Output transformer assistance

[dkevin]

Greetings to all!
I have asked these questions on other forums and all I've gotten so far is a lotta views but no responses.Perhaps my luck will change!
I have a Gibson GA-5 Les Paul Jr. amp (1956) that came to me with a number of (hacked) mods The original speaker was MIA along with the original OT. The OT supplied with the amp has 6 wires coming from it and was riveted to the chassis . Because this is a S.E. amp, I wondered why the OT would have 6 wires instead of 4 like the other S.E. amps I have.
Anyway, I am investigating new OT's and became quickly disoriented with the information. Some say the single-ended 6V6 needs a 5K primary while others say 8K. One source says the Asian designers chose primaries higher than American designers because they desired clean power as opposed to louder, with more distortion output. I believe my amp needs an 8K OT primary. Can anyone tell me where to find this info? Is it in the tube data sheets? Does it have somewhat to do with the plate voltage of the output tube?
Also, what is the effect of choosing an OT that is rated to operate at a higher output wattage than is required? For instance, if you have a 5W S.E amp using a single 6V6 output tube and you select a 10W 8K primary as opposed to a 5W 8K primary, what would it do (if anything) to the performance of the amp?
For this amp, I am considering an Edcor OT and an Allen amps Heyboer.
I know, neither one is exactly what came with the amp but I own several Gibsons and the OTs seem to be a weak point in the amps.
Please chime in if you can shed some light on these questions for a poor, lost newbie.
Thanks!!
dkevin

[guitician]

The ratings of transformers are general and should include a frequency range. If you use the 5w one for a narrow audio band, it will do 10w with no problem. Generally, bigger cores will provide less distortion, and are used in "Hi-Fi" amps.

[tubeswell]

Hi dkevin

Welcome to the forum

The primary zed through your OT can be within a range (of about 1/6th to 1/8th of your output tubes dynamic plate resistance (rp)).

You generally find the manufacturers recommended specs on the tube datasheet for the tube you are using.

If it is a single 6V6 in a single-ended amp, which is running a plate voltage between 300V-400V, then 8k reflected load is generally good. If you go to say 5k, you will get more compression and less power out of the tube.

[Bruce / Mission Amps]

I've always used 5K for my SE 6V6 Soulkicker jr. amps and I'm almost willing to bet money that yours was a 5K to 6K OT in the Gibson.
The highest, "cleanish" and good guitar amp sounding output power out I've gotten from my SE 6V6 amp with an 8 to 10 watt 5K output transformer is just around 4.5 watts.
Those amps used a small 50-70ma power tranny with a B+ of around 360vdc.
Similar to what you have.
I've never seen much more power then that out of any mass produced SE 6V6 amp either.... and usually much less, so I don't care what OT impedances others use, 5K is what I use.
I wonder if your replacement OT isn't a universal OT product with different taps for different speaker loads.... or it is a dual secondary OT with an ultralinear tap primary too.
*************

Take your ohm meter and tells us which side of the OT is the extra wires... primary or secondary, or both?

[dkevin]

More OT info

Thanks for all the responses. My head is swimming from all the questions/info. The transformer that came with the amp has 6 wires grouped in three pairs. The two on one side of the transformer by themselves (primary) were attached to the 6V6 and PT. There were two lower wires from the opposite side (secondary) of the OT that were floating around inside the chassis (bare on the ends) and unattached to anything - waiting to ground out on something. The last set of two upper wires were on that same side (secondary) were attached to the speaker. There are numbers stamped on the top of the transformer frame. They are 503456 over the top of 412539. I am not sure that this is an 8ohm transformer-only that it was hooked up to an 8ohm speaker.
Bruce, perhaps you could give me an idea of what you wanted me to measure with a multimeter. I am a bit lost here....
Thanks!
dkevin

[Suusi M]

http://www.drtube.com/guitamp.htm#Gibson

GA-5 Les Paul Junior, 5W 1x10" Combo
The Gibson amplifier model GA-5 has had many incarnations. The first model GA-5 Les Paul was introduced in 1954. The circuit of this amp is an almost identical copy of the Fender Champ (5C1 circuit). It was fitted with a Jensen 10" field coil speaker. It had a "TV" style cabinet.

