Selectable HT rails Voltage for two Gain Stages to run Tubes Hot (Higher Voltage) or Cooler (Starved Plate? even) [Pics Inside]

[reinw33]

Here are my new 6922 matched Tubes that I bought from a seller yesterday.
He gave me the opportunity to even to listen to a few, in DIY'ed PP and SE Phono-amps, and some other known good and even more expensive ones for over 100€. He even told about one that supposedly costs 800€, ofcourse we didn't test it
We listened to a few goodies of tubes and cheaper ones too, among those are the listed ones here below.
I still liked them, though he said they can be microphonic. But these here might not be as good as the best, but they are at least built like a tank and built to be portable so to speak. To survive transport from A to B.
I have one crucial question that is imperative to the designing process of my two Gain stages Mic preamp.
It's the TwinLine amp that I am trying to build, perhaps even prototyping first on wood by soldering point to point?
But some design question I have for you to get clarified to me beforehand please.


First let's have a look at my custom PSU transformer in process.

Please don't be dismayed by the exposed laminations of the Tx.
I wanted to show you a fully assembled picture but it's not imperative, it's somewhere on my phone probably.
It's a two legged core, capable of delivering 63W of Power and I am positive you can imagine the cubic vintage looks of it.

My question is this, like the title above:
Some commercial designs that are out there for mic preamps, have a switchable HT Voltage (I guess, I'll have to look up a single specific schematic) where you can switch the tubes running hotter or colder, right, don't get me wrong I am a beginner in tubes, and am currently learning the cathode bias, and the way how you can change the tube character.


Is this done also, in terms of higher HT voltage ( I guess) for hotter or lower HT voltage for perhaps even a starved plate operation, where you can switch from running the tube very hot, shorter life expectancy, to running the tube cooler.

My Question is this:

I have to know whether this is reasonable/feasable and done in this manner - just sanity checking here for the most part - to incorporate one more secondary, e.g. one that is for running both stages/or just one stage hot with an output Voltage let's say of 245V? (I dont know yet, or 185V?)
I faintly remember the limiting Anode voltage is around 300V for either the 5751 or the E88CC/6P23N, and I'll get to my available tubes now, I have a 12ax7 (chinese) a 5751 (TAD) as Mic stage and two matched 6P23N (russian) as line out stage.

I want to know this, do I have to accomodate a 4th secondary on the PSU, to achieve this above?
We have two options here, We can put out with 3 secondaries P1 280V, P2 48V for Phantom 100% certainly needed, 6,3V for heater 100% needed, and depending on your ansers tapping for a lower voltage., say 90V for starved plate design in parallel for example.

I apologize to you for my brief english and grammar mistakes, by chatting with you I improve myself in your language.

Because I want it to be in terms of driving the tube hotter or colder, switchable/selectable, for both gainstages, I don't necessarily want to change the headroom by changing the cathode bias, is that possible?
Or am I oversimplifying and misunderstanding the concept behind the switch that on some designs the lets the tubes run hotter or colder.
Do I need the HT Voltage and the lower HT voltage to come from tappings on the transformer or by a Voltage divider getting supplied by just one primary source?

If you can recommend to me Voltages at which those two stages can run at simultaneously that sound best that would be mega cool!
Because my schematic for the TwinLine amp IIRC doesn't mention the HT voltage, the tubes should be running at for recording vocals, because I have no clue, It's not ment as a distortion box.
I can probably look up on the internet what they recommend for the 12ax7, or 5751v as input stage and the E88CC in a system, and see where they overlap in their voltage ranges and that's the range how low and how high one can go with the HT rail.
I looks like nothing now, but it's just early, missing a lot of electronic parts that are about to get ordered, I don't want to make many small orders and pay 60€ just in delivery costs, makes not much sense to me.
It's a side project that is worthwhile, hopefully
Hopefully you understood what I want to achieve out of this project, and thank you for your replies.

