6550/KT88 Monoblocks for Tube Stereo 70+70W

[David Root]

I'm planning the above, based on a pair of One Electron BFT-1B PTs, which will idle the plates at about 525V or so. I'm thinking of a 5K OT and running the tubes at about 45mA cathode draw.

My question: Is it OK to exceed the design max plate voltage of 450V in UL from the datasheets to accomodate this PT, which I have a pair of in stock?
Assuming current production tubes, that is, as I'm not springing for a quad of MOV KT88s, even if I could find a set.

I could go for a quad of NOS GE or maybe Tung-Sol 6550s though.

I asked this question on diyaudio and got only a comment that "the KT88 is better suited to UL operation than the 6550", which I'm not sure I agree with because no reasons were given.

I did find one thread in which someone claimed to be operating KT88s in UL at "up to 550V". But no indication of tube life or audio performance.

[Reeltarded]

Marshall Majors on modern wall power at nipping at 690v on the plates, and UL. Take a look at Dr. Tube for what they had to do to make that work with KT88s to get the power up.

The ones I have left are placard amps with the 6550 warning and two of them are running GE 6550s.. so you underbias a little.. ehh..

KT-88 are better in these amps. My only experience with any amps sort of like what you are asking. I hate UL!

The OT Primary is 1k4... hmm..

[David Root]

Thanx reel. I didn't think to check the Major. I would never use UL on a guitar amp, maybe on a bass amp, but for stereo CD reproduction it's the way to go.

So 525V is no problem in UL it seems. So all I need to do now is work out the gain I need to get from a 1V CD player output to 70W out of a pair of KT88s with a small amount of GNFB.

I googled Dr. Tube but all I got was porn sites. Do you have their address?

[alvarezh]

http://www.drtube.com/marshall.htm

Don't know where he read the information dough.

Cheers.

[Reeltarded]

That was a little rude of me.

http://www.drtube.com/marshall.htm#Major

Shudda known. Do now. Will forget. #old #intheway

[gingertube]

David,
I have today responded to what I guess must have been your thread over at DIYAudio.

I recently repaired an ARANOV LS9100 Monoblock for a friend. A quad of 6550C in Ultralinear. It ran a B+ of 530V.
The max Vg2 you see on datasheets is for pentode (tetrode) mode.

With Ultralinear as anode current goes up and anode voltage pulls down the screen voltage comes down too (due to the tap on the OT). This moderates (but doesn't entirely eliminate) the usual increase of screen current as anode current goes up. You generally safely run Ultralinear at B+ of 20 to 25% higher than the Vg2 max rating on the datasheet.

I fitted Winged C Svetlna to that amp as that is what the other monoblock had in it. I biased teh output tubes at 62mA which is just under 33 Watts dissipation per tube. That was because of the "usual" max Rg1 abuse.

If teh output tubes are being driven with grids direct coupled to a cathode follower or mosfet source follower then I wouuld have no hesitation in biasing them hotter than that.

I put a link to my schematic trace on that thread.

Cheers,
Ian

P.S. Ignore the folk who say Ultralinear is no good for guitar amps. One or two folk stated they did'nt like the 135W Fender Twins which were Ultralinear and a bunch of copy cats then went raound saying Ultralinear is no good. The problem with those fender Amps was too much global feedback which had nothing to do with the UL. It may not surprise you to know that Kevin O'Connor (London Power) has been designing building selling amps for yonks all with ultralinear output stages. If you use ultralinear you need very little or none at all global feedback.

[tictac]

David,

The BFT PT's have two B+ windings and you can use them both as long as you don't exceed max ratings...

The 2nd B+ winding will give you around 360VDC; I use a BFT transfomer to operate 4 6550's with the 2nd B+ winding for the screen supply...

Not sure if this info is useful; you may have already known....

TT

[Reeltarded]

Cheers,
Ian

P.S. Ignore the folk who say Ultralinear is no good for guitar amps.

Cheeky!

[VacuumVoodoo]

This might be helpful, detailed design data up to 1000W power amps. It was for me
https://tubeamparchive.com/download/file.php?id=24508

[David Root]

Thanx guys, a lot of good info! I did find a few other anecdotes that documented UL above 450V so I've decided to go with 525V in UL. Gingertube, thanx for confirming that is good. tictac, I have the BFT-1B datasheet and considered using the 520VCT taps for the screen supply, but UL is just too easy! Vac Voodoo, I have not seen that book before, looks like a winner! Thank you.

I wonder why Dumble didn't use UL in his Steel String Singers/Dumblelands??

I am going to base mine on these schematics which are for a 50+50W integrated amp and associated PS, modified to 70W and the PS changed to suit each monoblock. Since I want to listen to blues and dinosaur rock mostly, I have dubbed them Rockoblocks!

Source of these is www.turneraudio.au . Patrick Turner is something of a GEB (Giant Earth Brain) in the field and his site is well worth a long look if you've not seen it.

Some questions remain:

1) Will changing the OT Zaa from 6K6 to 5K or 4K2 get me 70W or thereabouts? Yes I know it's not going to make much audible difference but it gives a reserve for bass & drum dynamics I think. It looks to me that there is plenty of available gain/drive potential.

2) Since my only source will be a CD player, do I really need V1A?

3) I did a draft layout, attached, for a 20x8x2.5 chassis. There is limited room for component boards so a conventional guitar amp board layout doesn't seem to work. I subsequently found a primer on hifi chassis layout, attached, from www.oldstockaudio.com, another good site, which I found useful. Anyone have any suggestions?

[Cantplay]

Would it make sense to put PT and OPT at opposite ends?

Lining them up close together seems like asking for interaction.

John

[David Root]

Then you have the tubes closer to the PT, which can't be good. As is now they are oriented with the bells at 90 deg. and the choke between the PT and OT.

BTW I didn't mention I'm going to use Edcor 100W OTs as they only make a 70W OT in 3K5 which is a bit low.

[martin manning]

Would it make sense to put PT and OPT at opposite ends?

Lining them up close together seems like asking for interaction.

The tubes and trannys are on display like statuary in this type of amp. They have to be arranged so as to be balanced and visually appealing ;^)

[Leo_Gnardo]

David, looks like you're planning a more muscular modern version of the good ol' Dynaco MkIII. FWIW those MkIII transformers can deliver a lot more than you might expect. You could look up their primary Z easily enough. And freshly made MkIII OT's can be sourced from Bob Latino tubes4hifi.com . If nothing else it's worth a look over there to see what he has, both hardware & information. (I'm not affiliated with, etc etc, just aware of a good source & you should be too.)

[Cantplay]

Bob also does an mono with 4 octals too.

http://dynacotubeaudio.forumotion.com/t ... tom-m125-s

John