OT for 2x5881

[Leevi]

What is the optimal primary impedance of the OT for JTM45 based amp using two 5881s.
B+ will be 450-480V.


It's difficult to find data since 5881 is rated to max. 400V.
However at least JJ's 5881s can have max 500V plate voltage.


/Leevi

[kagliostro]

May be you can use the same values of 6L6GB PP at 450V B+, many people say they are near the same tube



Franco

[HotBluePlates]

What is the optimal primary impedance of the OT for JTM45 based amp using two 5881s.
B+ will be 450-480V. ...

If you're building a JTM45, why would you not just use the same OT the JTM45 used? I'm not a Marshall historian, but sources which seem credible to me indicate the JTM45 had an 8kΩ primary for 2x KT66's and you would use the same for 5881 or 6L6GB/6L6GC.

[mresistor]

FYI  some info (not necessarily a buy recommendation)    the Marstran JTM45 OT is spec'd like the original   and I've heard the Heyboer is pretty nice too for the money...


http://valvestorm.com/Transformers

[Leevi]

If you're building a JTM45, why would you not just use the same OT the JTM45 used? I'm not a Marshall historian, but sources which seem credible to me indicate the JTM45 had an 8kΩ primary for 2x KT66's and you would use the same for 5881 or 6L6GB/6L6GC.

Yes, that was my first assumption and good to hear it's suitable for 5881's as well.


5881's were used in the first versions of JTM45 at least according to the Wiki.


https://en.wikipedia.org/wiki/Marshall_JTM_45


/Leevi

[sluckey]

The Hammond 1750Q is their replacement for a JTM-45 OT. It's rated for 7371Ω.

[mresistor]

according to this chart  the 62-64 JTM45 did indeed use 5881's and also KT66.  http://www.marstran.com/Historic%20Data%20Base.htm

[guitardude57]

Interesting...I didn't realize that the 5881 tranny was so close to the 6V6 specs.

[HotBluePlates]

Interesting...I didn't realize that the 5881 tranny was so close to the 6V6 specs.

It's not really "6V6 specs" it's power output at the given B+ voltage.

"Matching" to some mysterious tube spec is a myth. If you happen to see same/similar OT primary impedance (when not due to simple copying), it's because output tubes are often used to deliver the maximum output power feasible, at similar/high supply voltages, for a given class of operation and as a result the designs land at similar numbers.

Look at the chart Kagliostro posted:5.6kΩ at 450v B+. If you raise B+ you'll need a higher load impedance to take advantage of the extra voltage swing possible, and to keep plate current from climbing too high. A higher load also offsets plate current increase given the screen voltage will be higher in the amp than in the chart (though increased bias is also needed at idle).

[Leevi]

The Hammond 1750Q is their replacement for a JTM-45 OT. It's rated for 7371Ω.

I think I'll go for this Hammond.

according to this chart  the 62-64 JTM45 did indeed use 5881's and also KT66.  http://www.marstran.com/Historic%20Data%20Base.htm

Interesting and very informative chart


/Leevi

[foghornleghorn]

I would imagine Marshall originally used the same 5881 that Fender did at the time (1962), which was the Tung-Sol 5881/6L6WGB, an awesome sounding  tube that could handle 500 Vp all day, as it had to in the white Fender heads of the day. They probably switched to the KT66 because, being made in the UK, it would have been cheaper than the imported US tube, and capable of higher power too, even at a 7 to 8K primary.


For highest power output you would want a 4 to 5K or so primary, but for the JT45 tone you need the higher primary I think. Before you plunk down your $$ for the Hammond, check out Marstran as he blueprints his iron from original Marshall pieces and is highly regarded by Marshall build fans.


Just my $0.02.

[labb]

I would take a look at Magnetic Components 40-18039. 8K rated

[mresistor]

I would take a look at Magnetic Components 40-18039. 8K rated

I would probably lean towards this OT as well, it has a great price and the buried 100v tap for originality .

