Hello,
I've been messing around with some EL34 tubes the last few days. A lot of schematics I see just run the heaters directly from the ~6.3Vac take off from something like a Hammond transformer.
This is a bit too noisey for my liking so I am rectifying the Vac and passing it thru a Black Gate 1mF 50V capacitor for smoothing. The load resistor is roughly 5k but will be 10k in the final design.
What's making me wonder is that the rectified ac is appearing as roughly 9Vdc from the full wave 1N5404 bridge rectifier. No problem. But than when I attach the heaters, the voltage drops to 5Vdc as the load is added.
I had the transformer custom wound for me so I could keep everything seperate and simplified; it should be good for at least 3 amps. I am curious, do tube specficiations rate their tubes for the voltage that should be present with no load, or while the heater is in it's running condition? How close do people actually stay to the 6.3v recommendation and how many of you float the heaters around 50v or so to minimise hum?
Of coarse, with the lower voltage present, current rises, which is worrying me slightly as I had specified the transformer to run at 6.3v without a load. The supply must heat an EL34, which will require roughly 1.5 amps and two 12AX7's, which require a fraction of an amp each.
If a supply is going to be rectified and used to power heaters do designers usually accomodate for this in the input transformers voltages or do they just use a 6.3v heater transformer and live with the voltage differences?
I decided to go with just the Black Gate smoothing capacitor since a choke is going to start making things complicated and resistors are going to have to be very small to allow the correct current to flow.
In addition to these questions, I would also greatly appreciate any help with two others!
- Where in the UK sells well made, custom, mains transformers? I've used Newmark just recently and, while I'm not unhappy with them, I am not particularly impressed either.
- If my supply could possibly draw close to 3 amps should I switch the 1N5404's for something that can handle more continuous current to ensure there's no chance of them burning out over time?
I'm sorry if I've made some kind of stupid mistake somewhere along the way here... my mind seems to be in second gear mode at the moment and I'm making incredibly embarassing mistakes 24 / 7!
All the best,
John
I've been messing around with some EL34 tubes the last few days. A lot of schematics I see just run the heaters directly from the ~6.3Vac take off from something like a Hammond transformer.
This is a bit too noisey for my liking so I am rectifying the Vac and passing it thru a Black Gate 1mF 50V capacitor for smoothing. The load resistor is roughly 5k but will be 10k in the final design.
What's making me wonder is that the rectified ac is appearing as roughly 9Vdc from the full wave 1N5404 bridge rectifier. No problem. But than when I attach the heaters, the voltage drops to 5Vdc as the load is added.
I had the transformer custom wound for me so I could keep everything seperate and simplified; it should be good for at least 3 amps. I am curious, do tube specficiations rate their tubes for the voltage that should be present with no load, or while the heater is in it's running condition? How close do people actually stay to the 6.3v recommendation and how many of you float the heaters around 50v or so to minimise hum?
Of coarse, with the lower voltage present, current rises, which is worrying me slightly as I had specified the transformer to run at 6.3v without a load. The supply must heat an EL34, which will require roughly 1.5 amps and two 12AX7's, which require a fraction of an amp each.
If a supply is going to be rectified and used to power heaters do designers usually accomodate for this in the input transformers voltages or do they just use a 6.3v heater transformer and live with the voltage differences?
I decided to go with just the Black Gate smoothing capacitor since a choke is going to start making things complicated and resistors are going to have to be very small to allow the correct current to flow.
In addition to these questions, I would also greatly appreciate any help with two others!
- Where in the UK sells well made, custom, mains transformers? I've used Newmark just recently and, while I'm not unhappy with them, I am not particularly impressed either.
- If my supply could possibly draw close to 3 amps should I switch the 1N5404's for something that can handle more continuous current to ensure there's no chance of them burning out over time?
I'm sorry if I've made some kind of stupid mistake somewhere along the way here... my mind seems to be in second gear mode at the moment and I'm making incredibly embarassing mistakes 24 / 7!
All the best,
John
Hi,
Assuming 1 µF this way too low, try something in the order of 4.700µF or even 10.000µF.
