Valve Preamp
Joel,
I did say that the gain of a cascade double triode was theoretical, and in reality would never be achieved. The amplification factor of a valve is dependent on the equation Ra(Ra + ra) where ra is the internal slope resistance of the valve and Ra the external load resistance. Thus in the circuit described the full gain of the valve (ECC83 = 100 per section) could never be possibly achieved, especially because of the impedence conversion, which limits the values of the anode load and the cathode resistance, which is adjusted for optimum bias condition for the direct coupling. However, cascade circuits were widely used because of the high amplication factors available - this is why they used to be favoured in the front ends of TVs and VHF receivers instead of a single pentode pre-amplifier. In the circuit described the direct coupling would also help reduce low frequency phase shift.
For those colleagues in the UK who are interested in building one of the pre-amps discussed but want a cheaper source of parts, then it might be useful to contact a UK vintage radio supplier. Past Times Radio (http://www.pasttimesradio.co.uk/index.html) sell and strip down old radios, and sell mains transformers from valve sets for as little as £6.00. These transformers will usually be able to supply 250-300 volts at 50-60ma and 6.3 volts as well, more than enough for a pre-amp. Check their other components also - they sell new stuff at competitive rates.
Joel,
I did say that the gain of a cascade double triode was theoretical, and in reality would never be achieved. The amplification factor of a valve is dependent on the equation Ra(Ra + ra) where ra is the internal slope resistance of the valve and Ra the external load resistance. Thus in the circuit described the full gain of the valve (ECC83 = 100 per section) could never be possibly achieved, especially because of the impedence conversion, which limits the values of the anode load and the cathode resistance, which is adjusted for optimum bias condition for the direct coupling. However, cascade circuits were widely used because of the high amplication factors available - this is why they used to be favoured in the front ends of TVs and VHF receivers instead of a single pentode pre-amplifier. In the circuit described the direct coupling would also help reduce low frequency phase shift.
For those colleagues in the UK who are interested in building one of the pre-amps discussed but want a cheaper source of parts, then it might be useful to contact a UK vintage radio supplier. Past Times Radio (http://www.pasttimesradio.co.uk/index.html) sell and strip down old radios, and sell mains transformers from valve sets for as little as £6.00. These transformers will usually be able to supply 250-300 volts at 50-60ma and 6.3 volts as well, more than enough for a pre-amp. Check their other components also - they sell new stuff at competitive rates.
Valve Pre-amp
Nuuk,
As regards your 5687 pre-amp, offhand I would say that it would have good power handling capability, helped by both sections of the valve being joined together, lowish output impedence (the low anode resistance helping there) and probably not a great deal of gain. It is a simple circuit, so once you've got a power supply set up why not give it a try? However, I still think that using one section of the valve as an amplifier and the other as a cathode follower is your best bet.
Nuuk,
As regards your 5687 pre-amp, offhand I would say that it would have good power handling capability, helped by both sections of the valve being joined together, lowish output impedence (the low anode resistance helping there) and probably not a great deal of gain. It is a simple circuit, so once you've got a power supply set up why not give it a try? However, I still think that using one section of the valve as an amplifier and the other as a cathode follower is your best bet.
Re: Valve Preamp
B-ville,
You are confusing two distinct things - a "cascaded" gain stage, and a "cascode" amplifier. Two common cathode stages strung end to end are said to be "cascaded", and in this case the "theoretical" gain would be 100 times 100. But the SRPP circuit you were referring to is NOT of that nature.
Look at the top triode alone. Look familiar? (if you said "it looks like a cathode follower", you win the prize)
bournville said:
B-ville,
You are confusing two distinct things - a "cascaded" gain stage, and a "cascode" amplifier. Two common cathode stages strung end to end are said to be "cascaded", and in this case the "theoretical" gain would be 100 times 100. But the SRPP circuit you were referring to is NOT of that nature.
