Below 4.7 or 3.9 Volts zeners are actually stacks of series connected diodes. The Zener effect (reverse avalanche breakdown) doesn't work well at low Voltages.
Because they are stacks of diodes the zener current has a significant effect on the resulting zener Voltage. Check the zener current, your's probably aren't bad.
The dual tube connected for 6.3V is actually two seperate filaments connected in parallel, 1/2 the DC resistance over a single 6.3V tube is kind of to be expected. Filaments need a lot of current.
Because they are stacks of diodes the zener current has a significant effect on the resulting zener Voltage. Check the zener current, your's probably aren't bad.
The dual tube connected for 6.3V is actually two seperate filaments connected in parallel, 1/2 the DC resistance over a single 6.3V tube is kind of to be expected. Filaments need a lot of current.
Hi arold19,
I've been away for a bit, so I'm coming in late on this.
Your heater supply will draw twice the current at 6.3 V as it would at 12.6 V. Have you considered the increased heat from this extra current and voltage drop?
My question would be, why not drop from 14 VDC to 12.6 VDC using resistors? Split the resistance and filter capacitors and you will have lower peak currents and no burning hot regulators. The other option would be to use a switching regulator. This might be the better option as it also reduces your current draw from the transformer. That's a very good thing!
-Chris
I've been away for a bit, so I'm coming in late on this.
Your heater supply will draw twice the current at 6.3 V as it would at 12.6 V. Have you considered the increased heat from this extra current and voltage drop?
My question would be, why not drop from 14 VDC to 12.6 VDC using resistors? Split the resistance and filter capacitors and you will have lower peak currents and no burning hot regulators. The other option would be to use a switching regulator. This might be the better option as it also reduces your current draw from the transformer. That's a very good thing!
-Chris
Thank for the reply!
Anatech: the 2N3055 is a big transistor but you think it can burn to regulate the voltage from 15v to 6.3??? I use an heat sink to cool down the transistor...
My preamp is now finish, it work but the gain is a lot to high!!
I use a DACT 24 position attenuator, at the second position, the sound is to low, and at the third, it is to high....
My cd player is a Arcam Alpha7se, my power amp is a DIY Aleph30... and the speakers, DIY BLH FE103, I have also an 2 way 6.5inch speakers....
It is possible to change the value of some resistor??
It is real that I can put an 12AT7 tube insteed the 12AU7, the 12AT7 have less gain??
I am very happy with the ¨noise¨ there no humm, just a little noise when I am at 1 feet of the speakers...
I will post some picture of the preamp, and also make a report for the sound, now, the sound seem a little bit more hard that I expected...
Anatech: the 2N3055 is a big transistor but you think it can burn to regulate the voltage from 15v to 6.3??? I use an heat sink to cool down the transistor...
My preamp is now finish, it work but the gain is a lot to high!!
I use a DACT 24 position attenuator, at the second position, the sound is to low, and at the third, it is to high....
My cd player is a Arcam Alpha7se, my power amp is a DIY Aleph30... and the speakers, DIY BLH FE103, I have also an 2 way 6.5inch speakers....
It is possible to change the value of some resistor??
It is real that I can put an 12AT7 tube insteed the 12AU7, the 12AT7 have less gain??
I am very happy with the ¨noise¨ there no humm, just a little noise when I am at 1 feet of the speakers...
I will post some picture of the preamp, and also make a report for the sound, now, the sound seem a little bit more hard that I expected...
Hi arold19,
A 2N3055 is also not a high gain device, so the base current may be higher than you have allowed for. This may explain your low heater voltage.
I don't know all the details of your construction, only you do. Meanwhile, I'm trying to help - as you requested. If you make a poor engineering choice, it's up to other people to point that out to you. It's entirely your decision whether you listen to that advice or not.
-Chris
Why yes. Why do you ask? It depends on the size of your heat sink. You are also wasting more power than you need to. It is possible, depending on your transformer, that you may exceed it's heater winding rating.
A 2N3055 is also not a high gain device, so the base current may be higher than you have allowed for. This may explain your low heater voltage.
I don't know all the details of your construction, only you do. Meanwhile, I'm trying to help - as you requested. If you make a poor engineering choice, it's up to other people to point that out to you. It's entirely your decision whether you listen to that advice or not.
Yes.
Sort of. You can substitute the tube and you will have less gain (depending on feedback). The circuit really should be designed for the tube used. Different tube = different component values.
Cool! It's always great to see someone's project.
-Chris
Many attenuators have large steps at the extremes and smaller steps over the "normal operating" range. Changing the gain of the pre so that a more middle position is "normal" should be easy.arold19 said:
The PV-12 uses large film capacitors, in my experience these take some time to "break in". Also not all brands of tubes sound equally "smooth". Power supply details and grounding can all affect how "hard" a design sounds.
Anatech is right, many circuit topologies set the gain by design specifically to eliminate tube to tube variations.
Can you list the attenuator step sizes? Also an as built schematic would help us to help you.
My transformer is a Hammond 272BX 95VA 300-0-300 at 100mA, one filament 5V at 2A and one filament 6.3V at 3A.
