Interesting. Failure to put a cap on the input is significant. If there is any DC offset coming from the previous device (phone output, phono preamp output, etc.), then that offset will upset the biasing of the 6J1 tube. This is one of the reasons that old tube amplifiers with leaky coupling caps often have burned out tubes and transformer primaries. Over biased tubes due to DC leaking through the coupling caps.
Searching around a bit for schematics it seems not too uncommon to find the first stage tube on power amps to not have an input cap? IDK, I'm still a toob noob.
With the single ended topology that we are talking about here, the cap has to be there somewhere. Either at the output of the previous stage or the input of the first stage. Most other devices would probably have a coupling cap at the output, which is probably why they skipped the input cap.
As you suggested, and from what I can tell from looking at the board, it looks like they are using a regulated, single rail supply. Those look like 130V and 120V zener diodes connected in series, so my guess is a tightly regulated 250vdc supply. The tube filaments are connected in parallel and given a tightly regulated 6VDC. This is a big improvement over the other board, which connects the two filaments in series with half wave rectification and little filtering. This not only results in a very uneven filament voltage division between the two tubes, but passes alot of ripple onto the ground traces. The overall grounding scheme appears to be alot better as well. It is not a true star grounding scheme, but as long as B+ and filament ground return currents do not flow through the signal input ground return traces, you should be OK.
Got the 2nd batch of 24 tubes from Kornbread in the mail today. Thanks, Kornbread! Well packed and no damage. I'll start matching them up based on transconductance (gm), gain (mu), and microphonics. Kornbread also sent along a set of tube dampers. So I'll send everyone on the list a set of dampers to go with their matched tubes.
Never unwrapped any of the tubes and added bunches more padding before shipping them your direction. Kind of surprised there were no broken tubes from the trip across the pond as there was plenty of room in the box for them to rattle around.
I have found that tubes are tough little buggers! I few of those in the picture have some pretty dark looking pins. I wonder if that's corrosion and if a little rubbing with a Scotchbrite pad would brighten them up.
Ya, the pins on some of them look pretty dark. I'm carefully cleaning the pins on each tube to brighten them up a bit before each test. Then let them burn in for about 10 minutes or so to let the parameters settle. So far, the gain (mu) testing is looking very good for this batch. I seem to be getting quite a few good matches with fairly high "in-circuit" gain levels.
Spent all day today testing tubes. From the data, it looks like we will have more than enough good matches for everyone that wants a pair. Tomorrow I'll group them into pairs and give them a listen for sound quality and microphonics. Will also update the spreadsheet and sort the 34 tubes by gain (mu) and transconductance (gm).
I finished updating the spreadsheets and have attached them. The larger sheet shows all 46 tubes and the smaller one shows just the 34 tubes included in the matching process, sorted on tube gain (mu). As you can see, I was able to come up with 10 very good matched sets. These matches are based predominately on "In-circuit" gain (mu), not transconductance. Based on everything I could read on the subject, the transconductance measurement is not very useful for matching preamp tubes. My experience on this project bears that out. Transconductance (or GM) is useful for matching power amplifier output tubes only. In a preamp, a tube with a higher GM will result in a slightly lower output impedance but this will have little effect on the overall voltage gain. I ran frequency response spot checks on each matched set to verify this. The response is ruler flat out to 20kHz for all tubes, regardless of the GM measurement. So each pair is first gain matched to within about 1 to 2% and then grouped by GM. Tubes with low gain or high pin-to-pin leakage problems were skipped over.
I'm showing the following on my list for matched sets:
Kornbread, Spent some time today listening to music on my bench speakers, swapping out the ten matched tube sets. I'm feeding an old galaxy phone into my 6J1 preamp, then to a Lepai 2020 integrated amp. Speakers are a small pair of DC28F tweeter/Radio Shack 40-1022A two way sealed units with limited bass output. Stereo balance seems very good on all matched pairs to my old ears (confirmation bias?). I also put some unmatched tubes in for comparison and I could clearly hear the stereo balance shift one way or the other.
In terms of other sound quality aspects, the first batch of 10 tubes that you ordered seem to have a very detailed sound quality, compared to the 2nd batch of 24 tubes which seem more laid back and smooth sounding. The difference, however, is very subtle and my mind might be inventing a difference because I am expecting to hear a difference.
In terms of microphonics, all 34 tubes are very well controlled when I tap on them lightly with both the Lepai volume control and 6J1 preamp volume control turned all the way up. The first batch of 10 tubes, however, do not have as much internal shielding. When I bring my index finger close to the glass envelope it creates a low level hum type buzzing sound through the speakers.
