OK, my favorite "audio" tube is the KT88 or 6550. My choice for the best of those is the JAN GE 6550. They consistently post lower distortion that any other 6550 or KT88 that I have tried, but I haven't tried any of the super high $$$$ stuff. I got a quad of the GE's so long ago that they cost me like $20 or $25 each.
I "test" them here:
YouTube
Hi George,
When I moved to my house, I lost a brand new matched quad of Westinghouse 6550A tubes. Boxes looked perfect, each tube was cracked. I just threw them out last week actually. That after 17 years, the loss really hurt. My cost on those was probably on the order of $40 each when I bought them in the early 80's. They were being saved for a project that I scrapped after I lost the set.
I consider the 6550A and KT88 to be of the 6L6GC family.
-Chris
When I moved to my house, I lost a brand new matched quad of Westinghouse 6550A tubes. Boxes looked perfect, each tube was cracked. I just threw them out last week actually. That after 17 years, the loss really hurt. My cost on those was probably on the order of $40 each when I bought them in the early 80's. They were being saved for a project that I scrapped after I lost the set.
I consider the 6550A and KT88 to be of the 6L6GC family.
-Chris
Hi famousmockingbird,
Well, yes. It's a 42W plate monster with a long history. I'll probably use a pair for an 80 wpc amplifier at some point. More in keeping with the power levels an EL34 is used at, you could go with a 6L6GC having a little more power and lower distortion and a much lower heater current requirement (70% of what an EL-34 wants).
I don't get the "tube rolling" thing. A circuit is designed for a certain tube and that or it's equivalent should be used. So a 6L6GC has the 807 with a top cap for the plate, but it will run at high voltages compared to the 6L6GC. Or run with a beefier 7581A using a 35 watt plate. Same characteristics, longer life or higher power would be the trade-off there. Those tubes should sound the same as the 6L6GC unless you are taking that tube out of it's design center values (= a bad design in my book if intended for the 6L6GC).
I think a lot of people get confused by the MI trade magazines where they do change tube types to get a different sound. But that is an entirely different scenario than a home reproducer amplifier. They also tend to run small output transformers and higher plate voltages with low feedback so that the different tubes do sound, different. George could probably write volumes on that subject alone. But the important point is that what they are describing as far as the sounds of the different tubes do not carry over to what they tube will sound like in a well designed home audio amplifier. Completely different set of circumstances.
-Chris
Well, yes. It's a 42W plate monster with a long history. I'll probably use a pair for an 80 wpc amplifier at some point. More in keeping with the power levels an EL34 is used at, you could go with a 6L6GC having a little more power and lower distortion and a much lower heater current requirement (70% of what an EL-34 wants).
I don't get the "tube rolling" thing. A circuit is designed for a certain tube and that or it's equivalent should be used. So a 6L6GC has the 807 with a top cap for the plate, but it will run at high voltages compared to the 6L6GC. Or run with a beefier 7581A using a 35 watt plate. Same characteristics, longer life or higher power would be the trade-off there. Those tubes should sound the same as the 6L6GC unless you are taking that tube out of it's design center values (= a bad design in my book if intended for the 6L6GC).
I think a lot of people get confused by the MI trade magazines where they do change tube types to get a different sound. But that is an entirely different scenario than a home reproducer amplifier. They also tend to run small output transformers and higher plate voltages with low feedback so that the different tubes do sound, different. George could probably write volumes on that subject alone. But the important point is that what they are describing as far as the sounds of the different tubes do not carry over to what they tube will sound like in a well designed home audio amplifier. Completely different set of circumstances.
-Chris
Tube rolling can mean lots of things. I admit that I have been to sessions where several "golden eared audiophiles" gather around some expensive equipment and listen to different brands of the same tube and claim to be able to hear different sonic magic released by each one. DBT, whassthat......lets listen to the TFK 12AX7......Now lets listen to the Mullard 12AX7, can't you hear the crystal clear vocals on that record? Yeah, right.
