70's Vibrolux Ugly distortion

[abelljo]

Hi guys, I'm back. I excuse myself for reviving this post and not posting actively, but I travel every week due to my day job and only today I got back to this POS amp! Still have not found the problem. Things I got to doing after my last post.

Changed the bad octal tube socket

Relocated the power transformer center tab and first filter stage grounds directly to the chassis closer to the transformer.

Placed the .002 grid caps back in the amp thinking it might be the problem.

Looked at all the grounds again.

I don't know what else to do, but in the crappy video you can see how badly distorted it sounds even at low volumes. It affects both channels and I even swapped speaker in the middle of the video just to rule the speakers out.

http://www.youtube.com/watch?v=eWCXC0Z9aHU

[abelljo]

Hi guys, I'm back. I excuse myself for reviving this post and not posting actively, but I travel every week due to my day job and only today I got back to this POS amp! Still have not found the problem. Things I got to doing after my last post.

Changed the bad octal tube socket

Relocated the power transformer center tab and first filter stage grounds directly to the chassis closer to the transformer.

Placed the .002 grid caps back in the amp thinking it might be the problem.

Looked at all the grounds again.

I don't know what else to do, but in the crappy video you can see how badly distorted it sounds even at low volumes. It affects both channels and I even swapped speaker in the middle of the video just to rule the speakers out.

http://www.youtube.com/watch?v=eWCXC0Z9aHU

I guess I should sell it as is/parts only on ebay

[jestaudio]

Hi guys, I'm back. I excuse myself for reviving this post and not posting actively, but I travel every week due to my day job and only today I got back to this POS amp! Still have not found the problem. Things I got to doing after my last post.

Changed the bad octal tube socket

Relocated the power transformer center tab and first filter stage grounds directly to the chassis closer to the transformer.

Placed the .002 grid caps back in the amp thinking it might be the problem.

Looked at all the grounds again.

I don't know what else to do, but in the crappy video you can see how badly distorted it sounds even at low volumes. It affects both channels and I even swapped speaker in the middle of the video just to rule the speakers out.

http://www.youtube.com/watch?v=eWCXC0Z9aHU

I guess I should sell it as is/parts only on ebay

Out of interest have you tried the OT yet, i had a TW build with a similar problem which turned out to be the OT

[abelljo]

Hey, yes I changed the OT. I haven't messed with the choke, but I don't think there is anything wrong with it.

[Randy Magee]

Check the coupling capacitors for leakage... that's one of the few things left... you've tried tubes, different OT, changed the plate resistors...

[Structo]

Yes, indeed check the coupling caps between stages for DC voltage leaking through.
Ideally there shouldn't be any but sometimes a few millivolts.
But nothing large.

I take it you don't have a scope?

Forgive me if you have tried this but do you have another amp that you could input the preamp from the bad amp into?

And vice a versa, input a good amp into the power amp of the bad amp?

Sorry, just listened to the amp.
Wow, that is strange but I'll bet it is parasitic oscillation.

Or some sort of blocking distortion caused by power tube grids.

You need to clean up the lead dress and route wires more direct and away from each other.

Does this amp have grid stoppers on the input grids to the power tubes?

If not or if they are small value, try some that are between 1K and 5K.
Of course both resistors of the same value,

[CHIP]

Tom,
I think you meant DC leakage.
I do agree check the coupling caps for DC leakage, I recently replaced some coupling caps with some NOS ones and had the same type of problem. When I"d hit a few chords I was getting an annoying distortion on the clean setting of one of my D builds. One of the NOS caps was leaking DC voltage.

[Structo]

DOH!
Yes of course DC.

My brain said DC and my fingers typed AC.

[billyz]

I suppose you already checked your preamp tubes ? The PI tube in particular.

[abelljo]

Check the coupling capacitors for leakage... that's one of the few things left... you've tried tubes, different OT, changed the plate resistors...

The problem is on both channels though. Should I try the three .1 ufd in the PI? I have changed all plate resistors, tubes etc. while I was in there changed the PI and bias supply to BF specs.

I wish I had scope, but I don't anymore.