6.3VAC FWB rectified would give a theoretical maximum of 8.82VDC so in order to bring both the voltage and ripple down you need a Pi cel filter.
When executed properly AC heating an indirectly heated powertube should not give you any audible hum.
Cheers,
Assuming 1 µF this way too low, try something in the order of 4.700µF or even 10.000µF.
6.3VAC FWB rectified would give a theoretical maximum of 8.82VDC so in order to bring both the voltage and ripple down you need a Pi cel filter.
When executed properly AC heating an indirectly heated powertube should not give you any audible hum.
Cheers,
Hi,
Heater voltage is specified under load conditions. allowed tolerance is usually +-10%, even if it is probably better to be closer then that for best performance and longest life. 5V as you measure is way too low.
If you rectify 6.3V you probably end up with less then 6.3V DC unless you dont use a big cap due to losses in the diodes, according to PSUD I get roughly 6.3V using 10000 uF with 1A current draw and a transformer with normal spec. To use a black gate caps is to exaggurate a bit, a normal good cap will do, the larger the better.
Current should not be so much different with 5V or 6.3V.
The transformer has to be specified to give voltage under full load as it normally gives higher voltage under no load conditions. If you use DC for heater the transformer also has to be specified for higher AC current, normal is to calculate that AC current is 1.6 - 1.8 times DC current with a bridge rectifier and capacitors.
BTW, as you are using EL 34's I assume that this is for a power amp? I have never used DC heaters in a a power amp with in-direct heated tubes. Normally I separate the heaters for driver tubes and power tubes. For the power tubes I just tie one of the terminals to ground. For the driver tubes I use a hum pot of 100ohm with the slider connected to ground, (or a DC potential if the input stage is a SRPP or similar), the hum po is adjusted for min hum on the amplifier output. This method give me hum levels that are below the high frequency thermal noise and so low that I cant hear it even very close to my ~100dB sensitivity Lowther horns.
Please try to make the amplifier hum free without using DC for the heaters its should not be a big problem.
Regards Hans
Heater voltage is specified under load conditions. allowed tolerance is usually +-10%, even if it is probably better to be closer then that for best performance and longest life. 5V as you measure is way too low.
If you rectify 6.3V you probably end up with less then 6.3V DC unless you dont use a big cap due to losses in the diodes, according to PSUD I get roughly 6.3V using 10000 uF with 1A current draw and a transformer with normal spec. To use a black gate caps is to exaggurate a bit, a normal good cap will do, the larger the better.
Current should not be so much different with 5V or 6.3V.
The transformer has to be specified to give voltage under full load as it normally gives higher voltage under no load conditions. If you use DC for heater the transformer also has to be specified for higher AC current, normal is to calculate that AC current is 1.6 - 1.8 times DC current with a bridge rectifier and capacitors.
BTW, as you are using EL 34's I assume that this is for a power amp? I have never used DC heaters in a a power amp with in-direct heated tubes. Normally I separate the heaters for driver tubes and power tubes. For the power tubes I just tie one of the terminals to ground. For the driver tubes I use a hum pot of 100ohm with the slider connected to ground, (or a DC potential if the input stage is a SRPP or similar), the hum po is adjusted for min hum on the amplifier output. This method give me hum levels that are below the high frequency thermal noise and so low that I cant hear it even very close to my ~100dB sensitivity Lowther horns.
Please try to make the amplifier hum free without using DC for the heaters its should not be a big problem.
Regards Hans
Things just seem to have gotten easier... something has to go wrong...!
Thanks for the advice Hans and fdegroove!
It seems like I've been going the wrong way with all this. FDE, have you ever had any annoying levels of noise using AC for driver tubes or would you agree with Hans that the hum pot is easily enough?
Do you have any recommendations for smoothing capacitors? I know Elna's Cerafines have a great capacitance at low voltages, but they have been discontinued now so I would rather not use them if possible. Are there any other favourites for this part or should I just go with any moderately expensive brand?
Kind regards,
John
Thanks for the advice Hans and fdegroove!
It seems like I've been going the wrong way with all this. FDE, have you ever had any annoying levels of noise using AC for driver tubes or would you agree with Hans that the hum pot is easily enough?
