Differences Between EL34 and 6550...

[martinw]

Yeh the way Ive heard it from Joel at Electric is that they were once the sole US distributor but they then decided to break off and become their own company making their own amps and such.

Not quite. he was sacked as the US distributor after trying to register Matamp as a trademark in the US behind Jeff's back, and without his permission. A bit cheeky. Jeff took legal action, won and removed his distributorship.

[fps_dean]

6550s have a lower & tighter bass response than EL34s, there is nothing foggy/blurred about them at all.  think tight, punchy and lots of headroom.

EL34s have more compression and forward midrange presenatation.

I personally like 6550s for all metal styles, they're just ballsier.

 :twisted:

When pushed, 6550s will be quite foggy though....

The way I'd go is KT88s if you go that route... they are military grade 6550s and should be able to be used as a replacement for 6550s with proper bias adjustments.

[HTH AMPS]

6550s have a lower & tighter bass response than EL34s, there is nothing foggy/blurred about them at all.  think tight, punchy and lots of headroom.

EL34s have more compression and forward midrange presenatation.

I personally like 6550s for all metal styles, they're just ballsier.

 :twisted:

When pushed, 6550s will be quite foggy though....

The way I'd go is KT88s if you go that route... they are military grade 6550s and should be able to be used as a replacement for 6550s with proper bias adjustments.

I can't say thats what I've ever found with 6550s, as always ymmv.

KT88s back in the day of GEC could be considered military grade equivalents of 6550s, but not with current production KT88s.

With modern KT88s you're more likely to get 6550 guts in a KT88 bottle (i.e. the same valve)

[the_bleeding]

I can't say thats what I've ever found with 6550s, as always ymmv.

KT88s back in the day of GEC could be considered military grade equivalents of 6550s, but not with current production KT88s.

With modern KT88s you're more likely to get 6550 guts in a KT88 bottle (i.e. the same valve)

true say. the kt88 originally had a higher plate voltage than the 6550, but these days they're identical except for pins.
Brits invented the kt88 as an upgrade of the kt66 (which was the british answer to the bad construction of the 6l6), and 6550's were invented by the americans as an upgrade of the 6l6 after seeing what the brits can do. Yes theres more to this story, but thats the basics of it.

However, 6550's do get foggy, but i've only ever heard the fog in bass amps when they're pushing super low frequencies, like an ampeg svt. Everybody knows what an overdriven SVT sounds like. 6550's stay pretty clear for guitar though :)

[HTH AMPS]

I can't say thats what I've ever found with 6550s, as always ymmv.

KT88s back in the day of GEC could be considered military grade equivalents of 6550s, but not with current production KT88s.

With modern KT88s you're more likely to get 6550 guts in a KT88 bottle (i.e. the same valve)

true say. the kt88 originally had a higher plate voltage than the 6550, but these days they're identical except for pins.
Brits invented the kt88 as an upgrade of the kt66 (which was the british answer to the bad construction of the 6l6), and 6550's were invented by the americans as an upgrade of the 6l6 after seeing what the brits can do. Yes theres more to this story, but thats the basics of it.

However, 6550's do get foggy, but i've only ever heard the fog in bass amps when they're pushing super low frequencies, like an ampeg svt. Everybody knows what an overdriven SVT sounds like. 6550's stay pretty clear for guitar though :)

Ahh, we're talking overdriven 6550s in bass amps - well thats a different story altogether.  In guitar amps there really isnt the low end to get that fuzzy bass imo.

[fps_dean]

I can't say thats what I've ever found with 6550s, as always ymmv.

You just haven't pushed them hard enough... but that's hard to do.  It's all a matter of how much you abuse them.  Last time I did that (1969 Marshall Major with a Keeley Java Booster) it cost me a total of $600 in repairs and new tubes as I melted some tube sockets in the amp and some 2000 volt capable wiring.

KT88s back in the day of GEC could be considered military grade equivalents of 6550s, but not with current production KT88s.

With modern KT88s you're more likely to get 6550 guts in a KT88 bottle (i.e. the same valve)

Actually it is still as true in this day in age as it was 35 years ago... it's just the KT88s got cr@ppier, but so did the 6550s as well.

[jpfamps]

The EL34 is a pentode, whereas KT66, KT77, KT88, 6L6 and 6550 are beam tetrodes which work on a slightly different principal to a pentode. One feature of the beam tetrode is that the screen grid is wound concentrically with the control grid ie its in the "shadow" of the control grid. As less electrons strike the screen grid, beam tetrodes have lower screen currents than pentodes (which is why screen grid resistors are typically higher for EL34s than 6L6s). Lower screen current means more efficiency (screen current doesn't contribute to output power) and less third harmonic distortion, which I expect part of the reason that EL34 have a crunchier sound.  

EL34s have quite high transconductance, ie they don't require as much drive voltage as say 6550s or KT88s. Thus is you put 6550s in a Marshall you will get more head room as a greater signal is required to clip the output stage.