I showed up with one of my TSE's and waited patiently while several different flavors of 300B's from cheap Chinese to engraved WE's were run through it and pretended to go along with the crowd. Then....
I had brought several different pairs of OPT's, each housed in identical cardboard boxes, to be connected into the set up with Radio Shack grade clip leads. I got a couple of remarks about how using clip leads connected up to $200 speaker wire would ruin the test, but I pressed on. We listened to 5 different sets of OPT's on a few different types of music. The same music was used for each OPT and the "ears" were asked to write down their impressions. There was general consensus about the best and worst OPT, and it AGREED with my measured test data, but the middle three was a toss up depending on the music played, and the musical preferences of the listeners.
I created a HiFi amp called the SSE. It runs a 12AT7 with a CCS load driving an output tube of choice, which is cathode biased. The amp can run in pentode, triode, or UL which can be jumper selected, but many people install a switch. Cathode feedback from the speaker output of the OPT can be use if desired, and again some builders install a switch. It can run just about any audio tube with the usual pinout, although B+ and load impedance can really be optimized of one type.
Most builders chose one of the 3 popular types already mentioned. About 600 boards have been sold in 13 years, and I have heard from quite a few builders over that time period. The most popular tube is the 6550 / KT88. Virtually nobody runs them in pentode mode, the rest are about evenly split between UL and triode. The EL34 is a close second, but most users prefer triode mode. The 6L6GC runs a distant third place again with about a 50/50 split between triode and UL.
Most people prefer the EL34 without CFB, but CFB is reasonably popular with the 6550 / KT88 and 6L6GC especially in UL. These results mimic my own personal choices, and my choices have been stated several times, so this may skew the data.
There are also lots of different things called 6L6GC today that bear no resemblance to the original RCA design. Rolling some or all of these tubes through an amp like this is a valid endeavor and will yield audible differences, since these are vastly different tubes, and all will work reasonably well in this amp.
Stuffing a bunch of different TV sweep tubes into Pete's original Engineers Amp was an experiment I ran with over a 6 month period. I also ran several non sweep tubes through the amp using an external screen voltage supply. Those results are scattered throughout the original thread, but is why we have a 6HJ5 version today. It also allowed me to find and document the limits of several different tubes. That data is still being used today.
Again, I use the term rolling since it is widely understood, but I am testing several different types in a new design to gather statistical data on how the design handles different tube types, and what the tube to tube variations are on several tubes of the same type.
Again there are lots of tubes called 6L6GC and some are very different from others, and the differences are physically large enough where they could affect the sound of the tubes in a well designed HiFi amp.
A 6L6GC should sound like a 6L6GC in a given guitar amp when operated well below clipping. All bets are off when the tubes are driven well into the switching region. That is where different tubes WILL make different sound. Two identical pairs from the same batch can sound different depending on how they are matched, but there is so much BS surrounding that issue, that I won't even try to go there.
Lets just say that I exploited this heavily in the early 80's by making some amps with dissimilar output tubes, one 6L6GC and one EL34. Tune the operating points where they are both relatively linear at low power, but the higher Gm of the EL34 will start to dominate as the volume is cranked, making a push pull amp sound a lot like an SE amp (lots of 2H). Hammer it harder and both tubes become switches, but with different "on resistances."
Hi George,
That a 6L6GC is an unknown entity these days, I stuck with either 6L6GC's when I had them. For new repairs and designs I greatly prefer the 6L6EH as they are very hard to kill and they sound good in any home amplifier designed for real 6L6GC tubes.
For my amplifiers that use EL34's, the first thing I do after rebuilding them is to install 6CA7 tubes instead. I think a beam power tube sounds better that a true pentode in most cases. I do like the 6BQ5.
Being used in consumer home amplifiers, keeping them in the linear operating range is a given. Run them into clipping and it isn't music anymore.
