I picked up a small batch of 12C5 power pentodes with the "Coronado" brand. One has the octagonal "stop sign" with 12C5/12CU5 inside, one looks like typical Sylvania marking. Has anyone else encountered this brand. The parent company is Gamble-Skogmo out of Minneapolis.
You've probably already seen this but here it is anyway. I've never had any tubes with this brand because I'm in the NE. But I have seen lots of Silvertone and Airline products when I did repair work. Rebranding tubes was not uncommon.
Coronado brand appliances and electronics
Gamble-Skogmo marketed appliances, televisions and electronics under its Coronado private-label brand. Coronado-branded products were made by various companies that specialized in private label manufacturing such as Wells-Gardner, Belmont, Warwick, Detrola, Colonial, Arvin and Kingston Radio Corp. beginning in the 1930s. These companies also manufactured private label electronics for other retailers such as Sears (Silvertone), Western Auto (Truetone), and Montgomery Ward (Airline). It was not uncommon to see practically identical versions of Coronado-branded goods sold in competing retailers under the Silvertone, Truetone and Airline brand names.
Yeah, I saw that citation after a bit more searching. I think I have rebranded RCA and Sylvania tubes... The 12C5 will be another alternative output tube for use in my 12AQ5 partial feedback headphone amp. The 12C5 tube takes more filament current than the 12AQ5 (a lot more), so I suspect that the cathode is heftier, even though the plate dissipation rating is less for the 12C5 as compared to the 12AQ5.
The 12C5 is a 12 volt version of the 50C5 commonly found in AA5 radios. The 50C5 is what happened when the UL tried to make a hot chassis radio a little less hot. It is a 50B5 with its pinout rearranged. Both are miniaturized versions of the 50L6 which is a 6W6 with a 50 volt heater. All have a 7.5 watt heater.
The 12AQ5 is supposed to be a miniaturized version of the 12V6, which is a 12 volt 6V6. All have a 2.835 watt heater.
The plates in the 12C5 and the 12AQ5 are nearly identical in size. The 12AQ5 has the plate very close to the glass, so even a slight red plate can cause a localized hot spot resulting in sucking air. The 12 watt plate rating should be respected. Some poorly built tubes may fail at or below 12 watts due to hot spotting. The 12C5 has the disadvantage of having 4.7 extra watts inside the plate before B+ is even applied. Its 5.5 watt plate rating is however a joke. Look at the typical operating conditions, 50 mA on 110 volts, which is 5.5 watts. Who specs a part to be run "at the redline all the time." This is in a class A radio which was often left on for several hours every day in the 50's and 60's. When the radio died because of a weak tube, it was the 35W4, the rectifier. The output tubes lived for years. I have had no issues running 50B5's at 10 watts on the plates with as much as 350 volts. Just keep G2 at or below 120 volts or it will fry. A pair of these will deliver 20 watts in class AB2 on 340 volts and do it forever. Squeezing 20 watts out of a pair of 6AQ5's requires good tubes, and even then, it's hit and miss. The wimpy cathodes just don't put out enough electrons to get the plate voltage pulled down below about 50 volts, and they don't like AB2.
The 12AQ5 is supposed to be a miniaturized version of the 12V6, which is a 12 volt 6V6. All have a 2.835 watt heater.
The plates in the 12C5 and the 12AQ5 are nearly identical in size. The 12AQ5 has the plate very close to the glass, so even a slight red plate can cause a localized hot spot resulting in sucking air. The 12 watt plate rating should be respected. Some poorly built tubes may fail at or below 12 watts due to hot spotting. The 12C5 has the disadvantage of having 4.7 extra watts inside the plate before B+ is even applied. Its 5.5 watt plate rating is however a joke. Look at the typical operating conditions, 50 mA on 110 volts, which is 5.5 watts. Who specs a part to be run "at the redline all the time." This is in a class A radio which was often left on for several hours every day in the 50's and 60's. When the radio died because of a weak tube, it was the 35W4, the rectifier. The output tubes lived for years. I have had no issues running 50B5's at 10 watts on the plates with as much as 350 volts. Just keep G2 at or below 120 volts or it will fry. A pair of these will deliver 20 watts in class AB2 on 340 volts and do it forever. Squeezing 20 watts out of a pair of 6AQ5's requires good tubes, and even then, it's hit and miss. The wimpy cathodes just don't put out enough electrons to get the plate voltage pulled down below about 50 volts, and they don't like AB2.
