First, apologies if this is a repeated thread. I have searched but all of the words I'm looking for are very common.
The amp is a push-pull EL34 with bias set by adjusting a pot near each tube.
I warm the amp up for 15 minutes, then set the bias for 40mA, if I check again 10 minutes later it can be anywhere between 36 and 50mA. so I re-bias it, listen for a while and check it again, and it's usually changed on one particular tube position. All positions vary, but one is worse than most. It's so bad I'm actually quite surprised if it hasn't changed.
I've replaced the bias pots, checked all connections, done limited tube swapping (changing positions, not new tubes).
I'm using Valve Art EL34s at the moment, should I try a new set of tubes?
The amp is a push-pull EL34 with bias set by adjusting a pot near each tube.
I warm the amp up for 15 minutes, then set the bias for 40mA, if I check again 10 minutes later it can be anywhere between 36 and 50mA. so I re-bias it, listen for a while and check it again, and it's usually changed on one particular tube position. All positions vary, but one is worse than most. It's so bad I'm actually quite surprised if it hasn't changed.
I've replaced the bias pots, checked all connections, done limited tube swapping (changing positions, not new tubes).
I'm using Valve Art EL34s at the moment, should I try a new set of tubes?
What value are your grid circuit ('bias') resistors, brah, and what kind of EL34s are you using? Some EL34s can draw substantial grid current thermally, and the olde tyme 270K or greater value can give some bias drift. My best audio buddy loves GE fatboy 6CA7s in triode, and those buggaz can really pull the bias around.
It would help also to know what the driver tubes are, and what the plate resistors are on them, in order to balance out the circuit paramaters.
Aloha,
Poinz
Edit; punctuation. Sheesh.
It would help also to know what the driver tubes are, and what the plate resistors are on them, in order to balance out the circuit paramaters.
Aloha,
Poinz
Edit; punctuation. Sheesh.
I agree with Poinz that too high a value of "grid leak" resistor with fixed bias can cause bias drift. 220k or lower is usually pretty safe with EL34s, which are pentodes after all (with beam tetrodes it would normally need to be a lot lower than that). Don't forget to factor in the grid stoppers, because they form part of the "grid leak".
If you drop the values of the "grid leaks" you may need to increase the values of the coupling capacitors to compensate, to get the same roll-off frequency/time constant.
Of course, if you use direct-coupled cathode follower or source follower drivers, your "grid leak" resistors will already be low value and there will be no problem in that department.
If you drop the values of the "grid leaks" you may need to increase the values of the coupling capacitors to compensate, to get the same roll-off frequency/time constant.
Of course, if you use direct-coupled cathode follower or source follower drivers, your "grid leak" resistors will already be low value and there will be no problem in that department.
In no particular order:
No chance of getting anywhere NEAR a tube tester.
Drivers are 6N8P via 0.1uF then 1k into pin 5. The plate resistors on the drivers are 40k to 50k.
Bias supply is 100k off the wiper of the 20k bias pot. Bias supply impedance is less than 121k worst case. the 100k connects between the 0.1uF and the 1k mentioned earlier.
The output tubes are new production Valve Art EL34B (chinese, slightly more plate disspation than other EL34) with about 30 hours on them. They aren't particularly closely matched. When I swap the push with the pull the bias goes to 32mA and 56mA
All of the tubes are brand new chinese tubes.
This amp can use KT88 or 6550 as well, power supplies have plenty of current.
Perhaps I should leave it on for a few days and get back to you.
How many hours until they settle? 100?
No chance of getting anywhere NEAR a tube tester.
Drivers are 6N8P via 0.1uF then 1k into pin 5. The plate resistors on the drivers are 40k to 50k.
Bias supply is 100k off the wiper of the 20k bias pot. Bias supply impedance is less than 121k worst case. the 100k connects between the 0.1uF and the 1k mentioned earlier.
The output tubes are new production Valve Art EL34B (chinese, slightly more plate disspation than other EL34) with about 30 hours on them. They aren't particularly closely matched. When I swap the push with the pull the bias goes to 32mA and 56mA
All of the tubes are brand new chinese tubes.
This amp can use KT88 or 6550 as well, power supplies have plenty of current.
Perhaps I should leave it on for a few days and get back to you.
How many hours until they settle? 100?
I've had the amp on for about 6 hours so far this session.
I went out for 3 hours, before leaving I set all to 43mA.
One tube was 52mA, the rest were 45.
I've adjusted them all to 44.5, It's sounding bloody good. Giving it a bit of a workout with Imago by The Butterfly Effect with 2dB of boost at 32Hz.
Will leave the bias alone and report back tomorrow after a 15 minute warm up.
I went out for 3 hours, before leaving I set all to 43mA.
One tube was 52mA, the rest were 45.
I've adjusted them all to 44.5, It's sounding bloody good. Giving it a bit of a workout with Imago by The Butterfly Effect with 2dB of boost at 32Hz.