The 2nd batch of 24 tubes have much better internal shielding and do not hum or buzz, at full volume, when I bring my index finger close to the glass envelope. The sound is also a little smoother and easy to listen to (confirmation bias?).
When I look closely at the tubes with a magnifier, I can tell that the first batch of 10 tubes came from the same manufacturing run. They all have the two glass indentations on the sides, the same internal element structure, and the OTK stamp. The 2nd batch of 24 tubes, however, are from at least 3 different manufacturing runs with slightly different internal elements. This may have something to do with why these tubes have better shielding from external magnetic fields. Some of them have the OTK stamp, some of them do not (or it may be worn off). None of the 2nd batch of 24 tubes have the indentations on the sides of the glass envelope.
Thanks Bill for all the work you have done! I have decided that these will sound great and better than the first batch could have... So if you had confirmation bias, my confirmation bias confirms yours and I don't even have my tubes yet!
I just love the brotherly effort and contributions of those here on this site and support all of you with positive feedback and $$$ where applicable however small that may seem.
Keep going guys!
I am introducing new people to this place as they come into my life and they will join us as they feel motivated.
In terms of other sound quality aspects, the first batch of 10 tubes that you ordered seem to have a very detailed sound quality, compared to the 2nd batch of 24 tubes which seem more laid back and smooth sounding. The difference, however, is very subtle and my mind might be inventing a difference because I am expecting to hear a difference.
In terms of microphonics, all 34 tubes are very well controlled when I tap on them lightly with both the Lepai volume control and 6J1 preamp volume control turned all the way up. The first batch of 10 tubes, however, do not have as much internal shielding. When I bring my index finger close to the glass envelope it creates a low level hum type buzzing sound through the speakers.
The 2nd batch of 24 tubes have much better internal shielding and do not hum or buzz, at full volume, when I bring my index finger close to the glass envelope. The sound is also a little smoother and easy to listen to (confirmation bias?).
When I look closely at the tubes with a magnifier, I can tell that the first batch of 10 tubes came from the same manufacturing run. They all have the two glass indentations on the sides, the same internal element structure, and the OTK stamp. The 2nd batch of 24 tubes, however, are from at least 3 different manufacturing runs with slightly different internal elements. This may have something to do with why these tubes have better shielding from external magnetic fields. Some of them have the OTK stamp, some of them do not (or it may be worn off). None of the 2nd batch of 24 tubes have the indentations on the sides of the glass envelope.
Never unwrapped the tubes in the second order before passing them along. Looking at the ebay add copy, there is no otk or rocket stamp evident in the pics, but there does appear to be an indention along the sides. While I thought they were the rockets, apparently, they are not. The first order was for 6j1p-ev, the second order was 6j1p. Possible the 'ev' designates 'rockets'? Now I wonder if they are even Voskods as add copy clearly shows indentions in the glass. Sent seller a question regarding this discrepancy.
Double (triple?) boxing the tubes and getting ready for the mail out. Will try to get them in the mail by early next week. I found an on-line card box creation template that makes nice, little 1x1x2.5" vacuum tube boxes out of old 20 mil thick frozen pizza boxes (Chicago Home Run, Digiorno, etc.). After printing, I just crease and fold along the lines so that the pre-printed pizza label is facing inwards and the unprinted side is facing out. Unless you look inside, you would never know how it was made. Also found a local source for some really cheap little metal boxes that are perfect for holding a matched pair of tubes.
Kudos to @4thtry for going above and beyond. Sent these tubes to him in the same plain cardboard box the seller sent to me from Ukraine. The tubes were in there rolling around with only a single layer of bubble wrap for protection. Don't know how they didn't get destroyed on the trip across the pond. Anyhow, added some much needed padding and off they went to 4thtry.
Look how spiffy those little buggers look now that Bill got ahold of em.
Thanks guys. I'll pass out the remaining matched pairs at InDIYana. Everyone will also get a set of tube dampers that can be used to test for microphonics.
I assembled the high voltage kit a week or so ago and confirmed it passes sound. But I think the chinese tubes hold it back. I didn't mess with it past that yet.. Except I plugged it in last night when I got home and forgot about it until morning. The heatsink on the heater supply regulator was barely luke warm. So, with the 6vac coming from that transformer, I bet the heatsink is pretty safe to leave out if you don't want it. Also the kits don't come with output caps. It appears to spec 2.2uF, but larger may be prudent to allow more bass through? I did test it with 2.5uF temporarily before the 2.2uF caps shipped in. There is plenty of space on the board to accomodate. But be careful when hooking this up to other equipment since there are no input caps. And I don't see any bleeder resistor for the power supply cap/s so the spicy voltage could stick around for quite a while.
