'66 Super Reverb

[Mark Ross]

Hi all. I'm brand new but it seems that there is so much knowledge here I signed up to ask ask y'all about my amp.

1966 blackface super. My complaint is the highs are too piercing. If you attack the string at all it just produces a real sharp edge to the note. Tubes are older but not more than fifty hours and I've used it for 2 gigs. Everything functions, and the only "mod" I can see is a solid state rectifier by GT. Think the rest are 12AX7 and 6L6.

I've read about pulling V1 and another something about V4 or using12AT7 for either V1 or2.

I'd really appreciate any help!
Mark

[ES350]

What do you have for speakers? The stock CTS or C12R are not unusually bright speakers, but a lot of these have been swapped out over the years as they blow (as they do very easily). A SS rectifier will also stiffen things up and raise the B+, which may come across as brighter. Try a 5AR4 tube rectifier (stock compliment) and see if it calms down a bit...get it biased correctly (take to a tech if need be), and then see how it sounds.

[pdf64]

When was the last time it was given a thorough check over? If there a lots of original carbon comp resistors in there, they'll have drifted way off nominal value, expect some to have doubled. That's likely to cause tonal / functionality issues.
Maybe not all the speakers are even working - that would alter the load impedance and work the power amp harder.

[Chuck H]

+++ to both replies so far. Especially about the SS recto replacement. It's also worth noting that when played clean that is a bright amp. You can mod it internally to change the voicing, but I wouldn't. a couple of other things you can try that are non invasive are a Sovtek 12ax7WA in the first two preamp tube sockets. It's a good performer but a little dull sounding so it could help. You can also try doubling the length of instrument cable you use. Use a female/female coupler if you have to. This will increase capacitance a little at the input and roll off the glassy highs.

Different power tubes do sound different too. I'll mention Groove Tubes only because they have several types of 6L6's and a good bead on how they can be expected to behave. Email them and they can probably suggest a model that will be less bright in your amp.

Chuck

[Mark Ross]

Thanks guys. Very helpful! I'll try to answer your some of your questions. Three of the speakers are original with the blue Fender sticker,the fourth just says "Utah." All are working. The amp tech everybody uses here didn't know about leaving V1 empty "I've done thousands of fender amps and that couldn't possibly effect the tone! It would only disable the channel." so I don't know how much I trust him. The amp was retubed and biased about 20 years ago but then never used. That's when the SS rectifier was put in. I have less than 50 hours on it. I'm gonna try the longer cable.

What are your opinions about leaving V1 empty and, the other mod I've read about is using 12AX7 in V6 phase inverter instead of 12AT7.

Thanks so much, Mark

[Chuck H]

The old pull one tube trick has been done for decades. It changes the bias of the first stage in the channel your using. Makes it hotter and easier to saturate. I don't personally like the tone but it does sound fatter so it's worth a try.

There's no problem with the tubes after twenty years. Unused they can sit forever and not age. Of more concearn is the filter capacitors. If the amp was used regularly and then stashed for two decades your filters are probably toast. If your amp is humming more than usual, this is why. Those caps have a working life of about twenty years but actually age FASTER and less gracefully when not used. The SS rectifier puts the caps in even greater danger because it raises the voltage. You should probably replace all the electrolytic caps in the amp which would include the power supply filters, the bias supply filters (often forgotten but very important) and the preamp cathode bypass caps.

Your speakers being different may be of concearn. Unknown to many, old Jensen speakers were polorized OPPOSITE almost every other brand on the market. That means that if your Utah speaker is wired with the same + terminal as the Jensens it is OUT OF PHASE. That will thin your tone radically. Check that all the speakers are wired in correct polarity (who knows, they may not be) then swap the leads on the Utah speaker so the + lead on the other three is the - lead on the Utah. If the amp gets fuller sounding then that was part of the problem and leave it wired that way. If the amp gets thinner sounding swap it back the way it was.

Chuck

[pdf64]

'The old pull one tube trick has been done for decades. It changes the bias of the first stage in the channel your using. Makes it hotter and easier to saturate.'

As well as that, it also provides a ~6dB increased signal level before that phase splitter V6, due to the plate impedance of the normal channel being absent, thereby pretty much eliminating signal loss through the 220k mixer resistors.
Can you see a 6 digit code on the speakers (may be partially covered by the blue fender label)? That will indicate what the manufacturer/date is, see
More Codes
and so whether you need be concerned about the Jensen phase issue.

[Chuck H]

Indeed, the originals are most likely CTS. That's why I suggested the listening test. The swap leads and listen test may actually be quicker than finding the codes on the frames while the speakers are in the amp. But if a code is visible on one of the originals great. If they're Jensens the code will start with 220.

Chuck

[andyjazz]

I just finished working on a friends '66 Super. It was noisy, had lot's of horrible sounds coming from the reverb and also the vibrato didn't work. I found a couple of old resisters that were out of whack and replaced them. I also put in new ceramic tube sockets on the 6L6's and the rectifier. This was done just because he wanted ceramic sockets and the old ones were kinda loose. This probably solved alot of issues just by the incidental fact of desoldering and resoldering a bunch of connections. I also resoldered everything in the reverb circuit, rebiased the 6L6's and put a slightly different value cap across the "bright" switch.

