2Watt_BluesExpress_Version1 by timohp

[Mark]

These transformers won't make a 2 watt amp, they will make a 1 watt amp which will still be loud enough but will the bass response be enough?

http://www.classictone.net/Universal-Pr ... rmers.html

Please note the power transformer output is 380vac across the secondary. This will probably mean a rail voltage of 250VDC to 260VDC.

There may need to be some tweaking the circuit so the amp reacts properly.

[johnnyreece]

I'm guessing they might be under-rating those transformers. In the description, it says they're for the Firefly amp, so it might be worth popping over to AX84 to see how their circuit is set up.

[didit]

Datasheet says "3.5 WATTS MAX".

[Colossal]

FWIW, I've played a proper AX84 2W 4-4-0 Express which uses a 6SN7 dual triode as the power section. It was coupled with an AX84 designed oversized 1x12 loaded with an Alnico Gold and let me tell you, that was a loud and punchy 2W. As I recall, it used a custom 10W Heyboer OT with 22kΩ primary and the bass response was quite good.

[Mark]

Looking at the original circuit, it states the transformer set is. A Hammond 125C configured to 22K primary for the output stage, and a Hammond 269JX for the power transformer. Do you have any info such as the build number of the Heyboer output transformer?

http://www.hammondmfg.com/125.htm

https://www.hammfg.com/electronics/tran ... ic/200.pdf

http://www.ax84.com/static/4-4-0/AX84_4 ... 100216.pdf

http://www.ax84.com/4-4-0.html

The AX84 1 X 12" speaker cab.

http://www.ax84.com/static/corecabinets ... .08.03.pdf

http://www.ax84.com/corecabinets.html

The Amp Garage 4-4-0 Express build experience.

https://ampgarage.com/forum/viewtopic.php?f=1&t=5504

You Tube demo

https://m.youtube.com/watch?v=KLMjr2PfaNo

https://m.youtube.com/watch?v=Q7mLj5zyqgg

https://m.youtube.com/watch?v=hw3QToxi67k

https://m.youtube.com/watch?v=geC0cOc7G4Q

[Colossal]

Hi Mark,

I'm sorry I do not have the Heyboer part number. The amp was Cliff Chappell's and he had a run of OTs made to spec.

[romberg]

I built a 4-4-0 a while back using edcor transformers:

https://ampgarage.com/forum/viewtopic.php?f=1&t=21275

Don't remember the exact OT but I remember that it had a 10k primary. I mismatch the speaker load
by a factor of two which causes the tubes to see a 20k load. I figure it is close enough to the majick 22k .

The real key (in my opinion) to getting this to sound killer is to do what a fellow who goes by tillydog did and use split
load resistors off of the tail inverter. The whole amp up to and including the PI can be bone stock express. It is just that
the stock express PI will drive the 6SN7 too hard. One can tame that down to scale with the 6SN7's requirements by tapping the signal
off of the PI's load resistors somewhat in the middle of the stock values. If there is interest, I can try and dig up my notes on this.

Mike

[Mark]

That would be good Mike. An Express that sounds good and won't deafen you has to be a good thing.

[romberg]

Somewhere on the interwebs is a whole page with a long write up tillydog had on his tweaks to the 4-4-0. I can't seem to find it now .
However the key bit about the split load resistors on the PI can be found here:

https://ampgarage.com/forum/viewtopic.php?t=17732

Using a 6SN7 is not going to sound exactly like a full blown el34 driven pentode monster. But it does sound pretty
good IMO. Note that it is still very punchy and loud. Mine measured about 2.2W clean and cranked into overdrive (it is
an express after all) I measure around 5-7 watts. Through a 4x12 this is still loud enough to not be a bedroom amp. However
attenuating 5 watts or so is way easier than 50-75. I use a little weber mini mass with my 4-4-0 and run it into a pair of 1x12 cabinets.
It is close enough to my full size express for me. And allows me to practice without peeling paint off the walls.

Mike

[Mark]

I was thinking a post PI master volume might just be the thing for this amp.

This is the split load arrangement on the PI stage.

https://ampgarage.com/forum/download/fi ... &mode=view

The 39K with the 150K across it comes to roughly 30K. A 56K and a 68K will product a value in the same ball park and current running through them will be similar.

[Mark]

Here is the info on the Heyboer output transformers. An AX-84 member kindly gave me this info.

Cliff offered two versions of the custom Heyboers over the years.

Heyboer # HTS-9565 9960841
Push/pull 10W
Primary:
22k Blu-Red-Brn
Secondary: 0,4,8,16 (Blk,Yel,Grn,Org)
This transformer is vertical mount.

