New JTM45 build-low power output, early distortion

[mfreqmaster]

I have a new JTM 45 build I am debugging. The unit works, but is lacking headroom. Distorts very quickly and kind of sounds cool at low volumes but my tweed deluxe clone is louder. I know this isn't right. I have a list of voltages, using KT66 tubes:

1 2 3 4 5 6 7 8
V1 220V 1.7V 222V 1.7V

V2 186V 1.1V 328V 187V

V3 248V 43V 251V 43v

V4 453V 444V -44V 442V

V5 455V 444V -45V 442V


It appears my plate voltages are consistently higher than what I've found on the voltage charts online. Also my heaters appear to be 5.8V AC, so slightly lower than the 6.3V I was expecting. Thanks for any help in sorting through this!

[Pedro Vecino]

With only these data may have a very low bias adjustment. Maybe that's the reason for the increased voltage in the KT66.

[nosaj]

What schematic did you use?
V1 and V2 voltages look like something is off if they are 12a?7 tubes.

on v2 what pins is giving 187v?

nosaj

[mfreqmaster]

Here is the layout I used, more or less. Hopefully it comes up ok:

JTM45LAYOUT.pdf

Preamp tubes are 12AX7s. For V2 the voltages are pin 1 188V, pin 3 1.1V, pin 6 326V, pin 7 189V, pin 8 189V.

[g1]

With only these data may have a very low bias adjustment. Maybe that's the reason for the increased voltage in the KT66.

Good point. What is your voltage (DC millivolts) at pin 8 of the KT66's?

[pdf64]

Consider verifying that your meter measures Vac ok by checking the line supply.

Why does that stupid standby arrangement get propagated? It's not as if that Triode layout is 100% vintage correct.
Don't use the standby switch when implemented like that (ie hot switching the reservoir cap on to the rectifier), rather allow the GZ34 to do its thing and ramp the VB+ up smoothly.

[mfreqmaster]

I measured wall voltage at 120VAC. I have 1 ohm resistors at each pin 8 and ground. For v4's pin 8 I measure 31 mv. For v5's pin 8 I measure 41 mv. These are Tung-Sol KT66s & were supposedly matched.

[mfreqmaster]

I also switched the tubes, the problem seemed to follow the tube, not the socket. I did try a set of 6L6s since I didn't have any other KT66s around. No difference in volume. Amp starts to break up at 1.5 on the dial, not very loud. Would be pretty nice sound for a 5W bedroom amp but I know something must be off.

[loudthud]

The Voltages look good enough for the amp to work with the one exception of the heater Voltage being low. Check the heater Voltage at the first place the wires from the transformer go. The low heater Voltage could cause other Voltages in the amp to be higher than normal.

Check for some kind of short at the speaker jacks. This is hard to detect with a meter because the resistance of the output transformer windings quite low. Sometimes the easiest thing to do is disconnect the wire from the transformer to check for a short at the jacks. Does the amp have some kind of impedance switch?

The amp could be oscillating at an ultrasonic frequency. This will show up as an AC Voltage of several Volts at the speaker output when the amp is making no sound. The usual fix is to reverse the wires connected to pin 3 of the output tubes OR the wires going to pins 5 (or 6 if you have grid stoppers).

[mfreqmaster]

On the heaters, I have a fuse for each line connecting to the 6.3VAC windings on the power transformer. Did this at the recommendation of RG Keen I believe. The voltage going to the fuses was 6.5VAC, after the fuses was 5.8VAC. Swapped the fuses for another set I had on hand which happened to be 3 amp fuses. Have my 6.3VAC on my heaters now. Still no real change in clean headroom.

Disconnected the output transformer from the speaker jacks. Plugged my 8 ohm cab into the speaker jack to test, tested 6.8 ohms, so close enough.

Also checked for the AC voltage at the speaker output when the amp is not making a sound. I found negligible AC voltage at the speaker jack.

Did find that my feedback resistor was off, somehow the resistor measured only 4-5k ohms. I replaced with a 27k ohm the layout calls for. Also had a .01uF cap at the presence knob rather than a .1uf. I corrected these but no real difference in the clean headroom.

The amp has a nice sound, It's good enough and responsive enough I have a hard time stopping playing. It's loud enough that when I demonstrated for my wife, she had to leave the room when I turned up. (not because of my playing, because of the volume!) It's just that the thing starts to break up at about 1-2 on the volume knob & I feel my tweed deluxe clone has more clean headroom & I feel this is not right. If I could somehow coax more clean headroom and volume out of this thing it could be my next favorite amp.

[Chuck H]

Headroom is NOT a function of volume knob setting!!!!!!!!!!!!!!!!!!(need more room for exclamation points)

The number on the volume knob is an arbitrary representation of circuit function.

What you need to know is what the actual max clean AC at the speaker jack is. If it meets spec for your tubes at that voltage then it's the circuit function that has the amp clipping at a low volume knob number and NOT a lack of headroom. This is something hard to wrap your head around for some players and builders.

So, what is the AC reading at your speaker jack for max clean with a signal generator and a resistive load? Regardless of the volume knob setting.

[Enzo]

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!


There, Chuck...glad I could help.

[Chuck H]

Thanks Enzo. That did help (My left eye is twitching a little less).

And rather than edit, I'll add here that once it's determined that the amp is producing the correct amount of power there is usually much that can be done to idealize knob settings for aesthetic reasons and more intuitive user operation. I actually take great pains idealizing circuits to do this. So I'm not just blathering about knob settings not being important. I'm just saying that they're often a separate issue from actual performance.

[pdf64]

That 'headroom' word drives me nuts too!
It can mean several things, and may not be any kind of measurable fault.
It's obvious to the player what they mean but it can waste time if we assume we know what is meant.
Always best to get the 'issue' demoed; it may just be that they don't like the way the amp responds, or that a louder drummer has joined their band.
If can't get a demo, then ask them to describe the issue without using the 'headroom' word.

[J M Fahey]

Worst is that the word is used backwards.

A still somewhat inaccurate but more real description would be: "I replaced my 15W Tiny Terror which I could tastefully overdrive, by a 30/40/50/60W amp, now when I overdrive it band members yell at me; at earlier volume it's dry and lifeless and forces me to use buzzy Master or pedals, I HATE this extra headroom" .

And a few much more accurate ones might be: "earlier my guitar did sound like if I had some kind of overdrive always on, now it's clean, I LOVE this extra headroom"

But such descriptions are as rare as hen's teeth, the usual complaint is : "my amp starts dsistorting on 3, my bandmate's on 6 , he has much more headroom than me" , and the usual answer (ok, 95% of them) is to "plug a lower gain tube in V1 or PI" , usually one that will throw proper biasing away through the window.

Oh well.