New JTM45 build-low power output, early distortion

[g1]

Here is another way of describing it. You can't see the amp, someone else will turn the volume knobs up for you till you just barely start to hear it distort, then stop. Under these conditions, does the other amp still sound louder?

[Chuck H]

Here is another way of describing it. You can't see the amp, someone else will turn the volume knobs up for you till you just barely start to hear it distort, then stop. Under these conditions, does the other amp still sound louder?

Ow! My brain! Must.. try.. to.. un.. think.. now..

[mfreqmaster]

Alright, I apologize for my layman speak. Yes, my problem was that I did not feel the amp had enough volume before it started to break up. I don't have a function generator but I did find one on my DAW. I used a sine wave, fed the headphone output of my audio interface to the amp input. Wow, super loud before it started to breakup, much louder clean than my guitars. I measured the VAC at the output jack at the point where I felt the sine wave started to distort. (Using my ears only, didn't have a oscilloscope). The voltage measured was 4.5VAC. The output is going to an 8 ohm cab. If I did the calculation correctly, does this mean I have 2.5W of clean, ass whuppin' power? It was super loud though with the sine wave, hertz my ears, more so than the guitar signal ever could (tried several guitars, used same patch cable for sine wave).

I also updated the voltage readings at the valve pins, all heaters approx 6 VAC:

V1 - pin 1 224V, pin 3 1.6V, pin 6 219V, pin 8 1.6V
V2 - pin 1 191V, pin 3 1.2V, pin 6 330V, pin 7 189V, pin 8 190V
V3 - pin 1 238V, pin 2 28V, pin 3 43V, pin 6 246V, pin 7 26V, pin 8 43V
V4 - pin3 448V, pin 4 439V, pin 5 -43V, pin 6 442V, pin 8 47mV
V5 - pin3 448V, pin 4 439V, pin 5 -43V, pin 6 442V, pin 8 39mV

Again your input is appreciated, and I understand that even though my Marshall goes to 11, it doesn't mean squat.

[g1]

Ok, thanks for bearing with us as far as the inquisition .
Under the conditions where you had the 4.5VAC at the output, measure the AC voltage at the power tube grids (pin5).

[Jazz P Bass]

You would be well advised to measure the headphone out amplitude (Volts ac).

A 'typical' test signal amplitude setting on a guitar input jack is 100mv's.

[Chuck H]

I would expect 2.5W of sine wave to sound pretty loud. But not "super loud". Certainly shouldn't hurt your ears. But the test is a pretty simple one. Hard to get it wrong, so I expect it's correct. The test is "properly" done into a resistive load, but for our purposes that isn't a biggie.

Voltages look fine to me. I'm guessing you have 1 ohm resistors on your cathodes? Your readings there would indicate a pretty big mismatch in the standing idle current. ie: unmatched output tubes. If they were supposed to be matched tubes there may be a problem with one of them. That one at 47mV (47mA) would be drawing 85% of dissipation at idle. This is providing the 1 ohm resistors were selected as 1% tolerance parts as "I" always do when I bother to include them. With such a circuit, doing otherwise negates the function (unless your meter is accurate enough below 1 ohm to adjust the calculation) Please swap the tubes in the sockets and see if the mismatch follows the tubes or stays with the sockets.

Can you provide some gut shots? (BTW we've seen it all) the better the photography is the more we can help

[mfreqmaster]

A did the test again, this time making sure I had a 100mv input signal. For the 4.5VAC on the output, I had 6.8VAC on V5's pin 5 and 7.8VAC on V4's pin 5. 35mv on V5's pin 8 and 43mv on V4's pin 8. I switched the tubes from their sockets and got nearly identical results, meaning they followed the tube not the socket. I do have a set of spare 6L6 tubes available, but not sure I can bias them properly with my circuit? Also the box of Tung Sol KT66's have the sticker with number 21 written on each of them. Don't know what this means. Here are some gut shots, I know, what can I say? I never could get cloth diapers for my 4 kids to look good either. Fortunately they are beyond that age.

[g1]

Those 7VAC readings at power tube grids seem low, how did you have volume and tone controls set?

[mfreqmaster]

Most were middle way up, except for the bass, which is way to bassy, I barely have that up, it's just too much. I'm not all about that bass, at least in this particular situation.

[g1]

One more thing to clarify, pin8 voltages in post #18, the 39mV and 47mV readings were DC with no signal applied?

[mfreqmaster]

This is correct. The readings at the bottom of post #18 were with no signal applied.

[mfreqmaster]

Yes, the voltages at the bottom of post #18 were with no signal applied.

[g1]

The DC voltages and power tube voltages all seem ok. So the next thing would be to measure AC voltages on the preamp tube grids and plates.
Under the same conditions that gave you the 4.5VAC output, what do you measure for AC volts at pins 1,3,6,&8 of V1,2,&3 ?

[mfreqmaster]

For V1: p1 15mV AC, p3 .3mVAC, p6 6.4V AC, p8 .3 mV AC

V2: p1 27VAC, p3 2.5 mVAC, p6 10 mVAC, p8 27V AC

V3: p1 7VAC, p3 3VAC, p6 8VAC, p8 3VAC

[g1]

Terribly sorry, I forgot to ask for pins 2 and 7 also. Can you redo above including pins 2 & 7?