Electric Radio / Hammond modulation transformer

[AB2EZ]

Is anyone planning to try one of these Hammond (special production run) modulation transformers?

http://www.ermag.com/index.cfm?v_link=product_detail&v_key=638

I've had good results with the following two configurations of modulators using Hammond products in external modulators for my Ranger.

Configuration 1: Using a backward-connected Hammond 1629 SEA (single ended output transformer) in combination with a modern, low impedance (nominally 8 ohms) output, audio amplifier. The 1629 SEA weighs 11 lbs... which is much more than the Electric Radio / Hammond mod transformer weighs

Configuration 2: Using a Hammond 193C 20H 100mA-rated choke (about the same physical size as the Electric Radio / Hammond mod transformer) as a Heising choke, in conjunction with a backward-connected  Antek AN-0212 230V:12V ferrite/toroidal 19.2:1 step up transformer, and a modern, low impedance (nominally 8 ohms) output, audio amplifier, in a Heising configuration

This ER/Hammond transformer seems a little marginal ... but it probably would fit inside a Ranger as a replacement for the stock Ranger mod transformer (Note: it is not exactly the same size and form factor as the stock Ranger mod transformer). It would be interesting to see how it performs... compared to the stock Ranger mod transformer... particularly as a prospective replacement for a burned out stock transformer.

Stu
 

[W3GMS]

Stu,

I heard Janis, AB2RA saying she had purchased one.  From what Hammond said, it looks like they are selling so many and then that's it.  I guess you would call that a "limited production run"!   So far, I have never heard one on the air.  It would work great in the Ranger if it would fit.

Joe, GMS

[Chuck...K1KW]

It looks like you would have to use a mod reactor with this ER/Hammond transformer!  Examine the laminations in the picture and you will see that they are cross laminated, with no air gap to limit core saturation with DC current going through the secondary.  If all they did was to change the output winding to accomodate modulated stage impedances instead of speaker loads, it will not handle any significant DC, certainly not 100 to 150 mA that typical plate current might be.

What do you think?

Chuck 

[AB2EZ]

Joe: Thanks for the lead on Janis... AB2RA

Chuck:

I, too, am concerned about the transformer's ability to handle unbalanced DC in its secondary winding.

The ER description includes the following paragraph:

"They are modeled after the Hammond 1615 line, please see http://www.hammondmfg.com/1608.htm for the specifications. They are basically 15 watt audio transformers with 100 mA DC ratings, push-pull service for tubes such as 2A3, 6A3, 6AQ5, 6B4G, 6L6, 6V6, etc."

But the reference to 100mA DC may be from the primary winding's center tap to each push-pull primary side tube (i.e. balanced DC in the two halves of the primary winding). The schematic diagram of the transformer also suggests this.

I think I will send an e-mail to Ray Osterwald to see if he has more information on the unbalanced DC capabilities (if any) of this mod transformer. Likewise, I will send an e-mail to Janis (AB2RA).

Stu

[kb3ouk]

Why would they have based it on a push-pull audio transformer when they make single ended transformers that already are gapped to handle unbalanced DC, which would've been more appropriate for use as a mod transformer than the push pull one, just change the windings.

[N4LTA]

This is just their largest single ended transformer (125GSE) with a modulation winding instead of a speaker winding. I asked their engineering department to do this 3 years ago and make it a standard product.( I am a Hammond distributor) but they told me a single unit would be $500 or more and they did not see a market for doing them as a standard product.

The transformer is likely a very good product and can likely do more than 100mA DC. I have used the 125GSE driving the 8 ohm side with a solid state amp and it works well as a modulation transformer.

Pat

[AB2EZ]

Here is a response from Ray Osterwald (Electric Radio magazine):

Hi Stu,
Here is all I know about the transformer, from the fellow who is selling them on consignment, and from Hammond:

The transformer measures 11 Henries on the primary side and 11.5 on the secondary side, as measured with a Fluke 710B bridge inductance meter.
It's based on the Hammond number 1608 audio transformer. The 1608 has a 10 watt rating on the primary and secondary sides, and 100 mA DC, max. They suggest it to be used to match tube types 6AQ5, 6V6, 6BQ5, and EL84. The Hammond web page on the 1608 is: http://www.hammondmfg.com/1608.htm

Original design, per Hammond, was to modulate voice audio for a class C transmitting amplifier, and having 40% screen tap on the push-pull primary side. The secondary has 3 taps, one at 3k, 4k, and 5k ohms. The voltage for both audio and RF amplifier is 250-300 VDC and the tube originally designed for was is a dual pentode type 6360. The RF amplifier operates at 25 watts input DC.

I recommended this transformer for Tom Marcellino's 815 AM transmitter article in the December Electric Radio, but that 815 tube is running 175 mA at 500 volts, which is 25 watts over the 1608 ratings. What the review article said was "The December 2013 issue of ER will have an AM transmitter project that this transformer will be perfect for."

