National/Valco Model 1215 Build

[TubeJunkie]

Need a little help sizing a Power Transformer (PT) and matching an Output Transformer for an amp build I'm looking into.  I plan on doing a scratch-build of a Valco Model 1215 (schematic below), but want to make sure I'm doing things correctly.  The amp will use two 6SJ7 tubes for input, two 6SN7 tubes and one 6SL7 for the rest of the pre-amp section, two 6P3S-Es/5881s for the power amp section (I'll be subbing these in place of the 6L6G tubes, but using the specs of a 5881), and a 5U4GB rectifier (spec sheets below).

I read/heard in a few places that you look at the max characteristic ratings and add up the currents for each tube, and double it for triode tubes, and I also believe this is a 2-tube class A push-pull amp(?).  My calculations of the current requirements for the PT are listed below:

Tube            Current (mA)
=================
6SJ7                   2.3
6SJ7                   2.3
6SN7                20.0
6SN7                20.0
6SL7                  2.3
6SL7                  2.3
6SN7                20.0
6SN7                20.0
6P3S-E/5881     80.0
6P3S-E/5881     80.0
5U4GB              -----
            Total: 252.2mA

I used the specs of a 5881 tube because I found several examples of these 6P3S-E tubes run at 400-450V and more with no problems.  Here's a sample of what I found on the web:

"The 6P3S-E is commonly touted to have a safe dissipation well above 30W . . . with a high-voltage supply they will dissipate almost 42W before the plates start to glow. I've tested them as high as 40W (approx. 500V @ 80mA) . . . I've been using a quartet at 500VDC (with a healthy 45mA of idle current) for at least a year with no signs of weakening. No emission burn on the glass (even when viewed in front of a clean, white sheet of paper), no rattles, no bad sounds. They've been pumping out 35W/channel all this time."

Based on the above info, would these two transformers be a good match for this build?
   ClassicTone PT: 40-18065 (520-600V @ 300mA and I need the 6.3V @7.5A for the 6Sx7 tubes)
   http://www.classictone.net/40-18065.pdf

   ClassicTone OT: 40-18049 (30W)
   http://www.classictone.net/40-18049.pdf

Of course, I could easily be totally wrong about everything.

Schematic: https://el34world.com/charts/Schematics/files/Valco/National_1215_early1950s.jpg

[sluckey]

Hmm, I only see a need for 6.3V@3.9A on that schematic.

[TubeJunkie]

Hmm, I only see a need for 6.3V@3.9A on that schematic.

You're right, I went back to my spreadsheet and I put one of the values in wrong (3.3 instead of .3).  Here's my new calculation:

2 6SJ7 @ .3A ea. =               .6A
2 6SN7 @ .3A ea. =              .6A
1 6SL7 @ .3A =                    .3A
2 6P3S-E/5881 @ .9A ea. = 1.8A (same if 6L6s are used)
                              Total = 3.3A

My biggest concern is the current draw of the tubes, and whether or not I calculated that correctly.  If the calculation is correct, and given the new heater current value, another option for the PT would be:
http://www.classictone.net/40-18050.pdf (560V @ 270mA, 6.3V @ 6A, 5V @ 3A)

[sluckey]

2 6SN7 @ .3A ea. =              .6A

6SN7 needs .6A for filaments, not .3A. Two would need 1.2A

[PRR]

> 6SN7                20.0

6SN7 "can" pass 20mA, yes. And the datasheet boasts about it.

But that's not how Valco used them. There is a 100k in series with each plate. If 10mA were to flow in 100k, the B+ would have to be over 1,000 Volts! They are much more likely to be working near 1.5mA per triode (3mA/bottle).

6L6G can stand 23 Watts Pdiss max. (And for Valco Tone, you do not deviate much from this.) At 400V supply, two bottles, this is 115mA/pair, not 160mA.

I  think you have over-bought, something Valco didn't do.

[TubeJunkie]

2 6SN7 @ .3A ea. =              .6A

6SN7 needs .6A for filaments, not .3A. Two would need 1.2A

Thanks, I was looking at the 12SN7 . . . DOH  . . . new total = 3.9A

[TubeJunkie]

> 6SN7                20.0

6SN7 "can" pass 20mA, yes. And the datasheet boasts about it.

