Freaky Whistle in Guitar Amp

[Trout]

Haha, 2 steps forward, 1 step back here.

The Bypass caps were purchased from parts express 3-4 weeks ago, So I assume they are fairly new.
My Carbon comps are checked as far as correct value with my ohm meter and anything worse than 5% has been disposed of.

The whistle returned, or possibly never left I must have missed the magic combo of treble/presence/bass.
It is not as bad, but can still occur especially during certain low notes like a low G. it actually makes a rattling type whistle mixed with that note.
Also occurs with no notes being played.

I am going to do the pot bypass thing next.

The Amp is not near the speaker and chassis vibration has no apparent effect.

Here a couple pictures of the amp, This is amp 2 of 2 identical units.
Amp
Bottom
Sound Clip


The amp sounds fairly nice, Its just this annoying noise! lol
Gene

[Trout]

Originally posted by poobah
OK,

That pot value is your problem... you are probably sending your total gain up to high.

If your finger can change the sound it is definately a capacitive feedback type oscillation probably from components under the chassis. Keep in mind this problem will cure itself with the right pot probably.

Take the pot out of the circuit (temporarily) and just put a 4.7K... this will test the theory.

You hit it on the head, I replaced the incorrect pot with a temporary 4700 ohm resistor, Tested, Re-tested, Re-tested with the strongest tubes in my fleet.

NO Whistle.

I need to buy some more pots, I never have anything as low as that around here. I would have never guess it was that.

THANKS!!!!!!!!!!!!!!!!!!!!!!!!!




While on the subject of pots, 1 of my previous build calls for 1 meg volume pots, I find it to be very sensitive at the lower setting, Very non linear.

I have ben buyingThese

The Volume jump from 0-1 on the scale of 0-10 is about 60% of the total volume. This is ultra sensitive.
Would a log taper provide better control?
If so, do you know of a fast low cost source?
Gene

[Sherman]

Originally posted by Trout

...
While on the subject of pots, 1 of my previous build calls for 1 meg volume pots, I find it to be very sensitive at the lower setting, Very non linear.

I have ben buyingThese

The Volume jump from 0-1 on the scale of 0-10 is about 60% of the total volume. This is ultra sensitive.
Would a log taper provide better control?
If so, do you know of a fast low cost source?
Gene

A log pot would definitely give a more linear volume control if the amp was designed that way.

Check Antique Electronic Supply for pots of almost any value for gutar amps in both log and linear.

https://secure.tubesandmore.com/

[poobah]

Trout,

That's awesome... and it sounds awesome too... and you can play... right on!

OK, POT SCHOOL:

1. Whenever a pot is used with just 2 connections OR the wiper is connected to one end of the resistance element (still just 2 connections coming away from the pot); it is being used as a RHEOSTAT... big word for variable resistor... you can sub a resistor with any value between zero and the pot's value to approximate the value and the effect of the pot. For example a 2.5K resistor would sound the same as a 5K pot at 50%. IN ALL CASES, a rheostat must have the correct value to avoid changing the circuit (perhaps for the worst). In this particular case, you could could have just used the pot from the 0% to 20% settings and you would seen the proper performance (that would have been 0 - 5K ohms).

2. Whenever a pot is used with all 3 connections; it is being used a POTENTIOMETER. In this situation, it is SOMETIMES possible to use a diiferent value and still have a good result. It all depends on the circuit.

Question: before you run out buy pots... do you know about LINEAR versus LOG/AUDIO TAPER pots?

3. If you notice that at one end of the "dial" the pot seems to "do nothing" and at the other end of the dial the pot seems way "too touchy"; those are places where you likely want to use a AUDIO TAPER pot. These pots are built with the resistor "scrunched up" at one end to make it behave "evenly" as you turn the dial. Now, there is probably some half-4ss rule for which kind to use where... but it depends on the circuit. Most volume controls should be LOG/TAPER thoough. Tone controls are a whole 'nother deal. Post your schems and Tubelab or someone could tell you before you spend the bucks...

Cool to hear you and your amp dude...ROCK ON,



[edit, wow we where posting at the same time! no harm done]

[poobah]

Hey Trout,

Here's some links to DIGI-KEY and some good pots... these people are a joy to work with... mega corp for sure, but nice people, super prices, and stock on the shelf (millions), and they are not snotty about selling ONE.

Sorry but GOOD pots cost a bit more.


http://dkc3.digikey.com/PDF/T061/1283.pdf

http://dkc3.digikey.com/PDF/T061/1282.pdf

Later -

[bernification]

Nice work indeed Mr. Trout.
Guitar amps can be a real handfull, having power supply, output stage and high gain inputs in close proximity. I found sticking probes in the amp and finding out the most sensitive areas for this was a great help as to diagnose with out tools where my major problems were located.
Aspen Pittmans tube amp handbook is very usefull for this kind of work.
Good luck. I know the feeling of making an amplifier that I could play myself, and it's brilliant!

[Trout]

The Amp that has a REALLY touchy volume control is a cloned Fender Deluxe 5E3 I modded a bit back.

I swapped out a cap to get a Bright & Normal channel. Otherwise its basically the Standard Schematic

The both volume controls are ultra sensitive at 0-1, then gets very flat response after about the 3 posistion. Especially so on the bright channel.

I use that amp alot, Its got a very clean sound and works well for recording in a small studio/computer room. I can get plenty of tone without blowing down the walls and strapping pillows to my ears

This EL34 modded bandmaster based amp we just got fixed up is deadly!!
I manage to get a good overdriven sound and only have to push the closed back speaker cab facing out the door into the family room to sit in the comp room while I play it.

Of Course recording while the wifes home is nearly impossible lol

I Thank You All for the Help!!
Gene

[poobah]

Hey Trout,

I think we need to get PAID here... how's about you post a nice 3:05 instrumental for us (when you get a moment).

[Trout]

Hey, You guys think sorting this one out was fun, I got a real stumper for ya.

Here You go, Somethings hateing me on this one.
SE EL84 Champ thingy
Maybe I should make a new thread.

This ones plumeting into a distorted cut-off at max volume.

Plus It smoked at least 1 EL84, ( old austrian Tube of questionable quality and age).
Gene

[anatech]

Hi Gene,
Try watching the voltage on pin of the EL84 when cranking it.

The questionable EL84 poses a bigger danger to the amp than the other way around, might want to measure your 470 ohm resistor too.

-Chris