Power Transformer choice

[AleS]

I'm at a good stage of planning a build for a 18 watt-shaped amp, with a few clear differences but also a lot of things in common with the classic build.\

One thing that it has in common is the voltages - about 350 at the OT ct and only a few Vs less for B+ down to around 320ish at the preamp.

I've been looking for PT options here in the UK and found some options from Primary Windings which are very well price and offer a few options, though perhaps none would be considered the ultimate choice for an 18 watter, and would love to get a few opinions as to which might fit the bill best and how you would approach rectification (mostly merits of SS and tube in this context)?

The first is a non ct PT with 275V and 6.3V windings which would get a FWB rectifier, either fully SS or hybrid with two diodes and on EZ81 (no 5V tap, so that would do the job). I'd expect this to rectify in the 380s and drop further with either a sag/dropper resistor or the EZ81

The second does have an extra 5V winding but is only 250V which I feel might end up putting out too low a voltage. I'm also in two minds about the need for 5V for a rectifier as I wouldn't be able to use anything more droppy than a GZ34 and, in an 18w, I'm not sure how big a difference such an efficient rectifier would make (ie, no noticeable sag)

Lastly this on with 300 and 6.3V windings and more mA than I'll ever need for this build though I think it might end up needing a lot of dropping which, with the lack of a 5V winding, could only be done by adding resistors and/or zeners or, again with an EZ81. So while on the one hand it might be the most versatile (could try a wider range of voltages etc) it might simply end up needing more fixing. Many 18w schems call for a 290 PT but I think this is expecting lower wall voltages and rely on more tube rectifer drop.

I do like a little compression and tube sag, so like the idea of a rectifier valve, but am not too sure how saggy the EZ81 is (no experience with those) and how they would perform in that sense in the context of a hybrid FWB. Then again, it does seem sag resistors can do a decent job of adding that feel, so am not committed to one or the other technique and interested to hear what people's approaches in that sense are.

As a UK based buyer I feel there's a little less choice than over in the US and these tend to be about half price compared the Hammond 290PAZ and so forth, so would be keen to figure out if any of these might do the job just as well and I suspect a couple of them definitely could.

Thank you in advance for any suggestion and observation!

[Bieworm]

I’ve been using the Primary Windings PWAM01 almost exclusively for these amps. Very good results. Super light too. I used one on a 6V6 Bassman build a few weeks back, and this is a PHAT sounding amp!!!

Note: go easy on the solder terminals. They sit in a plastic bracket that easily melts when overheating it

[AleS]

I’ve been using the Primary Windings PWAM01 almost exclusively for these amps. Very good results. Super light too. I used one on a 6V6 Bassman build a few weeks back, and this is a PHAT sounding amp!!!

Note: go easy on the solder terminals. They sit in a plastic bracket that easily melts when overheating it

Very encouraging and confirms my expectations Out of curiosity, how do you rectify it and what voltage do you get out of it?

As an aside, that Bassman 6v6 sounds VERY interesting as a lower wattage JTM45-ish build! Do you have a schematic/layout/demo/build report posted anywhere by any chance?

[Bieworm]

I’ve been using the Primary Windings PWAM01 almost exclusively for these amps. Very good results. Super light too. I used one on a 6V6 Bassman build a few weeks back, and this is a PHAT sounding amp!!!

Note: go easy on the solder terminals. They sit in a plastic bracket that easily melts when overheating it

Very encouraging and confirms my expectations Out of curiosity, how do you rectify it and what voltage do you get out of it?

As an aside, that Bassman 6v6 sounds VERY interesting as a lower wattage JTM45-ish build! Do you have a schematic/layout/demo/build report posted anywhere by any chance?

I use a full wave bridge rectifier.. that’s the obvious solution with this transformer. I get about 340V DC after that
I do have a schematic and layout indeed. Watch your PM. Will post them tomorrow

[AleS]

I use a full wave bridge rectifier.. that’s the obvious solution with this transformer. I get about 340V DC after that
I do have a schematic and layout indeed. Watch your PM. Will post them tomorrow

Very grateful, thank you!
In terms of the FWBR, I was very vague with my question Is that including any sag/drop down resistor and/or have you ever experimented with an EZ81 or similar in a hybrid setup or always stuck to SS?

