Preferred OT primaries impedance for different tube types

[Gibson924]

what are the preferred OT primaries for 4x6l6,el34,kt66,6550/kt88
And what effect does the impedance have on tone.

After scouring tube manuals and lurking on here for months I can't seem to figure out the operating points of these. I have found some people say that 4000 is prefered for 2 6l6 and others that say 5000. so would that mean a primary of 2k to 2.5k would work for 4 6l6's and what would be the difference? volume/power or tone?

Is there a fairly wide plate operating point and are there boundaries where you don't want to use a specific tube? when buying ot's from mercury or others there are some for 100w6l6 or 100wel34/kt66/6550. so there are overlaps, but what are the prefered values for each type

Thanks
this place is awesome I've spent the last few months reading every post on here and this is now one of only a few aspects of amp building I don't quite understand yet

[jelle]

As far as I understand it is there a region in which the tube works good. The chosen point will have an effect on volume and tone. Lower imp will give higher power and more bass in the sound. Marshall uses low imp and rolls off the bass after the pi with those 22nf caps.

The JTM 45 uses an primary imp that is right on spec according to the tube manual and gets away with 100nf after the pi. This amp puts out less volume because of this higher imp.

Just hook up an 8 ohms fender amp to an 16 ohms load and listen....the sound changes and the volume drops somewhat.

I'm sure there are many others here who can explain this in more detail...
I do not have the impedances here right now. Maybe someone else will post these?

Interesting topic BTW.
I once read that the ideal primary is dependant on the voltage on the powertubes. Does anybody know about this?

Jelle

[dartanion]

A transformer doesn't have any set impedance, just the turns ratio. Thus, whatever you attach the primary leads to will set the secondary impedance based on the turns ratio.

You usually use the plate resistance to figure out what an appropriate OT would be.

It is also true that mismatching output impedance does affect tone. Matchless did this on purpose for some of their amps.

[Funkalicousgroove]

general rule of thumb/ known to work values:

pr 6V6--8K
qd 6V6--4K
pr 6L6--4K
qd 6L6--2K
pr EL84--8K
qd EL84--4K
pr EL34--3.5K
qd EL34--1.75K

There are a couple of books that are of great help with matching tubes to transformers:

The RCA recieving tube manual which is available through Antique Electronic Supply as well as many other sources,
and the GE essential Characteristics manual which is avaiable at the same place.

There is a huge range at which tubes will actually operate, it all depends on what you are trying to make them do, for example, a Trainwreck Express runs a pair of EL34's at a 6.6K primary, and makes about 35 watts class AB with about 400 volts on the plates, but an Aluminum pannel Marshall JMP makes over 50 watts out of a pair of EL34's with a 3.5K primary and about 475 Volts on the plates.


I had an ODS that I built and used a Marshall PT, so It made about 495 plate volts. I stuck a quad of JJ 6V6's in it and hooked it up to my scope and dummy load just to see what I got:

using a 1Khz Test frequency and a Mercury DO100 (2.2K:4,8,16)

I hooked it up to an 8ohm load with the impedance selector on the 8 ohm setting. It put out a respectable 38 watts before the onset of clipping. When I changed the impedance selector to 4 ohms and left the dummy load at 8 ohms (Effectively doubling the primary impedance to 4.4K) the amp put out an amazing 52 watts before the onset of clipping.

The same amp put out 95 watts clean with a quad of Svetlana 6L6's, but made 110 watts with a quad of JJ E34L's.

So you see, there are no set "Rules" only guidelines and records of what has worked previously.

[stoo]

Hey funk*
where does a KT66 fall in here?
Stew

[Funkalicousgroove]

Technically it is a 6L6 Equivalent, I had a pair of GE Tan Base 7581's that said on the box 7581/6L6WGC/KT66.

The KT in KT66 stands for Kinkless Tetrode, and it is supposed to be a 6L6 without the "Kink" or dip in the mids.

Here is the original MOV (Marconi-osram valve Co.) Data sheet:

[Funkalicousgroove]

FWIW, The JTM 45 Output transformer didn't offer a lower impedance than 6.6K, the kt66 designation and the 6.6K output impedance are a mere coincidence, originally the JTM45 was intended to have the Tung-Sol 5881, but due to their scarcity Ken Bran opted for the English Made KT66 which was an equivalent. When the price changed on the KT66, and production went down they switched to the EL34 because they were locally made, inexpensive, and readilly available, they also switched to the Drake Output transformers because they were less expensive than the radio-spares, and a more specialized transformer for the application. The RS in JTM45's was more of a utility/replacement transformer that could be wired for MANY different primary/secondary impedances.

[David Root]

Thanx, Funk for that data sheet. I have one from '77, the original (as usual) is better!

I have a 5k6 nominal OT I spec'd for a pair of KT-77s, using a PT with a 640 CT plate winding that gave me Vp 400V idle and only about 35W AFTER I switched the tubes to 6550s, forgetting that Vsat for a 6550 is 40V higher than a KT77 (cf EL-34) (duh!). I am now getting a new PT,
400-0 non-CT which I'll put thru a bridge rectifier for about 540 or so B+, about 490-500V Vp @idle.

I need to figure out the new nominal impedance of the existing OT (which I spec'd for 50W 20-20Khz) with the 6550s. I'm expecting to get about 55-60W or so prior to clipping. Is that reasonable?

[Funkalicousgroove]

what OT is it? with 500 plate volts under load and a 4K or so primary a pair of 6550's should put out at least 60W providing the OT is big enough.

Do you like the tone of 6550's for guitar? I find them a bit hard sounding myself, JMHO, and tend to lean towards KT88's myself.

[David Root]

I'm conservatively expecting about 460 or so Vp at full load, not 500. I custom ordered the PT from Trafomatic. It's rated 50W @ 20-20kHz bandwidth, so it should be OK for 60W+ with good bass too.

I'm using 6550s because the amp has an FX loop and 6550s are relatively flat frequencywise and low distortion, i.e. tonally "neutral" so they won't color the FX tone, also it's a 4-stage hi-gain preamp, which a lot of hi gain to metal players like in combination with 6550s, and in Marshalls at 3k4 Zpri. I just put in the higher Zpri to see what happens, lose a little odd harmonics by doing so, perhaps to improve articulation at high output levels. We shall see.

[Clyde]

Hi Funk,
I've heard the JTM 45 ran the KT66's on the 8K ohm tap of the multi-tap
Radiospares TX. Is this true and wonder what it would do to the tone?
I know it'd drop watts a bit though.

[David Root]

Funk seems to be offline for a bit. Yes, they used the 8k tap. I have heard so much about it I'm using KT66s with the 8k pentode in my next build. But different preamp, still in the thinking stage.

The 6k6 is a UL tap, so will be cleaner but probably about the same power. (UL is lower power than pentode mode, but 6k6 gives more output than 8k).

The 9k tap on the RS Deluxe was intended for EL84s. I imagine the AX84 guys use that one.