Need help about tubes, STR387

Talk about tubes and their effect on your tone

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TiPiMods
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Need help about tubes, STR387

Post by TiPiMods » Fri Oct 12, 2012 4:55 am

Hi guys
need some help or advices about tubes. Got a MkIIB 60W SRGX combo and recently tried some different tubes. Found some really nice Philips ECG STR 387 (double getter) in an old Acoustic G60T and put them in the Boogie. I measured 447V on the plates and got a reading of 31mA on both tubes (quite perfectly matched for this old suckers :wink: ) so that's biased quite cold. Does somebody know what the reading should be with Mesa STR415?

tried some other tubes to compare:
TAD 6L6 WGC short bottle, clear top: 39mA (37)
old Fender tubes (single getter, probably also STR387): 38mA (34)
EH 6L6: 25mA (29)

the Philips sounds very god, sparkling, most treble of all tubes I tried, the TAD is the loudest, more midrange ( probably its bias is close to 60% at idle), the old Fender tubes sound quite similar to the TADs, the EH is dark and midrangy, least appealing of all.

Somebody know, which tubes Mesa recomends for the MK series? On their page I only found the 6L6 STR440. I heard they have a colour code system, but didnt find any infos on their pages. Which color would be the best bias for a MKII.

Thanks, Thomas
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Re: Need help about tubes, STR387

Post by megavoice » Mon Oct 15, 2012 8:41 am

Hi,

I've been tought by MESA personally, that the colour makes no difference in sound at all. It only tells that that pair is matched. That's all.
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Re: Need help about tubes, STR387

Post by Triaxstasy » Mon Oct 15, 2012 10:31 am


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Re: Need help about tubes, STR387

Post by TiPiMods » Tue Oct 16, 2012 5:06 am

Thanks triaxtasy, very useful list, thats what I was looking for, and didn't find.
..und Grüße nach Deutschland :)

Anyone who has expierience with STR 415, are they biasing a bit warmer than the STR 387?
with the Philips ECG STR 387 I can really imagine how they would bloom, if they would bias a bit hotter.
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Re: Need help about tubes, STR387

Post by TiPiMods » Mon Oct 22, 2012 7:17 am

No STR415 user?
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Re: Need help about tubes, STR387

Post by 212Mavguy » Mon Oct 22, 2012 8:39 am

Warning! This is not the answer that the OP was originally looking for...

The amp is out of warranty. Bias mod kits are easily available to easily install by a quality tech. The mod is easily reversible so if vintage collectible resale value is important, well, it just doesn't need to be until selling time. First step is to set that amp free from tube prejudice. Give it the ability to use the tool's attachments to their optimum abilities, because the result is worth it.

As far as STR 387, they were a Sylvania tube. They are more tolerant of cold bias than some other 6l6gc, and they exhibit that great clarity described in the OP, and they also have a big and crispy low end to them as well, mid scooped a bit. As far as harmonic bloom in a hot bias, yes, you betcha. :)

Sylvania 6l6gc, Boogie STR 387, Jan Philips 6l6gc are all very similar, Sylvania products, but the most powerful 6l6 type is not the gc, it's a mil spec tube called the 7581a, simplifying explanation it's a 35 watt 6l6gc. Standard 6l6gc is 30 watts. All these mentioned tubes are scarce and expensive to obtain in unused true NOS or lightly used burned in vintage old stock, the VOS tubes being actually more desirable as well as less spendy.

To get those tones in my Dumble clones, I got some adapters from eBay and collected a stash of Sylvania 6bg6ga, their 7581a's guts and tone set in a different pinout with a plate cap on top. So in order to get the "now vintage" sounds that amp originally made, you might need to get creative. I have used 6bg6ga with great success in all but one of my 6l6 running tube amps. That would be bias kit, adapter sockets, and 6bg6ga tubes. The tubes are affordable, and if you wanted to use other kinds of 6l6 in your boogie, you can after installing the bias mod, and then if you are setting your own bias your ears will experience that bias adjustment setting is a global amp tone control in its effect. 55% bias setting, which is a tad on the cold side for 6l6gc is around 37ma at 447 plate volts. For 7581a 6bg6ga the 55% setting is around 43ma.

