70s build - some questions (new - added some clips)

[Funkalicousgroove]

the old tan base GE version sure was something to behold!!

[talbany]

the old tan base GE version sure was something to behold!!

The ones I tried were NOS perfectly matched Sylvania's..

I wanted to like them



T

[UltraHookedOnPhonix]

The Sylvania 7581A is the tube Dumble outfitted in #124. The Mesa Boogie STR-415 tube is a re-branded Phillips/Sylvania 7581A.

BTW, the older 7581A tubes look very similar to the Tung-Sol 6L6GC-STR.

[Max]

Max,

7581A tubes also make a difference, they are a ruggedized Military spec tube, and I believe they are actually a 35 Watt tube(sylvania version) so they "Should" make more power.

Fat Willie claims that he gets Derek trucks' Super 6 to make 120 clean watts using 7581's and 465v on the plates- Same amp with new off the shelf tubes and original PT makes 83 watts-

Maybe of course. I'll talk with the tech about all this and report.

Kind regards

Max

[talbany]

Let's get technicol.LOL!!..According to my 1979 Sylvania tube data, the 6L6GC is rated at
30 watts max. The 7581 is rated for 35.
This is the mil spec version of the Philips 6L6GC. It looks identical to the famous Sylvania 6L6GC and GC STR ..As for the STR designation. My understanding is it stands for Special Tube Request, that is, it is a custom design built to a particular customer's specifications.(In this case Boogie) So 'STR' by itself doesn't tell you anything unless it's qualified with the number following the 'STR', 6L6GC-STR387 being a well-known example. Since all three of these tubes look identical there is much speculation on people saying that it's all the same tube when we know the 7581 is constructed a bit different and spits out an extra 5 watts...To my ears I can tell the difference between the 2..
So do we know for a fact Boogie spec the 7581 version or used the STR as their designation and used the Sylvania 6L6 GC 30 watt...
I would think since they kept the 6L6 designation that it would be more likely a re branded GC version since you would need to know that as this directly relates to how the tube is biased...due to it's extra 5 watts plate dissipation on the 81's...HOWEVER!! since Boogie stopped putting in bias pots on most of their amps I wouldn't put it past them..DOH!!
Why does my Boogie sound gravely...Sound Familiar

I AM SO CONFUSED!!!

Bias 6l6 GC w/450 plate v set at 70%=46.5ma
Bias 7581 w/450v set at 70%= 38.2ma

Taste test 7581's
http://www.geocities.com/RainForest/3612/6L6tubes.html

Tony VVT

[Funkalicousgroove]

The Sylvania 7581A is the tube Dumble outfitted in #124. The Mesa Boogie STR-415 tube is a re-branded Phillips/Sylvania 7581A.

BTW, the older 7581A tubes look very similar to the Tung-Sol 6L6GC-STR.

I LOVE those tan based 7581/kt66's!!

[talbany]

Brandon
Here is an excerpt from the article I posted above...We are not alone..

German musiker szene magazine July 1999 6L6 Shootout

With the 7581A by Philpis/Sylvania we are back in the NOS region. These tubes are legend and are no longer available. Only clever collectors will be able to get specimens of these rarities. What we have here is a military grade version of the famous glass bulbs from the U.S.A. These used to be manufactured for military electronics and hence they are extra rugged, reliable, and long lasting. This stableness makes for a glassy clear sound with gorgeous attack. The 7581 types are very fast-responding, but somewhat harsh in details. In other words, they do not sound as sweet as the General Electric types or the Chinese STR ones. Yet when overdriven they always remain clearly accentuated. Only at very high volume levels sharp distortions appear. In the case of our test amp they work perfectly for adherents of straight clean sounds with explosive attack.

[talbany]

Double

[wjdunham]

It's been a little while since I stopped tweaking with my 70's amp, thinking I had it good enough. I made a few changes that had a nice effect, V2A coupler changed from .01 to .02 being the biggest. This weekend I was at a friends house doing some A/B comparisons to his amps, primarily a Mesa Lone Star. One thing became apparent pretty quickly, that the OD channel low end is lacking. It's very obvious if I just set the controls so the clean and OD have the same volume, and keep the OD pretty clean, switching channels just sounds like some turned the bass knob off.
Changing the V2A coupler in my mind really helped with the OD texture, but seems like the lows are still getting lost somewhere. I haven't had a chance to try a larger V2B coupler, which I will when I get home tonight. Other than that, I can't see where the lows might be getting dumped, the OD entrance in this flavor looks more like a divider, with a small cap to bleed the highs, but no series cap. Any other suggestions on this one? Increase bypass caps on V2 from 5uF to 10uF?
Thanks,
Bill

[Structo]

NA

[Structo]

NA

[wjdunham]

Tom, this one is based on the #40 schematic, which is pretty different from the DLite based versions, PI is really different, as are the couplers. It's not really a PAB/relay hookup issue, as I'm trying to get it dialed with the PAB off.

[Structo]

Ah, ok I didn't realize it was #40 based.

Those are some very different values on that amp.

[wjdunham]

Here's a few pics of the finished amp - I changed the V2 bypass caps to 10uF and both V2 couplers to .02uF, it still looses a little bit of the bottom end thump when switching from clean to OD, but overall it has a nice low end without any mud or flab. I think I'll call this one done for now. I probably spent more time on the woodwork than the innards.

[wjdunham]

Oh, and I'm planning on ordering a custom faceplate from BNP lasers as soon as I get the artwork done...