Basic question about stock HST-11a PSU

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trans4funks1

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Feb 4, 2013
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I don't have one to look at first hand yet, but I have noticed that many of the white market PCB kits include Power Supply retrofits.

I think I understand that the goal is to get a 120vDC B+ supply for the mic mods.

What is going on in the stock OEM supply? Is it a much higher B+?
 
I found a schematic for the Apex 460:

http://recordinghacks.com/pdf/apex/Apex460-circuit.pdf

and I found a schematic for the Apex 450:

http://www.gearslutz.com/board/attachments/geekslutz-forum/50139d1202159814-schematic-wrong-smapex450.pdf

I found various threads on the internet discussing these schematics as they apply to the Apex 460. The schematic for the power supply in the 460 document doesn't show the cable pin out accurately but apparently the 450 document does.

I have looked at the two schematics and I am still curious about the DC voltages with the mic hooked up as a load.

The Apex 460 PSU schematic suggests that there is 200vDC B+ and 12.6vDC for the heater.

The Apex 450 mic amp schematic suggests that there is 6.3vDC on the heaters and the B+ label is too blurry to read.


Question:
The mic amp schematics show the heater hookups for the mics in a way that confuses me. The Apex 460 schematic shows the heaters wired in parallel but lists the voltage as 12.6vDC while the Apex 450 schematic shows the heaters wired in series but lists the voltage as 6.3vDC. Shouldn't that be the other way around?

Question:
Does anyone know, from first hand measurement, what the B+ and the heater supply voltage are with the mic connected as a load?


Thank You.
 
Mine was 208V and 5.7(!) at the mic. Remember, the stock 7-pin mic cable is JUNK!!! It is THE reason for the low FIL voltage! The FIL wires are too small. Redco makes a good product for cheap as do several other manufactures. As with anything, higher priced products are "better" but, there is a point of diminishing returns. (Think Sommer tube cable, great but expensive!)
 
Thanks for the info, especially the part about the heater voltage not holding up across the length of the factory cable.
 
Is that the only reason to call the cables junk?
Might under-heating be desirable?
I know the connectors are likely a compromise...don't know about general durability and screen/shield,
Might be useful for different project or power supply:
either parallel conductors for less V-drop or make adjustable power supply
 
There is definitely some loss of the heater voltage along the cable, but I haven't tested a different cable to see if it would be any different. Regardless, with a modified PSU you can adjust heater so that it's a perfect 6.3V at the mic itself.
 
Gotham GAC-7 is specifically made for use with remote tube operation and it provides 2 oversize conductors in the 7 cable bundle for the heater voltage and ground return.

http://www.gotham.ch/en/index.php?section=docsys&cmd=23_details&id=20


The resistance spec for the GAC-7 heater cable is < 39 Ohm/km.

I hadn't thought about the possibility that you might have to boost the voltage at the PSU when using lesser grade cabling.

I wonder if anyone knows what the resistance rating for the common Alctron OEM cables are?
 
Melodeath00 said:
Does it matter much if you can just adjust until you measure 6.3V at the tube?

As I understand it, you will have to modify the stock PSU to have the capability to adjust the heater supply voltage. I don't think it matters much if you actually have the ability to make the adjustment and also have the current capability to sustain the extra load presented by the cable.

I'm trying to figure out if anyone knows what the current capability of the stock power transformer is.
 
shabtek said:
Is that the only reason to call the cables junk?
Might under-heating be desirable?
I know the connectors are likely a compromise...don't know about general durability and screen/shield,
Might be useful for different project or power supply:
either parallel conductors for less V-drop or make adjustable power supply

