I´m getting second thoughts about using E280F as input tubes.
They are great tubes for sure, but the plate current they require will probably heat my power transformer a bit too much.
In triode mode I´m worried about the Miller capacitance causing trouble when used together with my passive preamp and wiring them (correctly) as pentodes means extra work and components*.
The chassis are already cut for two noval sockets and there´s no room for upgrading to octal input tubes so I´m thinking about triode wired 6BX6/EF80 with cascoded bipolar CCS´s as plate loads.
According to these curves they should do well with a CCS load and Ia>8mA:
http://www.tubes.mynetcologne.de/roehren/daten/ef80pentode_as_triode.pdf
*Go achieve proper pentode operation the G2 voltage must me constant WRT the cathode. With a non-decoupled cathode resistor the G2 voltage can no longer be referred to ground but to the individual cathode, hence the need for extra circuitry.
They are great tubes for sure, but the plate current they require will probably heat my power transformer a bit too much.
In triode mode I´m worried about the Miller capacitance causing trouble when used together with my passive preamp and wiring them (correctly) as pentodes means extra work and components*.
The chassis are already cut for two noval sockets and there´s no room for upgrading to octal input tubes so I´m thinking about triode wired 6BX6/EF80 with cascoded bipolar CCS´s as plate loads.
According to these curves they should do well with a CCS load and Ia>8mA:
http://www.tubes.mynetcologne.de/roehren/daten/ef80pentode_as_triode.pdf
*Go achieve proper pentode operation the G2 voltage must me constant WRT the cathode. With a non-decoupled cathode resistor the G2 voltage can no longer be referred to ground but to the individual cathode, hence the need for extra circuitry.
the_manta:
Thanks for the reply! They´re considered junk here too, I threw away ~50 of them a while ago (before I saw Tom´s triode curves...
)
Since they come almost for free it might be worth the effort to gather up a bunch and test them for microphony, I´m planning to use EF80 as input tubes in a pair of OTL monoblocks too
(For some very specific reasons that doesn´t belong in this thread).
Thanks for the warning about the microphony!
Thanks for the reply! They´re considered junk here too, I threw away ~50 of them a while ago (before I saw Tom´s triode curves...
)Since they come almost for free it might be worth the effort to gather up a bunch and test them for microphony, I´m planning to use EF80 as input tubes in a pair of OTL monoblocks too
(For some very specific reasons that doesn´t belong in this thread).
Thanks for the warning about the microphony!
Changing my mind for the N:th time:
Upon further inspection it seems to be room for a pair of octal sockets instead of the novals. This allows me to use a tube that I´ve read good very things about: 7193 / 2C22.
Somewhat similar to 6J5 but with top caps for both plate and grid
and deadly linear with a CCS plate load.
Upon further inspection it seems to be room for a pair of octal sockets instead of the novals. This allows me to use a tube that I´ve read good very things about: 7193 / 2C22.
Somewhat similar to 6J5 but with top caps for both plate and grid
and deadly linear with a CCS plate load.
I forgot to drill holes for the plate and grid connectors to the 7193 input tubes before I painted the chassis, so guess what: I changed my mind one more time!
Instead of two 7193s I used a single 6SN7 and left the other octal socket unused.
The amp is almost finished by now and I listened to it a couple of minutes ago, except for a (quiet) hissing sound there is nothing to complain about. No audiable hum.
For the moment I run the output tubes fairly cool, about 270V plate to cathode and 50mA. The original plan was to push them a bit harder (300V 60mA) but I got nervous and installed cathode bias resistors (390R + 1000uF) to drop some voltage and get a mixed bias arrangment. I will increase the plate current later on.
The next step is to finish the chassis to make it cat-safe, then I will take a closer look at that hissing sound and start playing around with different bias points.
Instead of two 7193s I used a single 6SN7 and left the other octal socket unused.
The amp is almost finished by now and I listened to it a couple of minutes ago, except for a (quiet) hissing sound there is nothing to complain about. No audiable hum.
For the moment I run the output tubes fairly cool, about 270V plate to cathode and 50mA. The original plan was to push them a bit harder (300V 60mA) but I got nervous and installed cathode bias resistors (390R + 1000uF) to drop some voltage and get a mixed bias arrangment. I will increase the plate current later on.
The next step is to finish the chassis to make it cat-safe, then I will take a closer look at that hissing sound and start playing around with different bias points.