Please tell me if you don't like this idea.

Thank you for your reply!

 

[reinw33]

Ok, by the grace of this forum, I actually happened to find a similiar datasheet of the same type of tube 6922/6N23P/E88CC, that gives me reference Ra values for a given Plate Voltage. Great, I have a general Idea now where I should be at.
But I have not heard yet what sound what B+ Voltage actually distortion wise produces.
And that I have 3 secondaries, wait.... ja, i think 3 secondaries in total.
I am that new to valves with audio.

And thus the complexity of my question has been reduced to two things.

1,) How to properly make a proper plate voltage switch/mechanism/push button or poti that is safe to operate manually. Any suggestions? I have a specific type of switch in mind.

2.) At what Plate Voltages do you run your 6N23P actually? Where does it sound best for you?

I make a SE preamp with bought transformers that are shipping and wind some others with decades old audio iron later on, - it's not the one in the picture, it's army leftovers/surplus, but that's another project-
With the two transformers that I alreadfy bought I had to go international.
So it's really not easy and cheap to get the proper Tx in my experience here where I'm at.
How's your experience, all available and cheapo?

I can't even imagine, the different tone I'm going to get out of it.

Cheers.

 

[Tubetec]

I use a universal shaver transformer for preamp and mic HT ,
Single 230v primary , 230v (165ohms)secondary with 110v tap (100ohms)and as the taps are unequal you can effectively make three different DC voltage selections by swapping the transformer leadouts .
I have the transformer wires terminated with solder tags that screw down to terminals
A three position on-on-on toggle switch might be able to do it .

 

[reinw33]

I use a universal shaver transformer for preamp and mic HT ,
Single 230v primary , 230v (165ohms)secondary with 110v tap (100ohms)and as the taps are unequal you can effectively make three different DC voltage selections by swapping the transformer leadouts .
I have the transformer wires terminated with solder tags that screw down to terminals
A three position on-on-on toggle switch might be able to do it .

Hello Tubetec, nice, very interesting idea and approach.
I wonder how a difference of 110V to 120V and considerable 230V for a cool trick with tubes would sound like.
But from what I read so far, somewhere between 120V to 160V Plate Voltage should be the sweet spot long term, what do you think?
Is your universal shaver transformer used for a tube mic with an external 110V, 120V, 230V supply that you use for it?
The switch would have to be a shorting one, double throw tripple pole or you would have to power the unit off, before switching, doesn't it?
I do have 4-6 or so of those, with 6 pins, but I don't know whether they are shorting, and I think they might be too small and for audio not for power.

 

[reinw33]

I use a universal shaver transformer for preamp and mic HT ,
Single 230v primary , 230v (165ohms)secondary with 110v tap (100ohms)and as the taps are unequal you can effectively make three different DC voltage selections by swapping the transformer leadouts .
I have the transformer wires terminated with solder tags that screw down to terminals
A three position on-on-on toggle switch might be able to do it .

Hi Tubetec, Could you also answer this please, do you not also need a heater Voltage? Where do you get that from? But very nice idea, sparing yourself the winding. Really clever.

 

[ruffrecords]

The sound you get from a preamp depends at least as much on the topology you choose as it does on the tube operating points. Without seeing the schematic you are intending to build it is rather hard to answer these questions.

Cheers

Ian

 

[Tubetec]

https://www.thesocketshop.com/lap-2...ocket-115-230v-slate-grey-with-black-inserts/
Thats the shaver socket I use , I strip out the 2 pin socket assembly to make space for other components and house it up in a dual gang metal clad surface wall box ,
likewise the LT transformer ,bridge and capacitors are housed in another dual gang box ,mounted back to back with the HT enclosure .
I often find an LT transformer from old style wall wart charge adapters and power supplies , I can pick up them up for free at my local recycling facillity , its easy to find 4,6,9,12,15 V dc versions .

