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Old 10th August 2010, 03:32 PM   #1
sangram is online now sangram  India
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Default Repair and adjustment tips

Hi

I have a tube amp on my bench that blew the bias adjustment pot. The manufacturer is on vacation till the end of August, it would be a month or so till we got the correct replacement part.

I dialled the bias pots back all the way and basically have to run the entire adjustment procedure from scratch.

I've never worked with tube amps at this level, so was hoping I'd get a few pointers.

This is a KT88 PP amp with fixed bias, and two 12AU7s per channel. One for preamp duties, the second for phase splitting.

The bias for the output tubes are derived from a small negative supply generated by a series-pass transistor, and there is a pot that adjusts the level of bias. Each tube has a single pot, and one of the pots is blown (measures dead short) and the bias is set to minimum level, refusing to adjust upward.

User has already switched the tubes and the problem stays where it is, so the tube is probably fine. Amp has not been powered up since it came in, still on the workbench. The diagnosis was made by looking at the schematic and testing a few parts in-circuit and on their own. Relevant schematic section is attached:

Click the image to open in full size.

Part values:

R19, R510: 100K
R20: 1K
R505: 10K
R26: 100E
VR2: 22K (test shows 0ohm across the resistive element, all other pots test fine)

Do let me know if you need any more info/pictures.

The pot is not available locally. It's 22K multi-turn 1 watt wirewound with a 4mm shaft, and the only alternative is a 10K part or the right value with a 6mm shaft (which won't fit the panel hole). I'm guessing a 10K part won't work?

Here was the plan:

1. I was hoping to be able to power up the amp without any tubes, and check which pot position yields most negative grid voltage (to shut the tubes down) to prevent tube damage during the first power up. Is this at all possible? The tubes are Gold Lions, so was keen to prevent any wear and tear if I could help it

2. Once I get minimum grid voltage from pin 5 of KT88 to ground, I would insert tubes in the working channel and adjust bias to the manufacturer-recommended level of 300mV (test points E/F in the schematic, range recommended is 250-450mV, lower bias for first 100 hours).

3. Once this is done I would measure the resistance of VR3/4 (working channel) and insert a fixed resistor of approximately the same value in place of the fused part VR2. This would enable the owner to use the amp till I got the right part.

4. When the manufacturer returns from vacation in September, I would insert the correct part and remove the fixed resistor, and dial the bias to the long-term level of 450mV.

Awaiting your comments, feedback and suggestions. Help is much appreciated

Edit: here's the thread with the pictures of the part: Help identify this multiturn pot

Last edited by sangram; 10th August 2010 at 03:42 PM.
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Old 10th August 2010, 04:16 PM   #2
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I would use a VOM on the ohms scale on the good pots to determine which pot position will yield the max negative grid voltage. It's the direction that puts the B closest to the -62v buss. No need to power up.

But, may I suggest that you use the 10 K pot that you have on hand in series with a 12K resistor to make up a quick fix. The only question is which end of the pot to install the resistor. I'd place the pot closest to the -62 v bus (higher on the drawing) first as this would be most conservative. If that wouldn't allow for proper adjustment (high enough plate current), I'd swap them.

BTW- I totally approve of multi turn pots in this application. I'm just astounded to see how often manufactures use 5 cent single-turn pots in this application. It makes bias adjustment tedious and is a major pet peeve of mine.

Last edited by Captn Dave; 10th August 2010 at 04:20 PM. Reason: Added unnecessary rant
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Old 10th August 2010, 04:17 PM   #3
DF96 is offline DF96  England
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VR2 is a potentiometer, not simply a resistor, so it can't be replaced by a fixed resistor. It would have to be replaced by two resistors. You could use a 10K pot instead of 22K, if you change R505 from 10K to 3.9K or 4.7K. You can't assume that each valve will have the same setting - this is why there are separate pots.

I am puzzled why the pot has shorted - this is unusual. Is there a short in the wiring? Are you sure the pot has shorted? The symptoms you describe would happen if the pot slider has simply lifted off the track. Then R510 would apply the maximum negative bias - that is why R510 is present as a safety feature.
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Old 10th August 2010, 04:29 PM   #4
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Yeah, I like the idea of replacing R505 with a higher value. It's much cleaner. But that will only work if you are lucky enough to need the pot at the "top" of the circuit. But there is a way to improve your luck. You can rotate the tubes until you find the "hottest" tube (the one that needs the highest negative bias voltage) and place that tube in the "B" socket.
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Old 10th August 2010, 04:31 PM   #5
kmtang is offline kmtang  Canada
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Yes, you can have the KT88 tubes removed and set the negative bias voltage at the grid.

