1973 SF Champ more clean headroom possible?

[alberto2000]

I have a 1973 SF Champ running with a 12AX7A / ECC83 / 7025 preamp and EHX 6V6GT and Sylvania N6R and it starts to break up very early at around volume set to 4 playing with a Jazzmaster on neck pickup. Is this normal? What mods do you recommend to allow for more clean headroom? Swap the preamp tube? Check the bias?
Thanks a lot!

 

[SnidelyWhiplash]

Has the amp been serviced? Not familiar with a Sylvania N6R. The rec tube is supposed to be a 5y3. The amp is 5 watts with an 8 inch speaker. Quite limited in terms of clean headroom. Maybe a more efficient speaker???

 

[Dacious]

Yes. On a Tele it goes 5-6. You could swap a 5751 into the first preamp tube and I'd try a NOS or pull old tube for power.

A Weber sig ribbed ceramic speaker will give you a bit more volume. Ensure you get 4 ohm impedance.

 

[darkwaters]

Replace the 12ax7 with a 12ay7. I’ve done that with both my little amps and it makes them a lot more useable. As an added bonus, it also reduces hum.

 

[uriah1]

Yep. A 5751 or 12ay7
I tried a big watt Weber 8 and it gave overhead but lost something in tone
I ended up with Jensen ceramic in there. Bigger mag.

 

[Wally]

What is a Sylvania N6R? Tube Data Sheet Locator has no info on such a tube. When I do a search, the only reference to that tube is in this thread.
+1 on the question from @SnidelyWhiplash about the service record of this amp. What are your voltages and bias numbers? Where do you have your tone controls set?

 

[alberto2000]

Thanks all for your help. So it is normal for this amp to break up already at volume set to ~ 4 ?

I don't know the voltages not bias numbers. Would need to send to a tech.

On side note, maybe I should open a new thread though: I can't seem to dial in enough treble... I'm constantly at treble set to 10 and it still sounds not sparkly at all. What could the reason be here? Bass is set to around 3.

 

[Dacious]

Thanks all for your help. So it is normal for this amp to break up already at volume set to ~ 4 ?

I don't know the voltages not bias numbers. Would need to send to a tech.

On side note, maybe I should open a new thread though: I can't seem to dial in enough treble... I'm constantly at treble set to 10 and it still sounds not sparkly at all. What could the reason be here? Bass is set to around 3.

What shape are the electrolytics in? If they're original that could be the mud. Could also be the speaker.

Normally on a Fender preamp you don't want to go to ten. The treble control is like a master EQ. Turned too high you will get harshness and early brreakup.

Jazzmasters tend to be dark sounding guitars. When I had one and a Vibrochamp it was never as bright and sparkly as a Telecaster.

 

[Powdog]

Does it still have the original Oxford alnico speaker? Tiny magnet, paper thin cone. I’ve replaced many an Oxford with tattered cones. Weber, WGS and Jupiter all make very good replacements that won’t break the bank.

 

[InstantCoffeeBlue]

W

Thanks all for your help. So it is normal for this amp to break up already at volume set to ~ 4 ?

I don't know the voltages not bias numbers. Would need to send to a tech.

On side note, maybe I should open a new thread though: I can't seem to dial in enough treble... I'm constantly at treble set to 10 and it still sounds not sparkly at all. What could the reason be here? Bass is set to around 3.

Which Jazzmaster? Those pickups are typically a little hotter than vintage style Strat or Tele singles, and some of the more recent Jazzmaster models have overwound pickups. It's going to vary a little from amp to amp, but I'd say 4 is a normal breakup point with hotter single coils like P90s or the Jazzmaster soapbars.

If you're still running a stock speaker, try a WGS G8C.

 

[schmee]

Thanks all for your help. So it is normal for this amp to break up already at volume set to ~ 4 ?

I don't know the voltages not bias numbers. Would need to send to a tech.

On side note, maybe I should open a new thread though: I can't seem to dial in enough treble... I'm constantly at treble set to 10 and it still sounds not sparkly at all. What could the reason be here? Bass is set to around 3.

Yes. They are nasty little buggers.
It should be very trebly though, what speaker is in it?
I tried the last Champ I had with a bunch of different speakers, even a 12" + a 15" together. It didn't help cure the "nasty" that much! But did get a little more bottom end.

However I did build a Champ clone recently using a 15 watt output transformer ($23) and that amp is far more usable playing with others, not near as nasty. It has a bigger cabinet also which helps with low end.

 

[SnidelyWhiplash]

I see that the OP is in the EU. Does he have access to a tech that knows Fender circuits? The choices of components, speakers, etc. may be quite limited.

 

[68goldtop]

Hi!

I see that the OP is in the EU. Does he have access to a tech that knows Fender circuits? The choices of components, speakers, etc. may be quite limited...

