[triod750]

"Of course various tubes sound different due to being used outside of their design specifications; the Little Dots have fixed parameters that mean they will sound different."

Spot on, and this is why you can find excellent synergy where input and output tubes compensate for the deficiances in each other and in the recordings you are listening to.
Simple.

 

[Cognomen]

So has my understanding. I expected someone to tell me I'm wrong. I just wrote what I have been told. I have asked previously if it is possible to use the pentodes EL41 and EL42 triodestrapped in pairs as output tubes and the answer was no.

Because of their ic internal connection pins? I glanced at an EL41 datasheet on frank.pocnet.net and it had data for triode connections and triodes in push-pull. I have 6AM5 tubes that a datasheet specified had an ic pin 6: do not connect anything to it. When I looked closely at the tube I noticed nothing was connected to that pin on my Amperex 6AM5. So I ran it in EF91/EF92 mode without chopping off pin 6 and it ran well. Another datasheet, however, was more accurate and specified pin 6 was nc not connected to anything in the tube..

 

[triod750]

Because of their ic internal connection pins? I glanced at an EL41 datasheet on frank.pocnet.net and it had data for triode connections and triodes in push-pull. I have 6AM5 tubes that a datasheet specified had an ic pin 6: do not connect anything to it. When I looked closely at the tube I noticed nothing was connected to that pin on my Amperex 6AM5. So I ran it in EF91/EF92 mode without chopping off pin 6 and it ran well. Another datasheet, however, was more accurate and specified pin 6 was nc not connected to anything in the tube..

I can't answer this since I don't speak electric. I have just used EL42 as input tubes in Little Dot and briefly tried EL41 for this duty too, these with external heater PSU. Tonight I use EL42 'Sakuma style' as both input and output tubes, four of them, in an amp with output transformers. This is just an experiment to hear their tone this way.

 

[Scooter64]

New LD MKIII user here so please bear with me if this has already been covered…….theres a huge number of pages here to digest.
I’m moving the jumpers to change EF91 tubes to EF95. I get that I have to move the jumpers from pins 2 & 3 to 1 & 2 but am I right in saying that pin numbers are 1, 2, 3 from left to right when the amp is upside down and the front is facing me?
I know it’s sounds like a pretty obvious question but doing it for the first time, I do want to make sure I’ve got exactly right.
Thanks

 

[Cognomen]

New LD MKIII user here so please bear with me if this has already been covered…….theres a huge number of pages here to digest.
I’m moving the jumpers to change EF91 tubes to EF95. I get that I have to move the jumpers from pins 2 & 3 to 1 & 2 but am I right in saying that pin numbers are 1, 2, 3 from left to right when the amp is upside down and the front is facing me?
I know it’s sounds like a pretty obvious question but doing it for the first time, I do want to make sure I’ve got exactly right.
Thanks

(When installing or removing jumpers, the amp should have been off and cooled down which makes it possible the electricity harboring capacitors have discharged. IF you shut off the amp with music playing, it will continue playing music while the capacitors are discharging.)

It seems you are probably on the right track for moving the jumpers.

When I turn my version of the MKIII upside down (removing the tubes makes the job easier for me)
with the volume knob pointed toward me, both jumpers should be connecting the two pins to the left for EF95 mode and vice versa for EF91/EF92 mode. When I look with a flashlight, 5654 i.e. EF95 appears near the Leftmost jumper pins and EF91 or EF92 near the rightmost jumper pins.

There are different versions of the MKIII. They do not all look the same when it comes to jumpers. With a flashlight, one can probably find the version number by looking through the housing holes when it's upside down. If you don't already have a manual knowing the version might help if you opt to look online.

It is common for people to lose hold of a jumper when trying to install it. Turning the unit upright and shaking can get it to fall out. Having spares can be good. Some people have bought aftermarket long-handled jumpers. I install the standard tiny jumpers using cotter pin pliers which are like a needle nose plier but much, much narrower, even narrower than tweezers, and not stout.

(IF the jumpers are installed for EF91/EF92 and one powers up the unit with EF95 tubes, the amp will not work and should be quickly turned off because one is shorting the EF95 tubes and probably risking other problems if left on. It is a somewhat common error and people have shut it off soon enough to avoid damage. IF the jumpers are set for EF95 and one powers up with an EF91 or EF92 tubes, another somewhat common goof, the amp is likely to work well but may risk being unstable with some tubes due to an unconnected grid in the tubes. In addition when running EF95 tubes in my MKIII, per my user manual, it makes no difference whether the jumpers are installed for EF95 or completely pulled out of the unit. I think of installing them in EF 95 mode as a good way of storing them and avoiding losing them.)

