'65 Vibro Champ - lack of treble?

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JSett
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'65 Vibro Champ - lack of treble?

Post by JSett » Sat Sep 11, 2021 10:25 pm

So, my old BF VibroChamp has been pretty 'dark' sounding since I got it (I've alway had to have the bass down pretty low) but it seems to be getting noticeably harder to get much bite or top end out it recently. Even with notoriously bright Offset guitars.

The filter cap seems to be the original or just pretty old (strange choice to leave in place despite it clearly having other caps replaced on the board) so I'm in the process of swapping that over. Unfortunately my 65w iron isn't quite up to the job of desoldering the old can lugs so I'm only halfway through this until later today when I get my hands on a bigger one.

But, is there somewhere else I should be looking for reasons this might be so lacking in treble? I had a Silverface one before and it was WAYYY brighter. I've tried swapping out tubes to eliminate that as a source and it made no real difference (currently running a bunch of NOS glass).

On a related note, my Weber book shows the can should be connected as 20-20-20 and the one in there is 40-20-20. The replacement I have is a 20-20-20-20 so should I put it in as per the Fender schematic and leave the 4th lug unconnected?

Thanks in advance

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Re: '65 Vibro Champ - lack of treble?

Post by Sauerkraut » Sun Sep 12, 2021 12:08 am

It’s probably something else, because I think they hardly ever break, but have you already excluded the possibility of the treble pot being the culprit?

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Re: '65 Vibro Champ - lack of treble?

Post by JSett » Sun Sep 12, 2021 1:12 am

Sauerkraut wrote:
Sun Sep 12, 2021 12:08 am
It’s probably something else, because I think they hardly ever break, but have you already excluded the possibility of the treble pot being the culprit?
Yeah, I tested all the pots with the meter while I was in there.

The new iron actually landed at 9am today (thanks Bezos) so the new can is in and tested. Things definitely feel a little tighter. I went with 20-20-20 as per the original BF schematic and dropped in a 1/4" socket for the vibrato switch while I was there.

A bit of extra reading leads me to there being a difference in circuits over the years with the bypass cap from the second gain stage. Marked as 25uf on the originals and, supposedly, changed to 2uf in the 70s. Mine has a replacement at 10uf in there from a previous owner.

There seems to be more transformer hum now though than before :fp: or maybe I'm imagining it. Ergh.
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Re: '65 Vibro Champ - lack of treble?

Post by JSett » Sun Sep 12, 2021 2:01 am

Chassis back out, wired the extra capacitor section in, hum completely gone. In fact the amp is ridiculously quiet now too. It was already pretty good but the cap change has helped for sure. It was the original too (dated 1964) and actually had no signs of leakage or anything but not worth the risk considering I had a replacement ready to go.

I've been through with the multimeter and all the components that count seem to measure well enough in range of spec to eliminate those as an issue. Maybe I'm overthinking it. It has a Weber Classic Alnico in there (original in a box somewhere) that came in it to me...maybe that's the culprit??

Made such a huge difference having a decent sized soldering iron for the cap too. It's so much bigger than my small works one it's laughable. So much so I made a meme about it

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Re: '65 Vibro Champ - lack of treble?

Post by Sauerkraut » Sun Sep 12, 2021 8:49 am

johnnysomersett wrote:
Sun Sep 12, 2021 2:01 am
It has a Weber Classic Alnico in there (original in a box somewhere) that came in it to me...maybe that's the culprit??
That'd surprise me, but... you wouldn't happen to have a ceramic 8" lying around to compare?

I suppose the treble tone cap hasn't dramatically drifted out of spec? Or the 100k tone slope resistor? Guess you already checked though. If it wasn't obvious yet, I must admit I'm out of my depth here, because even if those did, I doubt the result would be as noticeable as what you describe.

A Fender amp that's not bright... it's weird. Glad to hear it's at least quiet now though.

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Re: '65 Vibro Champ - lack of treble?