Thank you very much for your thoughts!
Fortunately, the supply of components - and knowledge - about these amps is quite good around here (in Europe)!
Unfortunately, the supply of vintage Fenders isn´t quite as good

As to the original question...
Yes, I´d also recommend to have the amp checked out by a competent tech to see if everything is running the way it´s supposed to.
An sf Champ is a "clean" amp - up to about "6" or so. But it only has 6 watts, and it will break up pretty soon - thank god
A stronger speaker will help, but a Champ will be no Princeton or Deluxe.

I´m not a fan of "weaker" pre-amp tubes. Usually, they just make an amp sound more "lame" ...


cheers - 68.

 

[Singin' Dave]

To echo what others have said above:

A SF Champ is a 5 watt amp so you're not playing with much firepower here...

Step 1: Have amp fully inspected to ensure all specs/caps etc. are optimal and so you are starting at best baseline for steps 2 and 3.

Step 2: We are not aware of what speaker your currently using, but if its the stock speaker or a replacement with an efficiency rating below say 96/97db or so, then the very first thing I would try would be to find a trusted brand 4ohm 8" speaker rated at 99db or higher = should equal significant boost in perceived volume and may even help you get to 5 or 6 on volume before breakup (particularly if part of your problem is your old speaker breaking up). This assumes you do not want to try a 4ohm extension cabinet with a larger speaker (RCA to female 1/4 adapter + speaker cable)

Step 3: Source one decent ANOS (that's "almost" New Old Stock = a decent pull vs. NOS to save $) 12ax7, 5751 AND one 12ay7 pre-amp tube. Try each of them in V1 (aka "tube rolling") and see what happens. With the lower gain 5751 or 12ay7 you may really like that you have more range in your volume knob before breakup. BUT you may not like the tone as much with one of them at say "6" vs. a 12ax7 at "4".... its all taste. I suggest sourcing another quality 12ax7 as well as the lower gain tubes as no two are the same.

And...no idea what a N6R rectifier tube is nor what it would be doing in that amp

 

[Chiogtr4x]

W


Which Jazzmaster? Those pickups are typically a little hotter than vintage style Strat or Tele singles, and some of the more recent Jazzmaster models have overwound pickups. It's going to vary a little from amp to amp, but I'd say 4 is a normal breakup point with hotter single coils like P90s or the Jazzmaster soapbars.

If you're still running a stock speaker, try a WGS G8C.

I gotta watch my $$ right now, but I will be getting one of these WGS speakers (8 ohm), soon, for my little Monoprice 5-watter.

Amp sounds very good- clean 1/2 way up- nasty (good nasty') and raw, cranked.
I'd like a little more volume, but want to keep things portable ( no 10" speaker, no ext. Cab). This speaker seems like it will do just that.
Fun amp!

 

[68goldtop]

Hi!

I gotta watch my $$ right now, but I will be getting one of these WGS speakers (8 ohm), soon, for my little Monoprice 5-watter...

Nothing wrong with the WGS G8C - but it´s not super loud or sensitive... just saying´...

A Jensen RI C8R might be another good choice.


cheers - 68.

 

[Chiogtr4x]

Hi!

Nothing wrong with the WGS G8C - but it´s not super loud or sensitive... just saying´...

A Jensen RI C8R might be another good choice.


cheers - 68.

Yes, that seems like a popular pick too- don't have specs in front of me now, but thinking that ( specs) the WGS may be just a little more efficient?

No matter right now- as ' fun gear $$' has become 'gig gas $$', Ha!

 

[kuch]

My 70's SFVC starts to break up around 4 1/2 - 5. It sounds great at that level. A couple of years ago I got a weber speaker and replaced the original. It sounded the same. The biggest difference I noticed is when I replaced all of the tubes earlier this year. It "cleaned" it up a bit but not in a negative way. If I need any more headroom, I just plug into my PRRI.

 

[InstantCoffeeBlue]

I gotta watch my $$ right now, but I will be getting one of these WGS speakers (8 ohm), soon, for my little Monoprice 5-watter.

Amp sounds very good- clean 1/2 way up- nasty (good nasty') and raw, cranked.
I'd like a little more volume, but want to keep things portable ( no 10" speaker, no ext. Cab). This speaker seems like it will do just that.
Fun amp!

I think you'll love it! I've heard great things about the Jupiter 8SC as well.

 

[alberto2000]

Thanks all for your help. Some answers to your questions:

- Electrolytics have been updated 4 years back
- Jazzmaster has Antiquity II pickups and standard orange drop capacitors, 1M pot
- Speaker is Weber Signature Series

Updates:

- Swapped tubes to TAD 12AX7 preamp, 5V4GA / GZ31 rectifier, TAD 6L6GC power
- Placed 100pF ceramic capacitor in between volume lugs 1 & 2

The amp sounds beautiful now playing at volume 3 to 4 has a beautiful clean and sparkly sound and going through the -20db attenuator (Two Notes Torpedo Captor) I can play in my apartment!

I measured the dissipation:

pC = 36.7 V / 1000 ohm = 0.0367 mA
pV = 370 V
pD = 370 x 0.0367 = 13.579 W

Does that look right to you?

Thanks so much all of you!