Will 6V6 hurt a modern Fender 50 watt 6L6 amp?

[pablo_cruz]

Hi...new guy here...

I want to put 6V6 tubes in my 2005, 50w, 4x10, Concert Reverb Amp (2 x 6L6).

I don't have the knowledge base or technical savvy that most of you have, I just try things out for different tone. The 6V6's have a different tonal quality to them than the 6L6's and I might want to run them in the amp for awhile.

From what I have read, the 6V6's will hold up to the load. But I don't know for how long, or, more importantly, if any damage to the Fender circuit components is a risk in the short or long run. Thanks.

[Structo]

Does that amp have fixed bias for the 6L6?

If so the bias adjustment may not be in the correct range for the 6V6.

Also the primary impedance on the output transformer will not be ideal.

Plate voltage on the power tubes maybe too high for most 6V6GT
unless you use JJ 6V6's that most think of as 6L6 with a 6V6 label.

[The Ballzz]

It seems (please folks, correct me if I'm wrong) that at least some of the differences in tone from one type power tube to another is due to the differences in the circuit to use the tubes in an optimal manner.

For a number of reasons, simply "plugging in" 6V6 tubes into an amp designed and optimized for 6L6 tubes is not the best idea. It CAN be done, but is not a plug & play process!

My $.02 & Likely Worth Even Less,
Gene

[Stevem]

6v6 tubes are a beam type tube just like a 6l6 type and if you bias them in and run them with a OT of the needed impedance and cut back on the drive signal to what they want you will find that they sound very very much like there bigger brother !
I would recommend you get a set of China made Groovetubes in a number 2 rating and then bias them in and go play!
Note that just the act of installing the 6V6 tubes since they need less heater current will increase the heater voltage on all the other tubes also, and this in and of itself will change the nature of the amp!

[sliberty]

I haven't been able to find the schematic for the modern Concert reverb. But it seems likely that the 6L6's are running with plate voltages that are in the range mid to high 400's up to the low 500's based upon other Fender circuits with 4x6L6's. This is probably too high for most 6V6's. Additionally, these large amps tend to be fixed bias. So you would certainly have to adjust the bias, which may or may not involve circuit modifications (depending upon whether or not an adjustment pot was provided). So the bottom line is, I wouldn't just swap in the tubes. They might sound great for about 4 minutes and then cause some fireworks.

[martin manning]

I agree that 6V6 and 6L6 are similar types and will have similar sonic characteristics, but only if they are operated in a similar way. The power reduction using 6V6's will come from running a higher load impedance, and it will be very similar in effect to removing half of the output tubes and doubling the load impedance in a 100W amp.

As others have said, you will need to reset the bias and double the load impedance. Your amp must have an impedance selector, or if it has two speakers wired in parallel you can disconnect one of them. All of the voltages in the amp will increase due to the lower current draw from 6V6 vs. 6L6. That will change the sound, and you will definitely be on the outer edge for 6V6's.

The risk of damage to you amp is probably from the 6V6's failing and shorting, causing excessive current draw. I don't know how extensively fused your amp is, but fuses won't always prevent damage to expensive parts like transformers.

[Stevem]

I have used JJ brand 6v6s in a Highwatt 50 with over 500 volts on the plate and they held up fine and knocked the 60 watt RMS output down to 19 and I added resistors to keep the filament voltage at 6.5.
The Person I did the work for likes it, but for me the tone takes too much if a hit due to the impeadance mismatch which is what the guy likes. To each his own!

[David Root]

Stevem, how does increasing the heater voltage "change the nature of the amp"? I recently finished an Express and it sounded the same with heaters at 6.4-6.45V as it does at 6.75-6.8V.

[Structo]

It seems to make a difference in longevity of the tube most of all to keep it in spec.

But it would take better ears than mine to hear a difference.

[pdf64]

.. how does increasing the heater voltage "change the nature of the amp"?

Agree, from a tube theory perspective, RDH4 p3 indicates that the oxide coated cathodes used for receiving tubes operate with a massive surplus of electrons, in a cloud around the cathode. And that if so much current was drawn that the cloud was exhausted, damage to the cathode may result.
So tube characteristics shouldn't change according to the heater voltage, as long as it's within the tolerance band noted.

[strelok]

Edit: Martin said pretty much what I said below but I missed until after I posted. Just ignore me.

On the subject of the primary impedance you could always double the speaker load. Most 2x 6l6 amps have a primary of around 4k, at least that's the case in my fender bassman. So if you doubled the speaker impedance it would put the primary around 8k which would work very well with 6v6's. Since its a 4x10 I'm guessing the speakers are wired in parallel or series parallel so you could make the necessairy wiring changes and be good to go. Just keep an eye on the heater, plate and screen voltages as others have said, they will likely be higher due to reduced current draw. And of course the bias, hopefully it has a pot that will get in range otherwise you might have to swap out a resistor.