Little help with a JTM45 output transformer

[Joeythelid]

Hello all hope you are well?

I was going to start an introduction thread but thought I'd just combine it in with this

I'm Joe and I'm on my second amplifier build (jtm45)

I've got an elstone mr/30 output transformer - same as the original marshal number one amp I believe

My problem is I cant fathom what connections to use as it's a multi tap primary and secondary

Looking at the pictures of the original Marshall it seems his primary has 8 terminals were as mine only has 7?

The intention was to run this witn an 8ohm speaker but I'd be happy with 16ohm if need be

Anyway heres some pictures of the transformer and info I've found along with a pic of the original

Thanks in advance for any help

Joe

[Joeythelid]

And some more

[Joeythelid]

Sorry about the poor quality of the images

Heres the original jtm with the same Elstone transformer but with 8 taps

[shooter]

read through this thread and see if lights go off

http://el34world.com/Forum/index.php?topic=25698.0

[Joeythelid]

read through this thread and see if lights go off

http://el34world.com/Forum/index.php?topic=25698.0

Thanks mate that's a perfect

Please excuse my terrible deductive skills but from that I take that fir a 16 ohm load on the secondary I'm using terminals 1 and 4 on the primary and 3 as my CT

With B and C on the secondary as my output?

This is with EL34 output valves?

[Joeythelid]

Again thanks for the help in advance

[sluckey]

Please excuse my terrible deductive skills but from that I take that fir a 16 ohm load on the secondary I'm using terminals 1 and 4 on the primary and 3 as my CT

With B and C on the secondary as my output?

This is with EL34 output valves?

There are no such combinations in that chart. EL34s like to see 3K to 5K for a primary impedance, with 3400Ω being a very common impedance. So, look at the added columns in the attached pic and choose your poison.

A 16Ω speaker would reflect a 3844Ω impedance to the primary, if you connect the OT IAW the top row, ie, 2-3-4 primary and A-C secondary. Use the second row for an 8Ω speaker. Etc, etc.

[Joeythelid]

Thank you that's absolutely perfect

My mechanical engineering degree is a distant memory and so too the maths that went with it ha ha

[JB]

Interesting, I hadn't realised that first amp didn't use the RS Deluxe output transformer like the first production JTM45's did.  I was lucky enough be given an RS Deluxe, on the agreement that I build a '45 with it (so I did!).

Why EL34's though - surely if you're making a JTM 45 and have that OT you should use KT66's?  Or I think maybe the very first ones, possibly No. 1 included, used 5881's?  EL34's were quite a bit later.

[Joeythelid]

And just to be certain I don't need any of the link wires currently on there

[sluckey]

I would not remove those jumpers! If you want to be certain then connect a low voltage (I like to use 1V because it makes the math easy) to the secondary and measure the voltage on the primary. The voltage ratio will be the same as the turns ratio in your table.

[Joeythelid]

Thanks mate much appreciated

[Joeythelid]

Interesting, I hadn't realised that first amp didn't use the RS Deluxe output transformer like the first production JTM45's did.  I was lucky enough be given an RS Deluxe, on the agreement that I build a '45 with it (so I did!).

Why EL34's though - surely if you're making a JTM 45 and have that OT you should use KT66's?  Or I think maybe the very first ones, possibly No. 1 included, used 5881's?  EL34's were quite a bit later.

I really like el34's to be honest mate

[Joeythelid]

This table cant be right as with the above thread my terminal 4 is connected to nothing at all?

[sluckey]

That's probably true. That chart is for a MR/T2. Why would you think that's the same as a MR/30?

[Joeythelid]

Because I'm an absolute noob, dam I assumed it was mr series

Sorry mate I appreciate your help

Next time I'll stick to transformers I can actually find information on

[sluckey]

Turns out they ain't the same. You can't use that MR/T2 chart to figure the connections on that MR/30. Look at the attached. I found it in just a couple minutes.

Google may turn up a wiring diagram/table for your Elstone MR/30. Good luck.

[Joeythelid]

Thanks mate I've found that too but a much worse condition picture

I really didnt understand the info on it

[Joeythelid]

I also found a guy who sold one with the wiring information on ebay in 2017 but the pictures have been removed from the listing

[sluckey]

The pdf shows pics of both OT. And while they look similar, they also look very different. The number of terminals on the T2 agree with the number of terminals on the T2 chart. But the info you need is missing. The pdf says the actual wiring info is included with the OT.   

I also found that ebay listing. That's what gave me hope that google might help track down the MR/30. Be vigilant. Look in the google images as well as web links.

[Joeythelid]

The pdf shows pics of both OT. And while they look similar, they also look very different. The number of terminals on the T2 agree with the number of terminals on the T2 chart. But the info you need is missing. The pdf says the actual wiring info is included with the OT.   

