Push-Pull tube amp. Importance of matched tubes?

[RickyC34]

Hey all, I have a Muzishare x7 integrated tube amplifier. See on Amazon. I've had the amp for about two years and not long ago I noticed a substantial channel imbalance. (about 5 db). I started replacing the front tubes as these were the cheapest and it improved things to where now there's about a 2db difference.

My question. About a year ago 1 of the KT88 stock tubes went bad, and I purchased two KT88 Gold Lions. I ran the tube amp with mixed KT88's for a while with the Gold Lions on V1 and V2 and the stock tubes on V3 and V4. Could this have caused the imbalance? With a push-pull amp like the Muzishare, what is the best practice with nonlike tubes? If this is ok? Does it matter if they are both on one side or should I run one set on V1 and V3 with the others on V2 and V4? I hope this will be ok as I just replaced all the other tubs and two more KT88's would be a couple hundred bucks. At this point, I'm almost scared to use it in fear that I had messed things up with the non-matched tubes.

Thanks!

 

[RickyC34]

Looks like my link is an upgraded version. Here's the one I have.
aoshida-muzishare-x7.
china-hifi-audio muzishare-x7

 

[mhardy6647]

Really depends on the bias scheme they use.
Matching may be critical or the circuit may be quite tolerant of mismatched tubes.
Matching'll never hurt, though -- if it's PP stereo buy a matched quad of new, top quality tubes from a reputable vendor.
One gets what one pays for.
The effect of mismatched outputs in an amp that's picky about such things to maintain the proper bias is usually way more severe than a 2 dB (negligible) mismatch in level*, though. Do you know what redplating is?

The EL34 on the left in the (borrowed) image above is redplating rather severely. Run in this condition, it won't last long.
EDIT Actually the other one is redplating a bit, too. Could be bias, could be a bad coupling capacitor.

______________
* That's why preamps have balance controls!

 

[DVDdoug]

Isn't 1950's technology great!

Actually it sounds like a bad design. I wouldn't expect that to happen with a McIntosh tube amp. A good amplifier should stay in-spec as long as the tubes remain within tolerance.

Transistors & MOSFETs vary too, but they are more stable and don't age like tubes.

 

[Ruhled]

This is the nature of the beast unfortunately especially if these are zero negative feedback amps. Channel balance is often compromised with SET or zero nfb amps and owners often don't seem to realize it can be way off! Its important to measure these things with these sorts of designs. Separate input gains or a balance control are necessities but it goes against the ethos of "less is more" designs.

What can help if there's no provision to adjust channel balance is systematically swapping tubes from channel to channel and positive phase to negative in same or opposite channel in a push pull output stage if it only has a bias voltage adjust and not dc balance adjustment. You can sometimes nail down a particular combo that comes closer to perfect balance than where you started. 2db imbalance imho would still be annoying if you listen in one spot in the middle if the speakers. Anything more than maybe 0.5 or 1db would annoy me because it pulls the center off to one side in a pretty consistent matter at much over 1db in my system.

If it sounds like I'm down on tube amps please forgive me. I have many and have been using them for years now but this seems to be one thing people don't seem to readily acknowledge as a big issue sometimes.

 

[RickyC34]

Thanks for the replies.
@Ruhled I'll do the suggested tomorrow to help get me by until I replace the other two stock KT88s. I'll take some measurements and see what effect different combinations bring. Hopefully, improves that 2 db. Noob tube question here but when swapping tubes do I need to let anything cool down or just power down, use a glove, swap, and power back on? My best friend is visiting later this wk and is gifting me my first record player : ) the Audio Technica lp120xusb. He loves the Muzishare and I'm sure it will get a lot of use over the weekend.
The amp only offers bias adjust no dc. I do have the ability to adjust the channel delay and gain via Dirac though. With the tube amp I typically run Dirac with a very narrow correction window and correct phase.
I guess my worry/question is if something is wrong and causing the channel imbalance and I correct it digitally and continue to use it, I would think it could make the problem worse. Putting fuel on the fire?

