Small NOS OT - Question

[kagliostro]

Reading the Silvertone 1449 thread we have talk about the OT, so I remembered that a friend give me, some time ago, a small NOS NIB OT

and looking to it it is exactly 7K primary 3.2ohm secondary

the core is 17mm x 18mm

The measures are:

Large 54mm - Tall 46mm and the OT weight is 350 grams

wires seems to be 0.5-0.6mm diameter

(sorry for the decimal measures instead of imperial)

Do you think it can be used for a Champ or the OT is too small ?

How many W can I consider it is rated ?

Thanks

Franco

[PRR]

> weight is 350 grams

Compare with similar parts. I looked in Hammond. 350g is 0.66 pounds.

1750C Output 7W Champ 4.00 lbs
1750B Reverb Driver - Vibro King (6V6/6BQ5 @ >400V) 10W 1.00 lbs
1750A Output 3.5W Fender 12AT7 Reverb Driver 0.40 lbs

Weight also goes with bass response. The ssame iron might do 8W clean at 160Hz but 2W clean at 80Hz. This is probably why the "10W" reverb driver is smaller than the "7W" loudspeaker driver.

So I do not know what to say. It may be several Watts if there is not much bass, sub-Watt with heavy bass.

[DummyLoad]

use it for 6BM8/ECL82 reverb tank driver.


--pete

[kagliostro]

Many Thanks PRR & DummyLoad

Looking to the measures of the 1750A seems this can be a 4 - 4.5W OT

I was thinling to ask for a tube and B+ to use it

and the ECL82 @ 272V on 8K gives 3.5W ..... very good !

There is a single tube that can be a sub for the ECL82 pentode for the use with this OT ? 6K6 ? Others ?

Thanks

Franco

[DummyLoad]

use a 6V6. :-)


--Pete

[kagliostro]

   THANKS

OK, but you must explain me a thing

6V6 with 315V B+ give 5.5W on a 8.5K load

5.5W is a bit high respect to the estimated power of the OT

which is the right way to tame the power ?

a lower B+ (remaining on that "high" load value) or cathode resistor adjust ?

Thanks

Franco

[DummyLoad]

drop B+ and drop idle current. use a 6K6 and you'll be right where you want to be with 250V B+. without the specs of max bias current allowed for that transformer, you're really shooting in the dark. tune the driver so the output doesn't make 5W. use the ECL82 - should still plentiful in EU.

another thing to consider is that pics show that it's 7.5K ohm into a 3.2 ohm load - so 8 ohm tank load will be about 17K anode load.

--pete

[kagliostro]

Grazie ancora Pete

Franco

[PRR]

> ECL82 @ 272V on 8K gives 3.5W
> 6V6 with 315V B+ give 5.5W on a 8.5K load

Power goes as Square of voltage.

272V/315V is 0.86. 0.86 squared is 0.75. 0.75 times 5.5W is 4.1 Watts. Not so different than the 3.5W claimed.

_I_ think you should be nearer 200V and couple-Watt expectation. You can still use 6V6 at this level. It's like using a Chevy V-8 in a small boat. It is readily available cheap and is not strained.

> which is the right way

Same as Right way to play. With your teeth. On fire. Upside down.

[kagliostro]

Grazie PRR

Same as Right way to play.

unfortunately I'm not able to play 

--

Pete told 6V6 and I replied 6V6 but I was thinking 6П1П or 6AQ5

Franco

[DummyLoad]

Grazie PRR

Same as Right way to play.

unfortunately I'm not able to play 

--

Pete told 6V6 and I replied 6V6 but I was thinking 6П1П or 6AQ5

Franco

not really that critical what tube (9pin 7pin, small 8pin, etc.) you use with that OT. with 250-300V B+ with that load will work just fine. just bias accordingly. calculate max Pdiss, estimate bias point with datasheets and calculate Rk. start higher, work your way down to Pdiss ~~=80-90% max Pdiss.

EL95 is your EU 6AQ5 equivalent.

--pete

[PRR]

And 6AQ5 is just a small 6V6. Ratings a bit lower.

But if you have a 5W Pdiss 250V pentode you can use it.

Even (ahem) two parallel 6AU6 or other sharp-cutoff radio IF tubes.