Maybe it is bad but I hook up the two filament winding in serie tohave 11.3V ac, it give about 15V DC after the bridge... after, I regulate with the 2N3055 to 6.3 V...
Maybe it is a better idea to run the heater at 12.6V, but I'm realy not sure I have enough voltage..
So, how can I reduce de gain of this preamplifier???
My attenuator is a DACT CT2-100K-2, here is the specification:http://www.dact.com/html/attenuator_data_sheet.html
Maybe it is bad but I hook up the two filament winding in serie tohave 11.3V ac, it give about 15V DC after the bridge... after, I regulate with the 2N3055 to 6.3 V...
Maybe it is a better idea to run the heater at 12.6V, but I'm realy not sure I have enough voltage..
So, how can I reduce de gain of this preamplifier???
My attenuator is a DACT CT2-100K-2, here is the specification:http://www.dact.com/html/attenuator_data_sheet.html
It is a good idea to put a 25K or 50K resistor in serie with the 100 k pot??? It will change the impedance see buy the CD player the it will reduce the gain????
arold19 said:
I'm coming in late to this and there are 6 pages yet unread, but in case someone hasn't pointed it already, the line stage in this is -- except maybe exact part values and the quality of the parts -- the same as the Bottlehead Foreplay. Decent enuff sounding but can certainly be easily bettered. Has gobs of volyage output -- usually too much.
dave
Hi Arnold;
The first attenuator step is 10 dB that is quite a large step the next 4 steps are 4 dB each, still good sized steps. After that each step is 2 dB not good for fine adjustment, but probably usable.
To get the "normal" listening level we need to skip at least 5 steps (4 * 4dB + 10 dB) = 26 dB. Since the design seems to have about 85dB before the attenuator we could loose gain in both input stages.
I would start be changing the first stage plate resistor to 56K and both cathode Rs to 2K, on the second stage change the plate R to 75K and the cathode to 3K. This changes the total circuit gain for the first two stages to about 60dB. If additional loss is needed change the gain of the output buffer in much the same way (i.e. lower the plate to cathode resistor ratio). I spread out the changes over more than one stage in hopes of maintaining the PV-12 character.
My PV 12 did not have excess gain, is that schematic correct?
The first attenuator step is 10 dB that is quite a large step the next 4 steps are 4 dB each, still good sized steps. After that each step is 2 dB not good for fine adjustment, but probably usable.
To get the "normal" listening level we need to skip at least 5 steps (4 * 4dB + 10 dB) = 26 dB. Since the design seems to have about 85dB before the attenuator we could loose gain in both input stages.
I would start be changing the first stage plate resistor to 56K and both cathode Rs to 2K, on the second stage change the plate R to 75K and the cathode to 3K. This changes the total circuit gain for the first two stages to about 60dB. If additional loss is needed change the gain of the output buffer in much the same way (i.e. lower the plate to cathode resistor ratio). I spread out the changes over more than one stage in hopes of maintaining the PV-12 character.
My PV 12 did not have excess gain, is that schematic correct?
Hi Arold,
Even with tubes, preamplifier needs many hours of heating (24H at least) to obtain best sound.
What 12AT7 are you using? Cheap tubes could badly sounds.
Eric
Thanks for the picture, it's very nice of you.arold19 said:
What kind of capacitors do you use? Conrad Johnson use many large values of polystyrene capacitors (150nf, 2µF...). I don't think that you can find theses components on the market.
Even with tubes, preamplifier needs many hours of heating (24H at least) to obtain best sound.
What 12AT7 are you using? Cheap tubes could badly sounds.
Eric
I use Solen Metallized capacitor for the power supply, I didn't buy polystyrene capacitor because there are too much expensive... the 2uF output capacitor are Solen Film and Foil capacitor. My tubes are 12AU7 from Electro Harmonix...
Maybe a Mullard from U.K. will be better!!!
Maybe a Mullard from U.K. will be better!!!
Hi Arold,
Eric
Good choice. You mean Polypropylene Metalized capacitor. I didn't think (big) polystyrene capacitors still exist.
Electro Harmonix are not very sweat, but it stills good choice. Mullard probably be more warm and soft.
Eric
I buy my capacitors mostly from Michael Percy http://www.percyaudio.com/Catalog.pdf
He has a 2.0uF film and tin foil Polystyrene for $30.95 also almost as good sounding is a tin foil and polypropylene for $20.95. See page 5 of his pdf catalog.
Michael is helpful and knowledgeable, quick to ship and accepts PayPal. I have used him for years, a relationship worth cultivating.
Also Parts Connexion has an extensive selection of capacitors. http://www.partsconnexion.com/
For less money (and not quite as clean a sound) North Creek offers very large value metalized film capacitors http://www.northcreekmusic.com/Zen.htm
We found that 2 capacitors of 1/2 the value each connected anti-parallel (i.e. one outer film connected to the inner at one end and visa versa at the other end) sounded magically better. See also their Harmony capacitors
He has a 2.0uF film and tin foil Polystyrene for $30.95 also almost as good sounding is a tin foil and polypropylene for $20.95. See page 5 of his pdf catalog.