I also found that ebay listing. That's what gave me hope that google might help track down the MR/30. Be vigilant. Look in the google images as well as web links.

Thanks mate I'm going to keep looking

Worse case I use this output transformer with another future build

[Ed_Chambley]

Are you planning a 2 Power tube section or 4.  You mentioned preferring E34's, which is fine.  I commonly run reflected impedance to EL34's using a 6.6K to 4,8 or 16 ohms.  I own one of theses coffin case JTM45's and can assure you it had 5881's but I removed all the tubes as they were very nice when I got the amp in 1979.


It has a Radio Spares multitap OT and will provide either 6.6K or 3.3k to the tubes and since it does have 4 taps, you get 4, 8, 16 or 32 ohms and another "gray wire" that connects to nothing.  In 6.6k, setup with KT66 it is extremely loud and clean.  I to prefer EL34 in this amp, but I lower the voltage using a 5U4gb simply to get newer tubes to last.


To my ear, EL34's sound better running a higher load if you increase the Power Tube Screen resistors between 3 and 4K.  This will actually smooth the power tube saturation and add a little more compression.


These 30 watt amps are loud no matter how you do it.


If you want an original Drake in the box for a Plexi with 2 output tubes that is designed for EL34's or one with 4 output tubes, send me a PM.

[Joeythelid]

Are you planning a 2 Power tube section or 4.  You mentioned preferring E34's, which is fine.  I commonly run reflected impedance to EL34's using a 6.6K to 4,8 or 16 ohms.  I own one of theses coffin case JTM45's and can assure you it had 5881's but I removed all the tubes as they were very nice when I got the amp in 1979.


It has a Radio Spares multitap OT and will provide either 6.6K or 3.3k to the tubes and since it does have 4 taps, you get 4, 8, 16 or 32 ohms and another "gray wire" that connects to nothing.  In 6.6k, setup with KT66 it is extremely loud and clean.  I to prefer EL34 in this amp, but I lower the voltage using a 5U4gb simply to get newer tubes to last.


To my ear, EL34's sound better running a higher load if you increase the Power Tube Screen resistors between 3 and 4K.  This will actually smooth the power tube saturation and add a little more compression.


These 30 watt amps are loud no matter how you do it.


If you want an original Drake in the box for a Plexi with 2 output tubes that is designed for EL34's or one with 4 output tubes, send me a PM.

I bet that original is killer!

I'm going for two output valves on this one mate

Oh cool, I've just bought a replacement Heyboer output transformer to get the project moving but I'm always interested in vintage components cheers

[Joeythelid]

Actually speaking of vintage components I've got the Elstone power and output transformers going spare if anyone is interests?

I'm keeping the choke though (it's from 1944 and in great nick)

[Ed_Chambley]

Original is just that.  In sound tests recording Output Transformers there is a very little difference except a few amps.  Heyboer makes a fantastic version of the Drake part number for multitap 3.3K to 4,8,16 is 784139.  Dave Freidman uses only Heyboer as well as Metro "rebrands" them.  Same transformer!


Cost for 50, $71.62.  Cost for 1 is more than buying an original on Ebay.

[Joeythelid]

I was tempted by classic tone but the comparison they have up on their site is suspicious to say the least

I honestly just went Heyboer because I heard Friedman use them

[Joeythelid]

So I've finally got the correct information

[Joeythelid]

I've dragged this thread out long enough so heres my final calculation if anyone would be so kind as to check it for me

Going off that documentation

Ratio = primary impedance x speaker impedance (square route)

So

Square route (3600 x 8) = 21.2

And 3600x16 = 15

So closest ratios on the chart are 16-1 and 22-1

According to this if I attach H to B, G to C and F to D

I should be able to run two outputs off this transformer for either 8 or 16 ohm speakers?

Also little bit confused by the whole 3 and 5 going to the HT when it says the connections are 2, 5 and 7?

Thanks guys I know this is getting tiresome for you guys who know these calculations well

Joe

[Joeythelid]

I've dragged this thread out long enough so heres my final calculation if anyone would be so kind as to check it for me

Going off that documentation

Ratio = primary impedance x speaker impedance (square route)

So

Square route (3600 x 8) = 21.2

And 3600x16 = 15

So closest ratios on the chart are 16-1 and 22-1

According to this if I attach H to B, G to C and F to D

I should be able to run two outputs off this transformer for either 8 or 16 ohm speakers?

Also little bit confused by the whole 3 and 5 going to the HT when it says the connections are 2, 5 and 7?

Thanks guys I know this is getting tiresome for you guys who know these calculations well

Joe

Just spotted it should be ÷ not × but the math is still correct