1963 Fender Twin, Tonally Impaired. Assistance appreciated.

[keithb7]

Hi folks. I own a 1963 Fender Blonde Twin. 6G8-A circuit. It was pretty much all original when I got it. I did the basic cap job, all electrolytics including B+, Cathode and Bias caps. Installed a new grounded cord. Eliminated ground polarity switch and death cap. New WGS speakers installed. Tubes tested and seem fine. Amp works and sounds good on normal channel. Vibrato channel works however sounds kinda blah. EQ is lacking. Somehow the highs are lacking, and it sort of sounds like a blanket was placed over the speakers? Not sure, its hard to describe. Normal channel has the tone I like. The highs are crisp and the amp sounds great. Bass is full and thick too. A/B-ing the two channels, I am thinking maybe its that the vibrato channel is more mids pronounced somehow? Sounds like it. Vibrato works fine and sounds great on vibrato channel as it should.

I am a home amp hobbyist, self taught with help from great people on forums like this one. I have not ran into this type of problem before. Any guidance is appreciated. I suspect I have a coupling cap or a bright cap failing? Maybe a resistor? I can test resistors, as well as caps that start at 1 uF and up with my ESR. Tone caps are too small for my ESR tester. I want to avoid throwing parts at the circuit and hoping for a fix. I find this method is often a lesson in futility.
I will start by taking voltages and checking them against the layout and schematic. I will check all resistor values too and report back.
If you have some suggestions on where to go with tone stack caps, or any other advice in dealing with the amp's symptoms I would appreciate any guidance.

Schematic and layout are here for reference.



Schematic here:

http://www.prowessamplifiers.com/sch...6g8a_schem.pdf

Any finally here is my actual amp:



Thanks in advance. - Keith

[TigerAmps]

First off - VERY NICE AMP!

I had and re-capped a 62 concert with similar components. I am surprised that you are not getting resistor noise and other strange noises with components that old ...
I had all sorts of crackling, sizzling and even RF noises from the components in my amp. You could use a digital multi-meter to measure the blue cap values and the resistors.

Some of those blue caps could be leaking a little bit (especially after 50+ years); you could try swapping out some of the bypass caps and or the tone stack caps.
I would keep the originals though (be careful not to break them);
Also check all the components in the tone stack as well as the solder connections and make sure none of the leads broke off any of the caps and the connections are good.
It could be a simple connection or a lot of substitution. Also, a can of freeze spray could help identify questionable components.
I went thru an entire can, but when I was done, them amp was silent!

[Dave H]

Somehow the highs are lacking, and it sort of sounds like a blanket was placed over the speakers? Not sure, its hard to describe. Normal channel has the tone I like. The highs are crisp and the amp sounds great. Bass is full and thick too. A/B-ing the two channels, I am thinking maybe its that the vibrato channel is more mids pronounced somehow?

You may well be right. The vibrato channel has a lot of extra tone shaping. It's a strange circuit. One side of the 7025 is amplifying the bass and the other side the treble then the outputs are added together. That 0.003u cap across the plate load of the second stage won't be helping the treble. You could try reducing its value, that's assuming the vibrato channel is working as it should but you just don't like the sound of it.

[Randall]

Have you tried swapping tubes one and two to see if anything changes?

[keithb7]

Thanks for the replies so far folks. I see what you are getting at Dave H. The normal channel does not have the 0.003u cap off the second triode V1.
Maybe I will lift one end of that .003uF cap and see what happens?

No noticeable difference with a V1/V2 tube swap. Nasally more mid-heavy tone stays on vibrato channel.

Tiger Amps: No other crazy pops, noises, buzzes etc. Not too shabby for 1963. All the by-pass or cathode caps have been replaced with new S&A 25uf/25V.
All connections inspected good. No loose wires, or bad solder joints it seems. What is freeze spray? How does it work?

[Leo_Gnardo]

The vibrato channel has a lot of extra tone shaping. It's a strange circuit. One side of the 7025 is amplifying the bass and the other side the treble then the outputs are added together. That 0.003u cap across the plate load of the second stage won't be helping the treble. You could try reducing its value, that's assuming the vibrato channel is working as it should but you just don't like the sound of it.

That's it right there. The Fender vibrato, advanced for its time, operates by muting the lows, then the highs in the stage following those lo/hi filters in order to get its unique phaser-like warble. When the vibrato's off, the lo & hi pass circuits simply combine, the composite signal never is quite like what it was when it started out. I expect the problem has to do with phase changes as the signal passes thru the hi & lo filters, and that's unavoidable. It may be best to use an external A/B switch, use the clearer sounding non vib channel when no vib is called for. FWIW Fender was constantly trying different values of resistors in those filters in an effort to improve the tone on the vib channel so we're far from the first to get involved with that. I've certainly put in lots of time trying to solve it too, to no satisfaction. Of course if anyone has come up with a good solution, let's hear it! I expect one could bypass the filters & mix circuits with a relay or photocell switcher when vibrato function is not in use.

