You could try the bypass to see if the problem is somewhere else
Tried injecting a signal on the wipers of the balance control. Still have distortion to speaker terminals. Pin # 2 on the output tubes are clean.
You could try the bypass to see if the problem is somewhere else
It is a cathode feedback design, which is pretty sophisticated for the time. I do hope it's possible to salvage this one. Even if DC resistance is only half an ohm from cathode to ground, daveyh should find about 20mV across that resistance if the output tubes run at 40mA bias current. If the finding is truly zero mV, then the tubes are not conducting and we can conclude that those tubes are kaput. Well, assuming that the heaters are lit up.
It the distortion like a breaking up, or a cut in frequency response?
Have you tried Headphones? It also could be a dirty speaker switch.(this happened to me on a HK 230 Ballad integrated amp. it's confusing because it is both channels, having 2 bad transformers, or 2 or more bad output tubes is unlikely.
You still haven't confirmed bias current flowing in the output tubes. Use one of the methods discussed above. If normal bias current is confirmed, then the logical next step is to test the OPTs for internal shorts. If there's no bias current and voltages on all tube elements are nominal, then the tubes are bad regardless of readings on any tube checker. The only other possibility is bad tube sockets, which is extremely unlikely on every one in a set of four.
That depends on what you have available for test gear. One of the simplest ways is to use an inductance meter or LCR bridge. The method I often recommend for folks without that capability is to yank the output tubes and drive the OPT 8-ohm secondary backwards from a 5VAC or 6.3VAC source such as a small filament transformer, with a one-ohm current sampling resistor in series. The voltage across that resistor tells you how much current the unloaded transformer is taking. A good OPT will consume only 50~200mA of magnetizing current in this test, while a shorted one could easily take over ten times that much. Make sure the sampling resistor can handle at least 5W. Voltage on the primary side will normally be quite high during this test. For example, an 8K:8R OPT has a turns ratio of 32:1, so primary voltage would be 6.3 * 32 = 202V end-to-end or half of that from CT to either end. If magnetizing current is suitably low, then check primary voltages as confirmation of a good OPT.Test the OPTs for internal shorts? How do you suggest I do this?
Is this the 2R resistor in series with the 155V secondary on the power transformer? The voltages you reported previously on the power tubes were not at half nominal, they were very nearly normal. Please explain.
What's the voltage on the screens (pin 9)? If it's anywhere much over 300v, then I can see why you need so much negative bias voltage (by the datasheets, they need -15v bias with 400v plates and 300v screens).
432v is running the plates perilously close to the design max of the tube- so it's not at all surprising that so much negative bias is required. I'd be a bit concerned about the tubes wanting to run away (I've had a few 7189s do that, even Mullards and Amperexes), with that much voltage on them.
Regards,
Gordon.
Well, it's a great relief to know that your OPTs aren't blown. To keep them from being damaged and to get the best sound from your amplifier, you should now attempt to bring its operating conditions on target. One of the most common causes of excessive B+ voltage is AC line voltage above the original spec. You can check this by measuring heater voltages. They should be very near 6.30VAC if the unit is operating on the AC line voltage it was designed for.
Also, it would be smart to add current sensing resistors in series with each output tube cathode. I suggest 1-ohm 1% 1/4W resistors in this case. These resistors make it convenient to measure each tube's cathode current, so you can match push-pull pairs and set the bias trimmers for optimum performance, and they can act as fuses to help prevent OPT damage if power tubes fail shorted.[/QUO