[abelljo]

Yes, indeed check the coupling caps between stages for DC voltage leaking through.
Ideally there shouldn't be any but sometimes a few millivolts.
But nothing large.

I take it you don't have a scope?

Forgive me if you have tried this but do you have another amp that you could input the preamp from the bad amp into?

And vice a versa, input a good amp into the power amp of the bad amp?

Sorry, just listened to the amp.
Wow, that is strange but I'll bet it is parasitic oscillation.

Or some sort of blocking distortion caused by power tube grids.

You need to clean up the lead dress and route wires more direct and away from each other.

Does this amp have grid stoppers on the input grids to the power tubes?

If not or if they are small value, try some that are between 1K and 5K.
Of course both resistors of the same value,

There are two 1.5k resistors and two .002 caps that I replaced just to make sure I had not created a problem by removing them in the first place. To input the amp or input another amp, what do you mean? The only way I can think I could do this is running a long wire to the 270K input resistors from a known good amp's output. Is there an easy way to do this that I might not have a clue about? It is a good idea though.

[abelljo]

Yes, indeed check the coupling caps between stages for DC voltage leaking through.
Ideally there shouldn't be any but sometimes a few millivolts.
But nothing large.

I take it you don't have a scope?

Forgive me if you have tried this but do you have another amp that you could input the preamp from the bad amp into?

And vice a versa, input a good amp into the power amp of the bad amp?

Sorry, just listened to the amp.
Wow, that is strange but I'll bet it is parasitic oscillation.

Or some sort of blocking distortion caused by power tube grids.

You need to clean up the lead dress and route wires more direct and away from each other.

Does this amp have grid stoppers on the input grids to the power tubes?

If not or if they are small value, try some that are between 1K and 5K.
Of course both resistors of the same value,

There are two 1.5k resistors and two .002 caps that I replaced just to make sure I had not created a problem by removing them in the first place. To input the amp or input another amp, what do you mean? The only way I can think I could do this is running a long wire to the 220K input resistors from a known good amp's output. Is there an easy way to do this that I might not have a clue about? It is a good idea though.

So I have tried the suggestion above and confirmed it is after the preamp. When connecting the Vibrolux's output to my good working Vibrasonic's power amp input at the 220k resistor, the vibrolux's preamp sounds beautiful and of course super loud since I'm using an 85w power amp!

Now, when inverting the experiment and connecting the preamp of the Vibrasonic with a long wire from the reverb channel to the 220K input resistor of the Vibrolux the ugly low power distorted sound reappears.

So it could be anything at this point I suppose, bad caps in the PI, something in the power supply, maybe the choke? Where would you guys start?

Thanks and happy 4th of July to those here in the US.....

[martin manning]

Measure and post voltages around the PI and power amp: Pins 1, 3, 6, and 8 on the PI tube, and at the other end of its 470R cathode resistor. On the power tubes, measure pins 3, 4, 5, and 8. If all these look to be in line and stable at idle, then you might try replacing the three 0.1uF caps in the PI. If that doesn't do it then I'd suspect the OT.

[abelljo]

Measure and post voltages around the PI and power amp: Pins 1, 3, 6, and 8 on the PI tube, and at the other end of its 470R cathode resistor. On the power tubes, measure pins 3, 4, 5, and 8. If all these look to be in line and stable at idle, then you might try replacing the three 0.1uF caps in the PI. If that doesn't do it then I'd suspect the OT.

Thanks,

PI tube
Pin 1, 250v
Pin 3, 53v
Pin 6, 440v
Pin 8 connected to 3 /cathodes, 53v

Power tube
Pin 3, 457v
Pin 4, 456v
Pin 5, -51v
Pin 8, shorted/ connected to ground.

Hope the 3 caps are the problem. I swapped output transformers already.

Thanks again.

[martin manning]

Hold on, pin 6 on the PI tube should be pretty close to pin 1 (within a few volts). That triode (pins 6, 7, and 8) is not conducting. You're seeing the supply voltage at the socket (I assume that's where you measured) so check the socket contact with the plate and cathode pins.

And, the tube is good right? Fix this and I think you've got it.