Check My Chassis Layout 5F4 Head

[Rocketrob]

I'm about to start a new build; a 5F4 Tweed Super in a head configuration. I want to keep the size down to a small 18" width cabinet so I'm using a stand up PT (Hammond 276X) a Hammond choke (159Q) and a Allen Amps OT (TO35MT). I have not built an amp with a choke like this, so I am concerned about the location of these three components. I have seen amp layouts with the choke in the middle and some with the choke on the end. Does it matter where the choke is placed? Also , how much clearance should there be to avoid magnetic coupling? Lastly, is it OK to have the choke and OT facing the same way?

See attached drawing for reference.

Thanks, RD

[Chuck H]

IMHE you'd do better to place the PT and choke at the far end, squeeze the tubes together a little and orient the PT and choke the same while keeping the OT at 90*. This would make for a lopsided balance on a centered handle, but it would allow for more distance between any preamp circuitry and the OT, which is more important, again IMHE.

It's not really much of a change. And the gain level of that amp should probably be stable with the layput you propose. But if you up the gain a bit and run it cranked it's usually better to get the OT as far from the preamp as possible. Also, orienting the choke and PT on one end should allow for a smidge more distance between the choke and the OT which may reduce hum.

Why would you want to build this particular circuit??? With the choke oriented before the plate supply there will be huge amouts of sag and, of course, the need for a choke of very high capacity. If you heard a 5F4 and "loved it" and that IS your tone then I say go for it. But if not I think there are less sensitive builds with cheaper transformers that perform better by most standards.

JM2C

Chuck

[Rocketrob]

Thanks for your input Chuck. I redrew this with the laminations on the PT and Choke going the same way; then I rotated the OT 90* from them. I have 1" clearance between all three components. This arrangement does not seem to be much better though, as the PT is now even closer to the input section. Maybe I didn't understand your suggestion.

See new attachment.

As far as the circuit goes, I play alot of blues so I like the tweed sound. I aslo have blackface and silverface amps for that sound. I can always change it if I really don't like it; besides, I alreadt bought the trannies and choke RD

[Bruce / Mission Amps]

...

Why would you want to build this particular circuit??? With the choke oriented before the plate supply there will be huge amouts of sag and, of course, the need for a choke of very high capacity. If you heard a 5F4 and "loved it" and that IS your tone then I say go for it. But if not I think there are less sensitive builds with cheaper transformers that perform better by most standards.

JM2C

Chuck

No... I don't think you fully realize the circuit.
I use to do this amp as a kit and my 5-6Hy 200ma choke does not increase the sag.
It is the DC resistance if the choke that creates sag and the DCR is very low in good chokes (they are not that expensive) and if the choke looses inductance due to current saturation, in this amp all it does is allows the second filter cap to charge up to a higher ripple voltage which actually increases the B+.
Look carefully at the design and you should see why it works so incredibly well with only two 16uF caps.
It's a brute force PI filter....

[Rocketrob]

Thanks for the replies; however I'm still not sure wich is the better layout. Should I put the choke in the middle, or on the end with the OT in the middle?

I would also like to move the driver/PI tube close to the power tubes to shorten the grid wires to the board. Would that be OK? It seems that it would.

I've got all the parts I need to make this; but I need to finalize the chassis layout to get it made.

Thanks,RD.

[Chuck H]

OK... I was thinking of the typical choke input type when I saw this design has the choke ahead of the rail. I hadn't considered the first filter being before the choke. I've used Pi filters very effectively to stabilize current and I have to side with Bruce because he's da man when it comes to these amps.

If you change the choke position in the PS you will change the tone and response of the amp for better or worse, depending on what YOU think of the changes. Why not try it both ways, it's just a couple of wires (and perhaps a resistor and a bigger main filter). Look at examples of amps that use the choke after the OT to see how it's typically done compared to the 5F4.

Chuck

[cminor9]

Thanks for the replies; however I'm still not sure wich is the better layout. Should I put the choke in the middle, or on the end with the OT in the middle?

I would also like to move the driver/PI tube close to the power tubes to shorten the grid wires to the board. Would that be OK? It seems that it would.

I've got all the parts I need to make this; but I need to finalize the chassis layout to get it made.

Thanks,RD.

I'd recommend drilling the holes for the xformers' wires in the chassis (cause you probably know *generally* where they'll be), building the amp, and then moving the transformers around to wherever they are quietest. I think the best you'll get are some general guidelines, which it seems like you have a handle on. You know, keep away from pre amp, orient laminations 90 degrees from each other, etc.