Look at the top triode alone. Look familiar? (if you said "it looks like a cathode follower", you win the prize)
Valve Pre-amp
Joel,
I think the difference may be simply a matter of semantics, of different termonology used in the USA and UK, as the SRPP circuit would be called a cascode amplifier here simply because it is cascaded, anode connected directly to cathode,regardless of where the output is taken, from the anode of the second triode or the cathode (I've checked this in a couple of old Mullard audio books I have). I do agree with your explanation, the top triode is performing the task of a cathode follower which means that it will not contribute to stage gain, all the gain coming from the bottom triode (which in the case of an ECC83 would mean the maximum gain of the two stages would be 100). It is what I meant to say in my last posting, but you word it better than I did!
These differences in terminology do occur from time to time. I have a collection of Radio and Electronics Constructor magazines from the 1950s which often have different terms to the equivalent UK ones. At times US techie books released in the UK carry a supplement highlighting the difference in terminology!
Joel,
I think the difference may be simply a matter of semantics, of different termonology used in the USA and UK, as the SRPP circuit would be called a cascode amplifier here simply because it is cascaded, anode connected directly to cathode,regardless of where the output is taken, from the anode of the second triode or the cathode (I've checked this in a couple of old Mullard audio books I have). I do agree with your explanation, the top triode is performing the task of a cathode follower which means that it will not contribute to stage gain, all the gain coming from the bottom triode (which in the case of an ECC83 would mean the maximum gain of the two stages would be 100). It is what I meant to say in my last posting, but you word it better than I did!
These differences in terminology do occur from time to time. I have a collection of Radio and Electronics Constructor magazines from the 1950s which often have different terms to the equivalent UK ones. At times US techie books released in the UK carry a supplement highlighting the difference in terminology!
This is not a difference in terminology, actually. The SRPP circuit is a push-pull voltage amplifier - think of it as a single (weird) tube. "Cascaded" gain stages are two individual single-ended stages, one feeding the input of the next. Whether those are direct coupled or not makes no difference. I've never seen the term used any other way.
Also, in direct coupled common cathode amps - the plate is tied to the grid, not the cathode of the following tube.
All clear?😉
Also, in direct coupled common cathode amps - the plate is tied to the grid, not the cathode of the following tube.
All clear?😉
Brett said:
Right.
Yeah, that is a good set of articles. He's got down to earth advice on the site too. No voodoo.
Joel said:
Then what is the point? A CD player already has a low output impedence, so adding a CF circuit onto the end of it will just add noise, and lower your signal voltage by .9 or so.
Into what load? This is all dependent on the load. My 3db down point will not be 4Hz into a 100 ohm load.
Joel
Hi Joel
To answer your points:
1) the CD player almost certainly has a low impedance *but* not after you put the signal through an attenuator. Worst case output impedance from an attenuator = 1/4 of the attenuator value, this will be when the wiper is half way.
So if he's using a 50k attenuator, his SS amps with an input impedance of 20k will see a 12.5k source. Not good.
2) Nick said his load was 20k - hence -3db point f (Hz) given by 2*pi*f*R*C=1 (as I'm sure you know)
Cheers
Chris
General issue of valve preamp, ss power amp
Just wondering - anyone know if switch on surges from a valve preamp tend to damage SS amps downstream?
With a simple SE design, I'd imagine you'd get a rather large output voltage pulse at switch on...
Just wondering - anyone know if switch on surges from a valve preamp tend to damage SS amps downstream?
With a simple SE design, I'd imagine you'd get a rather large output voltage pulse at switch on...
SURGES.
Hi,
No...provided you use a bleeder resistor after your coupling cap at the output no voltage can develop at the input of the amp.
Gabe, are you listening?🙄
Cheers,😉
Hi,
No...provided you use a bleeder resistor after your coupling cap at the output no voltage can develop at the input of the amp.
Gabe, are you listening?🙄
Cheers,😉
Re: Valve Preamp
That would be a good sounding pre-amp; much better than ECC82 etc.
One problem, The upper ^SN7 should have its heater sitting on say 130V DC as the heater/cathode insulation will be stressed otherwise and could cause LF noise.
This would be easy in a stereo pre-amp since each valve could handle the two channels.
