Geloso G 211A negative bias supply advice

[Paul Mathew]

I am busy getting this amp back in shape and would like to replace the selenium rectifier supplying the negative bias to the grids of the 6V6 valves, with a suitable diode. I've just finished a AC/DC radio with selenium rectifiers and thought it prudent to replace them following the guidelines here: http://w3hwj.com/index_files/RBSelenium2.pdf
The author of this article suggests a resistor in series with the diode to simulate the equivalent resistance of the old selenium rectifier.
Not sure if it would be necessary in this application, but any advice would be appreciated.

[Nuvistor]

Is the old rectifier in good condition, if so I would leave it installed? Changing for a silicon diode may well require a series resistor to get the bias voltage correct if you decide to replace.

[Paul Mathew]

Looks ok to me, but I'd feel more comfortable fitting a silicon diode as I don't have any experience with them, and have read that they can fail and go up in smoke. I'm wondering why they didn't design the 6V6 valves with a common cathode resistor and by-pass cap to achieve the same result? Similar to the Quad 11.

[Nuvistor]

No problem with changing it if that makes you feel more comfortable, but adjust the additional series resistor to obtain the correct bias voltage.

Pros and cons of fixed and cathode bias, far too long ago for me to reliably remember, another forum member will probably give a much better explanation that I can.

[Ed_Dinning]

Hi Paul, if you search the forum you will see why fixed bias was often used in amps.

In this circuit the bias rectifier is under vey little stress. It could easily be changed to a 1N4002 and the change in forward voltage will make very little difference to the bias setting, which should be 17 +/- 1v

Ed

['LIVEWIRE?']

That type of bias circuit is more common than I thought. I've recently completed repairs to a Marshall Guitar amplifier (long thread on here about it!) which uses a 1n4007 in an almost identical circuit, except that there is a pre-set pot. in the Marshall, which allows the bias voltage to be varied, the idea being to set it at a suitable voltage for the output valves used.

[Diabolical Artificer]

Changing the selenium for a silicon is pretty simple, however while your at it you can make that circuit a lot better by using a pot instead of a dropper resistor.

If it were me I'd replace the two 100k grid resistors with a pot too, to get better balance and less THD and put a fuse before the diode and from the cathodes of the 6V6's to ground for protection.

A couple of 10 ohm resistors from cathode to ground wouldn't hurt and would allow you to measure current draw of the 6V6's to get better balance.

You get more power OP (output) with fixed bias, that is mainly why it is used, but this means you need matched OP valves to get the best result.

Andy.

['LIVEWIRE?']

Not sure if ready made types are available for 6V6 valves, but a device called a bias probe (Basically a valve base soldered to a valve socket, with either the anode or cathode pins brought out to leads allowing for the connection of a milliammeter) makes monitoring either the anode or cathode currents simple, the only caveats being that if the leads are in the anode circuit, the user must be aware that a DC potential of more that 200V, depending upon the circuit involved, will be present at the terminals of the leads used, and, if the probes are in the cathode circuit, allowance will have to be made for the screen grid current, usually only a few mA, when taking readings. A Bias probe can be made quite simply if not commercially available.

[Paul Mathew]

Quite a robust debate! I will spend some time following all the advice thanks. Meantime, I replaced the biasing circuit with new components and a 1N4007 silicon diode, and got the result in the pic attached. Interestingly, the circuit doesn't follow the schematic exactly. For example, only one half of the 12AT7 is shown in circuit, where in this amp it's an ECC83 (12AX7) and both triodes are employed in the first stage. The audio input is labelled 33 1/3 and 78, so I'm guessing it was designed for a turntable.

[Diabolical Artificer]

Neat job, nice, apropos of nothing are those caps Panasonic?

-15v seems a bit much, but I've not done any 6V6 OP stages, it's only -7 - 8v for EL84's . There looks room for a small trimmer/pot there on the left, end's of said pot to neg out and 0v, wiper = bias out, join the wiper to the neg supply with a 1M resistor in case the wiper looses contact, use something like a 50k.

You can check how much current each valve is drawing on the anode side of thing's, but 10r sense resistors make life much easier. You can get flush ish mount banana sockets so you can take take the cathode outside the chassis. also if you use a low wattage R, it can serve as a fuse in fault conditions, careful though, I've had R's go high rather than OC, not good. Two 10r (or 1r) R's and an inline fuse would be better.

Can't remember from schematic, but are there compensation filter's for the different speeds? Are there a few R's and C's or L's around the input?

Andy

[Nuvistor]

The circuits quoting -17 volts.

[trobbins]

If you are concerned enough to make the effort to swap out the selenium diode, then imho you should 10x to 100x more concerned that you know what cathode current is going through each 6V6 to ground (and hence the recommendations to add a cathode current sense resistor to each 6V6 cathode, or use a bias type plug insert tool).

If the 6V6's aren't set up properly, or there is a circuit fault developing, then it is all to common to see a future fault develop and cause major collatoral damage to your amp (damage to the output transformer primary windings, or the power transformer secondary HT (arancio) windings.

Hopefully your amp has a mains AC side fuse (ie. yet another schematic difference), and as you are confident enough to replace a diode, then imho you should really try and insert a 1N4007 in series with each 5Y3 anode - as a failing 5Y3 is another not-uncommon fault that could damage the power transformer, and that modification is just so easy to do nowadays that its a no-brainer for many restorations.

[Diabolical Artificer]

Just had a quick look at the 6V6 datasheet, bias is given as -15v or -19v for a Ik of 70mA in both cases, OP diss of 10 and 14w respectively. 14w is pushing it, I think best to run at a lower OP diss.

Looked again at the schematic and it has no voltages (apart from bias), what's the HT at the anodes?

Lastly any chance of some pictures?

Andy.

[Paul Mathew]

I've boxed this one up for the moment, but I remember the anode voltages being around 160V, a lot lower than the normal 250V.