Got a pair of small output trannies and some 6EW7 tubes in the mail today. The plan is to build a small SE amp using one of these tubes per channel, just for fun since I found those cheap surplus transformers that seems to suite the tubes.
The tube in question is a small 9-pin dual dissilimar triode with one low power/medium mu half and one higher power/low mu half. Seems really nice for this kind of project.
Since the transformers are surplus and of some unknown brand I felt that I had to test run one of them to see if they´re any good.
Said and done, I spended a couple of hours making a small breadboarded single channel prototype to see if it worked.
It did, but not that good. Low gain and floppy, distorted bass response. When I checked some voltages and currents I realized that something was very wrong.
In my hurry I hadn´t bother to double check the pinout for the tube, so I had mixed up the two (very) different halves!
The power triode is working as an RC-coupled gain stage and the driver triode is acting as a transformer coupled power stage.
I´m quite surprised that it did sound at all!!!
I´ll rewire the whole thing tomorrow night and then I will report how it works with the right triode at the right place.
The tube in question is a small 9-pin dual dissilimar triode with one low power/medium mu half and one higher power/low mu half. Seems really nice for this kind of project.
Since the transformers are surplus and of some unknown brand I felt that I had to test run one of them to see if they´re any good.
Said and done, I spended a couple of hours making a small breadboarded single channel prototype to see if it worked.
It did, but not that good. Low gain and floppy, distorted bass response. When I checked some voltages and currents I realized that something was very wrong.
In my hurry I hadn´t bother to double check the pinout for the tube, so I had mixed up the two (very) different halves!
The power triode is working as an RC-coupled gain stage and the driver triode is acting as a transformer coupled power stage.
I´m quite surprised that it did sound at all!!!
I´ll rewire the whole thing tomorrow night and then I will report how it works with the right triode at the right place.
6EW7
I can sympathise , my old Taylor 45D valve tester manual listed the high and low mu triode sections with the grid bias settings the wrong way round . I've tried a simple current source loaded 6EW7 SE and it sounded very nice , but very low power .
'...with the right triode at the right place'
very true ! I've tried to put my view across regarding this fact but some folks aren't listening (for example someone has decided that a 6sl7 section with 100k load , 175v HT 'providing' 70v p-p swing driving a 6080 with 220k gridleak will provide good sound
hehehehe) see 6080 parafeed thread )
316a
I can sympathise , my old Taylor 45D valve tester manual listed the high and low mu triode sections with the grid bias settings the wrong way round . I've tried a simple current source loaded 6EW7 SE and it sounded very nice , but very low power .
'...with the right triode at the right place'
very true ! I've tried to put my view across regarding this fact but some folks aren't listening (for example someone has decided that a 6sl7 section with 100k load , 175v HT 'providing' 70v p-p swing driving a 6080 with 220k gridleak will provide good sound
hehehehe) see 6080 parafeed thread )316a
Re: 6EW7
Ya know, you still didn't tell us why it wouldn't.
Tim
316a said:very true ! I've tried to put my view across regarding this fact but some folks aren't listening (for example someone has decided that a 6sl7 section with 100k load , 175v HT 'providing' 70v p-p swing driving a 6080 with 220k gridleak will provide good sound
Ya know, you still didn't tell us why it wouldn't.
Tim
I did some rewiring tonight and, big surprise, achieved some slightly better results
The bass response goes way deeper than expected. The OPT:s are quite small so I didn´t expect much on that point. Not the "tightest" bass I´ve ever heard maybe, but that must have something to do with the high DCR of the secondary winding.
The measured HF response is not that good, but it did sound OK so why bother? My hearing is far from perfect anyway...
Output power reaches 1,2 W.
Now I´ll have to find (or wind) a suitable power trannie, then it´s time to give it a go!
The bass response goes way deeper than expected. The OPT:s are quite small so I didn´t expect much on that point. Not the "tightest" bass I´ve ever heard maybe, but that must have something to do with the high DCR of the secondary winding.
The measured HF response is not that good, but it did sound OK so why bother? My hearing is far from perfect anyway...
Output power reaches 1,2 W.
Now I´ll have to find (or wind) a suitable power trannie, then it´s time to give it a go!
6EW7 Performance
I have a quantity of this type and indeed am using a pair in a temporary amplifier as drivers cum cathode followers to drive some triode-strapped dual beam tetrodes.
I have tested 6EW7 both individually and as differential pairs and can report that distortion is stunningly low.
