Hello all,
After reading this forum for a while and reading a few books on the subject I have finally built my first tube amp.
The problem is that it is not an amp at the moment
Just a complicated hum making machine.
Basically I have got to the stage where when turned on there is a squeal until the tubes heat up, this is then replaced by a very loud low frequency hum. Nothing coming in through the input seems to make it out the other end.
So far I have checked the power supply (with a dummy load) and measured the plate voltage on one of the EL34s (at 400V when the tubes have heated).
What should I be checking next?
After reading this forum for a while and reading a few books on the subject I have finally built my first tube amp.
The problem is that it is not an amp at the moment
Just a complicated hum making machine.
Basically I have got to the stage where when turned on there is a squeal until the tubes heat up, this is then replaced by a very loud low frequency hum. Nothing coming in through the input seems to make it out the other end.
So far I have checked the power supply (with a dummy load) and measured the plate voltage on one of the EL34s (at 400V when the tubes have heated).
What should I be checking next?
Before you go any farther, put some voltage equalizing resistors across the series filter caps in the power supply! This is a serious safety issue.
If you're getting squealing from your speakers immediately upon turn-on, focus on the power supply. A scope will be useful in seeing what's oscillating. As a diagnostic, you might want to try disconnecting all that power conditioning circuitry on the primary side and making a more direct mains connection to see if the trouble lies there.
The squeal and later hum may be related, but keep your eyes open- they may be two separate issues.
If you're getting squealing from your speakers immediately upon turn-on, focus on the power supply. A scope will be useful in seeing what's oscillating. As a diagnostic, you might want to try disconnecting all that power conditioning circuitry on the primary side and making a more direct mains connection to see if the trouble lies there.
The squeal and later hum may be related, but keep your eyes open- they may be two separate issues.
Guys, if it's squealing before warmup, it's not the feedback or misbiasing (there may be some problems there, but that's not the first layer of the onion).
Psysjal, you want to choose a resistor that's a decade lower than the cap's leakage resistance. If those are electrolytics, you'd be safe with 470K, 2W resistors across each cap.
Psysjal, you want to choose a resistor that's a decade lower than the cap's leakage resistance. If those are electrolytics, you'd be safe with 470K, 2W resistors across each cap.
Double and triple and quadruple check your wiring, especially tube connections. It is very easy to make mistakes soldering them, esp with octals.
I would also check your filter capacitors. Are they (too) old? Does the -ve connect solidly to ground?
Also try disconnecting the feedback loop from the output trans. See what happens if you just connect the secondary straight to a speaker.
Resistors across each B+ cap should be ~300K 1W.
I would also check your filter capacitors. Are they (too) old? Does the -ve connect solidly to ground?
Also try disconnecting the feedback loop from the output trans. See what happens if you just connect the secondary straight to a speaker.
Resistors across each B+ cap should be ~300K 1W.
Thanks for the help guys, I know have something that works.
It was two different problems in the end.
The squeal seemed to be down to a couple of bad joints in the power supply.
The loud hum was caused by a incorrectly labelled output transformer. Swapping the leads silenced it completely.
Going back to the value of the resistors across the filter caps. I am guessing that the value is chosen to allow the voltage even out across the caps without letting too much current bypass them. Is this a rule of thumb calculation?
Having never seen tubes in action before I have a few questions about how they should behave. They glow a nice orange colour at the moment and a slight blue can be seen in parts. Is this right or are they running a little hot?
Thanks,
James
It was two different problems in the end.
The squeal seemed to be down to a couple of bad joints in the power supply.
The loud hum was caused by a incorrectly labelled output transformer. Swapping the leads silenced it completely.
Going back to the value of the resistors across the filter caps. I am guessing that the value is chosen to allow the voltage even out across the caps without letting too much current bypass them. Is this a rule of thumb calculation?
Having never seen tubes in action before I have a few questions about how they should behave. They glow a nice orange colour at the moment and a slight blue can be seen in parts. Is this right or are they running a little hot?
Thanks,
James
Hi,
Certainly not in all cases and AFAIK from talking to just about anyone having worked in the electron tube manufacturing business I possibly could talk to, more often than not a sure sign of degraded manufacturing processes.
It sure as hell has nothing to do with the Cerenkov effect but it definetely does have to do with the purity of the glass used in manufacturing in some cases.
Ergo the gassier the glass the more the getter has to work, the sooner it will be exhausted and the sooner the tube will die.
That is just an example of a harmless case but a sure sign of worn out sealing pumps in a lot of cases anyway.
The effect has been discussed in a few threads before with various "opinions" forwarded. Few of which have any bearing with reality or fact.
Cheers,
Certainly not in all cases and AFAIK from talking to just about anyone having worked in the electron tube manufacturing business I possibly could talk to, more often than not a sure sign of degraded manufacturing processes.
It sure as hell has nothing to do with the Cerenkov effect but it definetely does have to do with the purity of the glass used in manufacturing in some cases.
