[chrisdrop]

I think most of this is pinout mapping (i.e.; female tube socket pin A > male tube pin B). Perhaps this is overly simplistic?

I was surprised to find not many posts on this topic here on head-fi. I am interested to explore: What are good parts for tube adapters? Where to get parts? Tips/ pinout mappings?

Thanks in advance for ANY guidance!

Best,
Chris

 

[randytsuch]

You really need to look at what specific tubes you want to swap.
Its more than pinouts, there is heater voltage and current to worry about, as well as a bunch of other parameters from the datasheet.
Max voltages, gm (transconductance), etc.
I'm far from a tube expert, know just enough to be dangerous. But I know enough to know you need to be careful here

Randy

 

[chrisdrop]

Some adapters I have:

voltage	source	destination	source-type	destination-type	source-base	destination-base
6.3v	2x 6J5	1x 6SN7	2x single triode	double triode	Octal	Octal
6.3v	1x EL32	1x C3g	pentode	pentode (in strapped triodes mode)	Octal	Loctal-8
6.3v	1x EL3N	1x C3g	pentode	pentode (in strapped triodes mode)	Europe side contact	Loctal-8

Triode-strapped pentode: A pentode can have its screen grid (grid 2) connected to the anode (plate), in which case it reverts to an ordinary triode with commensurate characteristics (lower anode resistance, lower mu, lower noise, more drive voltage required). The device is then said to be "triode-strapped" or "triode-connected". This is sometimes provided as an option in audiophile pentode amplifier circuits, to give the sought-after "sonic qualities" of a triode power amplifier.

Perhaps working through what would be interesting adapters, starting simply ideally; 12AU7 to 12SN7. These are both 12v. Both are dual triodes.They have different bases (12AU7 has Noval base and 12SN7 has an Octal base).

Here is the data-sheet for 12AU7. Here is the data-sheet for the 12SN7.

Here is the pinout of the 12SN7

Here is the pinout for the 12AU7

Is this one as simple as mapping pin-to-pin?

 

[randytsuch]

Quick look at the datasheets.
12SN7 uses .3A @ 12.6V for heaters
12AU7 uses .15A @ 12.6V for heaters.

If circuit is designed for the AU's, it might have a problem with the extra current, but then again it might be fine. It depends on the heater circuit design.
If you're going the other way, from AU to SN, then you're fine. Don't have to worry if going to lower currents. Although I would double check the heater voltage to make sure it doesn't go up too high at a lower load.
Please don't make measurements on a tube amp unless you know what you're doing, there are potentially lethal voltages developed in tube circuits.

I'm not smart enough on tubes to say if the rest of the parameters are close enough, they look pretty close to my un-expert eyes.
Tube designers would draw lines on the curves to find the operating points, but you need to know the operating voltages and currents of the amp to draw those lines. Then you can make sure the line intersection is at a linear point of the curve.

If you can get past the heater difference, diyaudio is a good place to ask tube circuit questions, but without a schematic its hard to answer questions on tube suitability.

 

[2359glenn]

You really need to look at what specific tubes you want to swap.
Its more than pinouts, there is heater voltage and current to worry about, as well as a bunch of other parameters from the datasheet.
Max voltages, gm (transconductance), etc.
I'm far from a tube expert, know just enough to be dangerous. But I know enough to know you need to be careful here

Randy

First you have to know how to solder.
Then mostly straight forward.
You can ask me any questions you may have.
Most parts are from sellers in China on E-Pay

 

[chrisdrop]

First you have to know how to solder.
Then mostly straight forward.
You can ask me any questions you may have.
Most parts are from sellers in China on E-Pay

Thanks Glenn. My soldering is OK as it happens and I have the minimal "stuff" (soldering iron, multimeter, etc). I'll try to sort out parts for some simple adapters I can use.

Deyan is great for building adapters. They are also available from China. That said, I'd like to do something so making a few adapters sounds like fun.

I am going to build a Bottlehead Crack for to learn a bit more. I will give it to one of the kids to use as I have your amp keeping me company I need to introduce them to the glow of tubes.

 

[L0rdGwyn]

Hey Chris,

The 12AU7 and 12SN7 are essentially equivalent tubes in different envelopes, so it pretty much is just a pin-to-pin mapping. The only quirk is that the 12AU7 as three heater connections which allows it to be heated from either a 6.3 or 12.6V supply. You would leave pin 9 (heater center tap) pin unconnected, then connect pins 4 and 5 to pins 7 and 8 of the 12SN7.

