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Lower Output - Voltages off - Debug help! (Read 5067 times)

MattG

Verified Member

Posts: 30

Lower Output - Voltages off - Debug help!
07/13/12 at 22:23:59

Hello,
I've hunted back thru the zkit1 forum, haven't found other posts addressing this, so I'm hoping for some debug advice!

I just finished up 2 zkit1s in the same box, each wired as a monoblock. I'm using the Edcor PTs and OTs. Reflector 6N1P and 6P15P-EV tubes.

Initial voltage check showed test point A - after 1st dropping resistor - to be a bit high, but others were ~ok, so I hooked it up and let it rip.

One of the amp's output is quite a bit lower than the other. I did some swapping of inputs, speakers, speaker cables, and tubes to make sure it was the amp itself, I'm pretty sure it is.

I put it back on the bench and took a bunch of measurements, here's what I see

Test Point Amp1 (ok) Amp2 (low)
Incoming VAC 330 330
After Diodes VDC 440 444
Test Pt "A" 362 382
Test Pt "B" 319 338
Test Pt "C" 10.75 8.35
Test Pt "D" 3.05 3.1
Output Tube Plate 354 375 (the blue wire feed from the OT)

As you can see, I've got a good 20V higher on the B+ rail, and well above the recommended 346 +/- 10. I'm not sure, but that probably also leads to the output cathode voltage being ~2V lower. And I'm hoping/guessing this situation is why my output is suffering?!

I double checked resistors of interest - the 1k/6W dropping resistors were bang on and both 6.8ks were close as well...all cathode resistors were matched.

My question is how do I fix this? Can I just increase that 1k resistor to get Test Pt A voltage down to ~350V?

Anybody else tuned the voltages on the zkit1?

Thanks!
-Matt

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dank

Seasoned Member

pair of dual 18
Imperials

Posts: 420

Re: Lower Output - Voltages off - Debug help!
Reply #1 - 07/15/12 at 00:23:40

Matt

Your amp2's output stage is not being biased properly. This results in low bias current thru the output tubes. The term "bias current" means the current flowing thru the tube with no input signal. I assume you have no input signal.

The resulting low bias current makes TPC low. This in turn makes TPA and TPB high.

Bias current thru both tubes should be TPC/Rcathode = 10.25/150 = 68ma or 34ma per output tube. Your bias current is 8.35/150 = 56 ma or 28 ma per tube which is down 18%.

Voltage of the control grid with respect to the cathode is going to be the primary thing controlling the bias current. Vg should be close to 0, and Vcathode should be TPC. The two other grids can have an effect on this too. What is on pin 6? - did you do the CCE mod? How about pin 9 - is it about the same voltage as the plate, pin 7? You can remove one tube and measure the other tube by itself. TPC will be about half because you removed one tube. With your 2 amps this should give you 4 sets of data and I'll bet one set will be off.

Dan

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MattG Verified Member Posts: 30	Re: Lower Output - Voltages off - Debug help! Reply #2 - 07/15/12 at 17:31:24 Thanks Dan! I will take some more measurements and report the 4 individual bias currents...hmmm, use the same tube? Yes, CCE caps are installed b/w pins 3 and 6...
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MattG

Verified Member

Posts: 30

Re: Lower Output - Voltages off - Debug help!
Reply #3 - 07/16/12 at 21:46:03

Dan, your individual tube/socket test did the trick!

When I got to the 2nd output tube on the low output amp, that's where I found the low cathode voltage, where the 1st tube/socket read ok...so I inspected the components attached to the socket and Boing! I see one leg of the 330k resistor just floating in space!! It was hiding kinda underneath one of the big honking 3.3uF Solen caps (this amp is the "PTP" version of the zen, wired to terminal strips around the tube sockets).

My guess is that when I wired up my (failed) standby switch, I ran the ground wire from the switch to the ground post on the terminal strip that the 330k resistor is also attached to, and in the process of soldering on the ground wire, the 330k popped free!

SO anyway, I reconnected the resistor, loaded all the tubes back in and now my voltages are very well matched b/w the 2 amps! They're a tad high still (TPA ~362, TPB ~322) but consistent...AND NOW the volume is matched and sounds great!! Cool

Dan, thanks again for the debug tip!

-Matt

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