Big bottle power amps

[Zozobra]

Hi guys,

Could I get a topic rolling on big bottle power amps? I'm slightly confused about the best way to operate the screens on these things. The maths and what looks like a good idea in principle tells me I should be dropping the screens to the region of 300VDC for a 6550/KT88, which to say I'm probably looking at ~500VDC on the plates with a 5K primary for a pair means a using a huge dropping resistor.

Heres a crudely drawn load line using a 5K primary:
http://dl.dropbox.com/u/8802910/JJKT88.png

Using the method described by the valve wizard, this shoots for about 80W output as an estimate.

A bit of digging about leads me to think that most people dont do this and they run the screens hot. Is this advisable? A discussion on this elsewhere informed me that the old model-t ran the screens of 6550's at 500V which is well over spec (thanks slayed if you read this). What is a sensible number to be gunning for? Certainly dropping it to near 400VDC gives me a good voltage to use for the preamp too, although I might experiment with a UL tap which will complicate that a little.

Thoughts and opinions very much appreciated. This one has bugged me for quite some time!

[Steve Conner]

There are two rival schools of thought. One likes to run the screens at half the plate voltage using a power supply with two taps. The other likes to run the screens higher and the plates lower, using a single supply with dropping resistors.

If gunning for really high power (100W/pair) then you'll need to run the screens at 300-400V and the anodes at 600, as the KT88 datasheet recommends.

If you're willing to settle for 60-80W per pair, as many designs do (and it's maybe wise with new production tubes) then either method should work.

The optimum load impedance depends on the screen voltage though, so make your mind up before ordering the OPT. High plate/low screen generates the power with high tube voltage and low current, so requires a higher load impedance.

The 6550 has a lower maximum screen voltage rating. But the KT88's 600V rating is given with ultralinear operation in mind, it doesn't mean that you can safely run them in tetrode mode with a 600V screen supply, any more than you could a 6550.

(there's another option to consider, ultralinear)

[clyde1]

I've been running a pair of JJ KT88's at 600v anode and 590v screen for about 5 years now in an amp that's gigged just about every weekend. No problems to report.

[Zozobra]

There are two rival schools of thought. One likes to run the screens at half the plate voltage using a power supply with two taps. The other likes to run the screens higher and the plates lower, using a single supply with dropping resistors.

If gunning for really high power (100W/pair) then you'll need to run the screens at 300-400V and the anodes at 600, as the KT88 datasheet recommends.

If you're willing to settle for 60-80W per pair, as many designs do (and it's maybe wise with new production tubes) then either method should work.

The optimum load impedance depends on the screen voltage though, so make your mind up before ordering the OPT. High plate/low screen generates the power with high tube voltage and low current, so requires a higher load impedance.

The 6550 has a lower maximum screen voltage rating. But the KT88's 600V rating is given with ultralinear operation in mind, it doesn't mean that you can safely run them in tetrode mode with a 600V screen supply, any more than you could a 6550.

(there's another option to consider, ultralinear)

I could think about using a voltage doubled set up if I can find a suitable PT (perhaps antek have something) for the screens at half the plate voltage route as thats easy to tap off and the screen current draw is small. I'd have to think about the preamp supply though, maybe some sort of regulator/stabiliser circuit would be the best route depending on the required voltage and how the plates are run, certainly 300V would be a bit skimpy so I would have to drop it from the anode supply. Ultralinear operation is something I want to try too and may well be something I have on a switch. As it stands I will probably get the OT from edcor and suck up shipping/import as they have a good range of OT's

I've been running a pair of JJ KT88's at 600v anode and 590v screen for about 5 years now in an amp that's gigged just about every weekend. No problems to report.

Wow, thasts pretty impressive! What sort of primary is that running with? I assume the screens are fed from a choke filter?

[defaced]

There have been a couple of threads on power supply design for this type of setup. They are worth the digging. Here's the main one I think about when this comes up: http://music-electronics-forum.com/t19103/

High plate/ half screens is my next stop along the high output tube amp road. Just need to get some other designs buttoned up first. I'm very interested to see what you come up with.

There has also been a recent thread on the KT-120 that might have some valuable info.

[Zozobra]

Thanks, I'll have a read. My aim is to get a good hearty amount of power but not at the expense of stability or having an amp that kills tubes and OT's like they 2 a penny I suppose that is always the dream! I'm interested in big bottle amps as over in UK land all we get are EL34/6L6 amps and I want something different. Blackstar are the only people I can think of offering KT88s as standard and that amp sounds shitty IMO.

This isn't quite my next project. First I'm building a preamp prototyping rig. I would have been well into it by now but USPS lost my damn power transformer and then antek didnt have any in stock to replace it. I ordered it in september and I still have no PT. Oh well, more time to think of preamp to try out I guess. I'll be posting a thread about it when all the bits are here. When I have a preamp I like then it'll be time to work on the power amp but I want to have a good think about it first.