[keithb7]

I pulled the .003uF cap out of the circuit. It made a little difference to the vibrato circuit, but not much. This amp's circuit is a little different it seems. Just so I understand it, are the first 2 tubes are normal preamp tubes for channel 1 and channel 2?
The next two 12AX7 preamp tubes control speed and intensity of the vibrato circuit, leaving 1/2 of a 12AX7 unused. The next 7025 is some sort of phase inverter for the vibrato circuit. The final 7025 acting a push/pull inverter for the
power tubes. Am I on the right path here? Thanks.

[Chuck H]

I agree with testing the coupling caps for leakage and resistors for drift. I also agree that the tonal difference between the channels is due to additional tone shaping in the vibrato channel. If you like the tone of the normal channel and you like the sound of the vibrato channel (when it's vibrato-ing) then simply don't use the vibrato channel for not vibrato-ed tones. That said...

You could try to modify the vibrato channel to sound more like the normal channel, but I can't say if this will or won't affect the channels stability or vibrato function or if you're just hoping there's a problem you can fix and otherwise do not wish to alter the stock circuit. But for the purposes of this post let's assume the amp is functioning correctly and you are willing to make a couple of small changes for tone.

There is a .003 cap in the vibrato channel that is bleeding highs. You can try lifting one end of this cap to improve top end. It's the one I've circled in red. Leaving the cap in there, but lifting one end makes this an easy mod to undo.

There is a 2uf/25V cathode bypass cap in the final stages of the vibrato circuit. Also circled in red. You can increase the lows passed by this circuit by increasing the value of this cap. That's easily done by just tack soldering another cap of 10uf to 22uf (voltage rating unimportant) parallel to the one in the circuit. Be sure to observe correct polarity. Also an easy mod to undo.

If either or both changes causes instability or changes the trem function undesirably simply undo the mods.

[Chuck H]

I pulled the .003uF cap out of the circuit. It made a little difference to the vibrato circuit, but not much.

There are two .003 bleeder caps on the vibrato channel. Removing the one on the tone stack may not make a huge difference. The one across the plate load of the tone stack recovery stage may not either, but it's worth a try. Removing BOTH is sure to make a noteworthy difference, provided the channel remains stable.

[Dave H]

The Fender vibrato, advanced for its time, operates by muting the lows, then the highs in the stage following those lo/hi filters in order to get its unique phaser-like warble.

Thanks for the explanation. I was wondering why it was done like that.

[Dave H]

Here's a sim of the 'extra eq' on the vib channel from the vol pot wiper to PI input. It's not exactly flat is it?

[TigerAmps]

"All the by-pass or cathode caps have been replaced with new S&A 25uf/25V."

You replaced the polarized caps (ones with +/-); the blue caps inside the chassis (without polarity indication) could have drifted or be leaking as well.
Also, did you try a new set of tubes (or at least 2 pre-amp and driver tubes)? As they age and wear, they can also result in a "flat" sounding amp as well.
You have a presence that should make a difference in the mids and highs; does it make a significant difference in the tone? It should.
Not as bright as a Marshall presence, but a noticeable difference.

[pdf64]

Here's a sim of the 'extra eq' on the vib channel from the vol pot wiper to PI input. It's not exactly flat is it?

[ATTACH=CONFIG]40086[/ATTACH]

Component value tolerance, and drift over the decades may be compounding this, bearing in mind that the lower value caps may be ceramic, possibly high K types, so variance with temperature too.

[keithb7]

Dave H that is a pretty cool simulation on the EQ. Not sure how to did that, but is there a chance you could provide the same chart for the normal channel to compare?

Tiger, the presence knob works and makes a difference, however not like the normal channel. I swapped and also replaced V1 and V2 tubes with new, known good tubes. No improvement.

Thanks to everyone's help here, this is all making more sense. The vibrato channel does its thing and can't put the signal back
to its original form. Makes sense and its a good explanation/theory. It is quite interesting to learn that Leo and Co. played with this vibrato EQ lots and tried to make it work better. Also that Leo_Gnardo here
also spent some considerable time, in vain, to improve the circuit's EQ.

I will play a little with the suggestions offered here. The .003 uF already pulled. Next I will try the 2uF cap on the final stage as mentioned by Dave H. I need some down time to measure values and research further.
Great group here. Thanks folks!

[Justin Thomas]

I've got a Concert with the same circuit (3-tube vibe). The Normal channel is a tad bit brighter, but not by much. The Presence does seem to affect the Normal channel more, but again, not THAT much. But I think at least half the reason Fender dumped this circuit is because it's just really finicky and requires too much dialing in. A beautiful thing if you're a boutique, but a production nightmare.

One note: whoever did a "cap job" on mine did a CAP JOB: <ALL> of them. Every single one, replaced with "cheap crap." But all the resistors are original or damn near close to it! Also has four new Celestions and the cab looks like it sat in a foot of water for SOME length of time... still sounds amazing and Fendery on both channels.

But it really is worth the time to get it right. Seriously. Don't quit!

Justin