1625 class A PP tube amp - long post, but good eye candy included

[markus]

Well, the last piece of the puzzle fell into place at the Kutztown Radio Swap Meet; the last *big* piece I needed was output transformers. I originally wanted to go Single Ended, but these really cool Army OPT's fell into my lap at Kutztown . . . but I'm getting ahead of myself

I'd heard great things about 807's (especially RCA's) and their half-brother the 1625. Supposedly the same electrically as a 6L6, but built to be abused by the military, HAM guys, and anyone else who wanted to play with higher voltage.

A couple years ago, at an MIT swap, I snagged 4 NOS/NIB RCA 1625's, with bases for $35. At another meet, I grabbed a box of tubes which included NIB/NOS Sylvania 6SN7W's - got the whole box for $40 From an old Kepco Variable power supply, and recent radio flea market, I have 14 8mfd PS oil caps, and two big 15 H chokes. At a more recent MIT meet, I grabbed two HUGE potted power transformers for $10 - secondaries are 850 vct and 300 vct; it also has a 5V and 6.3 V winding. Another flea market provided some HUGE 12V tranformers - I think they were for a field coil supply or something . ..1625's have a 12V heater. I'll need to source signal caps, resistors, underhood wiring, etc, but these are all the hard parts :yes:

Aaaaaaand the most recent, and important grab: two OPT's from our US Army
- WW2 vintage. One is a Stanley, the other UTC, but they both have the same military ID# TF 1A-13-YY, and *almost* the same ratings. These were pulled from the integrated amplifiers used for PA applications - not sure of the model ##'s. here's the ratings:

primaries are 9000 Ohms (perfect for Class A PP 6L6's / 807's / 1625's)
stamp on the side says they are flat (+ / - 1 db) from 50 - 10000 Hz. I'm betting they are flat + / - 3 db from about 30 - 20000 Hz :scratch2: their dimensions are exactly the same . . .

here's where they vary:

UTC is rated for 20 watts, Stanley for 30 watts - both rated at 142 MA DC

here's my question; if I go with the 1625's in Class A, they should be putting out 32.5 watts MAX . . . think it's safe to use the UTC ? do you think its under-rated for power ?? :scratch2:

check out the attached eye candy

I plan on making this a scratch build . . .ambitious, yes, since I have yet to really to complete a DIY amp :scratch2: but, I figure I'll just go off the 807 data sheet . . . Class A, PP, Va 500V, Vg2 300V, etc . . . does this sound like a good idea, or should I use an existing schematic ? I'd like to keep the circuit as simple as possible. One interesting issue, is the OPT's have no screeen tap -just straight P/P - pins 1, 2, 3 are for the two tube plates, and center tap; pins 4 through 9 are just connections for various speaker impedances from 30 to 600 Ohms . . I'm not too worried about the minimum 30 Ohm tap - yes, 8 or 16 Ohm would be ideal, but I plan to use this with my DIY line arrays, which means I could re-wire the speakers to present (about) a 30 Ohm load . . .

Any ideas or tips are GREATLY appreciated :thmbsp:

 

[Squidward]

Very nice, and I'll be watching with much interest. I think I have the exact same 1625 tubes, same boxes and all. They go CHEAP on ebay sometimes, a great reason to use a tube that's out of fashon: NOS can be reasonable!

 

[forty2w0]

I don't think you have to worry about the 20 watt limet 142 ma would be 71 ma per tube!...John

 

[ultralc]

I'm not too worried about the minimum 30 Ohm tap - yes, 8 or 16 Ohm would be ideal, but I plan to use this with my DIY line arrays, which means I could re-wire the speakers to present (about) a 30 Ohm load . . .

You can use other combinations of secondary taps for different output impedances. Someone maybe able to calculate more precisely, but I estimate 5 ohm between 60-30 ohm taps, 10 ohm between 125-60 taps, and 20 ohm between 250-125 taps. These are much closer to typical 4, 8, and 16 ohm speaker loads.

 

[markus]

I don't think you have to worry about the 20 watt limit 142 ma would be 71 ma per tube!...John

thats the funny part - maximum current draw (based on the data sheet) is about 120ma, so I've got a little room I had looked at some of the Hammond or Edcor OPT's, but most (inexpensive) ones top out around 80 or 100 ma. Granted, I won't be running this thing wide open all the time, but I'm a fan of overbuiling stuff :nerd:

You can use other combinations of secondary taps for different output impedances. Someone maybe able to calculate more precisely, but I estimate 5 ohm between 60-30 ohm taps, 10 ohm between 125-60 taps, and 20 ohm between 250-125 taps. These are much closer to typical 4, 8, and 16 ohm speaker loads.

that would REALLY cool, since I'd like to have a *little* flexibility with speakers - I'd love to hear my KLH 5's, 6's, Large Advents, and any other cool ones I come across in the future :thmbsp:

 

[Old1625]

While those are some nice looning OPTs my guess is that their original application was in line PA system use, where there was a smaller transformer attached to the individual speakers to do the final step-down to 8 or 4 ohms. Multiple taps on the primary were usually marked in terms of relative wattage--10,5,2.5, .625.... If the speaker was placed in a ceiling in a large room then the highest wattage tap was used. In a smaller room a lower tap would be used. The .625 tap might be used in a small restroom.

