The 6550 Experience

[daveweyer]

One thing you can find in the back pages here is a quote from engineer Robert Hovland about the GE 6550s. He notes they were the worst sounding of all the 6550s. Perhaps the TAD models have improved on the original, but the RCA big bottle types and the Tung Sol s from which the RCA was copied and even rebranded were by far the best.
Something to keep in my when spending $50 per tube.

[daveweyer]

Just checked, you can get NOS RCA 6550s for $70 per tube at ESRC. For original Tung Sols you'll have to call, they're probably about $120.

[Xplorer]

thanks a lot Dave ! i wrote a mail to them, we will see. i suppose that a quad of 6550 RCA would be better than a new tung sol quad, right ?

250 euros + shipping + taxs ... VS 160 euros ... mmmh , maybe if i can sell my strat neck, we will see.

is the difference huge ? some say it isn't.

[Roe]

yes, the GE 6550As sound too hard for most guitar work (better for bass though)

[Xplorer]

ok , GE version , eliminated !
now it's between an RCA quad and a new tung sol quad for cheaper. ( or even a big bottle RCA version ? ) . the old tung sol are way too expensive for me. but these tubes i just mentioned are the best 6550, right ?

i have a GEC kt88, i don't know if it works or not, but if i find a way to test it i might sell it. it looks ok.

[Xplorer]

ah , no , at escr they only have valve art 6550.

by the way, will i need some different kind of tube retainers, over the EL34 type ?

[daveweyer]

If you can't get NOS 6550s, Robert Hovland recommends the KT90, says it is every bit as good as the old 6550 but improved in cathode current and ruggedness. If you only intend to run 490 plate volts, these tubes will outperform.

[Xplorer]

mmmh , maybe these ? it seems to be a great one ( i couldn't judge anyway )

https://www.tubedepot.com/products/jj-k ... acuum-tube

and 6550 and kt88 are actualy the same ?

too bad i can't use maybe three like this, or some others, + the gec kt88 i have, or i should try to find a way to match all of them.

[daveweyer]

It would be best to match them, they are very similar but not exactly alike.
Good used 6550s are around too, so that may be an option.

[Xplorer]

yes, i keep this gec kt88 ... who knows ...

if i measure a tube, alone, ( i don't know which specs are interesting to read ) , is it possible to look on internet for some spare tubes that have the same specs, so once it's shipped at my home it's matched ?
or can you really match a quad only by testing them all at once, working together in a special machine or in an amp ?

the first solution would be great, to collect the right tubes ...

then in the end, if i have a matched quad of kt88, will it be the same electricaly and musicaly as a matched quad of 6550 ?
( not sure totaly, if 6550 is just another name for kt88 )

anyway, i've just purshased a quad of new tung sol 6550 !!
i asked for a little bit higher current as it was recommended to me by bill : better tubes.

so now, i'll just have to find a nice resistor ( 10 watts ? ) , of about 0,1 ohm, to help reducing the voltage on the heaters, Under 7 volts, as advised by bill and thorleif.

then swap the 27k with a 10k, use the 2k2, the 12at7 ... and bias the amp cold at 25 mA, if i'm correct.

[Xplorer]

juste received my tung sol 6550 quad !!

i'll go buy some 0,1 ohm 10 watts résistors, and near values if possible, to help correcting the voltage closer to 6,3 volts for the heaters, when i'll use the 220 v tap.

so i've got :

the 6550 quad,
the 2k2 10w resistors
the 12at7 from Dave,
should i change the 50/50 caps on the top of the SL, for some 100/100 ?
using a 10k instead of the 27k ?

and bias it down to 25 mA.
am i correct ?

it features already a split / shared cathode switch.
the preamp values are stock from the metroamp SL69 layout. maybe i should change some mica cap values, and remove the bright cap ? where is this famous one already .. i forgot

[Xplorer]

here are the mods, visualy ( i'm quite a visual guy , slower on the understanding of the amps ) .

do you think it's complete ? i'm not sure about the 2 x 220k , the 0,1 ohms in parallel .. the big caps values ..

the complete layout that you can download as a jpeg :
http://i.imgur.com/6pWgmcC.jpg

and the album :
http://imgur.com/a/wyvPa

some zooms :

[Roe]

I'd reduce the 220ks to 68k or 82k. and keep the stock filtering, perhaps except the mains

[Xplorer]

thanks roe !

i'll go down to 82k as a start i think, i'm not sure why it's required but i'm sure it's been answered earlier, so i'll re read the beginning a bit.
for the 100/100 mains instead of 50/50, i assume that it's for more power, but i can still do the mod without touching them ?

this is a temporary build , i'll make a west coast SL from scratch one day, with nos parts and the right transformers aside from this EL34 SL.

nothing chocks you, about the way i set the two 0,1 ohm résistors in parallel ?

[shakti]

According to Dave you'll be fine with 220ks, at least until the tubes get really worn. I am using 220k myself and no trouble so far. None of the purported Hendrix amps show evidence of the 220ks having been swapped.