KT-66s in my Twin?

[slowpokerhino]

I'm really impressed with the KT66s (Gold Lion) in my VM. It makes me a bit curious (okay, a lot curious) how they would sound in my 66 Twin.
Since they are in the 6L6 family I'm assuming they'd be safe to use.?? I obviously have no desire to damage the amp in any way.

I realize I'd have to re-bias (which is ridiculously easy with that amp by the way), any suggestions what I should bias them to?
Anything I should know or watch out for before I give it a try?

[Spotted Dog]

I run them in my Super Reverb Reissue. It came stock with some GTs and I replaced them with JJs, then re-biased a little hotter. The amp sounds better, more complex harmonics so to my ears the cleans are more pleasing; it is just a tad dirtier when pressed. The difference was not huge, but worth the effort. The question is with the two variables which one is to blame? Maybe its both variables working together, maybe Eurotubes just made a few bucks and a hotter rebias would have done the trick.

The big change in that amp came when I replaced the stock Jensens (which were broken in) with Webers. Two alnicos and two ceramics, it lowered the clean headroom a bit and the whole amp just opened up, the icepick highs went away, especially when cranked, and the bass is much fuller and tight as a crab's ass. When its cranked it is a beautiful tone.

I biased to 46.3 with a bias right, up from about 38 if recall correctly. I believe it was measured in mA(?)

[ClubAndCountry]

KT66s draw slightly more filament current than 6L6s, so it will put a little more load on the power transformer. Given how sturdy old Fenders are it's probably not a concern, but it might be an idea to check the power transformer for overheating after the amp has been on an hour or so - they can get quite hot anyway, and it's difficult to describe just how hot is 'too hot', but if you compare it before you do the swap it will give you an idea whether it's any worse.

The plate and screen voltages will be fine in a '66 Twin, or a 70s 100W Twin, but don't try this in a post-1976 'ultra linear' model - the voltages are substantially higher (well over 500V) and it may blow valves.

For biasing, I would probably go for about 35mA which is fairly conservative, but it's really up to you - I don't like the sound of running valves very hot, but some people do and generally it only shortens their life expectancy, especially in an amp like a Twin where you're unlikely to crank it fully into distortion. (If you're really driving them, sometimes they will fail if biased too hot.)

[slowpokerhino]

Thanks for the info. As soon as I have some free time I'll give it a shot.

I have all JJs (from Eurotubes) in there and it sounds noticeably better than before with Electro Harmonix and Groove Tubes I'd tried before them. I forget now what I biased it too but I remember I biased it hot which now seems silly since C&c pointed out what should be obvious (when do you ever push a Twin's power tubes?).

I've thought of doing a speaker swap. I like the original Jensens but a little experimentation wouldn't hurt. Less efficient spreakers would be nice. Anybody know the efficiency of the 50 watt Jensens Fender was using 43 years ago?

On the subject of the Twin. I've read that removing the tremolo circuit will add a bit of gain, is this true? Can it be done in a way that it can be reversed later if necessary?

[Spotted Dog]

Great info C&C. I think my SRRI is running noticeably hotter. Maybe I'll throw the GTs back in there and compare the heat. WIth the new speakers, maybe the GTs will sound just fine. I think overall the speaker swap has had the biggest impact on the amp's tone.

Can't comment on the original issue Jensen's but the newer ones just don't have the goods like the Webers. I saw recently that their build times are now a six week wait.

There is a lot of talk on various boards about simply pulling the tremolo tube to increase the gain. I think I need to try this as well. The tremolo does not get that much use in the Man Cave.

[slowpokerhino]

If you don't use the "normal" channel you can pull V1. It will add slightly more gain to the tremolo channel, not a lot but noticeable.
One of Derek Truck's tricks.

[ClubAndCountry]

Simply removing the tremolo valve won't do much - it's just the oscillator and is not connected to the signal path at all. The reason removing the tremolo *circuit* does is because the Intensity pot is still a parallel load on the signal path even when it's at zero, and absorbs some tone and volume. To fix this, the best way is to use either a 'switch at zero' pot or a push-pull in place of the stock Intensity pot, so you can have it out of the circuit normally but still have it available if you want it. If you *never* use the tremolo, you may as well just disconnect it though - undo the wire that goes to the top end of the pot (nearest the Speed control) and tape it safely so it can't short against anything.

Pulling V1 (the Normal channel preamp valve) also does increase gain on the Vibrato channel because it rebiases the Vibrato channel gain valve (V2) a little hotter - it's not much, but still noticeable.

You can also substitute a 12AX7 in the phase inverter position (V6, nearest the power valves) for a little more drive and harmonic complexity. If you've pulled V1 you already have one! (You need to leave the 12AT7 in V3 though, that's the reverb driver and a 12AX7 won't last long in there.)

Pulling V1 and/or V5 (the tremolo oscillator) will also reduce the filament current draw slightly - it might be a good idea if you're trying KT66s.

Some of the Jensen reissues are about the lowest sensitivity 12" speakers I know of, by the way - the P12Q is only 95dB and rated at 40W, which should be enough if you're not going to drive it flat out. I agree about them not sounding too great out of the box, but they're not bad after they've been used a bit.

[Spotted Dog]

If you don't use the "normal" channel you can pull V1. It will add slightly more gain to the tremolo channel, not a lot but noticeable. One of Derek Truck's tricks.

Ya f'ed that one up thanks for refreshing my memory.

You can also substitute a 12AX7 in the phase inverter position (V6, nearest the power valves) for a little more drive and harmonic complexity. If you've pulled V1 you already have one!

Definitely going to try this. Thanks.

Those Jensens improved noticeably during their year of use, but they have this ice-pick-shill thing happening at high volume. I just couldn't get past it, of course it could just be my ears! The ceramic/alnico combo took care of that.

[ClubAndCountry]

The stock C-series ceramic Jensens do have a shrillness I don't really like either - the P series are Alnicos, and sound sweeter. They're also noticeably quieter - I put a new P12R in a 70s Deluxe Reverb once and thought the amp wasn't working right, it was so quiet. The amp measured right on the correct power though, it was definitely the speaker.