A couple of mods on a Mesa TA-15...

[freefrog]

The amp was biased too hot in "dyna-watt" (IOW fixed bias) mode and its channels switched randomly, so it had to receive the upgrades evoked here: https://www.rig-talk.com/forum/threa...te-mod.109326/

While I was at it, I've added a few tone mods:

-the 120pF bright cap of the green channel has been changed for a 47pF, swappable thx to a push pull volume pot with a 1nF to ground. It makes the clean sound of the green channel less harsh and its drive sounds more musical to my ears when the amp is pushed;

-I've decreased the value of R85 thx to another resistor in parallel with it. The fixed tone cut is now at the same value than my favourite setting for Vox tone cut pots. Easy to do and reversible;

-I was thinking to add a mid pot to the red channel or to make switchable the 10k R39 resistor parallel to the 24k R40. Instead of this, I've opted for a mid shift mod, by modifying the value of the treble control: it's 250k when the pot is pushed and 100k when it's pulled, thx to a parallel resistive load. The effect is subtle but noticeable (with drive pedals, for example).

For the record, the two pots that I've changed for push-pull models were below their theoretical value: 220k for the supposedly 250k treble pot, 420k for the 500k volume control. I expected less deviance than 12% and 16% respectively...

FWIW.

[freefrog]

Response of the TA15 as captured from the 8 Ohm output, through a direct box between amp and cab. First pic = action of my mid shift mod, all control at noon (knowing that the Tweed mode involves a permanently enabled bright cap on the volume control). Second pic = effect of my 1nF to ground, compared to the response of the stock green channel (albeit with already a 47nF bright cap) and to the response of the H1 mode on red channel. Gain Cranked in the 2d pic.






And below is the response of the amp bone stock,all controls at noon : black = Green channel in "normal" mode, pink = "Tweed" setting on the red channel. Red= H1. Green = H2.

[g1]

The amp was biased too hot in "dyna-watt" (IOW fixed bias) mode and its channels switched randomly, so it had to receive the upgrades evoked here: https://www.rig-talk.com/forum/threa...te-mod.109326/

No actual information about the upgrades in that thread. Actually gave me the impression that the bias part of it was to make the tubes run even hotter.

[freefrog]

Sorry about the link, it's actually not this topic that I had in my mind. I'll have to redo my research of March 2022 on the Boogie Board and elsewhere.

If someone has issues with a TA-15, I'll share on request practical details about the upgrades applied after serial number 1287 (including the change of R69 for a 18k, making the amp colder biased in dyna-watt mode). The one that I've modded runs with them for 3 or 4 months without overheating nor switching randomly no more.

Now and as implied by screenshots about the frequency response, my topic was meant to share the tone tweaks that I find useful and not to talk about the upgrades which were just an occasion to mod the amp. Thx for your understanding.

[freefrog]

Update:

The switchable 1nF cap on the green channel has been reduced to 680pF for a less abrupt effect. I like it like that.

Have tried a push pull pot giving access to the stock 25k // 10k = 7,14k or to the 25k mid resistor alone for the red channel but it's not something that I'd recommend.

Swapped the Mesa branded JJ ECC83's V1 and V2 (short plates) for some vintage long-plate 12AX7's: a Tungsram in V1,an Ei in V2. If I forget that a first Tungsram died because of cracked glass, it might be the best "mod" I've done on this amp. YMMV.