Hoffman Plexi 50 questions

[circuitcrafter]

Hi all, and Doug too, if you are reading.
 
 I have a few questions about this Plexi 50 build.


1. My power tranny, (Weber) has a 6.3volt center tap. Do I still need to install the 100 ohm resistors in the circuit? It also has a
    feed for the bias, a red/blue wire. I am guessing that I would cap this wire off?


2. For the Hot Switch Mod, the caps C1a & C4b are .68uF. I have installed .68uF/50 volt electrolytics. I do have some .68uf/160 volt Sozo's. I am  just unsure what type and voltage rating of caps to use here. Any recommendations?


3.  There is a slight turret board and layout difference which I've included a picture. I have an alternative to make it work, I'm just pointing it out in case someone
     else should happen to run into this.


All help is appreciated so I can make up a parts list of things I need to complete.


Thanks all,


...Sean...

[TIMBO]

Hi Sean, Welcome aboard. That should be a great build.

1. The transformer has a centre tap on the heater winding, so no need for the 100r resistors. The 100r resistors make an artificial centre tap for heaters that have no centre tap.
The bias tap is needed for the power tubes bias. This is not needed if you choose a cathode bias for the power tubes, OR you use a Marshall type circuit that is taken off one of the HT legs.

2. The .68 caps will only see a couple of volts so 25-50v should be enough and they don't have to be Ecaps but if you don't have any thing else they will do.

Good luck.

[PRR]

> power tranny, ...has a 6.3volt center tap. Do I still need to install the 100 ohm resistors

In my opinion...

Get in the habit of trimming and shrink-taping the 6V CT, and blowing the 25 cents for two resistors.

The CT simplifies and cheapens manufacturing. It usually works good UNTIL a fix-bias Power tube develops a Heater-Cathode short. Then the PT smokes and dies.

With the 2*100r centering, with tube short, one resistor runs a tenth-Watt warm, the hum level may rise, but nothing smokes.

True, with cathode bias and a short you may not smoke a PT. Sounds like you plan cathode bias. However I would stay with 2*100r for consistency. In DIY you can afford the few pennies. 2*100r is a super-common thing which will not confuse any future amp-techs (including you in 2024 when you have forgot what you were thinking today).

Traditionally the PT CT was pennies cheaper, so was favored for mass production. However the cost of resistors has been steady/falling for 40 years, while the cost to stop a winding and bring out a wire has gone up. Many new factory amps use the 2*100r or similar, I assume because it got cheaper than a CT lead.

> so I can make up a parts list

For-sure put 2*100r on your order. When you actually hot-up the soldering iron, you can still do one or the other way, or try both. Won't hurt to end up with 2*100r "extra" for your next build.

[TIMBO]

I like PRRs reply. 

[lego4040]

Will be following this plexi and other plexi threads. My Hammond that I just got doesnt have the center tap and now that I know this about the 100 ohm res's I will keep that in mind

[shortfuse]

I like PRRs reply. 

Ditto

[EL34]

3.  There is a slight turret board and layout difference which I've included a picture. I have an alternative to make it work, I'm just pointing it out in case someone else should happen to run into this.

Hi,
It looks to me like you are using an older Plexi board with the new layout diagram
I changed the layout of the one ohm and screen grid resistors a while back

Not a big difference  in the two layouts

You have this layout
The screen grid resistors go to the lower lugs
The one ohm resistors go to the ground lugs

[EL34]

Do not start up that amp like you have it wired
It looks to me like you have the One ohms going to the power supply lugs


Look at this image
I point to the lugs where the screens and the one ohms go
And you need a buss wire going to the two screen grid lugs

[circuitcrafter]

Thanks for the replies everyone.

I did not think to check the old
layout to compare to this board.
I am aware of the jumper needed, just
haven't installed, and I'm out of 1 ohm resistors.
Those will be on a parts order soon.

I haven't had a chance to work on the amp in a week, so I will study the old layout as well.
Its my Friday today, so I will be able to get back into
"amp mode" again.
Thanks again,
...Sean...

[EL34]

The new layout is the same except for screen grid and one ohms got shifted to better utilize the spacing

You can use the new layout
I have already noted the differences in my photo above

[circuitcrafter]

Light bulb moment...


Yes, I see now, Doug. I received the board, and then a few weeks later you updated your layout diagrams. Really nice I might add. I looked at the older layout and that's what this board is.


