Hi,
My name is Richard and my main interest is valve (tube) stuff, primarily old radio but I'm prepared to 'dabble' in a range of things. I was brought up on valves but about 20 years ago my interest declined somewhat and all my stuff just lay about in the attic getting dusty. However, I recently completed a PhD and found that I had evenings and weekends free again, so I started to sort out and play with the sets, valves, amps etc.. I still had lying about. So far this month I've managed to trash two Mullard audio modules and a tuner-amp, but managed to repair Pioneer and Technics tuner-amps, so I'm getting a 40% success rate! In the words of the song, "Things can only get better."
My next project is to wire up a 120 watt audio module and sort out a valve pre-amplifier, and I thinking of using part of the spilt supplies (55 volts) as the HT, so I shall do a bit of experimenting. Checking through the forums there seems to be a bit of hostility towards the poor old ECC82/12AU7 and ECC83/12AX7 valves so I shan't suggest them! Hmm, will a 6Q7 provide enough drive I wonder? What about a battery valve, say a DAF96, which would run on the HT okay, connected to the module through a Ferranti intervalve transformer, connected the other way round to reduce the impedence? Or just forget it, and make an amp with push-pull 6V6s, which all my old mags insist is the peak of high fidelity?
So many (somewhat dubious) projects to try out, so little time! Seriously, if you have fun trying things out and learn at the same time, then why not?
My name is Richard and my main interest is valve (tube) stuff, primarily old radio but I'm prepared to 'dabble' in a range of things. I was brought up on valves but about 20 years ago my interest declined somewhat and all my stuff just lay about in the attic getting dusty. However, I recently completed a PhD and found that I had evenings and weekends free again, so I started to sort out and play with the sets, valves, amps etc.. I still had lying about. So far this month I've managed to trash two Mullard audio modules and a tuner-amp, but managed to repair Pioneer and Technics tuner-amps, so I'm getting a 40% success rate! In the words of the song, "Things can only get better."
My next project is to wire up a 120 watt audio module and sort out a valve pre-amplifier, and I thinking of using part of the spilt supplies (55 volts) as the HT, so I shall do a bit of experimenting. Checking through the forums there seems to be a bit of hostility towards the poor old ECC82/12AU7 and ECC83/12AX7 valves so I shan't suggest them! Hmm, will a 6Q7 provide enough drive I wonder? What about a battery valve, say a DAF96, which would run on the HT okay, connected to the module through a Ferranti intervalve transformer, connected the other way round to reduce the impedence? Or just forget it, and make an amp with push-pull 6V6s, which all my old mags insist is the peak of high fidelity?
So many (somewhat dubious) projects to try out, so little time! Seriously, if you have fun trying things out and learn at the same time, then why not?
You sound like my kinda guy 😱 😉 (Especially since you have spare toob stuff to mooch off of! 
)
Now seriously...
I have no problem with: 12AU7, 12AT7, 12AX7, 6V6, 6(anything!)6, old capacitors (long as they test well and don't explode! 😉 ), and so on. 🙂
Tim
)Now seriously...
I have no problem with: 12AU7, 12AT7, 12AX7, 6V6, 6(anything!)6, old capacitors (long as they test well and don't explode! 😉 ), and so on. 🙂
Tim
Tim,
I must confess that I'm a bit wary of old caps, especially electrolytics - I still have some unused ones dating from 1947-8, but I chicken out of using (reforming) them and buy new ones instead! I did manage to blow the odd one or two up years ago, and the results were pretty spectacular!
Seriously though, as regards the ECCxx tubes I suspect that in the 1950s and 60s they were possibly more widely used in European amps than in the US - I've got no objective evidence of this apart from circuits in old British and American radio/audio mags.
