Output Transformer Questions

[skyboltone]

the load determines the reflected impeadance that the tube sees
ie...... 6.6k and a 8 ohm load is a ratio 825 to 1..... so at 4 ohms the
reflected impeadance is 3.3K which may show sonic benifits of a lower impeadance
like a wider bandwidth..... or the sweet spot for the tube type
or the preferd tone for the ear of the beholder

I wonder why tube manufacturers/ design engineers recomend specific load impedences? I wonder why they post performance curves based on those impedences? What is a practical lower limit for plate load? Granted that some tubes are routinely used at extremes; say 6L6s at 4400 instead of 5000 or 6V6s at 6000. KF used 5200 for a quad of EL-84s but that's off the charts on the high side. Or with Ginger he used 6600 for the quad. That computes really to 13200ohms per pair at 325VDC on the plates. With cathode bias the curves show something more like 8800. If this is a new design paradigm I submit we ought to try a set of DC blocking caps at the plates and just plug it right into the cabinet. 8 ohms PP at say 0 output. Now that's bandwidth; +/- 0db from 0 hz to infinity.
I'm sorry, I just know that tubes are designed to push a specific load and most designers stay pretty close to that ideal. They build the circuit around the tube rather than forcing the tube to behave in a strange environment. I maintain that any 6CA7/EL-34 that sounds mushy at 6.6K PP load has some other handicap working. Especially an Express!! There are lots of ways to make an amp mushy in the low end, but I'd start with the preamp and tone stack first and move forward. Not just half the load impedence and power output of the output tubes. Come to think of it I've never heard an EL-34 mushy or flabby. Usually they come on pretty crisp. British tone and all that right?

[lewilson]

I have 373 volts on the el34s, 178 at the first triod, 202 on the second, 254 on the third and around 180 on the phase inverter. to me it sounds a mucky in the bass when I do not load it right. When I first built it I was running the el34 at 6.6 at was tuning the amp. Sounded o.k. but I was lowering grid resistors values and lowering kathode caps to get it more round and punchy with more headroom, trying different speakers. It would sound good with one guitar and not another. Then I ran the el34s at 3.3 and thinks became much nicer.I could put thinks back to the original trainwreck values and now it sound good with all my guitars. I think I waisted some time trying to tune it without the proper load. Could be my transformers? I didnt use the exact voltage or current ratings Ken did.

[skyboltone]

I have 373 volts on the el34s, 178 at the first triod, 202 on the second, 254 on the third and around 180 on the phase inverter. to me it sounds a mucky in the bass when I do not load it right. When I first built it I was running the el34 at 6.6 at was tuning the amp. Sounded o.k. but I was lowering grid resistors values and lowering kathode caps to get it more round and punchy with more headroom, trying different speakers. It would sound good with one guitar and not another. Then I ran the el34s at 3.3 and thinks became much nicer.I could put thinks back to the original trainwreck values and now it sound good with all my guitars. I think I waisted some time trying to tune it without the proper load. Could be my transformers? I didnt use the exact voltage or current ratings Ken did.

I'd have to check LE but the PI plate voltages don't look right to me. Have you verified that you've got all the right values in the PI resistor network? You might try several different tubes in the PI spot, I've had 12AX7s with some weird stuff going on inside that will pull the plates down like mad. What's the voltage on the PI cathodes? The reason I say that the voltage is off is because the PI voltage node on the dropping string is #3. Just by nature of a dropping string you wouldn't expect to see a lower voltage there than on tubes further down the line.

[lewilson]

good point. I will chek it again .

[lewilson]

Ive got 30 vdc at the pi cath. The plate voltage there is 198/194 not 180. My mistake

[skyboltone]

Ive got 30 vdc at the pi cath. The plate voltage there is 198/194 not 180. My mistake

Something's wrong. PI cathode should be over 50 and PI plates in high 200s.

[lewilson]

My power transformer is lower voltage than stock. I have 276 volts at b+3
The pi is drawing nearly 1ma per plate. Seems correct

[skyboltone]

Ok.

