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Old 1st August 2012, 02:00 AM   #1
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Default Build guitar amp with 2 6L6 or 4 6V6?

I'd like to build a ~40 watt Fender-inspired guitar amp, and want to install a half-power switch.

Options:
1) Two 6L6 with pentode/triode switch for high/low power
2) Four 6V6 with switch to disable two tubes for half power

Any advice on which would perform and sound better? I imagine that pentode/triode modes would have different tones, whereas option 2 would maintain the same tone in full and half-power. Is this true?

Thanks!
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Old 1st August 2012, 04:12 AM   #2
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A quad of 6V6 sounds different than a pair - still plenty of that 6V6 grind but much smoother (a bit cleaner) than a pair. I did a 4 x 6V6 amp with full powerscaling for a friend and each output pair switchable between fixed and cathode bias and between pentode and triode mode. He finds that he runs it mostly with one pair in fixed bias pentode mode and the other pair in cathode bias triode mode (Folk Blues and Country player). He sets the power scaling at about 80% and just leaves it there. That means the B+ is sitting around 285 to 290V (345V at 100% power setting).
Speaker I put into his amp was a Weber P12B "Bulldog" Alnico.

You won't get 40W from a quad of 6V6 without significantly abusing the tubes, more like 25 watts.

AFAIK - Laney Linebacker Amps from the 1990s had a quad of 6V6 and were capable of good tonal palette up to and including metal shread. Might be worth a search for some schematics.

Haven't actually ever used an amp with 6L6 in triode mode but triode mode is usually associated witha "darker" tone.

Cheers,
Ian
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Old 1st August 2012, 02:03 PM   #3
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Ian, thanks for the very helpful information!

I assumed that since a pair of 6V6 is about 20 watts that a quad would be almost 40w, but it sounds like that's not the case. I want to be able to get more clean headroom than a Deluxe Reverb (the prototype for my build). But then again it's nice to be able to go to low power mode and let it get dirty early like the Deluxe.

Wow, the amp you made for your friend was very flexible! Definitely food for thought.

-Drew
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Old 2nd August 2012, 07:32 AM   #4
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The amp I did for Neale (the friend mentioned above) was built to his design requirements where he wanted to be able to adjust just about everything. While I was actually really impressed myself with the result I feel it had too damn many "bells and whistles". I got 25 Watts RMS continuoous sinewave out of his amp at just below clipping. Datasheet value for a pair of 6V6 is 14 watts so that was about right. I used a 50W Marshall OT with an Raa of 3K4 which was probably a little low, 4K to 4K5 would have been better (Hammond 1645 is probably what I would use if I built another).

The amp I built for myself immediately after that one had most of the "bells and whistles" left off. I kept the 2 channel footswitched preamp design but I run a single pair of 1950's 6V6G for the output. I did put in an Ultralinear/Pentode Mode switch but find I never use that either, it stays in pentode Mode all the time. No powerscaling, just a Master Volume for playing less loudly.

My HiFi Amp runs the same 1950's 6V6G but in that case I do run them in Ultralinear for about 10 Watts for the pair.

I might take some "flack" here but I have found more recently that all off my favourite amps (mine and other peoples I've tried) run the output tubes quite conservatively at B+ values close to the datasheet values (285V for the 6V6).

I recently converted an old PA Amp (2 x EL34) into a "Junkbox Trainwreck" by rebuilding the front end. The local guitar god borrowed it 3 months ago and loves it so much he won't give it back, using it for all his gigs. It runs the EL34s at 375V for 35 Watts out.

I'm increasingly of the view that ALL output tubes just sound better when run more conservatively and I don't miss the extra 20% power that I might get at higher operating voltages at all.

Just one mans view, might be the old age thing, when I started doing amps 30 years ago I certainly used to screw every last possible watt out of anything I built. Reliabilty was often less than stellar as a result.

Cheers,
Ian

Last edited by gingertube; 2nd August 2012 at 07:36 AM.
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Old 4th August 2012, 03:48 AM   #5
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Ian, interesting that you find tubes sound better at more conservative voltages. I'll have to give that a try. Maybe there is a sweet spot there.

So I think I'm going to go with a quartet of 6V6s and include a half power switch which will remove two. I definitely do not want a darker sound as you mentioned happens when configured as triodes. That's not what I want a Fender-ish amp to sound like -- it must be bright and spanky!

