Calling Tube gurus

[Shub]

I just finished a KT-88 based tube kit. It is a modified Dynaco platform with bigger transformers, updgraded VTA board and rated at 60WPC (had it tested and it pumps out 76WPC!! )

Here is a link if interested in seeing more...

http://bob01605.50webs.com/

QUESTION!

So far I love it ....except one issue....

I noticed last night the the right channel is louder than the left. Also the tubes (Genelex Gold Lion KT-88) on the right channel are glowing brighter than the left.

I didn't have a chance to check the bias again (it's only been on a few hours since it was originally set to .550)

Could this be a bias issue? Or is there some other culprit that would cause this imbalance?

I know since this is a kit I could have messed something up - I did take it to an audio store that sells high end equipment (McIntosh, etc) since they also repair tube amps there. The co-owner was great - went through some preliminary testing... checked plate voltages, distortion characteristics, set the bias... This is also how I know it was capable of 76WPC. He had a nice suite of test equipment, o-scope etc...

Anyways, didn't notice the imbalance between channels. It could of course be either my speakers or pre-amp. I have it hooked up to my bedroom system to "break it in" for a while - I am paranoid to hook up to my ML's until I know it is not going to go up in flames :ROFL:

So right now it is hooked up to a set of 20yr old A/D/S M12 tower speakers --- I bought these when I was 18 and couldn't part with them - this stilll sound pretty nice if you ask me -- It's a pity A/D/S pretty much has faded away... I am using an Adcom preamp (same era). I suppose it could be either the speakers or preamp causing the imbalance between channels but I just thought it was a little too coincidental that the power tubes were also glowing brighter on the right channel.

Sorry for the long winded post!!

Thanks
Bjarni

 

[Spike]

If the tubes glow brighter on the right side, I suspect the right channel is being over-driven from the pre-amp. Swap the left and right interconnects between the preamp and the amp to see whether the imbalance moves to the left. If that is the case, then the problem is with your preamp.
If the imbalance stays with the right channels, put back the preamp-to-amp connection and focus on the connection between the amp and speaker. Swap the speaker cables to see whether the imbalance moves. It could be something in the passive circuitry of the speaker putting a drain on the amp, although I have not seen this happening.

Spike

 

[risabet]

Measure the heater voltages to the right and left sets of tubes. The KT88 voltages should be 6.3V. These should be identical. Also check the bias as a high bias will make tubes glow unnaturally, until they burn up.

 

[twich54]

As an addendum to Spike and risabet's reply, FWIW, when I rolled in my Gold Lion KT-88's last year It took approx 30-50 hrs before they stabilized. Since then I check them every other month and they have been spot on @ 40ma

 

[Tube60]

Is the brighter glow coming from the top of the internal structure, or is the plate structure itself glowing? Either way check your bias voltage! Also swap the output tubes side to side to see if the extra glow follows the tubes as it were. As mentioned before it can't hurt to check the output balance of your preamp.

 

[Shub]

Thanks

Thanks for the replies and advice -- I have some homework tonight! I tell you, I have been a little obsessed with this, but it has been fun. I really enjoyed putting the kit together!! So this is a little bit of a continuation of that process .... some fun trouble shooting

 

[JonFo]

I can help ....

Oops, wrong thread

Congrats on the kit though