[trobbins]

K, can you see the laminations enough to check if it is layered up as a transformer or with some air-gap?


It 'looks' like 7k PP with CT from wire size and assuming a single secondary winding.



Do you have enough test gear to check primary PP inductance - eg. apply 12VAC across primary from mains transformer and measure AC current ?

[kagliostro]

Ciao Tim

Thanks

I can't say if there is an air-gap because laminations (as far as I can see) are a kind of F laminations not the more standard E-I laminations



they are something like this (I can't see the core ofcourse)



So, if there is an air-gap it can't be seen

I'm 99.99999% sure it is an SE OT, you see 3 connection on the primary because the transformer has a part of the primary devoted to act like an incorporated choke, I've seen this configuration on many old recorder, radio and record player, yesterday I measured the resistance but today I didn't remember the values (the whole primary is  around 400R and the real primary is around 370R - the "choke" is the difference), the secondary is single so, 2 connections only

See this Grundig Como (Stereo Radio) schematic

Do you have enough test gear to check primary PP inductance

I've an impedance meter, but don't know if the measure is on the range of the instrument, I'll try and report to you

.... apply 12VAC across primary from mains transformer and measure AC current

This is necessary anyway or only if the impedance meter is out of range for the measure ?

(12VAC to the primary I suppose and no load on secondary ..... or you mean something other ?)

Thanks

Franco

[trobbins]

The old way to measure primary winding inductance was to measure the AC current (down at the 100uA to mA level) using a mains frequency low voltage supply - circa 5 to 20VAC - then use V = I.Z.  That is a much better way to measure transformer inductance, as the applied voltage affects the inductance level, and some inductance meters just use a low voltage level.  I guess you may measure something greater than 10H if there is no designed in gap.  And yes, all other windings are open.

Measuring turns ratio and checking if the impedance ratio is correct would be a good idea - but best to apply a voltage across the 'choke' part of the winding, so you get better accuracy of the 'primary' and secondary windings.

[kagliostro]

OK, summarize

Label say:

7K primary 3.2ohm secondary

Measures are:

Large 54mm - Tall 46mm and the OT weight is 350 grams

core is 17mm x 18mm

wires seems to be 0.5-0.6mm diameter

Resistance Measures: (Measured with a specific ohm meter, not the usual tester)

Primary ~395ohm - "Choke" ~16ohm - Total resistance ~410R

Secondary 0.395ohm

Impedance:

Primary 6.93H - "Choke" 10.39H - Total 7.46H

Secondary 4.31mH

Current flowing on windings (at 14.3V):

Total (Primary + "Choke") = 2.43mA

Primary = 2.58mA

Same measure with one other tester

Total (Primary + "Choke") = 2.44mA

Primary = 2.61mA

"Choke" = 0.59A (voltage Drop from 14.3V to 13.3V - the 12V PT I used is rated at 20VA)

Voltage Measures:

14.3v on Primary = 348.7mV on secondary

14.3v on Primary + "Choke" = 336.5mV on secondary

8.40v on Primary Secondary = 320v on Secondary Primary

8.38v on Primary + "Choke" Secondary = 331.1v on Secondary Primary + "Choke"

Impedance measured with CESVA MPI-3 (Line impedance meter):

3.2R resistor connected to secondary = 7K11ohm on Primary (measured at 1000Hz)

Are this data useful to understand better the spec of this OT ?

Thanks

Franco

[DummyLoad]

8.40v on Primary = 320v on Secondary




is that reversed? 320V on primary 14.3V on secondary?

as already stated: you have a 2-3 watt part based on the dimensions.


for reverb, copy a fender AB763/6G15 driver and recovery. use your transformer with an 8 ohm tank.

or build PRR 1/3 watt with 8 ohm load. <<< 


--pete

[kagliostro]

Thanks Pete

I corrected the post (it was 8.40v - 8.38v on secondary, the PT I used is only 20VA and the voltage dropped from 14.3 till 8.38 with that load)

OK for the W of the OT and for possible uses, the data I posted were in response to Tim questions/suggestions


Nice idea also the 1/3W

Franco

[trobbins]

K, I think a few of the reported values need some 'corrections'.


I have assumed your mains is 50Hz.  If you have 14.3Vac across the total primary winding (choke + pri), and measure 2.44mArms through the total winding, and have 410 ohm total resistance, then the total inductance is about 9.3H.


