Frying bacon noise

[RC_cola]

I have a Lectrolab R300 that has this noise. I have replaced some caps and resistors to get it back to working again to find this noise.

I noticed some wire connections did not match the schematic and relocated them to the locations per the drawing. This effort did remove the hum and buzz present only when the guitar volume was up.

I also notice many of the cap and resistor values are different prom the part list on the schematic. I looked at the schematic in the library on this site to see some values shown there dont match the list either as well as being different than values found on actual parts in the amp, whats up with that?


So what is a typical culprit for the frying bacon noise?

[sluckey]

Plate resistors is always a good suspect.

[jjasilli]

RC Cola.  Fizzy sound.  Hmmmm   

[RC_cola]

Fizzy, good one!

I replaced the resistors feeding the plates on the preamp tube, 6SN7. The sound might have slightly decreased after replacing those. The output tube, 6V6 does not have a resistor on the plate, its connected directly to the OT.

[Platefire]

I recently had that noise in my Allen Accomplice. I corrected my problem by replacing pre amp tubes.

[tubeswell]

Depends where the 'frying bacon' noise is coming from.

It can be a really bad thing if it is coming from your power transformer. In which case check for a short between the HT/B+ and the ground (which is typically what causes that noise in a PT).

[RC_cola]

I only had a power tube to swap out, no change. I dont have a spare pre tube on hand.

I checked all the PT leads to ground. The leads feeding the rectifier tube had about 340 ohms to ground on each lead but this pair has one tap of those windings connected to the case so I assume that reading may be normal?

I also noticed one end of the cathode resistor at the tube socket was connected to one of the heater leads at the next tube pin. I lifted the connection, no change. Looking at the schematic I see there is a second connection showing as this was done except the second point is to one of the three grid lines, not a heater lead connection.

[RicharD]

>So what is a typical culprit for the frying bacon noise?

Plate reisitor as Sluckey mentioned. 
Dirty tube socket.
Dieing (usually preamp) tube.  Power tubes tend to be a bit more exciting when exiting.
Bad solder joint.
Bad pot.
Could be a lotta thangs.

[kagliostro]

Hope the reason for the Frying noise isn't this 

Frying Vacuum Tubes3.AVI

Kagliostro

[RC_cola]

Nothing like that, so far anyway!

[kagliostro]

Nothing like that, so far anyway!

I'm sorry, I could not resist the urge to joke

Kagliostro

[RC_cola]

We all need a good laugh to keep from losing it sometimes! Love the pic, well as long as its not in my amp.

My antique radio buddy brought me some pre-amp tubes to try swapping out. I will try it later tonight.

[plexi50]

Hope the reason for the Frying noise isn't this 

Frying Vacuum Tubes3.AVI

Kagliostro

That is too freaky. I thought i was on the Enterprise with Kirk and Spock while watching this. Battling the Clingons

[RC_cola]

Well I swapped out with four different pre-amp tubes to have the same frying noise. I swapped out the rectifier tube as well for no change.

[tubeswell]

To reiterate, have you checked to see whether there is a short from the B+ to ground? When you have the amp on next time measure the DC voltages at all the filter cap nodes to make sure it is high and healthy. I'm wondering whether one or more of your filter caps could be (partially/totally) shorted, or whether the HT supply from the rectifier is touching the chassis somehow - dumping B+ to ground, and putting strain on the (secondary) High Tension winding.

[RC_cola]

http://www.el34world.com/charts/Schematics/files/lectrolab/lectrolab_r300.pdf


Here is the print for this amp. Where is the B+ lead you speak of?

I did check voltages at the filter cap. 406 volts at the rectifier output the upper 300's on the other points.

[tubeswell]

http://www.el34world.com/charts/Schematics/files/lectrolab/lectrolab_r300.pdf  Here is the print for this amp.

Those simple amps are cool aren't they?

Where is the B+ lead you speak of?

The B+/High Tension circuit is the (high voltage) power supply that starts at the HT winding and is followed by the rectifier and resistor/filter cap network.  Any point in that network where there is a short to ground will cause excessive current to be drawn from the power transformer. This is typically accompanied by a 'frying bacon'/sizzling noise and smoke and finally destruction of the transformer.

I did check voltages at the filter cap. 406 volts at the rectifier output the upper 300's on the other points.

Seeing at how all the power supply nodes look about right I'm assuming that you don't have a B+ short after all. So the frying bacon noise could just be that the rectifier is on the way out. Have you tried putting in a fresh rectifier tube?

[RC_cola]

I removed a rectifier tube from another amp with a clean sound and placed it in the Lectrolab, same noisey result. It was the same tube number.

I did meter from the chassis to all leads of the transformer. The only leads showing any response on the meter were the two feeding the rectifier tube but since that winding group has a lead connected to the chassis may be why I got a reading. It was about 350 ohms I think. I did remove all the tubes when I did that.

[PRR]

Which control changes the sizzle, Volume or Tone?

[RC_cola]

The volume does increase the sound of it. With the tone all the way down you can hear it, just more muted with the lack of high end so naturally the highs increase with turning the tone up higher. The level slightly increases with the tone at full rotation.

[PRR]

> volume does increase the sound

Then you know it is probably before (to the left of) R5 volume pot.

Assuming V1 has been chaged, that leaves dirty input jack, maybe bad R5 pot, and R1 R2 R3 R4 C1 C2 C3. Or very odd mis-wiring.

