6AK6 BF micro champ success

[bigdaddy]

As some of you know about my amp dilemma I have some info to pass on.

My last build was a firefly. I proved that indeed there is enough gain in a 12AX7 with a TS to drive the tube. You didn't need any extra gain stages unless you wanted a gain monster little buzz saw. The trick was to get the plate voltages up to the normal standard Fender BF ones. Which fall into that 180v-200v range. Since the 12AU7 was stout it could handle a bit more voltage then a 6AK6. Everything I had read about it stated that as a power tube it was to be best run at 180v max in SE operation on the screen and not much more on the plates. Well I came in a bit lower yet retained the correct voltage on my preamp. It's 165v on the 12AX7 plate and that's about as low as I like but it works well.

I have used this technique before to decouple dual pentode preamps. it made it easier to adjust their parameters. Just a plain old voltage divider. I must point toward (as usual) PRR for this idea and it's something that I always kept up my sleeve designing my amps. I knew that some way I had to get that voltage down yet not let it affect the plate voltage of the 12AX7. I think if I tried this with a bigger bottle tube it would NOT work. Since it's hard to get anything else in here I will give you the rest in the next post.

[bigdaddy]

This is what I used. A Mercury Magnetics champ PT but made to low voltage specs. I got it a few years ago to mess around with lower voltages on the bigger bottle power tubes to see if indeed they did sound better as per some suggestions by PRR(Paul at MM said it was a pretty useless PT, oh how wrong he was). With this PT I did find out a lot of information concerning sound and plate voltages in relation to primary impedances of output transformers and the tubes themselves. All subjective since I have no data. The preamp is pure BF Champ minus the NFB and a 1.5K grid resistor to the power tube.

The PT is 250-0-250 and using a 5Y3 gave me about 312v at the first leg of the rail. I decided to use a 470 ohm 10 watt resistor(I might remove this now) to a 8mfd Sprauge with a 220K resistor grounded as I do with all my amps. I felt it was not necessary to put a big cap there with this amp. Next I used a BFDR choke and a 20MFD F&T cap. I was at about 309v unloaded. I then used the voltage divider, 2-3K 5watt resistors. One went directly to a 20mfd F&T cap that feed the 100K plate resistors for the 12AX7. The other went to a 4.7K 5 watt with a 20mfd F&T that fed the plate and a 10K 3 watt from that leg(thanks Doug for all the parts and getting them to me so fast!!!) to the screen supply.

I first had a 10K resistor instead of the 4.7K and a 820 ohm cathode resistor. The amp sounded wonderful, like a hot rodded BF Fender would, only it's 1 watt. The 6AK6 sounds a lot like a 6V6. I have to try it with the higher voltages I'm getting now, 174v plate and 147v screen, it's about 100% dissipation so i think I have to maybe get the voltage up a tad and the wattage down. But I'm in Fender heaven with this amp and I never heard anything like it. It sounds exactly like a Fender should and does it at 1 watt. The OT is a 125E Hammond, I wanted to try a Allen OT but I don't have it to mess around with it anymore other then changing a resistor.

It took a long time and a lot of pain to get this amp right. It was 80% build already from another amp that was in there, a 10watt 6L6GC Champ. I had to do a little work but most of it was already done prior by myself.

I am now officially retired as an amp builder. All the parts and left over stuff is going in a big box and stored in a closet. I might if I feel up to it build another one, I have enough parts. I just have to dial it in a bit more and goodbye amp building.

These 2 last builds were brutal for me to do but they got done. I wish I was in better shape, this tube would probably sound amazing in a BFDR clone. 2-4 watts of pure BFDR......LOL. If anybody wants more info just ask, you can email me. I probably won't be around much anymore but to all those who helped me and were considerate and understanding I wish all the best to. To the others I also wish all the best to. And I must thank PRR who was the one person who no matter what I have to say triggered my brain into thinking and doing things with amps I never thought I could.

[zendragon63]

Glad you finally 'arrived' with your build. And you know it don't come easy... IMHO both the 6AK6 and the 6G6G can have a surprisingly good sound for home doodling if the rest of the amp is scaled appropriately. +1 on the lower voltage PT. Regards

dennis

[harryk]

I made a small guitar amp with marshallish preamp and 2x6AK6 power tubes giving some 2-3 W output power. This amp is aimed mainly for lead playing purposes in bedroom noise level. My voltages in 6AK6 plates are 274 V and screens 268 V without problems. I used VVR for two different purposes. One is to have dynamic power control and the other is to limit power stage plate voltages maximum to 280 V with VVR. That way I can run preamp voltages higher with a 2x250 V PT. Pictures and sound clips of my Bluetone Lead Jr. amp are here www.bluetone.fi/7 

[jojokeo]

Sound is really good even played back on my cellphone 
where did you get the tortoise tranmy w/ that voltage? Also what preamp structure/sequence are you running? Nice job there.

[harryk]

Sound is really good even played back on my cellphone 
where did you get the tortoise tranmy w/ that voltage? Also what preamp structure/sequence are you running? Nice job there.

Toroid PT is 2x230 V/65 mA. I bought it a long time ago here in Finland but cannot remember anymore where I bought it. I made another similar amp this Autumn to one customer and used Hammond 369JX PT with 2x250 V in it. Preamp schematic is based partly to some ideas which I found from net and partly to my own design.

[bigdaddy]

Yes that amp does sound excellent. A more modern sound but it does have tone which is to me the most important thing.

I think the 6AK6 is one of the best sounding tubes I ever heard. Better then the bigger version 6G6G which is similar but has the bigger glass.

I think bringing down the voltage made the tube sonically come alive similar to my experiments with 6V6 tubes. They have a sweet spot voltage wise. I think the 6AK6 is at 180v. The 6V6 is about 350v, 6K6GT about 300v. The thing about the 6AK6 is that from what I see the screen voltage likes to be lower then the plate by much more then a 6V6 for instance.

But the tone is there much more then using a triode for a power tube. I like this a lot more then the firefly which isn't bad.

If I feel up to it I think I might just try an EF86 in there to get that plexi gain sound being driven by a 6AV6. Who am I kidding, I'm done.......oh well. Wishful thinking I guess.