Recent build is DULL sounding

[HBamps]

I recently completed a build of an amp circuit I have made 4 times before. It's my take on a low wattage JTM 45. Essentially the same preamp (paralleled input, 12AT7 cathode follower, Marshall tonestack, LTPI) into a cathode biased 6V6 output, tube rectified. I consider this circuit 'stable' as it's sounded great the last 4 times I have built it, it's a great little classic rock sounding amp with great feel.

Problem is, this build sounds really dull, like there is a blanket over the speaker. The highs are muted and the mids, and even lows seem wolfy. Over all the amp sounds and feel sluggish as opposed to lively. It's a little tough to explain these symptoms accurately.

I have looked of the amp several times, taken all the voltages (all reasonably close), and racked my brain. I am stumped!!!

Could it be some sort of phase cancellation? How could that be remedied? At the PI for example, I have a 82k plate load on the input side and a 100k on the other. I had a build once where I had that switched and it made a huge difference.

Since it's paralleled, could I be getting a phase cancellation at the input?

Any thoughts or guesses would be appreciated. thanks for looking!

[loudthud]

Have you swapped the tubes for known good ones? Have you tried replaceing filter caps? Any chance of a parasidic oscillation? Connect DVM to speaker and set to AC volts. Reading should be low millivolts with no signal.

Any other possibilities you feel you have eliminated? Do you have an identical amp that you can jumper signals back and forth? Try feeding the signal from a known good amps' preamp to the poweramp and vice versa.

[Jazz P Bass]

Maybe you could try a different (known 'sounds good') speaker.

[HBamps]

Have you swapped the tubes for known good ones? Have you tried replaceing filter caps? Any chance of a parasidic oscillation? Connect DVM to speaker and set to AC volts. Reading should be low millivolts with no signal.

Any other possibilities you feel you have eliminated? Do you have an identical amp that you can jumper signals back and forth? Try feeding the signal from a known good amps' preamp to the poweramp and vice versa.

i will try a full tube swap this evening. I think I have tried already but will repeat.

I have not tried replacing filter caps though, wonder why you ask. Could you expand on that?

I do have another identical amp to compare. There is a couple minor differences between the two;
- one is a head, the problem amp a combo. For testing I am using combo speaker for both.
- Good amp has JJ cap can for first 2 filters, bad amp has separate (Illinois) caps (same value).
- Good amp has Hammond 270FX power trans, bad amp has Weber deluxe reverb power trans, both amps have approximately same voltage at all nodes.

How do I go about jumpering signals? Lift signal from one and run a wire to other?

Thanks for the feedback.

[loudthud]

I have not tried replacing filter caps though, wonder why you ask. Could you expand on that?

A bad filter cap can cause unintended feedback between stages and many strange problems. I would just try tacking another cap in parallel with what you have now to see if there is any change. Or, just check every node with a scope. Got a signal where there shouldn't be one?

How do I go about jumpering signals? Lift signal from one and run a wire to other?

An example would be to disconnect the PI from the tone stack. Now run one amps preamp into the others power amp. How does it sound? Is the problem in the power amp or preamp? Try to localize the problem to a small section of circuitry. No sense in changing an output transformer if the problem is in the preamp.

Did you make your own eyelet or turret board? Any chance you got some bad material? Ohm out any wires on the backside of the board.

[HBamps]

Thanks Loudthud! I will try a few of these things. This is what really drives me crazy about amp building, not being able to trouble shoot effectively. I end up hunting and pecking endlessly.

Yes, I built the eyelet board myself. It's a fiberglass material from McMaster carr. I have been using the same stuff since I started building amps. I guess if it's somehow conductive, it would really screw stuff up.....

A bad filter cap can cause unintended feedback between stages and many strange problems. I would just try tacking another cap in parallel with what you have now to see if there is any change. Or, just check every node with a scope. Got a signal where there shouldn't be one?


An example would be to disconnect the PI from the tone stack. Now run one amps preamp into the others power amp. How does it sound? Is the problem in the power amp or preamp? Try to localize the problem to a small section of circuitry. No sense in changing an output transformer if the problem is in the preamp.

Did you make your own eyelet or turret board? Any chance you got some bad material? Ohm out any wires on the backside of the board.

[loudthud]

Yes, I built the eyelet board myself. It's a fiberglass material from McMaster carr. I have been using the same stuff since I started building amps. I guess if it's somehow conductive, it would really screw stuff up.....

Fiberglass can sometimes absorb moisture, but it's unusual. You got it from a relliable source and it worked before so at this point we can assume it's ok.

This is what really drives me crazy about amp building, not being able to trouble shoot effectively. I end up hunting and pecking endlessly.