Keep doing what you're doing George. I learn a lot from you and it is always a pleasure to read your posts. Your web site is good too, but I haven't been there in a couple years now. I should pop in again.
-Chris
That a 6L6GC is an unknown entity these days, I stuck with either 6L6GC's when I had them. For new repairs and designs I greatly prefer the 6L6EH as they are very hard to kill and they sound good in any home amplifier designed for real 6L6GC tubes.
For my amplifiers that use EL34's, the first thing I do after rebuilding them is to install 6CA7 tubes instead. I think a beam power tube sounds better that a true pentode in most cases. I do like the 6BQ5.
Being used in consumer home amplifiers, keeping them in the linear operating range is a given. Run them into clipping and it isn't music anymore.
Keep doing what you're doing George. I learn a lot from you and it is always a pleasure to read your posts. Your web site is good too, but I haven't been there in a couple years now. I should pop in again.
-Chris
If I choose to use the 6LB6 (correct me if I´m wrong, but I believe the results will be very similar in terms of the 50 W and sound quality) on the first version of the board i ask your help in order to confirm if the correct connections are:
VT3
Pin 1 Socket: Heater - connect to (Pin 1) on the board
Pin 2 Socket: Cathode - connect to (Pin 2) on the board
Pin 3 Socket: Grid Number 2 - connect to (Pin 5) on the board
Pin 4 Socket: Beam Plates - connect to (Pin 4) on the board
Pin 5 Socket: Grid Number 1 - connect to (Pin 3) on the board
Pin 6 Socket: No Connection - Not connect to the board
Pin 7 Socket: Internal Connection - Not connect to the board
Pin 8 Socket: No Connection - Not connect to the board.
Pin 9 Socket: Internal Connection - Not connect to the board
Pin 10 Socket: Beam Plates - connect to (Pin 10) on the board
Pin 11 Socket: Internal Connection.
Pin 12 Socket: Heater - connect to (Pin 12) on the board
Cap: Plate - connect to (Pin 7) on the board
Pin 11 on the board connect to Pin 3 on the board
Pin 5 on the board connect to Pin 9 on the board
In relation to the other componentes/ values, I will not have to change anything else correct?
best regards and thanks for your help
Caetano
VT3
Pin 1 Socket: Heater - connect to (Pin 1) on the board
Pin 2 Socket: Cathode - connect to (Pin 2) on the board
Pin 3 Socket: Grid Number 2 - connect to (Pin 5) on the board
Pin 4 Socket: Beam Plates - connect to (Pin 4) on the board
Pin 5 Socket: Grid Number 1 - connect to (Pin 3) on the board
Pin 6 Socket: No Connection - Not connect to the board
Pin 7 Socket: Internal Connection - Not connect to the board
Pin 8 Socket: No Connection - Not connect to the board.
Pin 9 Socket: Internal Connection - Not connect to the board
Pin 10 Socket: Beam Plates - connect to (Pin 10) on the board
Pin 11 Socket: Internal Connection.
Pin 12 Socket: Heater - connect to (Pin 12) on the board
Cap: Plate - connect to (Pin 7) on the board
Pin 11 on the board connect to Pin 3 on the board
Pin 5 on the board connect to Pin 9 on the board
In relation to the other componentes/ values, I will not have to change anything else correct?
best regards and thanks for your help
Caetano
Your pin out data checks correctly.
I have a Sylvania 6LB6 here, so I looked at the "IC" pins to see what goes on there, and pin 9 is also grid1, and pin 11 is also grid2. (still flipped from the 6HJ5) No guarantee that other tube brands would be the same however.
It can be useful to put a short length of stiff copper wire on those internally used "IC" pins to help cool the internals. The 6HJ5 datasheet recommends connecting all the used pins to lead wires for additional cooling of grids. But don't overdo the 6LB6 tube pin 9 (grid 1) as it could pick up hum.
