Help with OT impedances

[thehoj]

Okay, so I have no idea why I'm struggling with this so much, but I'm working with an OT that doesn't exactly match up with my power tubes.

The OT in question is from a Marshall dal-401, which is designed for 4xel84's. I couldn't find out the specifics on it, but I'm assuming its roughly 4000ohm primary.

The amp I'm using this in is 2x6K6 at 285v with 400ohm shared cathode resistor, which according to the data sheet means I'm at 12000ohm.

So if I'm using the 8 ohm secondary tap, and hook up 12k to the primary side of the OT which is expecting 4k, then I would need to hook up a 24ohm load to that 8ohm tap keep the impedances matched. Correct?

If I run a 16ohm load on that 8ohm(24ohm) tap, then my primary side would see roughly 8k.. Does this seem right?

Is that a reasonable mismatch?

[thehoj]

And related to this, looking at this schematic, http://www.drtube.com/schematics/mar...02)%20iss9.pdf would you think w3 or w2 is the 8ohm tap?

[nickb]

And related to this, looking at this schematic, http://www.drtube.com/schematics/mar...02)%20iss9.pdf would you think w3 or w2 is the 8ohm tap?

W3 is the 16 ohm tap.

Yes, 3x the load impedance will give 3x the primary impedance.

As for the 4K guess, I'm not so sure about that.

[thehoj]

Why do you say that? Is that not typical for 4xel84?

W3 is the 16 ohm tap.

Yes, 3x the load impedance will give 3x the primary impedance.

As for the 4K guess, I'm not so sure about that.

[nickb]

Why do you say that? Is that not typical for 4xel84?

The DSL401 has 400v plate voltage and 40W out. I think the transformer might be nearer 3k ohm to achieve that, but I don't know for certain. The data sheets I see have 8K and 17 watts output for a single pair at 300V. That's why I added the caveat that I'm not sure. Not I think it's critical anyway. You can always measure the turns ratio easily enough.

[g1]

.
The OT in question is from a Marshall dal-401, which is designed for 4xel84's. I couldn't find out the specifics on it, but I'm assuming its roughly 4000ohm primary.

Sorry, but you can't assume the OT is designed for 4xEL84's. That OT (TXOP-00012) was used for the DSL401 (4xEL84), JTM30 (2x5881), and JTM60 (2xEL34). So which model it was actually designed for is just a guess.

Edit: This link seems to show it as being 6K: Marshall® 30 W Output Transformer JTM30, JTM60, DSL401 TXOP-00012 - Etronic Shop, Amplifier & Guitar Parts

[thehoj]

Poor choice of words.

Thanks for the link.

I guess my impedance will be right on then.. Using a 16 ohm load on 8 ohm tap.

Sorry, but you can't assume the OT is designed for 4xEL84's. That OT (TXOP-00012) was used for the DSL401 (4xEL84), JTM30 (2x5881), and JTM60 (2xEL34). So which model it was actually designed for is just a guess.

Edit: This link seems to show it as being 6K: Marshall® 30 W Output Transformer JTM30, JTM60, DSL401 TXOP-00012 - Etronic Shop, Amplifier & Guitar Parts

[Gingertube]

Yes,
16 Ohm speaker on the 8 Ohm tap will do the job.
Cheers,
Ian

[thehoj]

I'm kind of curious about nickb's guess that the OT might have a primary impedance of 3K... 6K is quite a long way from that.. That doesn't really seem right to me for 4xel84's.. Why would they use an OT like this for the dsl-401?

[Gingertube]

I'm guessing here but -
Standard Raa for a pair of Push Pull EL84 at B+ of 300V is 8K.
For a quad of EL84 at B+ of 300V you want 4K

"Traditionally" Guitar Amps have run somewhat lower Raa than the "ideal" to get a bit more power even though it is at higher distortion as well, so he may have assumed a value of something like 3K.

In this case , however, the B+ is much higher than than the HiFi value of 300V and so you need to INCREASE Raa to cope with that higher voltage (to keep signal currents within the tube capability), hence 6K rather than 4K.

Cheers,
Ian

[nickb]

I'm kind of curious about nickb's guess that the OT might have a primary impedance of 3K... 6K is quite a long way from that.. That doesn't really seem right to me for 4xel84's.. Why would they use an OT like this for the dsl-401?

Trust me I was curious too.. I have a little interactive widget thingy I wrote that allows me see the effect of various parameters graphically. Turns out I simply forgot to adjust correctly (in my head) for the parallel tubes. I made it even harder for myself because I used the plate resistance instead of the plate-to-plate resistance so there was another factor of four to contend with.

In such a case of unknown transformer I simply put a known signal on the primary (typically 100V rms) and measure the voltage on the secondary to get the turns ratio. I use an isolating transformer feeding a variac at 60Hz.

[thehoj]

Thanks for the info everyone. I ended up adding 2 more 6K6's, so my primary impedance is 6K now.. Perfectly matched for the OT. Amp sounds really great.
It's an older bogen preamp that I converted to a guitar amp. I decided to use the 6SF5's that were in it for the preamp section, as each one is a triode with the same gain factor as half a 12ax7. Pretty cool little amp.

[thehoj]

Just saw this post Voltage needed to drive power tubes

Interesting that the specs from Marshall indicate a 4.5K Primary impedance..
Either way I think I'm fine.



Although I'm also noticing in this post OT Image He posts an image of how he's wired the OT
And on the right side he has it wired as 16ohm at the top, 8ohm in the middle and ground at the bottom. On the amp I pulled this OT out of, w3 (which was indicated to me in this thread should be 16ohm) was connected to the middle lug, not the top lug.
Generally speaking about the physical layout of an OT like this, would it not make sense that 16ohm tap would be at the top, not in the middle?

As a very simple test, would measuring resistance between the common lug and either of the secondary lugs tell be a valid test? ie) higher resistance = 16ohm tap, lower resistance = 8ohm tap..

[g1]

As a very simple test, would measuring resistance between the common lug and either of the secondary lugs tell be a valid test? ie) higher resistance = 16ohm tap, lower resistance = 8ohm tap..

Yes, that is a valid test. The higher relative resistance reading will be the higher impedance tap.
However your meter will be giving very low resistance readings (likely less than an ohm), do it several times as there may be very little difference. A poor quality meter may not show a difference.

[nickb]

...... On the amp I pulled this OT out of, w3 (which was indicated to me in this thread should be 16ohm) was connected to the middle lug, not the top lug.
Generally speaking about the physical layout of an OT like this, would it not make sense that 16ohm tap would be at the top, not in the middle?
..

Another clue is that the 8 ohm terminal is a winding tap and you can see the two wires from the winding going to that terminal in your photo above.