I doubt that any significant power could be obtained (unless one paralled four together) but it is an excellent alternative to other medium-mu types, with the added benefit of a low-ra high-current section that could easily drive a big valve directly from its anode.
7N7
I have a quantity of this type and indeed am using a pair in a temporary amplifier as drivers cum cathode followers to drive some triode-strapped dual beam tetrodes.
I have tested 6EW7 both individually and as differential pairs and can report that distortion is stunningly low.
I doubt that any significant power could be obtained (unless one paralled four together) but it is an excellent alternative to other medium-mu types, with the added benefit of a low-ra high-current section that could easily drive a big valve directly from its anode.
7N7
DISSIMILAR TRIODES.
Hi,
You can also make nice regulators with this kind of valves.
To bad a lot of them require Novar sockets, other than that these are fine candidates for headphone amp service as well.
And they're available in a wide range of heater versions to boot.
I quite like'm for their compactness and if you haven't noticed yet a lot of them sport these lovely sounding graphite plates too.
Cheers,
Hi,
You can also make nice regulators with this kind of valves.
To bad a lot of them require Novar sockets, other than that these are fine candidates for headphone amp service as well.
And they're available in a wide range of heater versions to boot.
I quite like'm for their compactness and if you haven't noticed yet a lot of them sport these lovely sounding graphite plates too.
Cheers,
Measured distortion
My loadlines of this tube shows a lot of distortion using an impedance of 2000. But, I probably am not doing it correctly.
Is the calculated distortion for this tube as high as I think it is (75 v on the plate as grid goes negative from op point of -17.5 grid and 100 v in the other direction)?
Rick
My loadlines of this tube shows a lot of distortion using an impedance of 2000. But, I probably am not doing it correctly.
Is the calculated distortion for this tube as high as I think it is (75 v on the plate as grid goes negative from op point of -17.5 grid and 100 v in the other direction)?
Rick
Re: Measured distortion
My distortion tests were arranged so as to determine the inherent distortion that the valve produces.
A load of 2k would be far too low - N.B. I was not testing section II of 6EW7 as an output valve.
After all 10W is the absolute limit, so the most that you could screw out of it in SE would probably be about 2.5W
I suspect that something in excess of 4k would be required - perhaps 10k a-a for p-p.
7N7
fragman56 said:
My distortion tests were arranged so as to determine the inherent distortion that the valve produces.
A load of 2k would be far too low - N.B. I was not testing section II of 6EW7 as an output valve.
After all 10W is the absolute limit, so the most that you could screw out of it in SE would probably be about 2.5W
I suspect that something in excess of 4k would be required - perhaps 10k a-a for p-p.
7N7
Ooops...!
Too bad I didn´t bother to draw a loadline before I bought the stuff...I did it a minute ago and with a 2k load it looked...horrible
Sadly there´s no 4 ohm tap on the OPT, so I can´t cheat that way either...
Maybe it would work better with the output stage connected as a cathode follower? Would of course need a really "swingy" gain stage, but that could be arranged I guess.
Any thoughts about this?
I´m not going to buy new output trannies, I already have more amps than I´ll ever need and this one was only meant as a funny way to kill some sparetime.
Too bad I didn´t bother to draw a loadline before I bought the stuff...I did it a minute ago and with a 2k load it looked...horrible
Sadly there´s no 4 ohm tap on the OPT, so I can´t cheat that way either...
Maybe it would work better with the output stage connected as a cathode follower? Would of course need a really "swingy" gain stage, but that could be arranged I guess.
Any thoughts about this?
I´m not going to buy new output trannies, I already have more amps than I´ll ever need and this one was only meant as a funny way to kill some sparetime.
4k
Hi 7n7,
My mistake, I did use a load of 4k (25 ma:100v) for the distortion drawing.
My drawing shows Pout may be only a little over 1 watt in SE mode at 4k load.
I know that usually one can guess that a tube can produce about 1/4 the plate dissipation in SE mode but how can this be done?
What does "10k a-a" mean? "p-p" means push pull?
Thanks,
Rick
Hi 7n7,
My mistake, I did use a load of 4k (25 ma:100v) for the distortion drawing.
My drawing shows Pout may be only a little over 1 watt in SE mode at 4k load.
I know that usually one can guess that a tube can produce about 1/4 the plate dissipation in SE mode but how can this be done?
