first tubes pioneer sm 801

You all didn't think I was going to just go away?

So I've been listening to the Pioneer for a bit and Things are pretty good but not perfect.

The channels sound pretty well balanced but on the right side it sounds like there's a piece missing in the mid range. Its hard to describe but lets move on to voltages.

My driver tubes still have different voltages on them:

Out of C51 I get 423v before R81,R31 (both of which are in spec). At pin 1, I get 331v on the right channel and 300v on the left side.

Now I'm going to just ramble on so you can check my understanding of the basics. Pin 1 is the plate of the first tube section, it flows current as a function of the voltage difference between it and pin 3, the cathode.

Pin 2 is the grid, and that is going to have a D.C. voltage that is close to the voltage on the cathode, as well as the music A.C. signal coming in. This signal is what makes the grid swing more and less positive, acting like a valve between the plate and cathode.

O.K. so I have 95v on pin 3 in the right channel and 104v on the left.

Also 79v and 90v on the respective grids.

So for some more basics, On the 6an8 the signal starts in at pin 8, the grid of the pentode section. Then it goes through C45, R77, R79, C46 and then to the grid of the Triode section. Just trying to confirm my reading of the schematic.

My heaters are 6.6 vac on both tubes.

Pin 6 is the same as pin 2.

Pin 7 is 56v right and 54v left.

Pin 9 is 1.42v and 1.39v.

My guess is the hole in the sound is something amiss in the preamp section but I can't imagine that things are O.K. with the voltages so off.

Now that were on to new problems should I start a new thread? I'm afraid some people may be burned out on this saga.


Cheers
 
Hi all,

So I still don't understand the different voltages on the driver tubes. I am wondering if I should just replace all the resistors in that circuit? How can I tell what wattage a resistor is? I know the ones in the unit are .5 watt, I have a whole bunch of resistors that I got at an estate sale, they are smaller than the ones in the unit. They are the same size as the ones I got for the cathodes of the power tubes and I am already using them in a couple of places in the unit so they are probably O.K.

I have been looking at photos of other units with similar circuits and have noticed that many use physically larger coupling caps. Did I get the right ones?
 
Well I had some free time so I just went through and replaced all the carbon composite resistors in the driver tube circuit with the little carbon film resistors I had.
It sounds much better. Much more full, the bass is still a little thin, but that may be all there is.

I still have different voltages on the driver tubes. I put the old ones in and no change. I guess if its not hurting anything I will just listen to it for a while.
 
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