Greg,
Large bottles wear out like lightglobes and need expensive, heavy transformers. Have you considered little bottles for your voltage amplifying duties like 6SN7/6SL7?
Then you can connect the voltage gain stage to a power buffer, using SS in Class AB. Then you could have power with glory!!
I have some circuit, email me if you are interested.......
Hugh
Large bottles wear out like lightglobes and need expensive, heavy transformers. Have you considered little bottles for your voltage amplifying duties like 6SN7/6SL7?
Then you can connect the voltage gain stage to a power buffer, using SS in Class AB. Then you could have power with glory!!
I have some circuit, email me if you are interested.......
Hugh
Bit of activity on this thread overnight!
Thank you for sharing the link, that was very useful in understanding tube ratings - see my further comments below the next reply.
Also very interesting comments on the EL506!
I have to admit I got this idea from Morgan Jones' book who seems to advocate running SET amps at the absolute maximum anode dissipation.
Let me say I am not planning to run NOS tubes ever...I would rather those be kept in a museum. This leaves me with relatively cheap new production tubes, which I was planning on replacing every few years. I tried to get some understanding of how tube life relates to the ratings, and while I haven't been able to find anything on the KT88 I did find the below for the KT77. Based on 50% loss of power at 5000 hours I think I would have few years use before I got to, say, 90% power output, assuming 5hrs listening every weekend. So running close to the absolute maximum seems ok as long as mains voltage fluctuations are accounted for. What do you think?
EL506 & 6A3, do you have any insights into the correct procedure for sizing a screen grid resistor on a UL stage?
Good to hear from you. I always like your circuits, will send you an email shortly.
HI EL506,
Thank you for sharing the link, that was very useful in understanding tube ratings - see my further comments below the next reply.
Also very interesting comments on the EL506!
Hi 6A3,
I have to admit I got this idea from Morgan Jones' book who seems to advocate running SET amps at the absolute maximum anode dissipation.
Let me say I am not planning to run NOS tubes ever...I would rather those be kept in a museum. This leaves me with relatively cheap new production tubes, which I was planning on replacing every few years. I tried to get some understanding of how tube life relates to the ratings, and while I haven't been able to find anything on the KT88 I did find the below for the KT77. Based on 50% loss of power at 5000 hours I think I would have few years use before I got to, say, 90% power output, assuming 5hrs listening every weekend. So running close to the absolute maximum seems ok as long as mains voltage fluctuations are accounted for. What do you think?
I am intrigued by this idea. I really like the EL34, two of them would be brilliant....going to see what I can find out it. I don't suppose the glass audio article is available anywhere for download.
EL506 & 6A3, do you have any insights into the correct procedure for sizing a screen grid resistor on a UL stage?
Hi Hugh,
Good to hear from you. I always like your circuits, will send you an email shortly.
See attached - Audio Note no longer sells output transformers (parts?). An ECC81 with adjustment of cathode resistors and the B+ dropping resistor to the drivers works actually slightly better and ECC81 NOS is lots cheaper than NOS ECC83.
Regarding NOS versus new manufacture: depending on tube you might find that NOS has considerable longer life expectancy, sometimes up to four times. The number of hours is one measurement, many tubes' filaments are rated for 2000 times being switched on/off, in other words if you switch it on once per day then expect around 5 years. Filament burnout can be reduced by having a NTC resistor in series with the primary of the power transformer. If running a tube near their maximum operating voltage you may consider slow start B+ to avoid/minimise the chance of flash over. The KTxx tubes were designed for 5000 - 10000 hours yet many owners do not get much more than about 1500 hours out of them because the amplifiers have those tubes running beyond their ratings to get the maximum power out of them. Check out the specification for the KT66 (e.g. G.E.C.).
While large amplifiers may have a lot of maximum power many are running only at a level of less than 1 watt output (depending on sensitivity of speakers and the size of the room they are in). The larger the amplifier is, the more distortion is present at the mW levels. If you do not need to have this amount of bragging rights then you may use something smaller and have less future cost to replace tubes. There are only a handfull factories making tubes and the economics is not really there so it remains to be seen how long they'll keep operating. Not to mention the current uncertainties regarding continued supplies due to trade tensions.
I personally rather spend more money on better / more sensitive (large) speakers. Larger speakers invariably have better dynamics but that is another story.
This is all my personal opinion, YMMV.