 

[Scooter64]

(When installing or removing jumpers, the amp should have been off and cooled down which makes it possible the electricity harboring capacitors have discharged. IF you shut off the amp with music playing, it will continue playing music while the capacitors are discharging.)

It seems you are probably on the right track for moving the jumpers.

When I turn my version of the MKIII upside down (removing the tubes makes the job easier for me)
with the volume knob pointed toward me, both jumpers should be connecting the two pins to the left for EF95 mode and vice versa for EF91/EF92 mode. When I look with a flashlight, 5654 i.e. EF95 appears near the Leftmost jumper pins and EF91 or EF92 near the rightmost jumper pins.

There are different versions of the MKIII. They do not all look the same when it comes to jumpers. With a flashlight, one can probably find the version number by looking through the housing holes when it's upside down. If you don't already have a manual knowing the version might help if you opt to look online.

It is common for people to lose hold of a jumper when trying to install it. Turning the unit upright and shaking can get it to fall out. Having spares can be good. Some people have bought aftermarket long-handled jumpers. I install the standard tiny jumpers using cotter pin pliers which are like a needle nose plier but much, much narrower, even narrower than tweezers, and not stout.

(IF the jumpers are installed for EF91/EF92 and one powers up the unit with EF95 tubes, the amp will not work and should be quickly turned off because one is shorting the EF95 tubes and probably risking other problems if left on. It is a somewhat common error and people have shut it off soon enough to avoid damage. IF the jumpers are set for EF95 and one powers up with an EF91 or EF92 tubes, another somewhat common goof, the amp is likely to work well but may risk being unstable with some tubes due to an unconnected grid in the tubes. In addition when running EF95 tubes in my MKIII, per my user manual, it makes no difference whether the jumpers are installed for EF95 or completely pulled out of the unit. I think of installing them in EF 95 mode as a good way of storing them and avoiding losing them.)

Thank you for a comprehensive response - that’s really useful and gives me the confidence I have done it right.
I did do it with the unit off, and it had been overnight. I also used modelling long nose spring loaded tweezers so grabbing hold of the jumpers, and keeping them firm, was pretty straightforward.
I have a got a manual but it’s only a generic one I found on the internet so not specific to the model I have, although from what it says about jumpers and gain control, I think it does cover my unit.
Thanks again

 

[Scooter64]

Jumpers changed, tubes changed from Mullard 8161 EF91s to Voshkod 6ZH1P-EV EF95s and listening on my DT770 Pros……well impressed with the difference

 

[Cognomen]

Or you can build a Christmas Tree like me

These are Novosibirsk 6S2S, the Soviet version of the 6J5GT. As @triod750 mentioned, you can also use the 6C5 family of tubes. I have a pair of almost 80-year-old VT-65 Ken-Rad's that, in a blind test, I'm pretty sure I wouldn't distinguish from the 6HM5.

There are literally hundreds of triodes that can be used in Little Dot. For some of them, there are off-the-shelf adapters; for others, you'll need to build a specific one.

Here's the list of families I found compatible for drivers (6,5V, <= 450mA):
2C22, 37, 3A/167M, 437A, 5842, 6AC5G, 6AC5GT, 6AE5GT, 6AF4, 6AM4, 6AQ4, 6C2, 6C5, 6C8P, 6CW4, 6DS4, 6ER5, 6ES5, 6F12, 6F5, 6FQ5, 6GK5, 6HM5, 6HQ5, 6J4, 6J5, 6J5G, 6J5GT, 6K5G, 6K5GT, 6K5MG, 6L5G, 6P5G, 6S3P, 6S45P, 6SF5, 6T2, 7245, 7586, 76, 7895, 7A4, 7B4, 8556, 8627, 8628, 8808, 9002, A2521, A2599, CV5112, CV6, E20C, E88C, EC52, EC80, EC8010, EC8020, EC86, EC866, EC88, EC88C, EC90, EC92, EC97, NY67

 

[Cognomen]

Or you can build a Christmas Tree like me

These are Novosibirsk 6S2S, the Soviet version of the 6J5GT. As @triod750 mentioned, you can also use the 6C5 family of tubes. I have a pair of almost 80-year-old VT-65 Ken-Rad's that, in a blind test, I'm pretty sure I wouldn't distinguish from the 6HM5.

