Need some help troubleshooting a tube setup

dpowell

I recently purchased an Anthem Pre 2L to replace a B&K Reference 10. I've had it in place a little over a week. I really haven't been using the 2-channel system much since my Denon is still out for repair but tonight I was listening to some music playing on my Oppo. It is connected to my Pioneer HT receiver and piped out to the 2-channel rig through Zone2 outputs. Not ideal but much better than the junk CD player currently hooked up.

While listening tonight, I noticed what I thought were vocals not quite centered as I was used to but rather coming from closer to the right SDA speaker. I didn't think much of it until I turned the volume down quite a bit and sat back down centered and realized that the right speaker is playing louder than the left speaker.

I'm not quite sure where to begin troubleshooting. My first guess is that it's possible there's a tube failing in the pre since I don't recall (but can't state 100% for sure) hearing this same issue when the B&K was in place.

I don't have a tube tester so I don't know how to go about figuring out if I need a new quad of tubes for the pre.

Thanks in advance.

leroyjr1

Can you switch the tubes around to see if the problem follows.

dpowell

I was wondering the same thing. The Tubes are marked for specific slots so I don't know if they can be swapped or how to identify which tubes are pairs so that I swap them correctly.

thsmith

all 4 tubes are 6922 I believe. Just label them so you know where they came from.

Can you put the B&K back in place to see if the problem is still there ?

dpowell

I've already sold the B&K so no can do. This morning I pulled out the SPL meter that came with the SRT's and decided to see if it was hearing the same thing as I was. Indeed, the right channel is playing about 6-8db louder than the left channel.

Since the headphone output on the pre uses the same tube signal path, I tried the SPL meter on my headphones and from what I can tell, there seems to be a variance between the L/R channels on the headphones. It's not quite as noticeable but it does seem I am in for opening up the Pre and swapping the tubes. My assumption is that the LV1 and LV2 should be swapped straight across with the V1 aned V2 tube sockets?

Tracy, if you still have that quad, can you hold it for me?

dpowell

I've swapped the tubes in the Pre and still having the same problem. Decided to test the BIAS on the power amp and here's what I found. The tubes are supposed to test at .4 and the results are:

Tube 1: .35
Tube 2: .31
Tube 3: .38
Tube 4: .33

Tubes 1 and 2 are the left channel which might explain the difference in output. Not sure why they've dropped so much. I set the bias on them when I received the amp about 4 or 5 months ago. Do they usually require adjustments every few months? I'm setting them back to .4 but wondering if I'm looking at tubes that are losing their strength?

Tube 2 (left channel) seems to be having some difficulty sticking at .40 . I've adjusted it now probably 6 or 7 times and it keeps inching up or down on me.

dpowell

After setting the bias on the power amp tubes back to .4, I still have the problem. This is very strange. I still suspect a bad tube but don't know how to identify if that is the case other than what I'm seeing on the VOM. After over 1/2 hour of adjusting, Tube #2 on the left channel kept standing out as the tube that didn't want to play nice. I kept having to turn up the pot for that tube to keep it at .4, let a few minutes go by and then it would read to high. After a lot of the back and forth, I finally found a spot that it would stay fairly steady. The other tubes were much more compliant and listened to their master.

dpowell

Here's where I'm at. After talking with Tracy a bit this morning, I came away with some more ideas to try. In addition to changing the pre, I had also introduced some different interconnects and speaker cables. Here's what I've done:

Things tried for Left speaker lower db output problem: [

Swap L/R channel tubes in Pre-amp No change
Set bias on power amp tubes No change
Change Left Interconnect between Pre and Power amp No change
Swap speaker cables L-> R and R-> L Problem changed sides
Change both L & R speaker cables to original known good Monsters No change
Change both interconnects between pre & power amp to known good No change
Change both interconnects between source & pre to known good No change

Next and last on my list of things to try is swapping the EL34 Power tubes and see if the problem switches channels. If that doesn't provide anything to work with, I am at a loss. I am positive I both hear and can see an imbalance on SPL meter between the L and R channels. The fact that switching the speaker cables back and forth between L and R moves the problem back and forth makes me suspect the power amp. Any advise would be greatly appreciated.

tcrossma

Forgive me if I'm not correct, because I just sort of gave this thread a quick read, but if you swapped IC's between the pre and AMP without a change, and then swapped speaker cables *with* a change, then your issue has nothing to do with the pre.

thsmith

D, if you swap speaker cables and the problem moved the it is most likely your speaker cables.

Roll some different speaker cables even if it cheap zip cord.

george daniel

If the issue follows the swapped component,, I'm guessing that it's the culprit,,in this case,, the speaker cables.Good luck.

thsmith

Just talked to Doug and he only switched the amp end of the cable not the entire cable. He also rolled some Monsters in with the same effect.

He is going to roll tube 1/2 with 3/4 and see if he the same bias issue and the audio difference.

He also has some Sovtek 34s to try as well.

dpowell

OK, looks like I need some more expert help from tube gurus. I switched tubes 1/2 and 3/4. Tube 1 --> tube 3 and tube 2 --> tube 4 and vice vs. While letting the amp warm up and beginning to adjust the bias on tube 1, I saw and heard a spark/pop from the vicinity of tubes 3 and 4. I immediately powered off the amp and pulled the power cord out. I smelled that electrical burn smell for a brief period of time. Next, I grabbed a flashlight and went over the entire circuit board carefully looking for any fried components. Thankfully, I cannot find anything that appears to be toasted.

While beginning to adjust the bias on tube 1, I noticed that it was upwards of a .6 or .7 which made me wonder if one of the other tubes blew because the pot was set too high on it and I hadn't gotten to it in time to adjust the bias?

