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Hey guys!
I've been seriously considering the KT88 as the old only offering that might satisfy the Fender 400PS amp. So far, I was REALLY impressed by the GL KT77. The construction and vacuum quality were bare-none the best I've seen since NOS. I know the KT88 follows this same standard.
The 400PS runs some 700V at the plates, and 400V at the screen (with high current draw). By your experiences, what are the maximum voltages you have applied to the plates and screens without any run-away?
If you abused your GL KT88, tell me about it! I need to know!
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May your tubes be warm, long-lasting, and always lively. Holy be thy heater.
Follow Ups:
Your right, any cheap chinese tube lasted about an hour. The Old Svetlana 6550C seemed to hold up. I built the thing for a bass player but it was probably way to heavy to gig. The Gold Lions seem fairly durable, it is the 700 volts as much as the plate dissipation. As these amps are almost entirely Class B, how hard they are played matters a lot.
See if anyone responds.
8^)
One guy posted that he knows a friend that has the 400PS. And uses Gold Lion KT-88's. He bought the tubes from a buyer that selected the strongest tubes of the lot.
It's been in the amp for about 6 months. No problems. But, the guy does not do regular gigs.
The Bob Latino ST120 rated at 60 watts per channel has 520 volts on the plates with UL connected screens.
As I recall, that's the only way to get all the power. The speaker jacks are switching jack, each one turns on a pair of tubes.
I do! Not always, but if I can/need to, I do.
Ya, I heard of the "turning on of pairs" idea. But then, how does it achieve its 145W RMS to each pair?
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May your tubes be warm, long-lasting, and always lively. Holy be thy heater.
The three switching jacks each complete the cathode circuit for a pair of power tubes when a speaker is plugged in. When one cab is plugged in, two tubes operate. When two cabs are used, two pairs of tubes operate, etc. You can also insert a "dummy plug" into unused outputs, turning on other pairs to get more power.
If you operate with less than all tubes operating, I suggest rotating the tubes periodically so they all age evenly.
I wouldn't dummy plug anything because it doesn't count as an actual resistive load. Koerner himself said that accidentally pulling a load during operation would cause serious arcing and the possible destruction of the amp and the speakers. That implies an important electrical relationship between the load and the tube (which is why I thought it might act as a feedback of some sort).
Second, switching tubes gives me the hibbies as well. It wouldn't be the end of the amp, but the amp has three bias pots to consider - one for AC hum, a global bias feed to the phase-splitting transformer, and then another bias feed after the transformer to balance your left and right side. So, let's just say I wouldn't want to make any mistakes or assumptions about the effect of pulling tubes on bias, or mixing a side by accident and getting weird output or oscillations.
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May your tubes be warm, long-lasting, and always lively. Holy be thy heater.
I have my very first quad that came from Jim McShane in 2007 and I use and abuse the hell out of them and yes,I have overheated them when testing some old amps that had leaky coupling caps..The flashing on top is still almost perfect and,the flashing on the sides is starting to get smaller but these tubes have at least 3000 hours on them and the Gold Lion lettering is still in full view.I have run them at 600v on the plate and 300v on the screens in the modded Altec 1530s.As far as 700v and 400 on the screens go,if the GL KT77s can take it the KT88s can as well.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
Edits: 01/30/15
Tube-tastic, sir! This is EXACTLY what I needed to hear. Thank you. The 400PS is a beast. It will take these KT88 into the 60-75W RMS/tube depending on bias, and if I have all 3 4ohm taps occupied.
How it manages that output is still a little bit of a mystery to me. There's a regular-ish negative feedback going back into the 6L6/transformer driver stage from the OT. Yet, I can also see some sort of feedback from each 4ohm speaker jack (each a distinct output winding) back into the cathode of each 3 pairs of power tubes, .
I figured out standard Class A and AB, but the 400PS is a real oddball...
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May your tubes be warm, long-lasting, and always lively. Holy be thy heater.
Very special amp. Needing a bit more equipment to set it correctly. Good tubes are a must. More at this site.
Oh I'm very well acquainted with its needs. I own 5 of them - 3 operational and 2 others that need servicing. I have tried the JJ KT88 (while it was still a viable option I could depend upon). It's decent. But, I found the spectrum of sound was coloured heavy in the lows, and I'm always searching for newer/better options.
So, I'm wondering about the quality of the Gold Lion. To what voltages and current parameters have these KT88s been subjected?
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May your tubes be warm, long-lasting, and always lively. Holy be thy heater.
I am sure there is a community of 400PS owners on the Net. There just isn't many of these beasts, still running. Enthusiasm, for this amp as a part of vacuum tube amp history, is quite high. Ed Jahns is considered an iconic designer of his era. As you well know, these amps still go for very good money --- once stable.I know of no one that owns this amp. But, from what I've read --- only selected TS 6550 and GE 6550A are recommended. I have not heard of anyone succesfully running 1990's version of Svetlana 6550C in this amp. And those were very good tubes.
Do you have any photos of your amps? And the lab setup to get these amps running?
Thanks!
Edits: 01/30/15 01/30/15
Hey FenderLover!
Ya, I have one running 6550A's and I have a sextet that needs proper evaluation before I put it back into our 3rd working one (working, but needs biasing and benching).
I actually have a friend who helps me with this because I don't have the equipment necessary to do this on my own. And frankly, it scares the living crap out of me - I know electricity and what it is to get shocked. I wouldn't want to make any mistakes with this thing.
That's a photo of one of them.
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May your tubes be warm, long-lasting, and always lively. Holy be thy heater.
I would love to plug my Les Paul into that amp. Then, hit an open E-chord. Do you have those folded EV loaded cabs that came with the amp, when it was first introduced?
I thought the original cabs was loaded with a D150 or a K151... Anyway, ya, we built an oversized version of the cab with a D150. It's bananas. Running JUST that cab will shake stuff out cupboards.
And. guitar through a 400PS puts to shame smaller setups with an actual bass. Loud, smooth, delicious...
And thanks for the info on the Gold Lion's performance - I'll try a set!
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May your tubes be warm, long-lasting, and always lively. Holy be thy heater.
Could be OEM JBL's. The only folded cab I remember seeing had 18-inch EV in it. But, that could have been a replacement speaker.
The loudest amp I've ever played though is an Ampeg SVT into an 8 X 10inch cab. That was beyond loud, for me. I can't imagine what the 400PS with three speaker cabs hooked to it must be like. 400+ tube watts!
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