Which resistance and voltage checks do I use to troubleshoot after Integration?

[oguinn]

I was listening to a couple records today and noticed a couple issues:

• There's more surface noise than I remember
• When I turned the Smash's coarse attenuator up to -18dB there was a weird amount of bass or low frequency coming through at high levels that were way out of whack with the highs and mods.

I'm planning to listen to a few more records, but I want to troubleshoot the amp's voltages and resistances first to make sure something isn't going to damage the amp. However, I'm not sure which manual measurements are valid after the upgrade was installed. For instance, the Integration manual lists only a few spots to check voltages - surely those voltages aren't an exhaustive list - and no resistances. But I also removed resistors and added new parts in, so I know there isn't a clean combination of the base Reduction and Integration manual checklists.

Does anyone have a quick hit list of which are valid, which aren't, and which are new areas to check?

[Paul Birkeland]

I would recommend just checking the voltages listed in the Integration manual.  The Integration increases the gain of the Reduction a bit, so while there may appear to be more surface noise, there's also more signal to go with it!  The low frequency issues that you're mentioning could be subsonic coupling between your speakers and the turntable. 

[oguinn]

Oh, interesting, so that could be the cartridge picking up and re-amplifying low frequencies?

[Paul Birkeland]

Yes, there are tons of ways to deal with this that are all a bit dependent on your setup.  One of the worst case scenarios that I see is a pair of bookshelf speakers on a credenza with the turntable between them.  Though it looks stylish, it will not function stylishly. 

[oguinn]

I did avoid that setup, although my woofer is on the floor next to the shelving unit the turntable is on. Not really sure where else I can stow that...

[Paul Birkeland]

In that case many would recommend a turntable wall shelf, provided that the wall it would be mounted to is well supported (most are).

[oguinn]

That’s definitely on the list for the next place we live. I’ll play around with some of the rubber isolation pad material I have left over. Thanks, Paul.

[oguinn]

Checked voltages.

2 - 63.6
18- 75.9
26 - 75.7
31 - 53.5

31 in particular looks pretty low. Any cause for concern?

[oguinn]

Also rolled in a spare set of tubes. Those values were:

2 - 69
18 - 60.35
26 - 60.27
31 - 64.1

So a little lower on 18 and 26, then higher on 31. Are either set of numbers more desirable for some reason?

[Paul Birkeland]

Low just means strong emission, that's nothing to be too worried about.

[oguinn]

Paul,

I'm trying to understand why lower voltage means stronger emission. Let me know if I'm off base or misunderstanding.

Here's what I think I've figured out:

• Terminal 2 is connected to A1, which is the plate on the first half of the duo-triode
• Terminal 18 is connected to B1, which is the plate on the first half of the other duo-triode
• Terminal 26 is also connected to B1, which is why 26 and 18 are so close in both sets of readings
• Terminal 31 is connected to B6, which is the plate on the second half of the duo-triode
• Is it the case that these are the plate resistors that go to ground? If so, does a lower voltage here mean that more of the signal is being passed to the outputs than is being sent to ground?
• I measured pins that are connected to the plates on both halves of the B tube but not the A tube. Is there a reason for that?

I'm doing my best to piece this stuff together with the limited knowledge I have pieced together from watching YouTube videos, so I'm sure I'm wildly incorrect in some or all places.

[Paul Birkeland]

Is it the case that these are the plate resistors that go to ground?

With the integration installed, there are no plate loading resistors.

Since the 6922s are biased with LEDs, the bias voltage never changes significantly.  This is the mechanism that moves the plate voltages around so much.  In an old school circuit with cathode resistors and bypass capacitors, the cathode resistor will serve to stabilize the DC operating point in conjunction with the plate loading resistor. 

If you consider the case where the 6922 draws no current and is loaded by a resistor, the plate side of the resistor will be at B+.  If the 6922 draws all the current it can, the plate voltage would drop to nearly 0V DC (we would consider this a short).  Somewhere in the middle is the average expected value, with some tubes drawing a little less current and some drawing a little more current under these conditions.