odd problem with modded 50 watt Ceriatone OTS

[pine]

Amp was working fine until about two weeks ago, when I started getting a crackling static on lower, specific notes. The static actually crackled with the note's frequency. I changed the power tubes for some used tubes (that had admittedly been stressed at one point), and the static went away for two weeks then came back. Changed the power tubes again and it went away again. Could this be something more than just power tubes? The mod I referred to in the title of the thread is a 100 ohm 50 watt power resistor (for more sag) with a heat sink and thermal paste in series between the rectifier circuit and the standby switch.

Thanks in advance for the advice

[Structo]

Have you checked the plate voltage on the power tubes?

What are you setting the bias at?

Did you try different preamp tubes? You can get that crackling sometime with faulty preamp tubes.

Tubes should always be the first suspect.

That choke resistor is a bit too small IMHO.

I don't think anybody goes below 300 ohms on that.

Did you check your dropping string after installed the 100 ohm resistor?

List your voltages starting with the preamp tubes.

[ChrisM]

Id also check plate resistors and screen resistors. Might want to retention and clean tube sockets as well.

[pine]

Thanks for the replies, I'm at work so I can only answer a couple of those and will have to report back. Bias is set at around 33 mA. I didn't try different preamp tubes yet because the problem initially went away after the first power tube swap. It worked fine for a few months after I installed the sag resistor (incidentally I am still using the choke as well) so I was thinking it was logical to consider the amp as it was at that time as my baseline. I'll have to get the voltages when I get home.

[JD0x0]

Thanks for the replies, I'm at work so I can only answer a couple of those and will have to report back. Bias is set at around 33 mA. I didn't try different preamp tubes yet because the problem initially went away after the first power tube swap. It worked fine for a few months after I installed the sag resistor (incidentally I am still using the choke as well) so I was thinking it was logical to consider the amp as it was at that time as my baseline. I'll have to get the voltages when I get home.

i think the problem may be with your choke and/or choke resistor. someone at this site switched chokes to a 10H 100 ohm and immediately got that static that floats over the bass frequencys mostly, once he changed it back to 100 ohm 5H the static went away

[pine]

aha! that sounds promising. Why the time delay before it starts happening though?

[JD0x0]

aha! that sounds promising. Why the time delay before it starts happening though?

not sure. maybe when you switch the power tubes the new ones have a different bass response before they are "burned in" once you burn them in the tube's sound changes a bit and probably causes the "hash."

Just FYI all of my dad's marshalls have this static over the sound that youre explaining. He doesnt seem to hear it but it's very apparent to me

[markr14850]

choke resistor

Not sure what a choke resistor is, but anyhow, it doesn't sound like he's talking about a 100 ohm screen resistor. It sounds like he's talking about a 100 ohm resistor before everything, including the plates.

[talbany]

Something sounds Funky with respect to bias...@ 33ma on a 6L6 GC biased around 70% would give you a plate voltage around 625v...If that the case no wonder your chewing through tubes.

If your plate voltage is around 450V you need to raise it to around 46 ma..


Ive used rectifier sag resistors on many amps some on a switch and never have the issue your describing w/ a 5H and 10H choke...If you want to simulate the sag of a particular tube simple look up the tubes plate resistance...

5Y3 -60 volts @ 125 mA
5U4GB -50 volts @ 275 mA
5U4 -44 volts @ 225 mA
5V4 -25 volts @ 175 mA
5AR4 -17 volts @ 225 mA

They will sag most from top to bottom.


Tony VVT

[pine]

choke resistor

Not sure what a choke resistor is, but anyhow, it doesn't sound like he's talking about a 100 ohm screen resistor. It sounds like he's talking about a 100 ohm resistor before everything, including the plates.

That is the case, I was assuming that JD0x0 meant sag resistor, and the wirewound power resistor right after the rectifier circuit was my attempt to simulate the sag of a rectifier tube. What I don't get is why it worked fine like that for such a long while

[dogears]

I have no idea what you are saying Tony. Pine is merely saying he is biased at 33ma. Not that it is 70%. Nothing wrong with 33ma. In fact, many Dumbles are biased quite cool. I have to disagree with your 46ma comment. I don't know anyone with a 6L6 Dumble circuit at 450v, that likes bias that hot. A famous guy sent his amp in for service because it sounded like ass after a new tech changed the tubes. The problem? Biased at around 44ma..... Reset to 34ma and everyone was happy!

Something sounds Funky with respect to bias...@ 33ma on a 6L6 GC biased around 70% would give you a plate voltage around 625v...If that the case no wonder your chewing through tubes.

If your plate voltage is around 450V you need to raise it to around 46 ma..

[JD0x0]

460 on my plates biased at 37mA is a bit hot for me IMO his bias seems fine

[Structo]

I have my RCA 6l6GC blackplates @ 35ma with the plates at 435v.

I agree that my amp seem to like it on the cool side of things.

[JD0x0]

I have my RCA 6l6GC blackplates @ 35ma with the plates at 435v.

I agree that my amp seem to like it on the cool side of things.

i think in general 6L6's like it cooler so it doesnt emphisize the farty, crappy, loose bass that 6L6's usually have... which is why KT88's are now my favorite tubes

[talbany]

Tube bias No doubt is subjective and will vary slightly tube to tube and amp to amp..I wasn't trying to imply that the amp needed to be biased @ 70%..to sound good.. If it came across that way I'll step back (Delivery is everything)..I had the feeling that your issue could have been tube related and may have been biased to cold for that perticular set you had in there and didn't know you liked them cold.. If the amp had the problem and was biased cold that would be something I would consider....For those who dont know..Bias changes slightly with each variant..

6L6...29.5.ma
6L6GC.....46ma
6L6WGB....40 ma
5881.........40 ma
KT66-........38 ma

Plate voltage @ 450 biased 70% I know HOT!!!

Not enough to make a huge difference worth the mention..

Happy Holidays .............................. Tony