I have done that before on head builds, and it seems to work well. Haven't completed one with a choke yet else I'd tell you what worked for me. It's nearing completion though

I do suspect you'll have to balance noise with weight distribution. I found I had that problem with the few heads I have built. One is built with transformers on each end, one is built with them away from the preamp and all toward one side. The noise is negligible (it's pretty quiet) with the OT over the PA, and it's better to carry.

[tubeswell]

so far where I have used chokes, I put them as close as possible to the filter caps, (which is usually between the PT and the OT) and with the laminations oriented in different direct to the PT and the OT. I haven't had any noise problems yet (and I've done 4 amps with chokes this way)

[Rocketrob]

I'm starting to think it might not make as much difference as it seems. I've talked to a few well known builders and they all had different views on this. One said that the choke is so large in this circuit that it shoud be treated as another transformer; so rotate it 90* from the PT, and rotate the OT 90* from the choke. Another said the choke is part of the power supply, so keep it going the same way as the PT, and rotate the OT 90* from both of them. The original design places the choke on the end, facing the same way as the OT. All three layouts have proven to be successful, so who am I to argue?

I have decided to put the choke in the middle facing the same way as the PT, with the OT rotated 90* from them.

Still working out a couple more details for this chassis layout before I make it; I'll post another drawing soon.

Thanks for the help so far. RD

[Rocketrob]

Ok, here's the latest drawing of this chassis after a few adjustments. Just about ready to bend it up. Hey, I even have enough room for another pre amp tube, hmm...maybe tremelo and reverb somewhere down the line. RD

[tubeswell]

I even have enough room for another pre amp tube, hmm...maybe tremelo and reverb somewhere down the line. RD

If you can fit in two or three extra pre-amp tubes on board and if your heater winding is good for it (300mA per 12A_7), you may as well mount the additional sockets and wire the heater pins in now. 2CW

[Bruce / Mission Amps]

...
If you change the choke position in the PS you will change the tone and response of the amp for better or worse, depending on what YOU think of the changes. Why not try it both ways, it's just a couple of wires (and perhaps a resistor and a bigger main filter). Look at examples of amps that use the choke after the OT to see how it's typically done compared to the 5F4.

Chuck

Right...
another trick you can do is to run a small screen node choke (3-4Hy 90ma choke, as found in a 32-50 watt black-silver face Fender amp) ...but wire it in series with the lead wire from the second filter cap to the screen lugs, that feeds the screens.
Because the screen B+ node and the OT's center tap are fed B+ from the same place, you should look at a schematic and layout and you'll see how do it.
Or as suggested by Chuck you could even use a 500 ohm 5 watt resistor.
It does make a subtle difference in the driven tone of the amp... some players actually like it better.
You will have to rebias the power tubes when you do this because the lower screen voltage will reduce the quiescent (cathode/plate) idle current of the power tubes a bit.
However there is something to the interesting sound of these amps with the screen voltage being higher then the plate voltage and then when driven harder, the subsequent sag of the B+ through the OT but not so much sag of the screen voltage...
Yup, it probably is a design defect, as Chuck will point out, but it is what it is and those series of amps sound different because of nuance like that.

[Chuck H]

Yup, it probably is a design defect, as Chuck will point out, but it is what it is and those series of amps sound different because of nuance like that.

There are no design defects that "work". Meaning that something useful or positive is happening and there are no reliability issues. That's my take anyway. I call this sort of thing more of a happy accident. I've had a few myself. We all have. Sometimes they end up as proprietary circuits. Or if your Randall Smith, patents When the 5F4 was designed I don't think the designers expected players to intentionally over drive them. So any pleasant nuances in the clipped sound of those amps due to the circuit design would be a happy accident.

Chuck

[Rocketrob]

Well, being a novice builder(this is my fourth scratch build), I'm going to build the circuit stock for now then tweak it to my liking.

If you can fit in two or three extra pre-amp tubes on board and if your heater winding is good for it (300mA per 12A_7), you may as well mount the additional sockets and wire the heater pins in now. 2CW

There is only room for one more pre amp tube so I was thinking maybe a front end like a Princeton Reverb for a different build. I'm going to make three of these chassis so Ill see what else I can make with them. Three octals and three or four novals covers a lot of possibilities. RD