In any case, after finding nothing fried on the circuitry, I plugged it back in and powered it on after setting all pots to 0 and began setting the bias on tubes 1/2. Those I set without further incident and have them both at .4.

Tubes 3/4 are a different case. Tube 3 (formerly tube 1) tests normal however tube 4 reads out on the VOM as a solid 1 regardless of where the pot is set. I don't know what this means.

Help!:eek::mad::(:confused:

F1nut

It means you either have a bad tube or a bad bias resistor. I'm betting on the tube since you didn't have a problem biasing #4 previously.

george daniel

Like Jesse said,,,if you have a VOM--locate the bias resistor,and measure it,,usually they are either 5 or 10 ohms,,,cause the last thing you wanna do is keep feeding it tubes.I had a EL 34 power tube that acted similary as yours, turned out it was a bad cathode to to ground IIRC. Good luck,let us know what you find.

dpowell

Progress! I tested the bias resistors and sure enough, the bias resistor for tube 4 tested at a solid 1 where the other resistors tested at 10.5 (the manual says they are 10ohm resistors). Looking more closely (and underneath) the resistor, I see where it exploded downward. Very strange. I guess it's time for a repair. Anyone know a good technician in the San Diego area. I'm not sure that I'm willing to learn how to solder on such a nice amp.

george daniel

most any tube repair/dealer can fix it,,you are in Ca,, electronic Mecca--or you can replace it yourself,, but it might be a good thing to have it checked out by a reputable technician.Good luck,, congrats on your progress.

dpowell

I've found someone local who worked at Sony as an electronics tech for 17 years and will do the soldering for me. I was thinking if that goes well to have him replace the caps with something more musical. Any recommendations? I don't know if this is possible but I'd like to pull a bit more bottom end out of this amp. It's very musical but lacks a bit of punch in the bass dept.

dpowell

Even better. Just talked to him and he repaired tube amps for Fisher for 11 years. I'm heading over to his house today to see if I can get it taken care of.

dpowell

The local tech I found met me at Frys this afternoon to check the amp and schematics to ensure I got the right replacement resistors. After parting with $1.03 for a pack of 4 resistors, I followed him back to his house where he has a nice electronics workbench in his garage.

The guy clearly knows his way around electronics. He worked for 11 years as a tube amp tech at Fisher, then moved to Sony for 17 years and finished his career as an A/V tech for Tweeter. Having never seen the Anthem before, it was as though he was right at home testing the various parts out. As a side note, he was very impressed with the quality and well-thought out design.

Since the resistors are designed to work in pairs, I had him replace both resistors for tubes 3/4. After replacing them, he hooked up the amp to some test speakers and set the bias. He then connected a pre and we played some music for about 10 minutes and all looked well. Then suddenly the resistor for tube 1 blew!

I had him go ahead and replace the other two resistors. We then fired back up the amp and checked the bias again and listened to some more music. So far so good. I won't have a chance to test drive the amp until tomorrow to find out if there are any more fireworks. I hope that they were just worn-out resistors and that I don't have multiple tube failures.

dpowell

After getting home from work this afternoon, I plugged in the Anthem for its first test drive after replacing all of the tube bias resistors. So far so good. In fact, I don't know how this is possible but the balance problem described at the beginning of the thread seems to be fixed. Measurements with the VOM confirm what I'm hearing. Electrically, does this make sense?

dpowell

I was a good boy and played stuff at lower levels for a few hours on the new resistors with no incidents. This morning, I decided to up the volume a bit and there went one of the new resistors. I guess it's on to trying new tubes.

thsmith

Doug, can you relate the tube/resister that just blew with where the tube was when the first resistor blew ?

george daniel

dpowell wrote: »

I was a good boy and played stuff at lower levels for a few hours on the new resistors with no incidents. This morning, I decided to up the volume a bit and there went one of the new resistors. I guess it's on to trying new tubes.

I think that it's time to get that amp in the hands of someone who can diagnose and fix it,, throwing tubes and resistors at it is not the answer,,it sounds like you have other issues,,maybe voltage regulation,,or lack of-- time to unplug it and get it fixed--good luck.

dpowell

Nope, unfortunately I can't. Here is the response I got from Sonic Frontiers tech support.

From: Anthem Technical Support <tech@anthemav.com>
Date: Thu, 1 Apr 2010 14:45:01 -0400
To: Doug and Lisa Powell<dougnlis@gmail.com>
Subject: Re: Issue with Anthem Amp 1


Hi Doug,

Were output tubes changed at the same time as the resistors? They're the cause of nearly all problems, especially ones as you have described. One blown cathode resistor requires changing both output tubes in that channel.

In bad cases the bias pots can be burned - rotate from end to end with power off. OK if travel is smooth, replace if there's mechanical resistance. A visual inspection for burned parts (caps and resistors) is also a basic step.

We no longer service tube models but a schematic is attached for reference. Anyone who is good with tube guitar amp repairs (or of course tube stereo amps) should be ok to work on the Amp 1. Only a bad transformer would make it unrepairable, and that's a rare case.

Best Regards,
Nick

=======================

I am going to put the factory originals in there and see what happens.

george daniel

I owned one of these,, it is possible that a tube can take out a resistor,,usually a fuse,,I hope that I am wrong,,but I think it may be something else,if you run out of options,, ship it to me and I'll take it to Benny Amina in Atlanta if you like,,he's pretty good with tube circuitry.Good luck.

reeltrouble1

not exactly local but maybe Gary Dodd from Dodd Audio (texas) will help, he liked Polkies as a rule of thumb.

RT1