In the end the old Super came out sounding great. My friend had put a 12AT7 in place of the 12AX7 in the preamp section and he likes the sound - breaks up a little soon but...that's what he wants - so I just left that in. And the cap across the bright switch is quite different but totally reversable and most people don't use the bright switch anyway - he now uses it for a couple a songs when he wants that certain sound. Bottom line is I've always thought Super's have a kind of high-endy sound but you should take all of the advice mentioned by other posters and get it running right - then you can decide if a different pre-amp tube or speaker config is gonna be the ticket for you. But the Supers tend to be "trebly" - IMHO.

[Chuck H]

My friend had put a 12AT7 in place of the 12AX7 in the preamp section and he likes the sound - breaks up a little soon but...that's what he wants - so I just left that in.

Q... How does a 12at7 in the preamp make the amp break up sooner than a 12ax7???

Chuck

[andyjazz]

Well as I'm sure you all know the tubes in the "12AX7" family have different characteristics. They are interchangable in the sense that the pinout functions are the same but they have differences in terms of their basic output ratings. These differences can be expressed as amplification factor, cathode current, plate current... etc, I generally just consider the amplification factor which I'll abbreviate as AF. The ratings are as follows:

12AX7 - AF = 100
12AT7 - AF = 60
12AY7 - AF = 40
12AU7 - AF = 17

I'm sure there are others here who can describe electronically what/how these differences affect the actual function of a working circuit. I just notice the sound of the amps. I've also noticed a difference in how reverb tanks are driven. 12AU's just don't seem to work at all as reverb drivers. Of course there are other factors I might be missing, maybe there are other components in the circuit that one could change to accomodate a lower AF tube.

I also wonder if a lower AF value preamp tube will, say, cause your power tubes to fail sooner? Could it cause other problems in a circuit? I'd be interested to hear some ideas about this. In my experience swapping a 12AT7 or 12AY7 for a 12AX7 is a real easy way to experiment with the tonal characteristics of an amp. It will have little if any potential to harm the amp itself - and you don't even have to heat up a soldering iron!

[Chuck H]

Well as I'm sure you all know the tubes in the "12AX7" family have different characteristics. They are interchangable in the sense that the pinout functions are the same but they have differences in terms of their basic output ratings. These differences can be expressed as amplification factor, cathode current, plate current... etc, I generally just consider the amplification factor which I'll abbreviate as AF. The ratings are as follows:

12AX7 - AF = 100
12AT7 - AF = 60
12AY7 - AF = 40
12AU7 - AF = 17

It's important to note that even though these tubes can sub in the same socket they ARE different tubes and can be idealized with different circuit values.

As a lower "AF" tube an at7 in the preamp of an ax7 circuit should breakup LESS than an ax7 at the same volume knob settings IME. But I can't say for sure what the circumstances were around the percption, so...

12AU's just don't seem to work at all as reverb drivers.

This is a perfect example of my statement above. 12au7's are excellent reverb drivers. Probably better than the 12at7. But not in the "Fender" circuit. A reverb tank needs watts to drive it and the 12au7 is better suited (in a circuit designed specifically for it).

"AF" or gain (as in voltage gain) is not the same thing as watts (as in power, V*I) We need voltage gain for small signal amplification but we need power to drive transducers like speakers or reverb tanks.

I also wonder if a lower AF value preamp tube will, say, cause your power tubes to fail sooner? Could it cause other problems in a circuit? I'd be interested to hear some ideas about this. In my experience swapping a 12AT7 or 12AY7 for a 12AX7 is a real easy way to experiment with the tonal characteristics of an amp. It will have little if any potential to harm the amp itself - and you don't even have to heat up a soldering iron!

+++

I like to plug ay7's in the V1 socket of ax7 preamps sometimes. In an ax7 circuit the ay7 has the lower gain but seems to sound more "tubey". That is, it seems to react more dynamic in the mids when set for the same gain. Haven't done it yet but I do want to design specifically for an ay7 sometime. Does anyone still make (or make a reissue of) that tube???

Subbing alternative 12xxx tube in a preamp should has no effect on the longevity of the power tubes.

Chuck

[andyjazz]

Chuck,

Thanks for responding,

I figured you were going to point out something I got wrong.
Live and learn.

[Chuck H]

Your not wrong. A 12au7 plugged into a Fender reverb has no poop. I had no clue about the differences other than gain the first time I tried it and came to the same conclusions as you. So I'm just passing along my later findings for posterity.

Chuck

[andyjazz]

Thanks, After thinking ot over I'm sure that the reason my friends Super "breaks up" a little early is because he is still using the ORIGINAL speakers. I talked with him last night and he confirmed this. When I was working on his amp I was so concerned with what was happening electrically that I hardly noticed the speakers - other than they had the blue fender decal on them and they seemed to function in a reasonable fashion. The fact that he has a 12AT in the pre-amp, due to it's lower gain, is probably helping to keep the 'break up" from getting over-whelming.

Speaking of tubes in reverb circuits....maybe tomorrow when I have time I'll start a new post about the reverb in my GA-19RVT. The Falcon has a triode/pentode tube for the reverb driver (I forget the number).....more later.