Comments
Interesting. Failure to put a cap on the input is significant. If there is any DC offset coming from the previous device (phone output, phono preamp output, etc.), then that offset will upset the biasing of the 6J1 tube. This is one of the reasons that old tube amplifiers with leaky coupling caps often have burned out tubes and transformer primaries. Over biased tubes due to DC leaking through the coupling caps.
Searching around a bit for schematics it seems not too uncommon to find the first stage tube on power amps to not have an input cap? IDK, I'm still a toob noob.
With the single ended topology that we are talking about here, the cap has to be there somewhere. Either at the output of the previous stage or the input of the first stage. Most other devices would probably have a coupling cap at the output, which is probably why they skipped the input cap.
As you suggested, and from what I can tell from looking at the board, it looks like they are using a regulated, single rail supply. Those look like 130V and 120V zener diodes connected in series, so my guess is a tightly regulated 250vdc supply. The tube filaments are connected in parallel and given a tightly regulated 6VDC. This is a big improvement over the other board, which connects the two filaments in series with half wave rectification and little filtering. This not only results in a very uneven filament voltage division between the two tubes, but passes alot of ripple onto the ground traces. The overall grounding scheme appears to be alot better as well. It is not a true star grounding scheme, but as long as B+ and filament ground return currents do not flow through the signal input ground return traces, you should be OK.
Got the 2nd batch of 24 tubes from Kornbread in the mail today. Thanks, Kornbread! Well packed and no damage. I'll start matching them up based on transconductance (gm), gain (mu), and microphonics. Kornbread also sent along a set of tube dampers. So I'll send everyone on the list a set of dampers to go with their matched tubes.
Never unwrapped any of the tubes and added bunches more padding before shipping them your direction. Kind of surprised there were no broken tubes from the trip across the pond as there was plenty of room in the box for them to rattle around.
I have found that tubes are tough little buggers! I few of those in the picture have some pretty dark looking pins. I wonder if that's corrosion and if a little rubbing with a Scotchbrite pad would brighten them up.
Ya, the pins on some of them look pretty dark. I'm carefully cleaning the pins on each tube to brighten them up a bit before each test. Then let them burn in for about 10 minutes or so to let the parameters settle. So far, the gain (mu) testing is looking very good for this batch. I seem to be getting quite a few good matches with fairly high "in-circuit" gain levels.
Spent all day today testing tubes. From the data, it looks like we will have more than enough good matches for everyone that wants a pair. Tomorrow I'll group them into pairs and give them a listen for sound quality and microphonics. Will also update the spreadsheet and sort the 34 tubes by gain (mu) and transconductance (gm).
That's a lot of work. Thanks Bill!!
Thanks Bill, Kornbread - lots of shiny tubes!!!
Looks like you got the tube pins all cleaned up nicely!
Bill is doing all the work. I just hit the buy button on some cheap tubes.
I finished updating the spreadsheets and have attached them. The larger sheet shows all 46 tubes and the smaller one shows just the 34 tubes included in the matching process, sorted on tube gain (mu). As you can see, I was able to come up with 10 very good matched sets. These matches are based predominately on "In-circuit" gain (mu), not transconductance. Based on everything I could read on the subject, the transconductance measurement is not very useful for matching preamp tubes. My experience on this project bears that out. Transconductance (or GM) is useful for matching power amplifier output tubes only. In a preamp, a tube with a higher GM will result in a slightly lower output impedance but this will have little effect on the overall voltage gain. I ran frequency response spot checks on each matched set to verify this. The response is ruler flat out to 20kHz for all tubes, regardless of the GM measurement. So each pair is first gain matched to within about 1 to 2% and then grouped by GM. Tubes with low gain or high pin-to-pin leakage problems were skipped over.
I'm showing the following on my list for matched sets:
1) @ani_101
2) @a4eaudio
3) @Tom_S
4) @Kornbread
5) @jhollander
6) @DrewsBrews
7) @4thtry (myself)
Let me know if I missed anyone.
Kornbread, Spent some time today listening to music on my bench speakers, swapping out the ten matched tube sets. I'm feeding an old galaxy phone into my 6J1 preamp, then to a Lepai 2020 integrated amp. Speakers are a small pair of DC28F tweeter/Radio Shack 40-1022A two way sealed units with limited bass output. Stereo balance seems very good on all matched pairs to my old ears (confirmation bias?). I also put some unmatched tubes in for comparison and I could clearly hear the stereo balance shift one way or the other.
In terms of other sound quality aspects, the first batch of 10 tubes that you ordered seem to have a very detailed sound quality, compared to the 2nd batch of 24 tubes which seem more laid back and smooth sounding. The difference, however, is very subtle and my mind might be inventing a difference because I am expecting to hear a difference.