If you have 14.3Vac across just the 'primary' part of the primary winding (not including the choke), and measure 2.58mArms through the total winding, and have 395 ohm resistance, then the total inductance is about 8.8H.


If you put 14.3V across just the choke part of the winding, then that's not good, as you have saturated the choke and may well melt the wire 


The 'choke' portion of the total primary winding is then about 9.3 - 8.8 = 0.5H.


It is going to be a bit tricky to get a good impedance ratio measurement.  Using just the primary voltage of 14.3 and .3487V on secondary, gives about 7K to 4.2 ohm ratio.  But the magnetising current in the primary winding introduces a small error.  My only suggestion (if you are keen) is to apply a low voltage like about 3.15VAC (or something easy to get on the bench) across just the choke portion, and measure the AC voltage across just the 'primary' portion (not including the choke), and across the secondary - that will remove the magnetising current from the windings of most interest.

[kagliostro]

Ciao Tim

Thanks

I'll look for a ~3V AC source, then I'll try again

Franco

[trobbins]

6.3VAC heater with CT will give you 3.15VAC nominal 

[kagliostro]

Yes, I'll find one (I've more of one for sure)


Franco

[kagliostro]

Here I'm Tim

I connected 3.4V to the "choke"

there were 150.2mA flowing on the "choke"

I connected the Amperometer to the primary pins and there was a current of 7.6mA flowing

I connected the Amperometer to the secondary pins ant there was a current of 173.3mA flowing

(I hope I didn't do something wrong ???)

Franco

[trobbins]

That test is for voltage on the windings, which then allows the turns ratio to be calculated, and hence the impedance ration.

[kagliostro]

My only suggestion .......  is to apply a low voltage like about 3.15VAC ...... across just the choke portion, and measure the AC voltage across just the 'primary' portion (not including the choke), and across the secondary - that will remove the magnetising current from the windings of most interest.

So I was wrong, I measured current instead of voltage

OK I'll do that one other time

Franco

[kagliostro]

@ Tim

This are the new measures

3.5v on "choke" = Secondry 1.158v - Primary 47.5v

3.5v on Primary = Secondary 87mV

3.5v on Primary + "choke" = Secondary 84mV

3.5v on Secondary = "choke" + Primary 149.8v - Primary 144.5v

Ciao

Franco

[trobbins]

K, there is a spreadsheet (if you have MS Excel or a converter) to assist inserting the values and doing the calcs at:
http://www.dalmura.com.au/projects/OT%20calcs.xls


The 47.5 and 1.158V measurements indicate a 7k to 4.2 ohm ratio.
The 144.5 and 3.5V measurements indicate a 7k to 4.1 ohm ratio.
The 149.8 and 3.5V measurements indicate a 7k to 3.8 ohm ratio.


If you don't want to use the original configuration, then you could use the entire primary winding as an SE primary.   But you may find there is a benefit from using that choke portion, as it biases the core to allow more SE primary winding flux swing so you may notice a better low frequency response, and/or use it for a higher power output.  A quick calculation of ampere turns in the SE primary and in the choke portion of the winding would indicate how much 'bias' actually being achieved, as it may be practically insignificant.

If you have an SE output stage wanting 7k primary (eg. 5-10k range) then 4 ohm speaker is easy - you could also use an 8 ohm speaker if you needed higher than say 10kohm primary impedance.

[kagliostro]

Thanks for all this info

And for the xls file link 

I'll read all slowly later, now are 01:45 AM here, bed reclaims me

Grazie

Franco

[Raybob]

> weight is 350 grams

Compare with similar parts. I looked in Hammond. 350g is 0.66 pounds.

1750C Output 7W Champ 4.00 lbs
1750B Reverb Driver - Vibro King (6V6/6BQ5 @ >400V) 10W 1.00 lbs
1750A Output 3.5W Fender 12AT7 Reverb Driver 0.40 lbs ...

Wonder why Hammonds are so heavy.  I have a Fender replacement OT for Fender Champ from CE Dist. that weighs 0.6 pounds.  It's a little larger than a reverb driver.

[PRR]

> 0.6 pounds.

I was wondering also. 4 pounds seems like a lot for a cheap amp.

So maybe K's OT is a near Champ.