Post by JSett » Sun Sep 12, 2021 9:00 am

Sauerkraut wrote:
Sun Sep 12, 2021 8:49 am
johnnysomersett wrote:
Sun Sep 12, 2021 2:01 am
It has a Weber Classic Alnico in there (original in a box somewhere) that came in it to me...maybe that's the culprit??
That'd surprise me, but... you wouldn't happen to have a ceramic 8" lying around to compare?

I suppose the treble tone cap hasn't dramatically drifted out of spec? Or the 100k tone slope resistor? Guess you already checked though. If it wasn't obvious yet, I must admit I'm out of my depth here, because even if those did, I doubt the result would be as noticeable as what you describe.

A Fender amp that's not bright... it's weird. Glad to hear it's at least quiet now though.
All the components are still enough in spec to not be the culprit.

I do have 8" ceramics to hand but not in the abnormal 4 ohm that a Champ requires. It's a bit of an oddball in that respect.
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Re: '65 Vibro Champ - lack of treble?

Post by Sauerkraut » Sun Sep 12, 2021 11:44 pm

johnnysomersett wrote:
Sun Sep 12, 2021 9:00 am
Sauerkraut wrote:
Sun Sep 12, 2021 8:49 am
johnnysomersett wrote:
Sun Sep 12, 2021 2:01 am
It has a Weber Classic Alnico in there (original in a box somewhere) that came in it to me...maybe that's the culprit??
That'd surprise me, but... you wouldn't happen to have a ceramic 8" lying around to compare?

I suppose the treble tone cap hasn't dramatically drifted out of spec? Or the 100k tone slope resistor? Guess you already checked though. If it wasn't obvious yet, I must admit I'm out of my depth here, because even if those did, I doubt the result would be as noticeable as what you describe.

A Fender amp that's not bright... it's weird. Glad to hear it's at least quiet now though.
All the components are still enough in spec to not be the culprit.

I do have 8" ceramics to hand but not in the abnormal 4 ohm that a Champ requires. It's a bit of an oddball in that respect.
Both my old amps want 4 ohm too. One of them accidentally had an 8 ohm speaker for in for some months, but nothing was damaged.

If you have an 8 ohm speaker, I’d personally feel comfortable testing it in there just to determine if the speaker is the issue. The impedance mismatch shouldn’t cause any trouble unless you turn it up loud and leave it on long.

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Re: '65 Vibro Champ - lack of treble?

Post by horseblanket » Mon Sep 13, 2021 10:07 am

Isn't it ok to go 4 - 8?
I always thought that direction is fine but going 8 - 4 is where you will have problems.

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Re: '65 Vibro Champ - lack of treble?

Post by JSett » Mon Sep 13, 2021 11:37 am

Yeah, it'll probably be fine with 8 for testing purposes. I'll dig one out and try it at some point.

I've actually managed to improve the situation a little by, of all things, removing my volume pedal out of the equation. It seems that was sucking out a fair bit of high end from the guitars (yes, I had tried just direct too and it was still darker than desired).
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Re: '65 Vibro Champ - lack of treble?

Post by Sauerkraut » Mon Sep 13, 2021 9:58 pm

horseblanket wrote:
Mon Sep 13, 2021 10:07 am
Isn't it ok to go 4 - 8?
I always thought that direction is fine but going 8 - 4 is where you will have problems.
As I understand it, running a higher impedance speaker, besides lowering the overall output, can cause damage over time if you have a cheap, shoddy output transformer, or if the output transformer is old. I assume ‘65 Champs have very high quality transformers, but they’re hardly new.

Re: OP, I suppose one of those treble bleed mod contraptions would work in a volume pedal as well?

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Re: '65 Vibro Champ - lack of treble?

Post by JSett » Mon Sep 13, 2021 10:05 pm

Sauerkraut wrote:
Mon Sep 13, 2021 9:58 pm

Re: OP, I suppose one of those treble bleed mod contraptions would work in a volume pedal as well?
I think it was more the case that it has a 250/500k pot in it and my Jags/JMs/Mustangs run 1meg pots so it was slightly darkening the source when the whole setup was in play.