Do you have any recommendations for smoothing capacitors? I know Elna's Cerafines have a great capacitance at low voltages, but they have been discontinued now so I would rather not use them if possible. Are there any other favourites for this part or should I just go with any moderately expensive brand?
Kind regards,
John
I'll repeat John's comment again but with more straightforward language:
You lose voltage in the diodes. Open-load, the voltage should be 6.3 * 1.414 = 8.9. You won’t see the diode voltage drop open-load.
Once a load is applied, you are probably losing 0.7V per diode. Since the power has to travel through 2 diodes in a full wave bridge, you are losing 1.4V in the bridge. Now you are down to 7.5V usable voltage – even before applying full load.
You can’t convert 6.3VAC rms into 6.3VDC at full load. The power factor in the transformer will be awful, causing very high current spikes across the transformer and diodes. These spikes are much greater than the transformer's rating, so you’ll be losing additional voltage in the transformer’s resistive losses (otherwise known as regulation factor).
There’s where your voltage is going: diode forward voltage drop and inside your transformer due to a low power factor.
It is my suggestion that in order to use DC on heaters, the transformer secondary must output at least 8V rms. I use a 9VAC transformer for my driver stage tubes. I use a bridge rectifier and a voltage regulator, which accounts for at least 3.7V of losses.
To avoid losing 1.4V in the diode bridge, you can use synchronous rectification. I’d love to change every diode bridge in every piece of my audio gear to synchronous rectification. Over the next months, I’ll be developing a little circuit board to replace diode bridges with synchronous rectification. Synchronous rectification won't solve your power factor problem, though.
If you want more information on what the power factor is, and why it is affecting your voltages, let me know.
You lose voltage in the diodes. Open-load, the voltage should be 6.3 * 1.414 = 8.9. You won’t see the diode voltage drop open-load.
Once a load is applied, you are probably losing 0.7V per diode. Since the power has to travel through 2 diodes in a full wave bridge, you are losing 1.4V in the bridge. Now you are down to 7.5V usable voltage – even before applying full load.
You can’t convert 6.3VAC rms into 6.3VDC at full load. The power factor in the transformer will be awful, causing very high current spikes across the transformer and diodes. These spikes are much greater than the transformer's rating, so you’ll be losing additional voltage in the transformer’s resistive losses (otherwise known as regulation factor).
There’s where your voltage is going: diode forward voltage drop and inside your transformer due to a low power factor.
It is my suggestion that in order to use DC on heaters, the transformer secondary must output at least 8V rms. I use a 9VAC transformer for my driver stage tubes. I use a bridge rectifier and a voltage regulator, which accounts for at least 3.7V of losses.
To avoid losing 1.4V in the diode bridge, you can use synchronous rectification. I’d love to change every diode bridge in every piece of my audio gear to synchronous rectification. Over the next months, I’ll be developing a little circuit board to replace diode bridges with synchronous rectification. Synchronous rectification won't solve your power factor problem, though.
If you want more information on what the power factor is, and why it is affecting your voltages, let me know.
Hi,
Well, I'd use DC in phono stages and linelevel preamps.
In an amplifier using indirectly heated tubes I always use AC and never had hum I could measure or hear.
If you don't use a center tapped xformer you can create a virtual one by putting two 100R resistors in series across the filament winding and ground the midpoint.
Always use solid core wire no thicker than appropriate for the current it needs to pass, twist as tightly as you can and push it against the chassis away from other signal carrying wires and components.
That works very well too in addition of the above methods, just don't use the two resistors but connect the pot's wiper to ground.
This is often how it's done with directly heated tubes.
Regarding xformers in the U.K., have you tried Maplin?
I won't waste money on them here, you can use ordinary Nichicon, Philips, whatever you can easily buy.
Indirectly heated valves don't have their heaters in direct signal path.
Target for the correct voltage and make sure they can draw the current they need.
When I use DC for the heaters I also regulate their supply so it stays constant and the rail is squeeky clean.
Cheers,
Well, I'd use DC in phono stages and linelevel preamps.
