Calculating primary impedance on my original Express, help!

[geetarpicker]

Ok, most of you have heard my original '89 Express "Orphan Annie" in my clips and CD. I'm thinking about making a couple clones of her to use for live gigs so I won't worry about her. However I still have yet to play a clone that captures her sound AND her "feel" (the hard part) but having her right there on bench will surely make it more realistic to give it a shot...

My amp is of the later black "Pacific" variety.

Keep in mind I don't want to unsolder anything in my original amp. Truly I'm not even sure if this can be tested "in the circuit" without unhooking something (like the feedback loop, or pulling tubes, using an outboard test tone circuit, etc)...but here it goes.

So I made a small audio clip of an exciting sound, 60hz AC hum. I rolled off all the highs, and it looks like a nice 60hz sine wav in "Sound Forge" to me. Hooked up to the amp input I raised the gain (keeping it well below clipping) until I measered 1v AC output at the speaker jacks. The amp is hooked up to a 16ohm cab, and I isolated my Fluke Model 112 meter with an inline .1uf cap on each lead.

With a 1v output at the speaker jack w/16 ohm speaker load, I measure a primary input AC voltage where the leads come off the output tube sockets of 22.2v. I also tried the same test with a higher 5v output, and came up with a very similar ratio. I could hear that I was well below the saturation level of the amp, as the hum was still clean so I would assume I was not saturating the OPT transformer at all.

So, if I square that number 22.2 x 22.2 = 492.84 (impedance ratio)
Then times 16 (ohms of Marshall cab) I get a primary impedance of 7,885 ohms.

Seems high to me compared to published specs I've heard. Anyone done a similar test of other applicable trannys tested specifically at 60hz?
Just for grins I measured the DC resistance of the OPT primary, right from the tube sockets and got 107.4 ohm.

Maybe my backyard test procedures are WAY off?

[Jackie Treehorn]

Well, I tried your method with an unconnected Heyboer TW-11415 output transformer. I used my Fostex tone generator which was putting out .975 volts ac. Applying that voltage to the primary, and measuring the 8 ohm tap, I got .0335 which after the math gives 6776 or so. Now, I thought this was a 5200 transformer?!? So, I tried the same procedure on a custom made 5.2k Heyboer I had in an amplifier. When I measured with an 8 ohm speaker attached, it came out 5.8k or so which I knew was likely wrong. However, with the speaker disconnected and without the jacks shorted, I got 5211. So, the short answer is, don't measure with the speaker connected.

Also, sound forge can make perfect sine waves. It's under tools -> synthesis -> sine wave, I believe. You could feed the sine wave at 1 volt directly on the transformer and measure the smaller voltage on the secondary. You don't even need to turn the amp on.

So, for the past 3 and a half years I thought I had been using a 5.2k transformer! It appears to be a 6.6k...

[geetarpicker]

Thanks! I'm learning.

I think I got it. And thanks for the Sound Forge tip, I thought it did that but it was kinda hidden. Anyway I pulled the power tubes (probably not needed) and put a 1v 1khz signal to the primary of the output right at the tube pins. A headphone signal off my computer worked and Soundforge allowed me to dail the level in to just exactly 1v while connected. Then I got .05v (sometimes flipping to .049v) at the speaker jack, with nothing else connected. Just for grins the same test at 60hz got me the same readings...

So that nets me a OPT turn ratio of 20.
An impedance ratio of 20 x 20 = 400
And a net primary impedance @ 16 ohms of 16 x 400 = 6.4k

Sounds reasonable.

I think you are right that the speaker loading (and tube loading?) made my turn ratio measurements not quite correct in my first tests. I always first seem to try things the hard way. Thanks.

[Allynmey]

Glen, the earlier Expresses used the 6.6K tap (some Stancor 3801 and some custom). Later Ken used 5.2K for both the Liverpool and Express for a more aggressive sound. The tranny in yours (and francesca) were made for a more Bluesy sound....and slightly warmer. To get it exact, I made a tool that plugs into the power tube sockets and plugs into a variac. I measure the voltage in and voltage out (with the center tap unsoldered). When I get exactly 1VAC on the secondaries, I measure the voltage on the primary (variac) and do the math. It is usually spot on. Of course the trannys have about a 20% tolerance so you will only know what yours is and not what that factory made it to be.

Allyn

[tsl602000]

Later Ken used 5.2K for both the Liverpool and Express for a more aggressive sound. The tranny in yours (and francesca) were made for a more Bluesy sound....and slightly warmer.

Allyn

Sorry for my off topic question... the 6K6 version was used for the "more bluesy and slightly warmer sound"?

[paulster]

According to my notes that I've accumulated the Pacific transformer used was the 25525 which was 6K6 only, so this also corresponds with Glen's findings.

