Red-plating 7189

D

Diverted

Active Member
Hi all, I could use some help. I have an amp here (RealisticSAF40B) that has a moderate redplate in one of the four output tubes (push pull 7189s). It doesn't follow the tube; it happens in the same location, regardless of which of the two tubes in that channel I use.

What I've done:
I swapped out the electrolytics and paper/wax caps, all resistors checked and on spec. The usual cleaning pots etc., all tubes test good though not super strong. I haven't noted voltages on tubes yet but will after I write this. One thing I have not done yet is change out the selenium rectifier that rectifies 40 volts from the transformer and looks like it splits it off to power the heaters on the two preamp tubes and also the bias supply.
This is a fixed bias amp as there are no cathode elecrtolytics/resistors dropping straight from the cathodes/grids to ground. Instead they go into a resistance network and elsewhere in the set.

Unfortunately I do not have a good schematic, so I'm kind of winging it It's pretty fuzzy and low res. See pic below.

Anyway, there are no bias pots anywhere on this amp, so it's got to be set fixed bias. Assuming I've got the bias too hot, how then do I cool it off? (in general terms for now) hopefully more in depth as I get voltages and some guidance on how the bias setup in this amp works.

If you all could help me deal with this and maybe cool the bias off a bit, that would be fantastic. I would rather run it cool and conservatively and save the 7189s than push it too hard and cause potentially major issues.

Edit: Here's the best shot of the schematic I have. It's not great but better than nothing! Thanks. http://imgur.com/a/OYa23

Thank you!
SAF40B schematic.jpg
 
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I would check voltages on that tube socket and compare it to the others, if it's showing higher readings I would then trace it back and check resistors and components upstream.
 
Thank you for responding. Just ran some voltages, here's what I got. It looks like they're all pretty close so far.

Plate voltage on both 7189s in the channel that redplates are 419 after warmup (schematic calls for 415 but I am running at 122VAC, not the 117 called for in the schematic).

Center tap voltage to ground is 423. Voltage drop is 4 volts.
Resistance from center tap to each plate is 107.9. That gives me a plate current reading of 37ma? If I'm doing this wrong please tell me!

The voltage drop across the 30 ohm resistor connected to each cathode in the channel is 2.7VDC. The resistor measures 29.6 ohms so it's not fried or anything.

And on the selenium rectifier, yes I will replace that! Should I add some sort of dropping resistance after it? I assume I do but if so, how much?

Schematic: http://imgur.com/a/OYa23

Thank you!~
 
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Any idea why that one particular socket (not tube) is red-plating?

And Can you tell me what I did wrong to get37ma vs your 45?
 
Thanks. So I guess my next step would be to swap out the selenium rectifier for some 1N4007 diodes?

I think the amp was rated for 20WPC.
 
Diverted,
A couple of thoughts looking over your voltages and the schematic. First, I had a problem with red plating of a 7189 in a Scott 299B that was location dependent and traced it to poor contact of that tube socket with the tube pins. Don't know if you've cleaned and tightened the tube sockets, but that's a must. especially in the one that's red plating.

Second, what are the control grid voltages on each of the pins (pin 2)? The schematic shows -15.5 volts nominal, so you should be somewhat close to this and fairly consistent across the tubes. This voltage is what primarily determines what current the tube draws and the power dissipated. Try it with the output tubes out and in and see what this voltage looks like on each (the voltage will be somewhat higher--more negative--with the tubes removed). If it varies much, there could be an issue with the resistors in the bias network for that particular tubes circuit. Some variation is expected with the tubes in depending on the condition of the individual tubes, but it shouldn't be way different between tubes.

A problem with old amps in modern homes is high AC line voltage. These amps were designed mainly for 117 VAC (as is yours, per the schematic), but modern line voltages often look more like 122VAC (like yours) up to 125 VAC. The higher the line voltage, the higher your negative bias supply voltage and that means the pin two voltage will be more negative, which may make the tubes run cooler, but this might be offset by increased plate voltages. A help (not necessarily a fix) for this is to put a CL-80 thermistor inrush current limiter in the primary circuit of the power transformer. Even if it doesn't help the output tube draw much it will protect your caps, transformers, and power switch from the high voltage/current surge when you first turn on the amp.