Actually, I'm looking to try out a circuit that appeared in Radio Constructor in Feb 1953 - exactly 50 years ago - of an unusual push-pull amp in which there is no separate phase splitter but the first tube's cathode is joined to the second's cathode and bias is provided by a common un-bypassed resistor. The second tube's signal grid is connected to ground - the first's signal grid is connected to the preceding stage in the normal manner. I have not seen this circuit used anywhere else so I wonder if there are any problems with it - it does seem very simple and no doubt someone out there will have more info. The orginal circuit did use our old friend the 6V6 but I will use 7C5s and a 7C6 driver - no reason for this except that I've already got a chassis set up for these loctal valves.
If it works I'll post the results - it might be of use to anyone wanting to build a simple and cheap valve job, as there is nothing critical in the circuit.
I must confess that I'm a bit wary of old caps, especially electrolytics - I still have some unused ones dating from 1947-8, but I chicken out of using (reforming) them and buy new ones instead! I did manage to blow the odd one or two up years ago, and the results were pretty spectacular!
Seriously though, as regards the ECCxx tubes I suspect that in the 1950s and 60s they were possibly more widely used in European amps than in the US - I've got no objective evidence of this apart from circuits in old British and American radio/audio mags.
Actually, I'm looking to try out a circuit that appeared in Radio Constructor in Feb 1953 - exactly 50 years ago - of an unusual push-pull amp in which there is no separate phase splitter but the first tube's cathode is joined to the second's cathode and bias is provided by a common un-bypassed resistor. The second tube's signal grid is connected to ground - the first's signal grid is connected to the preceding stage in the normal manner. I have not seen this circuit used anywhere else so I wonder if there are any problems with it - it does seem very simple and no doubt someone out there will have more info. The orginal circuit did use our old friend the 6V6 but I will use 7C5s and a 7C6 driver - no reason for this except that I've already got a chassis set up for these loctal valves.
If it works I'll post the results - it might be of use to anyone wanting to build a simple and cheap valve job, as there is nothing critical in the circuit.
I had a rather interesting photo of an old radio (I got it from alt.binaries.pictures.radio), the area of the cabinet directly above the 'lytic had a white splattery pattern on it......
Wish I still had that, I'd attach it. 🙁 🙁
Yes, I've seen the LTP (Long-Tailed Pair) style output stage before. The truth is that the second tube can't possibly get as much drive as the first, so it's more like SE than PP operation.
If it were a true LTP (its tail is rather short at about 250 ohms (for 6V6s)), it would use a Constant Currrent Source below the cathodes, so that the total current is always equal (with drive applied to one tube, its current changes - resulting in a change of cathode voltage, which drives the other tube with an equal magnitude of signal).
This principle is used in preamp stages, but not output, since it's very costly to burn 100mA off in a CCS. (You need at least 50V (and 50*.1 = 5W burnt) of headroom, and that would need transistors (low headroom), something hard to come by in the 50s. 😉 )
The perhaps most revolutionary circuit I've seen from that period is multiplexed stereo. The jist is, you have a PP output stage. The balanced signal is tapped with a PP transformer, where common-mode signal passes the PP OPT and goes to a SE transformer (connected in series with the B+ supply). The two tubes act in PP for the PP output, and in parallel for the SE output.
A few details (including the driver circuitry) and you're done!
Clearly, it can only do class A, since class AB (etc.) has changing supply current, messing with the SE output.
Steve Bench did an article on this; I don't remember his website off-hand but a short search should return it.
Tim
Wish I still had that, I'd attach it. 🙁 🙁
Yes, I've seen the LTP (Long-Tailed Pair) style output stage before. The truth is that the second tube can't possibly get as much drive as the first, so it's more like SE than PP operation.
If it were a true LTP (its tail is rather short at about 250 ohms (for 6V6s)), it would use a Constant Currrent Source below the cathodes, so that the total current is always equal (with drive applied to one tube, its current changes - resulting in a change of cathode voltage, which drives the other tube with an equal magnitude of signal).
This principle is used in preamp stages, but not output, since it's very costly to burn 100mA off in a CCS. (You need at least 50V (and 50*.1 = 5W burnt) of headroom, and that would need transistors (low headroom), something hard to come by in the 50s. 😉 )
The perhaps most revolutionary circuit I've seen from that period is multiplexed stereo. The jist is, you have a PP output stage. The balanced signal is tapped with a PP transformer, where common-mode signal passes the PP OPT and goes to a SE transformer (connected in series with the B+ supply). The two tubes act in PP for the PP output, and in parallel for the SE output.
