Typical mistake to use too low gate stoppers (100-200R) for fast FETs.
I regularly use 1k -carbon comp- resistors.
I've always used metal film - anything wrong with that? Any need to change this to carbon comp?
With FET gate stoppers, the aim is to introduce some resistance in order to prevent oscillation. (The oscillations are suppressed when the R value is sufficient to spoil the Q of the unintended HF oscillator made from the inductance of gate and drain wiring, combined with the leakage capacitance between gate and drain portions of the circuit).
Usually, one can use any resistor, and the circuit will be fine, but the optimal practice is to use a resistor with minimised inductance. Through-hole Film resistors (metal film, carbon film) are constructed with a spiral of resistance element applied (or laser-formed) around a tubular substrate. This structure can add inductance - which increases with the diameter of the resistor.
Cheap thick-film Surface-mount chip resistors (for PCB use) and Carbon composition resistors are non-inductive, and are the best choice.
Film resistors usually work OK, but there are situations (e.g. where the R value is a little too low) when they will provoke oscillation in a circuit that was otherwise stable (or metastable). If you need to use a film resistor, choose a 1/8W for its small size.
The other precaution: make sure the amount of metallic conductor between stopper an gate is as small as possible (mount the R close to the gate) to reduce leakage feedback.
Also: don't make the resistor too large, especially with high voltage circuits. If the gate resistor is too high, the FET can break down due to fast transients coupling through the D→G capacitance, and running through the gate resistor (increases FET enhancement unintentionally). 1K is the highest value I would use on small FETs, and lower still with big high-current beasts.
Usually, one can use any resistor, and the circuit will be fine, but the optimal practice is to use a resistor with minimised inductance. Through-hole Film resistors (metal film, carbon film) are constructed with a spiral of resistance element applied (or laser-formed) around a tubular substrate. This structure can add inductance - which increases with the diameter of the resistor.
Cheap thick-film Surface-mount chip resistors (for PCB use) and Carbon composition resistors are non-inductive, and are the best choice.
Film resistors usually work OK, but there are situations (e.g. where the R value is a little too low) when they will provoke oscillation in a circuit that was otherwise stable (or metastable). If you need to use a film resistor, choose a 1/8W for its small size.
The other precaution: make sure the amount of metallic conductor between stopper an gate is as small as possible (mount the R close to the gate) to reduce leakage feedback.
Also: don't make the resistor too large, especially with high voltage circuits. If the gate resistor is too high, the FET can break down due to fast transients coupling through the D→G capacitance, and running through the gate resistor (increases FET enhancement unintentionally). 1K is the highest value I would use on small FETs, and lower still with big high-current beasts.
Just thought I should put in a report in this thread. I recently built the Bartola 01a line stage and after a couple of issues and an upgrade to the source follower boards I really like how it sounds. I had to do some individual selection of the tubes in circuit to find a couple of them that were reasonably well matched, both for gain and for frequency response. Some of them had a lot of HF roll-off with the 10k square waves very rounded on the leading edge.
Also had a funny thing happen yesterday. I had the line stage on a maple board on the floor in front of the plasma TV since the other audio gear is on the shelves under it. I hadn't ever had the TV on while I was using the 01a but this time I did. It seems I have a receiver and a line stage. The TV made it buzz like crazy and the level and pitch were dependent on the overall brightness of the picture. White, or light, colored screens were noisiest and the dark screens quieter but all made a very audible buzzing. I guess I may not be using it for movies. Any ideas? Is it the tubes themselves or the circuitry under the deck? I'm not too worried about it since I expect to use it for music, but I am curious. Haven't done any hunting yet to figure it out.
John
Also had a funny thing happen yesterday. I had the line stage on a maple board on the floor in front of the plasma TV since the other audio gear is on the shelves under it. I hadn't ever had the TV on while I was using the 01a but this time I did. It seems I have a receiver and a line stage. The TV made it buzz like crazy and the level and pitch were dependent on the overall brightness of the picture. White, or light, colored screens were noisiest and the dark screens quieter but all made a very audible buzzing. I guess I may not be using it for movies. Any ideas? Is it the tubes themselves or the circuitry under the deck? I'm not too worried about it since I expect to use it for music, but I am curious. Haven't done any hunting yet to figure it out.
John
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01a line stage experiences
Just thought I should put in a report in this thread. I recently built the Bartola 01a line stage and after a couple of issues and an upgrade to the source follower boards I really like how it sounds. I had to do some individual selection of the tubes in-circuit to find a couple of them that were reasonably well matched, both for gain and for frequency response. Some of them had a lot of HF rolloff with the 10k square waves very rounded on the leading edge.
