Which tubes with 5k 100W OT?

[andrew_k]

I'm building an October Stage (PP KT77), and wondering what other valves I could use with the 5k OT.

6550?
6L6/5881 seems like not too much of a stretch.
I would like to try KT66, but is the 8k primary listed in the GEC data too far off for the valves to be paired with a 5k OT?

Thanks

[Robert M. Martinelli]

Hi Andrew,

In a nutshell:
OTs don't have an inherent impedance, they reflect whatever impedance they "see" at one side on the other one according to their Ratio.

The formula is : Ratio( squared )= Zin/Zout, or, if you like it better, Ratio=Sqrroot(Zin/Zout).

So, if your OT is designed to match a 5K output stage to an 8 Ohm speaker, this means it has a ratio of 25 ( Sqrroot ( 5000/8 ) ).

Reversing the formula you can determine which load is the best match for a given output stage : Zout=Zin/(Ratio(Squared)).

This means that, with the same OT, an 8K output stage would be matched to a load of : 8000/625=12,8 Ohm, while a 6K output stage would be matched to a load of : 6000/625=9,6 Ohm.

Bear in mind that tube amps are far from perfect, and tolerances of 10 or even 20 percent are to be considered normal, so these amps tend to be rather forgiving.

Hope this helps

Best regards

Bob

[black_labb]

are you planning on running 2 or 4 output valves? if 4 then 4 6l6's would work very well with a 4 ohm load. (5k per pp pair really). el34's would be fine as well. the kt66 would probably do fine at 5k. valves tend to work well at a wide range of impedances. i dont have much experience with kt66's, but im quite sure they would be fine with 5k. probably get more power as well.

its common to think of lower impedances giving higher power and distortion. many fender amps were wired so that when an expension speaker was plugged in it was in parallel with the speakers in the combo already, which would halve the impedance (they reccomended the same impedance as the speakers in the amp usually). most people feel that most decent OT's can be run at half or double the rated impedance without issue, but that depends on how central the original rating is to what is reccomended.

[andrew_k]

Thank you Bob.
My interest lies more in what members would consider too far of a mismatch in load. 20%?

For example, I have a 6.6k OT in my 6G2, whereas according to spec, a PP pair of 6V6 should have an 8k load. Yet this sounds great.

[black_labb]

there isnt a set value, there is a range that is acceptable. the impedance will effect the way the tube runs and will change the sound. 6v6's for example are supposed to run well at anywhere between 5k and 10k. the values in the datasheets are just the values used for that test. they are not the "best" values, but they are within the range. if you look at a comprehensive datasheet they will usually show different impedances.

i have seen one datasheet that had a graph with impedance on the horizontal axis, and output power as well as distortion on the vertical axis. there was a huge range of usable impeadances.

[tubeswell]

"Typical" Plate Load Zeds

FYI - Hasserl posted this a couple of years back - (cheers mate - I grabbed it for my handy ref: cos I thought it was, well, ... handy - but that's a personal thing right?)

"Speed Racer from the old Ampage bbs once posted this info on common guitar amp primary loads that I saved (I think he also had it posted on the old OEI website):

Amplifier Primary Impedance

Marshall, 50W 2xEL34 3,400 Ohms

Marshall 100W 4xEL34 1,750 Ohms

Marshall JTM45 2xKT66 8,000 Ohms

Vox 30W 4xEL84 4,000 Ohms

Vox 15W 2xEL84 8,000 Ohms

Fender 50W 2x6L6 4,100 Ohms

Fender 100W 4x6L6 2,000 Ohms

Tweed-Spec Cathode Bias 6L6 p-p 6,600 Ohms

Matchless 15W 2xEL84 4,000 Ohm

Matchless 30W 4xEL84 4,000 Ohms

Park 50W 2xEL34 5,000 Ohms"

FWIW the DR has a plate zed of 6k6 for 2 x 6V6s, whereas the Tweed Deluxe and the PR both have 8k. I built a 5G9 for 2 x 6V6 or 2 x 6L6s that has an OT with a 5k load resistance - sounds okay (except I'm contemplating modding the pre-amp - but that's another issue). - What is the right reflected load? It depends on your goal - Tube charts tell ya one thing, but you've got to trust your own ears a bit too.

[Steve Conner]

As you can see there's a pretty broad range. I have an amp with two KT88s and a 6.6k OT that only has a 16 ohm output. It puts out the same power - 60W - into either 16 ohms, or 8 ohms which reflects a load of 3.3k to the tubes.

With EL34s it would do 50W into 16 ohms and 25W into 8, and the plates would go red at 8 ohms.

[Enzo]

And remember too that the numbers in the tube manual are not for guitar amps. The applications intended by the RCA manual and others are hifi or consumer audio. We might find some added bite and grit totally adorable in an amp, but mom and dad would not be nearly so happy with it in their table radio or record player.

So tubeswell is right, let your ears tell you, because unless you are building a table radio or hifi record player the "right" numbers might not add up.

Imagine you want to place a cabinet under the stairway in your house. Too far to the right, and it bumps into the underside of the stairs. Too far to the left and it covers the door to the laundry room. But anywhere in between is fine. You might like the look of it better more one way than the other, but it will still function just fine. Well that stairway is your tube curves, and that cabinet is the operating area of your amp circuit. You have room to massage things, your impedances don;t have to be precise numbers.

[andrew_k]

Thank you all for your informative replies, very kind of you to help out a newbie.

I'll be building this amp switchable to ultralinear, so it's good to know I should be fine running KT66,77 or 88. I've put together the power supply on a test rig and have 457V B+1, hopefully enough for about 70W from a pair of 6550 or maybe KT88 (dang those 88's are expensive!!!)

Thanks again!
-Andrew

[loudthud]

To get to 100W at 5K plate to plate you are going to need a high B+. 600V with a pair of 6550's. Ultralinear is going to be stressfull on the screen grids at that B+ without a pretty big screen resistor like 1.5K@5W. KT88's are a better choice at that high a B+.

[daz]

what would be awesome is if there was a chart that shows each tube type and number (IE: 6l6x2, x4, el34x2, x4 etc etc) and showed what the optimum and and min/max primary Z were and at what point as you go up or down in Z do you change the tap and which way. I bet there is a chart like that somewhere or a calculator, and it would be a great help to math morons like myself and even be helpful to those who aren't making for a quick glance instead of doing the math.

[andrew_k]

loudthud: thanks, but I'm not attempting to make 100W output. More like mid 60's. The October design calls for a 5K OT, and I could get the Hammond 1650R at a decent price so i went with that.