Regarding NOS versus new manufacture: depending on tube you might find that NOS has considerable longer life expectancy, sometimes up to four times. The number of hours is one measurement, many tubes' filaments are rated for 2000 times being switched on/off, in other words if you switch it on once per day then expect around 5 years. Filament burnout can be reduced by having a NTC resistor in series with the primary of the power transformer. If running a tube near their maximum operating voltage you may consider slow start B+ to avoid/minimise the chance of flash over. The KTxx tubes were designed for 5000 - 10000 hours yet many owners do not get much more than about 1500 hours out of them because the amplifiers have those tubes running beyond their ratings to get the maximum power out of them. Check out the specification for the KT66 (e.g. G.E.C.).
While large amplifiers may have a lot of maximum power many are running only at a level of less than 1 watt output (depending on sensitivity of speakers and the size of the room they are in). The larger the amplifier is, the more distortion is present at the mW levels. If you do not need to have this amount of bragging rights then you may use something smaller and have less future cost to replace tubes. There are only a handfull factories making tubes and the economics is not really there so it remains to be seen how long they'll keep operating. Not to mention the current uncertainties regarding continued supplies due to trade tensions.
I personally rather spend more money on better / more sensitive (large) speakers. Larger speakers invariably have better dynamics but that is another story.
This is all my personal opinion, YMMV.
Attachments
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Thanks for sharing the highly respected audio note design EL506.. And all your comments about new vs NOS tubes make good sense.
I can't imagine the current tensions in the former USSR are going to help with new tube supplies either, given the recent closure of one of the Chinese factories.
My current EL34 amp has all the power I need, but one has to constantly be striving for the next design in this hobby to stay interested! Maybe I need to go for something small and sweet like the audio note design. Actually the 6BM8 is a favourite of mine, so a little singled ended or PP design could be fun....anyway, I digress.
One thing tubes have taught me is that within reason, absolute THD measurements barely matter when it comes to sonic enjoyment (provided it is dominantly H2&H3.. I have some SS designs I am going to revisit with that in mind.....
EL506,
Thanks for the detailed information!
I do like the 7591, I use them in push pull UL mode, and Triode Wired mode.
Thanks for the detailed information!
I do like the 7591, I use them in push pull UL mode, and Triode Wired mode.
GregH2,
Sorry, I don't think anyone has ever found that issue of Glass Audio on the web.
This is not the only way to run parallel tubes, but here is one method:
Start with a good design: single output tube single ended (or for push pull a good two output tube design).
Keep the same B+ voltage, but 2X the current capability.
Change the output transformer to 1/2 primary Impedance (example 5k change to 2.5k), 2X the DC current capability, 2X the laminations (you are going to run 2 times the power out).
Increase the driver current 2X, lower the driver output impedance (can parallel the driver tube, and use 1/2 the original plate load). Now that driver will have 2X the input capacitance, so be sure the signal source can drive that.
For parallel tubes, Either use individual self bias resistors with individual bypass caps; Or use individual adjustable fixed bias and a current sense resistor (you can measure the voltage Either across the self bias resistors, Or the current sense resistors, to make sure the individual tubes are drawing the same current.
When possible use very well-matched tubes.
I hope that helps.
Sorry, I don't think anyone has ever found that issue of Glass Audio on the web.
This is not the only way to run parallel tubes, but here is one method:
Start with a good design: single output tube single ended (or for push pull a good two output tube design).
Keep the same B+ voltage, but 2X the current capability.
Change the output transformer to 1/2 primary Impedance (example 5k change to 2.5k), 2X the DC current capability, 2X the laminations (you are going to run 2 times the power out).
Increase the driver current 2X, lower the driver output impedance (can parallel the driver tube, and use 1/2 the original plate load). Now that driver will have 2X the input capacitance, so be sure the signal source can drive that.
For parallel tubes, Either use individual self bias resistors with individual bypass caps; Or use individual adjustable fixed bias and a current sense resistor (you can measure the voltage Either across the self bias resistors, Or the current sense resistors, to make sure the individual tubes are drawing the same current.
When possible use very well-matched tubes.
I hope that helps.
Not necessarily - extended Class A deliberately uses different operating configurations but imho it will only be satisfactory in PP.
EL506,
Yes, there are many ways to do parallel single ended and parallel push pull.
As I said: "This is not the only way to run parallel tubes, but here is one method:"
Yes, there are many ways to do parallel single ended and parallel push pull.
As I said: "This is not the only way to run parallel tubes, but here is one method:"
Yep, thanks very much. No real surprises there at all. This has definitely got me thinking...