A few details (including the driver circuitry) and you're done!
Clearly, it can only do class A, since class AB (etc.) has changing supply current, messing with the SE output.
Steve Bench did an article on this; I don't remember his website off-hand but a short search should return it.
Tim
Tim,
Thanks for the info. I have come across LTP (thats what it's called - I've forgotten a lot of the terms in my time away from the action) use in pre-amps as you have, but never a circuit like this in output stages. The stage is exceedingly simple, and that's probably the trouble - the first tube is wired up as normal class A but with the cathode joined to that of the second output tube and then through a common resistor to ground. As said, the second tube's signal grid is also connected to ground. The cathode resistor is half the normal value, presumably because twice the current is going through it, and the mag suggests 100 ohms for 6V6s. And that, as they say, is that! There is no constant current source so it may not work effectively, but otherwise the description of how it works is similar to what you say. The article explains the operation thus:
" When a positive voltage is fed to V1's grid, the current throught the valve increases, causing a rise in the voltage between it's cathode and HT-. Now V2 is held normally so that it's grid is negative to it's cathode by a certain amount, namely the voltage across Rk, and any increase in this voltage makes the grid of V2 more negative with respect to its cathode, so that it's signal drops; conversely, when a negative signal causes V1 to lower its current, the associated drop in voltage across Rk causes the current in V2 to rise. In other words, push-pull working is the result."
Anyway, I'll suck it and see - one advantage of using 7C5s is that I see them so often in old radios that I have an idea of what they are capable of so I'll know if two are doing more work than one! (not very objective I know). Other circuits I've seen used for avoiding a phase splitter is to insert a small resistor in g2 of the first output tube, and use a RC coupling to g1 of the second tube, alternatively using a split resistor load between the two anodes and taking a feed for the second tube from there (as also used occasionally is driver stages for push-pull).
As regards the multiplexed stereo circuit I think I remember seeing this in one of the old mags, Electronics World or TV & Radio News, or some such. I'm afraid that I did not read it properly, as I thought that all the windings were on one transformer, not a PP and separate SE being used. I'll have to find this article and read it again! Off hand, I don't think that this circuit was ever used commercially in audio equipment in the UK, manufacturers here tended to still to traditional circuitry throughout the 1950s and 60s.
Thanks for the info. I have come across LTP (thats what it's called - I've forgotten a lot of the terms in my time away from the action) use in pre-amps as you have, but never a circuit like this in output stages. The stage is exceedingly simple, and that's probably the trouble - the first tube is wired up as normal class A but with the cathode joined to that of the second output tube and then through a common resistor to ground. As said, the second tube's signal grid is also connected to ground. The cathode resistor is half the normal value, presumably because twice the current is going through it, and the mag suggests 100 ohms for 6V6s. And that, as they say, is that! There is no constant current source so it may not work effectively, but otherwise the description of how it works is similar to what you say. The article explains the operation thus:
" When a positive voltage is fed to V1's grid, the current throught the valve increases, causing a rise in the voltage between it's cathode and HT-. Now V2 is held normally so that it's grid is negative to it's cathode by a certain amount, namely the voltage across Rk, and any increase in this voltage makes the grid of V2 more negative with respect to its cathode, so that it's signal drops; conversely, when a negative signal causes V1 to lower its current, the associated drop in voltage across Rk causes the current in V2 to rise. In other words, push-pull working is the result."
Anyway, I'll suck it and see - one advantage of using 7C5s is that I see them so often in old radios that I have an idea of what they are capable of so I'll know if two are doing more work than one! (not very objective I know). Other circuits I've seen used for avoiding a phase splitter is to insert a small resistor in g2 of the first output tube, and use a RC coupling to g1 of the second tube, alternatively using a split resistor load between the two anodes and taking a feed for the second tube from there (as also used occasionally is driver stages for push-pull).
