Bijou All Tube Futterman Headphone Amplifier - Page 74

Yes, unloaded the B+ is fully adjustable. With the trimpot fully tightened from my previous attempts to achieve 250v loaded, my unloaded B+ voltage was at 278v which I promptly turned down to 250v.
**Again, something possibly relevant...B+ voltage hits a maximum, both loaded and unloaded, just after power up. After that peak, the voltage will slowly decline until it eventually settles on a median value. Unloaded the B+ is rock solid at 250v once it settles, seldom fluctuating only +-.1v. Loaded the B+ voltage will usually wander +-.3v, then every so often will wander up/down a few volts, and rinse and repeat.
Edit:Sorry to hear pabbi : (

The loaded PS with 100K, responding well, set at 250vdc is what you are looking for. Edit: If it isn't stable possible regulator problem.

Alright i'll try moving the heater wires around tonight.
i'll be also doing regal's mod to run the tubes hotter.
i might be buying some film caps to pop in.

edit: heres an internal picture. could the heater wires being tied to the 290VDC output from the transformers be the cause of hum?

Yes, I figured that even if my PSU could not hit the full 250v loaded, that it would atleast be adjustable between 190v and 230v or something of the sort. This to me says that something must be wrong other than needing the ez80.

And again forgive my noobdom, but what is a regulator? Is that the mosfet?

If you guys need any more voltage readings, etc feel free to ask. Many thanks for all the help.

On the AC side of the transformer, tying the wires together is no problem.

On the DC side, the heater wires should be twisted together and then run as close the metal chassis as possible wherever they go and as far away from all other wires as possible.

The mosfets are Q1 and Q2 in the PS schematic. They are attached to the big heatsinks and are the active devices in the power supply regulator circuit.

I think you'll have to replace these, both of them. I know it's really a pain because when I was debugging the prototype Bijou I killed mine a few times. But the behavior indicates that they are not working.

The mosfets die when the PS is shorted for any reason and no matter how short a period of time. Even just an instant.

After you replace the mosfets we can work on the other voltages. But you should still try to find an EZ81.

Ok, I'll replace them. Luckily EZ81 available local. Unfortunately soonest available mofsets can't ship for 10 more days ><. *braces for withdrawal*

If your B+ is holding at the lower voltage you can still run and listen. Nothing will break.

This post is to answer some PMs I have got recently regarding my suggestion to use 100µF film cap for the output capacitor C5. Sorry that I can’t answer your PMs individually guys… that’s very time consuming for me!

The desired capacitance (µF) of the output capacitor is depending on the impedance of the used headphone (Zheadphone). To ensure an acceptable bass response, we need a –3dB point of the high-pass filter (formed by the output capacitor C5 and R13//Zheadphone) at far under 20Hz (theoretically audible by human ears).

The –3dB point can be calculated using the following formular: f = 1/(2π.R.C)

=> f = 1/{2π. (R13//Zheadphone).C} = 1/{2πC.(10KΩ //Zheadphone)
=> C = 1/{2πf.(10KΩ //Zheadphone)} = 1/{2πf.(10KΩ.Zheadphone)/(10KΩ + Zheadphone)}

In real life, Zheadphone is of course not real Ohms, but… for the covenience we can assume it as lZheadphonel.

Obviously you can see from the formular that, the higer the lZheadphonel, the smaller the output capacitor is required, and… the lower the lZheadphonel, the larger the output capacitor is required. In some post back, I suggested using a good 100µF film cap for high impedance headphones with lZheadphonel = 300Ω which theoretically will result in a –3dB point at ~5.5Hz (at least).
It can’t hurt to use a larger film cap (220µF or 330µF) here if you can get it.

To sum up… depending on the impedance of your headphone, you can calculate that for yourself and determine what you are going to use for the output capacitor C5. The stock output capacitor C5 = 470µF is theoretically adequate for a wide range of headphones, even the ones with lZheadphonel = 32Ω . Keep in mind that many real world headphones are not able to reproduce low frequencies down to 20Hz !!!

Pabbi, also tight shots of the pots/wiring, as well as the input and output connections will probably help to analyze the situation.

Will do - give me a few hours, and I'll document in abundance.

Pics

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One thing, and I have tried this several ways - the shell of the XLR I have tied together, and have tried that grounded to the case, and not, and not tied together. No difference. Both make contact with the case, and are bolted to it with nuts / bolts.

Pabbi, carefully pack up the amp with bubble wrap and send it. We would need a super-macro camera to have any chance of finding a problem over the net. The pictures just don't have the resolution necessary to see what is wrong. My gut is that the wiring on the pots or input xlr have not been wired correctly but it just isn't apparent to me from the photos. Runeight? Ferrari? See anything? Namaanf/Amb are the Uber balanced mode experts, but are usually busy. So it's probably my shot, to figure this one out. If I had it by friday I would troubleshoot at Amb Laboratories, but I wouldn't want you to spend too much shipping. Although there are 2 high quality scopes available there, along with Amb's expertise. Edit: Priority mail is about $15.00 I think. If sent tomorrow, it might make it by friday. We would want tracking, if possible.

Unfortunately, I might have to agree. I can't discern enough from the pictures. Whereas, about 1 minute of scope measurements should diagnose the problem.

I hope that you guys can arrange to work this out.

Drained and boxed. Including a balanced 650 & cable, as well as the botched pot (one good channel) in case the wiring guidance is a contributor.

Thanks guys - seems like I'm always lowering the bar in DIY.