I saved wiring , space and the need for metal working (other than drilling a few holes) in the psu enclosure by not including any switches and using metal cable glands instead of IEC and multipin sockets .
There only two screws required to remove the lid to change the voltage selection ,
everythings hardwired , passive RC filtering, theres very little to go wrong.
Of course in the case of a tube mic you can include LT regulation as required .

I was trying to think of a simple way of incorporating a neon lamp so it acts as a dummy load for the HT line at initial switch on ,

There are reasons you might want a switchable HT supply , like for bench testing ,
maybe a Lorlin 4 position 3 pole , with an off position and the 3 HT selections ,
If you wanted continiously variable adjustment you could incorporate a ten turn pot in place of the final series filter resistance ,

Hope the recovery is going well Ian ,

 

[CJ]

You could build a Fairchild 670 regulator supply which has a pot for voltage selection.

Or if you want to get really low, copy a Heathkit variable HV supply circuit,

 

[reinw33]

The sound you get from a preamp depends at least as much on the topology you choose as it does on the tube operating points. Without seeing the schematic you are intending to build it is rather hard to answer these questions.

Cheers

Ian

Hello Ian,
always nice to hear from you.
It's gonna be SE. and I believe I'm going to try to build your twin line mic preamp modification.

At the 1 Gain, is this the RC LP filter for the Cathode of the 12AX7LPS?
It looks weird to me, why is it an electrolytic symbol and not a nonpolar?
Forgive me my ignorance.

Ian, I could find nothing meaningful in the datasheets of the tubes in relation to a clean sounding tube,
I made some brief calculations and in your design here the cathode current is somewhere in the μa, perhaps ~800 μa?

What kind of cathode current would you recommend for a clean preamp?

Please forgive me if should just read your manual, because it's in it.

I appreciate your input, do you recommend a stepped gain or just a poti at gain 1?

It's just too much of a fail ending up with a too low volts HT+ supply.

What can you recommend to those that want to build your modification? Honest question.

 

[reinw33]

You could build a Fairchild 670 regulator supply which has a pot for voltage selection.

Or if you want to get really low, copy a Heathkit variable HV supply circuit,

Wow CJ, I downloaded your disassemblies.
Thank you very much, but many are missing.
I just recently unwound a 2000 turn power transformer and will definitley need regulation.
for the 48V I probably a TLA783 and surrounding circuit, for the rest I will look at the fairchild 670, perhaps there is something in it that helps me, I have no idea what fitpiece is needed to regulate, do they only go up to 150V?, a clean plate voltage.
so far I only found a very pixelated unreadable fairchild 670 schematic.

 

[reinw33]

https://www.thesocketshop.com/lap-2...ocket-115-230v-slate-grey-with-black-inserts/
Thats the shaver socket I use , I strip out the 2 pin socket assembly to make space for other components and house it up in a dual gang metal clad surface wall box ,
likewise the LT transformer ,bridge and capacitors are housed in another dual gang box ,mounted back to back with the HT enclosure .
I often find an LT transformer from old style wall wart charge adapters and power supplies , I can pick up them up for free at my local recycling facillity , its easy to find 4,6,9,12,15 V dc versions .

I saved wiring , space and the need for metal working (other than drilling a few holes) in the psu enclosure by not including any switches and using metal cable glands instead of IEC and multipin sockets .
There only two screws required to remove the lid to change the voltage selection ,
everythings hardwired , passive RC filtering, theres very little to go wrong.
Of course in the case of a tube mic you can include LT regulation as required .

