I am hoping to build a small SE valve amplifier as a project. It is my first time building an amp and I am looking for some help. I posted another thread on here relating to a similar project salvaging parts from an (sadly broken) old tape recorder. However, it turns out I have different valves than I originally though and so, I have started a fresh with a new thread. I hope this is ok. I have chosen the schematic bellow as a starting point. I have a few questions relating to the components. Non of the resistors on the diagram a wattage rating. I'm not sure how to work this out in order to order suitable resistors.
For the HT 250v I'm planning to use a boost converter and I have an LM317 Buck converter for the heater's 6.3v supply.
I understand that in order to use the DC HT supple from the boost converter with the schematic bellow, some more work be required before the 1k resistors to remove ripple.
I'm using this build which uses boost and buck converters in place of a proper mains transformer as reference
The video above it described a build which uses a boost converter for HT supply. However, he uses a different tube and so his schematic is different to the one bellow.
In his build, after the boost converter before R3 and R7 he uses 2 x 100uf 400v caps and a 390 3W resistor (for smoothing?). I am unsure whether I would need to change the values here to suit the rest of my ECL86 schematic?
I'd be really greatful for any help figuring out how to adapt this to make it work with the schematic I've posted.
Kind regards!
For the HT 250v I'm planning to use a boost converter and I have an LM317 Buck converter for the heater's 6.3v supply.
I understand that in order to use the DC HT supple from the boost converter with the schematic bellow, some more work be required before the 1k resistors to remove ripple.
I'm using this build which uses boost and buck converters in place of a proper mains transformer as reference
The video above it described a build which uses a boost converter for HT supply. However, he uses a different tube and so his schematic is different to the one bellow.
In his build, after the boost converter before R3 and R7 he uses 2 x 100uf 400v caps and a 390 3W resistor (for smoothing?). I am unsure whether I would need to change the values here to suit the rest of my ECL86 schematic?
I'd be really greatful for any help figuring out how to adapt this to make it work with the schematic I've posted.
Kind regards!
The video you referenced does use the PCL82 tube; you want to use the ECL86 tube. They are very similar. The power supply section is the same, you still need the same CRC filter cell after the HT converter module to attenuate the audio frequency ripple that is common on those modules. If you want to follow that video, the only difference on the power supply section will be the filament supply. PCL82 tubes needs 16V on the filament, so they are connected in parallel. ECL86 need 6,3V so you need to connect them in series. You will then change the resistor value on the LM317 reference pin to ouptut 12.6V instead of 16V, and you need to increase the size of the heatsink because heat dissipation would be higher.
Thanks, Pcan. With regards to the resistors in the ECL86 schematic I posted, the designer didn't stipulate the wattage rating. I don't know whether the wattage rating is even relevant for this purpose and if it was, my understanding of the theory isn't good enough to work it out. Will resistors of a certain wattage be required?
Thanks
I use 1W resistor on my point to point Hi-Fi builds, because they are easier to solder and I don't need to check the dissipation and the voltage rating. I like TE connectivity ROX1 Flame-Proof Power Metal Oxide Film Resistors, rated a 350V max working voltage (600V peak overload). But 0.25w are technically ok on most situations.
I'm just getting my head around negative feedback abd biasing etc. I thought I'd build it as per the schematic to start with. If I can get that working I'll feel like I've achieved something. Then I'll look at negative feedback abd maybe some tone controls etc.
A 7k output xfmr might be better than a 5k output xfmr.
True, but the amplifier certainly will not put out 4 Watts, as was proposed in Post # 1.
But not so fast . . .
If the plate swings all the way to the cathode (it can not) . . .
250V B+, and at least 5V drop in the primary
Cathode bias ~ 7V
250V - 5V -7V = 238V
((238V) squared/7000 Ohms)/2 = rms power for a sine wave (but the amp will be clipped at this signal level).
Well, for a sine wave, that would be just over 4 Watts, and the clipping would make the rms power more than 4 Watts, but it would be hard to listen to music that way.
