Is 5.6V DC Too Low for 12AX7 Heaters? Peavey Classic 50

[Krwkka]

While checking out the bias supply on this amp I found that the DC on the heaters of V1 V2 seems low.

The voltage from the -27 volt supply is low at -24.4 vs. -27 on the schematic. I'm measuring the heaters correctly this is giving me 5.85V DC per heater.

When I switch to the dirty channel the -DC bias voltage drops further, leaving me with only 5.55VDC for the heaters.

I know it's not hugely off, but I want to know if this is right before I close it up. Should I be concerned?

By the way, 121.1 VAC from the wall.

[Mick Bailey]

It is a little on the low side, but not so off that it would seriously affect the amp. I wouldn't be concerned at all with 5.85v, but the drop when changing to the dirty channel doesn't seem right to me. What does the AC right off the PT do? See if this drops - if not you may have a high-resistance somewhere. FWIW I've recently worked on a Guytron amp and that had heater voltages just under 5.5v by design.

[mozz]

You shorten the life drastically at low voltage and high voltage, there is a chart on the web. Sure these aren't AC voltage?

[g1]

According to this schematic, there are a couple resistors in the -27V line. R61 & R62, are they good?
Agree with Mick that the heater voltage should not change when switching channels.

[Krwkka]

Thanks for putting up the schematic, G1. I shoulda done ...

The little resistors are fine. The AC doesn't budge when I switch channels.

The only thing I can think of is that something in the relay circuit is doing this. Does that make sense?

[g1]

The relay also runs off the -27V line, so if it affects the heaters, the line is not holding up like it should.
Are CR11 & 12 both good? C42 & 43? Is there any AC voltage on either side of R61?

[Enzo]

Where are you measuring 5.5v BETWEEN? if your 27v is down to 24v, that still leaves 24.4v for two 12v heaters in series. 12.2v per so 6.1v per side as series wired. measure from pin 4 to 5, not from something to ground. So the two resistors measure 11 ohms?

Is the reverb IC getting hot? It runs on the 27 too.

How much ripple is on the 27v rail?

[Krwkka]

Thanks for the suggestions, guys. Wife and I are getting ready for our daughter's 29th birthday dinner, so I'll pick this up tomorrow or Monday.

[g1]

Save us some cake.

[Krwkka]

No cake! Sad I am...

Anyway, Ripple is at 45mV. I did measure the heaters incorrectly. They're fine.

I would like to return focus on my original problem, red plating of the EL84s. I believe it is somehow related to the 27 volt rail, but over my head.

Do you mind going here? Red Plating