JTM45 volume compared to?

[axe-victim]

I finished the build some time ago, but never had a good speaker cab to go with it. Over Thanksgiving I built an EV TL806 cab loaded with an EVM12L. This rig sounds good, but it didn't seem to have the volume I thought it would. I A/Bed it with a SF DR. They had similar volumes with the DR on 2.5 and the JTM on 8. Seems odd to me. Any thoughts?

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[Janglin_Jack]

Yes seems odd. My JTM45 is loud. Using an EV speaker should be really loud. Something is up.

Mike

[axe-victim]

I did re-bias. It was a little high, maybe 80-85%. I put it at 70%. I'll recheck all the voltages and post them. Is there any other information I can get that may help?

[Janglin_Jack]

Are there any circuit mods that you did? Is there an impedence mismatch between the amp load and speaker cabinet? Maybe try another speaker to see if the volume is different??

I think testing voltages and double check the compenent values in the amp will be a good idea.

Mike

[axe-victim]

I've tried 2 different EVMs and the speaker in my DR with no change. All are 8 ohm. I'll double check the wiring to the speaker jack to ensure the wires are landed correctly. The only mod was to remove the bright cap on the bright volume pot. I have Marstran trannies installed also. I'll try to get the voltages measured before a gig tonight. If not, the gig is done at 10 and I'll do it then.

[neikeel]

I'm sure you are aware of the old chestnut in the PI where new builders put a 470k or 47k in between the two 1 Meg resistors instead of the intended 470R? Might be worth checking.

The other point is the non-linearity of some pots which mean you have to wind them further for the intended volume.

If it still not right post some well lit clear pictures, over view and close ups.

[axe-victim]

The voltage chart along with my measurements [below] is attached. I couldn't get the formatting right. All controls = 0, no input, cab connected.

When I touched each point, I heard no sound except for the following: V2-P1, V2-P8, V3-P1, V3-P6, V5-P5. An audible sound came through the speaker, especially V3-P6 which was much louder than the others. Does this indicate something bad?

I double checked the 470 resistor between the 1Ms. Yellow, violet, brown. I will disconnect and measure the actual value later.

I don't think the pot's tapers are involved. I had the amp dimed and it couldn't keep up with my DR.

Pictures will follow shortly.

[axe-victim]

Board details

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[axe-victim]

Preamp tube socket details

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[axe-victim]

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Power and rectifier socket details

[neikeel]

I confess I cannot see anything particularly wrong.
Build is of reasonable standard, voltages not far off (heaters a bit low - depends on your wall voltage and tap used) Brian's PT voltages are typically higher than the stock kit ones which many prefer but also usually means louder amp!

All I can think of is going over the solder joints in the signal line and reflow in case there is a high impedence joint.

Failing that a scope with a signal and see where you are not getting full amplification.

[danman]

Any chance you may have a weak tube in the amp? Have you tried switching tubes with other known good ones?

[axe-victim]

I did change all the preamp tubes from EH to Sovtek because I was hearing some microphonics. That went away, but didn't effect the power. I don't have extra KTs, so I'll start with all the solder. Probably order tubes also. Any suggestions? I have Golden Lions currently.

Thanks to all for the comments, suggestions and help to this point. As crazy as this world is, it's comforting to know there are such great people still out there!

[axe-victim]

Well, well, well. I found one problem! I was missing the 56K resistor in the tone circuit! It's way louder now! Sounds better also. Very touch sensitive. Tomorrow I'll A/B it with the DR again and post the results.

What I did tonight was basically repeat the build from the beginning without soldering. Instead, I checked the continuity. Everything checked fine until I started checking the component values. That's when I found the missing resistor.

Well, thanks again for everyone's help. Until the next build (or problem!)....

[Janglin_Jack]

Nice job finding it! Post your update when you compare to your DR

Mike