5E3 Deluxe Bias Question

[earache]

Hello

I'm working on a 5E3 Deluxe.

I understand that it is a cathode biased amp.

I'm not getting a negative DC voltage at pin 5 of the power tubes.

.Should there be a negative voltage measured at the cathodes?

The 250 ohm resister measures 247 ohms and the 25 uf capacitor is a brand new Sprague 25 uf x 50 V.

The grid stoppers on both power tubes measure within spec.

I haven't worked with a lot of cathode biased amps and I don't want to mess up the amp while testing it.

Amps with bias pots often have the negative voltage value printed on the schematic,so I know what to look for.

Where is the negative voltage derived from and which connections and/or components should I be looking at?

What should I expect to see when testing the bias of this amp?

Thanks In Advance

[Leo_Gnardo]

Zero volts on the output tubes control grids is normal on self-biased aka resistor-biased amps. The grid voltage is referenced to ground by 220K ohm resistors. What's important is grid voltage with respect to cathode voltage. The cathode voltage is what, about 15-20V? Then the grid voltage is negative relative to cathode voltage by the same amount. Alll is good. Nothing to worry about.

[Enzo]

As Leo said, the cathode is positive.

[pdf64]

...Amps with bias pots often have the negative voltage value printed on the schematic,so I know what to look for...

It's worth noting that in a fixed bias amp, a bias voltage noted on the schematic is a nominal value, for guidance only. For a real world amp on the bench, consider that its HT voltage (or power valve screen grid/anode voltage) may be a fair bit different to that on the schematic; we shouldn't ignore that and focus only on bias voltage, eg if the magnitude of the HT is 10% larger than nominal, then the bias will need to follow suit.
A good example of this is the 'brown' Fender Deluxe 6G3. Their schematic HT is 375VDC, anode and screen grids at 365VDC, bias supply output noted as -26VDC. That works out to about 40mA anode current, leading to an idle anode dissipation of 14.6W; that seems rather excessive but shouldn't result in immediate valve failure. A bias supply output in excess of -35V, bringing the idle anode current down to about 20mA, would be better.
However, their actual HT is typically over 450VDC. If their bias supply was adjusted for -26VDC, then their anode current may be around 80mA, and the power valves would overheat massively and probably be damaged in short order.
Hence when checking the suitability of power valve bias, it's a very good idea to check the power valve anode / cathode current at idle, even if a schematic with typical VDC is available. As the characteristics of real world valves may vary somewhat from their type bogey, and so their anode current can be expected to be perhaps +/-20% of that which the published charts indicate it should be.

To derive the anode currents from the electrode voltages above, I used nickb's handy online tool http://bmamps.com/ivds.html

[earache]

Thanks everyone, as always, for taking the time for these thorough answers.

Always a great experience posting a question here, always informative and thoughtful answers.

Much appreciated!

[Enzo]

Remember, this is just a guitar amp, not precision lab equipment. Nothing in them is precise. Look at the note on the layout drawing for the 5E3, all measurements within +20% and -20%.

[pdf64]

We've debated this before I think, but my view is that the 'All voltages +/-20%' note can't be taken as being reasonably applicable to every voltage in the amp; rather it's more of a 'downstream' thing, eg where the normal valve variance from type norm causes a perfectly acceptable variance of anode/cathode current, which will be reflected in resistively loaded anode VDC levels etc. Something of disclaimer, to avoid the Fullerton phone line being clogged up with queries about a 12AX7 anode reading 200VDC rather than the 170 noted on the schematic.
I don't think it's reasonable to apply it to 'upstream voltages', ie those at or 'close to' power supplies.
eg a heater VAC that should be around 6.3 but measures 5.1 or 7.5 should warrant further investigation; as should a 6V6 screen grid that draws 50mA at idle and hence measures 386VDC, rather than 405VDC.
Obviously a competent tech will do so regardless of those voltages being within the 20% tolerance.
But a noob etc checking voltages on a problem amp, having read the 20% disclaimer note and forum confirmation of that, maybe won't.

[Enzo]

The point is that we are trying to instill the idea in the OP that the voltages are not crucial. How many times are we asked this: My 15v is only 14.8, do you think that is why the fuses are blowing in my speaker? +/-20% just means don't take any noted voltage as gospel.