[MakerDP]

I've used this one for both a KT77 and a 5881 build. Great transformer.

http://www.classictone.net/40-18092.html

[Ed_Chambley]

The JTM45 Radiospares RS Deluxe OT in my 63 has many taps and configurations, but this amp came with KT66's.  It is factory connected 6K6, but also has the option of 8K and 9k as well.  It was considered a "inexpensive" HiFi transformer and also has multitap secondaries of 3,7 and 15 ohm and 30 watts.  These are now highly coveted.


Since getting an original is really expensive, I have tried numerous copied winds.  The one Monotone sells for the JTM45 has 3k3 primary with secondaries arranged to get the original connection.  The secondaries are 2,4,8 and 16 ohms so if you choose you can get 6K6 with 2 ohm connected as 4 and 4 as 8 and 8 as 16. This will be the configurations for the KT66.  It also works well with EL34 using it as is and simply tucking away the 2 ohm secondary if not needed.  It looks very much like Heybour made.  It too has a 100v secondary, which seems to be something desirable for reasons I do not know.


Of all the ones I have tried, using this one has gotten me the closest possible with using modern parts in a JTM45 to the original.  I too have read that the first ones used 5881 tubes, but I have never seen one and I did horse trade quite a few to finally get the one I have.


http://forum.marstran.com/viewtopic.php?f=2&t=128


The above link has some good information on the early Output Transformers.  What I have found is if I use KT66 with either a 4K connected primary or 8k primary the amps develop a noticeable increase in power out with the same speakers and mids become more prominent which makes getting the in-between clean more difficult to achieve and the amp sounds weak when the volume is low and the bugger gets loud quickly.  The original is supposed to be 36 watts due to the reflected impedance, but I have never actually checked.  I have been working with this PA configuration on a design for a couple of years now as it is my favorite of all amps.  The JTM45 has a wonderful tone with this arrangement, but due to the preamp it needs a treble boost or split cathode on V1 like the Plexi's.


It is a dual channel preamp that I have been working with and I am close, but still not satisfied.  I have added a Fender Reverb which is a very nice addition to get a little more depth since I never use a 4, 12's cab anymore.  If I had room to add tube tremolo I would.


On a side note.  After finding it is connected 6K6 I thought of a Fender BF Deluxe.  Well it does work very well and has makes the Deluxe sound huge.  I think if I were going to keep the KT66 in a Deluxe I might use the Mojo Transformer since I do not think the Deluxe would holdup to the additional wattage. That is the direction I thought P2Pamps was building when he said a KT66 Deluxe, but he selected a Bassman OT and it seems to be working well.

[Leevi]

It is a dual channel preamp that I have been working with and I am close, but still not satisfied.  I have added a Fender Reverb which is a very nice addition to get a little more depth since I never use a 4, 12's cab anymore.  If I had room to add tube tremolo I would.

My plan is to add a reverb as well. Furthermore there will be an overdrive mode using the 1. amplification stage of the 2. channel.
The amp will have only one preamp channel where you can switch between normal and overdrive mode. The overdrive mode needs own master volume.
Question about the reverb. Where in the signal path did you add it?


/Leevi

[Ed_Chambley]

It is a dual channel preamp that I have been working with and I am close, but still not satisfied.  I have added a Fender Reverb which is a very nice addition to get a little more depth since I never use a 4, 12's cab anymore.  If I had room to add tube tremolo I would.

My plan is to add a reverb as well. Furthermore there will be an overdrive mode using the 1. amplification stage of the 2. channel.
The amp will have only one preamp channel where you can switch between normal and overdrive mode. The overdrive mode needs own master volume.
Question about the reverb. Where in the signal path did you add it?


/Leevi

As I mentioned, the preamp has been changed some and V1 and V2 are pentodes so v3a plate before the coupling cap and is inserted at the grid of V3b.   The preamp has a plate driven tone controls from V3a and V3b is the reverb return.  As I mentioned it is different.


It is just like a deluxe reverb, but I did have to fiddle with the 1 meg resistor to ground on the 12at7.  I currently have a 500K dwell pot and have changed the tube to a 12Au7 and have surf level verb, but I did use an older Gibbs tank which to me has a better sound and feel.