As regards the multiplexed stereo circuit I think I remember seeing this in one of the old mags, Electronics World or TV & Radio News, or some such. I'm afraid that I did not read it properly, as I thought that all the windings were on one transformer, not a PP and separate SE being used. I'll have to find this article and read it again! Off hand, I don't think that this circuit was ever used commercially in audio equipment in the UK, manufacturers here tended to still to traditional circuitry throughout the 1950s and 60s.
Yep, I've seen the screen-phase-splitter technique too.
Some of those old guys sure didn't have much appreciation for quality. The All-Mighty Dollar, eh? 🙁
Tim
Some of those old guys sure didn't have much appreciation for quality. The All-Mighty Dollar, eh? 🙁
Tim
Yep, I suspect that in the 'old days' if your signal sources were AM tuners or crystal pick-ups then there was no point in making circuits more complicated (and expensive) to achieve better fidelity which might show up the limitations of your audio feed. Indeed, I think I first saw the screen-phase-splitter technique suggested to increase output of those small AC/DC sets that were common in the 1950s.
Still, I'll try the cathode coupled circuit out - if it performs well I'll start a new thread, otherwise I'll abandon that line of research - back to trying to use a 6Q7 to drive a RS 120watt amp module!
Still, I'll try the cathode coupled circuit out - if it performs well I'll start a new thread, otherwise I'll abandon that line of research - back to trying to use a 6Q7 to drive a RS 120watt amp module!
Hello Bournville, welcome back to audio electronics. I'm afraid I'm one of the guilty parties when it comes to slagging off ECC82 for distortion. 6SN7 is much better (but more expensive), whereas 7N7 is still a sensible price, and Billingtons and Antique Electronic Supply have PTFE Loctal bases.
I think I've seen your moniker on a number of threads already?
I think I've seen your moniker on a number of threads already?
Tim,
Yep, I've used the 6N/6SL7GTs on many occasions in the past and have a high regard for this tube - the first SE amp I ever built used one with a 6V6 for output. Later though I tended to use the ECCxx series, for convenience if nothing else. I've never used a 7N7, I've heard good things about them and perhaps in the future I'll get around to trying one out.
I've knocked up the circuit I mentioned using loctals - 2 x 7C5s and a 7C6 for a driver, with a 6J5 added later to provide extra gain (I replaced a loctal for a octal socket for this one - this is where I'd use a 7N7 if I had one to replace the 7C6 and 6J5 and save the trouble of a socket change). As far as I can tell it works OK - the output is definitely greater than a single ended 7C5, but whether it is as good as a couple in parallel, let alone using more conventional phase splitting techniques, is another matter. I don't have a scope at the moment, so my ears are the final deciders on quality and output. Perhaps knocking up an output meter will be my next task!
I have been browsing the threads as you say - I've picked up some good ideas that are worth trying and also dropped the odd comment, hopefully more to come in the future.
See you 'round the threads!
Yep, I've used the 6N/6SL7GTs on many occasions in the past and have a high regard for this tube - the first SE amp I ever built used one with a 6V6 for output. Later though I tended to use the ECCxx series, for convenience if nothing else. I've never used a 7N7, I've heard good things about them and perhaps in the future I'll get around to trying one out.
I've knocked up the circuit I mentioned using loctals - 2 x 7C5s and a 7C6 for a driver, with a 6J5 added later to provide extra gain (I replaced a loctal for a octal socket for this one - this is where I'd use a 7N7 if I had one to replace the 7C6 and 6J5 and save the trouble of a socket change). As far as I can tell it works OK - the output is definitely greater than a single ended 7C5, but whether it is as good as a couple in parallel, let alone using more conventional phase splitting techniques, is another matter. I don't have a scope at the moment, so my ears are the final deciders on quality and output. Perhaps knocking up an output meter will be my next task!
I have been browsing the threads as you say - I've picked up some good ideas that are worth trying and also dropped the odd comment, hopefully more to come in the future.
See you 'round the threads!
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