[Bieworm]

I use a full wave bridge rectifier.. that’s the obvious solution with this transformer. I get about 340V DC after that
I do have a schematic and layout indeed. Watch your PM. Will post them tomorrow

Very grateful, thank you!
In terms of the FWBR, I was very vague with my question Is that including any sag/drop down resistor and/or have you ever experimented with an EZ81 or similar in a hybrid setup or always stuck to SS?

I always put a 100 ohm 25 watt resistor in series with the + on the rectifier
An EE81 doesn’t sag much and I’ve had troubke with reliability. In a Marshall type amp SS is king

[Bieworm]

here are my docs. This started out as a build for a guy who wanted a 6L6 version with switchable bias. I changed the schematic to my recent build with 6V6 and the PWAM01 transformer, which is bridge rectified and has only cathode bias. You can not get negative bias with a full wave bridge rectifier, so cathode biased is the only option with the primary windings transformer. The layout you will have to figure out for yourself regarding the rectifier and bias. I can send you pictures of my build if it would help.

6V6 tweed bassman tremolo.pdf

Layout Broesman T V.20230711.pdf

[AleS]

I always put a 100 ohm 25 watt resistor in series with the + on the rectifier
An EE81 doesn’t sag much and I’ve had troubke with reliability. In a Marshall type amp SS is king

Late due to holidays, but thank you once again, for the 6v6 Bassman/JTM plans and the intel on the PW transformer.

Is the 100R sag resistor also responsible for the voltage drop? I'd expect that 275 to rectify quite a bit higher than 340, especially with a nigh-on-dropless SS rectifier, possibly in the 380s? Or does that go down somewhat because the current draw is in the higher end of what's available on tap?

Just curious to figure that one out, as there doesn't seem a hard and fast rule to calculate rectified voltages since many different things (current being one) seem to come into play and figures seem to vary by quite a bit!

[Bieworm]

Don’t forget the plate dissipation pulling the voltage down factor.
The 100R will drop little voltage, but there is a big difference in B+ in an amp with or without being under load

FWIW I drew a new schematic of the amp in fixed bias and blackface reverb. That will be my next build, with another type of power transformer ofcourse.

IMG_8378.jpeg

[Paul-T]

I'm in a similar boat, I like Primary Windings and am thinking about the PT for a future 18W build for which I already have the OT. In my case, I'm set on a tube rectifier - my build is to replace my 1979 Park 18W so I want a bit more sag than that old beast, which was wonderful but very aggressive . The idea of SS with a sag resistor obviously has a lot of benefits, too, but most of my previous builds were done that way, I want an earlier Marshall feel and have never tried a 6.3V rectifier.

If you do opt for the EZ81 I am pretty sure PW would do a custom order of the PWHT02 with 290V secondaries for not a lot more. They're very helpful. Modulus also do a specific 18W PT, well-made but a bit pricy, and Valvepower/Steveuk, formerly very active and well-regarded on this board, does a specific PT with, from memory, 295V on the secondaries and two 6.3V taps to allow for the EZ81, which is a bit of a bargain, though you might have to wait until he/his people make a new batch. Finally, although I have no experience of them, Luke at Transformer Equipment Limited in Sandwich, Kent, has a good rep with the very best UK amp repairers, and I noticed that they do an 18W PT - photo attached. I think when I push the button it will be steve first, then PW.

Update: I msj'd TEL and their 18W is "£87.60 + carr £12.50 + vat" so similar cost to Modulus. Although Brexit means lots of transformer sources are problematic, for 18W at least we have a decent choice.

[Bieworm]

You will be disappointed of the tiny sag an EZ81 yields. If you want real sag, bite the bullet and order the 18w universal power transformer from TubeTown germany. These have a 5V winding to use real rectifiers with sag like a 5U4 or something.