I was trying to think of a simple way of incorporating a neon lamp so it acts as a dummy load for the HT line at initial switch on ,

There are reasons you might want a switchable HT supply , like for bench testing ,
maybe a Lorlin 4 position 3 pole , with an off position and the 3 HT selections ,
If you wanted continiously variable adjustment you could incorporate a ten turn pot in place of the final series filter resistance ,

Hope the recovery is going well Ian ,

Hi tubetec
This is another great idea!
So the neon lamp would be sitting open in parallel if I understand you correctly, and turn on briefly when you switch the B+ to B+ No.2 And then turn off when you've switched all the way over to another B+ No. 2?
It really makes me think about this concept, thank you!
But wouldn't this work only when your B+ was higher then the turn on voltage range, or else would it conduct sufficiently as dummy load?
I assume we are always with tubes above 90V?


EDIT: Now I understand you. It's for warming up the tube on startup. Does it make a a real noticeable difference in any way?

 

[My3gger]

It's the TwinLine amp that I am trying to build, perhaps even prototyping first on wood by soldering point to point?
But some design question I have for you to get clarified to me beforehand please.

Why not build it on pcb? It is simpler, doesn't get dismantled after it is done, requires less metalwork, support is good, etc.
I regret prototyping Classic Solo because it works very well as is, mine was dismantled when i needed the same board for something else...
Twin Line amp or Classic Solo probably won't work well with much lower HT, less so starved plate.

 

[reinw33]

Why not build it on pcb? It is simpler, doesn't get dismantled after it is done, requires less metalwork, support is good, etc.
I regret prototyping Classic Solo because it works very well as is, mine was dismantled when i needed the same board for something else...
Twin Line amp or Classic Solo probably won't work well with much lower HT, less so starved plate.

Not familiar with classic solo, is this a guitar amp?
I can feel you though.
Reason being I don't have a laser printer yet for PCB production.
Solder point to point, imagine a big, thick piece of wood and mounted onto it, to try things, have some space, until I like what I hear and it works.
And then perhaps put it on perfboard or some ordered proper coated PCBs from the schematic from china.

You have to keep in mind it's my first tube project, I don't know what things sound like, how this preamp can sound like, or any at all.
Except those that I heard for the first time at the local NOS tube seller here where I live.

Concerning HT, man, I don't mind spending hours winding the Transformer until I get 300V, and make it as fantastic as possible, it's just the datashets, they don't mean anything, they say 1ma 2ma,.. 4ma Cathode current, I have no feel for the % THD values either. From looking the schematic for the TLA,I assume it should be around 800μa roughly speaking, how does one with 400μa c.c. sound like? I have no idea whatsoever.

EDIT: In hindsight, from the top of my head, a 400μa Cathode Current in that schematic above, would translate to roughly estimating a 150V B+, instead of 300V for 800μa.
What transistor values should I try out?Those that sound good, which sound good respective to B+? What's a good ballpark range for cleanness?
I have still questions, and I don't like having to look up datasheets all of the time, that don't tell me what sounds good and clean. I'm still not familair with all tube terms intimitaley. I definitly want a high enough HT.
CORRECTION: Assuming a 300V B+ with the twinline amp, the Cathode Current would be 1.1 mA. I'll need to read the manual thoroughly someday, but not when tired. But that's less than 100V for the tube.

 

[My3gger]

Classic Solo is on the same page you can find Twin Line Amp:
https://www.customtubeconsoles.com/diy
Pcb's are sold on the forum. In triode, cathode current is the same as in anode, that is 10mA for each Classic Solo gain stage. Output stage can be wired normally as mu follower, like first stage, or SRPP which gives more sound. Did you ever design power transformer with three taps needed for project like this? If not i suggest buying toroid for your first project, preamp also needs both signal transformers.

 

[reinw33]

Classic Solo is on the same page you can find Twin Line Amp:
https://www.customtubeconsoles.com/diy
Pcb's are sold on the forum. In triode, cathode current is the same as in anode, that is 10mA for each Classic Solo gain stage. Output stage can be wired normally as mu follower, like first stage, or SRPP which gives more sound. Did you ever design power transformer with three taps needed for project like this? If not i suggest buying toroid for your first project, preamp also needs both signal transformers.