There are literally hundreds of triodes that can be used in Little Dot. For some of them, there are off-the-shelf adapters; for others, you'll need to build a specific one.

Here's the list of families I found compatible for drivers (6,5V, <= 450mA):
2C22, 37, 3A/167M, 437A, 5842, 6AC5G, 6AC5GT, 6AE5GT, 6AF4, 6AM4, 6AQ4, 6C2, 6C5, 6C8P, 6CW4, 6DS4, 6ER5, 6ES5, 6F12, 6F5, 6FQ5, 6GK5, 6HM5, 6HQ5, 6J4, 6J5, 6J5G, 6J5GT, 6K5G, 6K5GT, 6K5MG, 6L5G, 6P5G, 6S3P, 6S45P, 6SF5, 6T2, 7245, 7586, 76, 7895, 7A4, 7B4, 8556, 8627, 8628, 8808, 9002, A2521, A2599, CV5112, CV6, E20C, E88C, EC52, EC80, EC8010, EC8020, EC86, EC866, EC88, EC88C, EC90, EC92, EC97, NY67

While listening to RCA Made in Great Britain 6HM5, I looked up this list of single triodes (and a few nuvistors) and queried them on all posts on this forum. This post was the first time approximately 85% of them were mentioned. In the past people tried and posted impressions of some triodes: 6AQ4, 6J4, EC90, short bottle 6HM5/6HA5/6HQ5 and perhaps EC92 and they did not bubble up to the top on lists of favorite tubes. More recently a limited number of people listened to the 6C5 and 6J5(GT) triodes and liked them. They are precursors of the twin triode 6SN7 which is used in a wide variety of good quality audio kit. Double triodes have been extensively auditioned on this forum both as driver tubes and as output ("power") tubes and some of them in addition to the 6SN7 incorporate similar tubes available as single triodes so the 85% 'statistic' errs on the high side.

The RCA Britain 6HM5s (Mullard Blackburn same year) have getter flashing running far down its side like the Yugo. Upon close examination the RCA has about 1 mm less. I also noticed very similar internals except for a slight difference in their plates. While the Yugo 6HM5 emits a flash of light when powered up, the RCA filaments burn very bright for the first second or so and then calm down. I find the RCA is slightly more forward than the Yugo in terms of mids and highs which suits my somewhat laidback speakers, ADS L620, well. I also found the RCA slightly warmer/euphonic. The RCAs sounded better than the Yugos on old Benny Goodman Big Band jazz recordings. Perhaps the Yugo whose mids sounded a bit harsh sometimes in comparison more accurately reflected the quality of 70 to almost 100 year old recordings, though. I like the warmer euphonic presentation for some genres of music: some classical and some jazz that I don't enjoy nearly as much on solid state. The RCA might be very, very slightly rolled off in the lower bass region (i.e. electronica and lower notes bass guitar) than the Yugo. Sometimes when switching back and forth I wondered about it and sometimes felt I could not hear a difference. I listened to them with RCA dissimilar double triode 6JK8s as output (power) tubes with my Little Dot MKIII. Due to brain and/or tube burn in I'm still learning about the sound of the 6JK8 which I consider competent to very competent so far.

 

[Cognomen]

I have been experimenting successfully with dissimilar double triodes as output ("power") tubes in my Little Dot MKIII. Many of them sound very good. I have tried 6 types and believe that many other types should work well also. Many of these tubes were designed for American televisions. An electrical engineer friend with tube audio and ham radio build experience had a hunch dissimilar double triodes might not work well because the amp has a push-pull facet. When I mentioned they worked well he said that the amp tolerates that much difference between the triodes. Below you can find the most exhaustive list of dissimilar double triodes that I can provide. Let me know if you know of any additional types. I found them on the web's pocnet tube database which sometimes misclassifies double dissimilar triodes as double exact same triodes.

