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#26 pre amp
#26 pre amp
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Old 2nd January 2018, 06:29 AM   #4561
euro21 is offline euro21  Hungary
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#26 pre amp
Peek current of AZ1 (in this schematic) about 55mA.
If one half of rectifier is weak, sparking is possible.

Try to increase Rt up to 400R (max. 800R).

Rt= Rsec+n2*Rprim+R1

n2= square of ratio
Rprim: DCR of primary
Rsec= DCR of secondary (if it is CT, the half of secondary)
R1 practically DCR of first L + optional -equal- resistors series with AZ1 anodes.
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Old 2nd January 2018, 08:21 AM   #4562
Vbenonisen is offline Vbenonisen  Norway
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Thank you euro21.
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Old 2nd January 2018, 08:39 PM   #4563
Vbenonisen is offline Vbenonisen  Norway
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I found two 47R resistors and soldered them to the secondaries, but it didn't do diddly-squat. I'll try two more in series (as I have several) and see what happens. Perhaps there is no way to "rectify" (pun intended) the rectifier when it has sparked several times already.
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Old 2nd January 2018, 09:13 PM   #4564
euro21 is offline euro21  Hungary
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#26 pre amp
1.) Are you sure, that first and second capacitor are quite good?
Replace its temporary to another ones.

2.) Try to put temporary small (few 100nF) capacitor after AZ1 (cLCLC). Output voltage will be increasing a bit.
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Old 2nd January 2018, 09:31 PM   #4565
euro21 is offline euro21  Hungary
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#26 pre amp
Sample voltages.
Attached Images
File Type: jpg AZ1 hybrid choke input.jpg (79.0 KB, 385 views)
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Old 2nd January 2018, 10:09 PM   #4566
Vbenonisen is offline Vbenonisen  Norway
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I have ordered a couple of new and smaller capacitors (2 x 20uf) to replace the first and second with. I will try that first. Thanks again euro21.
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Old 14th January 2018, 06:16 PM   #4567
adam1016 is offline adam1016  United States
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Hello all

I have been gathering parts for an 01A preamp, which should be similar enough to the 26 to share this thread (for now, at least). I've probably posted about it before, but it's pretty much a direct schematic copy of Ale Moglia's 01A, using his gyrator boards and coleman regulators.

I'm planning on using the attached image to create the B+
power supply, though maybe changing the rectification a bit.
Hopefully this will give about 200V on the B+.
Click the image to open in full size.

Before I build, there are two things I am wondering about and would like to clear with the DIYAudio community. First, I am having some trouble designing the coleman filament regulators to give me 15V at the output, as all the designs for this voltage seem rated at much higher currents, I'm starving the filaments at 200ma, and using filament bias. I have the raw supply kit, and was planning to use a hammond 266L28 transformer, with two 14V windings, and then on the raw DC supply, to have C1&C2 be 10,000uF, 35V, and have each dropper resistor be a 5W wirewound, and I think I've decided on .5 Ohms. Can somebody who has used these boards chime in with some advice? I've emailed rod, and this is pretty much what I arrived at from his reply.

The second question I have is how I should setup the power. I'm going to have the power supply in a separate chassis, and in past amps, I have applied filament, and then applied B+, but the coleman regulator manual seems to suggest the opposite. What's the proper power-on sequence?

Here's the schematic (Thanks Ale!) for the amp:
Click the image to open in full size.
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Old 14th January 2018, 07:00 PM   #4568
andyjevans is offline andyjevans  United Kingdom
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That's one of my PSUs you have there, and it was indeed meant for Ale's Gyrator, so should work. I should point out that for the gyrator you need some 01A in good condition. I suggest the ST versions which generally work better with the gyrator. I have a number of globe 01As and they don't work at all with the published design of the gyrator - I'm getting 200v on the anode. They may be weaker - I do have a small number of 01A that work, but I do have many that don't. Maybe the values of the gyrator can be changed - I haven't gone into that.

For order of turn-on I turn the filaments on first. For Rod's regs 200mA gives you a large sense resistor value of 4.5R at 1 watt. You should be OK with 4.7R. You design the filament supply to give you the required voltage. With Rod's latest boards you should be OK with 13v which gives you a headroom of 4.5v. 14v would also be OK. Note the 22k resistor to ground which I used with choke input to satisfy critical inductance. This wouldn't be needed with just dropper resistors, but I'd recommend choke input if you can - smoother.
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Old 14th January 2018, 08:09 PM   #4569
euro21 is offline euro21  Hungary
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#26 pre amp
As you can see, 14V AC is enough for filament biased, starved (200mA) 01a.

The raw supply DC voltage is a bit high (higher R.C. dissipation), 5-6V headroom is enough to R.C. regulator.

Instead of 0.5Rs use for example 4.7Rs, so raw supply will be about 16.5V.

In my preamps filament and HV powered at the same time (for a decade). Low power preamp tube usually not sensitive to cold cathode-rising anode voltage problem (few mA anode current).

The situation is different with larger power tubes, my 10/10Y/801/801a tube's filament start first, then HV.
Attached Images
File Type: jpg 01a filament 14VAC raw, R.C starved.jpg (94.6 KB, 268 views)
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Old 14th January 2018, 09:02 PM   #4570
Rod Coleman is offline Rod Coleman  United Kingdom
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#26 pre amp
The Raw DC for 01A that Euro21 presents should be perfect for 01a preamps. The 5V bias should be enough for a preamp.

In general, I prefer to heat the filaments before applying HV. Please let me know the paragraph № that hints differently, I'll check & amend it!
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