The bandwidth on such systems was generally pretty well limited.

Just the same you might be able to tickle some hi-fi from these units--who knows....? :scratch2:

 

[PakProtector]

hey-Hey!!!,
Looks like a sweet project. It is always refreshing to see Class A amps underway, as the Texts always offer up the AB designs. If your OPT's don'e carry the LF the way you'd like, call Heyboer TX in Michigan and see what they can do for you. They have at least two Peerless 20-20 10k a-a designs that they can build... and with thise you can get U-L taps nearly anywhere you'd like.
cheers,
Douglas

 

[DaLarry]

.....
check out the attached eye candy .....

Deifinitely soft core for geeks! :tresbon:

 

[markus]

hey-Hey!!!,
Looks like a sweet project. It is always refreshing to see Class A amps underway, as the Texts always offer up the AB designs. If your OPT's don'e carry the LF the way you'd like, call Heyboer TX in Michigan and see what they can do for you. They have at least two Peerless 20-20 10k a-a designs that they can build... and with thise you can get U-L taps nearly anywhere you'd like.
cheers,
Douglas

THAT'S a good idea . . . what's a pair of those going for these days ?

I have yet to dive into this project - still honing my tube skills - rebuilding a power supply on a little SE Sylvania EL84 amp . . last night I rebuilt the CRC filter on a little turret board, but went with 80 instead of 40 mfd on the first cap - very pretty light show inside the rectifier tube Soooooo. . this week I'll redo it, and it *should* start making music after a long hibernation.

If I successfully resurrect this little guy, then first on the list for scratch building is the JE Labs SET 45 amp, then I'll dive into this 1625 PP build.

 

[PakProtector]

THAT'S a good idea . . . what's a pair of those going for these days ?

I have yet to dive into this project - still honing my tube skills - rebuilding a power supply on a little SE Sylvania EL84 amp . . last night I rebuilt the CRC filter on a little turret board, but went with 80 instead of 40 mfd on the first cap - very pretty light show inside the rectifier tube Soooooo. . this week I'll redo it, and it *should* start making music after a long hibernation.

If I successfully resurrect this little guy, then first on the list for scratch building is the JE Labs SET 45 amp, then I'll dive into this 1625 PP build.

A single pair of S265Q built with M6 Si-Steel core goes for ~$220. You get to specify tap location in 10% increments, and get more than one pair...

Your light show in the rectifier was due to more capacitance than the tube could stand. Either put some smaller caps in the first position, or some good inductance to get peak current demand down.

My firend George built the JE 2A3 and added a switch to change the cathode R to 1k5 and run 45's. He's built the schematic that runs a cathode follower, so careful choice of which section gets used for the input will allow rolling in stuff like the 6DN7 and 6EM7. My first suggestion would be to uprate the choke in the PS, and get the best OPT's you can afford. This amp sounds quite good...
cheers,
Douglas

 

[dspear99ca]

last night I rebuilt the CRC filter on a little turret board, but went with 80 instead of 40 mfd on the first cap - very pretty light show inside the rectifier tube

You'll fry your rectifier if you exceed the max cap input. Most EL84 designs would have a rec like a 5U4GB which specs 40uF max. Put the higher caps after the first resistor and you'll have no trouble.

 

[markus]

You'll fry your rectifier if you exceed the max cap input. Most EL84 designs would have a rec like a 5U4GB which specs 40uF max. Put the higher caps after the first resistor and you'll have no trouble.

yup - just a little DIY Audio Dyslexia on my part (DOH!)

 

[Tom Bavis]

While it's TRUE that the 30-60 Ohm connection will give you 5 Ohms... the coupling will be worse, and the high frequencies may suffer a little. Best to use as much of the secondary as you can.

Great to be able to build it all with mil-spec parts! I wouldn't worry about the power rating of the UTC - there won't be much more than 20W in class A.

 

[markus]

While it's TRUE that the 30-60 Ohm connection will give you 5 Ohms... the coupling will be worse, and the high frequencies may suffer a little. Best to use as much of the secondary as you can.

Great to be able to build it all with mil-spec parts! I wouldn't worry about the power rating of the UTC - there won't be much more than 20W in class A.

how much do you think the frequency response would suffer if I merely used a 16 ohm driver on the 30 ohm tap ?

although I must say, I'm leaning towards using some of the Peerless spec'ed Heybors for OPTs, and use these 9K mil spec OPT's for my SET 45 project . . . with 16 ohm speakers, they'd show a nice 4.5 load to the output tubes . . not too mention a lil' ol' 45 tube would have little chance of saturating them