6V6 Tubes in a Fender Blues Deluxe Reissue?

[titanxt]

Since I am having my amp repaired anyway, I thought that I would replace the tubes. Can I put 6v6 tubes in my Fender Blues Deluxe Reissue that was made with 6L6's without major mods?

Thanks!

[MWJB]

The JJ6V6 will go in with just a rebias (20-22mA per tube). You could also rewire the output transformer secondary wire from the 8ohm tap (green) to the 4ohm tap (green/yel) to correct impedances, just a case of removing & insulating one wire & replacing it with another.

[titanxt]

Is it worth it? I have read that sme don't like the sound and some have - I know it is up to each person to decide on what they like. However, is the tone difference worth the effort?

[MWJB]

That's a very subjective question. What are you looking for regards change inn tone?

[txstrat]

The 6V6 is sometimes called "the 6L6s little brother". Don't expect a big change in tone as it is.
The changes are more a kind of earlier distortion due to less output power.
But even that is not a BIG difference, IMHO.

[BackwardsBoB]

You'll hear a difference in the sound. The 6V6 is lower gain, and it's in the negative feedback loop, so you'll get less negative feedback. The result tends to sound warm at low volume, with blooming bass as you turn up the amp, and easy power-stage overdrive.

It's a simple matter to boost the negative feedback, perhaps by moving to a higher impedance output tap or adding more 6V6s (heater current is less than half of a 6L6's). You can experiment with a smaller blocking cap going into the PI input to roll off some bass as well. Fixing the negative feedback tightens things back up. They'll still be the warmest sounding tubes around.

You'll be well over the plate and screen ratings, so consider stiffer screen resistors, at least 1K. Don't be too fearful. I think that 6V6s were used in TV horizontal deflection circuits sometimes. This probably isn't the best application for early NOS 6V6s, but the JJ's in my favorite amp are running well at 475V.

[Old Tele man]

Originally Posted by BackwardsBoB

The 6V6 is lower gain, and it's in the negative feedback loop, so you'll get less negative feedback...

...the 6V6 has plate amplification factors (mu) of 9.8 & 205, respectively, for triode & pentode operation, while the 6L6 has plate amplification factors, resectively, of 7.99 & 135, which makes the 6V6 have higher, not lower, gain than the 6L6.

...however, the 6L6 has a higher transconductance value (gm = 0.0060 mA/V) than the 6V6 (gm = 0.0041 mA/V).

...did you mean transconductance (gm) instead of gain (mu)...or, were you meaning in-circuit voltage amplification (AV) gain?

[BackwardsBoB]

After a little research, it appears that we're both wrong, so let's try that again.

For the same operating conditions, a tube with a higher gm (pardon the lack of subscripts) will have a higher voltage gain, since the mu will be the gm times the load impedance. Your spec numbers won't reflect this, since the load in the specified operating conditions will be higher for the 6V6GTA than the 6L6GC. For example, with Va=360V (a voltage spec'd for both tube types), push-pull, you might find Ra-a of 3.8 kOhms for the 6L6GC, and 7.6kOhms for the 6V6GTA.

In the case in question, we're using the same output transformer, which knocks the mu of the 6V6GT way down, approximately in half.

Now if we set the speaker output impedance switch to 4 Ohms for an 8 Ohm load when we use the 6V6GTs, we double the plate load the tubes see, and approximately match the Ra-a spec for a 6V6S push-pull pair at 360V. With this mod, both tube types require about 45V p-p to achieve full power of 47W for the 6L6GC pair and 23W for the 6V6GTA pair with Va=360V. This is about half the power for the 6V6GTAs compared to the 6L6GC, and power is V squared divided by R, so the p-p voltage to the speaker is about the square root of 2 times as much for the 6L6GC compared to the 6V6GTA.

So far, so good. For the swap to the 6V6GTA, moving the output tap maintains the drive voltage requirement, and corrects the plate load. Doubling the number of tubes should work as well, at twice the output power. There's a bit of fudging here, since the screen voltages spec'd at Va=360V differ by 45V in the specs (6V6GTA Vg2 is higher), but I think it's close enough for rock 'n roll.

My mistake is basing my post on my experience. I see now that I'm subbing 6V6S for 6L6GC, but I'm doing it in grey fuzzy Carvin X-series. The original EL34s were swapped for 6L6GCs before I swapped the 6L6GCs for 6L6S. I think this invalidates my experience with negative feedback, and I'm not sure how to calculate what changes to it will be required when replaced 6L6GC with 6V6S in a typical 6L6GC circuit (it's late). I do know that an amp that can't overdrive 6L6GCs on the clean channel with most of my guitars can certainly overdrive 6V6s, so it's reasonable to assume that more negative feedback is required. In any case, given the dramatic differences between the tube types, emperical tuning of the NFB is a good idea.

[Steve Conner]

The in-circuit gain of a pentode or tetrode is roughly just gm*Rload anyway, since rp is high enough to ignore.

In other words, the "amplification factor" of one of these tubes, measured in the conventional way, would be nearly infinity and very non-linear. To get round this, two values of mu are quoted for these tubes. Inner mu is when using the screen grid as a plate. It expresses the extent to which unbypassed screen resistors will reduce the tube's effective gm.

There's also the outer mu, measured using the screen grid as control grid, and the plate as the plate. I guess this one would express how unbypassed screen resistors affect the speaker damping, or something.

[rockon1]

JJ 6V6's are rated at 500pv and 450 on the screens-just like thier 6L6. Bob