Some things I've found, about 6l6 fender blackface design

[azzaaa]

Hi to all. This is a first time post although I've been a regular visitor. Anyway I have finally come to a point with my 6l6 blackface design where i am actually happy with the sound. There are a few things I've come to realise with that design that i would like to share to see if any of you have had similar experience with.
1. 25uf bypass caps on triode gain stages over .68uf
2. Attatching tone caps directly to pots as in G.Weber and also changing the layout of the phase inverter so the grid leads are shorter and plate leads longer. This seems to add some clarity and reduced hum.
3. The phase inverter design around the 12at. I found i could not make it sound right. I lowered the plate load resisters to 20k(approx twice internal plate resistance) which seemed to access a more linear part of the curves. I also lowered the bias resistor to 250ohm which raised the current to about 5ma per triode. This redesign seemed work the winged c 6l6 so much better.
4. Lowering the coupling caps in power amp. Currently 4700pf orange drop.
5. Increasing screen resistors gave the amp increased sustain and sag.
6. Increasing dropping reistors to preamp so voltage on plates was >200v
7. The tung-sol 12ax7 reissue has a very funky almost tweed sound. And was the best for this circuit. (nos excluded)
8. If the clothes make-ith the man the speaker make-ith the amp.
9. Brand and size of filter caps have a very noticeble effect on tone and performance.
In the end i seemed to have arrived at a design that is original? I also will add that i also approached it with a Ken Fischer philosophy in that it is single channel no reverb etc and was trying to extract max tone and performance. I would love to hear some responses from your experiences. thanks azz

[Darkbluemurder]

4700pf (=0.0047uf) as PI output coupling caps? That is a drastic reduction from the stock 0.1uf. Are you sure that you didn't want to say 0.047uf?

Interesting change on the 12At7 PI. That tube must be cooked quite hard at 5ma.

For the cathode caps I like 4.7uf or 10uf.

Cheers Stephan

[ChrisM]

1. 25uf bypass caps on triode gain stages over .68uf

Ok, they were 25mF to start. Never seen 680nF in a BF Fender.

4. Lowering the coupling caps in power amp. Currently 4700pf orange drop.

You sure, 0.0047mF? Seems a bit small, no?

6. Increasing dropping reistors to preamp so voltage on plates was >200v

Increasing the value of the dropping resistors would lower the plate voltage. What did you really do here?

7. The tung-sol 12ax7 reissue has a very funky almost tweed sound. And was the best for this circuit. (nos excluded)

Hmm, I always found them pretty clear and pristine, not a Tweed tube for me .

9. Brand and size of filter caps have a very noticeble effect on tone and performance.

If you do a search on this forum (or any opther) you'll see a ton of talk on this subject. Some interestring stuff out there.

In the end i seemed to have arrived at a design that is original?

Nah,
You modded a Fender like all of us have. Put props for getting something you like.

[azzaaa]

Thanks for the reply. I'll explain that I had a real farty bass problem and a harsh top end. So some normal changes are to lower the bypass caps on the preamp i just didnt like it in this circuit. But i found that the lower voltages on the 12ax7 definately softned the upper register. The 4700pf in the power amp really helped tighten the bass to where i wanted it. As far as the 12at is concerned the max current for that valve is around 10ma so i figured at half of that i should be fine. Thats why i couldnt work out why fender ran such a high current valve at such low idle. The main concern was the wattage rating of the dropping resistors.

[Bob-I]

Cool stuff. I've tried many of those mods but not all. I lowered the bypass caps to 5uF but never as low as .68uF. Congrats on getting it to where you want it, always a good feeling.

I'm still looking for a way to get more clean headroom out of my Princeton Reverb build. I upped the PT using an Allen oversized xformer, upped the OT sith a deluxe OT and dropped the bypass caps. All helped but I believe it can be cleaner still.

Thoughts.

[tubeswell]

I'm still looking for a way to get more clean headroom out of my Princeton Reverb build. I upped the PT using an Allen oversized xformer, upped the OT sith a deluxe OT and dropped the bypass caps. All helped but I believe it can be cleaner still.

Thoughts.

Using a 12AT7 in the PI would be a simple way of starting

[azzaaa]

If your talking about the 1164 princeton reverb then the phase inverter is a cathadyne phase inverter? Have not had any experience with them but I think they are very difficult to get a clean output from. One option could be to play with the bias resistors on it or to change it to a long tail pair phase inverter but this would essentialy change the whole amps vibe to a deluxe. Also you could try upping the bias reistors to around 2.2k if there was distortion in the preamp this would centre bias the 12ax7. A few thoughts anyway.

[Cygnus X1]

I'm still looking for a way to get more clean headroom out of my Princeton Reverb build. I upped the PT using an Allen oversized xformer, upped the OT sith a deluxe OT and dropped the bypass caps. All helped but I believe it can be cleaner still.

Thoughts.

Are you running 6L6's in that?
You might be able to do it given the beefier iron.
Also try SS rectifier if it doesn't already have one.

[orrong65]

One of the things I do on a twin channel blackface is to set the normal channel up ultra clean with massive headroom by using a 5751 tube and a 63uf bypass cap, everything else standard. Talk about tele twang with a thumping bass through a 15" alnico speaker!

If I really want to tweak it further I remodel the tone circuit to a more "tweed" sound, by replacing the 100k slope resistor with 56k, and the tone caps with .022. Leave the treble cap at 250pf, or you can increase it to 330pf for a lower shoulder. This increases gain and offsets the gain loss from the 5751 tube.

As all of this is sacrilege with a vintage amp, it can be easily reversed of course!

[AllenBradley]

I'm still looking for a way to get more clean headroom out of my Princeton Reverb build. I upped the PT using an Allen oversized xformer, upped the OT sith a deluxe OT and dropped the bypass caps. All helped but I believe it can be cleaner still.

Thoughts.[/quote]

I've had some luck getting more headroom from a PR in a few ways:
#1 is speaker choice. A good clean 10" upgrade. If you really need to go cleaner, put in a 12" of choice. HUGE difference.
I agree with the PI rewiring - convert to a standard BF long-tail PI - again HUGE difference in headroom. You would also use a 12at7 after rewiring.
And, of course, try different tubes. That 6L6 idea with the bigger trannys sounds superb. I do this all the time in my BF Deluxe - AWESOME bloom vs. the 6v6s.

[skyboltone]

If your talking about the 1164 princeton reverb then the phase inverter is a cathadyne phase inverter? Have not had any experience with them but I think they are very difficult to get a clean output from. One option could be to play with the bias resistors on it or to change it to a long tail pair phase inverter but this would essentialy change the whole amps vibe to a deluxe. Also you could try upping the bias reistors to around 2.2k if there was distortion in the preamp this would centre bias the 12ax7. A few thoughts anyway.

Cathodyne inverter is unity gain or less. You don't need much toob their/ Use a 12DW7 with the AU7 side for the cathodyne. JJ makes them. The lower GM makes a smoother distortion free inverter.