ID	Test Results in Little Dot	Base	V, Ah	mu (amplification factor) triode #1	mu #2	Adapter Needed	Other Voltages of same tube	Notes
6JK8	Works Well (Gain set at 5)	9 mini	6.3, .4Ah	55	70	Not Needed Plug and Play!	8, 17	Designed for tuners
12DW7	Works Well (Gain set at 5)	9 mini	6.3, .3Ah	100	20	12AU7	12	1/2 of a 12AX7 plus 1/2 of a 12AU7
6EM7	Works Well (Gain set at 5)	Octal	6.3, .925Ah	64	5.4	6SN7	10, 13	Note 1
6EA7	Works Well (Gain set at 5)	Octal	6.3, 1.05Ah	66	5.5	6SN7	12, 15	Note 1
6GL7	Works Well (Gain set at 5)	Octal	6.3, 1.05Ah	66	5	6SN7		Note 1
6DN7	Works Well (Gain set at 5)	Octal	6,3, .9Ah	22.5	15.4	6SN7
6BX7GT	Untested	Octal	6.3, 1.5 Amps. needs external power supply	?	10	Same pinout as 6SN7		Recommended as sounding similar to esteemed and expensive Melz 6N12S
6DR7	Untested	9 mini	6.3, .9Ah	64	6	Yes, Note 2	10, 13, 19	Some people with different equipment especially like this double triode.
6FR7	Untested	9 mini	6.3, .925Ah	68	5.4	Yes, Note 2	10, 13	Note 3 (Unobtanium?)
6FD7	Untested	9 mini	6.3, .925Ah	64	6	Yes, Note 2	10, 13
6EW7	Untested	9 mini	6.3, .9Ah	17.5	6.0	Yes, Note 2	10, 15	Note 4 (Low mu)
6DE7	Untested	9 mini	6.3, .9Ah	17.5	6.0	Yes, Note 2	10, 13, 19`	Note 4 (low mu)
6DA7	Untested	9 mini	6.3, 1.0Ah	20.0	6.3	Yes, Note 2	10	Note 3, 4 (Unobtanium? Low mu)
6CS7	Untested	9 mini	6.3, .6Ah	17	15.5	Yes, Note 2	8	Used in a fancy Leben integrated
6CY7	Untested	9 mini	6.3, .75Ah	68	5	Yes, Note 2	8, 11
6BN7	Untested	9 mini	6.3, .75Ah	28	12	Yes		Only 4 listed on eBay; a dealer advertises having some.
6CM7	Untested	9 mini	6.3, .6Ah	21	18	Yes	8
6FJ7	Untested	Compactron	6.3, .9Ah	22.5	15.4	Yes, Note 5		Compactron version of 6DN7
6FM7	Untested	Compactron	6.3, 1.005Ah	66	5.5	Yes, Note 5	13, 15	Compactron version of 6EM7
6FY7	Untested	Compactron	6.3, 1.05Ah	65	6.0	Yes, Note 5	11, 15	Compactron Version of 6FM7
6GF7(A)	Untested	Novar	6.3, .985Ah	64	5.4	Yes	10, 13	Novar Version of 6EM7
10EG7	Untested		10 Volts	17.5	6.0			Note 4 (Low mu)
12AE7	Untested	9 mini	12 Volts	13.0	6.4			Note 4 (low mu), Note 6 (Caution: Probably incompatible.)
12DW8, a diode with two dissimilar triodes	Untested	9 mini	12 Volts	9.5	6.4			Note 4 (low mu), Note 6 (Caution probably incompatible)
6FQ7 See Note 7
6B5 and 6N6 See Note 8

Note 1: 6EM7, 6EA7, and 6GL7 were interchangeable tv tubes and some tubes have all three identities marked on them. I have rolled half a dozen different ones. They all sound very good and similar. A current personal favorite is Curtis Mathes 6EM7. It's a Raytheon Japan rebrand and the only 6EM7 I have rolled. It makes me wonder if the 6EM7 might be better than the other two.

Note 2: The 6DR7, 6FR7, 6FD7, 6EW7, and 6DE7 all have the same pinout. Pin #2 is G2 and Pin #3 is G2. The 6CS7 and 6DA7 are the same except pin 2 is not connected. The 6CY7 is the same except pin 2 is an internal connection. Could an adapter with pin 2 not connected to anything work for all of them? If I eventually order adapters, I will double check all these pinouts, bases etc!

Note 3: These tubes may be rather unobtanium; either none or only one was listed on eBay. A dealer who claims to have the largest inventory in the world does not advertise having them except for "call/email" regarding 10DA7.

Note 4: mu (amplification) may be too low. The stock 6N6P output ("power") tubes have two triodes each with 20 mu and the 6AK5 may have 20 mu as a triode. It is unclear to me whether or not one could compensate for low mu output ("power") tubes with high mu drivers such as 6HM5 that have 80mu. The highest mu tube I can recall is the 12AX7 with two 100mu triodes. I have rolled drivers with 10 mu with stock output tubes, and it decreases the amplification overall. It's still enough for me perhaps because I have rather sensitive Beyer wired Amiron 250 ohm headphones and tend to listen at low volume and mostly listen to speakers anyhow with a cap-coupled power amp with the input pots turned way down because it is capable of producing 100+ more watts per channel than I need in my small listening room with 92db sensitive speakers.