In terms of microphonics, all 34 tubes are very well controlled when I tap on them lightly with both the Lepai volume control and 6J1 preamp volume control turned all the way up. The first batch of 10 tubes, however, do not have as much internal shielding. When I bring my index finger close to the glass envelope it creates a low level hum type buzzing sound through the speakers.
The 2nd batch of 24 tubes have much better internal shielding and do not hum or buzz, at full volume, when I bring my index finger close to the glass envelope. The sound is also a little smoother and easy to listen to (confirmation bias?).
When I look closely at the tubes with a magnifier, I can tell that the first batch of 10 tubes came from the same manufacturing run. They all have the two glass indentations on the sides, the same internal element structure, and the OTK stamp. The 2nd batch of 24 tubes, however, are from at least 3 different manufacturing runs with slightly different internal elements. This may have something to do with why these tubes have better shielding from external magnetic fields. Some of them have the OTK stamp, some of them do not (or it may be worn off). None of the 2nd batch of 24 tubes have the indentations on the sides of the glass envelope.
Thanks Bill for all the work you have done! I have decided that these will sound great and better than the first batch could have... So if you had confirmation bias, my confirmation bias confirms yours and I don't even have my tubes yet!
I just love the brotherly effort and contributions of those here on this site and support all of you with positive feedback and $$$ where applicable however small that may seem.
Keep going guys!
I am introducing new people to this place as they come into my life and they will join us as they feel motivated.
Never unwrapped the tubes in the second order before passing them along. Looking at the ebay add copy, there is no otk or rocket stamp evident in the pics, but there does appear to be an indention along the sides. While I thought they were the rockets, apparently, they are not. The first order was for 6j1p-ev, the second order was 6j1p. Possible the 'ev' designates 'rockets'? Now I wonder if they are even Voskods as add copy clearly shows indentions in the glass. Sent seller a question regarding this discrepancy.
xx
I was peeking around at other tube models. it seems ev versions are potentially military longer life spec?
Double (triple?) boxing the tubes and getting ready for the mail out. Will try to get them in the mail by early next week. I found an on-line card box creation template that makes nice, little 1x1x2.5" vacuum tube boxes out of old 20 mil thick frozen pizza boxes (Chicago Home Run, Digiorno, etc.). After printing, I just crease and fold along the lines so that the pre-printed pizza label is facing inwards and the unprinted side is facing out. Unless you look inside, you would never know how it was made. Also found a local source for some really cheap little metal boxes that are perfect for holding a matched pair of tubes.
Using recycled material is a great idea. Thanks Bill!
Seller states I was sent the tubes in the picture, which, lacking the glass indentions, are obviously not the same as the ones in the picture.
@4thtry you've went above and beyond. A huge thank you goes your way.
If there are any unclaimed matched tubes, I'd like to try a pair.
No problem, John. There are quite a few matched sets to go around. Many thanks go out to Kornbread for re-ordering so many tubes for me to pick from.
@4thtry Bill, I didn't check my messages till today - I can collect them from you at Indy.
Got matched pairs back.
Kudos to @4thtry for going above and beyond. Sent these tubes to him in the same plain cardboard box the seller sent to me from Ukraine. The tubes were in there rolling around with only a single layer of bubble wrap for protection. Don't know how they didn't get destroyed on the trip across the pond. Anyhow, added some much needed padding and off they went to 4thtry.
Look how spiffy those little buggers look now that Bill got ahold of em.
You da man Bill!
Best Packing Job Ever!
Golly Wally, you got a pair of tube condoms.
Everybody, let me know what you think of the sound of these tubes, and if the dampers make any difference.
Thanks guys. I'll pass out the remaining matched pairs at InDIYana. Everyone will also get a set of tube dampers that can be used to test for microphonics.
Thanks again guys!
I assembled the high voltage kit a week or so ago and confirmed it passes sound. But I think the chinese tubes hold it back. I didn't mess with it past that yet.. Except I plugged it in last night when I got home and forgot about it until morning. The heatsink on the heater supply regulator was barely luke warm. So, with the 6vac coming from that transformer, I bet the heatsink is pretty safe to leave out if you don't want it. Also the kits don't come with output caps. It appears to spec 2.2uF, but larger may be prudent to allow more bass through? I did test it with 2.5uF temporarily before the 2.2uF caps shipped in. There is plenty of space on the board to accomodate. But be careful when hooking this up to other equipment since there are no input caps. And I don't see any bleeder resistor for the power supply cap/s so the spicy voltage could stick around for quite a while.