Hi, just registered here. Some background - I'm retired, worked in the microcomputer field most of my professional life, now returning to the magic of tubes. I build a fair amount of simple ham equipment with tubes, and recently I built a stereo amp with a pair of 6JE8s driving (do I dare type it?) Radio Shack 70V line transformers as output transformers. It all sounds much better than it should to my retired ears. Now I'm thinking about how to improve that amplifier, and what I might build next using the tube stash I have, which includes 5670 and 5965 dual triodes, lots of triode-pentode types, 5763 and 6CL6 pentodes (OK, the 5763 is a beam power tube), and a batch of 7C5s. Also various dissimilar pentode compactrons, and lots of other tubes, most of which are intended more for RF use. All I have going for me is a fair understanding how how tubes work, and a very nice scope. 🙂 I am pretty determined to just have fun with the tubes I have, but clearly I have to spend some money on OP transformers at a minimum. My question is, is such bottom-end equipment a suitable topic of discussion here? Thanks!
IIRC, type 5763 is from the 6AQ5 (6V6-ish) family, but spec'ed for mobile transmitters. You could make a nice push-pull audio amplifier with a pair per channel - plenty of circuits floating around for 6V6's that'll work great. 6CL6 is overkill for drivers, but "nothing exceeds like excess".
All good fortune,
Chris
All good fortune,
Chris
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They are meant for signal at no DC such as the output tube plate current.
And the load presented to the output tube is much too low.
A proper output transformer in each channel will result in a big improvement to your amplifier
The 7C5 makes quite a good output tube, any circuit with a 6V6GT will work. There are many.
The Loctal socket could be a problem, but someone here will know a good source.
Best of luck in your retirement, John L Stewart. 🙂
On a technical level audio line transformers are not a good match for most any audio tube.
They are meant for signal at no DC such as the output tube plate current.
And the load presented to the output tube is much too low.
A proper output transformer in each channel will result in a big improvement to your amplifier
The 7C5 makes quite a good output tube, any circuit with a 6V6GT will work. There are many.
The Loctal socket could be a problem, but someone here will know a good source.
Best of luck in your retirement, John L Stewart. 🙂
Yes, I know the line transformer is the worst possible choice, but I wanted to see what could be done with them, and I had them. I have them wired up to present an 8k load, the highest that they will go, and the current through them is only 20mA. But certainly I will want to track down and try better transformers.
(This also brings up the question of deciding on a proper transformer load value for tubes not rated for audio use. I will have to explore that question.)
As to loctal sockets, I do have them, from when I was making 7C5 one-tube transmitters. 7 Watts out and a sweet, pure note. 🙂
(This also brings up the question of deciding on a proper transformer load value for tubes not rated for audio use. I will have to explore that question.)
As to loctal sockets, I do have them, from when I was making 7C5 one-tube transmitters. 7 Watts out and a sweet, pure note. 🙂
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Everybody is welcome! A nice pair of output transformers (OPTs) makes a big difference, and then start combining your tubes in all sort of topologies to drive the OPTs for nice music!
Brimar actually published a 5763 data sheet with a lot of audio application numbers, which gives me confidence that I can hope for some success with them.
Can you bang up a B+ supply of 250 to 300 VDC at about 250 mA or so? Plus some 6.3VAC at 5 or 6 amps? If so, all you need are two output transformers with maybe 8K or 10K Ohm primaries, a chassis, and some small parts. Rock 'n roll.
All good fortune,
Chris
All good fortune,
Chris
Mike, welcome.
Some folks are using inexpensive AC line toroids as output transformers. Check some of the threads where Lingwendel and Kodabmx have chimed in. If you are in the USA a good inexpensive source for trannies, both power and opt is Antek. See example below.
Best, Steve
https://www.antekinc.com/mp-10w80-audio-output-transformer/
Some folks are using inexpensive AC line toroids as output transformers. Check some of the threads where Lingwendel and Kodabmx have chimed in. If you are in the USA a good inexpensive source for trannies, both power and opt is Antek. See example below.
Best, Steve
https://www.antekinc.com/mp-10w80-audio-output-transformer/
When I first started playing with tubes I wired up an old power transformer from an oscilloscope as an output with some push pull 6BQ5s and it worked pretty well. Made me wish I had a second one to build a permanent stereo amp.
Hah, that's pretty adventurous! So you connected the speaker to the heater winding?
Here is a simple 3W amp that would work with your 7C5s.
Built this one in 1968, its still running here in the workshop.
For an output transformer I used a Hammond 125D Universal I had in my stash. 🙂
If I built this today I would use a Hammond 125CSE, made for singled ended use.
No toobz visible as many folks like to see now.
But this build is all in one, power supply, power amp,
Baxandall tone circuits & low level front end for magnetic phono or microphone.
The program sources are Eico HFT90 FM, Eico HFT94 AM & Nexxtech CD player.
Built this one in 1968, its still running here in the workshop.
For an output transformer I used a Hammond 125D Universal I had in my stash. 🙂
If I built this today I would use a Hammond 125CSE, made for singled ended use.
No toobz visible as many folks like to see now.
But this build is all in one, power supply, power amp,
Baxandall tone circuits & low level front end for magnetic phono or microphone.
The program sources are Eico HFT90 FM, Eico HFT94 AM & Nexxtech CD player.
