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Discussion Starter · #1 · (Edited)
New Power Tube Day!





These just came in last night. I ordered them because my Sunn 1200s (70s, not the Fender reissue) crapped out 2 jams ago and the tech said the (vintage, likely original) GE 6550A power tubes should be replaced just in case. One of them definately tested weird last I checked so....

Debated replacement options, including KT90s and 120s (but whoa, those are spendy, and I don't need, nor probably want, that much headroom) and then came across these (very good deal - cheaper than new JJs or EHs, which is amazing). It’s a matched quad of NOS military grade Teslovak ( aka Tesla, which folded in 1989 - JJ started in 1993 using some of the old Tesla equipment, but is otherwise a completely unrelated company) KT88s. Being military surplus, they are rather heavy due to thicker-than-normal glass (nice for added peace of mind in a guitar/bass amp that will be gigged) and where most likely QCed to a higher standard than regular audio stuff. They ain’t no NOS Gold Lions (like, who can afford those anyway), but I expect good things; can’t wait to fire up the Sunn with these puppies installed.

Sunn amps originally used KT88s (were a clone of the Dynaco MkIII HiFi power amp - with an instrument preamp tacked on the front). They switched to 6550s to save money in the early-mid 70s. Either will work in any Sunn amp of any era*; some swear NOS GE 6550s are the only thing that sound right in them, others swear by KT88s (a large contingent using JJs). I’ll soon see about that, but I suspect the people who prefer 6550s do so because they break up earlier than KT88s, and in a Sunn (especially the 4 power tube models vs 2), it can be hard to get power section to overdrive (think Fender Super Twin; which also has a high wattage ultralinear power section).

* there were a few EL34 based amps in the mid 70s (Sentura and Solarus - same amp with a different speaker cab)
 

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Discussion Starter · #3 · (Edited)




Retubed and biased. Tech said found a an issue causing the mid boost switch to not act right in off mode; fixed that (no details) but then said there was some noise in mid boost mode which he traced to a ground issue. Traced that to a loose rivet (holding down one of the turret boeards) used as chassis ground that was giving an intermittent connection. He drilled that out and replaced it with a screw. Looks good; sounds good (haven't had a full volume jam yet), but: the trem don't work (it used to). When you turn the trem on it modulates the idle noise and not the signal (if you hit a note, it rings clear as a bell as the noise wobs in the background instead; mute the instrument and the wob continues). It occurred to me: could this be due to the input and ground connections to the trem module (discrete transistor and optical based) getting switched around? Got so preoccupied testing that out to see what the behaviour was that I totally forgot to check if the reverb still works.

[sigh] I don't wanna take it back; was hoping to start some recording this weekend.
 

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Just off a quick peek at the schem, trem seems to be tied in with reverb. Is reverb all working right?
 

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Hope it's back to full strength soon!

I opened the thread, saw the KT88 and went "oooh" out loud. Big glass!

Would you be able to explain to me the difference in KT88 versus 6550? I always thought they were the same tube under a different name.
 

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Discussion Starter · #6 · (Edited)
Just off a quick peek at the schem, trem seems to be tied in with reverb. Is reverb all working right?
Haven't been back to the studio since to check - but ya done know that's the first thing I will try tomorrow when I get there. ... I didn't see that in the schem tho - but to be fair I am looking at the Spectre/Solaris etc schems for the preamp and the 2000s for the power section. I saw an actual 1200s schem once, but haven't been able to track it down since.

Hope it's back to full strength soon!

I opened the thread, saw the KT88 and went "oooh" out loud. Big glass!

Would you be able to explain to me the difference in KT88 versus 6550? I always thought they were the same tube under a different name.
They are compatible and interchangeable (bias adjustment applies even when changing to tubes of the same type) but not the same tube. 6550 came first (American) and the KT88 was developed later in Europe to compete withit vs having to import American stock. Since developed a bit later they were able to make some 'improvements' (whether they are or aren't depends on what you like) or maybe because it is not a copy but a different tube with very similar specs there are just some differences. Cleaner; more headroom; tighter bass; more hifi generally. Also takes a higher max plate voltage IIRC.
 

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Sorry, which one has the higher headroom - KT88?

Also, you should be seeing my pedal this weekend with any luck. It's technically no longer mine so Dan needs to hustle haha. Back to tubes!

I also ask because his JCM800 runs 6550's.
 

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Discussion Starter · #8 ·
Yes, the KT88 has higher headroom.

I'm ready for the pedal (I thought it wasn't coming but then I saw your for sale thread). If we knew it was gonna take this long we coulda orderred the official replacement part by now, LOL.
 

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Yes, the KT88 has higher headroom.

I'm ready for the pedal (I thought it wasn't coming but then I saw your for sale thread). If we knew it was gonna take this long we coulda orderred the official replacement part by now, LOL.
Fortunately my friend who bought it isn't in a rush haha.
 

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I saw an actual 1200s schem once, but haven't been able to track it down since.
Here you go. Not sure if the footswitch enables or disables it. Point 'F' to the lower right of the opto module connects to the F just below the .005 at upper right corner of schematic.

Edit: sorry, I was looking at that wrong. Point F is actually the 22V supply rail. So the trem isn't really connected to the reverb but they do share the same power supply.
 

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Discussion Starter · #11 ·
Thanks.... OK I meant a legible version; how are you reading that, holy moly ;P

It is mostly the same as the other Sunn trem+reverb models' preamps, from what I see so far. Somehow never noticed that F point before. ... connected to the recovery section of the reverb circuit. Interesting. Also interesting is that full signal is always fed to both fx but fx level (or wet /dry mix) is controlled by the amount of recovery/output gain. Usually that's done at the sending end, no?

The output jack wiring is not accurate. Some of the other, Sunn amps did that (spkr and ext spkr jacks, where plugging a jack into ext spkr switches the tap used on the OT to a lower Z) but the 1200s (mine and all others I've seen) has it like the 2000s (jack per tap on the OT - earlier ones were 16 and 8, later they used 8 and 4 like mine).
 

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I was expecting New "Power Transformer Day". Lol. See, its on my mind. I pick up my new PT in 2 days, then finally my 1963 Vibrolux will come to life.
 

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Discussion Starter · #13 ·
So it turns out that the reverb works.

Interesting thing about the trem I didn't notice before: the depth control works in the opposite direction (again, on the niose; the signal stays dry).
 

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Discussion Starter · #15 ·
I haven't measured this one, but my other one, the Solarus (also 6550, not the EL34 version) was 500V (measuerd twice, once 491 and the other 499). From looking at various model schems that seems to be pretty usual for Sunns.
 
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