Thanks for the help!
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Old 4th August 2012, 12:24 PM   #6
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My own mucking about years ago leads me to confirm what Ian has said.

However, you say "bright and spanky", and that would make me try 6L6s with a healthy B+ (not 'out of spec', but not 'low'). Of course, that would give you more of a Super Reverb sound than a Deluxe....

Triode mode can still be bright, it's a subtle thing, but I'd call it "softer" rather than "darker".

I'm sorry, don't mean to muddy the waters, but ask yourself just what tones you want. "Fender" covers a big range, from Tweed Deluxes to Blackface Twins. The last of course, is the epitome of "bright and spanky", but it's also the epitome of "too heavy" and "too loud".
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Old 4th August 2012, 01:57 PM   #7
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Keriwena, thanks for your input!

I'm getting more and more familiar with the Fenders, but still have a ways to go. I really like the Deluxe tone, but it just got too crunchy too early. So I thought a quad instead of a pair of 6V6s would do the trick.

But, from what I understand the 6L6 based amps are cleaner and brighter (or "glassier"), is that fair?
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Old 4th August 2012, 07:22 PM   #8
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Yep, that's fair.

I suspect 6L6s will offer you more of what you want, as a quad of 6V6s will only have +3dB more headroom. That's not so much.

I like Deluxes because they get crunchy, but I'm a dyed-in-the-wool Marshall gal. Still, there are tones only a Fender preamp can dial in.
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Old 13th August 2012, 04:18 AM   #9
ivan H is offline ivan H  Australia
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Quote:
Originally Posted by gingertube View Post
The amp I did for Neale (the friend mentioned above) was built to his design requirements where he wanted to be able to adjust just about everything. While I was actually really impressed myself with the result I feel it had too damn many "bells and whistles". I got 25 Watts RMS continuoous sinewave out of his amp at just below clipping. Datasheet value for a pair of 6V6 is 14 watts so that was about right. I used a 50W Marshall OT with an Raa of 3K4 which was probably a little low, 4K to 4K5 would have been better (Hammond 1645 is probably what I would use if I built another).

The amp I built for myself immediately after that one had most of the "bells and whistles" left off. I kept the 2 channel footswitched preamp design but I run a single pair of 1950's 6V6G for the output. I did put in an Ultralinear/Pentode Mode switch but find I never use that either, it stays in pentode Mode all the time. No powerscaling, just a Master Volume for playing less loudly.

My HiFi Amp runs the same 1950's 6V6G but in that case I do run them in Ultralinear for about 10 Watts for the pair.

I might take some "flack" here but I have found more recently that all off my favourite amps (mine and other peoples I've tried) run the output tubes quite conservatively at B+ values close to the datasheet values (285V for the 6V6).

I recently converted an old PA Amp (2 x EL34) into a "Junkbox Trainwreck" by rebuilding the front end. The local guitar god borrowed it 3 months ago and loves it so much he won't give it back, using it for all his gigs. It runs the EL34s at 375V for 35 Watts out.

I'm increasingly of the view that ALL output tubes just sound better when run more conservatively and I don't miss the extra 20% power that I might get at higher operating voltages at all.


Cheers,
Ian
Thanks 4 confirming about lower B+, I am just staring a jTM45/50 pre, switcheable cascading & went with Brluesbreaker re issue PT as it supposedly runs lower B+. Cheers
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Old 13th August 2012, 03:08 PM   #10
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My quad 6V6 amp is running PP parallel with the plates at ~390V through a Fender Super Reverb OT, which I think is about 16k with an 8-ohm load (so each pair sees 8k). I'm using JJ 6V6S's and the amp sounds great, and is quite loud into a 2x12 cabinet. I also have a quartet of 15 year old Sovtek 6V6's that I tried in this amp and they sounded terrible compared to the JJ's. No bottom end, too much midrange and no glassy sparkle. For comparison the tried the Sovtek vs JJ in a Vibro-Champ and the same tone difference was observed. Originally I had a switch to lift the cathode of one pair (both pairs are separately cathode biased) to cut the power in half but I preferred the tone of the quartet over the pair, and there didn't seem to be reduction to half power, maybe a 30% reduction by my ears.
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Last edited by doozerdave; 13th August 2012 at 03:11 PM.
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