No High Frequency Response in Tube Pre

I just built two "electric crayons" tube mic pre's on one (old homemade tape recorder electronics) chassis. It's point to point and follows the schematic except I didn't include a high/low switch.

http://www.tangible-technology.com/tubes/altec.html

One channel works fine, the other has nothing above about 1K. The parts are identical, and the voltages are correct. I switched the transformers and tubes and nothing changed. I'm a newbie, so I thought it might be my soldering and rebuilt it twice, but no change. On the bad channel, I bypassed the 1st stage, and wired the input directly to the pot. After I did this, there was no high end at lower settings, but with the pot turned all the way up it sounded good. The pots are AB 1meg audio taper and both measure about 1.2meg. Any suggestions on where the problem might be? Thanks

Are all the d.c. voltages about correct?

Yes, the d.c. voltages are very close to what the schematic says. Also, the working pre and the one with no hf read about the same.

Do you own, or can you borrow, a capacitance meter? If so, measure the capacitance from Pin 2 of V! to ground. Also measure the capacitance from Pins 2 & 7 of V2 to ground, although this seems like a less likely problem spot. The crucial clue is that when hooked up directly (via the "low gain" setting) the frequency response rolls off when the pot is set to less than maximum level. (Let me guess: the worst is at -6dB?) If there's about 530pF from V! Pin 2 to ground, that would be about right for a rolloff of 1kHz at a setting of -6dB on the pot, which would have an output impedance of 300k at that point.

Peace,
Paul

[quote author="pstamler"]Do you own, or can you borrow, a capacitance meter? If so, measure the capacitance from Pin 2 of V! to ground. Also measure the capacitance from Pins 2 & 7 of V2 to ground, although this seems like a less likely problem spot. The crucial clue is that when hooked up directly (via the "low gain" setting) the frequency response rolls off when the pot is set to less than maximum level. (Let me guess: the worst is at -6dB?) If there's about 530pF from V! Pin 2 to ground, that would be about right for a rolloff of 1kHz at a setting of -6dB on the pot, which would have an output impedance of 300k at that point.

Peace,
Paul[/quote]

I would add (although you are probably sophisticated enough to know this already) is do this with the power off and the supply caps fully discharged!

Thanks for the help guys. (Paul, I was reading and rereading your articles before I even bought my soldering iron.) I might have access to a capacitance meter later this week. I'll post when I get some results.

I got to use a capacitance meter yesterday, and V1 pin 2 to ground measured 75pF. No reading on V2 pins 2 & 7. By measuring capacitance, am I looking for a bad connection, whereby the pot acts as a tone circuit? I assume that the path from V1 pin 2 to ground is to the wiper to the grounded end of the pot to the chassis. What would be the relationship be of V2 pins 2 & 7 to ground since the is no direct connection?

(Let me guess: the worst is at -6dB?)

Click to expand...

Excuse my ignorance, but is 0db the pot at its maximum level? The hf rolloff is worst at the lowest gain, and seems to gradually increase as the gain is increased. Thanks for the help.

[quote author="starfish"]I got to use a capacitance meter yesterday, and V1 pin 2 to ground measured 75pF. No reading on V2 pins 2 & 7. By measuring capacitance, am I looking for a bad connection, whereby the pot acts as a tone circuit? I assume that the path from V1 pin 2 to ground is to the wiper to the grounded end of the pot to the chassis. What would be the relationship be of V2 pins 2 & 7 to ground since the is no direct connection?

(Let me guess: the worst is at -6dB?)

Click to expand...

Excuse my ignorance, but is 0db the pot at its maximum level? The hf rolloff is worst at the lowest gain, and seems to gradually increase as the gain is increased. Thanks for the help.[/quote]

Have you checked to make sure the ground end of the 1M pot is really grounded? 1M in series with that 75pF of capacitance will roll off at 2122Hz, and will behave the way you describe. Except that the gain control won't actually change the gain much under those circumstances.

Check all connections on the pot.

In answer to the other question, pins 2 and 7 on V2 should have very little capacitance to ground, and should measure very high resistance to ground as well. Which they do, so that part of the circuit's working right, which makes sense. The pot and the tube stage following it are the obvious suspects.

Peace,
Paul

Hi, after trying several different ground points without any luck (I also resoldered most of the connections in this part of the circuit), I ended up replacing the cable to and from the pot. It works! :grin: Thanks so much for the help!

So maybe it was a nearly-melted-through shield-center conductor causing exceptionally high capacitance.

Soldering cables without damage is tricky.