Attachments
Wow, this thread turned out to be a great way to spend a Sunday morning. I'm just now in the process of gathering parts for a sweep amp and this is the best reading I've found so far. Tubelab has done an excellent job of documenting his experiments, and for that I thank him. The outputs I plan on using have a primary Z of ~6K and came from an old Fisher that claimed 35 watts per channel using 7591s (no UL taps). I went to my tube shed yesterday and gathered up a few 6AV5s, 6DQ6s and 6BG6Gs for experimenting. I'm working on this with a friend (who is way past me as far as tube circuit design) and we haven't actually settled on a driver yet. I was thinking something in the 6BK4/6CG7 family. I mostly just wanted to thank the original posters in this thread and see about any drivers that have been tried with good results. Thanks, Jay
jaymanaa said:
Given your OPT, your best bet here is to go with the 6BG6. This type is the octal version of the 807, and will definitely work with a 6K P-2-P load. It will also like a bit of local NFB to tame the open loop pentode harshness. Most of these horizontal deflection PAs like a much lower P-2-P load. I considered the 6DQ6 for a project, but that type gave a bit more power than I was looking for, and would definitely stress the power xfmr I had in ye olde junkbox, so I opted for a 6BQ6GTB design instead. Still, nearly 40W out, and it pushes that power xfmr to the limit. As for loads, the 6DQ6 came in at under 4K P-2-P, and the 6BQ6GTB likes 4K4 P-2-P.
Here, you lost me. The 6BK4 is a high voltage regulator that's designed to handle up to 60KV, and the characteristic doesn't begin until 5KV. It is definitely not a driver type, and is completely different from a 6CG7. As for grid drivers, I've used cathode followers of the 6SN7 and 6FQ7. The 6CG7 is comparable to the 6SN7, and the main difference between the 6FQ7 and the 6CG7 is that the latter type includes a shield between the triode sections that the 6SN7 and 6FQ7 lack. These will make excellent cathode followers for grid drive for any Class AB1 pentode.
^^^^
Follower duty isn't all that demanding, and the 6BK7 ought to do well there. Like a 6BQ7, it looks like it would also make a fine cascoded LTP as well.
The Pd= 2700mW means that getting enough current to drive the Cgk + Cmiller + Cstray at high frequencies should not be a problem either.
Follower duty isn't all that demanding, and the 6BK7 ought to do well there. Like a 6BQ7, it looks like it would also make a fine cascoded LTP as well.
The Pd= 2700mW means that getting enough current to drive the Cgk + Cmiller + Cstray at high frequencies should not be a problem either.
Hey JAY!
Interesting meeting you over here, I'll get with yu soon!
;
;
;
;
Anyway I had been contemplating building a 6AV5 amp for a while now, finally did it!
These tubes are very nice, (thanks for all your hard work Tubelab) mine sounds great!
Got a few cosmetic things to finish, like routing/painting the base and output trannie board.
Here's a couple pic's if anyone is interested.
Interesting meeting you over here, I'll get with yu soon!
;
;
;
;
Anyway I had been contemplating building a 6AV5 amp for a while now, finally did it!
These tubes are very nice, (thanks for all your hard work Tubelab) mine sounds great!
Got a few cosmetic things to finish, like routing/painting the base and output trannie board.
Here's a couple pic's if anyone is interested.
Attachments
amp lodown
Jay
I'm at work and don't have a copy of the circuit,
Shoot me an email and I'll send it to yu from home.
It's nothing to Fancy, at the moment no solid state parts and no Global Fbk.
The driver is the beefy/low mu triode of the 6FM7/6FJ7/6FY7 compactron tubes.
The Phase splitter is an Schmidt type LTP using the 6bk7/6bq7/6bz7 type tubes.
As of now I'm cathode biased using 2 resistors in series with the larger one
on the bottom cap bipassed and top smaller not, creating a bit of cathode fbk.
The red switch on the front turns on or off the cap bipass on the driver tube.
300v supply, Choke input running LC,LC to the Outputs then an old ST-70 choke
feeding the front end from there then RC for each triode section, it's a pair
of 5V4 rectifiers wired in paralell for the dc.
Plate to cathode voltage is about 250v (I'll convert to fixed bias later) and
52ma per tube, the outputs are from a Sansui 1000A at I believe about 6k P - P.
(Outputs are wired in triode, no ul taps on the Sansui trannies)
The output tubes are running fairly mild (I have plenty of the Sylvania's) but
it sounds wonderful, full spectrum from top to bottom powering some polk 10B's. (New Polk Tweet)
(Tried the amp on a few other sets of speakers and works just fine there as well)
I'm sure there's still some tweeking to be done, but it's rock solid and very quiet.
Overall I'm very happy with the way it turned out.
Later,
Jay
I'm at work and don't have a copy of the circuit,
Shoot me an email and I'll send it to yu from home.
It's nothing to Fancy, at the moment no solid state parts and no Global Fbk.
The driver is the beefy/low mu triode of the 6FM7/6FJ7/6FY7 compactron tubes.
The Phase splitter is an Schmidt type LTP using the 6bk7/6bq7/6bz7 type tubes.
As of now I'm cathode biased using 2 resistors in series with the larger one
on the bottom cap bipassed and top smaller not, creating a bit of cathode fbk.
The red switch on the front turns on or off the cap bipass on the driver tube.
300v supply, Choke input running LC,LC to the Outputs then an old ST-70 choke
feeding the front end from there then RC for each triode section, it's a pair
of 5V4 rectifiers wired in paralell for the dc.
