tweed ohm question

[andyEvans692]

Hi all.

Im pretty new to building amps, so far I have built a couple of stewmac kits, champ, and tweed deluxe , I have decided to build a Princeton tweed , using a small kit bundle from here in the uk .. ( no speakers included )

So looking at both the diagrams of my stewmac champ , and the Princeton tweed they are very close with only a couple of slight differences ..

So I half expected the Princeton to be 4ohm the same as my champ , but on the circuit diagram its listed 8ohm , I checked their Champ diagram that is also showing 8ohm ..??

Can anyone please help me to understand how we get to 4ohm and 8ohm , would this be to do with the transformers ?

as I mentioned im pretty new to building these , I will add the pics of the diagrams for comparison .. thank you. .

 

[corliss1]

Yes, this is dependent on the output transformer you're using.

 

[jrblue]

Those kits must must not be exact clones or the speaker output would be 4 ohms. Though, as you may know, a 4 ohm stock, real Champ will drive an 8 ohm speaker with no problem. Fender amps can generally work with double (or half) the specified speaker ohms w/ no problem.

 

[andyEvans692]

cheers , this is where I was getting confused .. I looked at their output transformers and they are using 8ohm , they listed it saying that it give a wider selection of cabs ..

Shame they haven't kept to the period correct 4ohm output , but then saying that its going to have a 10" speaker in it , and they would have only had 8" I think..

im guessing with the ohm its not going to matter as long as I select an 8ohm speaker ..

Now there's a whole other issue , what speaker ??

 

[Michael Smith]

Oddly enough, a real Fender Champ calls for a 3.2 ohm speaker load. Unless they had the speakers custom made, I'm not aware of any 3.2 ohm speakers, so I assume they actually used a 4ohm.

 

[Paul G.]

The output transformer determines the speaker impedance. The ratio of turns (input to output) takes the low voltage/high impedance signal from the power tubes and transforms (yup) it into high voltage/low impedance. The power tubes have an output impedance, and the turns ratio determines the reflected impedance.

The kits probably use a transformer for 8 ohm output because nowadays 8 ohm speakers are much easier to get than 4 ohm. Match your speaker to the kit maker's recommendation since it is related to the output transformer included in their kit, not the original spec.

 

[2L man]

When Negative FeedBack is used its series sample resistor value sometimes need to be changed if OT output is changed. From 4 to 8 ohms about 1,4 times higher.

 

[Lowerleftcoast]

Now you know it is the OT that determines the difference.

FYI
The 8 Ohm OT connection to the 22k NFB resistor makes a circuit change. To maintain the same level of NFB the resistance would have to be larger than 22k.

Some OTs come with 4-8-16 Ohm taps. If you were to only use an 8 Ohm speaker the 8 Ohm tap would be used for the speaker. To maintain the same level of NFB the 4 Ohm tap would connect to the 22k NFB resistor.

The level of NFB is a choice. Fender made a choice for a 22k resistance. OMMV.

 

[Powdog]

A 33K Ohm NFB resistor with an 8 Ohm speaker will give you about the same amount of NFB as a 22K Ohm with a 4 Ohm speaker. You can also put a potentiometer in the NFB loop for adjustable NFB, kind of a pseudo-Presence control. A slightly larger OT and 10” speaker will give you a different sonic experience than your 5F1, which as you’ve already mentioned is basically the same amp.

I installed an adjustable NFB control into one of the input jack holes on this 5F2a, a “VooDoo” control complete with a skull knob.

 

[King Fan]

All good info here. If you already built a 5F1, you might especially consider an 8-ohm OT and 10" speaker in the bigger 5F2a cab; a small-speaker 4-ohm 5F2a is awfully close to a Champ. As mentioned, there are tons more choices in 8Ω (and 10") speakers, and the NFB adjustment is easy (and even optional, I suppose).

If you went with the Mojo/StewMac wiring diagram, the *build* could also be boringly similar to the 5F1. If you want to up your game and build a slightly safer, quieter amp, you could improve on their standard power entry and grounding.