And, I estimate that the plate might be able to swing to within 50V of the cathode, not to 0V from the cathode.
250V - 5V -7V - 50V = 188V
((188V squared)/7000/2) = 2.52 Watts
Take a typical inexpensive 7000 Ohm output transformer that has 350 Ohms primary DCR, and 8 Ohm tap that has 0.4 Ohms DCR.
That output transformer's insertion loss is 1dB (0.79 x the power that the output tube sends to the transformer primary, will come out the secondary).
2.52 Watts x 0.79 = 1.99 Watts.
2 Watt amplifier
True, but the amplifier certainly will not put out 4 Watts, as was proposed in Post # 1.
But not so fast . . .
If the plate swings all the way to the cathode (it can not) . . .
250V B+, and at least 5V drop in the primary
Cathode bias ~ 7V
250V - 5V -7V = 238V
((238V) squared/7000 Ohms)/2 = rms power for a sine wave (but the amp will be clipped at this signal level).
Well, for a sine wave, that would be just over 4 Watts, and the clipping would make the rms power more than 4 Watts, but it would be hard to listen to music that way.
And, I estimate that the plate might be able to swing to within 50V of the cathode, not to 0V from the cathode.
250V - 5V -7V - 50V = 188V
((188V squared)/7000/2) = 2.52 Watts
Take a typical inexpensive 7000 Ohm output transformer that has 350 Ohms primary DCR, and 8 Ohm tap that has 0.4 Ohms DCR.
That output transformer's insertion loss is 1dB (0.79 x the power that the output tube sends to the transformer primary, will come out the secondary).
2.52 Watts x 0.79 = 1.99 Watts.
2 Watt amplifier
You forgot that the plate can swing by the same amount upwards (that is above B+).
Cheers
Ian
ruffrecords,
Power supply +250V before any DC voltage drop in the output transformer DCR, and before the DC voltage drop of the self bias cathode network.
Even with no drop of B+ and self bias voltage; -250V to + 250V swing = 500V peak to peak
500V peak to peak / 2.828 = 176.8V rms.
((176.8V rms)squared)/7000 Ohms = 4.47 Watts, not a sine wave, but instead an extremely clipped waveform.
4.47Watts on a perfect day, but wait . . .
1. The tube plate voltage can not pull all the way down to the cathode voltage. Look at the zero volt grid line, and the plate to cathode voltage is not 0 volts. You will end up driving the grid very positive, have grid current, charge the coupling cap, etc.
2. The output transformer drops the 250V supply to less than 250V.
3. For a simple inexpensive amplifier, an inexpensive output transformer often has about 1 dB insertion loss; 4.47W x 0.794 = 3.55W
4. If, in spite of all the above, you could get a 4.47W sine wave, and used an output transformer that has a 0.5dB insertion loss,
4.47Watts x 0.891 = 3.98 Watts.
Purchase a Monolith Magnetics $$$ output transformer for the amplifier proposed by the original poster.
That just might push him about 10X over budget.
5. For a sine wave, 500V peak to peak is exactly the same as 250V peak. They are both 176.8Vrms.
No, I did not forget that the plate swings both up and down.
Power supply +250V before any DC voltage drop in the output transformer DCR, and before the DC voltage drop of the self bias cathode network.
Even with no drop of B+ and self bias voltage; -250V to + 250V swing = 500V peak to peak
500V peak to peak / 2.828 = 176.8V rms.
((176.8V rms)squared)/7000 Ohms = 4.47 Watts, not a sine wave, but instead an extremely clipped waveform.
4.47Watts on a perfect day, but wait . . .
1. The tube plate voltage can not pull all the way down to the cathode voltage. Look at the zero volt grid line, and the plate to cathode voltage is not 0 volts. You will end up driving the grid very positive, have grid current, charge the coupling cap, etc.