In an amplifier using indirectly heated tubes I always use AC and never had hum I could measure or hear.
If you don't use a center tapped xformer you can create a virtual one by putting two 100R resistors in series across the filament winding and ground the midpoint.
Always use solid core wire no thicker than appropriate for the current it needs to pass, twist as tightly as you can and push it against the chassis away from other signal carrying wires and components.
That works very well too in addition of the above methods, just don't use the two resistors but connect the pot's wiper to ground.
This is often how it's done with directly heated tubes.
Regarding xformers in the U.K., have you tried Maplin?
I won't waste money on them here, you can use ordinary Nichicon, Philips, whatever you can easily buy.
Indirectly heated valves don't have their heaters in direct signal path.
Target for the correct voltage and make sure they can draw the current they need.
When I use DC for the heaters I also regulate their supply so it stays constant and the rail is squeeky clean.
Cheers,
This is one of those moments where I should be kicking myself...
Do you hand make a regulator for the supply yourself or buy a prefabricated version? I read an article by one amp designer who mentioned that he used the LM723 for his supply regulation. Is it worth putting together voltage regulation circuits myself or are the prefabricated ones just as good?
This isn't directly related to my initial question but... I was reading about this idea ealier and found someone asking about whether or not it could be used on a directly heated cathode. One of the replies suggested that unless the resistors where 0.1% precision values and the transformer was perfectly balanced then it would allow hum thru and would also damage the sound quality. I guess an adjustable hum pot solves this.
That sounds like a good idea. Would you use similar capacitors in the high voltage supply for the plates or would you go with a more expensive brand? I know Black Gate make the Wkz series specifically for the LC filter in HV supplies. It's 220uf + 220uf but it's retail is something around 55 pounds without VAT if I remember rightly.
Compared to Nichicon's, would I be right in thinking Black Gate believe they can charge more for their Wkz series due to some improvement in their transient response or filtering abilities?
My thanks to everyone who's helped out so far,
John
Hi,
It's a ******** filament supply for crying out loud...Relax, if it doesn't hum it's O.K.
The off the shelf regs are fine unless someone's going to tell me that heaters ar feeding back into their cathodes that is...get a grip, please...???
Inversely, you can tap signal of a heater circuit....ah, ah....
Sorry if I sound rude but I start to get fed up with elemantaries...
Buy a book, read Tubecad, in short educate yourself...
Make an effort and learn, maybe one day we'll be buying your stuff?
Ah well,
It's a ******** filament supply for crying out loud...Relax, if it doesn't hum it's O.K.
The off the shelf regs are fine unless someone's going to tell me that heaters ar feeding back into their cathodes that is...get a grip, please...???
Inversely, you can tap signal of a heater circuit....ah, ah....
Sorry if I sound rude but I start to get fed up with elemantaries...
Buy a book, read Tubecad, in short educate yourself...
Make an effort and learn, maybe one day we'll be buying your stuff?
Ah well,
No problem, I understand what you're saying! I don't like asking questions like these, but I'd been having quite a lot of trouble finding accepted answers to them.
I'm currently making my way thru the Naval electronics manual. It's good, but then it gets to points, like heater supplies, and it does nothing other than tell you that tubes need one; well, perhaps slightly more.
Is Radiotron's 4th Edition worth the $40 - 50 it's currently selling for? I'm having a bit of trouble finding a copy in the UK. Postage from the US is substantial for a book that weighs this much.
I would greatly appreciate any links to sites online specifically related to tube theory if you can suggest any others.
Regards,
John
Quote:
It's a ******** filament supply for crying out loud...Relax, if it doesn't hum it's O.K.
John
There comes a time in everyones life when they need to put trust into what others say. I will be blunt and too the point. Frank has the credentials, experience,knowledge to know what the hell he is talking about. Electronics is like a game of chess with Frank except he is always 5-6 moves ahead of his opponent. Frank won't steer you wrong. If I understand him correctly he is telling you not to waste your money on caps/components that aren't going to help you. If you don't have hum or hear hum don't put yout money into extra parts when you don't need them. Take that money and put it into other areas in the amp that will show a sonic/performance difference.