Note 5: It seems possible the same adapter could work for the 6FJ7, 6FM7, 6FY7 - not all the exact same pinout so that would need to be considered - somewhat akin to differences in note 2.

Note 6: This tube was designed to run directly off of a 12 volt car battery in hybrid transistor tube audio. Its plate is rated for 12 volts, 16 volts max. I believe our Little Dots operate at much, much higher plate voltage. I am concerned this tube is incompatible and that deploying in a Little Dot might not end well at all. The 12AE7 tube has a center tap but 3 datasheets I read do not provide information about using it to change voltage. It has the same pinout as a 12AU7. I have listened to car audio 6ES6 drivers whose plates are rated 50volts max (much lower than the little dot) and they are the worst sounding tubes I have auditioned in it. Perhaps I'm lucky they sounded bad and rolled them out rather quickly.

Note 7: The 6FQ7 is sometimes listed as a dissimilar double triode. While the plates have different voltage ratings, both triodes have mu 20. I think of it as a 9mini version of the double similar triode 6SN7.

Note 8: I don't see a way to use 6B5 and 6N6 but am an electronics novice. If someone sees a way to use them, please let me know. They are available in different voltages also.

 

[Cognomen]

I'd like to learn more about converting 12 volt to 6 volt tubes such as the 12AU7 and 12A_7 tubes whose pins 9 are hct, heater center 6 volt conversion taps. The 12AU7 etc have the same pinout as a stock Little Dot MK III 6N6P output ("power") tubes except pin 9 in the 6N6P is an internal shield while pin 9 in the 6AU7 is a hct, heater center 6V conversion tap, used to transform it from a 12 volt heater tube to a 6 volt heater tube. When running at 12 volts one heats pins 4 and 5. Does running at 6 volts require simply require bridging pin 9, the hct, heater center tap, to the pin 4 or 5p and heating any two of pins 4, 5, or 9? Can I run 12A_7 tubes with pin 9 hct heater conversion taps as output ("power") tubes in a Little Dot Mk III etc as stock like the pinout of 6N6P with a wire mod connecting pin 9 to pin 4 or 5 with heating 6V on whichever of the 3 pins? Or, do I need to specifically heat pin 9 and specifically pin 4 or 5?

 

[TrollDragon]

I don't think it's worth using 12A?7 tubes as outputs but your ears your gears.

In order to use one at 6.3V you need to join pins 4&5 together and use pin 9 as the other heater connection. This would require an adapter (commercial or DIY) to use them as outputs.

 

[Cognomen]

I don't think it's worth using 12A?7 tubes as outputs but your ears your gears.

In order to use one at 6.3V you need to join pins 4&5 together and use pin 9 as the other heater connection. This would require an adapter (commercial or DIY) to use them as outputs.

Thank you, I wanted to learn about how it works. I have output adapters that work with 12A?7 that I have used for the excellent E80CC and sweet 12BH7's. I have about 80 used "junk box" 12A?7 that I am in no rush to sort through for cosmetic pairs due to more intriguing output tube listening prospects. I know they have been tried rather extensively here as output tubes and are not generally favorites in that capacity. Perhaps they are better suited as drivers. As part of my dissimilar double triode odyssey, I listened to probably NOS 12DW7, 1/2 12AU7 plus 1/2 12AX7, for the last few days. While they work well electronically, I was glad to roll them out, perhaps hasty in terms of burn in/getting used to them. Curtis Mathes hi fi, Raytheon Japan, dissimilar triode 6EM7 sound much better. I drove both with GE tall bottle 6HM5 "made in MEX" whose midrange is excellent with acoustic music and vocals. The mids are just slightly louder than the bass with other music. The bass in rock and electronica is a bit rolled off and inarticulate. When powered up, one of them briefly flashes like a Yugo ~. They beautifully play Hilary Gardner's Trail of the Lonesome Pines especially imaging her voice. She's an old-fashioned jazz vocalist that mined old popular music and perhaps Broadway for country music crossover songs. Then she turned them into somewhat laconic jazz with well-paced vocals with C & W instrumentation including pedal steel. It makes me recall other very bright country crossovers I have especially enjoyed such as Ray Charles Modern Sounds in Country and Western Music which I would classify as a pop album,, and The Byrds starring Gram Parsons in Sweetheart of the Rodeo, a landmark country rock album. Get along little dogies...