Hope this is helpful.
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Re: Need help about tubes, STR387

Post by TiPiMods » Wed Oct 24, 2012 9:29 am

thanks Maveguy.
Can I use the 7581A with Boogies fixed bias or are they too different?
How do they sound? Since they are even stronger I suppose they stay clean a bit longer.
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Re: Need help about tubes, STR387

Post by 212Mavguy » Wed Oct 24, 2012 10:38 am

You could use them, but it would be risky, the risk compared to the cost of installing the bias mod that could easily be set to either Mesa or other values is unacceptable. It's out of warranty, but that doesn't mean one should take chances just to save some pennies, you have a very nice amp. Mesa selected certain tubes and rated/graded/color coded them for their current draw, and naturally, powerful pentodes can draw at quite different amounts in the same circuit and values right out of the new box...

If you want to do a mad science project, turn all the lights off in a darkened room, fire the amp up with the questionable tubes in it, wait at least seven minutes, if the plates don't start glowing red and the amp sounds right, you're ok, Mesa bias not too hot for that set. Too cold and the amp will sound too bright and scratchy.

If you don't know what the plates are don't do this, read up some more tube theory, your tone will thank you. "Tnside Tube Amps"... Dan Torres, "The Art of Tone"... Kevin O'Connor, and "Designing Tube Preamps for Guitar and Bass Amps" by Merlin Blencowe.

Get the mod done, you won't regret it and it's as easy to learn how to set bias as learning how to tune a guitar.
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Re: Need help about tubes, STR387

Post by TiPiMods » Wed Oct 24, 2012 5:39 pm

Hi Mavguy
thanks, I know tube amps since my boyhood, I repair and tune tube amps and as you can read in my first post in this thread, I also have a bias probe and can measure the plate current at idle.
I thought you tried some 7581A and can tell if they draw more current than average 6L6GC.
I probably will install a bias pot in the Boogie, I did it with some other Amps including my Hiwatt 50 and the Kitty Hawks, and its a lot more confortable to set the bias to my liking.
regards
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Re: Need help about tubes, STR387

Post by 212Mavguy » Fri Oct 26, 2012 8:11 am

Thanks for the correction and clarification, and please accept my apology. For a person with your knowledge my words probably sounded rude at times.

Yes, I have tried 7581a Sylvania and they can possibly draw more at idle, but that depends on who is making the 7581a. The Sylvania build is capable, but the relabeled Russian one I have only has 22-25 watt plates. I have not run gE 7581a.

I'd treat a 7581a just like a 6l6gc if from the same maker with no distinction between the two bias wise unless my ears could detect differences. Theoretical difference is about 6-7ma. My ears can tell the difference between the top and bottom of that range. When I ran them I also ran 6l6gc Sylvanias, I start with the formula but use my ears for the final setting when adjusting bias as well as PI trimmers. Settings used for 6l6gc Sylvania and 7581 Sylvania were fairly similar in the amp I tried them in if I recall correctly. The tubes matched within the pairs about 5-7ma spread at a b+ of 450-460 volts in my D-clone.

I have never heard or played thrugh a Kitty Hawk, I am curious about them because of the Dumble connection at one time. The ones I have seen have very big iron.
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Re: Need help about tubes, STR387

Post by TiPiMods » Fri Oct 26, 2012 8:47 am

No problem, thanks for your answer.
I found some 7581A on ebay europe but they are quite expensive (99€, but the auction ran out without a buyer), may be I can get them for less.

The early Kitty Hawks are amazing amps, I have a Standard with Mahogany enclosure, which is said to sound quite similar to a Dumble ( I never had the pleasure to play one) and the other one is a Junior 50W, quite similar to a Boogie MkI with its two inputs. Both have a beautiful, bold clean sound, and a great lead with more bite and edge than my Boogie MkIIB. Especially the Standard is unbelievable powerful, I think it could blow the EV12 easily. The power trannie is huge. I had to recap it and changed all the pots, but now it works flawless.

some pictures, maybe I'll upload some more: http://imageshack.us/g/3/1002779sb.jpg/
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Re: Need help about tubes, STR387

Post by 212Mavguy » Fri Oct 26, 2012 12:54 pm

They are expensive in US too. That is why I went the 6bg6ga route with adapters via eBay USA and bought quite a few until I had decent main and spare pair matches in gray and black plate (yes!) 6bg6ga Sylvanias. That might not work for you in your location.

There is an online dealer that sells the Sylvania 6bg6ga's NOS plus parts to DIY the socket adapters. Cost in US $$ for a quad of prematched 6bg6ga plus adapter kits will be less than for the same guts inside a pair of NOS 7581a's

http://www.vacuumtubes.com/6BG6.html

The length of the tube including plate cap and adapter can be an issue in some head cabs, and in some instances the adapter might put the plates too close to a speaker magnet in some combos. But they sure look cool with the wires going up to the plate caps and the sound... :shock: :lol: 8)
212Mavguy

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