73,
Ray Osterwald, NØDMS
Electric Radio Magazine
Ray@ERmag.com
720-924-0171

[w1vtp]

If the transformer is cross laminated, I seriously doubt its DC carrying capability.  If you don't care about "yellowy audio" no problem.  Several years ago I did a test for one of my 3 EICO 720s using a cross laminated 60 watt AF transformer and used a Hammond 193C 20 HY reactor with great success.  I have attached the test results using that combination.

Just a note: the relatively poorer low frequency THD (if you call 7.5% poor) was probably due to a combination of phase shift difficulties from the reactor 20 HY / blocking cap 2 MFD combination and the power supply output capacitance.  The choke demonstrates good characteristics at AF frequencies.  If you do not need more than 100 MA, it's an excellent choice at a reasonable price.

Al

PS:  I recommending floating the choke from ground and protecting from accidental contact.

[w1vtp]

More observations: I used the 1650 for my tests which is rated 200 ma per side.  That's the rating for push pull operation. That other transformer is rated at 100 per side.

The original calculations were modeled using PSpice.  It show somewhat of a rise at the bottom end. However, given the components that were used including the EICO PS the rise in THD was predictable

Al

[VE3AJM]

This transformer wouldn't be suitable for use as a replacement in a Johnson Ranger as is then, given the specs.

For $165, you can buy an exact replacement Peter Dahl mod transformer for the Ranger right from Hammond.

Yes, you could isolate this transformer above ground, and not run unbalanced DC on the secondary by the use of modified heising etc., but that being the case, I wouldn't buy it for a Ranger. I do have access to other/better modulation transformers that Hammond made in the 50s and 60s.

I would be curious to find out why Hammond and/or ER chose the 1608 as a model for this transformer. Are these things flying off the shelves?

Al VE3AJM

[KI4YAN]

It was designed to fit a specific article's specs, probably. A small, low-power, hi-fi transmitter.

I have two of the 1608's and two 1615's here, I use them for audio work obviously but they are VERY under-rated by Hammond. They keep flat from 20hz to 20Khz well in excess of their ratings, as long as the impedance match is reasonably close.

The 100mA DC doesn't bother them much, since it's on the secondary side only, the primary side is push-pull so it's cancelling.

I am currently running a pair of 6BQ6GA's in triode-mode at 250v, 50mA standing current, class AB1 operation, into the 1608 that this is based on, and am getting a clean 22W+ output with less than 0.4% distortion after a little global negative feedback. The amp sweeps clean from 20hz to 19khz, starting to drop off a little early with the extra power. If I adjust the bias a little at a time to make sure I keep it all balanced out, I have extracted over 36 watts from this same amplifier, at 2% distortion! The iron is being worked there, though, it does start to warm up on a sine-wave test.

Remember...the 1608 is rated at 10W of audio, 30hz to 30kHz +/- 1dB referenced to 1kHz.

[AB2EZ]

In the Viking Ranger application (replacing the stock mod transformer), or a similar application:

The primary side of the modulation transformer is driven by a push pull audio amplifier... with the DC plate current for each tube balanced out (more or less) in the two halves of the primary winding. Therefore... there will be very little DC H-field produced by the unbalanced remnant of the two primary side DC currents.

The secondary side has to handle the unbalanced DC of the 6146 (plate and screen current).

Note that the Hammond 1608 and 1615 are both 4-16 ohm output (secondary) transformers. If one were to put 100mA of DC through the secondary (for whatever reason), the resulting H-field would be relatively small... because the number of turns on the secondary side is relatively small (about a 10th of the number of turns on the primary side). The DC H-field is proportional to the DC current passing through the secondary winding x the number of turns in the secondary winding.

If you modify a 1608 or 1615 to produce a transformer with a 5000 ohm output impedance, then you must increase the number of secondary turns by a factor of about 10. This means that the DC H-field resulting from 100mA of DC current passing through the secondary will be about 10x larger in the modified transformer (versus what it would be in a 1608 or 1615)

Therefore, if you simply modify a 1608 or 1615, by increasing the number of secondary turns for use in an application where the output load impedance is expected to be 5000 ohms (i.e. the modified transformer has the same number of turns on the output side as it does on the input side)...and if you don't make any other changes in the transformer's design... it is likely that the modified transformer will saturate if you put 100mA of unbalanced current through the secondary winding.

Stu

[VE3AJM]

The ER online description etc. and possible applications for its use as a replacement in low or medium power AM transmitters including the Johnson Viking, may? mislead/be misleading.

"This makes them the perfect transformer for any real AM project in the low and medium power class. They are ideal as a replacement part for Viking transmitters with burned out modulation transformers, or any homebrew transmitter project with a push-pull output using 6AQ5 through 6550s."

http://www.ermag.com/index.cfm?v_link=product_detail&v_key=638

And then there's ERs reply to Stus latest inquiry, which is different from the above:

"Original design, per Hammond, was to modulate voice audio for a class C transmitting amplifier, and having 40% screen tap on the push-pull primary side. The secondary has 3 taps, one at 3k, 4k, and 5k ohms. The voltage for both audio and RF amplifier is 250-300 VDC and the tube originally designed for was is a dual pentode type 6360. The RF amplifier operates at 25 watts input DC.