 The big 1k 5w's are slightly lifted, and then I was going to put the 1 ohms slightly below. But after looking at your older layout, I will do it that way, so I don't have the left 1 ohm resistor slightly angled to meet the turret. This is the spot that I was confused, and therefore laid it out this way. I studied the older diagram and said "oh, ok!!".


The only 1 ohms I have are 1/2 watt, so I did not put those in.


For the 1k ohm 5w resistors, will this type work (in pic), or would it be better to put the white square ceramic type? I have not shifted them yet (in pic).  Also, the older BOM lists 1 ohm, 2 watt, and the newer BOM lists 1 watt. Any particular preference?


I do have 10/uF 100v caps to go in next, but they are radial. I know they'll work, but I don't want to spoil the clean layout with those.


Here is a pic with work in progress, as of last week. I still yet have to pull apart the rest of the chassis components, and still making a parts list. The Weber has a cap board, vertically squeezed next to the main board, but I am going to
go with the 50/50 cans. So much cleaner!!


With all this research, I actually think I've figured out why I could not get the Weber to work, but since it's torn apart now, I'm going to continue with this really nice Hoffman.

[EL34]

The one ohm wattages do not matter
use whatever you have

[PRR]

> The only 1 ohms I have are 1/2 watt, so I did not put those in.

What Doug said. You can't get 1 Ohms "too small" for this.

A BIG tube could draw 100mA or 0.1 Amps. In 1 Ohm that will drop 0.1V (100mV). ).1 Amps times 0.1 Volts is 0.010 Watts (1/100 Watt).

Your half-watt jobs are 50 times bigger than they need to be.

> older BOM lists 1 ohm, 2 watt, and the newer BOM lists 1 watt. Any particular preference?

Whatever you can-get/have.

You could probably find 0.012W parts in an SMD catalog; but if you drop one you will never find it again.

1 Ohm is an odd size for 1/4W parts. Tube cathode monitoring is an odd application. Most other folks using 1 Ohm resistors run a lot of current in them. So they need 1W, 2W, 5W or more. These are the most likely sizes of 1 Ohms on the market. Any of them is over-kill for us, but the price is not a problem and size is usually no real problem.

> I have are 1/2 watt

Use them.

[drew]

Why do tube guitar amp builders use 1 ohm resistors for this purpose, while tube hifi guys seem to favor 10 ohm?  I'm dreading that the explanation may involve math, but I want to know the answer anyway.

[circuitcrafter]

Thanks Doug, Timbo, and PRR.


I've seen those 1 ohms at different wattage values in some schematics, and then also straight to ground on others, so that was the basis of that question. I have a bunch of 1/2w - 1 ohm resistors which I'll use, and a lesson learned.


I will also cap the center tap and use the 2, 100 ohms on the heater supply, and I think I'll use the radial caps I have for now. I can bend them in to look nice.


 This was my first amp build. I've built 3 completely from scratch since and those worked fine. This one has been on my bench for several years now, and now I feel I can finally finish it.




Thanks for all the help. I am only a "highly experienced beginner", so all your help guys is greatly appreciated!

[PRR]

> guitar amp builders use 1 ohm resistors for this purpose, while tube hifi guys seem to favor 10 ohm?

1) Tradition.

Hi-Fi guys have checked bias current for decades. Until digital meters, readings much below {math ahead!!} 1V were fuzzy. 50mA in 1 Ohm is 0.050V, way too low on a 1V scale. 50mA in 10 Ohms is 0.5V, easy to read on 1V or 1.5V scale.

Guitar amp users "never" cared about bias current. If the tube didn't melt, they played. Somehow the idea of checking bias current has gotten trendy. But this is after digital meters became common. The basic DMM reads 199mV, so it can very easily read 0.050V.

2) Tone

The more resistance you stuff under the cathode, the lower the distortion but the more drive you need. Hi-Fi guys are not afraid to add 10 Ohms here. You really have to go over 20-50 Ohms to make any real change. However guitar gurus are very afraid of any change in classic designs, and pick the lowest useful value for the least change of tone.

3) History of Parts

Until transistor power amps, resistors smaller than 10 Ohms were very very rare, and never stocked at the local parts store. Transistor amps use 0.1 Ohm to 1.0 Ohm emitter resistors, so as these burned-up the parts stores started stocking the low-Ohm parts. Now you could actually buy a 1 Ohm.

[circuitcrafter]

I was thinking about that after Drew's comment...
Tone. If I'm not mistaken, would there be a wider frequency demand on an output section compared to a guitar amp, requiring a larger draw on the tube?  Just listening in, and curious myself..