I will be running these tubes at 150V plate voltage or below, for a single-ended headhphone amp, so I won't be terribly concerned with plate dissipation. I may take things even further and use a Russian submini power pentode. A lot of the headphones I check out have a max input of 200 mW or so, which will give me a whole bunch of power headroom for the tubes in question.
I saw also that the 12C5 has higher transconductance than the 12AQ5, so maybe a better choice for a partial feedback triode emulation amp. Ukfortunately, the pinout is different, so I would have to do some layout changes, or some ugly cut-and-jumper action.
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Are you committed to a 12 volt heater? If not, the following tubes have the same 7BZ pinout as the 12AQ5:
7BZ pentodes - 6AQ5 6DS5 6HG5 6HR5 6BF5 6BM5 6DL5 50B5
Depending on how you did your PC board layout, there may be other possibilities. If pin 7 was left open, there are lots of choices with a jumper from pin 7 to pin 2. The 6AH6 has lots of Gm and can output a watt of audio in SE. The 6AK6 is a low power audio output tube that makes a watt in class A. I made a little battery operated guitar amp with a push pull pair for almost 3 watts on 175 volts.
7BD pentodes - 6AG5 6AN5 6AJ5 6AK5 6CE5 6BC5
7BK / 7cc pentodes - 6AK6 6AU6 6AH6 18FW6 6BA6 6BD6 6HR6 6HS6 12AU6 12BA6 12BD6
7CM pentodes - 6AS6 6BH6 6BJ6 6BZ6 6CB6 6CF6 6DC6 6DE6 6DK6 6EW6 6GM6 6JH6 6JK6 12AW6 12BZ6 12DK6
7BZ pentodes - 6AQ5 6DS5 6HG5 6HR5 6BF5 6BM5 6DL5 50B5
Depending on how you did your PC board layout, there may be other possibilities. If pin 7 was left open, there are lots of choices with a jumper from pin 7 to pin 2. The 6AH6 has lots of Gm and can output a watt of audio in SE. The 6AK6 is a low power audio output tube that makes a watt in class A. I made a little battery operated guitar amp with a push pull pair for almost 3 watts on 175 volts.
7BD pentodes - 6AG5 6AN5 6AJ5 6AK5 6CE5 6BC5
7BK / 7cc pentodes - 6AK6 6AU6 6AH6 18FW6 6BA6 6BD6 6HR6 6HS6 12AU6 12BA6 12BD6
7CM pentodes - 6AS6 6BH6 6BJ6 6BZ6 6CB6 6CF6 6DC6 6DE6 6DK6 6EW6 6GM6 6JH6 6JK6 12AW6 12BZ6 12DK6
Yes, I am committed to a 12V heater - 2 X 12 = 24, and that's the voltage I'm using to drive the series filaments and the self-designed DC-DC converter I'm using to generate the B+. I can get a good deal on a bunch of 12B5s, so I think it will be the tube of choice for this project from now on.
One other proviso - no small-signal pentodes need apply - I'm looking for enough power to drive headphones that may be as low as 16 ohms impedance, though with 30-50 ohms being the sweet spot.
It also occurs to me that I could use a standard 7-pin chassis-mount socket with solder lugs and space the board far enough off the chassis so I can jumper the socket to the PCB to conform to whatever pinout I choose. That's probably what I'll do in the short term, at least. Since I have a solid-state front end, there will only be two tubes up on top in the first place.
