Tube problem?

[richidoo]

Good discovery!

With the sub disconnected is it merely "better," or is it totally cured, with no distortion and balanced channel levels?  

Is the subwoofer something you added recently which caused the change in SQ or has it always been there?

If you are using the SP-8's tape output for the sub, then does the Monitor/Input switch position affect the problem? Maybe the tape amplifier is not on unless the switch is up. The tape output and the main output have separate cables, it is not the same signal, so there could be a tape buffer (current amplifier) circuit, which if not engaged or if broken could cause this.

The cable length will not affect the load significantly. If the cable is healthy it should not be an issue.

[jimbones]

Sorry Rich, I think I was editing as you were responding. It turns out it is not the sub. It is the pre. I will have no choice but to feed a signal in and measure the output across the frequency range and compare left and right. It's not a huge difference but is noticeable.

[richidoo]

No problem Jim

Measuring the output is a good confirmation that the problem lies in the preamp. Do you have a test tone? I can make one for you if you need it.

Did you even get a chance to find and measure the small 1/2watt cathode resistors? If there is level difference between the channels that would be a good thing to check, although likely hood of a film resistor being broken is small. It should be near the big wirewounds that you already measured.

[jimbones]

There are generator software as freeware online that I have used before. Thats what I'll start with. I'll used a constant input into my Aux 1 input of 1V? 500mv? It seems like the difference is in the mid/high end.

[richidoo]

I usually use 2Vpp for test signal, but CDPs make 2Vrms with a full scale signal, which is about 3Vpp. Try to use signal level typical of your own sources to start. If your source only makes .5V then use that to best simulate your actual situation. You can try up to 3Vpp input. The preamp can take higher input voltage but no consumer gear is louder than 3Vpp = 2Vrms.

I think the cathode resistor would affect the whole bandwidth, not just highs.  And if a coupling cap shorted internally I think it would reduce the LF more than HF.

I don't know what else it could be other than dirty volume pot. At least you have AR service if you need them, or maybe a good local tech?

[jimbones]

OK so I pump in a signal and it begins a very slow roll off at 10K and by 19K it is pretty shot. Also the right channel has higher output than left side.  I'll try again with different input level because this was a low voltage input.

[jimbones]

OK So I spoke to Scott at SG and he says he listened to the preamp, sounds fine, but when he measures it the left/right balance is off. The frequency response is fine though. He said it is slight enough that he doesn't hear it but can measure it and recommends I leave it alone. (which I will) He said the only thing to fix the balance problem is to install a ALPS Pot which would be very expensive.

[jimbones]

BTW, I retubed the SP8 while I was at it. went from Tungsol to Electro Harmonix. Me likey better