Will leave the bias alone and report back tomorrow after a 15 minute warm up.
Oz...It's common to come across EL34 p-p circuits using fixed bias resistor values of 100K or so. If you are using standard AC heater supplies and unregulated B+ everyone will get several mA adrift as everything gets hot and may never really settle down. Some tubes i.e 6550 types are notoriously sensitive anode current vs. heater voltage variations.
If you are absolutely sure there is nothing going wrong with the amp then it seems you have been given ropy tubes. Replacement is the only cure.
A simple check I often do first on a suspicious behaving amp is with no signal on the input, simply connect amp to an old speaker and just listen as it warms up. The pop-corn and residual noise may be alarming. Disconnect the global nfb loop and tube currents should be the same. Beware checking the B+ and signal areas, the clicks and noise from probe contact can be very high.
One might get abnormally high hum from the speaker without global nfb connected, this can be caused by mismatched output stage tubes not nulling B+ ripple in the output transformer.
2dB boost at 32Hz is nothing. Give it 20dB and make the tubes work their loins and shake the building.
richj
If you are absolutely sure there is nothing going wrong with the amp then it seems you have been given ropy tubes. Replacement is the only cure.
A simple check I often do first on a suspicious behaving amp is with no signal on the input, simply connect amp to an old speaker and just listen as it warms up. The pop-corn and residual noise may be alarming. Disconnect the global nfb loop and tube currents should be the same. Beware checking the B+ and signal areas, the clicks and noise from probe contact can be very high.
One might get abnormally high hum from the speaker without global nfb connected, this can be caused by mismatched output stage tubes not nulling B+ ripple in the output transformer.
2dB boost at 32Hz is nothing. Give it 20dB and make the tubes work their loins and shake the building.
richj
there are three unregulated filament supplies, left, right and phase splitter/drivers.
this amp has two bias supplies, one for left and one for right. each is schottky bridge rectified with CRC filtering. off each of these supplies is two 20k 10 turn pots, then 6.8k to ground. That is each el34 has it's own bias adjustement off a supply shared with it's mate.
the wiper of each of those pots goes via 100k onto pin 5 of the EL34s.
I'm not sure if anything is going wrong with the amp, that's why I'm asking here. It's most likely the tubes, but could it be the OPT?
the +2dB is actually -2 on everything else, my dac clips with any gain.
It sounds reasonable to me, but I know it should be about +10 according to the calculations on my speakers.
this amp has two bias supplies, one for left and one for right. each is schottky bridge rectified with CRC filtering. off each of these supplies is two 20k 10 turn pots, then 6.8k to ground. That is each el34 has it's own bias adjustement off a supply shared with it's mate.
the wiper of each of those pots goes via 100k onto pin 5 of the EL34s.
I'm not sure if anything is going wrong with the amp, that's why I'm asking here. It's most likely the tubes, but could it be the OPT?
the +2dB is actually -2 on everything else, my dac clips with any gain.
It sounds reasonable to me, but I know it should be about +10 according to the calculations on my speakers.
I thought it was my sinuses, but I confirmed it by turning around. the channel with the dodgy bias is slightly muted in the treble.
it does a quiet little crackle sometimes as well. particularly when warming up.
Watching the bias current it's as if that tube position takes a long time to warm up.
Will shuffle some tubes around tomorrow.
it does a quiet little crackle sometimes as well. particularly when warming up.
Watching the bias current it's as if that tube position takes a long time to warm up.
Will shuffle some tubes around tomorrow.
Mike, this is what I would do:
First, clean all the tube pins and sockets, and make sure the socket connectors in there are firm and snug.
Flop that puppy, and very carefully inspect all the solder joints and connections, especially those to the sockets and all those in the bias supply.
Now, put your tubes in and fire up. Adjust the bias, adjust it again after a few minutes.
If you're still having the problem (electrical and sonic), now's the time to swap tubes in the output stage from channel to channel, to see if the problem follows the tubes. Be systematic, and keep notes; often it's only one tube.
Chahge 'em, brah,
Poinz
First, clean all the tube pins and sockets, and make sure the socket connectors in there are firm and snug.
Flop that puppy, and very carefully inspect all the solder joints and connections, especially those to the sockets and all those in the bias supply.
Now, put your tubes in and fire up. Adjust the bias, adjust it again after a few minutes.
If you're still having the problem (electrical and sonic), now's the time to swap tubes in the output stage from channel to channel, to see if the problem follows the tubes. Be systematic, and keep notes; often it's only one tube.
Chahge 'em, brah,
Poinz
You shouldn't take any measurements until the tubes warm up for at least an hour, then shut down your source material and wait an additional 10-15 minutes. After a few days, they should be fine. If not, it's possible that they will always drift. (I've had a set of KT88's that never stabilized).
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.