The purpose of 6.6k Primary on Wreck OT's

[JamesHealey]

Why?

What's the point? what does this achieve? I like it, but I want to understand it more?

What are the reasons for it?


Is this a HiFi transformer spec?
Is the volume drop compared to say 3.5k Marshall OT the only reason?


etc.. etc.. real tech answers if possible.

[Structo]

Because that is what Kenny used?

Sorry, not very technical but from what I have read here, the people that stray from Kenny's designs are the ones that have the issues.

[lumox0013]

I think what he is asking is what are the effects of changing the primary. I have wondered this for a while not just for this design but in any for instance why is the rocket 4.3k and the liverpool 5.2? anyone who can explain, much appreciated!!!

[RJ Guitars]

Why?

What's the point? what does this achieve? I like it, but I want to understand it more?

What are the reasons for it?


Is this a HiFi transformer spec?
Is the volume drop compared to say 3.5k Marshall OT the only reason?


etc.. etc.. real tech answers if possible.

Output transformers are not really at a fixed impedance but rather are windings in a fixed ratio that matches up the impedance of the output tubes with the impedance of the speaker. The actual impedance of the output tubes is typically a few K ohms, but changes constantly. The specific rating is actually more like an average impedance. So when we say a transformer has an impedance of 6.6K ohms, what we mean is that it matches up a set of tubes with a rated impedance of 6.6K
ohms to a speaker of 4, 8, or 16 Ohms, depending upon which output winding we select.

6.6K is a typical specification for a pair of 6V6's and a host of other tubes, but even that number changes depending upon the operating point of that tube. This includes factors like the B+ voltage you apply, how the tube is biased, etc.. A pair of EL-34 tubes typically use an output transformer with an impedance around 4K ohms, but that is not really anything more than a good average value to work from.

The STANCOR transformers that were common in some of the early Trainwreck amps typically had a primary impedance of 5.2K or 6.6K Ohms. These were commonly used as HiFi transformers but were not exclusively set aside for that purpose. Many other companies produced transformers with similar specifications and the Heyboers that we associate with the Trainwreck builds are extremely similar to those STANCORs. There are several other AGF threads that give the bits and pieces of the transformer history... look for posts by rhinson.

Gerald Weber published a couple good pieces of info on matching up output transformers... one key item being that output transformers are not really precision or particular devices and if you match things up within 100% of their rated values, that is close enough to call it a good match and how it sounds is the ultimate indicator of how well everything matches up. So using that criteria, the difference between 5.2K and 6.6K isn't significant in the matching of things but has more to do with subtle differences in how things sound...

Plenty more technical info to be said if you are interested... but the bottom line for me is that the reason Ken Fischer chose the 6.6K value in the Express and the 5.2K in the Liverpool is that he liked the sonic results, the impedance match was plenty close enough.

thanks,

rj

[JamesHealey]

Weren't some Express' shipped with 5.2k primary especially the later ones? or were only liverpools 5.2k?

RJ I'd be very much interested in as much technical jargon as a man can take on the matter if you want to go into it.

[JamesHealey]

if you match things up within 100% of their rated values, that is close enough to call it a good match and how it sounds is the ultimate indicator of how well everything matches up.

by this your essentially sayin 8k and 2k would be suitable loads for x2 EL34 in the extremes?

[j-po]

The highish 6.6k impedance causes more screen current to flow when the tube hits saturation thus making the distorted sound rounder. The low B voltage helps the screens survive the larger current.

[JamesHealey]

so if u ran a wreck with a B+ of 490v and 6.6k Primary.. your screens would be burning up pretty fast?

[j-po]

I guess the screens would be glowing orange with a B of 490V. The Komet has voltages in that range. What reflected impedance and tube life I don't know.
If you want to use a transformer with a B of 490V you could always drop the screen voltage with a string of 5W zeners. Then you obviously would need to up the reflected impedance even higher than 6k6. A rough guess would be 490 plates, 360 screens and a 9k OT. You would end up with a more powerful amp.

[M Fowler]

JamesHealey,

Based on what I have researched there is no answer only basic guidelines.
Primary impedance is matched to the plate resistance. Some say the primary impedance for pentodes is 1/5th to 1/10th the plate resistance of the tube.

The primary impedance increases as the plate voltage rises.

I have built the TW Express with 4k in combo, 5.2k head and 6.6k head. I prefer the 6.6k for the Express. In other amps I go with recommended primary impedance known to have been used and established long ago.

Mark

[JamesHealey]

I guess the screens would be glowing orange with a B of 490V. The Komet has voltages in that range. What reflected impedance and tube life I don't know.
If you want to use a transformer with a B of 490V you could always drop the screen voltage with a string of 5W zeners. Then you obviously would need to up the reflected impedance even higher than 6k6. A rough guess would be 490 plates, 360 screens and a 9k OT. You would end up with a more powerful amp.

I've got a string of 5W Zener dropping the B+ on HT CT again, now it's running to wreck voltages again and comfortably.

[RJ Guitars]

if you match things up within 100% of their rated values, that is close enough to call it a good match and how it sounds is the ultimate indicator of how well everything matches up.

by this your essentially sayin 8k and 2k would be suitable loads for x2 EL34 in the extremes?

I can dig up the exact quote, but the gist of it is that generally nothing is going to burn up... that may not mean that it sounds good. You can get an idea on what the differences might be from those times you accidentally left your amp on the wrong tap and played away. Using the 4 ohm tap rather than the 8 ohm tap just changed that ratio by 100%... how did it sound?

IMPORTANT NOTE OF CAUTION - Gerald Weber said this and I believe that it is typically true, but it has some risk associated with it. An under-rated transformer performing on the edge of capability might just go away if you push it harder. Also, some manufacturers are already using up some of the 100% safety margin that he mentions. If you go 100% further, you might be too far outside the safe operational boundaries.

The comments on increased distortion and other tonal changes start to give you some insight into the subtleties and complexities of transformer selection. If you go one way and the distortion seems to be a little higher, can you expect the amp to clean up if you go the other way? These things can all be explained technically, but they have many interactive and dynamic properties and sometimes it's just easier to use your ears until you like what you hear... which may be different than the next guys preference. Ken Fischer was famous for swapping out transformers (all from the same lot) until he liked what he heard.

What I really think is the important thing to take home with you from this discussion is that the 5.2K and 6.6K differences are one slight difference in a transformer. How they are wound, what type of steel they are using, and many other of these things also matter a bunch. Once a good match has been established there is wisdom in following suit... thus the notion that messing with the Express standards might cause you grief.

I tend to learn stuff the hard way which I think definitely causes me grief but ultimately makes me wiser

rj

[John C]

Isn't the key word here "damping"?

[funkmeblue]

damping factor http://en.wikipedia.org/wiki/Damping_factor

[RJ Guitars]

Isn't the key word here "damping"?

damping factor http://en.wikipedia.org/wiki/Damping_factor

Thanks guys! A great technical reference... say more and help us connect the dots back to the original post? Can you put this into context for understanding the 6.6K Ohm Wreck standard... how does this relate to what our ears hear, what is the impact on distortion, frequency range, volume, etc..

thanks again,

rj