Even a simple amp like a 5F2a gives you many choices -- and their consequences -- way beyond 4 v. 8Ω, cab size, speaker and speaker size. What PT and OT? (These are important.) Does your PT have a 6.3V center tap? Can you fit a separate power switch (*really* good idea)? Can you make that a UK/EU-correct DPST switch?

We've had a few recent 5F2a build threads, some from east of the Atlantic, that would be worth reading. For layout purposes -- and I'm not saying you have to do any of this -- I once drew up a 5F2a version that might be close to what I'd personally build these days. Just food for thought:

 

[andyEvans692]

thanks guys , some really interesting things you've all mentioned Im pretty excited for this build , Thinking its going to make a good indoors amp , I find the tweed deluxe a little loud , the champ is ideal , now with the Princeton tweed with a 10" should fill the gap ...

 

[peteb]

Oddly enough, a real Fender Champ calls for a 3.2 ohm speaker load. Unless they had the speakers custom made, I'm not aware of any 3.2 ohm speakers, so I assume they actually used a 4ohm.

I am pretty sure they are referring to the 3.2 ohm DC resistance that a 4 ohm impedance speaker has.

I agree that 4 ohm speakers are common and 3.2 ohm speakers are not. The exception is that Weber makes a 3.2 ohm speaker, in modern times, to meet this spec.

why call out DCR? Because it is easier to measure DC resistance than it is to measure AC impedance.

I have one of the old speakers from a champ that calls out 3.2. I am near certain that it measures 3.2 ohm DCR.

 

[peteb]

Can anyone please help me to understand how we get to 4ohm and 8ohm , would this be to do with the transformers ?

i think it has all been explained well above.

I just want to put it another way.


if the OT turns ratio were 1000:1, a 4 ohm speaker would create a 4K ohm impedance load for the power tube to work into.

the same 1000:1 OT could be paired with an 8 ohm speaker to create an 8K ohm impedance load for the power tube to work into.

 

[Paul G.]

I guess somebody left some doors open, because we got a lot of interesting answers to questions not asked. Just follow the spec's given to you for this amp by the supplier as they provided your output transformer, and know the turns ratio. If you want to confirm, just look up the spec's on the provided part. There will be a model number.

 

[peteb]

I believe I made an error in post 13.

the voltages on the primary and secondary windings are proportional to the windings turns ratio.

the impedance multiplier is equal not to turns ratio, but turns ratio squared.

so the 1000:1 impedance ratio OT must have the square root of 1000, 31.6:1 winding turns ratio.

 

[King Fan]

I guess somebody left some doors open, because we got a lot of interesting answers to questions not asked. Just follow the spec's given to you for this amp by the supplier as they provided your output transformer, and know the turns ratio. If you want to confirm, just look up the spec's on the provided part. There will be a model number.

LOL, I kinda figure doors *should* be open to try to help someone who says they're 'pretty new to building amps.' Post 2 basically answered the question asked -- but that question clearly had company. For example, the OP was worried in post 4 that he was departing from vintage correctness, and so folks stepped in to note that going from 4 to 8Ω wouldn't hurt and could help a lot choosing speakers and speaker size.

Then more folks offered helpful insights on the one thing that would change, the NFB ratio, which he wouldn't consider if he "just followed" the Modulus plans.

And we already see in post 1 he's tempted to look back at his former Mojo/StewMac plans -- a mashup like has been a problem for many builders.

I'll be the first to admit that the Modulus layout is better (and UK/EU correct) on power entry and grounding than the Mojo or StewMac. And for sure my old drawing isn't needed -- as long as he isn't mixing old plans into his new build. For ease of viewing here's the Modulus layout he included in his first post. But no small-shop (or Fender) drawing can be trusted to be unambiguous -- the diagram implies isolated input jacks, but the note implies Switchcraft style. Oh well.

Finally, their website doesn't seem to have any details on his OT except it's 7K:8Ω and *seems* to go in both their 5F1 and 5F2a kits. Let me know if I've answered too many more unasked questions...