But that's not how Valco used them. There is a 100k in series with each plate. If 10mA were to flow in 100k, the B+ would have to be over 1,000 Volts! They are much more likely to be working near 1.5mA per triode (3mA/bottle).

6L6G can stand 23 Watts Pdiss max. (And for Valco Tone, you do not deviate much from this.) At 400V supply, two bottles, this is 115mA/pair, not 160mA.

I  think you have over-bought, something Valco didn't do.

So . . .

Given this new info, the calculation would look more like:

Tube       Current (mA)
======================
6SJ7           2.3
6SJ7           2.3
6SN7           1.5
6SN7           1.5
6SL7           2.3
6SL7           2.3
6SN7           1.5
6SN7           1.5
6P3S-E/5881   57.75
6P3S-E/5881   57.75
5U4GB        -----
      Total: 132.1mA

Is this correct, and does this mean a PT that gives me ~400V at B+1 with ~140mA total draw?

[PRR]

Is there ANY clue what the original B+ or OT impedance was in these things?

6L6B and 5881 datasheets just echo the very-old 6L6 datasheets, ignoring the higher ratings. 6L6GC only has a few high-rated suggestions. I like to turn to 7027 which is late-spec 6L6 guts on different pinout and bottle.
http://www.mif.pg.gda.pl/homepages/frank/sheets/049/7/7027A.pdf
Self-bias ratings run from 24W at 8K load to 44W at 4k load, quite a spread. The age of the Valco does not suggest 5F6a Bassman output (they could not find a speaker to swallow that). So your 4k OT for a VOX 30 seems less likely than a 6k or 8k 20W OT. This also agrees with the trend toward 250ohm/pair cathode bias.
Not the same guts but clearly originally aimed at working in the same sockets is KT66.
http://www.mif.pg.gda.pl/homepages/frank/sheets/126/k/KT66.pdf
Bottom of page 2 conditions seem reasonable for vintage amp if B+ is some below 400V instead of some above to account for lower Pdiss rating.

Difference of "tone" between 4k and 8k can be significant.

[DummyLoad]

i prefer 6.6K with 6L6 in cathode bias. B+ around 380-430V.


Hammond 274AX works with PSU2 designer sim with a 5U4GB rectifier. the 6.3V filament winding is rated at 3.5A - IMO that's not a issue. PSUD2 sim shows about 415V to plates and about 320V to screens with the loads given previous posts.


for the OT - suggest use of the hammond 1760H 6.6K 20W part, or equivalent classictone 40-18087, or if you want a better bottom end, the classictone 40-18064, the classictone 40-18087 probably my first choice. the mojotone mojo791 is equivalent to the classictone 40-18064. i have tested both and prefer the classictone part.

some of the mojo of that amp is probably in the skimped on iron so, the 20W hammond or classictone parts would likely get you closer to that valco tone that you seek. looking at the pics of one 1215 example; the 20W part looks closer to what you'd probably want.   


--pete

[TubeJunkie]

Is there ANY clue what the original B+ or OT impedance was in these things?

6L6B and 5881 datasheets just echo the very-old 6L6 datasheets, ignoring the higher ratings. 6L6GC only has a few high-rated suggestions. I like to turn to 7027 which is late-spec 6L6 guts on different pinout and bottle.
http://www.mif.pg.gda.pl/homepages/frank/sheets/049/7/7027A.pdf
Self-bias ratings run from 24W at 8K load to 44W at 4k load, quite a spread. The age of the Valco does not suggest 5F6a Bassman output (they could not find a speaker to swallow that). So your 4k OT for a VOX 30 seems less likely than a 6k or 8k 20W OT. This also agrees with the trend toward 250ohm/pair cathode bias.
Not the same guts but clearly originally aimed at working in the same sockets is KT66.
http://www.mif.pg.gda.pl/homepages/frank/sheets/126/k/KT66.pdf
Bottom of page 2 conditions seem reasonable for vintage amp if B+ is some below 400V instead of some above to account for lower Pdiss rating.

Difference of "tone" between 4k and 8k can be significant.