[Paul-T]

You will be disappointed of the tiny sag an EZ81 yields. If you want real sag, bite the bullet and order the 18w universal power transformer from TubeTown germany. These have a 5V winding to use real rectifiers with sag like a 5U4 or something.

Good advice, I love tube-town. I'm probably set on the EZ81, just for a change from other builds, it's true to the design, and most of all I have one in my stash.

[AleS]

Thank you for chiming in Paul, very helpful as I was not aware of most of the sources you mention!

Also thank you Bieworm once again for contributing with your experience with the bits and pieces here discussed. On the subject of sag, do you find sag resistors to differ noticeably from tube rectifiers?

On the theme of Tube Town, have you placed orders from them since the $*1tstorm ahem, brexit? Curious to figure out how much might get slapped on top should I ever need/want anything from them as they seem extremely well stocked.

I've given a nudge to John at Primary Windings, so I'm hoping that'll resolve the PT situation. Now I just need to decide whether to go for an 18w OT (which would allow for the Matchless mismatch for an effective 4k primary) or an AC15 one (which seems to have a LOT of extra heft compared to the 18w OTs but doesn't lend itself to mismatching being rated lower at 6.2k).

Decisions decisions....

[Paul-T]

Sounds promising, looking forward to seeing this develop and how you get on with John at Primary Windings.

Bad news re tube-town I'm afraid - I used them for a few years after Brexit when there was the interim trade deal - but now they have a minimum order of 140 euros to ship to the UK. Same with most other smaller dealers. Others, like piemme, told me, with apologies, they don't ship to the uk at all. It's possible Banzai might ship without minimum orders.

Primary Windings obviously do a good 18W OT; I went for the Valvepower Radiospares Repro as it has a 4 Ohm tap which I might need (see pic); others on here have praised it and it's a bargain. I'm sure I know one other uk supplier, I'll add to the post if I remember, but I think they're pricier.

[Bieworm]

Thank you for chiming in Paul, very helpful as I was not aware of most of the sources you mention!

Also thank you Bieworm once again for contributing with your experience with the bits and pieces here discussed. On the subject of sag, do you find sag resistors to differ noticeably from tube rectifiers?

On the theme of Tube Town, have you placed orders from them since the $*1tstorm ahem, brexit? Curious to figure out how much might get slapped on top should I ever need/want anything from them as they seem extremely well stocked.

I've given a nudge to John at Primary Windings, so I'm hoping that'll resolve the PT situation. Now I just need to decide whether to go for an 18w OT (which would allow for the Matchless mismatch for an effective 4k primary) or an AC15 one (which seems to have a LOT of extra heft compared to the 18w OTs but doesn't lend itself to mismatching being rated lower at 6.2k).

Decisions decisions....

TubeTown is practically my neighbour. I’m from Belgium
My shipping costs 9€, so I’m better off than you guys. I’m a pretty good client there, looking at my orders..

About the vox OT. The lower the primary impedance, the lower the distortion. That’s a big reason why an 18w is getting grittier sooner. That, and the PI grid load resistors.

[AleS]

Sounds promising, looking forward to seeing this develop and how you get on with John at Primary Windings.

Bad news re tube-town I'm afraid - I used them for a few years after Brexit when there was the interim trade deal - but now they have a minimum order of 140 euros to ship to the UK. Same with most other smaller dealers. Others, like piemme, told me, with apologies, they don't ship to the uk at all. It's possible Banzai might ship without minimum orders.

Primary Windings obviously do a good 18W OT; I went for the Valvepower Radiospares Repro as it has a 4 Ohm tap which I might need (see pic); others on here have praised it and it's a bargain. I'm sure I know one other uk supplier, I'll add to the post if I remember, but I think they're pricier.

