7581 power tubes Mark V with 6L6 option

I have had the 7581 tubes in my am for a while. I play through the amp every day, on week ends sessions can last for several hours. Out of the box the 7581 sounded great. Every time I powered up the amp it just kept getting better. Finally go to the point of hearing the full potential of the tubes. It is that point where you find out that you either like them or hate them. The full quad of 7581 were performing well, until today. They turned to the dark side. It was gradual not alarming in any way. At first I thought it may be the walnut guitar I was playing through so I tried a different guitar with more brightness in character (maple and alder). Amp still sounded dark. The clean channel was great in tonal character, channeld 2 and 3 just seemd to be lacking something, but what I find hard to put into words. I had a matched pair of Mesa 6L6 just waiting for abuse so I replaced two the 7581 (primary power tubes V8 and V9). That was a great move. The 6L6 have a bit more top end character in terms of brightness. 10W and 45W modes run the 6L6's for that full chime or shimmer of tone. Of course the cocktail of preamp tubes has contributed somewhat to cure the over bright character of the 12ax7a's originally seated into thier respective sockets at the time of assemby (not sure if Mesa ships amps with the tubes in place). 90W mode is now tailored to my liking. I have retained the brightness and chime of the 6L6 and broadend the tonal character in the other extreme with the 7581 as the secondary power stages. Deep tones from the 7581 with all of the chime of the 6L6 ready to strip life from your eardrums. 7581 after many hours of use tend to hang in the dark zone, harder to saturate and overdrive in to clipping, I would assume that some of the higher order harmonics are reduced considerably causing a softer clip and a darker tone. However, if you use 6L6 as the primary power tube, they saturate nicely and operate very well with the pentode mode of channel 3 and at least generate 4th order harmonics with ease (I have run a signal analyizer at Penn State University in 1994 on 6L6 power tubes to study the soft clipping effects of valves, at that time I was designing a solid state amplifier as a senior design project, the goal was to preserve or replicate valve character with FET's which are similar to vacuum tubes ).

I will see how well or how long this works out, before I change to something else.

I decided to confirm that the 7581's were the cause of the change in tone. Out of the box they were comparable to 6L6GC's in tonal character but not as bright. It did not take long from sounding great to overly dark. Now I am waiting for their replacements (GT6L6-R2 which are old stock SED winged =C= ). Sooner or later I will find the tubes that I like.

Pulled the chasis from its shell, and placed it inverted onto the 412 cab. Removed all preamp tubes and replaced them with the original Mesa tubes. Also removed the last remaining 6L6 tubes I had left and put the 7581's back into V8 and V9.

There is still plenty of power in the low to mid frequency range. Not much there in the upper range. Reinstalled the 6L6 into the V8 and V9 which was much better. While playing (without the reverb) I noticed a flare up but was uncertain if the arch occured in V10 or the 5U4G until it happend again. V10 was having issues. I found some old tubes I had in the spare box of parts I had on reserve for the Mark IV. Forgot I even had them. They were STR420 so in they went into V10 and V11. Back to the beginning, super bright again. Reinstalled the coctail of preamp tubes but changed the arrangement. First I wanted to experiment with all Tung Sol 12ax7. That was very bland. V1 is now EH12ax7, V2 EH12ax7, V3 JJ HG tube, V4 Tung Sol, V5 Tung Sol, V6 was the ruby HG12ax7AC7, V7 Sovetek LPS. More closer to the Mark IV tone and gain response. I was supprise to get a similar tone response with the EH in V1 as I did with the 5751. The only difference is the gain control is similar to the Mesa 12ax7A but not as bright. I will get around to trying the 5751 in the PI eventually. For now, I have found the tone I like.

I cannot say that the 7581 are a bad choice. They sounded great when they were working properly. Also, I am uncertain if there are issues with my Amp at this time. It is only a few months old and burned up the original tubes very early.

I guess the next step is to verify bias current of the AMP just to confirm that it is within specifications.

If I discovery anything of value I will make mention of it.

bandit2013 said:

...I guess the next step is to verify bias current of the AMP just to confirm that it is within specifications...

Click to expand...

This is of the utmost importance, especially when changing any output tubes. If the bias is not set properly, it can lead to premature tube failure. :!:

I would agree, however, have no means to adjust bias on my amp. I really do not plan on voiding my warranty at this time to change it. I have a set of new tubes now, SED's WGDC, old stock GT's. No tone change, or any issues yet and almost the same amount of usage. In both cases, I have indicated to the vendor what amp the tubes will be used in. Also the 7581 were new construction, so difficult to say if they were up to par with old stock tubes, also they were not 7581A.

Having the bias checked would be a good start just to verify the amp is operating properly.

If you are worried about the warranty, you might want to read it. It states; "This warranty becomes void if any tubes other than genuine Mesa tubes are used." So technically, you've already voided your warranty. :shock:

You can adjust the bias in your amp, or any other fixed bias amp that has no adjustment pot, by changing the value of the bias resistor, or replacing the bias resistor with a pot.

I would agree.