 1.35 V. But I think that the difference is too small to indicate a problem with the tubes (or something else).

A2: I'm not sure because there are different opinions on this subject when this is discussed on forums and untill now I didn't find literature in which this possible problem is quantified. I personally would not like a triode with a moderate amplification factor like the 6AK5 (mu = 28 in triode mode) to work at such a low grid voltage because a little bit of grid current will flow in the positive going parts of the input signal (true with input levels like from a CD player). The point where grid current starts is - 1.3 V (valid for almost all tube types). The amount of grid current is very small, but it varies with input (the more positive the input voltage swings, the more grid current will flow; in the negative going part of the input signal no grid current flows).

A3: At start-up the tubes are not conducting right away. Because of that there will be no voltage drop over the 3K anode resistors untill the tubes warmed up and start to pass current. So during this time the full 100 Vdc is over the coupling capacitors. Probably they will be able to stand this, but I personally would prefer a little higher maximum voltage for those capacitors.

A4: In my opinion 100 uF is fine (large enough for the low frequencies not to roll-of + no problem if the value is higher than necessary).

A5: I don't have experiences with the 6J1P-EV. The values in datasheets are valid for an average tube. The values are the result of measuring more than one specimen of the same tube type. So individual tubes can/will show a bit different values than the datasheet tells us.

A6: The bias being 0.5 V off (so per tube being about 0.25 V off) sounds like a lot to me at a grid voltage of - 1.35 V. Doesn't look like 'matched' tubes to me. But if they were realy matched, than I wonder at what working point(s) the seller (of manufacturer) matched the tubes. I think that matching tubes at a certain working point doesn't garantee that they will be matched at all other working points. In your case we're discussing a pentode connected as a triode. Maybe the matching was done with the tubes connected as pentodes.

_________________
Greetings,
Robert

http://otl800.blogspot.com/

Thanks for your reply, appreciated.

I've now finished the amp have have made all the schematics available:

Schematics and BOM available here:
https://easyeda.com/Ratti3

Blog with more info on this project:
http://ratti3.blogspot.co.uk/

Video of the project, 3 parts, this is part 1:
https://youtu.be/28DwrHB3h8w

I've made the amp board in such a way, so that it can be used without the additional modules or batteries. All you need is 12VDC and line in and headphones out.