EL34 to 6L6 change

[mikeboone]

What would I need to do to a cathode biased EL34 amp before swapping tubes to 6L6? My bias caps/resistors are 220uf and 300 ohm per tube on a 2xEL34 configuration.

It's an amp that I built and I copied the output section of a Matchless Chieftain. I adjusted the R to 300 ohms to calm it down just a tad.

It sounds really nice but the tinkerer in me wants to experiment but I am not quite sure.

Thanks,
Mike

[Chuck H]

As it's wired for EL34's now, you should be able to plug in 6l6's and pinout correct. Not always the case going the other way. So with your amp it should just be a matter of bias. The 6l6's may require more bias voltage. Which means a bigger cathode resistor. But, analogous to the higher bias voltage, 6L6's reqiure more drive voltage. That is, if you want the same level of clipping. Obviously the amp will have plenty of drive from the preamp and PI to achieve max clean volume. And perhaps a cleaner, more 6L6 vibe is the goal. So adjust the power tube cathode resistor and your done. If you do want to bump he gain back up, that's a little tricky. Because the Chieftan doesn't really have any voltage dividers for taming signal (other than volume controls). But you could change the cathode resistor for V1, V2 or both to 1k instead of 1.5k. This should still be linear enough and will increase drive a tad.

[mikeboone]

Awesome... Thanks. I found my worksheet from when I figured how much W this guy is making in its present state. I can work through the same steps using the new values.

Always good to have a sanity check.

[mikeboone]

With no changes to cathode bias R, I put a pair of 6L6GC tubes in just to see what it would sound like. The clean tones were pretty nice as you would expect. But the high gain was kind of fizzy on the high end and flabby on the low end. If I were doing a blues amp, this would be the ticket. But I am after a more traditional rock tone.

It reminds me of what I didn't like about the Mesa Rectifers when they were 6l6 only. The newer models have a bias switch and the amp sounds pretty good as long as you stay away from that modern high gain setting. The vintage setting is very much like a high gain sort Marshall circuit.

In case you tweakers are wondering, this amp is a normal channel form a blackface mated up to a cathode biased EL 34 output stage. I modified the tone stack and preamp cathode caps, in a Marshall'esque way, to clean up the low end. It's a rocking little machine.

[Chuck H]

A correct bias may help a tad with the tone. FWIW the fizz could be crossover distortion that, for whatever reason, isn't as pronounced with the EL34's. The specific bias condition and OT specs, etc. But basically this is the change I would have expected. It's a different sort of sound. If you like the clean tone just check that the bias condition is within safe parameters and plug in the 6L6's whenever you want to record a clean track or do a jazz gig

[mikeboone]

The 6L6's were Getting kind of hot, more so than the EL34's so I do firmly believe that adjusting the cathode R is required to for long term use. But it did tell the tale and was kind of what I expected them to sound like.

If you have the time there is a you tube link below that I recorded to test the PPIMV that I installed. I understood the LARMAR to be a good way to get consistent drive through a wide range of volumes. This was recorded with my iPad laying on the bench at a volume I could probably talk over. I layer the iPad lens down so my creepy mug isn't showing .

All the changes are on the guitar volume pot and pickups. It's a music man axis super sport HH, for reference.


PPIMV - YouTube

[TransLucid]

Don't forget that if your amp has feedback, you may need to adjust the feedback factor to maintain the overall output stage gain.

[Chuck H]

I think that's a fine sounding amp. It doesn't sound like anything "classic" but still valid in the same way. And with way more gain on tap if you want it. Chucky likey.

[Chuck H]

Good point. It wouldn't be dramatic, but still relative.

[mikeboone]

Thanks Chuck... I was after a rock tone other than Marshall or Orange, my two personal favorites BTW... Its been a neat experiment and I have tried all sorts of transformers and actually arrived at using an OT from a Fender Pro Sonic. Its really agressive.

To TransLucids point... I have tried a few different things on the NFB, if thats what are are referring to, and gotten some cool results. First was a really low value R (820 ohm) like fender used in the AB763. It stifled the amp way too much. I removed it completely, which is how you hear it now, and its a nasty little machine. It does get a little hard to control at higher volumes though. Most recently I experimented with different values and found that anything above say 8.2K didnt really continue to have any useful affect. So with a 10K in there, I put a bypass cap (.001uF) across the R to let some mids and highs bleed by and I am kinda digging that - for now .

Keep in mind I am not a EE, but an old RF communications tech. I mimic elemets from different amps and hack at it until I find something I like. So, there is no real method to my madness other than straight up experimentation to tried and true circuits.

Thanks guys for all the input... Its cool to have you guys to bounce ideas off of. My nieghbors dont care much for our hobby though!!!