Incidentally Marshall made an amp called the Park 70 which used two KT88s, which from what I can see from working on one is simply a Park 45 which uses EL34s, with KT88s in it.

By the way the new JJ-KT77 is a genuine beam tetrode as I have spoken to someone who broke one open and disassembled the internal structure and NOT a repackaged EL34. I think the rumours that the JJ-KT77 was actually a pentode came from a valve vendors web-site that statesd that as the plate curves did not have a kink in them it couldn't be a tetrode (true but it could be a BEAM tetrode). KT actually stand for "kinkless tetrode"! I notice they have now amended their web-site.

By the way we have been doing quite a lot of listening to different valves, especially comparing the JJ-KT77 with EL34s. We really like the sound of the KT77 as it has the extended bass and smoother sound of a beam tetrode. KT77s will work fine in all EL34 loaded amps, so its good to have another option available.

[HTH AMPS]

jpf,

Are the new JJ KT77s using standard pin sizes now? - the first run were using thinner (mil-spec according to Eurotubes) pins and many amps needed their sockets retensioned.

[jpfamps]

Yes the JJs are fortunately now using standard sized pins, although I expect there is still some old stock knocking about.

The problem with the pins on the JJ valves was that there was no taper on the end of the pins so if you were not careful about how you inserted the valves into their sockets you could damage the socket. JJ's answer to this was to make the pins thinner! We stopped using JJ octal valves because of this as even if we retensioned the sockets we couldn't guarantee that the valves would not fall out. My guess is that the main motivation for the change in the pin size came from the hi-fi community who don't tend to throw their amps into the back of transits etc. I think Groove Tubes supply their own tapered pins for the JJ-Valves they re-brand.

Whether the thinner pins are Mil-spec or not, well who knows.  I haven't had any dealings with Eurotubes, however as they only stock JJ tubes I suspect they are NOT the most unbiased source of information on JJ-tubes. Incidentally I installed a quad of JJ-KT88s into a JCM800 that where supplied by Eurotubes  and the matching was all over the shop, although this could have been due to being clattered around in transit.

[viking]

By the way we have been doing quite a lot of listening to different valves, especially comparing the JJ-KT77 with EL34s. We really like the sound of the KT77 as it has the extended bass and smoother sond of a beam tetrode. KT77s will work fine in all EL34 loaded amps, so its good to have another option available

.     My earlier post about the KT77 didn't work,so.....The JJKT77 has been a saviour for me .They sound like a cross between a 6L6 and a EL34.Or halfway between them.Nice.

[JamesHealey]

EL84 - Compressed british lots of mids, sparkling cleans think Bryan May
EL34 - Classic Rock, 70s and 80s Heavier Styles
6CA7 - EL34's American Military version, as used by EVH.
KT77 - Higher plate voltage version of EL34, Extended bass and highend
KT88 - Huge plate voltage handlin extended bass and highs kinda scratchy
6V6 - Fender Deluxe Reverb, sounds like smaller 6L6's
6L6 - Vintage 6L6 sound, think country etc.
6L6GC - High power handling version, think Mesa/Boogie etc..
5881 - European 6L6 as used by ENGL
6550 - American KT88, harsher less mids than KT88 still huge sounding

[sambo]

Interestingly, the Orange Thunderverb head uses 6550s I think...

[Henk]

Very interesting read, allthough i kind of expected to find more talk about


MULLAAARDS !!!!

Ok cant help it, just needed to get out.......

[the_bleeding]

i want to post a note about kt88s vs 6550's.

ORIGINALLY the kt88 was designed to have a higher plate voltage.
Presently, most current production kt88's have the same plate voltage as 6550's, the only difference between them being a pin connection.

[jpfamps]

The pin out of 6550s and KT88s is the same.

The original data sheets for the KT88 and 6550 list the maximum plate voltage as 600V (later data sheets list 800 V for the KT88), however the main difference between these valves is that the 6550 has a much lower screen voltage rating of 400 V versus the KT88 (600 V). The KT88 was designed for ultra-linear operation, and in this mode the screens and plate are at virtually the same voltage.

Amps that use 6550 with high plate voltages use much lower screen voltages. For example I was working on a Marshall 2001 bass amp which uses 8 (yes eight!!) 6550s to generate 375 Watts. The plate voltage was around 670 V (careful now), however the screens were run at around 350 V. Ampeg use a similar arrangement in the SVT and Orange do the same in the Thunderverb.

The Marshall Major runs the output stage in ultralinear configuration with a B+ of around 650 V, so it is not advisable to run 6550s in this amp as the creen voltage is way too high.

So the 6550 and KT88 are not equivalent valves. There is always a suspicion with Sovtek valves that the 6550 and KT88 are the same valve in a different package; as there are no datasheets for these valves you can't tell. However the winged-C and JJ-KT88s have the expected parameters for a KT88. Also the winged-C 6550 has the expected lower voltage ratings.