[rooster]

OK, Andy, thanks for the input. I run the TSluts OT and I use the 6.6K taps. So I wonder how everybody else is using the 6.6K tap with EL34s - are you using the 4 ohm OT tap on an 8 ohm cab, for example? I did read that Glen has the 6.6K primary on his original and two clones....Glen? Do you make this adjustment?

As far as impedance goes and various tube and OT manufacturers *suggestions*, this is the more *correct* thing to do, given an EL34 tube. Meaning given a 6.6K primary, change the OT load (4 @ the OT to 8 @ the speaker cab) to give the EL34s 3.3K impedance.

But you know what? KF DOES NOT make his labels to read "8 - 16 - 32" on the Expresses that are using the 6.6K OTs - something that would have to happpen if he intended such a match. ...Or did he just assume people would know this? ...Or did he mean for his customers to use either 6V6s or EL34s with the same OT taps? ...Or did he even care?

For a guy who got so anal about even the wire in his amps, it strikes me as odd that he skips this issue. Or did he and I missed it?

[geetarpicker]

Yes my original '89 Express has a 6.6k primary Pacific, and so do my two clones of her. I find the amp sounds best matched like the ohms label says.

I do however run my '68 Superbass 100 Marshall with the offset decribed as it just runs the tubes easier that way and seems happier in the red plating department with modern tubes.

[rooster]

Double post, sorry

[rooster]

Glen - Well, you got TONE so I won't argue with that. Thanks for weighing in.

When you have a tube amp that is built around an impedance that compliments the specs of the given pwr tube , if you go to the high side with the load, i.e. the amp says 4 ohm and the cab is 8 ohm - regardless of volume increase or decrease - you are doing the pwr tubes a favor. So this is the worst thing that is happening with an Express when you run an 8 ohm cab on the 8 ohm tap.

On the other hand, if you go to the low side, i.e., extreme case, 16 ohm at the amp and 4ohm at the cab, then there are all kinds of bad things that can happen - and especially with an EL34. (O'Connor's books are full of this type scenario, BTW.)

So the question remains: exactly what is happening when you run the 4 ohm tap into an 8 ohm cab on the Express with the 6.6K OT? I don't know. It seems like it would be close enough for gtr playing actually. ..........But then I think about how loud the amp is already, and then there's those expensive Mullards, and then there's Glen's documented TONE. Hmm, is this really a hard decision?

[lewilson]

The tubes would see 13.2k impedance by running a 8 ohm load on a 4 ohm tap with a 6.6 ot ?

[Andy Le Blanc]

6.6k \ 8 .............. = 825:1

6.6k \ 4................ = 1650:1.......x8 = 13.2k

an output tranny is a matching device
know your load
Ive project in mind to use a 100w marshall tranny to idealy match a pair
of 6550 that does just that.......connect an 8ohm to the 4ohm tap
it works out to be really close to RCA values and has a lot of extral iron
in it so I can bias up to design center maximums and may be get the most
out a pair of 6550...... its a useful bit of info
mind the minimum impeadance ..... so if its biased at eight you can go up
but you dont want to go down...... without proper matching

[rooster]

Andy - I really don't understand your data? 3.3K is the primary impedance of a 6.6K OT with its tap set at 8 ohms - with a 4 ohm load applied. Likewise, 3.3K is the primary impedance of a 6.6K OT with its tap set at 4 ohms - with an 2 ohm load applied. Likewise again, 3.3K is the primary impedance of a 6.6K OT with its tap set to 16 ohms - with an 8 ohm load applied. Etc., etc..

At 400VDC plate, an EL34 pair running in Class AB1 is looking for an impedance of approximately 4K at the primary side of the OT. Hence it follows that an OT with a primary of 8K running the same scenario above would be looking pretty perfect with an 4 ohm load being applied to the 8 ohm tap.

Where are you getting this 13.2K impedance and what does it mean as we talk about swapping loads on the 6.6K OT used in the Express?