 

[RickyC34]

@mhardy6647
Thanks for the reference, my tubes don't look like that and I've never seen anything like that in person. I'll replace the other two stock tubes with 2 more Gold Lions if not a fresh quad as suggested. Things are just a bit tight this time of year.

 

[Keith_W]

Many years ago, I had a conversation with an engineer who worked on tube amps. The conversation went like this:

Me: "I need to buy a pair of matched tubes for my push-pull valve amp"
Him: "I have NEVER seen a pair of matched tubes that are matched like the manufacturer says"
Me: "Why?"
Him: "Because of manufacturing, and different tolerances from one manufacturer to another. Valves are hand manufactured components, they are inherently difficult to manufacture consistently, so they all come out measuring differently. Depending on the company, they may have different tolerances before rejecting the tube. Some only test if the tube works, then they sell it. Some of the more expensive tubes have tighter manufacturing tolerances. A manufacturer has to go through a large batch of manufactured tubes to find a matched pair. A third party reseller offering a matching service has a smaller pool of tubes to find a matched pair from. Matching has its own tolerance, independent from manufacturing tolerance. Also, tubes degrade with use, and tubes may not degrade at the same rate. So even if a new pair of valves came as a matched pair, by using them they will eventually not match".
Me: "If it is impossible to find a matched pair of tubes, why do designers design push-pull valve amps knowing that the valves on push and pull might be poorly matched?"
Him: "The job of an engineer is to do his best given the limitations. In the past, a push-pull valve amp was required for higher power. Nowadays, it is a marketing imperative. We do our best even though we know that an imbalance might result".

And in the same conversation:

Me: "Is there such a thing as tube burn-in?"
Him: "A new tube will take a few hours before the rate of change of its performance stabilizes, and then it slowly degrades. The testing done in the factory is enough to achieve this burn-in, any further burn-in at home is pointless unless your new valves weren't run in the factory for too long".

And finally:

Me: "if tube amps have so many problems, why do you manufacture tube amps?"
Him: (grinning) "I have to make a living, and I know how to design and manufacture tube amps. People want them".

I thought he was refreshingly honest.

 

[Ruhled]

Thanks for the replies.
@Ruhled I'll do the suggested tomorrow to help get me by until I replace the other two stock KT88s. I'll take some measurements and see what effect different combinations bring. Hopefully, improves that 2 db. Noob tube question here but when swapping tubes do I need to let anything cool down or just power down, use a glove, swap, and power back on? My best friend is visiting later this wk and is gifting me my first record player : ) the Audio Technica lp120xusb. He loves the Muzishare and I'm sure it will get a lot of use over the weekend.
The amp only offers bias adjust no dc. I do have the ability to adjust the channel delay and gain via Dirac though. With the tube amp I typically run Dirac with a very narrow correction window and correct phase.
I guess my worry/question is if something is wrong and causing the channel imbalance and I correct it digitally and continue to use it, I would think it could make the problem worse. Putting fuel on the fire?

The channel imbalance is likely normal. Nothing to worry about. Dsp is exactly how i correct mine if I can't get it acceptable by swapping tubes around. Also note the imbalance will change as the amp warms up. So 2db when its still relatively cold might become 3db or 1db in an hour or two. But usually if you get it in the ballpark cold it doesn't seem to change a lot.

 

[RickyC34]

I swapped tubes, is this behavior normal?
The previous layout is Left side V1, V2 Gold Lions. Right V3, V4 Stock
I moved previously V4 stock tube to slot V2. and moved the previously V2 Gold Liion to slot V4
So now : V1 GL , V2 Stock, V3 Stock, V4 GL.
When I turned the amp on the bias on V4 Skyrocketed, V3 went to about 3/4 of where it needed to be, V2. is barely above 0 all the way to the left, and V1 is about 1/2 the way of where it needed to be.
My concerns are V4 and V2 is it normal for things to be THAT different? Should I just crank the bias, adjust it, and keep an eye on it?

 

[Saponetto]

X7s have a recognized QC problem related on bias pots.
Before replace tubes with a good ol' matched quad, do a careful check on the efficiency of the pots.
If you can't do it yourself, my suggestion is to book a visit to a service center.