Plate to cathode voltage is about 250v (I'll convert to fixed bias later) and
52ma per tube, the outputs are from a Sansui 1000A at I believe about 6k P - P.
(Outputs are wired in triode, no ul taps on the Sansui trannies)
The output tubes are running fairly mild (I have plenty of the Sylvania's) but
it sounds wonderful, full spectrum from top to bottom powering some polk 10B's. (New Polk Tweet)
(Tried the amp on a few other sets of speakers and works just fine there as well)
I'm sure there's still some tweeking to be done, but it's rock solid and very quiet.
Overall I'm very happy with the way it turned out.
Later,
Hi all, I thought I might get a few opinions here regarding output TXs for this project. I'm planning on a 12DQ6 power tube (PP) and 12SN7 driver. It just worked out that I have a lot of these so I thought I would put them to use. My only concern is that the lowest Z output TXs I have are a pair of Fishers that came out of a KX200 amplifier. They are rated at 35 watts (by Fisher) but the Z is 5600 ohms. Is that too high? and if so, is output power the only thing that will suffer, or could I expect other problems as well? I started out on this project with the plan of EVERY part being from my different "scrap" piles, but if need be I could definitely pony up for some outputs. I will re-read his post, but I think Miles addressed this issue a few posts ago, and suggested using the 6BG6with these outputs. The other part of the puzzle is the fact that I primarily use an all horn speaker system with a sensitivity of about 103 by my best guess. 2A3s are almost enough power for me, but 300Bs at 6 or 7 watts is better. Given this, I would think that 30, or even 20, would provide a lot of headroom overall. I appreciate everyone taking time to give me advice on this. I have lots to learn, and although I have read tons on tube circuits, it seems that I learn so much more by doing, and listening to folks in the know (like you fellas). Thanks, Jay
jaymanaa said:
That's not the problem. A 5K6 OPT means that each 12DQ6 will be loaded by:
5600 / 4= 1400R
The characteristic maxes out at 350mA (Class AB1 operation here) giving an output of:
Irms= .35 / sqrt(2)= 248mA
For an output of:
I= .248^2 * 1400= 86W
And a plate dissipation of:
(2 * .35) / pi= 223mA (ave)
This load will require a Vpp= 550Vdc:
Pdc= .223 * 550= 122.65W
Pd= (122.65 - 86) / 2= 18.33W / VT
The 12DQ6 can certainly handle that.
However, you will be way overdriving that OPT, and way underutilizing the finals. Given that you said that you have efficient speeks, and don't need more than 20 -- 30W out, the 12DQ6 is overkill.
Now you can see why I dropped this type from consideration in favour of the 6BQ6GTB. I didn't need some 80+ W either.
There should be no reason that you can't use a 12DQ6 with a 5.6K ohm load. Just go easier than 550 VDC for the B+ and the tubes will last a lifetime.
I still have several 6DQ6's, 12DQ6's and 17DQ6's. I would try them in triode mode with a B+ in the 250 to 300 volt range. If the screen grid doesn't like the voltage, I would then try pentode mode. I have not yet had the time to explore the upper limits of these tubes, some day.
When I was a teenager I used to make guitar amps from old TV sets. A trip to the trash dump would yield a months supply of parts for free. I learned to make tubes glow at a young age. Back then the 6BQ6 and the 6DQ6 were probably the most popular "free" sweep tubes. I made a lot of amplifiers with them.
I made SE guitar amps with 6DQ6's using the power transformer from the donor TV and the vertical OPT used as an audio OPT. I used a modified Fender Champ schematic which runs the output tube in pentode mode with the screen voltage about 10 volts less than the plate voltage. I can't remember what the B+ or operating points were back then, but I remember that the amp would just scream (a cool sounding distortion) with a 6BQ6 installed, and have a warm clean sound with the 6DQ6. I would guess that the B+ was in the 350 volt range, since that was about where the old black and white TV's of the early 60's ran.
I still have several 6DQ6's, 12DQ6's and 17DQ6's. I would try them in triode mode with a B+ in the 250 to 300 volt range. If the screen grid doesn't like the voltage, I would then try pentode mode. I have not yet had the time to explore the upper limits of these tubes, some day.
When I was a teenager I used to make guitar amps from old TV sets. A trip to the trash dump would yield a months supply of parts for free. I learned to make tubes glow at a young age. Back then the 6BQ6 and the 6DQ6 were probably the most popular "free" sweep tubes. I made a lot of amplifiers with them.
I made SE guitar amps with 6DQ6's using the power transformer from the donor TV and the vertical OPT used as an audio OPT. I used a modified Fender Champ schematic which runs the output tube in pentode mode with the screen voltage about 10 volts less than the plate voltage. I can't remember what the B+ or operating points were back then, but I remember that the amp would just scream (a cool sounding distortion) with a 6BQ6 installed, and have a warm clean sound with the 6DQ6. I would guess that the B+ was in the 350 volt range, since that was about where the old black and white TV's of the early 60's ran.
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