Michael is helpful and knowledgeable, quick to ship and accepts PayPal. I have used him for years, a relationship worth cultivating.
Also Parts Connexion has an extensive selection of capacitors. http://www.partsconnexion.com/
For less money (and not quite as clean a sound) North Creek offers very large value metalized film capacitors http://www.northcreekmusic.com/Zen.htm
We found that 2 capacitors of 1/2 the value each connected anti-parallel (i.e. one outer film connected to the inner at one end and visa versa at the other end) sounded magically better. See also their Harmony capacitors
It is a good or bad idea to put a 25K or 50K resistor in serie with the 100 k pot??? It will change the impedance see buy the CD player the it will reduce the gain????
Most modern CD players are solid state and they will drive a 2K Ohms or more just fine, but 25K in series with the 100K pot will only reduce the gain by about 2 dB or not even equal to that first step.
Hi Herman,
Many thanks for theses links.
Personally I use SCR (French) capacitors.scr I buy it directly to the manufacturer. Conrad use that components in their preamplifier's power supply.
A link to my last prototype (model 4 preamplifier).
Eric
Many thanks for theses links.
Personally I use SCR (French) capacitors.scr I buy it directly to the manufacturer. Conrad use that components in their preamplifier's power supply.
A link to my last prototype (model 4 preamplifier).
Eric
Hello Hermanv.
It is my first tube project and there a couple of thinks I don’t understand..
I don’t think you have the same schematic than mine for the PV-12...
Do be sure, I have to change the 82.5K on the plate of the first stage for a 56K.
I have to change the 3.32K on the Cathode of the first stage for a 2K.
For the second stage, you want to change the value of the plate resistor to 75K, but I cannot find any plate resistor on the second stage in the schematic...???
After, I have to change the 40.3K Cathode resistor of the second stage for a 3K, That a lot difference, it is 13 time smaller value.
Help me to understand !
It is my first tube project and there a couple of thinks I don’t understand..
I don’t think you have the same schematic than mine for the PV-12...
Do be sure, I have to change the 82.5K on the plate of the first stage for a 56K.
I have to change the 3.32K on the Cathode of the first stage for a 2K.
For the second stage, you want to change the value of the plate resistor to 75K, but I cannot find any plate resistor on the second stage in the schematic...???
After, I have to change the 40.3K Cathode resistor of the second stage for a 3K, That a lot difference, it is 13 time smaller value.
Help me to understand !
arold19 said:
The 12AT7 has more gain than the 12AU7. Mu (12AU7) = 17, mu (12AT7) = 60. Anatech in the subsequent post (#64) must have had a very rare slip of memory. But I did not look at the circuit now; quite corrrect that one cannot just "roll" tubes without adjusting circuit values for optimal performance.
Hi Johan,
Oops. You are right. I was thinking he was comparing a 12AT7 to a 12AX7 (as is normally the case). The mind is both a wonderful and scary thing!
Hi arold19,
Just to make a comment here. I have found that Electroharmonix sound fine, not "brittle" at all. Understand that this depends on the circuit they are installed into. Also, new Mullard is made by the New Sensor group. I like their all tubes a lot. Good quality and they tend to be the same. Very good manufacturing.
When tubes sound less "bright", I suspect that their plate resistance may be higher than normal (rolling off the highs). Still, your circuit has the most to do with sound quality as long as you are using feedback. Otherwise, the variations in gain and sound quality from tube to tube will be a nightmare.
-Chris
Oops. You are right. I was thinking he was comparing a 12AT7 to a 12AX7 (as is normally the case). The mind is both a wonderful and scary thing!
Hi arold19,
Just to make a comment here. I have found that Electroharmonix sound fine, not "brittle" at all. Understand that this depends on the circuit they are installed into. Also, new Mullard is made by the New Sensor group. I like their all tubes a lot. Good quality and they tend to be the same. Very good manufacturing.
When tubes sound less "bright", I suspect that their plate resistance may be higher than normal (rolling off the highs). Still, your circuit has the most to do with sound quality as long as you are using feedback. Otherwise, the variations in gain and sound quality from tube to tube will be a nightmare.
-Chris
I used the schematic on the first post of this thread. I do not have a C-J schematic. Perhaps I misunderstood because you said "here is the schematic". If you have another schematic please tell me where it is posted or re-post it for me.arold19 said:
In a simple circuit, without feedback, the gain of a tube stage is Plate R divided by Cathode R (this is only the part of a cathode R that is not bypassed with a capacitor). Once you have the gain of a stage, you multiply it by the next stage and so on to get total gain. Next for me, it is easier to understand if I then convert to dB, which is 20 * (log)gain (log is base 10).
So an increase in plate resistor, will increase gain and reduce current through the tube. A increase in cathode resistance will decrease the gain (and current) by changing both you can change gain while keeping the current about the same. The tube operating point (current) will have some effect on the sound quality so it's usually smart to keep it the same as the original designer. Reducing gain by adding resistors in series with a volume control will work, but might cause increased noise.
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