Metropoulos Forum

Hi – First post but have been lurking here, literally, for years; so much useful information. I now need some advice, so time to post…

I am a “build by numbers” guy and have built a couple of Egnater Seminar amps (basically a modded JCM800) after being impressed by them on You Tube – I got hold of a layout & schematic and got there in the end with the help of a local tech, and the amps sound fine. I then got very interested in the TopHat Emplexador which seemed to me to combine plexi and 800 type tones in the same head, with a great hard-wired tag-strip build (2xEL34). I’m in the UK and took the plunge and imported one from the USA. I rewired the PT for 240V and it’s a great, great amp – very versatile and touch sensitive. Fantastic build quality as well.

If I understand things correctly, the Emplexador is basically a plexi with a MV and voicing switch that brings in an extra gain stage for the JCM tones? I thought I’d clone it by cannibalising the PT and OT from my Seminar build: They’re both Mojotone JTM45 Heyboers - exactly what Bruce specifies for his seminar amp. OT is Mojo 7021300 and PT is Mojo 7020300 (350V at 200mA).

I have had some fun building this: I copied the Emplexador exactly and simply dropped my transformers in – consequently, I now know the difference between a full wave bridge rectifier (Emplexador) and half bridge (the JTM45 PT has a centre tap; the Emplex doesn’t)! It also oscillated like crazy at first but switching the OT primaries seems to have fixed that and it now does what it’s supposed to. However, it’s a little “stiff” sounding compared to the Emplexador and is a bit less responsive/touch sensitive. Clearly, given that they’re different, I’m wondering if this is down to the PT and OT.

The plate voltage on the Emplex with its full wave rectifier is 442V (valves out) versus 517V for my build; much hotter. As might be expected given this, the pin readouts on the valves are hotter on my build than the Emplexador. I’m left thinking that the amp might sound a bit looser if I lower the plate voltage, e.g. by getting the “45 style low voltage” PT from Mojotone that is 300-0-300 rather than the 350-0-350 I am currently using? Also, what effect would replacing the OT with a JTM50 type OT have? I really have no idea how the OT for the JTM45 and JTM50 are different, and what sonic difference that might make if using the 45 type in a 50 type build.

Very grateful for any help you can offer before I go splashing out big money on new transformers, especially if I don’t need to. My amp is nice for sure, but I’d like it to be as good as its “parent”.

Cheers,
Steve

Hi Steve

Sounds like an interesting project. I found a schematic for one of these a while back and have contemplated building a clone using a 2204 as the basis. Did you take notes/pictures? There were some bits that were not clear (MV switching and voice switching) and so much else going on it has remained on the backburner.

I am certain those high voltages will affect the tone and feel, plus EL34s of new production probably will not cope with them for long. So yes lower voltage target of 400-420v sounds about right. The JTM45 OT will probably have 6.6k primary whereas a JTM50 or for that matter a JMP50 OT will be 3.4k. You can use either output tube with them but be aware you may need to adjust the output impedance settings of the JTM45 OT amp to get EL34s happier.

I can recommend both the metro -139 and the Marstran -128OT if that helps!

Thanks for chipping in Neil.

I didn't take any notes while doing the clone because I had the original amp sitting right in front of me so I simply copied the layout and parts, with the exception of the transformers since I already had those (I still have the Emplex). I guess the most complex bit is the vintage/modern voicing - that's achieved via a 4PDT switch, only 3 poles of which are used. Very fiddly wiring it since there are a load of resistors coming from the switch to an adjacent preamp tag strip.

From what you say it sounds like I'll have to change the transformers, certainly the PT? You mention the different value primaries of the JTM45 OT primary vs the JTM50. For a noob like me, what impact does impedance have on the tubes you can use and their sound? I've tried, but can't seem to find too much on that topic?

I see you're in the UK. Can I ask where you get your trannies from? I got the Mojo/Heyboers from the USA and the shipping was crippling. Do you know of any good UK suppliers? I've used AmpMaker (top man!) and Modulus, but they don't do the bigger Marshall trannies. Thanks!

Cheers,
Steve

I have bought transformers from various sources:
-Metro when they were in stock as George is great to deal with but never from Heyboer direct
-Marstran is also a great resource and now where I would buy from if getting a new clone
-Majestic transformers in Dorset are pretty good value, not tried their output transformers yet
-I quite like using originals that I have collected over the years for particular projects.

I have also bought from Chris ****** as well (a 45/100 OT was great0 but he is not a particularly reliable contact in my limited experience (he still owes me for a PT I paid for and ignored my polite emails......butI am not going to the US to get it back!) he has some particular devotees here but I will not deal again.
I have not tried the classic tone ones from the US - they are certainly cheaper and get good reports. I agree Ampmaker is a decent bloke to deal with too but not had trannies from him. Steves UK is also pretty good for 18watt bits.

Hope that is an honest UK perspective.
Regarding the type of OT different suppliers have different specs. Ideally you match the primary load to output valve type but you can get around a mismatch by selecting a different output tap to the load of the cab so the reflected load is correct. Many threads on the subject. Ideally if you are running EL34s a Dralke 784-139 type is a good choice.

Thanks Neil; you are a true Gent.

I'll get some new trannies and see what happens. I may take the opportunity to do a complete rebuild taking a bit more care and attention (some of my chassis work was a touch sloppy). You know how addictive this stuff is...

Cheers,
Steve