Schematic with 1x 6SJ7, 1x 6V6 & 1x 5Y3, (Gibson)

Schematic with 1x 6SJ7, 1x 6V6 & 1x 5Y3, (Joe Piazza)

Fender Champ Amp, 5C1 schematic, (Fender Musical Instruments)

[dkevin]

Yes, but....

This is probably true about some of the Les Paul GA-5 amps but most certainly not mine. The earliest amps had the TV front with a 5x7 Rola speaker that had the output transformer mounted to the speaker frame. The baffle of my amp still has the mounting cutout for the 5x7 even though someone mounted an 8" speaker to it. It does have the tube complement of 6SJ7-6V6 and 5Y3. It probably is 5 Watts or so.
The short/long version of this amp is that someone tried to convert it to a later version GA-5 Skylark (12AX7-6V6-5Y3). I am in the process of trying to restore it to the original tube lineup. In the process of trying to convert the amp,(or sometime before) the original speaker was removed and along with it the original OT (or so I speculate). I am almost certain that this is not the original OT. From all the pictures I have seen, I can only see 4 wires. Someone correct me if I am mistaken....
dkevin

[tubeswell]

You can't measure the impedance of the OT with just using a multimeter. You need two meters and an AC supply.

You need to unhook the primaries (except for the centre tap) and the non-grounded secondaries (leaving secondary that is attached to the chassis ground). With the amp plugged into the wall but not switched on (so the chassis which you have your primary centre tap grounded to is at ground potential), you use a low voltage AC supply (like from a variac or a model railway AC transformer supply), and you hook up each loose secondary - one at a time - to one side of the AC supply, and the other side of your AC supply to the chassis ground. You measure the VAC across the secondary with one meter, and you monitor the VAC across each end of the primary with the other meter. (For the most accurate measurement, you need to monitor both sets of AC voltages simultaneously, so you need two VAC meters). You will get a ratio of voltages, primary to secondary, in each case. The impedance ratio will be the square of the VAC ratio of primary-to-secondary.

So say you used a 16VAC from a model railway transformer AC supply (which you need to measure to make sure you've got the right ratio) on your secondary, and you got a primary of 459.52VAC on the primary. That is a voltage ratio of 28.72:1, which will translate into an impedance ratio of 825:1, which would be 6k6:8 (6k6 primary zed with an 8R speaker). Whereas a voltage ratio of 31.62:1 would translate to an impedance ratio of 1000:1, which would be 8k:8 and so on.

You have to be careful measuring the VAC across the primaries - don't go shorting them and don't kill yourself.

[black_labb]

i've used a 5v supply from a transformer in an amp (which was partially taken apart). works well

[Steve Conner]

Hey, go easy on the poor guy.

If the transformer had two wires coming out one side and 4 out of the other, then it's most likely a single-ended type, with one primary, and a tapped secondary for 4, 8, and 16 ohms.

If you're lucky, the wires might have the standard RMA colour coding:
http://www.nostalgiaair.org/Referenc...s/audxfrmr.htm

I never noticed much difference between different plate impedances in these small SE circuits. But one thing that I found really matters is the size of the output transformer. A bigger one seems to give more volume and more bass. I think the designers of these little tube amps often deliberately undersized the OPT to restrict the bass, so they could use a cheap and nasty speaker in an open-backed cabinet, without the cone flapping out of control.