There will of course be some grid biasing difference between 6HJ5 and 6LB6, but that is adjustable.
Many other TV Sweeps will fit the 6LB6 wired socket too.
You could also just flip (crossed) the connections of resistors R22 and R30 for tube VT4, and resistors R23 and R29 for tube VT3. No foil cutting required then.
I have a Sylvania 6LB6 here, so I looked at the "IC" pins to see what goes on there, and pin 9 is also grid1, and pin 11 is also grid2. (still flipped from the 6HJ5) No guarantee that other tube brands would be the same however.
It can be useful to put a short length of stiff copper wire on those internally used "IC" pins to help cool the internals. The 6HJ5 datasheet recommends connecting all the used pins to lead wires for additional cooling of grids. But don't overdo the 6LB6 tube pin 9 (grid 1) as it could pick up hum.
There will of course be some grid biasing difference between 6HJ5 and 6LB6, but that is adjustable.
Many other TV Sweeps will fit the 6LB6 wired socket too.
You could also just flip (crossed) the connections of resistors R22 and R30 for tube VT4, and resistors R23 and R29 for tube VT3. No foil cutting required then.
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Thanks a lot for the help.
I just got the pcb's from Pete and as before everything went perfectly.
In the coming 2 weeks I will be on vacation and I will take some time to decide which valves I will use, if the 6LB6 (easy to find but very expensive) or the 6DQ5 that until now i also couldn´t find a supplier with available stock.
In any case I´m thinking of using the sockt's atached to the chassis in order to be able to connect the different pins to the board.
If I can buy the 6DQ5, I will ask you again to see if the connections are correct (even more complex for me than the 6LB6)
..
Caetano
I just got the pcb's from Pete and as before everything went perfectly.
In the coming 2 weeks I will be on vacation and I will take some time to decide which valves I will use, if the 6LB6 (easy to find but very expensive) or the 6DQ5 that until now i also couldn´t find a supplier with available stock.
In any case I´m thinking of using the sockt's atached to the chassis in order to be able to connect the different pins to the board.
If I can buy the 6DQ5, I will ask you again to see if the connections are correct (even more complex for me than the 6LB6)
..Caetano
The problem is exactly to find these tubes in europe.
The final choice will probably be the 6DQ5 (Vacuumtubes.net where I bought tubes a few times and everything went very well) if I can use the pcb's without foil cutting (is it possible ?), as in the case of 6LB6 where I can bolt the socket´s to the chassis and make the connections to the board.
Caetano
The final choice will probably be the 6DQ5 (Vacuumtubes.net where I bought tubes a few times and everything went very well) if I can use the pcb's without foil cutting (is it possible ?), as in the case of 6LB6 where I can bolt the socket´s to the chassis and make the connections to the board.
Caetano
My build isn't staying biased at 400mv. I really don't want to bother Pete with this again because he's already been really generous with time. There is a slow upward drift on the bias which eventually results in the 500ma at F1 fuses blowing. This has been happening for a while, I thought it had stabilised, but I realise it was because I was doing shorter listening sessions, this drift only really happens after 4-5 hours when the amps are nice and hot.
I have tried the following things:
1. Replaced R13 & 14 with 47k resistors.
2. Replaced tubes multiple times, actually have enough to get the 6HJ5 pretty much matched without any biasing, so I did this. Also switched out the 6GU5.
3. Replaced C5 & C6, they're currently 0.22uf 550v (50 Volt under spec).
4. Put a 100 ohm resistor across output, also tried a 33 ohm 0.1uf Zobel.
4. Disconnected NFB.
5. Tried different pre-amps.
I'm driving a set of Paul Carmody Sunflower Redux. Sunflower [Redux] - undefinition
I live in a country where our currency sucks, so I had to get the power transformers locally, according to the exact Hammond spec. I trust the guy who produces them, he's made plenty of iron for me and others on our local forum. B+ measures at 430V, which is a obviously a bit high, but I don't think it's drastic.