 

[RickyC34]

X7s have a recognized QC problem related on bias pots.
Before replace tubes with a good ol' matched quad, do a careful check on the efficiency of the pots.
If you can't do it yourself, my suggestion is to book a visit to a service center.

Thank you. What tools do I need to perform such a test? Others have stated that a 2bd difference isn't a big deal. Does the bias behavior suggest that I may have a problem?

 

[Saponetto]

Erratic bias behavior (e.g. going full clockwise/counterclockwise trying to compensate bias value) strongly suggests that a maintenance/replacement of the pots is required.
The intervention is, as per many tube amps, mainly a delicate mechanical issue.
To make things worse, the X7 has two metal subchassis, aimed at hold the pots in place, that are bolted on the upper panel so there is risk of cosmetic damage.
My suggestion still is to service your amp by some trusted professional that will eventually replace the imho subpar stock pots with something well-suited for its critical role.

 

[fpitas]

Really depends on the bias scheme they use.
Matching may be critical or the circuit may be quite tolerant of mismatched tubes.
Matching'll never hurt, though -- if it's PP stereo buy a matched quad of new, top quality tubes from a reputable vendor.
One gets what one pays for.
The effect of mismatched outputs in an amp that's picky about such things to maintain the proper bias is usually way more severe than a 2 dB (negligible) mismatch in level*, though. Do you know what redplating is?

View attachment 333541
The EL34 on the left in the (borrowed) image above is redplating rather severely. Run in this condition, it won't last long.
EDIT Actually the other one is redplating a bit, too. Could be bias, could be a bad coupling capacitor.

______________
* That's why preamps have balance controls!

A friend built a KT88 amp and the bias wire from the supply fell off (yes, his construction techniques were less than stellar). I noticed a lot of light coming from the back of the amp. The anodes were all well past red. Miraculously, they all survived.

 

[mhardy6647]

bias is very important.

 

[fpitas]

bias is very important.

Uh...yeah. So is not using old 24AWG telephone wires everywhere. And just twisting the connections instead of soldering. The things poor high school students do

 

[mhardy6647]

Erratic bias behavior (e.g. going full clockwise/counterclockwise trying to compensate bias value) strongly suggests that a maintenance/replacement of the pots is required.
The intervention is, as per many tube amps, mainly a delicate mechanical issue.
To make things worse, the X7 has two metal subchassis, aimed at hold the pots in place, that are bolted on the upper panel so there is risk of cosmetic damage.
My suggestion still is to service your amp by some trusted professional that will eventually replace the imho subpar stock pots with something well-suited for its critical role.

If this is the case (and I certainly wouldn't be surprised!) -- yes, this is probably the best place to start, given that the output tubes are not inexpensive.

 

[mhardy6647]

Uh...yeah. So is not using old 24AWG telephone wires everywhere. And just twisting the connections instead of soldering. The things poor high school students do

Not my project -- some of mine are yet scarier.

EDIT: But do note the fuses! Safety First!

 

[RickyC34]

If this is the case (and I certainly wouldn't be surprised!) -- yes, this is probably the best place to start, given that the output tubes are not inexpensive.

Erratic bias behavior (e.g. going full clockwise/counterclockwise trying to compensate bias value) strongly suggests that a maintenance/replacement of the pots is required.
The intervention is, as per many tube amps, mainly a delicate mechanical issue.
To make things worse, the X7 has two metal subchassis, aimed at hold the pots in place, that are bolted on the upper panel so there is risk of cosmetic damage.
My suggestion still is to service your amp by some trusted professional that will eventually replace the imho subpar stock pots with something well-suited for its critical role.

I switched the tubes back for now and played music for about three hours. The bias stayed steady and where it should be. Correct replacement tubes are not inexpensive but probably less expensive than having the unit serviced. Although there would be a probability I would end up at service anyway after replacing the tubes if the problem persists.

 

[fpitas]

View attachment 333772

Not my project -- some of mine are yet scarier.

EDIT: But do note the fuses! Safety First!

My parents' recurring nightmares probably looked like that. I never told them about the times I did touch the B+