Next, looking at the voltages you reported, the 7189's are indeed running hot. If you use the conventional way to calculate watts of idle plate dissipation (no signal into the amp--I presume your measurements are made with no input signal--i.e., quiescent) is to determine the cathode current for each tube and subtract about 3 mA for the screen grid draw. With your measurements, this means that you are seeing cathode current of 2.7 VDC / 30 ohms = 0.09 amp (90 mA) per pair (there is a 30 ohm resistor for each pair, so the voltage you measured is for both tubes). Dividing the current by two, you get 45 mA per tube. Subtracting 3 mA for the screen grid, you get 42 mA estimated plate current per tube. At 419 VDC on the plate, this means you are seeing 419V x 0.042A = 17.6 Watts quiescent plate dissipation per tube versus the spec max of 13.2W. That is hot! At the 37 mA plate current you determined, that would still be 15.5 watts dissipation, or 17% over design max.

Most thinking today is to run the tubes at around 80% of the max quiescent dissipation, or about 10.6 watts. Reversing the calculation, running at this quiescent dissipation level means you would want to see the cathode voltage at about 1.7 VDC. Getting the cathode voltage down to this level (and therefore the tube, or cathode, current draw) means that your pin 2 voltages need to go UP in negative voltage (that is, become more negative so the tube conducts less).

Then, to your original question and assumption, you would need to increase the bias supply voltage at the exit of what appears to be R30, the 10K resistor that is setting your bias supply voltage by lowering the value of this resistor. If you change out the bridge rectifier, it may well raise the nominal (negative) output voltage anyway as you suspect compared to the -44 V shown on the schematic. Keep an eye also on the filament voltage to the 12AX7's to keep it around -23 to -24 VDC. Base all this on actual measurement of the voltage exit the new bridge rectifier with the 12AX7's in place. These voltages may already be low if the bridge rectifier is failing, as others have noted.

So, summing up, you would appear to be running the 7189's really hot as you suspected. Expanding on Steve's advice, It would be nice to know the pin 2 voltages for each output tube and the voltages you measure for the bias supply at the exit of the bridge rectifier (where the schematic shows -44 VDC), at the 12AX7 filament supply exit R31 (against the voltage shown on the schematic-- -2x VDC), and at the exit of R-30 (10K) at the labeled bias supply point that goes to the tube pairs (I can't ready the label).

AND, be sure the redplating socket is clean and tight--you may also want to reflow the solder joints on this (maybe all) of the output tube sockets in case there's a bad solder joint.

Dave

NOTE: Edited to clean up bias voltage references, per below. Dave
 
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Oops, after all that I REVERSED the thought on the pin 2 voltages. You would need a MORE negative voltage (less positive) to reduce the tube current draw, as 6DZ7 says. So, he may well be right that your bridge rectifier is producing low voltage and needs replacement to RAISE the negative bias voltage and cool off the tubes. Call it electronic dyslexia. Sorry about that!
Dave

Note: edited post #8 to correct. Dave
 
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6DZ7 said:
With 2.7v through 30R you're pulling 90ma. For two tubes that's 45ma per. Multiply that times the plate voltage and you get idle current/power of @19W plate dissipation. See data sheet.
Click to expand...

Yah, but that includes screen current, right?
 
No problem.
Going to swap it out for some diodes now. One thing: Since the new silicon rectifier will be more efficient would I then need to add resistance afterwards to get
 
Just replaced the selenium rectifier with four 1N4007s. I'm now getting 430 on the plates and 433 at the output center taps. Divided by the voltage drop I'm getting about 27ma (I think, if my math's correct). This looks to be OK. Don't see any redplating either, but then again I didn't last night either until I was playing the amp with the lights off.

