Output transformer substitute for the bench........

[Jack_Hester]

I picked up a very nice EICO 1030 in 'non-working' condition, cheap.  The fuse was missing.  Owner stated that it came out of an attic, where it resided for quite a few years.  Inside, it looks in very new condition.  Same outside, except where the dust had been washed off.  Apparently, it had been covered or wrapped in something, as there was no dust inside.  Just a factory-new look.  Warmed it up and calibrated it by the simple instructions.  Looks like I have a fine bench supply.  We have one at work, identical to it, but made by Precision.  It is used on an average of once every decade or so.  They won't sell, as it is needed those few times.  I used it quite a bit back in the 70's, but not for it's intended purposes. Just as a DC supply, when the few other SS supplies were in use.

Anyway, to my real point.  Is there some substitute instrument to be had (or preferably built) to use in place of an audio transformer, while experimenting?  As I have so little experience in the audio world, I am unaware of how to search.  I'm still looking, but hope the solution is already at hand, here.  Thanks.

Jack

[jjasilli]

I have a similar Heathkit PS.  400VDC @ 100mA.  I've used it for some testing.  Not enough current rating for a pair of big bottle tubes.  Also, guitar amp supplies are usually poorly regulated, which has become part of their charm.

There is no substitute for an OT.  One size does not fit all.  The OT should be matched to the power tube(s) plate impedance on the primary side; and handle sufficient voltage, current and wattage.  It could be purposely under-rated for specific design goals

[kagliostro]

For the bench you can ask to someone to build for you a multi tap OT with some primary windings and multiple secondary windings

you can also "play" with the reflected impedance, if an 8ohm impedance speaker is connected to a nominal 8ohm secondary winding, giving a reflected impedance of 4,000ohm in the primary, the same 8ohm speaker connected to the 4ohm secondary winding will give a doubled reflected impedance of 8,000ohm

so connecting the speaker to different output winding will give you the ability to use the primary winding for a couple of el84 (8,000ohm) or a quartet of the same tube (4,000ohm)

You must have one OT for SE and one OT for PP because the SE transformers are also rated for DC component (they have a gap between E and I in the core), the PP transformers don't need this ability because the current is acting in the two middle side of the winding in opposite so avoiding magnetisation of the core

Kagliostro

[Jack_Hester]

Well, I didn't think that there would be such an animal.  But, it was worth the asking.  And, I still haven't given up on looking.  The multi-tap suggestion is along the lines that I'm tending.  As to the reflected impedance, I found this interesting discussion:

http://www.radioremembered.org/outimp.htm

Sure takes a while to search.  Have to get out and stretch my eyes for a bit.  Been in the shop for the past 3 hours.  I'm back in the house, giving the back a rest.  Thanks a bunch, all, for the replies.  I'll post anymore info that I may come up with.  Have a good one.

Jack

[kagliostro]

Well, I didn't think that there would be such an animal.

An OT with Ultra Linear taps is also an OT which has 2 different Plate to Plate impedances

if you find an old OT used for PA amplifiers they, very often, has many secondary windings

and if you use the secondary windind "moving" the connected speaker from one winding to the other, you can obtain a large primary impedance bounch of options

here an example, the Geloso output transformer connection charts, as you can see, there are so many output winding that you can use with the same speaker to obtain a lot of options about reflected impedance

Kagliostro

[PRR]

If you accept that bandwidth and power/bass limits won't be "perfect", use any large OT (two: one P-P one SE) and a collection of 8 ohm many-Watt resistors.

A Bassman 4KCT:4 is a common and useful part.

Load with 8 ohms to get "8K", suitable for 2-6V6 etc. The bass response may start an octave higher, the bass cleanliness may be "better" than a proper 2-6V6, but it will play (or not-play, if the rest of your amp is way-wack). The 2-6L6 Bassman will also work for 4-6L6 Super/Twin, but run a bit warm and be 10%-20% down on actual Watts due to excess DCR.

[jjasilli]

Yes, but with the Eico PS which I assume maxes out @ 100mA, then there's not enough current rating to power a pair of 6L6's; and only 400VDC.  So I guess we're limited to 6V6's and el84's which simplifies the selection of an OT to one with an 8K ohm primary for PP, and preferably a multi-tap secondary.  Other small bottle tubes are also options.

Max wattage for PP = 400² / 8K = 20W.  So a 20W OT will due for PP.  But again, a 20W amp with a regulated PS with no sag and a fully rated OT is not the usual recipe for a guitar amp.  Though maybe Pete Traynor would beg to differ, as he tended to use big iron

[Jack_Hester]

Yes, but with the Eico PS which I assume maxes out @ 100mA

Manual says 150mA @ 400vDC.  Probably splitting hairs, but shouldn't this be enough to experiment with SE amps?  I haven't graduated to the PP models.  And, as I have an autotransformer (not a Variac model), I may just use a PT with a larger output rating than what I may ever need, and supply it with a variable input.  Is this a practical choice?  Seems to me I saw this stated in another thread.

Jack

[jjasilli]

OK, 150 mA is good for big bottle tubes in SE, and borderline OK for 6L6 in PP at max signal, at about 350VDC or lower!    Check the specs for the particular tube type, or post if you have a question.  Sounds like a fun project.

[Jack_Hester]

I only take stated ratings with a grain of salt.  It may be good for 150mA, but I'll prove it to myself, first.  Figuring on 100mA and finding out that it can do more, is a better approach.  The one at work has never been used for more than 50mA, as that's all the old transmitters/instrumentation were good for.  Anyway, SE will be the only trials for now.  PP will come later.  I'm swamped with work.  That's a good thing.

Jack

[DummyLoad]

Manual says 150mA @ 400vDC.  Probably splitting hairs, but shouldn't this be enough to experiment with SE amps?


most smaller 5-15W SE plans - absolutely!

SE OTs have an air gap. this facilitates the devices ability to stand a DC bias preventing core saturation.
 
as PRR says, a 4K OT is a handy thing... a 4k and 5K device should get you close enough to just about any ratio you'd need with popular tubes with speaker load adjustments. a 3K, a 4K, and a 5K with multi-tap secondaries will cover just about any impedance you'd probably want to experiment with - just be mindful of the current rating of the winding(s) and power handling capacity of the core. UL taps can be fun to experiment with as well.

in an ideal world you'd use what you're going to build with.

 

[Jack_Hester]

most smaller 5-15W SE plans - absolutely!

I'm in business!  Thanks everyone for the feedback.  Now, to find the time to put it to use.

Jack