Thanks for the info and the links . . . I'll read through those datasheets.  I've looked and looked for several months and haven't found anything about B+ Voltages, tube voltages, or specs on the PT or OT.  I'm going to keep searching and see if I can maybe find someone on a forum with an original model 1215 that could give me their voltages and possibly the specs on the PT and OT, but so far no luck with that either.  Of course, where's the fun in doing this if it doesn't have challenges?

THANKS!!

[TubeJunkie]

i prefer 6.6K with 6L6 in cathode bias. B+ around 380-430V.


Hammond 274AX works with PSU2 designer sim with a 5U4GB rectifier. the 6.3V filament winding is rated at 3.5A - IMO that's not a issue. PSUD2 sim shows about 415V to plates and about 320V to screens with the loads given previous posts.


for the OT - suggest use of the hammond 1760H 6.6K 20W part, or equivalent classictone 40-18087, or if you want a better bottom end, the classictone 40-18064, the classictone 40-18087 probably my first choice. the mojotone mojo791 is equivalent to the classictone 40-18064. i have tested both and prefer the classictone part.

some of the mojo of that amp is probably in the skimped on iron so, the 20W hammond or classictone parts would likely get you closer to that valco tone that you seek. looking at the pics of one 1215 example; the 20W part looks closer to what you'd probably want.   


--pete

I hadn't heard of the PSU Designer II program . . . it looks like a great tool.  I'll have to download it and see how it works.  Thanks for the info on the PT and OT.  I'll see if I can confirm your suggestions . . . you've given me a great place to start.  I've also been looking for posted pics of the 1215s chassis, but haven't found anything yet with enough detail to tell for sure, but your pic of the OT does show that it is on the small size, so probably closer to a 20W than a 30W.

[TubeJunkie]

i prefer 6.6K with 6L6 in cathode bias. B+ around 380-430V.


Hammond 274AX works with PSU2 designer sim with a 5U4GB rectifier. the 6.3V filament winding is rated at 3.5A - IMO that's not a issue. PSUD2 sim shows about 415V to plates and about 320V to screens with the loads given previous posts . . .

I have a MOJO British 45 PT laying around that's similar to the Hammond 274AX.  Does this look like a good candidate?

[DummyLoad]

i prefer 6.6K with 6L6 in cathode bias. B+ around 380-430V.


Hammond 274AX works with PSU2 designer sim with a 5U4GB rectifier. the 6.3V filament winding is rated at 3.5A - IMO that's not a issue. PSUD2 sim shows about 415V to plates and about 320V to screens with the loads given previous posts . . .

I have a MOJO British 45 PT laying around that's similar to the Hammond 274AX.  Does this look like a good candidate?

way too hot - that's a 200mA part.

valco probably used 100-120mA part for the PT - the 1215 output was likely around 15-20W max. that marshall PT is for a 50W amp and is overkill. a 150mA part is likely to be overbuilding. 

a good part of the tone of the 1216 amp is the loose power supply and the screen drive coupled with the minimalist OT. buy parts that meet spec. or are barely under spec. for this amp.

--Pete

{EDIT - removed over-quoted quote -- PRR}

[TubeJunkie]

DL - Makes sense . . . thanks for all the great input.

[bmccowan]

In my files I have the following PT specs for a 1952 all Octal 2x6L6 Valco/National:
340-0-340 @150MA
5V @ 3A
6.3 VCT @ 2.7A
Schematic attached
Not the same amp as it has 1 6SC7, 1 6SL7, 2x6L6, 5U4GB - but thought it might be useful info

[TubeJunkie]

In my files I have the following PT specs for a 1952 all Octal 2x6L6 Valco/National:
340-0-340 @150MA
5V @ 3A
6.3 VCT @ 2.7A
Schematic attached
Not the same amp as it has 1 6SC7, 1 6SL7, 2x6L6, 5U4GB - but thought it might be useful info

Interesting . . . it looks like they went about 115% of the actual current load for those tubes - 150mA for an ~130mA load.

I think I'll look for more old National/Valco schematics, they might add more info to solving the puzzle.  This does help, thanks!