It gets better with experience. Keep your eyes open for a cheap scope and signal generator. They will really help you see what's going on inside your amps. Right now we have no idea where the problem is. By splitting the amp up, we'll try to narrow down what part of the amp has the problem, making the assumption there is only one problem. Then parts substitution well probably find it.

I've gotten much better at troubleshooting since I started helping people on these boards.

[Enzo]

And if we look at something long enough, it tends to look familiar. Take one of the known good amps and open it up next to this dull one. Now go down the eyelet board part by part. Are they the same? Make sure there were no assembly errors like a cap and a resistor swapped places. Also verify each part. Measure the resistors. I know some places the circuit will skew the readings, don;t unsolder them. If a part measures funny, compare the reading to the good amp. If both of them read a 47k resistor as 11k, chances are that is OK. If it happened, you would not be the first or last guy to install a 470 ohm resistor where a 470k resistor belonged. Or something similar. Likewise reread the values on each and every cap in the signal path.

Chances are the wiring is OK, but check that too, wire for wire. Especially on the controls.

I know this sounds dumb, but believe me, we all have made mistakes. I have pulled wrong value resistors from a labelled drawer - I always check. I have received bags of resistors from suppliers that were picked wrong. Bag said 470k but the contents were not. Always check. SO it doesn;t matter whose mistake it might have been, we just need to find any that were made.

And compare DC voltages amp to amp. DO the tube socket pins. You may find some tube is way off its normal operating point, and the result is funny tone.

You have other working amps? Good. Use one as a signal tracer. Google "signal tracer" and find a zillion takes on the theme. Basically all are just a probe wired to the input of an amplifier. The probe is mainly just a series cap to block DC, and a terminating resistor. YOu can make up a simple probe and plug it into your other working amp. Using this tracer, you can input a signal to the amp, and "listen" to the signal at each stage through the amp. What signal? I just use a feed from my shop stereo receiver set to a music station. Or play a CD. Music is a full range signal, so you can easily hear if treble or bottom are missing.

A scope, and the experience of using it are valuable, as Loudthud says. But a tracer is something you can bang together in a few minutes and it is a useful tool as well, especially in the absence of a scope.

[HBamps]

I looked over this amp for a while last night and can't find anything. I did a full tube swap with the known good one with the same results. I read it somewhere but it sounds like it's a high and high mid roll off. just tubby. If I didn't know this circuit sounded good, I would think it's time for redesign. I can't help but to suspect the PT. The 'bad' amp is using a Weber WO25130 and good one using a Hammong 270fx. The specs are pretty much identical but the Weber tranny in much smaller and lighter.

I have always assumed that if you have the voltage and available current, it shoudn't make a difference but I am suspecting something here.

Over the weekend I plan to hook up the good amp's power supply (off the rectifier and power switch) to the bad amp. Or maybe the entire power section (split the amp at the PI). Hopefully this will be telling.

FWIW, I have a scope but haven't learned how to use it yet.

thanks for the tips thus far, this forum rocks.

john

[loudthud]

I would put a power supply problem at a low probability if the voltages are within reason. I would check any small cap (100pF?) near the volume control, the treble cap (270pF?) on the tone stack, and the 47pF cap in the phase inverter.

[HBamps]

I would put a power supply problem at a low probability if the voltages are within reason. I would check any small cap (100pF?) near the volume control, the treble cap (270pF?) on the tone stack, and the 47pF cap in the phase inverter.

Already clipped the 47p at PI and paralleled a cap across the 500p in the tone stack. The second changed the sound, as it should have, added more upper mids, but it didn't open the top end up.

[Chuck H]

I wouldn't move to modding the amp as it is yet because that could confuse troubleshooting.

Other than one being a combo with individual filters and the other being a head with a multi can filter, are there any differences to the layout or ground scheme between the two amps?

Do you notice any other symptoms? Like the amp gets duller at some sudden point on the volume control. Or a static click when the treble knob is advabced to a certain setting. Does the presence control work as it should?

[stokes]

[QUOTE=loudthud;264899]A bad filter cap can cause unintended feedback between stages and many strange problems. I would just try tacking another cap in parallel with what you have now to see if there is any change. Or, just check every node with a scope. Got a signal where there shouldn't be one?


I like this suggestion as a starting point,if you dont have a scope or cant use it yet,try checking each filter for a.c. voltage at each node on the B+ rail.The first or main filter will show as much as 5vac and each one down the line should show less than 1vac,any more and you have a bad filter.I have seen a lot of the newer Fenders,Devilles,Hot Rod Deluxes etc with various symptoms,including yours with a bad filter.I think the asians like the Illinois had a bad run in the '90's and early 2000's.Like Loudthud said,a bad cap can cause a lot of various symptoms.Its gotten to the point when someone calls and says they have a problem with one of these Fenders its always a bad cap.Not saying its definately your problem,but you gotta start somewhere and its an easy test.