Plate voltages seem too high (415 is spec). Maybe build a bucking transformer to drop six volts off the AC?
 
Only problem with that is that the gz34 only requires a two amp winding. I'd worry about a 5u4 in there with its 3a requirement. I don't have any specs on the power transformer so I don't know if it could handle it.
 
Diverted said:
Just replaced the selenium rectifier with four 1N4007s. I'm now getting 430 on the plates and 433 at the output center taps. Divided by the voltage drop I'm getting about 27ma (I think, if my math's correct). This looks to be OK. Don't see any redplating either, but then again I didn't last night either until I was playing the amp with the lights off.

Plate voltages seem too high (415 is spec). Maybe build a bucking transformer to drop six volts off the AC?
Click to expand...

I'm not sure I'd worry that much about this amount of over-voltage if the dissipation is in-spec.
 
I haven't seen any readings of pin 2 to ground on the output tubes. A leaky coupling cap could bring the bias voltage up (more positive, less negative) causing increased current flow. I have found brand new caps to be leaky at times.

Shelly_D.
 
Dave 451,

Thanks for the lengthy reply. Somehow I missed it earlier but just read through it.
Here’s some updated info:

The control grid (pin 2) voltages range from 15.59 to 15.61 (-15.5 nominal) with tubes in.

As for the line voltage, I think I will rig up a 6.3v bucking transformer to bring down the AC volts. As to your CL-80 suggestion, I have some on hand. But are they necessary when using a GZ-34? I could certainly add one; no trouble at all.

The bias supply voltage is -46.6.

Finally, yes I scrubbed up the tube pins and tried to cinch them tighter. Thank you!


Went over the numbers again:

Pin 3 (cathode) ranges from 1.79VDC on one channel to 1.35 on the other.

Pin 7 (plates) 437
Pin 9: 403 (grid 2): 403

Center tap to ground: 439.5
Resistance from center tap to ground: 112.3

If I’m doing this right, it looks like 2.5vdc drop divided by resistance gives me 22ma, which seems cool enough.


See anything wrong with my calculations here? Thank you again!
 
Looks like your new bias bridge rectifier is doing the trick. As 6ZD7 said, it looks like the old Bridge was failing and under-biasing your tubes causing them to draw excessive current. The control grid numbers look good. At 1.79 volts on the cathode, this gives exactly 27 mA on the one set of tubes by calculating from cathode current, matching your plate current number and giving 11.7 watts dissipation (88% of max rating).

The 1.35 volt side is a little light, giving about 8.5 watts quiescent dissipation at 437vdc. With the control grid bias voltages being so close, this may mean this pair of tubes is a little weak. A power test might be helpful.

As for the CL-80, I do install them even with a tube rectifier to cut the line voltage down a bit and help save the power switch contacts.

How doe your filament voltage look for the 12AX7'S?
Dave
 
Dave and all, thanks again for the help. I am getting about 25V across C22 so it looks like the 12AX7 filaments are about right. The other 12AX7s in the set that are fed by the 6.3v winding are right on spec as well. Put in a CL-80 on the hot side of the line too. Thanks for the tip.

As for the lower wattage channel, I kind of expected that. When I got the amp it had two 7189s and two 6BQ5s, one of each per channel. Pulled the 6BQ5s and found two 7189s in my stash. When I tested them luckily the numbers were good for two more or less matched pairs, but one pair (the one with lower output) were weaker. So that's what I figured would happen. I've been listening to the amp and it sure does sound nice. Plates are nice and black as well, so so far so good. Looks like the rectifier did the trick. Cheap fix!

Thanks again,

Ted
 
Well, the plot thickens. Have the amp on again and giving it a close look, and that one tube is still red-plating, it appeared after a few minutes on. It's very slight, not very noticeable but in a darkened room I can clearly see it. I'm wondering what I should try next?
 
If you move the tube now , does the red plating tube move or does it stay with socket.
If it moves with tube its the tube. If not you probably have a resistor or two that drifted. That should show by voltages
 
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