[bmccowan]

There are a bunch of them out there - and a lot of repeats Valco/Airline/Supro/National/Gretsch/Oahu and some other house brands. Have fun looking, but I would not fret (no pun intended) too much about hitting an exact spec. As others have said, you want to cover the amperage requirements, but do not want to go over too much. If you do, you will end up jumping through hoops to get the B+ voltage down. I do not believe for a second that Valco did any careful calculations to size transformers. It was more likely going to the parts bins and saying, "this should work." I have 2 all octal Valcos from that time period. Both are 6V6 amps so the requirements are a bit different, but I can agree with others that the transformers are not oversized, and the B+ voltage is relatively low (under 400v.)

[PRR]

> I do not believe for a second that Valco did any careful calculations to size transformers.

I would suspect they called a transformer shop and just asked for "iron for 2x6L6". Transformer suppliers used to offer standard sets for the usual set-ups. If Valco balked at the price, they knew where to shave a few pennies. 

[DummyLoad]

PT suggestion:

http://www.classictone.net/40-18060.pdf

OT suggestion:

http://www.classictone.net/40-18038.pdf

--pete

[TubeJunkie]

> I do not believe for a second that Valco did any careful calculations to size transformers.

I would suspect they called a transformer shop and just asked for "iron for 2x6L6". Transformer suppliers used to offer standard sets for the usual set-ups. If Valco balked at the price, they knew where to shave a few pennies.

I'm of the same belief . . . Valco probably found the best price for what was "close enough" to the specs to give a descent sound without the amp also producing smoke and flames 

[TubeJunkie]

PT suggestion:

http://www.classictone.net/40-18060.pdf

OT suggestion:

http://www.classictone.net/40-18038.pdf

--pete

Thanks DL, I was looking at these as possible candidates as well . . . this helps confirm I'm looking in the right direction.  My two current candidates for the PT are the Hammond 274AX and the ClassicTone 40-18060, and for the OT, the ClassicTone 40-18038 and the the ClassicTone 40-18087 (if I decide I also need 16 ohm).  Should I be concerned that the 40-18060 only provides for a 125mA load, or is that "close enough?" Between the Hammond and the ClassicTone PTs, the Hammond is low on the Heater Current, but the ClassicTone is low on the HT current.

[DummyLoad]

the classictone PT would be what i'd run. simulation with PSUD2 shows around 415V with the hammond. we don't have DCR and NL volt specs for the classictone part, so simulation of the classictone loaded is purely guess work. i little shy on B+ is better than low filament V.

--pete 

[TubeJunkie]

There are a bunch of them out there - and a lot of repeats Valco/Airline/Supro/National/Gretsch/Oahu and some other house brands. Have fun looking, but I would not fret (no pun intended) too much about hitting an exact spec. As others have said, you want to cover the amperage requirements, but do not want to go over too much. If you do, you will end up jumping through hoops to get the B+ voltage down. I do not believe for a second that Valco did any careful calculations to size transformers. It was more likely going to the parts bins and saying, "this should work." I have 2 all octal Valcos from that time period. Both are 6V6 amps so the requirements are a bit different, but I can agree with others that the transformers are not oversized, and the B+ voltage is relatively low (under 400v.)

Thanks for the info.  What amps do you have, what works has been done on them, what tubes are they using, and do you have any measured voltages?  I'd like to look at the schematics and maybe ask a few questions about the ones you have.

[bmccowan]

I have a:
National Sportsman 1202 (6SJ7, 6SL7, 2x6V6, 5Y3) 2 8" Jensens
a gretsch Electromatic Artist 1655 - 6SQ7, 6SC7, 2x6V6, 5Y3 = 10" speaker
I also have a Valco branded version of the above Gretsch
I have done the typical - new filter caps; replace drifted resistors, replace leaky coupling caps, replace weak tubes, clean/lubricate pots, clean and tension the tube sockets. No mods.

[TubeJunkie]

I have a:
National Sportsman 1202 (6SJ7, 6SL7, 2x6V6, 5Y3) 2 8" Jensens
a gretsch Electromatic Artist 1655 - 6SQ7, 6SC7, 2x6V6, 5Y3 = 10" speaker
I also have a Valco branded version of the above Gretsch
I have done the typical - new filter caps; replace drifted resistors, replace leaky coupling caps, replace weak tubes, clean/lubricate pots, clean and tension the tube sockets. No mods.

Thanks!  This helps.