Attachments
TV Vertical Sweep tubes like 6V6, 6CM6, 6AQ5, 6W6 (and similar 12L6, 25L6, 50L6, 6DB5, ...) 6JZ8, 17LD8, 6KY8, 6HC8, 6LU8, 6LR8, 10/6JA5
and the dual dissimilar Vertical triodes: 6CS7, 6CM7, 6CY7, 6DA7, 6DE7, 6DN7, 6DR7, 6EA7, 6EM7, 6EW7, 6FD7, 6FJ7, 6FM7, 6FY7, 6GF7, 6GL7, 10EG7 .... make good linear output tubes. Usually these are cheap. (Ebay, or Vacuumtubes.net are good places to check prices, no affiliation) 29GK6 is a 6BQ5/EL84 clone.
A higher current tube will work better with the usual real-cheapo OTs. Among the Verticals, 10/6JA5 handles 110 mA DC max, usually 3.3x that for peak current. (it also has virtually the same plate curves as the acclaimed EL506) Many of the TV Sweep type tubes can be up-rated by x1.333 Watts or so for audio use. (these tubes run continuously at max Watts in TV service, so are conservatively rated)
If you try power xfmrs for an OT, then the higher current Horizontal Sweeps (with local N Fdbk) are the go to types. 12GE5, 21HB5, 6EX6, 6CB5A, 21LG6A, 21JV6, 38/12HE7 .... are some cheap ones. 38HE7 can often be found very cheap. By using 21V across pins 10 and 12, the pentode only can be powered up (for the tubes with series internal heaters), The pentode can be up-rated to around 15 Watts then. 6CB5 and 38HE7 have decent triode mode curves. 12GE5 and 12JN6 not far behind. (keep the B+ down for triode mode, due to low V rating of screen grids)
and the dual dissimilar Vertical triodes: 6CS7, 6CM7, 6CY7, 6DA7, 6DE7, 6DN7, 6DR7, 6EA7, 6EM7, 6EW7, 6FD7, 6FJ7, 6FM7, 6FY7, 6GF7, 6GL7, 10EG7 .... make good linear output tubes. Usually these are cheap. (Ebay, or Vacuumtubes.net are good places to check prices, no affiliation) 29GK6 is a 6BQ5/EL84 clone.
A higher current tube will work better with the usual real-cheapo OTs. Among the Verticals, 10/6JA5 handles 110 mA DC max, usually 3.3x that for peak current. (it also has virtually the same plate curves as the acclaimed EL506) Many of the TV Sweep type tubes can be up-rated by x1.333 Watts or so for audio use. (these tubes run continuously at max Watts in TV service, so are conservatively rated)
If you try power xfmrs for an OT, then the higher current Horizontal Sweeps (with local N Fdbk) are the go to types. 12GE5, 21HB5, 6EX6, 6CB5A, 21LG6A, 21JV6, 38/12HE7 .... are some cheap ones. 38HE7 can often be found very cheap. By using 21V across pins 10 and 12, the pentode only can be powered up (for the tubes with series internal heaters), The pentode can be up-rated to around 15 Watts then. 6CB5 and 38HE7 have decent triode mode curves. 12GE5 and 12JN6 not far behind. (keep the B+ down for triode mode, due to low V rating of screen grids)
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These are fine for push pull amplifiers, albeit not as good as OPT's purpose built for tube amps. I have a pair of the old Rat Shack 70 volt line transformers in a push pull 11BM8 amplifier that has lived outdoors on my deck, tucked under the eave of the roof, but otherwise completely exposed to wx, for considerably more than a decade.
The Bogen T-725 line transformer is also useful and has a considerably wider selection of taps for more transformation ratios.
edit: the "center tap" is usually two taps down from whatever end tap you are using.
Win W5JAG
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You know, I actually did a lot with the 70v line transformers. I found that if you use the primary 8k and "0" ends and the 2k as the center they had very ggod balance for low power push-pull operation. I drove the secondary with a 5 volt filament winding to find that they were very well balanced. Only issue is the windings are not balanced with regard to DCR, which isn't a huge deal at low current.
smoking-amp,
I have a good number of those tubes, especially a variety of sweep tubes (again, obtained for possible RF use), but also 6005s, 6W6s and 12L6s. I'll tell you a story about my 12L6s. Years ago, as I was getting back into the tubes I played with as a kid, I saw 12L6s at a much lower price than 6L6s, so I though I would be clever and buy some. You know the rest. Lesson learned.
BTW, my very first tube amplifier was for an electric guitar that my mom brought home from her job at Sears. It was a sweep tube from one of the junk TVs that relatives gave me, possibly a 6BQ6. I was probably 11 or 12. How I got it to work, I can't imagine, but I did! Magic times!
Anyway, thanks (to all) for the info.
I have a good number of those tubes, especially a variety of sweep tubes (again, obtained for possible RF use), but also 6005s, 6W6s and 12L6s. I'll tell you a story about my 12L6s. Years ago, as I was getting back into the tubes I played with as a kid, I saw 12L6s at a much lower price than 6L6s, so I though I would be clever and buy some. You know the rest. Lesson learned.
BTW, my very first tube amplifier was for an electric guitar that my mom brought home from her job at Sears. It was a sweep tube from one of the junk TVs that relatives gave me, possibly a 6BQ6. I was probably 11 or 12. How I got it to work, I can't imagine, but I did! Magic times!
Anyway, thanks (to all) for the info.
I got the 70V transformers on clearance years ago to use as output transformers for simple receivers, as in 1 and 2 tube regens, and they do work just fine in that application at low current. I even went so far as to create a spreadsheet calculating the reflected load values between all the different taps, if anybody has any interest in such a thing. For both the 10/5/2.5/1.25/.625 versions and the 8/4/2/1/0.5 versions.