2. The output transformer drops the 250V supply to less than 250V.
3. For a simple inexpensive amplifier, an inexpensive output transformer often has about 1 dB insertion loss; 4.47W x 0.794 = 3.55W
4. If, in spite of all the above, you could get a 4.47W sine wave, and used an output transformer that has a 0.5dB insertion loss,
4.47Watts x 0.891 = 3.98 Watts.
Purchase a Monolith Magnetics $$$ output transformer for the amplifier proposed by the original poster.
That just might push him about 10X over budget.
5. For a sine wave, 500V peak to peak is exactly the same as 250V peak. They are both 176.8Vrms.
No, I did not forget that the plate swings both up and down.
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Do you have the schematic of the tape recorder? I have worked a bit on Tandbergs, and the secondary on those also provides a supply for the motor, so there is spare capacity with those transformers if used indepedently of the mecahnical parts. Heaters could be an issue, but a small heater transformer allows you to switch to TV tubes if you like, use PCL86 instead.
I would not dispute any of the above and i apologise for missing the fact you used peak instead of peak to peak which means of course you did not forget the plate swings above B+.
However, the originally posted design has a 5K load which gives a best case power output of 6.25W. I can believe that with a low cost output transformer and the practical swing of the output pentode that 4W might well be achievable. What the distortion would be, especially open loop as in this design, I would hesitate to say, but the typical application of an ECL86 back in the day was most likely aiming for loudness rather than quality.
Edit: However, I have fallen into my own trap of assuming the plate can swing as far as it likes when what more often than not limits output power is the available current swing. The ECL86 data sheet shows a nominal quiescent current of 36mA which is therefore the maximum peak current swing. Interestingly 7K at 250V implies a current of 35.7mA so clearly 7K is the best load to use. The (Phlips) ECL86 data sheet actually shows figures for a class A output stage operating from a 250V supply into 7K and gives the maximum output as 4W with 10% distortion. Bottom line, I agree with you.
Cheers
Ian
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ruffrecords,
I apologize, I am often very critical and impolite.
Thanks for your response.
An interesting detail about Post # 1 is that the Video shows a circuit that is using Schade plate to plate negative feedback.
Whereas, the original poster's own schematic on Post # 1 has no negative feedback.
Most of my amplifiers either use Ultra Linear as the only negative feedback, or use Triode Wired Pentode/Beam Power tubes as the only negative feedback. Or, I just use real triodes and no negative feedback.
Few threads call Ultra Linear a form of negative feedback, and even fewer threads call Triode Wired Pentode/Beam Power tubes call that a form of negative feedback.
Newbies, here is negative feedback that you may, or may not, have realized is negative feedback.
I do most of my listening to my vacuum tube amps at near field; either at the computer, or in the bedroom.
The point is, with near field listening, I most often listen to those amplifiers at 1 Watt or less. A few watts less than the 8, 6, 4, or 2 watts that those vacuum tube amplifiers can put out is not a problem for me.
The far field listening is for the TV and DVD/TV in the living room, using a solid state amplifier (Horrors!).
Everyone . . . keep Hobby-ing, keep listening, keep enjoying!
I apologize, I am often very critical and impolite.
Thanks for your response.
An interesting detail about Post # 1 is that the Video shows a circuit that is using Schade plate to plate negative feedback.
Whereas, the original poster's own schematic on Post # 1 has no negative feedback.
Most of my amplifiers either use Ultra Linear as the only negative feedback, or use Triode Wired Pentode/Beam Power tubes as the only negative feedback. Or, I just use real triodes and no negative feedback.
Few threads call Ultra Linear a form of negative feedback, and even fewer threads call Triode Wired Pentode/Beam Power tubes call that a form of negative feedback.
Newbies, here is negative feedback that you may, or may not, have realized is negative feedback.
I do most of my listening to my vacuum tube amps at near field; either at the computer, or in the bedroom.
