Possible OT choice for 36W project?

[markd]

While searching for 36W OT choices I came upon these photos of a 1959 AC30/4. This is the very earliest AC30 with the EF86 preamp. There has been nothing but rave reviews of this amps tone.

http://www.vintageamps.com/showroom2.html

Scroll down about half way and look at the output tranny. It is clearly a Radio Spares "DELUXE" OT as used in the early JTM45. This is the big brother to the Radio Spares "EL84" OT used in the Marshall 18( see reprint of RS catalog on pg.19, History of Marshall book)
You can see it is laid over on it's side and the secondary is set for 16ohms(check the RS deluxe spec sheet attached below). With an 8ohm load(two 16ohm speakers wired in parallel?) and the primary set up for 8K(only choices are 6.6K, 8K, and 9K) this would give the correct 4K load a quartet of EL84's require.
Perhaps this is a replacement OT? Has anyone else seen a AC30/4 with this OT? Never the less it is working in this amp and could perhaps be a good choice for a 4x EL84 marshall style circuit. Might just give "the tone". There are several copies of the RS "Deluxe" OT available, so it wouldn't be too hard to test this theory.
markd

[stevesuk]

Mark,
Are you intending to join the 36 Watt club ?
I can thoroughly recommend it. I won't be building anything else for the near future.
I used an OT from Graydon for the first one I built, I am just about to finish one with an OT from Shinrock and then I have an unlabelled 50W OT I am going to try on the third, I will then settle on one of those. I did try Vox for a reissue PTP Vox AC30, but it was really expensive.
I am trying to upload sound files But I seem to have hit a block with FTP files and they are just too big for the site uploads.

I have an ex Rolling Stones 1964 Vox AC30 grey panel with top boost and reverb in my amp room (absolutely untouched inside) and the 36 Watt I built sounds better by far !!
Steve UK

[markd]

<Are you intending to join the 36 Watt club ? >

Steve,
After building 3 18Ws it seems the thing to do. I will probably use some version of the Vajra preamp. Maybe with a more "normal" layout. I already have two RS "Deluxe" OT clones, the OEI and the Mercury Magnetics. I also have an AC30 PT clone laying around here somewhere.
With an 5AR4, it should give the voltages and sag I am looking for. What was your EL84 plate voltage with 220v at the wall? What brand 5AR4 did you use? That can make a huge difference. An old Mullard 5AR4 will give the lowest B+ in my experience.
markd

[stevesuk]

Mark,
I can't remember the voltages I'll test them.
I used a GZ34 which is easier to source in the UK.
Steve

[markd]

Steve,
GZ34/5AR4- This is one of those I say tomato, you say to-ma-to sort of deals. Same tube. If you have two different brands see if they change the B+. If you have a Mullard GZ34 in some old amp give it a try.
Tonewise, my "mojo" meter maxes out on those Mullard GZ34s. ha! Please don't give me a blindfold test though cause' it's probably just an old valve wankers superstition.
markd

[Graydon]

I used a dual diode rectifier on my 36W with a 125ohm dropping resistor and after tweaking the B+ supply resistors a little, I ended up with almost exactly the same plate voltages as the 18W everywhere. My 36W sounds just like an 18W but with more headroom, more bottom end and more "tone."

[Plexi]

This may not have alot to do with the 36 watter..but i just finished a cross between a JTM45 and a Plexi. So one can use the GZ34 or the SS recto.

the difference between the 2..i have 150v on the first preamp,and 450v on the plate of the EL34,useing the GZ34..

Using the SS recto i get 10v difference..or 10v higher..160 preamp and 460v on the EL34..

The 36 watter i'm building will also be using the GZ34 recto tube.

Richie

[AmpMad]

Hi Markd,

Thanks for posting that connections diagram for the RS de luxe. I had been looking for that for a while. I have an old 'ali-panel' JTM45 that I'm fixing up and I was curious as to how the output impedance might be altered.

Thanks again!