Champ circuit preference? (pls vote even if you don't post)

[Colossal]

For those of you that have some experience playing Fender Tweed Champs, I'm wondering if you have a preference between the earlier 5E1 or later 5F1 circuits...

The major differences are:

5E1
1) Higher gain
2) V1a fully bypassed
3) Entire B+ supply is choked
4) 6V6 plate and screen fed from same B+ node.

5F1
1) Lower gain
2) V1a unbypassed
3) B+1 feeds 6V6 plate, B+2 feeds 6V6 screen
4) 10k dropping resistor between B+1 plate and B+2 screen supply

Presumably, for the later 5F1 model, Fender dropped the choked supply in favor of the 10k dropping resistor as a means to save some money. I'm wondering how the 5E1 and 5F1 might differ tonally and with regards to feel. There is some variance in the record as to the PT secondary voltages these amps used. For the sake of argument, let's presume both a 5E1 and 5F1 utilize the typical Fender 330-0-330 '59 Tweed Champ PT and a 5Y3GT rectifier. The major difference then is going to be the sound of the choked supply with almost equal plate and screen voltages vs. the 5F1's C-R-C power supply and unbypassed (lower gain) V1a.

Thoughts and opinions?

Thanks in advance guys.

[Firestorm]

Just to muddy the waters, every 5F1 I have ever seen (about a dozen of them) had V1a bypassed in the conventional way. I can only guess that Fender assumed they'd get enough output from the increased B+ of not choke filtering the whole supply, drew up the schematic, then changed their minds. Who has seen an unmolested 5F1 with V1a unbypassed? (It would likely have to be one of the earliest ones).

[ampdan]

And to muddy the waters even more, my completely stock '55 5E1 has the 5F1 circuit..

[echuta13]

I haven't seen a 5F1 without a bypass cap on the first stage either. Fenders schematics are not always accurate... I would take the 5F1 circuit over the 5E1, but I would be tempted to add the choke to the 5F1 design, go to SS rectification, and bump up the filtering a bit.

[Firestorm]

And to muddy the waters even more, my completely stock '55 5E1 has the 5F1 circuit..

ampdan,

So is V1a bypassed on yours?

[ampdan]

Yep.

[surfsup]

Not sure the effect but the 6v6 plate is filtered better and the 10k will drop more voltage than the choke too?

Well actually the choke drops the plate current too.

[Cliff Schecht]

The difference between the two circuits is pretty negligible. When you consider that Fender actually bypassed V1a in the production amps, it makes them even closer.

The difference between a choke and a 10k resistor is about the only difference, and in a small SE amp the difference will be negligible. The choked version shows 320-->305 and I suspect that the 10k resistor will have similar numbers using the same power transformer. If the preamp draws say 3mA then through the 10k resistor you are only dropping about 30V. This isn't much different than the choke and I suspect that you could put the two versions side by side and not hear or feel a difference (all other factors being the same of course).

[Colossal]

Super cool guys, thanks for the discussion on this. Please keep it comin'.

1) Yes, I know there is a fair amount of variance and the Fender schemes are not always accurate so we have to take that for what it is.

2) I too have seen a seen a Champ labeled 5E1 that was clearly a 5F1.

3) the difference in the power supplies is what interests me specifically. If we assume that an ideal 5E1 and 5F1 both have a 330-0-330 PT, then the 5E1 will have a better filtered HV overall with the C-L-C filter before the power tube and preamp. Second, the 5F1 with its 10k dropping resistor on the screen supply would have a slightly lower screen voltage and I would suspect that under load this voltage would be somewhat lower than the 5E1's.

Both amps will not experience sag being single ended, but their equilibrium voltages at idle and under load I would suspect be slightly different.

My main question though is, can you tell a difference between the two when cranked or do both have a pretty similar spongey growl reminiscent of the Reverend Billy G's iconic works. I guess I am trying to determine if there is a significantly audible difference between one circuit and the other. I've generally observed that the 5F1 seems to be the more popularly cloned circuit. That could be for the lower cost due to the lack of choke and easier for the majority of (first time) builders. Most people seem to bypass V1a anyway or make it switchable to 1k5 (alone) - 0.68uF - 22uF.

[echuta13]

I would think that the choke would stiffen the power supply and minimize 120hz noise, so not sure how spongy the amp would be versus one without it. The 25uf cathode bypass cap on the output tube would probably be where some of the sponge is coming from. Popping a 1000uf/50v cap in its place would verify that.

[rdjones]

Is the choke the same (or similar) as the later "small" choke as used in Blackfaces ?

rd

[Colossal]

I would think that the choke would stiffen the power supply and minimize 120hz noise, so not sure how spongy the amp would be versus one without it. The 25uf cathode bypass cap on the output tube would probably be where some of the sponge is coming from. Popping a 1000uf/50v cap in its place would verify that.

Yes, the choke filtered supply of the 5E1 certainly is going to do a better job of filtering. With the choke at 5H ~400R DCR and C2 at 8uF before the 6V6, I get -26dB reduction in ripple and a drop of about 3V across the choke. This puts the plate and screen at roughly the same voltage, but with much lower ripple.

The 5F1 power supply would be just the capacitor input feeding the plate and with the 10k/8uF filter feeding the screen, a -36db ripple reduction to the screen voltage with a hefty voltage drop. The large reservoir cap at 16uF would make up some for the reduced ripple filtering compared with the choked 5E1 supply.

The 5E1 circuit should have a higher preamp voltage because it only sees the plate and screen voltage reduced by the 22k dropping resistor where the 5F1 sees 22k-8uF-10k, a larger drop.

One major variant that I've seen is that some Tweed Champs are reported having a 250-0-250 PT and others 330-0-330, both with 5Y3GT rectifiers. Obviously, the 330VAC secondary is going to have a significantly higher plate voltage. Both circuits are biased with 470R/22uF. The cathode voltage according to Fender is 19V (37mA plate current).

[Colossal]

Is the choke the same (or similar) as the later "small" choke as used in Blackfaces ?

rd

RD,

The choke data I have for the Tweed Champs is 5H and about 400ohm DCR, rated to about 40mA conservatively. The choke is quite tiny, about the size of a Fender reverb transformer.

[LeftyStrat]

Dang, I was going to vote, but didn't see the option "All Single Ended Amps Suck."

Just kidding. I wasn't going to really vote.

[Alexo]

It's really like asking if Chinese food or Thai food tastes better. Thai food.

Seriously though, choke vs resistor in that slot will change the tone and feel, I find that in general, a choke gives you more definition and clarity all the way down the line, but if you're plugging into a tweed champ, you are probably not doing it for the definition and clarity.

I strongly recommend just setting up a rig that lets you switch between the choke and resistor and seeing which comes closer to what you're looking for.

...personally, I like the sound of a champ with the screen tied to the plate supply as in the 5E1, more cluck and bite...