DMM causing voltage drop on EL34 plates – and other delights

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DMM causing voltage drop on EL34 plates – and other delights

Postby Leif B on Mon Jan 23, 2017 8:42 pm

Hey all, I have a bit of a head-scratcher here

Sound City L120
Losing power/volume when played. Sounds a little like compression, or tremolo. No scary clicks or pops, just a considerable dip.

While trying to find a voltage drop somewhere (either plates, screen, or bias increase) I noticed that when I try to measure the plate voltage, it dips, and the amp goes eerily quiet.

Pulled V4, cutting preamps off, and then was able to get steady voltage readings on the EL34s (nice and quiet power amp)

tried new PI coupling caps – no joy

disconnected NFB – no joy

traced DC on coupling caps, replaced – no joy

replaced a few plate & cathode resistors – no joy

After more testing, I've found it's definitely in the preamps. There's some chittering/squawking with the treble triode down low, the mid triode pumps out mids *and* hum, and when I scope the signal path, its looks like the bass triode makes things unstable. All very strange to me.

I've just put new power supply caps in, so there should be enough draw for lots of gain, but even when quiet, it sounds terrible.

Any insight, much appreciated.
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Re: DMM causing voltage drop on EL34 plates – and other deli

Postby Vortexion on Mon Jan 23, 2017 10:37 pm

Leif B wrote:I noticed that when I try to measure the plate voltage, it dips, and the amp goes eerily quiet.

Just off the top of my head...

I wonder whether it's possible that you've got some parasitic oscillation being generated in the pre-amp/EQ stages somewhere (not necessarily all the time), and that maybe, by putting your DMM between the EL34 plates and chassis earth to measure the DC Volts, you're exacerbating it, making it go completely ape? As you'll know, parasitic oscillation is so high frequency as to be inaudible, but will keep your output valves very busy indeed. Consequently, you get loads of current draw, which causes a drop in the plate voltage, and the "eerily quiet" phenomenon could be evidence that your tubes are busting a gut to amplify radio frequencies - until you disconnect your DMM and things return to almost-stable. If there is parasitic oscillation, it could be down to a number of factors, one of the most common being dodgy lead-dress.

If your 'scope has decent bandwidth, it might be worth trying to replicate the DMM-on-plates scenario with the scope attached, to see if you can reveal any RF happening.

Also, are you sure that your DMM is not faulty? Have you got another DMM to compare it against?

Some food for thought, anyway. :)
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Re: DMM causing voltage drop on EL34 plates – and other deli

Postby Leif B on Mon Jan 23, 2017 11:10 pm

Thanks for the reply

Yeah I was thinking parasitic oscillation, and you've given me food for thought.
For sure some of the leads could be shielded.
Forgot to mention also – I can hear the valves themselves sort of squeaking... if that's the right word.
I've observed this before with an amp turned up loud on a dead load, but this is happening at low volume. Wasn't sure if it was power valves or preamps, but parasitic makes the most sense.

DMM should be good. But I did think of that, and checked with another – same deal.

You know, when I first noticed the eerie quiet thing, it was more one side of the OT than the other.
I wonder if maybe a few of the 'swamp resistors' need to be replaced. I thought about it, but figured they don't see much stress.
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Re: DMM causing voltage drop on EL34 plates – and other deli

Postby Vortexion on Tue Jan 24, 2017 12:06 am

The L120 is certainly a prime candidate for parasitics on account of its complicated pre-amp/EQ circuitry.

Shielding: AFAIK, the only wires that should be shielded are the ones from the input sockets to the first stage grids. Beyond V1, good lead dress should be enough.

Other possible causes of parasitics: dry solder joints, earth wires being taken to the wrong point in the chain, a poorly manufactured or failing pre-amp valve...?
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Re: DMM causing voltage drop on EL34 plates – and other deli

Postby pdf64 on Tue Jan 24, 2017 9:49 am

My experience is that oscillation is to be expected when probing power tube plates (with an unscreened / open chassis); the additional wiring in the plate circuits with the meter and leads greatly increases opportunity for coupling back to earlier stages.
There's no reason to be concerned or surprised by things going wacky; it's just a practical demonstration that lead dress matters.
You've already found the only way to measure it reliably, that is by muting the signal at their grids, eg by removing the tube of a preceding stage.
I suggest the LTP V5.
But really, unless there are strong indications that the OT primary is bad, there's no point in measuring the power tube plate voltage; it will be within a few volts of the HT.
Any voltage difference between plate and HT is immaterial and it's safer to measure the HT (blunt probe tips tend to slip off tube socket terminals especially when pressing hard to break through surface crud), so just measure the HT and be done.

Regarding the hum, I've found that the waxed fabric insulation used for some wiring in Sound City amps can become leaky, eg between runs of adjoining wires.
Can't take anything for granted with old amps :say_what:
Last edited by pdf64 on Tue Jan 24, 2017 10:07 am, edited 2 times in total.
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Re: DMM causing voltage drop on EL34 plates – and other deli

Postby Leif B on Tue Jan 24, 2017 9:52 am

This great, thanks muchly you two
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Re: DMM causing voltage drop on EL34 plates – and other deli

Postby Unit_1 on Tue Jan 24, 2017 6:26 pm

Whenever I find things happening that seem unlikely or impossible I buy a new battery for my DMM and then check the voltage of the battery that was in there. In most cases I find a battery under 8v, and the weird things go bye bye.

How old are your DMM's, could it be both batteries are on the way out?
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Re: DMM causing voltage drop on EL34 plates – and other deli

Postby Leif B on Fri Jan 27, 2017 10:00 am

Not a DMM issue for sure.

So here's the story: I pulled everything out of the amp and rebuilt the power section. Measurements are all good, nothing scary. But then... It seems the OT is bad. Sigh.

Now, it's a bit of a weird OT – never seen one like it.
The primary has four wires – as if it's two single-ended primaries. The two black wires are joined together to make a CT, and from there we get a familiar three legged OT primary.

The secondaries (plural) are strange too. It LOOKS as if there might be two separate windings for two speakers – like perhaps this OT could be used in a stereo hi-fi amp with two single-ended outputs. To be honest I'm still trying to wrap my head around it, so my data is a little muddled, but all in all, I think this is one more culprit in the amp, making it sound terrible.

It definitely **sounds** as if there's a short in the OT secondary, but I can't put my finger on it completely...

It's as if there are two 'sides' to the secondary – they are electrically separate from each other, BUT, when wired for play, each colour is joined like-to-like – as if perhaps it's two 60-watt windings combined to handle 120 watts.

The colours are (2 x) black (common), orange, brown, yellow, red
Measuring DCR from the common, both 'sides' give familiar readings for each respective tap – until you get to red.
So it looks like this:
Side 1:
Bk (comm)
Or – 0.6ohm
Br - 0.7ohm
Y - 0.9ohm
R - 1.6ohm

Side 2:
Bk (comm)
Or - 0.5ohm
Br - 0.7ohm
Y - 0.8ohm
R - 3.8ohm

Repeat: there is no short from side 1 to side 2. They remain isolated.
Curious, no?

I'd love to hear some similar experiences if any of you have one
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