It also occurs to me that I could use a standard 7-pin chassis-mount socket with solder lugs and space the board far enough off the chassis so I can jumper the socket to the PCB to conform to whatever pinout I choose. That's probably what I'll do in the short term, at least. Since I have a solid-state front end, there will only be two tubes up on top in the first place.
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Just to make matters more clear, the board would be mounted below the chassis with all components except for the tube sockets hanging down - no real mystery to anyone who has built one of these things before...
Small signal pentodes may have enough power - with the right transformer.
Video output pentodes are a good compromise - most are good for 50-80 mA peak currents and 5 watts of dissipation. Some come with fairly decent free triodes in the bottle. More gain and prettier curves than 50C5’s, for the most part.
Yes OPT... But with the proviso that it has to be an obtainable transformer. The one that I used with this particular project was a Transcendar beast intended for headphone output - it might not be available for a long while, as Transcendar looks to be taking a long time getting their affairs in order. GG's wife looks to have taken over the reins at the company, and their web site is still "under construction". I may look at getting a custom done at Musical Power Supplies with the proper size (and hi-fi rather than guitar quality) for headphone work.
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One of the problems I would have with the video pentodes is their transconductances are extremely high at the currents I will probably want/need to use for low-impedance headphone driving, making the inner feedback loop slippery and hard to compensate. I ran into that problem with a 6EW6 and a 6L6GG used in a partial feedback configuration to emulate triode behavior. I had to back off and use a 6AH6 biased at lower current to peel some gain away so I could not only stabilize the thing, but also be able to close the outer loop without having all this gain to burn off, a situation you don't want when you have a reactive element like a transformer in the loop. Some of those video pentodes have transconductance almost twice as high as the 6EW6.
Is the 12B5 the correct type number? I can't find any info on that tube number online, though I didn't dig too deep. If there is a 12 volt 50B5 out there I could be interested in "testing" some.
Stan had over 100,000 50B5's when he passed unexpectedly. I was going to get a few hundred real cheap. My plan was to run a pair of 50B5's in push pull on 330 volts of B+ obtained from a voltage doubler on an isolation transformer. The driver tube was a 12AU6 in the prototype, but there are several other possibilities. Heaters were series wired across the transformer secondary. I saw 20 watts at just over 3% THD from the test amp which survived full power operation overnight.
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Or 12CA5 - another little pentode designed for 110-volt-ish operation (and on special at vacuumtubes.net for the cheapskate). Not as much power as the xxC5 (sorry, George) but enough to make smoke come out of your ears in headphones.
Those video output pentodes are probably better as triodes rather than in some other feedback arrangement.
Those video output pentodes are probably better as triodes rather than in some other feedback arrangement.
Not 12CA5 - that one is in a nine-pin package and doesn't get to play with the 7-pin tubes. I looked at that one last night as a possibility, and nixed it because of the radically different pinout. The 12C5 and 12AQ5 both come in 7-pin packages, albeit with different pinouts.
The 12C5 is a 12 volt 50C5. Both are based on the 6W6GT octal tube as is the 50B5. The 7.5 watt heater gives these tubes plenty of emission, so they play like miniature sweep tubes. Much like their sweep tube brethren, the plate voltage spec can be ignored within reason, but the screen grid spec MUST be respected. Keep it at or below 120 volts. The 20 watt prototype board runs the screens at 120 volts. These tubes work well in UNSET configuration too, which is used in the proto amp.
I thought *CA5 was 7 pin, inter changeable with **EH5. The old 50EH5’s sounded better *to me than 50C5’s in single ended pentode, run off the usual 110 to 130 volts in table radios, phonographs, and the like. I had several growing up. Nobody wanted them and I liked the way they sound so I ended up with them. Not a lot of power, though.
In the Tungsol data sheet, the 6W6GT is billed as a vertical sweep tube. Got A2 (Vg2>0) curves in pentode and triode mode too. That will let you keep the g2 down and get current.
In the Tungsol data sheet, the 6W6GT is billed as a vertical sweep tube. Got A2 (Vg2>0) curves in pentode and triode mode too. That will let you keep the g2 down and get current.