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phil 15th March 2013 04:14 AM

6v6 fixed bias
I'm building the ax84 output stage, October 10.10.10 club version. 6v6 fixed bias push pull. Can anyone recommend a front end and tone stack for a clean sound. Any comments on this type of output stage would be gratefully received.
I have an output transformer built by Electraprint for a pair of EL84's in ultra linear and was not going to use the ultra linear taps. Is this a good idea and will it work ok?
Thank you,

gingertube 17th March 2013 11:55 PM

My own amp is a 6V6 push pull. Like you I used an 8K Raa Output Tranny with Ultralinear taps. I wired it with a Pentode Mode / Ultralinear switch. I and everyone who has used the amp prefer it in pentode mode. If building again from scratch I would not bother with Ultralinear Option - just insulate the ends of the UL tap wires with heatshink and stowe them out of the way or wire them to some unused terminals on a tag strip and not connect them. So your intention not to use the UL taps is in my opinion and practical experience the correct decision. Very very few guitar amps use Ultralinear although more Bass Amps do.

For simplicity you could use the clean preamp from AX84 site or the ICON of clean, a Black Face Fender preamp.

I used a London Power Standard Preamp (2 channel, clean and lead) with minor component value changes to accommodate the fact that I used 6SL7 octal twin triodes rather than 12AX7. The component value mods where intended to address the higher grid current levels of the 6SL7.

Hope this is of some help to you,

phil 18th March 2013 01:48 AM

6v6 fixed bias
Hello Ian, thanks for the reply, I was hoping you would read it I am always most interested in your postings and the advice you pass along.
I will take your advice re the ul taps.
I have a nice pair of nos 6sl7's which I would love to use instead of the ecc83 would it be possible for you to direct me to the circuit that you built using these please.
Thanks again,

Struth 20th March 2013 07:02 PM

Hi Guys

It should be noted that UL sounds perfectly fine for guitar, just not the way Fender combined UL with other circuit attributes. Fender amps sound sterile enough with their low feedback circuit values, but further adding UL makes the amp even more lifeless. This is nested feedback and works well for hifi.

If you take one of those amps and open the NFB loop, the amp comes to life. Try it, you'll like it.

As always, change the screen-stop to 1k-5W for est tube reliability.

All of my amps use UL and they sound fabulous... or so I'm told... hehe

Have fun
Kevin O'Connor

JRGuitarGuy 20th March 2013 09:14 PM

UL is nice for Big Bass amps etc, 6v6 and fixed bias though pentode sounds the best to me its more important with NFB use really low value or none at all swear makes huge differences.

Struth 20th March 2013 10:22 PM

Hi Guys

Let your ears lead you to your "tone" decisions rather than relying on public opinion. I build my amps the way I think is best and the customers either like it or not. The ones that do are the only ones I need care about.

Saying "something is only good for one thing" sounds like parrot talk. If you actually believe it - that's great... for you, just as my beliefs are great for me.

Guitar amp designers follow hifi rules and continue to limit their potential for "tone creation" by lethargically staying on that path. We use tube safety rules as our guide, which allows much more lattitude for discovering new paths to great sound. If you want your amps to be suitable for condo use then definitely be categorical in the design paradigm. You may not have as much that way.

Have fun
Kevin O'Connor

JRGuitarGuy 20th March 2013 10:41 PM

if you want a tighter sound go higher with the NFB and see what you like, I just like it lower or none at all it feels more alive and open to me that way. its almost a sacrifice though if you like both like I still find I want more tightness but then you miss out on that openness increasing values etc

Struth 21st March 2013 04:06 PM

Hi Guys

JR, your "tighter sound" guide is entirely hifi based and not exactly true to what creative MI design is about.

"Tightness" of sound has a different connotation in MI than for hifi. In hifi, it is an issue of clarity - of actual fidelity - and the use of feedback and/or more of it is a step in the right direction. For a guitar or bass amp, tightness is also a clarity but in this case is one of not having excessive bass. Truly extended bass response in MI usually causes a woofy bass tone that interferes with midrange intelligibility. The fix for this is to roll of bass so that the harmonics will dominate a bit more. This is Guitar Amp Design preschool stuff.

Also, do not confuse a high feedback "factor" with high values for the feedback elements. The factor is the amount of sacrificed openloop gain that gives the loop its power to reduce distortion and stabilise gain. The same closedloop gain can be achieved using decades of difference of component values, as in the example in the previous post.

When it comes to being creative with circuitry to find your guitar tone, there are pretty much no rules - except that the parts should be used within their safe limits. So, when I see people saying that UL is bad for guitar, or that only such-and-such a circuit can get a given sound, I want those people to remove their blinders and see the range of possibilities. Electronic diversity allows you to get to your tone by any of countless paths. In fact, all paths lead you to your tone - some have more hills than others.

Have fun
Kevin O'Connor

JRGuitarGuy 21st March 2013 04:36 PM

Maybe I don't mean tighter I'm sorry, I have a hard time deciphering what exact tonal changes sometimes, it might be clarity what I'm hearing and that woofy bass response, for rhythm stuff I like a lot more NFB but lead stuff its just amazing to me with lower NFB like I'm not sure if its just clarity or a wider frequency spectrum, its really noticeable to me, its just like harmonics/overtones are jumping out at me more, that aliveness etc, Then again after re-capping the stealth and scroring sylvania black-plate coin base 6v6 and some early GEs I stick a 12AY7 in V1 omg just beautiful cleans so much more transparent and touch sensitive to my picking, then I stuck a I61 black burn mullard ECC83 another OMG hi-fi smoothness I was missing out on lol, for PI I put a GE 12ATY I know the way Higher trans and you can tell it really pushes, also play around with this old RCA black-plate 12AU7 in V1 its waaay quieter obviously but omg the Dynamics and transparency yet again is amazing. It's really a night and day difference compared to every current production tube I stuck in there lol, Something about these GEs I found though, They are contributing a lot to that openness and warmth I believe, then again I might be mistaking that for tightness or clarity I'm not sure all I know is it just changed the voicing very dramatically, I never really experimented much with lower gain preamp s might be that too, I love how they just roll off all that fizzyness and harshness.

JRGuitarGuy 21st March 2013 05:26 PM

Testing New tubes/Caps - YouTube

The stealth was a very controversial design I know lol. This is the best I ever got it sounding right here lol, you guys tell me what the hell I'm hearing in here that is this amazing? is it the GE clarity? but that low mid/middy bloomy depth/detail is here in spades now it was gone before lol, those couplings I changed helped ALOT too, the GEs really smooth the sucker out really took out some of that wicked ice-pick/harshness. Alot of it I think is those GEs, and those Sylvania's are magical too lol. It just sounds magical all over now lol

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