I recommended this transformer for Tom Marcellino's 815 AM transmitter article in the December Electric Radio, but that 815 tube is running 175 mA at 500 volts, which is 25 watts over the 1608 ratings. What the review article said was "The December 2013 issue of ER will have an AM transmitter project that this transformer will be perfect for."

Are/can both descriptions be accurate?

Al VE3AJM

[N4LTA]

This transformer is nothing like the 1608 - 1650  series which are push pull audio transformers. Read the specs. It is the same transformer core  as the 125GSE  which is a SINGLE ENDED audio output transformer rated at 100 mA DC. It is made for 100mA DC current unbalanced. The 1608 is a series of push pull transformers that are rated for ZERO DC current unbalanced.

The 125ES series is a specialty series of transformer made for single tube single ended Class A output circuits. They also make a series of High Fidelity single ended output transformers (1647 series).

[AB2EZ]

The physical dimensions of a Hammond 125GSE (each lamination is 4.15 inches x 3.46 inches and the spacing between the mounting hole centers on opposite ends is 2.5 inches) are larger than the physical dimensions of the ER/Hammond transformer (each lamination is 3.06 inches x 2.5 inches, and the spacing between the mounting hole centers on opposite ends is 2.5") shown on the ER web site. Therefore, the 125GSE appears to have a lot more (87.5% more) "iron" than the ER transformer.

In addition, the transformer shown on the ER web site has end bells (like the 16XX series), and has dimensions that are almost exactly the same as the Hammond 1620 and 1645F.

The ER web site description, as well as the reply E-mail from Ray Osterwald state that this transformer is based on the Hammond 1615.

Where is the reference that states that this transformer is based on the 125GSE?

[N4LTA]

OK then Hammond Mfg (which I am a distributor for)  are lying. In the past, the photo that I saw did not have end bells. They also make a 1627 series Single ended transformer with end bells with the smallest one good for 100 mA DC. But you guys obviously know more than the Hammond engineers. Don't buy one.

[WD5JKO]

I wonder what the possibilities are using this ER/Hammond transformer as an auto-transformer? The presence of screen taps on the primary immediately made me think about that. Then there is the possibility of phasing a portion of the secondary to improve the turns ratio, AND to buck some/all of  the DC magnetizing current?

The little transformer in the Retro-75 was barely capable of 80% modulation, and even at that, only down to 500 hz before saturation kicked in. Re-configuring that to an auto-transformer, and phasing in the secondary improved the modulation capability to about 140%, and lowered the saturation point to around 300Hz.

My point here is however this ER/Hammond is configured, perhaps we can exploit the application possibilities with an auto-transformer configuration. There is a similar Tim-Tron conversion that "turbo-mods" the Heathkit Apache using this concept.

http://amfone.net/Amforum/index.php?topic=27820.msg212458#msg212458


Jim
Wd5JKO

[AB2EZ]

Jim

If you use a spare winding, or a portion of a winding to cancel out (balance out) the H-field produced by the DC current in the normal mod transformer secondary winding, then that balancing current will have to be fed from a source that has a high impedance at audio frequencies. Otherwise the spare winding will draw audio power.

To obtain this high impedance, at audio frequencies, while keeping the DC impedance low, one would require an inductor that is comparable to the inductor that would be required in a Heising configuration.

Alternatively, one could try to use a feedback controlled current source to provide the balancing current... But that is equivalent to using a (low efficiency) single ended audio amplifier on the primary side of the modulator.

If you use the auto transformer approach, you can balance out some of the DC, but you will create a mismatch between the audio amplifier on the primary side and the RF amplifier on the secondary side. Of course, if there is a mismatch to begin with, regardless of what input and output taps you use, you can use the auto transformer approach to improve the match. This may be what you accomplished in the case of the Retro 75.

With respect to unbalanced DC secondary current associated with the RF stage's DC plate current, I think you are up against the law of "conservation of aggravation".


Stu

[VE3AJM]

This transformer is nothing like the 1608 - 1650  series which are push pull audio transformers. Read the specs. It is the same transformer core  as the 125GSE  which is a SINGLE ENDED audio output transformer rated at 100 mA DC. It is made for 100mA DC current unbalanced. The 1608 is a series of push pull transformers that are rated for ZERO DC current unbalanced.

I have a 1608 Hammond transformer and data sheet here and have looked at the specs.

The ER transformer has a push pull primary, i.e. it has a primary winding which is centre-tapped, and is further tapped in from each end for screen grid connection , and has a single multitapped secondary winding. My 1608 has the same physical construction as the ER transformer with the end bells.

These configurations are the same as with the ER mod transformer. I don't have access to what the core material is. Maybe it is the same as this 125GSE. OK. This is your assertion.

The 125GSE is a single winding primary with no centre tap, no screen taps, and has a multitapped single secondary winding.

However, it is Hammond that claims that this ER transformer is based upon the 1608 or 1615 series of push pull output transformers, and that this transformer has a 100ma current primary and secondary capability.

We are going on what Hammond has published.

Al VE3AJM