I'm also using Edcor CXPP60-4.2K for the OPT, again, to keep costs down, but this is obviously out of spec of the design.
Any thoughts would be appreciated.
I have tried the following things:
1. Replaced R13 & 14 with 47k resistors.
2. Replaced tubes multiple times, actually have enough to get the 6HJ5 pretty much matched without any biasing, so I did this. Also switched out the 6GU5.
3. Replaced C5 & C6, they're currently 0.22uf 550v (50 Volt under spec).
4. Put a 100 ohm resistor across output, also tried a 33 ohm 0.1uf Zobel.
4. Disconnected NFB.
5. Tried different pre-amps.
I'm driving a set of Paul Carmody Sunflower Redux. Sunflower [Redux] - undefinition
I live in a country where our currency sucks, so I had to get the power transformers locally, according to the exact Hammond spec. I trust the guy who produces them, he's made plenty of iron for me and others on our local forum. B+ measures at 430V, which is a obviously a bit high, but I don't think it's drastic.
I'm also using Edcor CXPP60-4.2K for the OPT, again, to keep costs down, but this is obviously out of spec of the design.
Any thoughts would be appreciated.
I'm experiencing the same thing even after replacing R13/14 with 47K. The drift isn't as fast after replacement, but it still happens. I've just accepted it and keep an eye on the bias voltage to make sure it doesn't get too high. Pete suggested 55ma bias current (schematic says 50ma). I set it ~53ma, and have seen it creep up to 58ma after 6-9 hours of continuous use. I get nervous and turn them off above 60ma.
I've pretty much pinned it down to it being heat related. They will drift higher/faster on warmer days and will behave nicely on milder days. Having cross ventilation from a fan over the amps totally eliminates the issue and bias stays rock solid. I'm considering installing 120mm computer fans in the chassis bottom and drilling ventilation holes around the tube sockets. I have the parts on hand, but now that the weather is getting milder, this may wait
Regarding your OPT, you'll need to optimize the NFB cap C1 by feeding a 1khz square wave and adjust C1 until square wave is just underdamped. I believe this is how Pete explained it to me, lost that email though. The point is you don't want any ringing and you adjust the cap accordingly.
I've pretty much pinned it down to it being heat related. They will drift higher/faster on warmer days and will behave nicely on milder days. Having cross ventilation from a fan over the amps totally eliminates the issue and bias stays rock solid. I'm considering installing 120mm computer fans in the chassis bottom and drilling ventilation holes around the tube sockets. I have the parts on hand, but now that the weather is getting milder, this may wait
Regarding your OPT, you'll need to optimize the NFB cap C1 by feeding a 1khz square wave and adjust C1 until square wave is just underdamped. I believe this is how Pete explained it to me, lost that email though. The point is you don't want any ringing and you adjust the cap accordingly.
Thanks for the response and the tip on C1, I will check it out this next weekend.
I was also thinking heat could be an issue, and I did a check of components, with both an IR gun and a thermocouple probe. I need to run the amps for a good 4-5 hours and then do the checks again, U1 got up to around 60* Celsius quite quickly, I might strap a larger heatsink to it. I'm also thinking of raising the Power transformer a couple of mm to get some more airflow.
I was also thinking heat could be an issue, and I did a check of components, with both an IR gun and a thermocouple probe. I need to run the amps for a good 4-5 hours and then do the checks again, U1 got up to around 60* Celsius quite quickly, I might strap a larger heatsink to it. I'm also thinking of raising the Power transformer a couple of mm to get some more airflow.
So I have a couple hundred hours on my set now. I have not experienced the bias drift that others have noticed. Per Pete's suggestion, I did leave out C1 all together. So far the units have been super stable. They are driving a set of Klipsch Chorus speakers at mid to high volume with no issues. I did opt to remove the bottom cover from the chassis for better air flow. Lovin em so far.