Recently got a 18w OT from SteveUK, what to build?

brooom

So I recently got a 18w output transfomer from Steve Uk. Kind of looking for ideas on what to build, as I have an aluminium chassis from modulus and an edcor power transfomer.

I'm thinking either a super simple (normal channel only) ac15 or a mini jtm45 with 6V6 output.

Any opinions are welcome.

equalsql

Do you like tweed amps like a 5e3 or 5e7? Solid state or tube rectification? Got any piccies of the chassis you can post up?

brooom

I have a 5E3 and a clark 5E7. My edcor has a 5v tap so I'll be wiring up a rectifier for this one.

It's a small chassis 35 cm wide, but I can fit it all in there.

ecc83

6V6s with a sstate rect. Add a 'sag' resistor if you want. You could even run EL34/6L6 wiith appropriate biasing.

"tap" or isolated winding?

Dave.

brooom

Sorry, I meant isolated windings, not a tap. I'm happy running small bottles (el84/6V6) the OT is also made for an 8k primary which is ideal.

So ac15 or jtm type of circuit?

Modulus_Amps

JTM with a fixed bias power section and presence control, will sound like a mini black flag amp and kill most 18w amps.

You have to get the feedback right, a variable pot would help with that.

ecc83

brooom said:

Sorry, I meant isolated windings, not a tap. I'm happy running small bottles (el84/6V6) the OT is also made for an 8k primary which is ideal.

So ac15 or jtm type of circuit?

Good! Needs to be isolated to run a GZ34 which IS overkill for 18W but should last a long time. I am however not keen on 'modern' thermionic rectifiers (the TAD is about the best GZ I found) and if you go for diodes your traff and amp will run cooler.

Dave.

brooom

Thanks guys... Ok so I need to make a parts list for modulus.

Looks like it could be a fun little amp

Modulus_Amps

I built an 18w hot rodded M style amp, fixed bias and thought it was the best 18w amp I have built so far. 

You can hear it cranked up in this video, but it does amazing clean tones too
https://www.youtube.com/watch?v=_A4MjfH1Vn8&t=423s

brooom

I'm looking more at the "bass circuit" end of the spectrum but that does sound nice, thanks for the video.

I'll give that a go, I'll probably need to adjust feedback and maybe the power rail nodes a bit.

Rich210

What about something like a princeton, but with middle and presence? Maybe add a bit of a lush spring reverb. 

RiftAmps

8k Pri @ 18w?

Lots of options but with a 275-0-275 PT and GZ34 rectifier you'll get a B+ of approx 375v at the reservoir cap, assuming three preamp valves.

I'd do an 18w JTM with Reverb, easy.

brooom

Bingo @RiftAmps my edcor pt is 275-0-275 and 150ma so definitely enough for a jtm circuit.

The OT is 8k primary around 18w: https://www.ebay.co.uk/itm/265238831413?hash=item3dc1776135:g:RGQAAOSwAPZg-Uz2

@Rich210 - not something I'm considering as I already have a Magic vibro prince

ICBM

Modulus_Amps said:

JTM with a fixed bias power section and presence control, will sound like a mini black flag amp and kill most 18w amps.

+1

Mini Black Flag with either EL84s or 6V6s.

brooom

Cool, thanks guys, looks like that's the topology I'll be aiming for.

Thanks to the guys who suggested fenders as well. I live those circuit, but I've got them covered already.

pluckbuddy

I have the same OT and am planning a JTM, superbass, tweed bassman, Vox top boost mashup - they all use the same basic topology - with a 2x6v6 power section and no negative feedback. 

Merlin's preamp book has some suggestions to reduce blocking distortion which i might incorporate. Mainly they reduce bass and change the phase inverter bias. 

If I can bring myself to drill another preamp tube hole in the chassis, I'll add reverb with the reverb recovery stage going into the second PI input. I hate drilling chassis though - too much hard work and concentration required!

ecc83

pluckbuddy said:

I have the same OT and am planning a JTM, superbass, tweed bassman, Vox top boost mashup - they all use the same basic topology - with a 2x6v6 power section and no negative feedback. 

Merlin's preamp book has some suggestions to reduce blocking distortion which i might incorporate. Mainly they reduce bass and change the phase inverter bias. 

If I can bring myself to drill another preamp tube hole in the chassis, I'll add reverb with the reverb recovery stage going into the second PI input. I hate drilling chassis though - too much hard work and concentration required!

Then find  a small engineering shop and bung them a few quid to punch the hole for you.

Dave.

brooom

My chassis is 2mm aluminium... So routing some holes isn't really that much of an issue.

pluckbuddy

Yes mine is an ampmaker alu chassis of modest gauge and should be manageable. I have access to a pillar drill and the right drill bit too. And yet I'm still expecting it to be... ugly. lol. 

I should just do it.

brooom

Oh I just drill it with my standard black & decker drill and then expand the wholes with curved/circular/tapered files. I don't have the proper tooling. But it's usually only a couple of routes so it doesn't really take too long.

ecc83

brooom said:

My chassis is 2mm aluminium... So routing some holes isn't really that much of an issue.

See if you can borrow a Qmax chassis punch then grease it well.

Dave.