T-Bone Retro Tube PSU modified for AKG Solid Tube

[Super8]

Hi, new to the group and wondering if anyone has any ideas about this

Like many, I have an AKG Solid Tube -- 2 in fact -- with dead psu units. I also have a cheap and decent-looking PSU for the Thomann valve mics. I noticed on the Thomann website that a purchaser comments that he has adapted this PSU simply and successfully for the AKG mic. However, Thomann will understandably not share the customer's contact, so I can't ask exactly how he did it! Can anyone have an educated guess?

The details are: Solidtube: anode voltage quoted 205v, heater 12.6v (from a single pin).
Thomann PSU: transformer supplies 175v regulated to 150v via zener diodes anode, plus 6.3v each to 2 pins for the heater.

It'd be great to find a solution and I'm sure it could be used by many in a similar situation.
thanks to you all,
Mike

 

[Tubetec]

Welcome to the forum Super8,
The tube heaters would need rewiring for 6.3v for starters , xlr pinouts are more than likely different , chances are you can get some extra HT voltage by changing the zener and series dropper resistors . Solid tube schematic is easy to find ,compare it with the Thomann psu and figure out the changes needed . One thing about tube mic cables is the heater wires are normally thicker so theres less voltage drop , draw out diagrams with the correct colour codes to avoid mistakes . Establishing a good grounding scheme to get lowest induced noise can be tricky . Contributor Ian Thompson Bell has an excellent document on grounding techniques . As the output stage of the solidtube is a cathode follower you may benefit from an elevated heater supply ,but you'll need to compromise to some degree so neither tube section is run beyond safe limits . Anyway you've come to the right place , lots of tube mic builders here who are very helpful.

 

[Super8]

Welcome to the forum Super8,
The tube heaters would need rewiring for 6.3v for starters , xlr pinouts are more than likely different , chances are you can get some extra HT voltage by changing the zener and series dropper resistors . Solid tube schematic is easy to find ,compare it with the Thomann psu and figure out the changes needed . One thing about tube mic cables is the heater wires are normally thicker so theres less voltage drop , draw out diagrams with the correct colour codes to avoid mistakes . Establishing a good grounding scheme to get lowest induced noise can be tricky . Contributor Ian Thompson Bell has an excellent document on grounding techniques . As the output stage of the solidtube is a cathode follower you may benefit from an elevated heater supply ,but you'll need to compromise to some degree so neither tube section is run beyond safe limits . Anyway you've come to the right place , lots of tube mic builders here who are very helpful.

Thanks a lot for the reply. I've got the pin-outs and drawn out some diagrams, assembled appropriate plugs and cables.

One basic thing I'm wondering: psu transformer voltage is marked 175v, then regulated by the zeners to arrive at 150v to anode, so even without that reduction would this need to be stepped up somehow to achieve the 205v as quoted on the solidtube schematic?

my remaining solidtube psu actually went up in a puff of smoke today with one vocal left to do for a project I'm using it on, so keen to get the soldering iron on! ;-)

 

[Khron]

One basic thing I'm wondering: psu transformer voltage is marked 175v, then regulated by the zeners to arrive at 150v to anode, so even without that reduction would this need to be stepped up somehow to achieve the 205v as quoted on the solidtube schematic?

If the transformer is marked "175V", that will be AC. Rectifying and smoothing that will get you about 1.4x that, so maybe 245V or so. Potentially more, depending what current that 175V winding is rated for (since mics rarely draw more than 1-2mA from the high voltage supply.

 

[Super8]

If the transformer is marked "175V", that will be AC. Rectifying and smoothing that will get you about 1.4x that, so maybe 245V or so. Potentially more, depending what current that 175V winding is rated for (since mics rarely draw more than 1-2mA from the high voltage supply.

OK, great -- that's very helpful. So on the anode voltage side just a question of identifying the zeners and the resistor and changing them accordingly. If I know those figures and the 150v current output it should be possible to arrive at 205v.

Thanks a lot.

 

[Khron]

You might also need to reduce the values of the series dropper resistors. Probably easiest to just solder ones in parallel with the stock ones, though.

 

[Super8]

You might also need to reduce the values of the series dropper resistors. Probably easiest to just solder ones in parallel with the stock ones, though.

yes! Thank you. I think with the zeners I'll have to take them out just to read the markings... unhelpfully soldered so they're largely on the underside of them ;-)

 

[Super8]

You might also need to reduce the values of the series dropper resistors. Probably easiest to just solder ones in parallel with the stock ones, though.

An update on this little project: had a look inside the Thomann psu and traced the line back from the high voltage output pin and we appear to have:

2 x 75v zener diodes (marked 1N4761A) and an array of 4 little 0.6v ones further down the line.

60 k ohms total resistors ( 2 x 20.6k, 3 x 6.8k).

As the goal is to increase the output from 150v to 205v, I was thinking of just replacing one of the 75v zeners with a 20v (BZX85c20) which I can get quickly and easily here in France. For the resistance, should it drop proportionally to 40k ohms so? I thought about just bypassing one of the 20.6k resistors?

Would be great to know people's thoughts.

 

[Tubetec]

Then you only end up with 100v , you need to drop more volts across the zener chain not less .

 

[Tubetec]

Its in parralel with the load after all

 

[Super8]

Its in parralel with the load after all

Ah of course, yes, sorry! And decrease the series resistance proportionally to the increase in volts in the zener chain I guess?

 

[David König]

Can you please send me or publish the pin-outs and drawn out diagrams? Thanks in advance!

 

[thequesobandit]

Hey Super8 did you ever get it working? I have one solidtube that I would like use the thomann power supply. Would you be willing to post your pin outs and mods to the PSU?

thanks!!

 

[scotland]

Hey Super8 did you ever get it working? I have one solidtube that I would like use the thomann power supply. Would you be willing to post your pin outs and mods to the PSU?

thanks!!

Hi, did you manage to find out how to comvert the Thomaan power supply for the solid tube?

Many thanks....