TA-15 Intermittent Gain Drop in Channel 2

I have an intermittent problem with my TA-15 head. The problem appears to only happen on Channel 2 in 15W and 25W power modes, and seems to be less problematic or non-existent in 5W mode. It typically happens after the amp has been in use for 30 minutes or longer. The gain will suddenly drop very significantly while I am playing. Typical settings are gain at 2:00 and master at 9:00. Tone controls all around 12:00. Problem happens in all Channel 2 modes – Tweed, Hi1 and Hi2. The problem is not accompanied by any unusual noise or microphonics, just a sudden gain drop. Gain typically returns to normal if I allow the amp to cool for a few hours and then turn it back on, but will again drift after around 30 minutes of playing.

I have not yet tried to replace or rotate any pre-amp or power tubes, as the problem that I am experiencing does not seem to be consistent with the manual’s description of typical power or preamp tube problems. Would appreciate some feedback from other users who may have experienced this problem, or from do-it-yourselfers that have repaired these amps. I bought the amp here in Hong Kong and warranty is already expired, repair services here are questionable, so I am motivated to fix it myself if I can.

HK Expat said:

I have an intermittent problem with my TA-15 head. The problem appears to only happen on Channel 2 in 15W and 25W power modes, and seems to be less problematic or non-existent in 5W mode. It typically happens after the amp has been in use for 30 minutes or longer. The gain will suddenly drop very significantly while I am playing. Typical settings are gain at 2:00 and master at 9:00. Tone controls all around 12:00. Problem happens in all Channel 2 modes – Tweed, Hi1 and Hi2. The problem is not accompanied by any unusual noise or microphonics, just a sudden gain drop. Gain typically returns to normal if I allow the amp to cool for a few hours and then turn it back on, but will again drift after around 30 minutes of playing.

I have not yet tried to replace or rotate any pre-amp or power tubes, as the problem that I am experiencing does not seem to be consistent with the manual’s description of typical power or preamp tube problems. Would appreciate some feedback from other users who may have experienced this problem, or from do-it-yourselfers that have repaired these amps. I bought the amp here in Hong Kong and warranty is already expired, repair services here are questionable, so I am motivated to fix it myself if I can.

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It certainly sounds like a failing preamp tube. This would be the first thing to check, by swapping in a new/known good preamp tube into each position - one position at a time - and see if you can pin point the suspect tube.

Only happens on Ch2? Thanks!

Hi and thanks for your input, Mark II!

Yes the gain drop issue appears to only happen on Channel 2. The problem is quite intermiitent at this stage, and only started happening a few days ago. Seems to be more likely to occur after the amp has a chance to get hot, say after being in use for an hour or so. I tried it briefly last night with 15W setting in Ch 2 and the problem did not occur - the gain was normal and the tone was nice and fat. If the problem persists then I plan to try swapping out the 12AX7's (V1/V2) one at a time as you suggest, starting with V2 as it handles the Ch2 input gain stage.

A question for you on the choice of replacement tubes. I rather doubt that the local dealer here in Hong Kong stocks all of the required tubes for Mesa's. Finding another good 12AX7 should be quite straight forward here, but if it is really best to use the tube kits from Mesas then I will need to place an order and wait for them to ship. I did note from the owner's manual that although Mesa does suggest that you use their tubes and none other, I also noticed that they are more vocal about that point when it comes to the power tubes and less so when it comes to preamp tubes. What are your thoughts on using good quality AX7's other than the ones that Mesa provides? If by chance you are associated with Mesa Boogie in any way, then my apologies if this is an awkward question to answer.

HK Expat said:

Hi and thanks for your input, Mark II!

Yes the gain drop issue appears to only happen on Channel 2. The problem is quite intermiitent at this stage, and only started happening a few days ago. Seems to be more likely to occur after the amp has a chance to get hot, say after being in use for an hour or so. I tried it briefly last night with 15W setting in Ch 2 and the problem did not occur - the gain was normal and the tone was nice and fat. If the problem persists then I plan to try swapping out the 12AX7's (V1/V2) one at a time as you suggest, starting with V2 as it handles the Ch2 input gain stage.

A question for you on the choice of replacement tubes. I rather doubt that the local dealer here in Hong Kong stocks all of the required tubes for Mesa's. Finding another good 12AX7 should be quite straight forward here, but if it is really best to use the tube kits from Mesas then I will need to place an order and wait for them to ship. I did note from the owner's manual that although Mesa does suggest that you use their tubes and none other, I also noticed that they are more vocal about that point when it comes to the power tubes and less so when it comes to preamp tubes. What are your thoughts on using good quality AX7's other than the ones that Mesa provides? If by chance you are associated with Mesa Boogie in any way, then my apologies if this is an awkward question to answer.

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I am a customer service rep with MESA. We recommend using MESA tubes for a few reasons - primarily power tubes - which are all selected fur current draw that is optimum for the fixed bias in our amps. Also, if you are within your warranty period we will not cover any damage done to the amp caused by non-MESA tubes. Preamp tubes are somewhat different as they don't involve the high current that power tubes do - so its not as crucial.

Its important to check whether your issue happens in one channel or the other, and also at what wattage settings - as this can be a factor in determining whether its preamp or power tube related. Let me know if you've got any questions or need assistance in any way. Thanks!

Thanks again Authorized Boogie for your helpful feedback! The problem is very intermittent at this stage, and the symptoms in terms of the power modes and channel affected were all listed in my posts above. I will probably wait a bit until the problem is more persistent and consistent before I start trouble-shooting and swapping out tubes. When the time comes to start trouble-shooting then I will come back to you with the results, if that is OK with you. I can understand your replies pretty well as I am an elec engineer, although these days I am more on the manufacturing side of digital products and as such I enjoy learning more about the analog side in this forum.

I am taking delivery of a new Express 5:50+ today so I will probably be focusing on the new amp for the next couple of weeks and will then re-visit the TA-15. I want to create a setup for recording where I can mic my TA-15 and Express+ at the same time and see what kinds of tone I can get. I will say that my guitar playing buddies have been amazed at the sounds I am getting with Pro Tools recordings of blues/rock tunes with the TA-15 thru a single 12" speaker at bedroom volumes miced up with a good condenser or SM7; they all think that I am using stacks of Marshalls cranked in a big room! This is the best tone that I have been able to get in my 40+ years after playing live and in studio with amps from Marhall, Hi-Watt, Peavey, Fender, Line 6 and so on. Very pleased with the Mesa gear!

Follow Up Note to Authorized Boogie:

The intermittent gain drop problem in Ch2 finally became more constant, so I swapped V1 and V2 (V2 being suspect and V1 being known good). I then let the amp get hot and Ch2 was stable and OK. Over the course of two days I powered it up now and each time Ch2 was OK. I was however not able to clearly replicate the same gain issue in Ch1 with the suspect tube in V1 position. Although results were not totally conclusive I went out and picked up a couple of new EHX Gold Pin 12AX7's (one-year warranty in Hong Kong expired over a year ago). I placed one of the new 12AX7 in V2 and put the known good original V1 tube back in V1 position. The original V2 tube is set aside and marked as suspect. After two weeks of playing both channles are stable and sound great.

Just thought I would update this to all Boogie members and especially to Authorized Boogie as he had given me some good support and advice and wanted to hear back on the results.

HK Expat

Thanks for the follow-up, and glad to hear you got it sorted out!