Replacing 6L6 with 6V6: what changes needed?

[AmadeusMozart]

I can get my hands a good looking PP 6L6 amp. However I've got the impression that the output transformers are relatively small and have been thinking about replacing the 6L6 with 6V6.

With Google I found a guitar thread that discussed this swap but this is a standard amp intended for stereo use. I read that the feedback loop needs to be moved from the 4 Ohm connection to the 8 Ohm connection and that the 4 Ohm speaker connection becomes the 8 Ohm speaker connection to make up for the changes in feedback requirements and increased impedance.

Is this a feasable route to take or is it better to ditch this idea and perhaps some can chime in what the impact on the performance of the amp may be?

Many thanks for your inputs.

[DigitalJunkie]

What's the B+ voltage? It might be a bit on the high side for 6V6's.

[Eli Duttman]

I can get my hands a good looking PP 6L6 amp. However I've got the impression that the output transformers are relatively small and have been thinking about replacing the 6L6 with 6V6.

Instead of changing O/P tube types, you are (IMO) much more likely to succeed by switching to triode wired "finals". You avoid the PSU issue already mentioned and a gain structure issue too. The 6V6 is very easy to drive, not so the 6L6.

Another tweak that caters to the lack of "iron" bulk is installing a 1 pole high pass filter at the amp's I/Ps, with F₃ tuned just below 20 Hz. Keeping infrasonic trash out of the amp provides protection against O/P trafo core saturation.

[AmadeusMozart]

Eli, thanks for the suggestion. I've gone ahead an ordered the amplifier (MeiXing MC34-B) after I had a good read up about the characteristics of the 6L6 series and studying the tubecharts. Looks to be the 6V6's bigger brother and a darn fine tube. Time will tell.

Was unable to find specific details on a 6L6 in PP triode mode, only item I did find was someone remarking he got 6 W out in this mode (enough for me).

I always regretted selling the Leak that I restored (EL84's and very large OPT's) and I hope that I be able to get something that gets close.

There are some discussions about PP missing the second harmonic distorting the waterfull decay of the intermodulation and that the ECC81 is rich in 2nd harmonic. I also picked up suggestions that the selected drivers are not the best.

While I am waiting for the amp to arrive I'll be looking at modification options and wonder what the best bang for the buck may be. Have some Russian 6N2P-EV and 6N3P-EV as well I have somewhere a rather new ECC83 and some OK ECC82's. One thing I like to try out is the triode modification but am relucatant to do this without tube (factory) specifications lets I go over the ratings for g2.

If you have any suggestions then I would greatly appreciate those. Thanks.

[Eli Duttman]

Yes, the 6L6 is venerable. It's the very 1st beam power tetrode and has been produced continuously, in 1 incarnation or another, since RCA introduced it in the 1930s. Hats off to THE longevity champion.

6L6GCs are the "norm", these days. Between the GE and RCA data sheets, you'll be quite busy.

An interesting fact about the 6L6 is that it is specifically rated for a positive grid current regime. You can squeeze a fair amount of power out of a triode wired pair in either A2 or AB2.

As I stated previously, the 6L6 has substantial drive requirements. I'd go for a Mullard circuit using a 12AT7/ECC81 section as the voltage amplifier and an ECC99 as the LTP. If you opt for positive grid current, buffer the LTP with IRFBC20 source followers DC coupled to the control grids of the "finals".

Circuits with GNFB loops should (IMO) be executed with high gm types, to obtain resistance against slew limiting. The types I suggested are high gm.

[AmadeusMozart]

Eli, Thanks for that info, much appreciated. Have heard some suggestions of changing the Chinese tubes for something else. Quite some conflicting information on the web and a lot is from people that are "wannabee". I rather go by the opinion of someone like yoursself: are there any 6L6GC that I have to stay away from and any that you recommend without going overboard? Thanks.

[Ian444]

Amadeus, did you get the amp and change the tubes? Just wondering as I have one too and fitted 6V6 recently (Russian 6P6S). The OPT's on my amp were around 3K9 a-a for 8 ohm loads and 5K3 for 4 ohm loads. I fitted some 8K OPT's and they sound great with the 6P6S, and the power tranny now runs a lot cooler.