Ergo the gassier the glass the more the getter has to work, the sooner it will be exhausted and the sooner the tube will die.
That is just an example of a harmless case but a sure sign of worn out sealing pumps in a lot of cases anyway.
The effect has been discussed in a few threads before with various "opinions" forwarded. Few of which have any bearing with reality or fact.
Cheers,
Hi James,
As I am in the process of building a couple of EL34 amplifiers at the moment, I was interested in your circuit diagram. I would be interested in knowing more details of this amplifier if you are able to provide same. I am interested in knowing how it compares with a couple of other schematics I have on hand.
-Eric
As I am in the process of building a couple of EL34 amplifiers at the moment, I was interested in your circuit diagram. I would be interested in knowing more details of this amplifier if you are able to provide same. I am interested in knowing how it compares with a couple of other schematics I have on hand.
-Eric
@psysjal
Just some thoughts:
I prefer fixed bias as opposed to cathode (self bias) in an amp such as this though it's a little more expensive/complicated but not very much so, and a few more watts and there's always a little tradeoff: more distortion, but not a lot more. I do think self bias does sound better in some instances. I run my outputs around a conservative 35mA. Not to mention crossover distortion. I think self bias is somewhat less reliable as I've seen quite a few EL34 amps go cherry with that config. You could blame that on EL34's w/questionable quality or just an overall bad design. How about using a common/shared cathode resistor of say 235R - 270R? Using fuses in series with the cath resistors could be a good idea if you are worried about thermal runaway. Of course using high quality tubes you should be ok. Another upside or downside to the self bias is that you can use a higher value grid leak on the EL34 , 470k as opposed to appox. 270k for fixed bias.
Also IMHO that ECC83/12AX7 would have to go, but that's just me. I'm not knocking your amp, a Mullard 5-20 variant. I'm always trying to fix something that ain't broke! 
Hmmm I also see there's a Zobel network on the output, tried that on my current amp. Got rid of a lot the ringing, but didn't like the sound. That was with my Polks, but they're (tweeters) crispy critters right now but hey that took about 375W of sand "hard clipping" watts of Jimmy Page! Haven't scoped it with my current DIY'ed spkrs yet. Different spkrs will present different "kinds" of load to your amp. I would also swap out the 5U4G for a 5V3A both have a 3A heater current (a 5V3 draws 3.8A). More rugged tube, higher ratings, more balls. I wish current tube manufactures would make'em but they prob wouldn't stack up to NOS.
A little BLUE glow is usually nothing to worry about but lots of it could mean not so good as Frank pointed out. Purple or Indigo is usually a bad sign. As is also glowing red or orange plates. As for the equalizing resistors I would/do use 100k - 150k 3W across each cap. I've seen some designs using zeners instead. I've never tried using zeners for that, yet. Good to hear that you got your amp to be an amplifier instead of an oscillator!
Wayne
Just some thoughts:
I prefer fixed bias as opposed to cathode (self bias) in an amp such as this though it's a little more expensive/complicated but not very much so, and a few more watts and there's always a little tradeoff: more distortion, but not a lot more. I do think self bias does sound better in some instances. I run my outputs around a conservative 35mA. Not to mention crossover distortion. I think self bias is somewhat less reliable as I've seen quite a few EL34 amps go cherry with that config. You could blame that on EL34's w/questionable quality or just an overall bad design. How about using a common/shared cathode resistor of say 235R - 270R? Using fuses in series with the cath resistors could be a good idea if you are worried about thermal runaway. Of course using high quality tubes you should be ok. Another upside or downside to the self bias is that you can use a higher value grid leak on the EL34 , 470k as opposed to appox. 270k for fixed bias.
Also IMHO that ECC83/12AX7 would have to go, but that's just me.
I'm not knocking your amp, a Mullard 5-20 variant. I'm always trying to fix something that ain't broke! Hmmm I also see there's a Zobel network on the output, tried that on my current amp. Got rid of a lot the ringing, but didn't like the sound. That was with my Polks, but they're (tweeters) crispy critters right now but hey that took about 375W of sand "hard clipping" watts of Jimmy Page! Haven't scoped it with my current DIY'ed spkrs yet. Different spkrs will present different "kinds" of load to your amp. I would also swap out the 5U4G for a 5V3A both have a 3A heater current (a 5V3 draws 3.8A). More rugged tube, higher ratings, more balls. I wish current tube manufactures would make'em but they prob wouldn't stack up to NOS.A little BLUE glow is usually nothing to worry about but lots of it could mean not so good as Frank pointed out. Purple or Indigo is usually a bad sign. As is also glowing red or orange plates. As for the equalizing resistors I would/do use 100k - 150k 3W across each cap. I've seen some designs using zeners instead. I've never tried using zeners for that, yet. Good to hear that you got your amp to be an amplifier instead of an oscillator!
Wayne
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