Help! new Tung Sol 6l6gc Tubes make terrible"sqwaking" distortion sound on loud notes

[bbrownsbrick1]

have a push/pull cathode biased (with a bias pot) amplifier with a tube compliment of one 12ax7, one 12AT7 , and 4 6l6 (when i got it used it had 3 sovtek 5881's and one mismatched Mesa 5881). i just got a weber bias-rite for my birthday and decided to test those old tubes i got in my used head. mismatched indeed. i bought a new matched quad of tung sols 6l6GC's and tested them. when turning the pot high past 25.5 ma at first, they really squeeled, putted out and made a ton of outer space noises. then i tested them one at a time, and the plate voltage was around 254, and the tubes were well matched at around 80-81 Ma for bias. I keep messing with the bias pot on my amp, but everytime i turn the amp up half-way and up and strike loud notes, i get a weird squawk, and weird crossover distortion that sounds ungodly. i then take them out and put in those old sovtek 5881's crank it up, and the squawking on the loudly struck notes was gone, still getting around 254 plate voltage on the old sovtek 5881's.

are the new tubes damaged, or is it an internal problem with a cap, or a resistor value that wont match the tubes output, especially for an amp thats 80 watts with a plate voltage that low. in serious need of help!

[bnwitt]

Ok, first of all you do not have a cathode biased amplifier if you have a bias pot. What you have is adjustable fixed bias. Secondly, what amp is this? Manufacturer, model? Thirdly 254 volts is a very low plate voltage for 6L6GC power tubes. Are you measuring that voltage from pin 3 of your power tube sockets to chassis ground? Lastly, do you really feel comfortable probing around inside a chassis that could kill you dead?

[bbrownsbrick1]

it is an audiozone model 6. its a hand-built amp, and i talked directly with the builder and he said it was cathode-biased, even though it has a bias adjustment pot. yes, the plate voltage is actually that low. i have no idea why. he then said the output transformers were made for el-34's, but 6l6's would work also. is this why the 5881's(more rugged) work and the 6l6gc's dont?

[mikeboone]

You need to figure out why the voltage is that low. And you need to be safe doing it.

I would remove all the tubes and measure and record all DC voltage on all pins of all tube sockets. Also measure it at the supply on the filter caps. Its either low coming from your DC supply or being severely loaded along the way. Divide and conquer.

List all that info here and you will get some help.


Be safe doing it. I have a small wrap of electrical tape around my positive meter lead exposing only the tip. This will keep you from arc'ing pins together while measuring. Use a clip to ground your other lead and keep your other hand away from the circuit. DC travelling from hand to hand towards ground is very uncool.

[bnwitt]

he said it was cathode-biased, even though it has a bias adjustment pot.

That makes no sense unless the pot is a very high wattage unit and is in between the power tube cathodes and ground acting as a variable cathode resistor. How about posting some digital photos of the guts of the amp?

[bbrownsbrick1]

im not experienced at doing this, so how do i measure dc of the tube sockets? should i ground one lead to the chassis? i dont know where the chassis ground is on my amp. also, wouldnt measuring the dc voltage of the filter caps just tell me how much is stored up in them? what if it's an output transformer issue? i will take pictures as soon as i get off of work and post them. thanks

[bnwitt]

Triple B, take pics and post before you do anything else. We don't want to see your heart stopped. The measurements we're talking about have to be taken with the amp turned on so it can be dangerous unless you take the readings with the tubes pulled and at the socket pins outside the amp.

[bbrownsbrick1]

thanks for all the help guys, but after talking to the builder and not getting many solid answers, i decided to just bite the bullet and take it to my amp guy. he was puzzled as well. on another note, im going to start a new post on troubleshooting a non-functioning vibrato on a BF 65' fender twin reverb i just got (for 365 bucks!). everything else functions as it should, but the vibrato doesnt work. im thinking its probably the LDR oscillator. thanks!

[gtrdad40]

i would still like to see thoes pictures of the guts! very intresting.

[bnwitt]

i would still like to see thoes pictures of the guts! very intresting.

Yeah me too. Along with your amp guys comments.