J
It's a ******** filament supply for crying out loud...Relax, if it doesn't hum it's O.K.
John
There comes a time in everyones life when they need to put trust into what others say. I will be blunt and too the point. Frank has the credentials, experience,knowledge to know what the hell he is talking about. Electronics is like a game of chess with Frank except he is always 5-6 moves ahead of his opponent. Frank won't steer you wrong. If I understand him correctly he is telling you not to waste your money on caps/components that aren't going to help you. If you don't have hum or hear hum don't put yout money into extra parts when you don't need them. Take that money and put it into other areas in the amp that will show a sonic/performance difference.
J
I’ve done the sâme with 6.3V execpt there was no need to get another tranny..... .....desperation....except I used Schottky diodes (SB340 etc) with lower forward voltage losses than standard diodes in discrete bridge
The input stage in my amp uses 2x7199’s, initially the hum level was shocking. The heater current for the pair is about 0,8A, so I lifted the system 6.3V 20A off ground and used Schottky diodes to fullwave rect into a 10,000uF 10V elect. The DC volts available to the 7199’s is somewhat higher, and I used lo ohm resistors either side of DC to bring the down to 6.3V with another 3300uF slapped on. There was no need to use Lo-drop-out-reggies as the in /out differential was to low. After doing this, I biassed the whole heater chain to minus 10V. That cured every snippet of hum and now I can use different makes of tube. Bear in mind 7199’s vary alot but.... this recipe may not hold for other tube types. Botch it it up first.......
The input stage in my amp uses 2x7199’s, initially the hum level was shocking. The heater current for the pair is about 0,8A, so I lifted the system 6.3V 20A off ground and used Schottky diodes to fullwave rect into a 10,000uF 10V elect. The DC volts available to the 7199’s is somewhat higher, and I used lo ohm resistors either side of DC to bring the down to 6.3V with another 3300uF slapped on. There was no need to use Lo-drop-out-reggies as the in /out differential was to low. After doing this, I biassed the whole heater chain to minus 10V. That cured every snippet of hum and now I can use different makes of tube. Bear in mind 7199’s vary alot but.... this recipe may not hold for other tube types. Botch it it up first.......
Sinc apol's for the confusion.......remember other foreigners also read these sites......patience....some of us can't be expected to get all intreprets perfect.......as those who have english as the cue language (mine is 2nd)....I see that reply as an insult.....
Each day I get loads of multi-lingual emails from other sites, some are trivial natured but I do promise sincere replies.
tschüs
rich
Each day I get loads of multi-lingual emails from other sites, some are trivial natured but I do promise sincere replies.
tschüs
rich
Since I last posted I've ordered a copy of Valve Amplifiers. I have 6 pounds in book vouchers to spend, so it seemed like a good way to use them! I'm really looking forward to having something in paper to read for the next few weeks.
I also got some part thru for a new supply this morning. RS is quite possibly the most incredible retailer I've ever delt with. They're only really matched by The Hi-Fi Collective, in terms of response alone. I'll be on first names basis with the guy from Omega who delivers RS's parts soon enough.
The supply works perfectly now. So far I'm really happy.
One strange thing happened though.
[If you really have an empathy with tubes, leave the thread now]
I have a number of old El34's sitting around doing nothing. They're very old and have taken some serious wear in an A/B amplifier. In fact, they're so old and worn, they just blow the fuses when they're run. It's a Marshall amp, don't worry!
I decided to see how high the heater voltage could be taken before the heater's life time was exponentially shortened.
Well, the supply only manages 10Vdc, but the tube was glowing like a light bulb. It felt so nice to sit in an almost dark room light only by the heater of a tube. Which gives me an idea for a really cool lighting project... but anyway...
The tube began to tick as the voltage was lowered back towards 6.3v, perhaps around 5 - 10hz. I assumed it was simply moving as the temperature changed, but a minute or so later the ticking still hadn't stopped. So I increased the voltage again. From 6.3 - 8 volts the ticking began to decrease again.
The tube was not ticking before I raised the heater above it's normal operating voltage.