Quad 6SN7 output version
Heyboer # HTS-10021 9961001
Push/pull 10W
Primary: 23K (Blu-Red-Brn)
& 11.5K (Blu/wht-Red-Brn/wht)
Secondary: 0,4,8,16 (Blk,Yel,Grn,Org)
This transformer is horizontal mount.

I'll see if I can't get a price on these transformers.

I think this could be a good amp for around home, but I hope it is a thousand times better than a ZVex Nano. I think given the choice of using ZVex Nano or a Tom Schotz Rockman, the Rockman would be the nicer sounding option. The Nano seems to have a very narrow sweet spot.

[johnnyreece]

Somewhere on the interwebs is a whole page with a long write up tillydog had on his tweaks to the 4-4-0. I can't seem to find it now .

Here's a snippit from where you posted on my 4-4-0 thread. I *think* this might be what you're looking for.

Reporting back. Shorty story: It is very much worth doing.

It is not my idea and I point any that are interested here for Tillydog's pioneering work:

http://www.ax84.com/bbs/dm.php?id=436562

The following are my findings on my amp.

I've got my 6sn7 biased at around -13V. So, it starts to clip with a PI signal around 25V or so. And the signal was clean all the way through that point. When fully cranked the PI was slamming the 6sn7 with 125Vpp waves. And when the PI make the transition into overdrive it developed a funky looking lower half. My output had fairly steep looking slopes similar to Tillydog's.

After switching to 40% split plates the power tubes still clip at exactly the same point. But now when they are cranked they are only getting hit with around 60Vpp. The funny looking waves at partial clipping were gone. Finally the partially clipped output waves had a slope that resembled a sin wave.

Most importantly how does it sound? I did not think it sounded terrible before. But there is no way I'm putting it back. It does the whole clean to mean thing much better now. And the cleans are available over a wider portion of the guitars volume.

I feared this may alter/lower the gainyness of the amp. But it does not. Just makes it smoother (I think due to the slopes not being pure vertical until way cranked up). It also sounds nicer in the transition region of partial clipping.


Below are a few before and after scope shots

Mike

[Mark]

Thanks for that Johnny Reece. That's good info on all accounts.

Here is the reply from Heyboer, the transformers are still available.

"I found the info
Primary is 9,200 & 23,000 ohms: 4-8-16 ohms
Not a stocked item but we can make some
1-2 pcs $120.00 ea
3-5 pcs $95 ea
Lead time approx. 4 weeks"

[Mark]

Does anyone have pictures of the layout change with the split load PI plate resistors?

I ask as the board is pretty tight in that region of the layout.

[tristanc]

"I found the info
Primary is 9,200 & 23,000 ohms: 4-8-16 ohms
Not a stocked item but we can make some
1-2 pcs $120.00 ea
3-5 pcs $95 ea
Lead time approx. 4 weeks"

I tried to contact them on Monday as I'd be interested in one if they'd ship to the UK for a sensible amount of money. I've the little 4W OT from PrimaryWindings on hand, but very tempted by the larger OT.

I've spent quite a bit of time looking at the 4-4-0 and October Studio PI & output design. I read through the long thread here and at AX84 on its development. From what I can make out, both designs were tweaked by Cliff & Brian with changes pushed back and forth between the designs.

One of the issues was the PI outputting too much signal for the 6SN7. TillyDog did the split plate route. Cliff & Brian's solution was to swap the 12AX7 for the 12AY7, keeping pretty much everything else the same to lower the gain. Looking at the load lines (yellow line is AC load line, black is DC) you can see the stage is roughly center-biased with the 12ax7, but (to my eyes) warm biased with the 12AY7. The output swings are roughly the same, but the input swing is ~doubled to get that - so lower gain overall.

Would it not make more sense to either keep a 12ax7 as PI and increase the tail resistor to 47k or higher? Or, keeping with the current schematic, use the 12AY7 but bias is centrally (something like 2k R_b) and increase the tail resistor to reduce the swing available?

If you're going to push the PI hard (I think this is the case) according to Merlin's chapter on the LTPI it's better to centrally or cold bias to avoid too much shift / blocking etc. Which is exactly what could happen if substituting a 12AY7 for a 12AX7 happens with no other component changes.

Any thoughts?

I am going to build another dual October / high gain preamp with the 2W output in the coming weeks for use around the house. This will have the same 'Scale' control on the LTPI, a MV, and switchable fixed / cathode bias to take advantage of a bit of extra compression. Basically, unlike my October Club which is for loud and pedals, this one is for lots of distortion and mean to clean. So lots of tweaking...