It's more time than cash to replace them all. (C9C10C3 3-pack, you could disconnect the C3 section and wire a separate 10uFd-22uFd to do C3's job.) While replacing you may find some real problem like bogus solder, cracked resistor, oozy cap, crap in jack, ancient beer-scum on tube socket.....

[PRR]

> some values shown there dont match the list either as well as being different than values found on actual parts in the amp, whats up with that?

Just from the flavor of the plan, I'd guess LectroLab was a bottom-feeder, selling amps made as CHEEP as possible.

And most of these parts values are un-critical.

R3 could be 10Meg, 15Meg, 5Meg, even 3Meg or 22Meg and still "work". Maybe with almost no difference. If the parts warehouse had excess inventory of 6.8Meg and was dumping them cheaper than 10Meg, that's what would be used.

Likewise R4 could be 100K for somewhat lower gain, 1Meg for more bass but relatively dull in treble, or 390K if a box of 390K resistors fell off a truck. (I once designed near everything with rows of 68 ohm resistors because a box DID fall off a truck in the street.)

The different values will perhaps give somewhat different tones. Fender mostly (NOT always) stuck near the values which gave the tone he wanted. Manufacturers selling much cheaper than Fender were not selling "that tone", they were selling "an amplifier". If it amplified, the buyer should not complain.

[RC_cola]

I replaced C2 in order to restore the volume function prior to signing up here. The volume would only sound good about 1/3 rotaion, beyond that you only heard a splat sound when tsriking the strings hard. The frying noise was not present then.

I metered the volume pot while the intial condition was present. I didnt see anything odd with the reading. I did restore a better connection to the chassis of the two pots since the leads of the pots shown going to ground are just soldered to the pot case.

I replaced R4 earlier with a 100K, thats what was in it.

C1 is a .005 disc cap and C4 is a 20mfd in a can with C9 and C10. I do have a 450v 33mfd cap handy right now. Can I use that as a test cap for C3?

[phsyconoodler]

I'm thinking it's cap bleeding DC to the pots.That .005uf cap might be the culprit.the 33uf cap will be excessively bassy.

[RC_cola]

Unsure if I mentioned this, this noise is present with no plugs inserted to the input jacks.

[RC_cola]

Last night I removed the input jacks with the two resistors and cap. I found where the resistors and cap connected together one resistor could be moved within the connection. I had two 100K resistors on hand and replaced the original. I also put in a different disc cap than the original. The frying noise seems to have deminished a good bit but its still not real quiet.

The part list shows it as a .005 paper condenser cap but it has a disc cap of that value in it. There is one more disc cap used also listed as a paper condenser. Whats the difference between disc caps and paper or mylar rolled caps?

[jojokeo]

The disc/ceramic cap is fine. I'd change the two 100k resistors down to half that. Your highs will be clearer and you'll get more tone from your guitar. Those are high values for input resistors. The cap can even be removed but it's there just making sure no DC gets into the input of the first stage and throwing the bias off. Most modern amps don't have this. There's a lot of searching & reading to do on your own for your cap question but quickly, not a whole lot of difference for what you have and how it's used.

[RC_cola]

I often see remarks about a "death cap", what is that referring to?

[jojokeo]

It hooks up to the mains wiring to the power supply for back in the days when they didn't have 3-pronged plugs and wall recepticles w/ a ground.

[PRR]

> The {input} cap can even be removed but it's there just making sure no DC gets into the input of the first stage and throwing the bias off.

It can be removed; but you may not like it.

Note that there is NO cathode bias network. In this amp, bias happens because the 10Meg grid resistor is "too big" for the small grid current but in a good direction for a little bias. Without the cap, the pickup would set bias to about zero, the tube would distort badly for all but the softest strum.

Leave that cap there (0.005u ceramic is in good taste for a cheap-amp; paper/mylar is more audiophile) *OR* rip it apart and copy a Tweed Fender input stage.

> I often see remarks about a "death cap", what is that referring to?

Completely different cap. One side of the wall-cord to the chassis. C4 in your plan. In days before 3-hole wall outlets, we flipped the plug for lowest buzz. Sometimes this gave no shock, sometimes it gave a strong shock if you touched a radiator or a grounded PA mike. I would prefer that you discard the antique power cord and install a proper modern 3-pin cord, green to chassis.

[RC_cola]

There is one more disc cap in use, the rest were paper tubular's. Do you have a schematic for the tweed copy you referenced?

When replacing with a three conductor power cord, is the cap still used? Would doing the power cord upgrade help with cleaning up anu noise in the amp? I do realize its done more as a safety issue.

[RC_cola]

It seems I may have moved up one rung of the ladder to success! I cleaned the tube sockets. R3 had a 390K resistor and I replaced it with a 5 meg I had on hand. Pin 6 on the preamp tube is shown directly connecting to ground but it has a 2.7K (actual measured value)resistor there instead. I resoldered that resistor.

Now the crackling frying sound seems to have deminished leaving mostly a hum noise now. I will get some caps to test bypass the existing filter caps next.

R11 at the first filter cap is shown as a 270 ohm but the actual value in place is a 4K.

[RC_cola]

I thought I asked this before but I dont see it so here it goes.

The cathode resistor connects to the PT on pin 8 but I noticed the lead from the resistor passed through the pin 8 tab and also connected to pin 7 which is a heater lead. Is there a reason for that configuration?

 The drawing shows the resistor connecting to pin 8 to the cathode and also shows a line connecting it to the third grid line. Is that an internal connection within the tube?