I now looked over at my scope. There was a lot of pulse like modulation on the voltage with a drop to a flat, but elevated, plane every 5 - 15 pulses. If I remember rightly, the scope was set to 20ms per division for it's time base and 5v per division for it's amplitude.
The ticking was loud enough that you could hear it in a quiet room from roughly 2 foot away. But anymore than that and it was almost impossible to hear.
I figured something must have been going wrong in the supply or the heater itself was going. I swapped the tube for an equally worn out EL34 and switched the supply back on. No ticking, and no nasty modulation.
The scope is a Fluke 123, which is probably the coolest thing in the entire house right now. When it's connected to an input, in automatic mode, it fine tunes itself to display whatever majority signal it detects. But with the new tube it simply sat still and didn't bother going any further.
Okay... I know the tube is almost certainly damaged; it couldn't have been used for anything other than an ornamnet anyway. I've smashed a few of them just to look inside. But what could have happened to the heater to produce these effects? I believe the tubes are gassed quite badly. Possibly something to do with the effect?
The ticking seemed to be quite close, in frequency, to the flat parts of the modulation.
I wish I had the SCC and Scope View option now. I could have sent you a piccy of it! The pulses d
I have some questions regarding the LM350T I am using for the voltage regulation. If anyone would be willing to help me with them I'd really appreciate it.
Now I'm stuck until the B9A sockets arrived really.
You've all been a great help so far!
My best wishes,
John
I also got some part thru for a new supply this morning. RS is quite possibly the most incredible retailer I've ever delt with. They're only really matched by The Hi-Fi Collective, in terms of response alone. I'll be on first names basis with the guy from Omega who delivers RS's parts soon enough.
The supply works perfectly now. So far I'm really happy.
One strange thing happened though.
[If you really have an empathy with tubes, leave the thread now]
I have a number of old El34's sitting around doing nothing. They're very old and have taken some serious wear in an A/B amplifier. In fact, they're so old and worn, they just blow the fuses when they're run. It's a Marshall amp, don't worry!
I decided to see how high the heater voltage could be taken before the heater's life time was exponentially shortened.
Well, the supply only manages 10Vdc, but the tube was glowing like a light bulb. It felt so nice to sit in an almost dark room light only by the heater of a tube. Which gives me an idea for a really cool lighting project... but anyway...
The tube began to tick as the voltage was lowered back towards 6.3v, perhaps around 5 - 10hz. I assumed it was simply moving as the temperature changed, but a minute or so later the ticking still hadn't stopped. So I increased the voltage again. From 6.3 - 8 volts the ticking began to decrease again.
The tube was not ticking before I raised the heater above it's normal operating voltage.
I now looked over at my scope. There was a lot of pulse like modulation on the voltage with a drop to a flat, but elevated, plane every 5 - 15 pulses. If I remember rightly, the scope was set to 20ms per division for it's time base and 5v per division for it's amplitude.
The ticking was loud enough that you could hear it in a quiet room from roughly 2 foot away. But anymore than that and it was almost impossible to hear.
I figured something must have been going wrong in the supply or the heater itself was going. I swapped the tube for an equally worn out EL34 and switched the supply back on. No ticking, and no nasty modulation.
The scope is a Fluke 123, which is probably the coolest thing in the entire house right now. When it's connected to an input, in automatic mode, it fine tunes itself to display whatever majority signal it detects. But with the new tube it simply sat still and didn't bother going any further.
Okay... I know the tube is almost certainly damaged; it couldn't have been used for anything other than an ornamnet anyway. I've smashed a few of them just to look inside. But what could have happened to the heater to produce these effects? I believe the tubes are gassed quite badly. Possibly something to do with the effect?
The ticking seemed to be quite close, in frequency, to the flat parts of the modulation.
I wish I had the SCC and Scope View option now. I could have sent you a piccy of it! The pulses d
I have some questions regarding the LM350T I am using for the voltage regulation. If anyone would be willing to help me with them I'd really appreciate it.
Now I'm stuck until the B9A sockets arrived really.
You've all been a great help so far!
My best wishes,
John
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