[geetarpicker]

Speaking of Pacific. If you call them and simply ask for output transformer part #11415 will they still make one for you?

I'm 90% sure that is the proper Pacific #.

Or have they moved on to bigger fish these days and require large minimum orders?

[paulster]

Glen

My specific note about the Pacifics reads as follows:
Pacific (1-800-325-0501)
PT 25475 $98.29
OT 25525 $79.15

The fact that I have prices listed suggests that they either do (or did). Worth the phone call I reckon.

[riscado]

Hi Glen, isn't there a schematic of your amp running around the web?

I've seen "orphan annie" talked about in the old firebottle foruns...

[geetarpicker]

Not that I know of. But that doesn't mean something's not out there! Don't put this in stone, but I seem to recall mine isn't any different than Francesca but I need to really check for sure.

[riscado]

Glen check out the begining of page 7 of that 2003 express thread... "little orphan annie" is mentioned as an express from which a copy of the circuit was taken, here is a little quote:

Even the exact clones report the same, Callahan amps were based off of Little Orphan Annie(one of the schems we have-can't remember which one) and from what I've heard and read they sound nothing like a 'Wreck.

link to firebottle ampage express thread

that's why I wondered if this was your amp

offtopic: While I'm at it Glen, I can't thank you enough for the great express clips you've been posting, but another amp I'd really love to see a desmonstration from you is one of you non-reverb early blackface deluxes, I love those amps and would love to see you do a demo one day if possible.

thanks

[drz400]

Well, I tried your method with an unconnected Heyboer TW-11415 output transformer. I used my Fostex tone generator which was putting out .975 volts ac. Applying that voltage to the primary, and measuring the 8 ohm tap, I got .0335 which after the math gives 6776 or so. Now, I thought this was a 5200 transformer?!? So, I tried the same procedure on a custom made 5.2k Heyboer I had in an amplifier. When I measured with an 8 ohm speaker attached, it came out 5.8k or so which I knew was likely wrong. However, with the speaker disconnected and without the jacks shorted, I got 5211. So, the short answer is, don't measure with the speaker connected.

Also, sound forge can make perfect sine waves. It's under tools -> synthesis -> sine wave, I believe. You could feed the sine wave at 1 volt directly on the transformer and measure the smaller voltage on the secondary. You don't even need to turn the amp on.

So, for the past 3 and a half years I thought I had been using a 5.2k transformer! It appears to be a 6.6k...

Dont forget that you have to measure after you hook the leads up
You cant measure first and then hook it up, the transformer will load the generator.

[Richie]

Glen check out the begining of page 7 of that 2003 express thread... "little orphan annie" is mentioned as an express from which a copy of the circuit was taken, here is a little quote:

Even the exact clones report the same, Callahan amps were based off of Little Orphan Annie(one of the schems we have-can't remember which one) and from what I've heard and read they sound nothing like a 'Wreck.

link to firebottle ampage express thread

that's why I wondered if this was your amp

offtopic: While I'm at it Glen, I can't thank you enough for the great express clips you've been posting, but another amp I'd really love to see a desmonstration from you is one of you non-reverb early blackface deluxes, I love those amps and would love to see you do a demo one day if possible.

thanks

Glen owns a few of those,so maybe he'll post something from one of his amps.

[novosibir]

Measuring the ratio of an OT, installed in the amp has to be done with open secondary and also requires to disconnect the NFB loop. Although the 47K or 100K NFB resistor in series with the 4.7K, what's electronically connected across the secondary, is a nearly negligible load, though it's influencing the reading.

I'm usually measuring the OT's ratio with a maybe fairly tough method. I disconnect the primary's CT and the NFB loop, pull out the output tubes and apply the pure wall voltage across the primary - fused of course. Keep in mind, that since I live in Germany, that it's even 230 VAC

I'm additional measuring the induction's current by including a mA meter in the supply voltage, what let's you in addition with a primary inductivity measurement with 120 Hz and 1 kHz determine the used sheet metal quality for comparision's purpose.

The above mentioned measurement I'm by principle doing on every OT, I get for building amps - and since meanwhile I do have nearly 20 years experience in building tube amps, I already know fairly exactly before, how an OT will sound in the finished amp, although I haven't heared it yet.

The funny thing with OT's btw. is, that when you get 5 or 6 OT's from a transformer company - and though they're all from the same run - each OT is different to the other.

Larry

[dartanion]

Pacific is still in business?!?!?!

I thought that they would have went under YEARS ago. My MUTANT18 watt amp that I built a couple of years ago was from an old mono HiFi amp that has Pacific transformers in it. That OT sounds better than most RS clones out there. I'm going to have to call them to see if they still make that set. Anyone know if they have a website? I will have to google it.