There's still not a lot of treble on tap going straight in, but from further research that's apparently not all that uncommon - and a capacitor was changed in the circuit in the 70s that brightens them up. Maybe that's what I'm basing my analysis on and subconsciously building it up to be worse in my head.
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Re: '65 Vibro Champ - lack of treble?

Post by Sauerkraut » Mon Sep 13, 2021 10:17 pm

johnnysomersett wrote:
Mon Sep 13, 2021 10:05 pm
Sauerkraut wrote:
Mon Sep 13, 2021 9:58 pm

Re: OP, I suppose one of those treble bleed mod contraptions would work in a volume pedal as well?
There's still not a lot of treble on tap going straight in, but from further research that's apparently not all that uncommon - and a capacitor was changed in the circuit in the 70s that brightens them up. Maybe that's what I'm basing my analysis on and subconsciously building it up to be worse in my head.
If that’s the case, I’d go ahead and swap that cap. I’d do that even if it had been all-original and untampered with, cause I’m a godless brute (and can’t get along with dark amps), but that one clearly already had quite a bit of work done to it anyway.

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Re: '65 Vibro Champ - lack of treble?

Post by JSett » Mon Sep 13, 2021 10:29 pm

Sauerkraut wrote:
Mon Sep 13, 2021 10:17 pm
johnnysomersett wrote:
Mon Sep 13, 2021 10:05 pm
Sauerkraut wrote:
Mon Sep 13, 2021 9:58 pm

Re: OP, I suppose one of those treble bleed mod contraptions would work in a volume pedal as well?
There's still not a lot of treble on tap going straight in, but from further research that's apparently not all that uncommon - and a capacitor was changed in the circuit in the 70s that brightens them up. Maybe that's what I'm basing my analysis on and subconsciously building it up to be worse in my head.
If that’s the case, I’d go ahead and swap that cap. I’d do that even if it had been all-original and untampered with, cause I’m a godless brute (and can’t get along with dark amps), but that one clearly already had quite a bit of work done to it anyway.
Yeah, I likely will. All the replaced components are at stock values and have likely just been changed through general maintenance. I'm switching the transformer this week so it runs on 240v and I can ditch the step-down. I might change that out while I'm at it
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Re: '65 Vibro Champ - lack of treble?

Post by sookwinder » Tue Sep 14, 2021 7:21 am

I have spent a lot of time working with the VC amp circuit, back in the day electrolytic capacitors were very expensive hence Leo chose the smallest available cap, so those very early VC have far less filtering than later model. For me what ever the year SF or BF I always replace the cap can with a 40/20/20 version (whether that is a 40/20/20 or a 20/20/20/20 and two of them in parallel to create the 40 it doesn’t matter)

There are a shit load of genuine VC schematics out there , even within various up grades to the VC, I like the last BF version through to the early SF version but with the proviso to have the 40/20/20cap can

As far as the 8ohm speaker yes it will make it sound a little darker then the correct 4ohm one
But it won’t kill the tranny
Have you got two 8 ohm 8” speakers?
Connect in parallel and see how it sounds

Also stick a shorted RCA plug into the vibro female RCA foot switch connector and turn the vibro pots down to zero and that will give you more (non vibro) gain

I have had a treble tone cap die on me in one of my VCs, things don’t last for ever

Later SF versions had an extremely small cap across to if the 6V6 pins to stop oscillation
Yours would not have that and should not need it
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Re: '65 Vibro Champ - lack of treble?

Post by luau » Tue Sep 14, 2021 7:30 am

sookwinder wrote:
Tue Sep 14, 2021 7:21 am
Have you got two 8 ohm 8” speakers?
Connect in parallel and see how it sounds
Do it!

Sometimes I run mine through a pair of vintage C12Rs in a Bandmaster cab and it sounds immense and awesome.

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