A few miscellaneous questions.

[robnyc]

"It also has pretty heavy and muddy bass" There are two factors here. First, did you replace the components that add equialization with a simple trim pot between the two sections of the 12AX7? This is essential in allowing you to adjust the preamp stage gain so as not to overload the AGC stage and not add additional bass boost. Remember that this first stage is just a flat gain buffer. When the AGC rectifier tube (SL7) delivers too much DC to the grid-1 (pin 4) of SK7 so as to cause it to go positive re: the cathode the system loses control and you get phasy distortion and way too much bass.

Play a loud record, while it is playing touch the + of your meter to C26 (1mfd). You should not see more than approx 40vdc. As i may have mentioned, the meter will load the circuit and this reduce the voltage, but if you are quick in taking the measurement (dependent on the meter response time) the time constant of C26 will keep the voltage within the true value long enough to be "close enough for rock 'n roll". Reduce the trim pot setting to a point where there is no more overload, if any exists.

As outlined before, these machines originally fed all frequencies to the 8" sides. This was too much bass and often damaged them. I added a capacitor of approx 220-330 mfd in series them to limit bass. The cap can be a standard type with a voltage raring of 35vdc or more. Since there is no DC here a polarized is OK.

Rob

[nyjb]

Thanks Rob.

I ordered the trim pots and in the meantime created a "trim pot" using a discarded speaker switch. This let me compare a few different resistance values. When setting the trim pots, what settings for volume, bass/treble, and AVC should I have? I'm really excited about it, as I've often felt like there was not quite enough power. This reminds me of the scene in Spinal Tap in which Nigel explains how an amp that goes to 11 gives him enough to go "over the cliff"

I've been trying to reduce the hum(s) in this amp and others. I often find small DC voltages where I don't expect to, sometimes at ground points and sometimes following coupling capacitors. It's usually fewer than 10 mVs. How much should I accept? Even points directly wired have different amounts. I understand that some hum is inevitable, and eventually I'll get a sense of what's realistic (and good enough for rock n' roll). In the meantime it's like stamping out a fire. Is there a likely suspect for this after the mod?

Is there any point in tweaking the resistors in the bass switch, or in the rectifier tube bias?

Regarding the 8" crossover, does it matter which post of the speaker the caps are wired to? It seems to make sense to put it between the speaker plug and the first speaker, but the parallel/series speaker circuit sometimes confuses me.

And lastly, I've been getting a pop from time to time when making selections while records are playing. I haven't been able to find a thread about it, though. Your thoughts?

You are racking up a lot of good karma.

[nyjb]

The only 1 mfd cap is C16, which goes to the cathode of the second stage of the N. C22 goes to the 4 pin of the N from the plate of the K, but it has a value of .06 mfd. It's an MRA5-L6.

A-ha! C20 & C31 are each .5 mfd, and are in parallel. That's 1 mfd.

Is there any reason for having two caps here instead of one? They are grounded in two different places, but I figured that's originally a matter of getting the bulky old caps to fit.

Also, there was a green wire running from the plate of the 5869 to the bias circuitry of the N. I wasn't sure what to do with it after the tube swap. At the moment it is disconnected. Does it need to be reconnected, and if so, which plate of the 12AX7 would it go to? (My guess would be the second.)