I've sent John a nudge and emailed Steve from Valvepower, so we'll see what comes of it. I guess I'll now find out in the new week! Again, thanks for the intel

I have a thread over on the Hoffman forum about my project but, since it's a very 18w-shaped design, I might start one over here as well as it might be of interest to current users and future browsers, and I might reap some extra suggestions and words of encouragment/wisdom throughout...! (More on the design below as I reply to Bieworm)

TubeTown is practically my neighbour. I’m from Belgium
My shipping costs 9€, so I’m better off than you guys. I’m a pretty good client there, looking at my orders..

About the vox OT. The lower the primary impedance, the lower the distortion. That’s a big reason why an 18w is getting grittier sooner. That, and the PI grid load resistors.

I'm looking to build a two channel Matchless Spitfire/Nighthawk mashup, which shares a lot of the topography of the 18w but I'm definitely looking to get more headroom from it, hence the interest in either running it at 4k or going down the Voxy 6.2k route.

The Vox OT looks nice and hefty which is inviting, but those 18w OTs get a good rep and would effectively allow for, so despite my current tinkering I ultimately think either would do a good job

[AleS]

Got sidetracked by a very busy week at work. No signs of life from Primary Windings and TEL quoted me 112 + postage and VAT for the 18w power transformer, so higher than Hammond options.

Found this guy however which looks great for the application, price is ultimately in line with a Hammond or the Modulus offering (about a tenner cheaper) but has 5v winding and a generous current limit, more than an 18w might need but could be of interest for anyone working on a larger amp build needing more juice.

They also have an 18w/8k primary OT for around £50, also made by OEP - I've not heard of those in amp builds, though I think they are sought after by people building consoles and outboard gear. No cheaper than the tried and tested Hammonds though.

[Bieworm]

Got sidetracked by a very busy week at work. No signs of life from Primary Windings and TEL quoted me 112 + postage and VAT for the 18w power transformer, so higher than Hammond options.

Found this guy however which looks great for the application, price is ultimately in line with a Hammond or the Modulus offering (about a tenner cheaper) but has 5v winding and a generous current limit, more than an 18w might need but could be of interest for anyone working on a larger amp build needing more juice.

They also have an 18w/8k primary OT for around £50, also made by OEP - I've not heard of those in amp builds, though I think they are sought after by people building consoles and outboard gear. No cheaper than the tried and tested Hammonds though.

I think that PT is a pretty dangerous one because it’s going to be mounted outside the chassis. Even when all lugs are heatshrinked or isolated.. an accident lurks in a dark corner. Or one would have to use special brackets to enable to use the transformer with the connections through a cutout in the chassis

[AleS]

I think that PT is a pretty dangerous one because it’s going to be mounted outside the chassis. Even when all lugs are heatshrinked or isolated.. an accident lurks in a dark corner. Or one would have to use special brackets to enable to use the transformer with the connections through a cutout in the chassis

You mean dangerous from an exposed HT connections standpoint? I've seen contact cover blocks for these which cover it all up.

Ultimately there's still the option to bite the bullet and route a hole in the chassis from a drop through PT, which would make the PW a viable option as it is, though the fact that they've gone MIA for over two weeks makes me worry as per how long it might take for it to actually be made and dispatched...

[Paul-T]

No signs of life from Primary Windings and TEL quoted me 112 + postage and VAT for the 18w power transformer, so higher than Hammond options.

Found this guy......
They also have an 18w/8k primary OT for around £50, also made by OEP - I've not heard of those in amp builds, though I think they are sought after by people building consoles and outboard gear. No cheaper than the tried and tested Hammonds though.

Those are Carnhill I believe. I don't know the full story, they are very reputable and made Ampmaker's transfomers. This looks different from the Ampmaker version although that might just be looks.

I thought I'd seen frame transformers used as lay-down or drop-through transformers, especially on 18W builds; ie remove one end of the frame, drop through, then replace the frame. Did I hallucinate, Bieworm? Or was it that you wanted to use it as a stand-up, Ale?

My Steve/Valvepower OT was £28 btw. Don't know why TEL quoted you more.