Thanks for the link. Really good stuff. I once downlaoded all of those recently.

Yes, I'm aware of that, that the HT needs tappings, so far I came up with 3, of something like 12,6V for LT, of sth. like 48V for phantom and sth. like 300V for B+.
I'm getting surer, but yet not written in stone, as regulators need to be take into account.

Do you have any fitting psu suggestions for the TLA?

In terms of e.g. LT., Dropping from 48V would need a 18 watts resistor for both heaters of the e88cc IIRC for one tube, so that's a no go.
That's just some numbers I crunched, and I can handle 12,6V for both tubes in total roughly saying 0,9A, means it needs a two Watt resistor for both valve half's heaters, one tube.
I go very deep into the designing process of the PSU transformer, as well as the audio transformers and regulators and surrounding circuitry.

I'm not fully professional at it yet, because of the fact that, also I try to make it happen in one go with one specimen of transformer, and as fast as possible.

I do have 3 dedicated 48V CTed toroid transformers though too. But I want everything in one go, one transfromer.

But I'm very positive it'll work out just fine with time.

I assumed that the cathode impedance of the 12ax7 LPS is rather high, I assuemd 75k ohm, also assuming a 300V B+, thus:

In that schematic above, the TLA amp, the plate voltage sits at 85-ish Volts. Feel free to correct me if im wrong.

Can you please give a cathode current, if your knowledgeable enough, that sounds clean for a 12ax7-(LPS) and a 6922 (E88CC)? I assumed a 75k impedance of plate to cathode, is that far off?

Thank you everyone for your replies, it's nice to disgust this first, then later on realising design errors.

 

[reinw33]

EDIT: I'll try to find time to read and learn more about tube bias. Initially I put this to the side, but as the design plan of the transformer has advanced enough, it becomes more urgent to get the tube circuitry stuff understanding going asap.

 

[ruffrecords]

Some general pointers on the TwinLineAmp (TLA) and the Classic Solo.

Both these designs are very tolerant of HT voltage. You can run them on anything from 250V to 340V. This means you do not really need a regulated supply. All you need is a well smoothed one. Three RC stages of smoothing after the reservoir cap will generally be able to achieve 90dB of ripple reduction which iis plenty.

If you limit the HT to 250V and use 6922EH tubes then you do not need heater elevation. You can connect heater 0V to analog 0V. This is handy for running 12V relays from the heater supply.

I don't use the 12AX7LPS any more. They are too microphonic. My current favourite is the Sovtek 12AX7WA - nice and quite and readily available

The TLA is basically a pair of primitive tube op amps. The 12AX7 cathode is the -ve input. This has 47K connected to the output and another 47K connected to ground. Hence the basic closed loop gain is 6dB. Gain is increased by the addition of the GAIN1 and GAIN2 resistors/pots which act in parallel with the 47K from cathode to ground. He 100uF cap provides dc blocking. The GAIN1 or 2 set to 430 ohms you will get 40dB of gain.

It is a little bit more subtle than a simple op amp. The 47K from 12AX7 to gound also sets the gain of the first stage. As you reduce the value of the gain1 resistor you not only increase the closed loop gain but also increase the gain of the 12AX7 stage. The reason for this is to ensure the amount of negative feedback remains reasonably constant no matter what the gain setting. This is essential in order to maintain stability as you vary the gain over such a wide range. THis configuation is sometimes called a TransAmp.

The Classic Solo is much simpler. No negative feedback, just fixed gain stages. You can configure each stage either as an SRPP or a mu follower. The SRPP is almost identical to the TLA output stage and runes at about 5mA to 6 mA. The SRPP has excellent drive capability but quite high distortion.

The mu follower configuration has lower distortion than the SRPP but not the drive capability because it is single ended rather than push pull like the SRPP. To improve the mu follower drive capability you pretty much have to double the quiescent current.

I hope the above answers most of the basic questions about these two designs.

