Carvin Belair 2x12 Gain Stage

[bobloblaws]

The pot serves as the grid leak for the V2. I'd suggest resistance checks from wiper to ground through it's rotation. You could jumper the pot and still lose the grid reference to ground.

In the message you just posted you imply I can do these checks with the pot in circuit, do I have that straight? I just want to make sure.

Ok, but make sure the grid (pin7) has 500K or less to ground at all pot settings when measured with your meter.

So in other words make sure it doesn't show an open line, correct?

[g1]

Correct.

[g1]

Out of curiosity, how do you determine what voltages to expect on the cathodes? Also, based on what you said about expecting different plate voltages based on different cathode resistor values, are the plate resistor and cathode resistor the only factors determining what the plate voltage will be (given the same B+ and same tube type?).

I wanted to answer this eventually, you don't have to read it now but I'll get it in here anyway.
I used your plate voltages to determine the cathode voltages. The current is the same at both ends of the tube. So the supply voltage less the plate voltage gives you the voltage drop across the plate resistor. Voltage divided by resistance gives the plate current. Same current through the cathode times the cathode resistance gives the cathode voltage.

This is very handy for verifying schematic voltage listings, I've found several errors this way. Comparing to known 'standard' circuit values verifies any gross faults. (Fender blackface tend to be the most typical well known 'standard' values).

Yes, with same B+, tube type, and circuit configuration, cathode and plate resistors are the only thing that affects the plate voltage (aside from slight variance between tubes of the same type).
So right off the bat, I knew V2a and V2b plate voltages could not be the same due to different cathode resistors. Your numbers seemed correct (compared to standard values) and the schematic values could not both be. Further calculations proved your readings to be correct.

By the same methods, if you were only shown the cathode voltage, but knew the value of cathode and plate resistors, and the voltage at the supply node, you could calculate the plate voltage.

[bobloblaws]

Well this is kind of strange. I went ahead and did some measurements as suggested. I didn't see anywhere where the grid lost the ground reference. Checking resistance from the wiper to ground perplexed me somewhat. Each end of the pot has virtual continuity with ground. As you can see on the schematic, the lug connected to C12 is connected to ground through the L1 relay. The max resistance I measured toward the middle of the travel of the pot was 135K which was a bit of a surprise since the pot is supposed to be 500K and I would have expected closer to 250K at the highest point. I don't know if that in itself is reason enough to change the pot. Anyway, then I tried jumping the top lug to the wiper/V2.B grid. The result also surprised me as the pot still worked, albeit with more signal passing to the grid. I'm assuming this is because in effect I'm connecting the grid directly to ground via the relay? It's all a little above my level of understanding. In any case, I discovered that when I took out the jumper and returned to the baseline test condition the hiss is virtually gone! Without having changed anything. I did give the pot a blast of Deoxit at some point yesterday, can't remember when exactly, maybe it was after I did the scoping and probing or maybe it took awhile to do it's thing, but it's seems OK now. There is still some noise, but much less than before and it comes on more gradually. So thanks a lot G1 and JM, the advice is always appreciated, and if you don't mind explaining why the design has both of the outside lugs on the drive pot connected to ground I'd appreciate that as well. Also, please let me know if you think the pot needs to be changed anyway based on the mas 135K measurement. Thanks!

[J M Fahey]

Maybe you measured with Drive channel set OFF.
In that case *both* pot ends are grounded, and wiper set at exact center will measure upper half (250k) in parallel with lower half (250k) for a combined wiper to ground measurement of 125k .

135k is well within tolerance for a nominal 500k pot.

[bobloblaws]

Maybe you measured with Drive channel set OFF.
In that case *both* pot ends are grounded, and wiper set at exact center will measure upper half (250k) in parallel with lower half (250k) for a combined wiper to ground measurement of 125k .

135k is well within tolerance for a nominal 500k pot.

Right, I didn't think of it in terms of parrallel resistors, it makes sense now. Also right re: the grounded lugs, 'cept the issue was I measured with the amp off instead of on, in which case both lugs were grounded in both channel switch positions

[g1]

the issue was I measured with the amp off instead of on, in which case both lugs were grounded in both channel switch positions

Yes, sorry I missed that, the relay is closed when the amp is turned off so the pot is grounded at both ends.
It's purpose is to kill the signal coming from V1A while Ch.1 is selected.
I hope the pot cleaning is what cured it. Otherwise, if the problem recurs, there is probably an iffy solder connection that got temporarily cured from being jiggled while you were working in there.

[bobloblaws]

Hey guys, I have another problem with this amp. I'm not sure if the preferred thing to do is start a brand new thread or carry on in this one. Maybe someone can let me know?

Anyway, the "Acoustic Presence" control doesn't seem to be affecting the tone of the amp at all, or if it is it is imperceptible to me. I've done some troubleshooting but kind of at a dead end. The control is meant to work on the clean channel only. I verified that the relay is grounding the one pot lug when switching to the dirty channel. So the default would be the clean channel with presence/negative feedback. I cleaned the pot and it seems OK but it doesn't go quite down to zero ohms, the range is something like 40 ohm to 28 Kohm. C70, R70, and R71 are in spec and I can scope a test signal on either side of C70. On the pot side of C70 the amplitude of the signal is affected by turning the presence control, however, on the R70 side the signal barely changes.

Can anyone suggest next steps?

Edit: For the record I created a new thread and this was answered thusly:

From the manual:

"The rear ACOUSTIC PRESENCE control adds a sibilance to the high frequencies of your guitar.
Most presence controls work in the 3k to 4k range. However, the VINTAGE TUBE’s presence starts
at a very high 8k Hz delivering 10 dB at 12k Hz and continues to 20k Hz which extends all the upper
harmonics of your guitar. The amount of sibilance will depend on the speakers used. To keep both
channels totally independent, the ACOUSTIC PRESENCE is switched by relay only into clean channel
1. The effect of the ACOUSTIC PRESENCE will seem ever so slight, however, the result is added
sibilance only to the ultra-high frequencies."

I suspect it's working and, as the description says, ever so slight. If you're running a typical guitar speaker, frequency response may not even include 8K or higher and you would hardly notice it at all since we're in tweeter range.