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EL 34 Red Plating - What to do?

Speakers don't go bad, they get blown up. If that happens on full power with a tube amp you could damage the tubes or OPT or both because of excess voltage and arcing and the like. Tube amps don't like to be run without load. Semiconductor amps don't care.
 
PE9ZZ said:
Speakers don't go bad, they get blown up. If that happens on full power with a tube amp you could damage the tubes or OPT or both because of excess voltage and arcing and the like. Tube amps don't like to be run without load. Semiconductor amps don't care.
Click to expand...

:)

I agree that you need to always have a load on a tube amp and that (almost all) SS amps don't care. For tube amps that load gets reflected back to the output tube via the output/impedance matching transformer (the result of which is a high impedance load to the output tube) and keeps the output tube from drawing too much current and dumping heat into the primary of the output transformer. Kinda like flooring a car in neutral - not a good idea. Don't even turn it on without a load. It might not get damaged the first time, but why test fate?


Lotsa ways speakers can go bad without being blown up, but being blown up is a leading candidate.
- Lead wires can get disconnected between the terminals and the VC. This can happen for a number of reasons including metal fatigue, manufacturers defect, over-current, over-extension and corrosion. There is probably a few other reasons. I've managed to save a couple of speakers by reconnecting the leads between the terminals and VC. In fact voice coils can fail randomly - mostly due to manufacturers defects or age/corrosion/material failure.
- Of course there are mechanical failures that can occur that hurt the speaker, but don't open-circuit the VC. Those can be: VC bottoming that deform or damage the VC, spider sag, surround sag, surround failure (foam rot, etc.), magnet shift, corrosion, manufacturers defects, kids with fingers, cats with claws, Pete Townshend, etc.

I hope this is helpful.
 
Think I found the problem, there might be a couple.

(I made some serious mistakes when recapping I fear.....)

Hope this makes sense...

On tube #one, the .22 cap is connected to pin 7. Pin 5 is connected to pin 6 via a 1.3K resistor. .22 cap is not connected to pin 5 at all.

On tube #two, the .22 cap is connected to pin 6, which is the junction to pin 5 via a 1.3K resistor

On tube #three, the .22 cap is connected to pin 7. Pin 5 is connected to pin 6 via a 1.3K resistor. .22 cap is not connected to pin 5 at all.

on tube #four, the .22 cap is connected to pin 6, which is the junction to pin 5 via a 1.3k resistor. The 1.3K resistor has also come loose.


So, I suspect I am supposed to connect .22 on tubes #two and #four to pins 6 like tubes #one and #three instead of pins 7???

And, of course I have to reconnect 1.3K resistor on tube four between pin 5 and 6

Thoughts?

(Other than me being a dolt...)
 
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How does your tube # refer to the circuit diagram you posted before?

What tubes are being used on the amp?

I don't see 1.3 K resistor on the circuit you posted?

So to make sure we are on the same page, do you count the pins as shown below:

upload_2020-3-15_11-42-3.png
 
I am going by the pin #'s on the bottom of the actual tube, EL34's

There are four of them, going from left to right on the amp.
 
The caps are connected to pin #6 because it isn't connected to anything in the tube:
el34.png

This makes it a convenient support point. The resistor from pin #6 to the grid (pin #5) is to prevent oscillations. Pin #7 is connected to the heater and not to the capacitor.
 
Interesting, according to the EL34 pin assignment below, pin 7 is one of the heater connection and pin 6 is not used. And pin 5 is the control grid.

upload_2020-3-15_11-53-40.png

How could the amp be working with the connections you posted? It's beyond me...

BTW, how long the amp been working properly before red-plating after the recap?
 
Thanks! I figured thats what they were using pin 6 for :)

It looks like they have the .22 cap on tube 1 connected differently than tube 2 in the schematic???

The schematic is for one channel only...
 
Uhm, yeah. I don't understand why its been working either??? I had a local shop "fix" it a couple of years back - it had been working Perfectly prior to that???

And I thought I just replaced caps as they had been put in before?
 
Assuming the design is using the unused pin 6 to hold the 1.3K grid resistor, so the .22 cap should connect to pin 6. But please check if the 220Ks are connected to pin 6 and ground for all tubes.

Also, pls let me know which tubes are red-plated.
 
Perkri said:
Thanks! I figured thats what they were using pin 6 for :)

It looks like they have the .22 cap on tube 1 connected differently than tube 2 in the schematic???

The schematic is for one channel only...
Click to expand...

I really don't see the connection difference in the schematic... must be me..:-)
 
According to the statement "On tube #two, the .22 cap is connected to pin 6, which is the junction to pin 5 via a 1.3K resistor" in the previous post, tube # 2 connection seems okay to me. But yet it's the one red-plated. We may be fighting multiple problems.

For now, can you measure the grid voltages (pin 5) and the cathode (pin 1 or 8, BTW is pin 1 and 8 connected together) DC voltage for all tubes to see if we can find out what's odd with tube #2?
 
I wonder, if tube one not being connected properly could cause #2 to redplate?

Suspect I should start by connecting the .22's to the junction on pin 6 as they were before?
 
Quick question. The 1.3K resistor on tube 4, from pin 6 to pin 5 is broken. How many watts does it need to be?
 
Perkri said:
I wonder, if tube one not being connected properly could cause #2 to redplate?

Suspect I should start by connecting the .22's to the junction on pin 6 as they were before?
Click to expand...

If we make change now without measuring the voltages I suggested above, we may never understand the cause of the problem.

But if you are not comfortable working with high voltages on tube gears, I fully understand. 50V+ is high voltage to me too..:)

1/2 W 1.3K resistor should be fine.
 
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