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been reading a lot of info on this as I do the rebuild, have run into a couple of problems. One, the sound coming out is not balanced, scoping it shows that to be the case, it would seem the vol. pot is not tracking well, is this a common problem? I can use the balance to get the correct lf/rt balance, but this is the first amp I have run across that I had to use the balance for a fair amount of imbalance. The second problem deals with the 6BL8 phase splitter/amp, I am getting 90-100 volts on both pins 1 and 6 whereas the schematic notes 65 v. All the resistors measure correctly and the voltage into the dc balance is correct, replacing the tubes I still get that or higher voltage on the two plates. This is the later 299A version with the trannies along the back of the amp, not sure if the 6BL8 had a change from the schematic I have, which is for the earlier version... thanks
Follow Ups:
cleaned the vol & bal pots by disassembly, thorough cleaning and a topical application of D5 to the carbon tracks. Reassembly and voila! balance is now excellent, no need to use the bal pot to get the proper sound balance. But even better, the sound really opened up, much improved clarity with attendant imaging, soundstaging and punch. This also points to one resto topic wherein the restorer found that new (or nice and clean) socket pins and I/O jacks made a huge improvement in sound quality. I would note that my EL84 sockets are somewhat corroded looking, so a replacement of those will be in order, the other tube sockets look very good and will only need a light cleaning to be spot on...
did a breakdown and cleaned out both the balance and vol pots, the balance pot had some goo inside it, the vol pot was fairly clean, but I cleaned the tracks with IPA and then with D5 applied topically to the tracks. Running the vol pot on the scopemeter it tracks pretty close, about 3-4k difference between the two sections. After the epoxy dries (had epoxied the new F & T caps onto the old wafers for the cans, the one on the end has popped of twice when I knocked against it from the side...) I will run it on the scope and track the signal voltage to see if it is better now...
I am planning on replacing the slide switches on my 299, I found what I hope will work that has gold contacts and is sealed. This 299 was really clean until I opened the control cover, then I saw what looked like white/yellow goo all over each of the two bandswitches, had to completely take them apart and clean them, only way I could get the goo off, looks like some of that same goo may have gotten into the balance pot...
How do you take apart those pots?
Dave
carefully lift the tabs after removing the little shoulder ring around the shaft, the pot then is open for cleaning. Removing the shaft ring allows the rotary contact to be removed so that a thorough cleaning can be had, wasn't able to do that for the dual pot as the rotary contact on the upper pot was stuck on the shaft, other dual pots I have done has the upper rotary contact easily removable. Yes, this is a little extreme but will result in the best performance you can get from cleaning a pot, although I would like to replace them with a top quality pot. Unfortunately, finding pots with taps, like quality audio pots from Alps, etc. can be challenging and searching that will be for another day...
I agree with the other post.
Those switches in the preamp section will often give rise to issues. I find this in lots of integrated amps. As to the volume pot, you can check with a meter to see how well (or not) the two sides are tracking relative to one another.
If you suspect a selector switch, or if you just want to rule it out, bypass it with alligator clipped wires and see for yourself.
I had a similar mistracking with my 299A. There was no one cause...lots of little additive things seemed to contribute to it.
The biggest culprits on my unit:
1) The phase reversal circuit adds a lot of uncertainty to Channel B. I removed it in its entirety which eliminated a lot of trouble.
2) The phono input slide switches also add a lot of unwanted noise and variability to the phono preamp. I bypassed these and just use phono 1
3) The volume control was a total fail and no amount of deoxit would fix it. I was lucky to have a reasonably matched pot with loudness taps to replace it. I replaced the phase reversal switch with a new slide switch and use that as the power switch.
4) I gave the phase inverter sockets a good cleaning and replaced these tubes with a matched set of 6GH8
One other weirdness on my unit- you might find this as well....the front panel has a slight amount of flex to it and IIRC, there are some rigid leads that connect the volume control right to one of the tube sockets. Turning the unit on and off would put enough torque on the front plate to disturb that socket and cause noise and possibly add to the imbalance. Moving the power switch off the volume control and cleaning that socket helped a lot.
Regarding the phase inverter voltages- the HH guys seemed to stir the pot very often. I wouldn't get too concerned about the higher voltages. Sounds close enough.
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