As far as the heater current draw, it is unlikely to be an issue. These preamp tubes draw little current and it is uncommon for a transformer heater winding to be rated for less than 1A, usually quite a bit more. It should be able to supply the required current, but you can always look at the transformer datasheet to be certain, just add the current draw for all of the tubes connected to that winding and make sure it doesn't exceed the rated value. For example, if the winding is rated for 12.6V 1A and and a pair of tubes has the heaters wired in parallel, it would draw 0.6A for the 12SN7 and 0.3A for the 12AU7, both under the 1A rating. Unless you have some sort of voltage dropping resistors on the heaters, you don't have to worry about them being run over voltage at a lower current, the transformer secondary will supply the required current at its rated voltage.

Now if you want to make an adapter for two tubes that have very different operating characteristics, it becomes more tricky and requires more understanding of the amplifier circuit. You would have to determine where the adapted tube is going to be biased in the circuit that it was not made for. To figure that out, you would have to have some understanding of load lines for a common cathode amplifier circuit. The operating point would be determined by the B+, plate load, biasing scheme, etc. For input tubes in the GOTL, they are plate resistor loaded (22K I believe, at least in the one I had) and cathode biased. Glenn could let you know the B+ at the top of the plate resistor, the value of the cathode resistor, wattage ratings, etc. then you could draw a load line for the tube you want to adapt and be sure that A) it is going to be biased at a reasonably linear operating point and B) none of the tube maximum ratings will be exceeded. Making a triode adapter for a pentode tube gets more complicated as you have to triode strap the pentode within the adapter, then determine where it will operate in the circuit when triode strapped. Many pentode tube datasheets also include operating characteristics when run as triodes.

If you do a Google search for "tube amplifier load lines", you will lots of hits for resoures going over how to draw them, it is one of the most important skills for understanding/designing a tube amplifier.

Very cool you are going to build a Crack! Looking forward to hearing about it

 

[chrisdrop]

Hey Chris,

The 12AU7 and 12SN7 are essentially equivalent tubes in different envelopes, so it pretty much is just a pin-to-pin mapping. The only quirk is that the 12AU7 as three heater connections which allows it to be heated from either a 6.3 or 12.6V supply. You would leave pin 9 (heater center tap) pin unconnected, then connect pins 4 and 5 to pins 7 and 8 of the 12SN7.

As far as the heater current draw, it is unlikely to be an issue. These preamp tubes draw little current and it is uncommon for a transformer heater winding to be rated for less than 1A, usually quite a bit more. It should be able to supply the required current, but you can always look at the transformer datasheet to be certain, just add the current draw for all of the tubes connected to that winding and make sure it doesn't exceed the rated value. For example, if the winding is rated for 12.6V 1A and and a pair of tubes has the heaters wired in parallel, it would draw 0.6A for the 12SN7 and 0.3A for the 12AU7, both under the 1A rating. Unless you have some sort of voltage dropping resistors on the heaters, you don't have to worry about them being run over voltage at a lower current, the transformer secondary will supply the required current at its rated voltage.

Now if you want to make an adapter for two tubes that have very different operating characteristics, it becomes more tricky and requires more understanding of the amplifier circuit. You would have to determine where the adapted tube is going to be biased in the circuit that it was not made for. To figure that out, you would have to have some understanding of load lines for a common cathode amplifier circuit. The operating point would be determined by the B+, plate load, biasing scheme, etc. For input tubes in the GOTL, they are plate resistor loaded (22K I believe, at least in the one I had) and cathode biased. Glenn could let you know the B+ at the top of the plate resistor, the value of the cathode resistor, wattage ratings, etc. then you could draw a load line for the tube you want to adapt and be sure that A) it is going to be biased at a reasonably linear operating point and B) none of the tube maximum ratings will be exceeded. Making a triode adapter for a pentode tube gets more complicated as you have to triode strap the pentode within the adapter, then determine where it will operate in the circuit when triode strapped. Many pentode tube datasheets also include operating characteristics when run as triodes.

If you do a Google search for "tube amplifier load lines", you will lots of hits for resoures going over how to draw them, it is one of the most important skills for understanding/designing a tube amplifier.

Very cool you are going to build a Crack! Looking forward to hearing about it

Thanks for the very comprehensive (as usual) comment.

I was surprised there wasn't much here on head-fi regarding DIY tube adapters given they are widely used for many amps.

I just went over to eBay and ordered a few different styles of bases/sockets for a simple 12AU7 > 12SN7 adapter. Humble beginnings, in line with my humble level of knowledge here.

Will report back ...

 

[mmoraw]

One more thing to add, above what has been said already here,
I've just finished drawing new pcb for my new tube supply, and the way I've designed it is that heater voltage is provided by lm350, and although I can push close to 3A through it, I don't think the temp management I've implemented could take it. Make sure you are will within the margin what psu can handle.