The point is, with near field listening, I most often listen to those amplifiers at 1 Watt or less. A few watts less than the 8, 6, 4, or 2 watts that those vacuum tube amplifiers can put out is not a problem for me.
The far field listening is for the TV and DVD/TV in the living room, using a solid state amplifier (Horrors!).
Everyone . . . keep Hobby-ing, keep listening, keep enjoying!
No problem
I am intrigued by the funny names that have been given to various forms of NFB by tube afficiandos. Unfortunately the names give you no clue as the the effects of the NFB. RDH4 reminds that there are really only four different types of NFB and they all have apt descriptive names like series-derived/series-applied, series-derived/shunt-applied, shunt-derived/shunt-applied and shunt-derived/series-applied. You can usually tell by inspection which one is actually being used. In the case of Schade it is shunt-derived/shunt-applied.
I agree. NFB is probably the most misunderstood field of electronics
Agreed 100%
Cheers
Ian
If you draw a 5 kΩ loadline into the plate curves set, you'll find that it cuts the 9 W dissipation hyperbola. Remember that ECL86's/6GW8's have become very expensive these days, so you should avoid this.
Best regards!
I've decided to let the tape recorders live on as tape recorders as its not worth the hassle of trying to strip them. Having looked inside it turns out the valve sockets are PCB mount types. The output transformers are only 3ohm and the mains transformers have the cooling fan's attached to them. So effectively all I'd be able to salvage is the ECL86 valves and maybe some wire. I think its best to let someone have the recorders who might want to use them as tape recorders. Maybe they could make a working one out of the 2.....
Anyway....
I've ordered a pair of matching, new old stock PCL86's and some chassis mount McMurdo sockets. I already have all the resistors and cap's needed plus a brand new pair of 5k:8ohm OT's ready to play with.
I have a quick question regarding the LM317 adjustable Voltage regulator. Going back to the video I posted earlier (which I have taken inspiration from for the power supply side of my build), he incorporated a slow start circuit which he explains is to prevent "damage by turn on hot spots"on power up.
I'm not sure exactly what turn on hot spots are, why (if at all) they are more likely then when supplying the voltage from a voltage regulator as opposed to traditional MT, and if this circuit is strictly necessary? The LM317's I bought come as a modules and have the pot for adjusting the output built in as well as the screw terminal blocks. I would have to build the slow start circuit on another little board and mount it on the underside of the chassis next to the module if it is necessary to have it.
I'm also wondering about wire gauge. I've got a lot of stranded 1mm wire which i believe is around 17 AWG. I hoping this is sufficient for the HT. I've got enough of this to wire everything but I've got some thinner stuff 15-15ish AWG I can also use.
Looking forward to getting started on this. As soon as all the bits I've ordered arrive I will make a template from card and offer everything up to work out positioning.
Anyway....
I've ordered a pair of matching, new old stock PCL86's and some chassis mount McMurdo sockets. I already have all the resistors and cap's needed plus a brand new pair of 5k:8ohm OT's ready to play with.
I have a quick question regarding the LM317 adjustable Voltage regulator. Going back to the video I posted earlier (which I have taken inspiration from for the power supply side of my build), he incorporated a slow start circuit which he explains is to prevent "damage by turn on hot spots"on power up.
I'm not sure exactly what turn on hot spots are, why (if at all) they are more likely then when supplying the voltage from a voltage regulator as opposed to traditional MT, and if this circuit is strictly necessary? The LM317's I bought come as a modules and have the pot for adjusting the output built in as well as the screw terminal blocks. I would have to build the slow start circuit on another little board and mount it on the underside of the chassis next to the module if it is necessary to have it.
I'm also wondering about wire gauge. I've got a lot of stranded 1mm wire which i believe is around 17 AWG. I hoping this is sufficient for the HT. I've got enough of this to wire everything but I've got some thinner stuff 15-15ish AWG I can also use.
Looking forward to getting started on this. As soon as all the bits I've ordered arrive I will make a template from card and offer everything up to work out positioning.