What does "10k a-a" mean? "p-p" means push pull?
Thanks,
Rick
Re: 4k
A-a = anode-to-anode i.e. the total promary impedance of the primary of a push-pull (p-p) transformer.
It is worth noting that the manufacturers of the PX25 (ra = 560 ohms) recommended 10k as the optimum load.
Personally I am inclined to think that 6EW7 is best reserved for driver service rather than as a combined driver and output valve.
I know that some people have claimed success with 6DN7, but again I imagine that they have 105dB/w loudspeakers!
Configured as a diff pair, with cascode CCS and using the second sections as self-biased cathode followers driving a 30k pad, I measured THD of 0.025% (-63.5dB) at +28dB output (19.5v rms).
Since my oscillator's THD is -64.5dB I have concluded that this experimental circuit produced negligable distortion.
7N7
fragman56 said:
A-a = anode-to-anode i.e. the total promary impedance of the primary of a push-pull (p-p) transformer.
It is worth noting that the manufacturers of the PX25 (ra = 560 ohms) recommended 10k as the optimum load.
Personally I am inclined to think that 6EW7 is best reserved for driver service rather than as a combined driver and output valve.
I know that some people have claimed success with 6DN7, but again I imagine that they have 105dB/w loudspeakers!
Configured as a diff pair, with cascode CCS and using the second sections as self-biased cathode followers driving a 30k pad, I measured THD of 0.025% (-63.5dB) at +28dB output (19.5v rms).
Since my oscillator's THD is -64.5dB I have concluded that this experimental circuit produced negligable distortion.
7N7
I´m getting a bit discouraged here...
No opinions about the cathode follower output idea?
No other ideas about what I can do with these tubes and transformers, together or in different projects?
I thoght for a while about using the transformers together with trioded EL86:s. Maybe they would match the 2k load better?
The 6EW7 looks alot nicer with a horizontal load line, which I guess represents a CCS or plate choke load.
Could be a nice n´grunty driver stage for a big triode amp or maybe even a hybrid
The fact remains: This cheap tube/transformer combo does indeed sound quite nice, so I have to admit that I´m a bit tempted just to carry on as planned, ignoring all the technical stuff
No opinions about the cathode follower output idea?
No other ideas about what I can do with these tubes and transformers, together or in different projects?
I thoght for a while about using the transformers together with trioded EL86:s. Maybe they would match the 2k load better?
The 6EW7 looks alot nicer with a horizontal load line, which I guess represents a CCS or plate choke load.
Could be a nice n´grunty driver stage for a big triode amp or maybe even a hybrid
The fact remains: This cheap tube/transformer combo does indeed sound quite nice, so I have to admit that I´m a bit tempted just to carry on as planned, ignoring all the technical stuff
Minutes ago I took a better look at one of the output trannies
and noticed something really strange: there´s no airgap!!
Ok, not that strange maybe cause the trannies obviously weren´t made as SE OPT:s. I thought they where when I bought them...
The weird part is that the amp seems to work quite good with 50 mA DC running through the trannie!
Well well, at least I got a new idea: What about some kind of parafeed amp with a EL86 pentode CCS as a plateload for the 6EW7? A big coupling capacitor would then block all the DC from the OPT, which must be a very good thing, right?
Would also solve the problem with the very different voltages required for the different halves of the 6EW7. The input section asks for a lot higher voltage than the power section, and the PSU design would be alot easier if there´s a CCS that drops aome 150V on top of the output triode.
Both tubes can tolerate quite high heater-cathode voltages, so there shouldn´t be any problem with the heater windings.
Any thoughts, ideas, opinion???
and noticed something really strange: there´s no airgap!!
Ok, not that strange maybe cause the trannies obviously weren´t made as SE OPT:s. I thought they where when I bought them...
The weird part is that the amp seems to work quite good with 50 mA DC running through the trannie!
Well well, at least I got a new idea: What about some kind of parafeed amp with a EL86 pentode CCS as a plateload for the 6EW7? A big coupling capacitor would then block all the DC from the OPT, which must be a very good thing, right?
Would also solve the problem with the very different voltages required for the different halves of the 6EW7. The input section asks for a lot higher voltage than the power section, and the PSU design would be alot easier if there´s a CCS that drops aome 150V on top of the output triode.
Both tubes can tolerate quite high heater-cathode voltages, so there shouldn´t be any problem with the heater windings.
Any thoughts, ideas, opinion???
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.