[defaced]

Well, as a start, consider taking a standard 6L6/EL34 amp and adjust the circuit for KT-88s. Make sure the screen resistors are in the 1.5k to 3k range, bias feed resistors are 100k, and that the bias circuit can get the tubes in their operating range. That will give you an idea if you like the KT-88 sound. I run JJ KT-88s at a slightly lower voltage than Clyde. I'm in the 535v range on the plates and about 5v less on the screens, 1.9k load for four, which I'd like to see a little higher but that's what I have setup on my prototyping rig. I have not bothered to measure output power. That's a pending item because I have some AB2 stuff to look at.

[Zozobra]

Thanks for the tip. I have a D60 schematic I can compare to other amps.
My prototyping rig is going to use 6L6s for the mean time. I have one of Merlins 100W OT's I bought a long time ago to use which has 3k5 and 4k primaries. The PT I have on the way is an Antek AS-1T300 which has dual 300V, 0.17A secondaries and 6A of heaters to play with. Perhaps not ideal for KT88s but I might try some in there at some point for giggles.

I'm about half way into the dual rail thread, lots of interesting stuff in there. I'm going to have to read it a few times to get thinks to sink in. Certainly the regulated screens idea doesn't sound like a terrible idea for getting the two rails. I have Merlins power supply book so I'll revist the relevant chapters, which I sort of glossed over previously as I had no context for it at the time.

[clyde1]

Wow, thasts pretty impressive! What sort of primary is that running with? I assume the screens are fed from a choke filter?

JJ's data sheet lists screen max as 600v, 800v for the anode. Primary impedance is 5k, using a Hammond 1650R. After the choke there's a common 680r 10w resistor feeding 2 100r 5w screen resistors. The amp is also switchable to UL and the 100r resistors stay in circuit.

[soundmasterg]

When I hear someone talk about big bottles, I think of something like an 813, not a KT88! That said, many of the Sunns are running 560+v on plate and screen in a UL circuit and the JJ KT88's hold up just fine under those conditions. I have Winged C 6550C's in my SVT which is upwards of 600v plate and around 300 screen and they've been going strong in there for 10 years with constant use.

Greg

[Zozobra]

JJ's data sheet lists screen max as 600v, 800v for the anode. Primary impedance is 5k, using a Hammond 1650R. After the choke there's a common 680r 10w resistor feeding 2 100r 5w screen resistors. The amp is also switchable to UL and the 100r resistors stay in circuit.

Thanks for the help. I'm going to start drawing up a power amp design soon. I can get that Hammond here for okay(ish) money. I kind of split between that and an equivalent edcor though.

When I hear someone talk about big bottles, I think of something like an 813, not a KT88! That said, many of the Sunns are running 560+v on plate and screen in a UL circuit and the JJ KT88's hold up just fine under those conditions. I have Winged C 6550C's in my SVT which is upwards of 600v plate and around 300 screen and they've been going strong in there for 10 years with constant use.

Greg

Good to know. I may well have been somewhat over cautious in my approach. I had this worry that I'd put something together based on pre-existing circuits just to be greeted by smoke and sparks! Unfortunately KT88s would be considered big bottle tubes by pretty much anyone in the UK!

[mooreamps]

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

[imaradiostar]

Count me as one vote AGAINST running KT88's with high screen voltages. In my case I didn't have high enough screen resistors- I was using 300 ohms per screen grid and watching the scope instead of the glowing screens! I'd imagine the 1.5k to 3k range mentioned above might have saved them.

jamie

[Zozobra]

Actually now I come to think of it I have my Laney AOR100 on the bench currently to fix up (signs and suggestions so far point to the jacks being gummed up, which I'll replace tomorrow - fingers crossed). I should probably try it with some KT88s before perusing this too much as suggested earlier. I know people have modded AOR's to run 6550's before, seemingly without too much issue. It runs about 460V on the plates and not s great shake less on the screens (choke filtered). I don't know what the primary is, I'd guess something like 2k, maybe 1k7 at a push. I'll draw some load lines later to see how sensible they look. If they don't look silly I'll try some JJ KT88s in it. The transformers in this amp are pretty damn beefy and look like they can take a fair bit of punishment.

As for the mods this is my thinking; The screen grid resistors will probably need increasing as they're only 1k; I'll go with something 2-3k. The bias circuit may need adjusting too. The schematic looks to have a range of up to -42V which might be a little skimpy; Also to get the primary looking more sensible for a pair of KT88s I'll mismatch a 16ohm load to the 8ohm tap to give something like 3.5-4k. Does this sound like a sensible start?

[Steve Conner]

KT88s should be a straight swap into it. You just won't get a great deal of extra power, if the supply is only 460V.