No, that is not the only reason to call it junk! The first, and most glaring, problem is the undersized FIL conductors. The MAIN problem, regardless of conductor function, is flat out low quality conductors. Feed some various voltages through the cable and read. Low and behold, they are all low. I can not remember all of the voltages I tested but, the PSU FIL voltage was higher than 6.3V, more like 6.7V. So, with that said, that is a one volt or a 15% loss! YOW! A 15% loss through the mic cable has to be generating some kind of heat not to mention possible noise. Now, start throwing 200V plus through it and one could run into a potentially dangerous situation not to mention noise(again).
As for adjusting the voltages to read correct at the mic: Yes, this could be beneficial. But, again, the problem is not the PSU, it is the stock cable! Adjustments made to the PSU voltages are merely a "Bandaid" to the real problem. As for running FIL at lower power, this is a terrible practice that has gone on for years because Neumann did it for years in their U47. Yes, FILs can be ran at +/- 10% of their rated voltage(in most cases!) but, it is not a good idea to do so. Running the FIL at a lower than rated voltage CAN extend the life of the tube but, not always. Plus, the sound/tone of a tube/valve FIL being ran at a lower voltage can start to sound "mushy" IF the voltage drop starts to get too low. On the other hand, running a tube/valve FIL at a higher voltage results in that valve's life to be shortened by as much as 50%(!) and it does not sound that good. Hard to describe really. It makes the sound/tone a little "hard" in the midrange. That is the best way I can describe it. I have done extensive testing on an Epiphone Valve Junior which comes with it's FIL jacked way up. I measured 6.8V on mine and it had a terrible tone even after some mods. After I soldered a resistor in there, the FIL voltage lowered and the tone improved. Not night and day but, markedly better (in the mids mainly).
On the other hand, some tubes FIL have a wider voltage range namely compactron tubes. This allows an 11V FIL to be ran at 9V, an 8V to ran at 6V ect. One can increase the life of a tube by slightly lowering it's voltage. By lower I mean 6V for a 6.3V tube/valve, 12V for a 12.6V and so on. Notice that it is a 5% drop not 10% plus! The slightly lower voltage will not effect the tone/sound.

  Before any of you start with your "defaming" of what I said, go out and buy Merlin Blencoe's books on tube amps. Yes, it is not microphone book but, loads of valuable tube info that applies to ALL tubes not just amp circuits. So please, DO NOT take what I say as truth, go find and read for yourselves!
 
If memory serves the stock Apex 460 schematic is wrong:  the heaters are wired in series in the mike, and the stock PSU uses a 12V regulator to power them.

I can't answer the secondary voltage question (e.g. why it's so high):  it's possible that they were just using a turn-key transformer and that's what was available for the cheapest.
 
I just recently modded a hst-11a PSU and it certainly does NOT have the FIL ran at 12V. It had a cheap 6V regulator jacked up with a diode for a grand total of 6.7V. Running the tube at 12V would make much more sense as the FIL noise would be cut in half. I have been trying to source a transformer with more appropriate voltages that fits inside the stock PSU box to no avail. Has anyone sourced such a transformer?  The stock transformer sucks by the way!
 
The stock transformer does not really cause any problems. It is just a run of the mill, off the shelf part. A much nicer transformer could be use such as a toroidal. The stock transformer should be more purpose built to suit it's use, that's all. Having a transformer with a slightly lower HV(95~125V) and a higher voltage FIL(12~18V) with a bit more current capacity would be nice. It should run a little bit cooler than the stock one. Mine runs a bit north of warm. It could stand to be a little cooler. This is with the modded stock PSU with a pair of 62V zeners. I have yet to measure voltage loss with this setup. As I said before, I have yet to source such a transformer and any info would help.
 
mabell313 said:
I just recently modded a hst-11a PSU and it certainly does NOT have the FIL ran at 12V. It had a cheap 6V regulator jacked up with a diode for a grand total of 6.7V. Running the tube at 12V would make much more sense as the FIL noise would be cut in half. I have been trying to source a transformer with more appropriate voltages that fits inside the stock PSU box to no avail. Has anyone sourced such a transformer?  The stock transformer sucks by the way!


As I mentioned in the OP, the schematics for the 450 show a 7806 regulator and a 6.3vDC supply and the schematics for the 460 show a 7812 regulator and a 12.6vDC supply... but then the accompanying schematics for the mic circuit both show wiring schemes on the heaters that would work with the opposite regulators and vDC values.

It seems like documentation isn't consistent so perhaps the mic and PSU assemblies are not consistent either?

At first I thought it would be useful to figure out what a actual Apex 460/Alctron HST 11a circuit mapped out as and prepare an accurate schematic, but if they vary from time to time it seems like t would just further confusion to share a schematic that is wrong half of the time.

Thanks for the info.
 
I've seen 460's wired both ways - one was very old however.  I fully expect that Alctron will use whatever method saves 0.00001 cents that day based on parts on the grey market.  :D
 
Hi, Does anyone know what  are the expected Polarization Voltages from the PSU in the Apex 460 mic in stock form?
I'm looking for info on what might the voltages be for Cardioid, Fig 8 and Omni.

Thanks you so much
 
By default, the polarization supply is 120V to match the B+ line.  The backplate of the capsule is 60V (taken from a resistor divider), and the front capsule is grounded.  Hence you have for polarization voltages:

0V = omni mode
60V = cardioid mode
120V = figure 8 mode

Since the stock supply has a 9-position switch, there are 6 extra patterns between those above.  Starting at omni mode, each click of the switch raises the polarization roughly 15V.
 

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