DC blocking capacitor in AC 'amplifier' output on mVM

[6SN7WGTB]

I have an item of valve test gear which is an AC millivoltmeter (1mV - 300V ranges).

It has an 'amplifier output' on the meter driver stage - which produces ca. 150mV at FSD on any range, intended as typically an oscilloscope preamplifier.

It is taken from the cathode of the final amplifier/meter driver valve (an EF184), via a 100Ω resistor, with then 51Ω to ground giving it its characteristic impedance.

FWIW the cathode is connected to the suppressor grid. The cathode has ca. 3-4V DC on it, exactly per spec.

My question is this: the schematic shows a 6µ8 electrolytic in series with the output, presumably for DC blocking as well as any pass filtering.

However my unit is missing this but from every detail appears entirely original.

Should I install one?

[Silicon]

I would use a film capacitor in place of the electrolytic if I intended to make much use of the amplifier outlet.

[Diabolical Artificer]

The EF184 will be configured as a cathode follower so there'll be some DC offset without the blocking cap. Can you see if it looks like the cap was fitted and removed, are there any clues? It could be they forgot to fit it at the factory.

Pretty sure I have some 6u8 axial's just waiting for the day when I need an odd value.... if you want one I can pop one in the post though a 10u will probably do just fine.

Andy.

[ronbryan]

The TF2600 schematic on the Radio Museum does not show a DC blocking capacitor in series with the output.

Ron

[6SN7WGTB]

Originally Posted by Diabolical Artificer

The EF184 will be configured as a cathode follower so there'll be some DC offset without the blocking cap. Can you see if it looks like the cap was fitted and removed, are there any clues? It could be they forgot to fit it at the factory.

Pretty sure I have some 6u8 axial's just waiting for the day when I need an odd value.... if you want one I can pop one in the post though a 10u will probably do just fine.

Andy.

Very kind offer Andy, thank you. I've 8µF and 10µF axial in stock so will either do that or go film.

[6SN7WGTB]

Originally Posted by ronbryan

The TF2600 schematic on the Radio Museum does not show a DC blocking capacitor in series with the output.

Ron

Very interesting.

Looking at the schematic in my printed original manual, it's evident that the blocking cap is a later drafting change - the weight and scaling are different to the rest of the schematic.

I might be over-sleuthing it, but the component list looks like C42 is a later edit - it's the one component whose value is out of line.

Not to say it might, later still, have been removed I suppose...

[RichardGM]

If the intended purpose of the output was for an oscilloscope, the assumption might have been that the scope would have an AC-coupled option. But then, when they decided to include the capacitor, if it was for a scope why have several μF when the scope would have a high input impedance?

[6SN7WGTB]

The manual also mentions pen recorders which might not like DC up 'em.

[barrymagrec]

We used to use the amplifier output to feed an audio monitoring amp, useful for knowing where you are on test tapes and checking erasure into the noise, for instance.

Would definitely expect a blocking cap on the output.

[6SN7WGTB]

Just measured the DC on the output and it's 0.5V.

Have ordered 4µ7 50V film cap...