For the beginners, a heater supply must always be referenced; usually this is to 0V i.e. the -ve terminal of the HT supply, either to a centre tap on the heater supply or via a pair of say 100 ohm resistors one connected to each side. For the upper 6SN7, instead of connecting the centre tap or mid-point of the two resistors to 0V you obtain in this case say 130V using a potential divider; this could be a couple of say 68k resistors connected between the HT rail and 0V. You will need a capacitor across the lower resistor - 10uF at 200V would do nicely. The mid-point between the two 68k resistors will be 125V assuming your HT to be 250V; this you connect to your heater centre tap; no currtent will be drawn - it's just a reference to keep your precious valve happy!
7N7
Paulr said:
That would be a good sounding pre-amp; much better than ECC82 etc.
One problem, The upper ^SN7 should have its heater sitting on say 130V DC as the heater/cathode insulation will be stressed otherwise and could cause LF noise.
This would be easy in a stereo pre-amp since each valve could handle the two channels.
For the beginners, a heater supply must always be referenced; usually this is to 0V i.e. the -ve terminal of the HT supply, either to a centre tap on the heater supply or via a pair of say 100 ohm resistors one connected to each side. For the upper 6SN7, instead of connecting the centre tap or mid-point of the two resistors to 0V you obtain in this case say 130V using a potential divider; this could be a couple of say 68k resistors connected between the HT rail and 0V. You will need a capacitor across the lower resistor - 10uF at 200V would do nicely. The mid-point between the two 68k resistors will be 125V assuming your HT to be 250V; this you connect to your heater centre tap; no currtent will be drawn - it's just a reference to keep your precious valve happy!
7N7
HEATERS.
Hi,
While it usually is, it is not an absolute necessity.
For the SRPP: I'll post a pic to show how to polarise the heaters with respect to the cathode.
Cheers,😉
Hi,
While it usually is, it is not an absolute necessity.
For the SRPP: I'll post a pic to show how to polarise the heaters with respect to the cathode.
Cheers,😉
HEATER TO CATHODE INSULATION.
Hi,
LF?
IME, this manifest itself with a HF noise...a screeching oscillated sound that when left to the element will take out your tweeters within seconds and ultimatily will short out cathode and heater. (they will stick together...one hell of a way to make your own DHTs.
)
Cheers,😉
Hi,
LF?
IME, this manifest itself with a HF noise...a screeching oscillated sound that when left to the element will take out your tweeters within seconds and ultimatily will short out cathode and heater. (they will stick together...one hell of a way to make your own DHTs.
)Cheers,😉
Attachments
Re: HEATER TO CATHODE INSULATION.
Frank,
I meant before the "diode" becomes a short circuit!
The AVO VCM 163 tester that I have provides a insulation test; certain types seem more prone than others to leakage; I have found 6CK4s and to a lesser extent, 6DN7s naughty in this respect. I also I had a pair of Tung-Sol 6BX7s that were very bad.
I have cured them all by the usual brutal means of applying about 10v for a minute or two; that sorts them out. I once reluctantly had to do this to a Philips E88CC, gold pins and all. Even that one survived the "take that you pig" treatment!
I remember an early pre-amplifier that I built in a hurry where I forgot to reference the heater supply to 0V; there was a really horrid noise; rather like a faulty (and cheap) guitar lead!
7N7
fdegrove said:Hi,
LF?
IME, this manifest itself with a HF noise...a screeching oscillated sound that when left to the element will take out your tweeters within seconds and ultimatily will short out cathode and heater. (they will stick together...one hell of a way to make your own DHTs.
Cheers,😉
Frank,
I meant before the "diode" becomes a short circuit!
The AVO VCM 163 tester that I have provides a insulation test; certain types seem more prone than others to leakage; I have found 6CK4s and to a lesser extent, 6DN7s naughty in this respect. I also I had a pair of Tung-Sol 6BX7s that were very bad.
I have cured them all by the usual brutal means of applying about 10v for a minute or two; that sorts them out. I once reluctantly had to do this to a Philips E88CC, gold pins and all. Even that one survived the "take that you pig" treatment!
I remember an early pre-amplifier that I built in a hurry where I forgot to reference the heater supply to 0V; there was a really horrid noise; rather like a faulty (and cheap) guitar lead!
7N7
SCROOOCH....
Hi,
I must be the lucky guy...it only happened to me once with a dodgy ECC82 (of course)...net result: a set of ribbon tweeties blown.
That was in an SRPP setup BTW.
