Problem with this fizzy sound

[Silvio55]

Ok Pete, Iīll try that also. I donīt have 1M right now but I have a 500k audio pot, can I try with this one?

[pdf64]

Yes, 500k should be fine.

[jmaf]

There are none in the UK on eBay, but thanks anyway.

You're right, the listings presented were "alternative", after I clicked on "UK Only". I should've known you had looked there. How about Tube Amp Doctor, isn't he in Europe?

Another idea, I've seen Laney use Skot : FT Transformers Ltd - Custom Built Transformers
Maybe they'll ship a single choke unit over to you.

[sickman82]

How about Tube Amp Doctor, isn't he in Europe?

See Hammond transformer range at Bluebell

Great.. thanks for all the info guys. I will look in to the choke once Christmas is out of the way, it is a cheap enough modification to see whether or not there is a benefit to it. I have found 'Tube Town' sells chokes as well as 'Tube Amp Doctor'. Although, Bluebell seems the cheaper of the three. Sorry I haven't been quite as active this week with the thread, it's been manic at work leading up to Christmas.

I'm going to order in a new valve complement for the amp in New Year as well.. I've always meant to try some different valves in this amp, but never got round to it. The 12AT7 is something I have considered before with regard to cleaning up the amp a bit for better use with pedals. I hear JJ are a pretty good match for this amp. I'll try a few and see what I like.

Silvio, how are you getting on?

[Silvio55]

Well, I didnīt make any further mods in the amp these couple of days. Today I was boring and with the multimeter in my hand I did something I should have done before: measure the voltages, and I think found something. While the voltage at the wall socket itīs ok (228 volts), the voltages at the plates of the pre amp tubes are all aprox. 20 volts below of the schematic readings. The voltages of the power tubes plates are ok at 331 volts. Is that something to worry about or itīs just normal?

[Chuck H]

the voltages at the plates of the pre amp tubes are all aprox. 20 volts below of the schematic readings. The voltages of the power tubes plates are ok at 331 volts. Is that something to worry about or itīs just normal?

I can'r say if it's normal. It could be a post schem revision or some such. Maybe sickman82 can tell us if his has the same situation.

If you could take voltage readings at all the pins we can tell if some triode is drawing excessive current and dragging those voltages down.

And, FWIW, I think the idea of soldering in a temporary pre PI master is a good one to help isolate the ugly distortion. It's probably the PI, but if I were trying to fix a problem I would be certain to isolate it first. Since you don't have a scope the pre PI master will help with that.

[Silvio55]

Here are the voltages at bright mode:

V3: Pin 1:106, Pin 3:0.8, Pin4:2.98, Pin5:2.98, Pin 6:115, Pin 8:0.83, Pin 9:2.98
V4 Pin 1:210, Pin 3;46.7, Pin4:2.98, Pin 5:2.98, Pin 6:160, Pin 8:3.15, Pin 9:2.98

I will be trying the pre PI master these days.

[Chuck H]

That looks OK to me. I mean, other than the plate voltage being low. But operationally things look normal. So I'll guess it may be a part value revision that happened after the schem.

If the pre PI master reveals that the crackle is not from the PI I would definitely increase the preamp voltages to see if that helps. This may be the mysterious modification to the power supply that that was discussed. Since upping the preamp voltage will also increase gain a little it would be an idea to try 150k plate resistors if you do increase the preamp voltages. That too may help. I wouldn't trouble with it unless you find that the problem isn't coming from the PI though.

[Silvio55]

Yeah, but doesnīt these voltage changes afect the bias condition of the PI? The bias resistor of the PI is still 1k5 in my amp, the same as the schematic.

[Chuck H]

Yeah, but doesnīt these voltage changes afect the bias condition of the PI? The bias resistor of the PI is still 1k5 in my amp, the same as the schematic.

Yes it does. But very little. The PI voltage isn't that low compared to the schematic. And the circuit is "self biasing". Which is sort of a rough description since there are limitations to how much the self biasing nature of the circuit can correct itself. But in your case it's really negligible. That isn't to say a different resistor won't change the bias. It will. But the circuit would then become self correcting to a new bias point.

The MB amps I mentioned before that had the same crackle decay also had lowish plate volts in the preamp (about 120 IIRC). I was pretty green then but I do remember being perplexed by it at the time. I'm thinking the slightly hot bias and low voltage of the preamp may be the problem. But you should install that pre PI master and find out if the noise is in the preamp or the PI.

[Silvio55]

Ok thanks Chuck! So I will try the pre PI MV, but how do I conect the pot? the wiper to the output of the 3rd gain stage, one leg to ground and the other to the input of the PI? after or before C10?

[Chuck H]

Just solder a 1M pot over R19 with one leg and the wiper. This will change the load at full up to 500k but that shouldn't make much of a difference. Besides, the point is to turn the input to the PI down for testing purposes anyhow. Ideally you would replace R19 with a 1M pot across it's outr legs and feed the PI with the wiper.

When testing to see if the preamp is responsible for the crackle be sure to turn the existing master full up.

[sickman82]

It sounds like you guys are getting somewhere with the problem. I'll email this guy as soon as I can to see if I can get any more information out of him to confirm whether you are heading down the right track with increasing the voltages. It sounds pretty plausible to me, but then I'm no electronics engineer, so what do I know!

I'll try to get some voltage readings for you over the next couple of days as well. I'm so busy with work and other obligations at the moment that I'm not getting much free time to help out. I'm checking the thread when I get the chance, and I will try to put as much in to it as I can. It's that time of year again, thankfully it is only once a year!

EDIT: Just emailed the guy regarding the plate voltages and the PI. Let's see what we get back. He's usually pretty good with replying, but there is a time difference.. so I'd expect to have an answer by this time tomorrow.

[Silvio55]

Well, didnīt have a 1M pot, so I just soldered a 1M resistor in parallel with R19. Nothing again, the fizz is still there, so I guess the problem is not the PI...

[Chuck H]

No. That's not right. The reason to use a 1M pot is that it WON'T change much BUT will allow you to turn down the drive to the PI. You can't do that with a resistor. What you want is a way to turn the amp down before the PI and then crank the post PI master up full.