[6267]

bnwitt--Cathode bias with adjuster is not all that hard, a larger resistance in parallel with a smaller one (the larger being the pot plus series R, the smaller being the cathode resistance) looks like a lower resistance. You'd have to do the math for your application, but a range of bias adjustment is possible with little dissipation occurring in the pot. If the pot fails, the circuit will see the highest Rk value, so it won't melt. I do have a 2k 5w pot that I'll use to find the sweet spot on 6v6 or el84 amps, and then put in something fixed.

[6267]

255v isn't ridiculous to run a 6L6 or EL34 on if it a true class A design. It's more "Radiotron" than it is "Marshall/Fender/Mesa" though. I'd ask the designer what it's supposed to be before worrying about the voltage. The sound you are describing could be a speaker problem, power supply problem (filter caps have had it), cathode circuit problem (cathode cap has had it), feedback problem (insufficient damping), or microphonics. Can you tap the new tubes and get loud noises? If so they are bad, send them back. Otherwise, some more description of the sound is probably in order.

[bbrownsbrick1]

ok thanks for all the help! the amp builder got back to me, here is what he had to say (really nice guy, btw)
6l6 and el-34 tubes have very close to the same impedance. a transformer for 4 el-34s is about 1800 ohm and one for 4 6l6s is about 2000 ohm, basicaly the same.

that 81 milliamp for each 6l6 tube number just sound way too much to me. it is way too high compared to the way i build amps. to me, thats sounds like a good number for a pair. that 81 milliamp per tube number is even too high for a class a circuit at 275v plate voltage. the book i use lists 65ma for a class a circuit bias number. my book also lists 80ma at 225 volt for class a circuits.

the amps power transformer does have enough power to give four 6v6 tubes 81ma each and i think if you are doing that you might be driving the tubes too hard. when i built and sold those first model 6 amps they were biased at about 40ma or 45ma per tube. el-34 tubes require more milliamp current and at that voltage it would be about 60ma or 55ma per tube. my book does not reccommend using them below 300v plate voltage, although i have and i didn't have any problems when i did it. i'd stick with the 6l6 tubes for that amp because thats the way i sold it.

the amp is not cathode bias. it has a bias supply with an adjustment control. it is near the power transformer. i'd lower the bias number to 40ma or 45ma per tube with the bias control located near the power transformer, forward of the power tubes

the amp is class a/b, push/pull circuit. the power transformer is a russian made one and is the same as was used in the sovtec 120 watt amps. it has more than enough supply to run four 6l6 tubes and i'm thinking your first thing to be sure of is that you are not running them too hard/hot --rick

[bnwitt]

bnwitt--Cathode bias with adjuster is not all that hard

I didn't say it was. It is just highly unusual in guitar amplifiers.

I do have a 2k 5w pot that I'll use to find the sweet spot on 6v6 or el84 amps, and then put in something fixed.

You'd have to have alot more than a 5 watt pot to do this on 4 6L6GC tubes running at 75 watts

255v isn't ridiculous to run a 6L6 or EL34 on if it a true class A design.

His amp is listed as 75 watts with 4 each 6L6GC tubes on the internet. You can't get 75 watts out of 4 6L6GC tubes in class A or Class A push-pull with 255 volts on the plates. That would be more like 26 or 58 watts.

[bnwitt]

the amp is not cathode bias. it has a bias supply with an adjustment control. it is near the power transformer. i'd lower the bias number to 40ma or 45ma per tube with the bias control located near the power transformer, forward of the power tubes

the amp is class a/b, push/pull circuit. the power transformer is a russian made one and is the same as was used in the sovtec 120 watt amps. it has more than enough supply to run four 6l6 tubes and i'm thinking your first thing to be sure of is that you are not running them too hard/hot --rick

Ok, this is starting to make a lot more sense. It seems that this amp was designed (really) for 6V6GT tubes with an oversized PT and OT. Is that correct. I did see a Harmony Central review which said the amp was a 4/6V6GT topology but found another reference saying it was 75 watts. Those two statements don't jive.

The builder is saying it can handle 6L6's as well. Be aware though that 6L6's are not designed to run at that low of a voltage in AB push pull configuration. Also 4 6V6's at 250 volts have an impedance of 5,000 ohms plate to plate. Going to 4 6L6's or EL34's as you've been told has an impedance of half that. I am not surprised the amp squeals and squawks with that mismatch if this is the case. Dose it have multiple secondary (speaker) settings like 8 ohm 16 ohm etc.