6L6WGB's anyone ?

for Dynaco Mark II/III/IV and DIY PP monoblocks

6L6WGB's anyone ?

Postby SteveH » Tue Sep 14, 2010 6:52 am

I found a quad of 6L6WGB's (JAN) for a price I couldn't pass up (eg - free). They tested good, and I would like to toss them into my MKIII's wired up in triode. Unfortunately Im running a B+ of about 500v - so Im getting something like 475 on the plates...About 75v past the WGB's limit. The excellent TungSol data sheet even lists the curves in triode:
http://www.mif.pg.gda.pl/homepages/frank/sheets/127/6/6L6WGB.pdf

With typical operation at 360v. Im going to get a set of 5R4's to sub in for the 5AR4's Im using now and see where that gets me. Unfortunately my chassis are stuffed to the gills; so there isn't anymore room for me to add another RC filter off of the PSU. Any other thoughts ? Any experiences in running these hot ?

Thanks,
Steve
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Postby DeathRex » Tue Sep 14, 2010 8:02 am

Remove the first filter capacitor and wire it back in after the choke, so instead of CLC, you'll have LCC. Might loose 20-25V that way, but hum might increase. 5R4 will be happy tho.
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Postby Ty_Bower » Tue Sep 14, 2010 8:04 am

When I first powered up my Mark III, they had a quad of cheap Chinese 6L6 in them. I think I paid about a dollar each for the tubes. They seemed to perform surprisingly well, and no smoke came out. Of course, you can't really see what the screens are doing. Maybe they look like a bad toaster underneath the plates.

Image

Then there is the Russian 6p3s-e. Their datasheet specs the maximum voltage at something ridiculously low (250V?), but that spec was clearly meant to be violated. If you get some good ones that don't have any gas in them, they will cheerfully run all day long at 450V.

Image

Some tubes run just fine past their maximum ratings.
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Postby SteveH » Tue Sep 14, 2010 8:12 am

Thanks guys !

I really don't want to remove the first filter cap; as it is a beast to get to (using the SDS Cap board; which is hidden below a bunch of stuff) - but that is a good idea.
I'll put in the 5R4's and measure what things are looking like. I may just have to stick to using these in an ST70...

Thanks again,
Steve
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Postby EWBrown » Wed Sep 15, 2010 12:31 pm

I'm sure that the power trannie's 5V filament current rating is sufficient, but check it, just to play it safe. 5AR4s consume 2A at 5VAC (actually 1.9A) and 5R4s, 5U4GBs, etc consumer 3A at 5VAC.

The Russian tube specs are intentionally, conservatively rated ridiculously low, IIRC they rated 6P14Ps at 250V on plate and G2. We all know those can go to 380VDC and even up to 400 Vwithout exploding ;)
Not to mention those amps which run them at 700V on the plates and 350 on the screens, albeit at a significantly lower plate current (15 mA)

The 6P3S are decent power tubes, but they still fall well short of the hefty capabilities of good well built 6L6GCs. The Chinese 6L6GCs and variants are generally fairly good tubes, even though many of their more modern (EL34, 6550, KT88) power pentodes and BPTs are, at best, of "variable" quality. 6L6s/6L6GCs have been around long enough, so that everyone around the world has had plenty of time to get them right O:)

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Postby SteveH » Wed Sep 15, 2010 3:39 pm

Thanks ! So...Im going to try to snag a pair of 5R4's...But in the meantime it is oh-so-tempting to toss these in and bias them down to slightly below their max dissipation to see how they sound...
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Postby SteveH » Wed Sep 15, 2010 5:27 pm

Ok, so this is what Im working with....

My MK3's are throwing 510v on the plates. I biased the 6L6's at 40mA for a total of about 20w dissipation (25w max). Im sure the tubes aren't super happy about this kinda voltage (especially wired in triode)...but they sound OK. Im not that thrilled with the sound to be honest - the high-mids and highs sound great, but the low end to midrange has something missing. Of course; Im comparing it to KT88's that I run about 60mA on...I'll snag a pair of 5R4's and see how dropping the B+ by a couple of volts helps.

-Steve
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Postby DeathRex » Wed Sep 15, 2010 7:19 pm

SteveH wrote:My MK3's are throwing 510v on the plates...the high-mids and highs sound great, but the low end to midrange has something missing.
-Steve


Like half of the plate, that has melted off. (sick)
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Postby DeathRex » Wed Sep 15, 2010 7:28 pm

I just finished a pair of MK III clones, well almost finished. Was testing them today and with 499 on the plates and 110ma for both was getting 28VPP on 4 ohms with a 1kHz sinewave. I'm too scared to drop a set of 6P3Ss in. I've got a set of KT77s coming in though.
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Postby SteveH » Wed Sep 15, 2010 8:20 pm

Post up some pics !

My Winged 'C' KT88's sound really nice @ 60mA.
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Postby DeathRex » Thu Sep 16, 2010 5:02 pm

Nothing fancy, just a Mark III.

The tube line up is: 2 X JJ KT88, RCA 6AT6, RCA 6CG7, Russian 6C4S. The chassis is a Hammond aluminum 12 X 10 X 2 cut to 10 X 8.5. The transformers are Edcor CXPP60-MS-4.2K outputs and XPWR033 power, painted with an automotive flat black and the transformers just happen to be the same size. The capacitors are old ones that have been restuffed with a 60uf at 800V, 47uf at 450V, and 33 at 450V. There's also a 60uf at 800V underneath. The circuit is wired inbetween a Kara Chaffee modded Mark III, a Eico HF-89, and the Eiclone.
I still have to cut the bottom, and put feet on it.
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Postby SteveH » Mon Sep 20, 2010 8:30 pm

Looks good !

I did put some 5R4's in the amps, and it brought my B+ down to about 490v on the plates. I left the tubes biased at 40mA. The sound has greatly improved....But it is still much more laid back than the KT88's (which is fine on my current speakers which I find too bright). My gut feeling is 460v at 45mA would be the sweet spot.
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