Big bottle power amps

[Zozobra]

Ah okay, I've done the mismatch as I only want to run a pair rather than a quad. I'm more interested in how they sound, although I suspect its a bit of a false test at the lower plate voltages. I've drawn the class B part of the load line for a few of the set ups mentioned in this thread to see how they look anyway. As a crude approximation it looks like a pair will give about 80W for the 4k load at 460V after making some fairly wild assumptions about the plate curves.

[Steve Conner]

That's about right, in my own experience when you put KT88s into an amp meant for 6L6/EL34, you get about 60-80W per pair.

Even though the output power isn't much increased, they do sound different, you may like it.

[Zozobra]

I've certainly gravitated towards a cleaner poweramp sound over the last few years, thus my interest in higher output valves. I bought the AOR as a project amp but I got on with it too much to gut it. It does clean and loud better than any of my other amps which is great for me as I like using an array of dirt pedals to get my tone. I mainly play sludge/doom stuff in B. I had a superbass for a few years and it never quite worked for me.

[Steve Conner]

Oh, well the KT88s are definitely worth a go. As far as my subjective impression goes, they make the amp sound cleaner.

I don't see the point of only using a pair, though. The output power will end up less than it was, so that'll make it sound less clean at a given volume, all other things being equal.

[Zozobra]

Its purely that I dont want to drop 100 quid on a quad to see them go up in smoke so I'll try a pair first!

[Steve Conner]

Well, the downside of that is that you won't get a matched quad, if you order the other two tubes later. Unless you have an understanding tube supplier who will give you another pair from the same matching bin as your previous ones.

I think the odds of them going up in smoke are slim, they can handle any abuse that an EL34 or 6L6 would. So if the amp wasn't burning those, it won't burn the KT88s.

The thing most likely to burn is the heater winding on your transformer, if the amp was designed for 6L6s.

[Zozobra]

Fair point. The amp is designed for EL34s and currently has 6CA7s in. I'll most likely have to go with hotrox if I go KT88 as voc-rock dont sell KT88s, only 6550EHs. I'm not mad keen on hotrox either as they've never had great matching in my experience so I might be better off with 6550s from voc-rock. I'll mull over a pair vs. a quad after consulting the wallet. If I go with two pairs in the long run then hopefully it wont matter so long as each half of the PP circuit pulls more or less the same current draw right?

[Steve Conner]

Yes, in fact some people say matching is overrated for guitar amps altogether.

I wonder if there is really that much difference between a JJ KT88 and an EH 6550 at the end of the day. JJ are the odd one out, most of the other Russian big-bottle-sized tubes seem to use the same set of innards.

[defaced]

Make sure you have enough physical room if you decide to go with 4 KT-88. Many EL34/6L6 amps have the power tube sockets too close together for KT-88s. This will become even more important if you decide to build an amp that really takes advantage of their capabilities. I think minimum recommended spacing is 4" (100mm) center to center for these guys at full power.

I agree with Steve of most everything else. I don't think these will go up in smoke on their first journey into the great divide. If you want to keep them in the amp, then start messing with stuff to make them "right".

[Zozobra]

I'll check that. My trace elliot speed twin has the sockets really close together, even 6ca7ehs were a bit of a squeeze. They sounded like shit in that amp though!
I know people have run 6550's in the AOR before but I'll bare the 100mm spacing in mind. Hell I have to get the thing playing working again before I start putting big bulbs in. Hopefully is just the jacks otherwise there will be new thread in the trouble shooting forum!

This is a picture with 6CA7s in. They look reasonably well spaced.


My speedtwin on the otherhand doesnt (6L6EHs)

[soundmasterg]

I do have a chassis in my shop with a couple of 813 tubes on it, running class A push-pull. I just don't know what to do with it.... Probably too much power for a guitar amp now that I drive it with my 5 watt IPA LTP driver stage.... Maybe a bass amp...

-g

Thats pretty cool! What kind of power does it make and what voltages and OT impedance are you running? My grandpa was a ham radio guy and had started to build something with 813's, but never finished it. I inherited it along with some used and new 813's and had been thinking of making a bass amp out of it, but need to learn more about using those big bottles. I've heard with 1500-2000v plate that you can get 500 watts out of a pair of 813's?

greg

[mooreamps]

Well, these are power tubes that you really should be using a split rail HV power supply on... 1.4 KV on the plates, and 750 volts for the screens, running resistor bias class A at -90 volts bias ; in push-pull into a 100 watt Hammond O/T... I spent too much time on the filament power supply ; 10 volts DC at 5 amps... The tone is just "big"... Sounds like your playing through a 4x18 for loudspeakers.... I really could not drive them that hard with 12ax7's, with resistor plate load in a conventional LTP. But,,,, using a pair of EL-84's in an IPA configuration,,, heh, heh, heh......Makes me think it was born about 30 years too late ; if you wanted to be a Who Tribute Band....

I don't know yet what it is capable of. I'm not done with it yet.... I was thinking of adding combination bias to drive the power tubes into Class A/B, and maybe add another pre-amp channel.....

I am definitely of the mind to think ; little bottles = little tone... Big bottles = "big tone"...

-g

[Steve Conner]

Champ 1000 Watt Tube Amp

John Chambers makes his own transformers, which is about the only way you're going to get a transformer that can do 813s justice.

Transformer design is a compromise, and running off a high B+ voltage just makes it harder. I asked him what the HF bandwidth of his transformer was like, and he admitted that it couldn't cover the full audio band to 20kHz. But it didn't matter, as the amps were going to be used to drive the bass bins in a reggae sound system. I bet that was one fat sounding system

[R.G.]

'Course, if all it's going to do is to drive the bass bins, a 1kW Class D would do the job nicely with only a tiny fraction of the trouble.

But we're not here because it's easy, are we?

[mooreamps]

It might do the job nicely, but isn't that also solid state ???? I wonder about a bass amp, running a pair of 813's into an 8x10 cab would sound ?... I mean just to put some kind of a franken amp on stage with a pair of 813's glowing inside the chassis.... I think people in the audience would "freak"...

-g