Cheers

Ian

 

[reinw33]

Some general pointers on the TwinLineAmp (TLA) and the Classic Solo.

Both these designs are very tolerant of HT voltage. You can run them on anything from 250V to 340V. This means you do not really need a regulated supply. All you need is a well smoothed one. Three RC stages of smoothing after the reservoir cap will generally be able to achieve 90dB of ripple reduction which iis plenty.

If you limit the HT to 250V and use 6922EH tubes then you do not need heater elevation. You can connect heater 0V to analog 0V. This is handy for running 12V relays from the heater supply.

I don't use the 12AX7LPS any more. They are too microphonic. My current favourite is the Sovtek 12AX7WA - nice and quite and readily available

The TLA is basically a pair of primitive tube op amps. The 12AX7 cathode is the -ve input. This has 47K connected to the output and another 47K connected to ground. Hence the basic closed loop gain is 6dB. Gain is increased by the addition of the GAIN1 and GAIN2 resistors/pots which act in parallel with the 47K from cathode to ground. He 100uF cap provides dc blocking. The GAIN1 or 2 set to 430 ohms you will get 40dB of gain.

It is a little bit more subtle than a simple op amp. The 47K from 12AX7 to gound also sets the gain of the first stage. As you reduce the value of the gain1 resistor you not only increase the closed loop gain but also increase the gain of the 12AX7 stage. The reason for this is to ensure the amount of negative feedback remains reasonably constant no matter what the gain setting. This is essential in order to maintain stability as you vary the gain over such a wide range. THis configuation is sometimes called a TransAmp.

The Classic Solo is much simpler. No negative feedback, just fixed gain stages. You can configure each stage either as an SRPP or a mu follower. The SRPP is almost identical to the TLA output stage and runes at about 5mA to 6 mA. The SRPP has excellent drive capability but quite high distortion.

The mu follower configuration has lower distortion than the SRPP but not the drive capability because it is single ended rather than push pull like the SRPP. To improve the mu follower drive capability you pretty much have to double the quiescent current.

I hope the above answers most of the basic questions about these two designs.

Cheers

Ian

Yes, it did Ian. Thank you very much for your reply.
Should the heater elevation voltage be chosen for the plate voltage when the tube is conducting or not conducting?
The 12ax7 has a max. of 180 Vhk and the 6922 200 Vhk potential.

 

[ruffrecords]

Yes, it did Ian. Thank you very much for your reply.
Should the heater elevation voltage be chosen for the plate voltage when the tube is conducting or not conducting?
The 12ax7 has a max. of 180 Vhk and the 6922 200 Vhk potential.

It is a class A circuit. The tubes are all conducting all the time.. Since the centre point of the SRPP output stage is at half the HT voltage this means the lower cathode is close to 0V and the upper cathode is close to half HT volts. Heater elevation therefore needs to split this voltage in half hence heater elevation should be 25% of HT voltage.

Cheers

Ian

 

[reinw33]

... heater elevation should be 25% of HT voltage.

..
Of course ruffrecords, I see now why.
Thank you.
The PP stage is in series, so it's 25% instead of 50% elevation. Makes total sense after your explanation.
It is more subtle than an opamp definitely.
Below is the transformer in process for you.

In the 19" 3RU rack, there is enough space for another dedicated P48 Tx.
I think about winding a prototype bobbin and use the other dedicated P48 Tx. as preliminary.
Reason being, I'm very confident I get the windings right, I just don't want to commit the one and only "original" bobbin yet, that came with it, until the windings have worked out for 6,3V and +250V on the prototype.
At least I think so.

Recently I read that someone says there are better sounding tubes than the 6922,
Ian, someone offers 7 or so E88CC NOS, should I avoid stocking more of those and look for a project with "better sounding tubes" at the ouput that is?
I'll try to come up with those alternative tubes in a second.
I made a screenshot but it's black and the tube info is lost.