[dkevin]

OT help

This is all helpful information about how to identify an unknown OT and determine its suitability for future use. But I still wonder about the questions raised in my original post.
When it comes to choosing an OT for this amp (for instance) if I choose a 10W OT, 8Kprimary, 8ohm secondary- what effect will that have compared to choosing a 15W OT, 8K primary, 8ohm secondary? So far I hear you saying that the effect will be more bass, run cooler and cost more(not to mention larger size).
There still seems to be some disagreement/difference of opinion about the best primary to select for a single-ended 6V6 OT. Bruce chooses to build his amp with a 5K primary. Others stick to the 8K primary suggestion. Do I hear you say that the effect of choosing smaller primaries is cleaner power with less distortion- or is the 8K better for getting the max. power out of the amp? And when it comes to the two alternative replacement OTs I mentioned-the Edcor vs the Allen amps champ OT(made by Heyboer)- do you see a clear favorite here?
I am deeply indebted to those of you who can translate the voodoo of electronics for a noob like me.
Thanks again!
dkevin

[Suusi M]

To basics

You are planning on restoring this amplifier so that it sounds and behaves just like it did when it was first built.

Looking at the data sheets, in class A the most your going to get out of a single ended class A 6V6 is about 6 watts @ 12% distortion MAXIMUM. It will draw 34 ma at zero signal and 35 at max power

To get that you need a 6 watt, 8k single ended transformer.

Transformers back then were physically bigger coz the iron core was not as good and saturated more easily.

if you use a 10 watt transformer then the windings will be different, with regard to saturation of the core.

In transformers, Power is conserved so:

6 watts into 8 ohms needs 6.9 volts
6 watts into 8K needs 219 volts

10 watts into 8 ohms need 8.9 volts
10 watts into 8 ohms needs 283 volts

Although the ratio is the same the DC component is almost doubled

Cutting out a lot of crap about theory, Single ended transformers have to cope with a large DC component, which also effects their construction.

Thus get the 5 or 6 watt 8 K single ended transformer NOT the 10 watt one


NEXT we need to discuss if your amplifier originally used a field coil speaker or not?

If you cannot get an original field coil speaker then you will have to improvise with a modern speaker and a choke.

If it did, then someone has prolly replaced the field coil choke with a resistor. If you can find out from Jenson what the inductance of the field coil was @ 35 ma The inductance of a choke is dependent on the DC current flowing though it. Replace the resistor with the correct choke, then the amplifier will behave like it did when it was new and unmodified.

REF 6V6GT Data sheet

[dkevin]

More OT

Hi Suusi,
This amp did not have a field-coil speaker originally. It had the OT mounted on the frame of the 5x7 speaker. I had the same concern that you have when I first saw pictures of the amp. I did all kinds of investigations into replacing a field-coil with a permanent magnet before someone pointed out that I was not looking at a field-coil but a permanent magnet with an OT mounted on it. Make sense? I read your previous post from the man who said it had a field coil but I do not think the info is totally correct.
I am grateful for the help in selecting the correct OT. I believe I will go with the 5-6 watt, 8K primary, 8ohm secondary OT that is touted as a Champ replacement/upgrade by Allen amps. It is made by Heyboer and I think they have a good rep in the OT business.
My concerns all along have not been confined to this one amp's restoration. Having a limited understanding in these matters (along with a limited vocabulary)in the "technical crap" side of things is frustrating to all concerned. However, I am working toward being able to understand these matters and find the appropriate information from the authoritative sources in order to make informed decisions.
I can easily find people who want to make these decisions for me. It's a lot more difficult to find people who can take the time to explain the process to me. I am thankful for the help wherever I find it!!!!
dkevin

[domocaster]

GA 5 output transformer.

Hi there,
I own a GA 5T, although it is not quite the same as the GA 5, the output transformer wireing is the same on both.
ie; these circuits use a feedback resistor loop,that's why there are six wirescoming from the output tranny.
the two extra wires are for the feedback loop,
the wire colours and their conections are as follows;[ from the top of transformer],red;5Y3, blue; power tube,
[bottom of transformer],black to feedback resistor, green to feedback resistor,
the other black and green to the speaker.[8 ohm].
Mercury Magnetics do an exact replacment for the GA 5.
I hope this has been of some help, check out Vintage Gibson Amps on Yahoo, it is a good group page.
Any prob's just email,
Cheers, Dom.