For a traditional JTM45 you can use V2B plate and insert at V2A like Doug has in the library on adding verb to a western electric circuit.  I have it this way on a Bassman I built for a friend.  On this one I used the 1 tube reverb for the first time and I was pleasantly surprised at how nice it sounds.  Some say you cannot get a dripping wet verb using 1 tube, but it has more than I ever use even if playing surf style stuff.


Here is the link on Doug's site: http://el34world.com/charts/reverbmod.htm


It is fairly simple providing you have the space.  You can also use the Belton Brick, but if you do desire this route get the one that is made for wet reverb as the other one does't really get deep enough.  FUCH amps uses one in his dumble copy with 2 5881's and it is nice.


You can also consider a FX loop. Since I have added so many tubes I used a Solid State FX loop I got from Metro which seem much better than the one Mojo sells, but it is $20 more.  The one from Mojotone is fine, but I had to fiddle with it to get it working as I wanted.


This has gotten me to thinking.  Don't get me wrong, I really like the Fender Spring Reverb, but I am considering removing it and focusing on a tube loop as that will allow me to use more effects and also I can use a plate verb. 
kagliostro posted a tube FX loop I have tried and really like it.  The problem gets down to how much crap can I put into the chassis.


I have gotten really impressed at some of the pedals, especially the Strymon Big Sky, but it is $500.  It is the best verb I have heard.  I like to have a loop.  Decisions Decisions.

[MakerDP]

This has gotten me to thinking.  Don't get me wrong, I really like the Fender Spring Reverb, but I am considering removing it and focusing on a tube loop as that will allow me to use more effects and also I can use a plate verb. 
kagliostro posted a tube FX loop I have tried and really like it.  The problem gets down to how much crap can I put into the chassis.

I've been thinking of using the Belton brick for amp reverbs too. One less pedal on my board for not that much more $$$ per amp.

As far as loops and space inside the chassis goes... consider just doing a simple out/in  that interrupt the signal chain with jacks wired straight through and build something like a Dumbleator external effects loop. You could even build a tube reverb or belton brick into it for even more flexibility.

[Leevi]

Thanks for the replies

I have experience in Belton Brick but this time I'm going to use 17" reverb pan and reverb transformer.
I will add the reverb after volume and will use one 12AX7 (1/2 - 1/2) as a driver. I have implemented several reverbs with this concept
and find it pretty good.


/Leevi

[tubenit]

Leevi,

This is how I would approach the reverb & insertion point.  There is a lot of information in ARCHIVES on one tube reverbs if you need more.

And there are editable SCH versions of 5F6-A Bassman (original, reverb, & FX versions) here:
http://el34world.com/Forum/index.php?topic=571.0

Since you are not doing a standard original Bassman, the editable SCH versions could be useful to you.

With respect, Tubenit

[Leevi]

Thanks for the schematic tubenit. This is another option I was thinking.
I have used that in some VOX based amps where it worked pretty well.
Anyhow I can easily try both ones and select the best one. The reverb sector is very much the
same I have used before, I just try it now in different insertion points.


/Leevi

[Ed_Chambley]

Leevi,

This is how I would approach the reverb & insertion point.  There is a lot of information in ARCHIVES on one tube reverbs if you need more.

And there are editable SCH versions of 5F6-A Bassman (original, reverb, & FX versions) here:
http://el34world.com/Forum/index.php?topic=571.0

Since you are not doing a standard original Bassman, the editable SCH versions could be useful to you.

With respect, Tubenit

Oh yea, I have used this.  Small Bassman chassis I added a board and made a cover to keep it outside the small access tweed cassis.  Very nice verb, but after the tonestack and everyone preference differs.  With a little tweaking it sound great and is fairly simple.  It actually took me longer to make a chassis doghouse to cover the circuit than it dod to wire it uo.


I am a fan of the 1 tube verb and I feel if you want more verb than this a simple loop with a EQ booster infant of the effects really help.  I use the Boss,but they are noisy in stock condition.  Upgrading the caps and resistors males the very quiet providing you are not boosting too much which you really never will as the effects get way to strong.


As always, lots of ideas.  Noe to decide what will work best for you.