Once this is done, power off the amp and put the tubes back.

Power it up and check the negative bias voltage is present and further adjust the KT88 idle current as required.


Johnny
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Old 10th August 2010, 04:46 PM   #6
DF96 is offline DF96  England
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It all depends whether you want to do a proper repair, or a temporary lash-up. To do a proper repair you need either the correct component, or a simple compensation for the 'wrong' component (i.e. 10K pot instead of 22K pot, with corresponding reduction in the series resistor R505). If you are charging a customer then you need to do a proper job.
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Old 10th August 2010, 05:43 PM   #7
sangram is online now sangram  India
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Wow, thanks for all the replies.

@DF96: The amp is within warranty, so no, we're not charging for the repair but keen to get it back to him so he can enjoy it till the proper part arrives, which is why the lash-up is being planned. We would like to get the amp to factory condition (no authorised tech in my country, I'm hopefully being compensated for the repair but by the distributor. Client is not paying a dime.) To your question, the pot is measuring a short across the track. Even after disassembly, with the wiper out of the circuit. I agree it is most unusual for that to have happened as the current through the pot is miniscule.

@Johnny, thanks so much - that's very helpful and probably answers my questions extremely well. The schematic indicates a desired grid voltage of -55V, which I guess I can measure on the tube sockets. I do understand that each tube will have slightly different cathode voltage (which is really the exposed test point). The manufacturer has numbered the tubes from 1 through 4 already, I assume this is done as part of initial factory setup. This is very helpful, thanks so much. I was worried of blowing something if the tubes were not in the circuit.

@Capt Dave: Thanks for the very helpful reply The pot is in parallel with one of the resistors, I assume the top of R510 is where I would get maximum grid voltage, so would a reading close to zero across R510 indicate I have my target? Which would also probably mean that the added 12K would have to be toward the ground (more positive voltage) to keep the bias on the lower side?

Appreciate the great help, all

I do have another possibility, using a single-turn, small sized (1/8W) volume control pot. If I get a dual, say 47K, and parallel the two gangs I could get about 1/4W of power handling and a value close enough to the original, would this work? The only problem is that these usually have carbon tracks and will fail to open circuit, putting the tube at full conduction or close enough The original was rated at 1 watt (I think) but I don't think the grid really draws too much current - would this work? Too risky?
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Old 10th August 2010, 06:07 PM   #8
DF96 is offline DF96  England
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The pot carries about 2mA, and the grid should draw almost no current at all, so power rating should not be an issue. The main risk, wiper losing contact, is already taken care of.
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Old 10th August 2010, 06:22 PM   #9
sangram is online now sangram  India
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Thanks, I guess I'll go with the mini 47K dual pot then seems like the best option to me!
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Old 10th August 2010, 06:29 PM   #10
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The exposed test point, "B" for example, is the grid voltage. I think you understand that changing this grid voltage changes the idle current which you measure across the cathode resistors as cathode current. I just want to be sure we're clear on that.

V505 and your fouled up pot in series form a voltage divider. Your pot is at the top of the voltage divider so the wiper picks up voltage at the top of the range. R510 has a very important function: it prevents the destruction of the very expensive gold lion KT88 if the wiper in VR2 should lift. If it should, the grid would see -62 volts which would put the tube at cut-off. Without that resistor, the grid would see no bias voltage and the tube would be lost. R510 does not have another function in the circuit per se.

The added 12 K resistor could be in either position. Remember the circuit is a voltage divider. You can only access the voltage through the wiper so the pot needs to be installed in a range of the divider where you expect to find the required voltage. Perhaps a better suggestion would have been to place a 6K at each end of the pot. Do you follow that? That's why I mentioned rolling the tubes to find the "lucky" tube. Each one will need a different grid voltage to produce the spec cathode current - that's just the nature of tubes as imprecise devices.

You could use a single 47K pot and get away with that too. Leave the R505 in the circuit. Be careful, the single turn pots can be touchy.

Having said that, you should be careful not to try anything you do not fully understand because those Gold Lion tubes are very expensive and it doesn't sound like your customer or distributer are fully committing to paying for the repair let alone a replacement tube. You don't want to be caught holding the bag.

Last edited by Captn Dave; 10th August 2010 at 06:37 PM.
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