Help Me Diagnose a Tube Issue

[prspastor]

Hi Everyone,

I've been enjoying my Express build I completed about a month or so ago. From the beginning, I've had 6v6 tubes in it since that is all I've had on hand.

I finally ordered some EL34s to try. I got a set of Winged C and Tung Sols.

I tried the Tung Sols first and got some pretty nasty distortion when I played bass notes. The characteristic of the distortion was of the kind that sounds like when you play and turn your amp off. It is that dry, thwacky, crackly kind of distortion. Not musical at all.

I installed the Winged C's and all was great with them. Very nice sound and overdrive characteristics.

I'm assuming this was just a bad set of tubes (Tung Sol)? If so, I wonder what was the problem with the tubes...

Question 2:
I noticed that my power transformer was warmer with the EL34s than with the 6v6. I'm assuming this is because of the higher current draw for heaters and power output? I could touch the PT and leave my hand on it, but it was definitely hot. Does this seem normal for all you Express folks?

Still learning everyday,
John

[Reeltarded]

1. Funky tubes? Could be. Hard to say from here. The description sounds like cut-off though. What is the bias on the yucky tubes?

2. Warm is fine. Play it to bits!

[prspastor]

Thanks for the feedback Reeltarded.

Well, they were cooler tubes/low headroom tubes.

My B+ for the EL34s are 398v
I think I was running around -30v of bias voltage (although my memory could be wrong)
Cathode current at 40ma

[statorvane]

I too recently replaced some 6V6 tubes w/EL-34s. I'm thinking you may have to re-bias between switching the EL34 pairs.

I got two pairs of matched tubes a few weeks ago - one set for my son's JCM800 clone and one for my 4-hole Marshall clone, which was running JJ 6V6s. When I swapped the EL34s between pairs, in either amp, the bias settings needed to be adjusted. The tube sets are Mullard RIs and some cheap Chinese EL-34Bs. The Mullards sounded pretty good (my son really likes them). I am left with the Chinese EL34Bs which are loud, but sound... meh. Of course they cost... meh too.

[prspastor]

Oh yes, I did of course do a re-bias. Set these particular tubes at 40ma idle current.

I think they were just bad tubes - something was wrong with them. The =C= were just fine. I've got another set of Tung Sol coming my way. I think my interest is what in these tubes was broken to sound this way. Improperly wound grid, screen, poor connection in the tube, or we'll never know!?


Props to Tube Depot for their courteous service.

[prspastor]

Just an update. I received another pair of Tung Sol EL34s and installed. I encountered the same issue. The crackle distortion really shows up when the tubes are biased at 65% (~40ma) or higher. If I bias them much cooler, say around 28 ma, the crackle goes away.

I also notice that it is bass notes from the 5th and 6th strings around the 2nd and 3rd frets that really cause this crackle sound to happen. The distortion occurs much more on my neck pickup rather than the bridge pickup because it is more of a full sound/bassy. If I turn down the bass tone knob, this reduces the occurrence of the crackles.

I popped in an entirely different set of 6v6 than I had been using. Biased them up and they sound fine. No crackle distortion.

I went through and exchanged out every preamp tube one at a time and tested to make sure it wasn't a preamp issue. No change.

I don't have time to record the sound as I'm leaving on vacation this morning. However, do you all have any thoughts as to what is going on? Do you think this is a bad solder joint? Another bad set of tubes?
Probably to hard to diagnose without the sound.

It is funny that it only happens with EL34s.

[redshark]

Just before you blame it on the power tubes completely check the solder joints that go to ground. Diferent power tubes generate diferent frequencies and some of this frequencies will make certain joints vibrate.

[passfan]

For some reason your amp refuses to supply higher currents without crackling up. Recheck your joints in the plate and screen connections and bias pins. You do have pin 1 and 8 tied ? Check your flyback protection diodes as well if you installed them. You do have the recommended current ratings on your transformers, right ? Does it play ok at lower volumes and crack up at higher ones as well ?

[prspastor]

Thanks all for the quick responses. I'll check into these suggestions. I really appreciate the help as I prefer the EL34s.

Yes indeed, the crackle becomes more apparent when I turn up the volume. Plays better at lower volumes - sometimes there is a small crackle on bassy clean double stopped notes, but there are less crackles at lower volumes/turned back on the guitar.

I do have the Heyboer spec transformers in this amp. 300ma if memory serves correctly.

Thanks again for the help.

Here is a picture of my power tube sockets.

[img:640:480]http://lh4.googleusercontent.com/-TEM4X ... M_0183.JPG[/img]

[redshark]

Ok man, run this test now. It might seem extreme but it will tell you if it is a solder joint or not. I did it to both my express and rocket.
Get a rubber mallet, then with the amp on (only chassis) and volume half with no instrument plugged in start hitting the chassis gently with the rubber mallet all over the edge.
If there's a bad joint you will hear the crackling on the speakers. If not there might be an issue with the tube or something else.

[Gibsonman63]

Short of the rubber mallet, you may try moving the amp head off of the speaker cabinet and see if the problem goes away. If so, then it is vibration related.

[Randy Magee]

I had a similar issue as you... I have external bias points on my amp and was having the crackle sound you describe. In trying to diagnose what was causing it, I removed the flyback diodes and the crackle went away. I sought help on the forum as to why this was happening and nobody really came up with a viable explanation, so I've just left the diodes out... I'd rather have the convenience of adjusting the bias without having to remove the chassis from the cabinet than the protection the diodes afford.

[passfan]

If you whack it too hard with the mallet you could run the risk of damaging a tube heater. Kind of like striking a light bulb when the element is hot.

[Super_Reverb]

I tried the Tung Sols first and got some pretty nasty distortion when I played bass notes. The characteristic of the distortion was of the kind that sounds like when you play and turn your amp off. It is that dry, thwacky, crackly kind of distortion. Not musical at all.

This sounds like parasitic oscillation to me. Mild cases sound like a non-harmonious overtone (mosquito buzzing at times). Serious cases can involve ultrasonic oscillation that can throw off the bias levels in preamp and power amp and sound like you are not getting power. I have had both over the years.

If your problem is parasitic oscillation, it is due to a regenerative (positive or in phase) feedback condition. If it is parasitic oscillation and you have no wiring errors, your layout and/or lead dress are the problem. High gain amps are obviously very finicky about layout. Since voltage levels are high, the chance for capacitive coupling or magnetic coupling between stages is higher as compared to a lower gain preamp.

Grid stoppers and grid to cathode capacitors is how some people correct oscillation. These work by limiting high frequency response and can become audible with large Rs and Cs. Fender after CBS got involved had some disastrous experiments with huge Cs on grids of power tubes.

Another thing you can do is place 100-150pF across preamp plate resistors to attenuate gain at frequencies above 10KHz.

Am not saying this is your problem: if it is, you could tack in capacitors and add grid stoppers. If this tames the beast, then you know what you are up against.

Long term solution would be to figure out where you are getting parasitic feedback.

My most extreme case was due to preamp signal wiring too close to OT secondary leads.

rob

[prspastor]

Thanks for everyone's thoughts and help. I'll go through and re-check all of the solder joints and look at lead dress issues as well.

I had read your post, Randy, about your problems with the diodes. I'll try with and without when I get home.

I didn't have the amp on the speaker cab in any of these playing situations - so I'm not sure that vibration is the culprit although high SPL might be contributive and cause some vibration.

I'll post my results in about a week when I get back from vacation.

Best,
John