O.K. here comes the trick:
I never polarize tube heaters and often surpass the rated heater to cathode voltage ratings (usually rated 100 to 150V) by feeding the heaters with a well filtered and regulated DC supply and leave it floating with respect to ground.
These valves never gave trouble and still measure the same after ten years of professional abuse.
By professional I mean day in day out 10 to 12 hours of average service, the preamp was rarely ever switched off .
Naturally you don't have to "push" things over the top either and I don't mean to say that polarising them is a bad thing at all.
BTW,I always recommend to polarise the heaters on DHTs as shown in the above posted pic trick, it often kills that last bit of residual hum.
On the heaters?
While that may save the tube it will also kill the transconductance by the same process.
No free lunch I am afraid,😉
Hi,
I must be the lucky guy...it only happened to me once with a dodgy ECC82 (of course)...net result: a set of ribbon tweeties blown.
That was in an SRPP setup BTW.
O.K. here comes the trick:
I never polarize tube heaters and often surpass the rated heater to cathode voltage ratings (usually rated 100 to 150V) by feeding the heaters with a well filtered and regulated DC supply and leave it floating with respect to ground.
These valves never gave trouble and still measure the same after ten years of professional abuse.
By professional I mean day in day out 10 to 12 hours of average service, the preamp was rarely ever switched off .
Naturally you don't have to "push" things over the top either and I don't mean to say that polarising them is a bad thing at all.
BTW,I always recommend to polarise the heaters on DHTs as shown in the above posted pic trick, it often kills that last bit of residual hum.
On the heaters?
While that may save the tube it will also kill the transconductance by the same process.
No free lunch I am afraid,😉
I'll admit i cannot prove you otherwise...
But does this mean that Alan Kimmel is a liar?
Or do you and I interpret him differently?
Cheers,
Bas
Re: SCROOOCH....
I think you mean it can reduce gm.
Of course I limit the high voltage to shortish bursts except in desperate cases. My VCM163 tests Ia and gm simultaneously; a simple check has always shown gm to be unchanged after the treatment
I suppose I should say here that one should not attempt this unles one has a decent valve tester that can measure leakage, vary heater voltage in small steps and measure mutual conductance..
Of course I accept that there is a risk of damamging the cathode's emissive surface, but it's worth remembering that valves subjected to this treatment are being rescued from a journey into the bin!
7N7
fdegrove said:
I think you mean it can reduce gm.
Of course I limit the high voltage to shortish bursts except in desperate cases. My VCM163 tests Ia and gm simultaneously; a simple check has always shown gm to be unchanged after the treatment
I suppose I should say here that one should not attempt this unles one has a decent valve tester that can measure leakage, vary heater voltage in small steps and measure mutual conductance..
Of course I accept that there is a risk of damamging the cathode's emissive surface, but it's worth remembering that valves subjected to this treatment are being rescued from a journey into the bin!
7N7
Thanks for all the input chaps. On behalf of Ian, Paul and myself may I remind you that we are complete beginners with valves and it's all getting a bit too technical.
Appreciate it is all relevent to building a good preamp but it can be a bit daunting at the outset. 😕
Appreciate it is all relevent to building a good preamp but it can be a bit daunting at the outset. 😕
No gain without pain
Nuuk,
I appreciate that you are just starting, but equally I have to say that the referencing of heater supplies is not very "technical" relatively speaking.
If it's any consolation, I have books here at home, large sections of which I do not understand and will never understand!
I suggest that you buy a couple of suitable books to start you off in acquiring a basic understanding. This is especially important with valve amplification since high voltages are involved and care must be taken.
I am sure that there have been threads on this and a quick search will provide you with all you need to know.
Best of luck
7N7
Nuuk said:
Nuuk,
I appreciate that you are just starting, but equally I have to say that the referencing of heater supplies is not very "technical" relatively speaking.
If it's any consolation, I have books here at home, large sections of which I do not understand and will never understand!
I suggest that you buy a couple of suitable books to start you off in acquiring a basic understanding. This is especially important with valve amplification since high voltages are involved and care must be taken.
I am sure that there have been threads on this and a quick search will provide you with all you need to know.
Best of luck
7N7
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