Also had a funny thing happen yesterday. I had the line stage on a maple board on the floor in front of the plasma TV since the other audio gear is on the shelves under it. I hadn't ever had the TV on while I was using the 01a but this time I did. It seems I have a receiver and a line stage. The TV made it buzz like crazy and the level and pitch were dependent on the overall brightness of the picture. White, or light, colored screens were noisiest and the dark screens quieter but all made a very audible buzzing. I guess I may not be using it for movies. Any ideas? Is it the tubes themselves or the circuitry under the deck? I'm not too worried about it since I expect to use it for music, but I am curious. Haven't done any hunting yet to figure it out.
John
Just thought I should put in a report in this thread. I recently built the Bartola 01a line stage and after a couple of issues and an upgrade to the source follower boards I really like how it sounds. I had to do some individual selection of the tubes in-circuit to find a couple of them that were reasonably well matched, both for gain and for frequency response. Some of them had a lot of HF rolloff with the 10k square waves very rounded on the leading edge.
Also had a funny thing happen yesterday. I had the line stage on a maple board on the floor in front of the plasma TV since the other audio gear is on the shelves under it. I hadn't ever had the TV on while I was using the 01a but this time I did. It seems I have a receiver and a line stage. The TV made it buzz like crazy and the level and pitch were dependent on the overall brightness of the picture. White, or light, colored screens were noisiest and the dark screens quieter but all made a very audible buzzing. I guess I may not be using it for movies. Any ideas? Is it the tubes themselves or the circuitry under the deck? I'm not too worried about it since I expect to use it for music, but I am curious. Haven't done any hunting yet to figure it out.
John
Hi John,
Great to hear the feedback, I've been using the UV-201a preamp for some time now and I really like what I hear. The clarity of the 01a is there plus additional driving capability from the source follower which helps with the HF response.
The tricky thing with these very old valves is that, despite they may measure within spec (or close to it) some have some poor HF response with a roll off of 10-12kHz. This probably may have discourage builders from the 01a for sure. Unfortunately, you can only figure this out by either listening or running a FR test. I test most of my 01a stash before using them.
Is not surprising what you have experienced. Any TV, has a digital circuit operating at high frequency and will likely interfere with equipment nearby. In my case, it doesn't. However, DHTs are quite prone to pick up hum and interference from its plate. Some people had to shield the valves (many cases reported with the 26 for example). I meticulously select my valves to ensure I don't get any one like those.
If the interference is not evident when you place it far away from the noisy sources, then I wouldn't waste time to screen the valves and you will miss the lovely glowing view
Enjoy the fantastic sound of the 01a.
Cheers
ale
Great to hear the feedback, I've been using the UV-201a preamp for some time now and I really like what I hear. The clarity of the 01a is there plus additional driving capability from the source follower which helps with the HF response.
The tricky thing with these very old valves is that, despite they may measure within spec (or close to it) some have some poor HF response with a roll off of 10-12kHz. This probably may have discourage builders from the 01a for sure. Unfortunately, you can only figure this out by either listening or running a FR test. I test most of my 01a stash before using them.
Is not surprising what you have experienced. Any TV, has a digital circuit operating at high frequency and will likely interfere with equipment nearby. In my case, it doesn't. However, DHTs are quite prone to pick up hum and interference from its plate. Some people had to shield the valves (many cases reported with the 26 for example). I meticulously select my valves to ensure I don't get any one like those.
If the interference is not evident when you place it far away from the noisy sources, then I wouldn't waste time to screen the valves and you will miss the lovely glowing view
Enjoy the fantastic sound of the 01a.
Cheers
ale
Your construction skills are beginning to approach legendary status. Anyone who can destroy all their tools on extra hard grade aluminium top plates is either bonkers or a bit of a genius.
Hi Ale,
I would be thankful if you could answer my question regarding Rmu value.
I have finished my 01a preamp using BF862 and 1,5K R7 (Rmu) resistor. However some time ago I noticed that you recommended to use 330-470R for R7 with above mentioned FET.
What is the consequence of using 3-5 times higher Rmu resistor?
I would be thankful if you could answer my question regarding Rmu value.
I have finished my 01a preamp using BF862 and 1,5K R7 (Rmu) resistor. However some time ago I noticed that you recommended to use 330-470R for R7 with above mentioned FET.
What is the consequence of using 3-5 times higher Rmu resistor?
Just for you, Andy. No drills were harmed in the making of these cages. 
20161023_175917 by Richard Crawley, on Flickr
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
20161023_175917 by Richard Crawley, on Flickr