B-15N preamp design

[GregS]

I did B15 type pre-amp with 6SL7's coupled to a Traynor Custom Special type power amp. The amp puts out 160W clean. The phase inverter is a long tail pair circuit so it is definitely do-able with a 6SL7.

[txstrat]

I did B15 type pre-amp with 6SL7's coupled to a Traynor Custom Special type power amp. The amp puts out 160W clean. The phase inverter is a long tail pair circuit so it is definitely do-able with a 6SL7.

Which of the triodes in your LTP have you connected the signal input to? Grid 1 or grid 4?
I've learned that grid 4 is less sensitive to hum. That's why the second triode is the first in the original schematic.

[überfuzz]

I guess the SS thingy might tighten up the tone a bit. What size do you have on the filter capacitors at rectifier and pre amp stages? Small sized at the pre amp stages often give a, lets call it, looser tone.

[catalin gramada]

I always ended to keep the tone stack between first and second stage like in early circuit and put the volume control in the end of tone stack like Orange circuits do. The floating paraphase could be very precisely balanced till reach clipping condition tweaking resistors values over the plates . Lot of headake with 6sl7. better to use 12ax type if you dont have access to high quality ones. This is the way I modified this circuit to have a manageable B15 tone style. clean sine from 45 to 60 w output depends what I put in power stage
Happy Christmas to All !!!
Catalin

[txstrat]

I guess the SS thingy might tighten up the tone a bit. What size do you have on the filter capacitors at rectifier and pre amp stages? Small sized at the pre amp stages often give a, lets call it, looser tone.

The first two are 100uF the others 47uF.
Think that's OK even with four 6L6s.

[txstrat]

better to use 12ax type if you dont have access to high quality ones.

I put in some Tung Sol reissues and they are way better than the pair of no name brand 6L7s I'd used before.

Merry christmas from me to all too.

[catalin gramada]

one more sugestion I never tried for 6sl7 but it works for me in 12ax cicuits very efective helps to reduce hum filaments to absolutely none: use two separate filament windings. one 6,3v center taped for power stage and one 6.3v with humdinger for preamp. both have to be referenced with separate wires to ground. It even worth to try with a small separate transformer for preamp to see how it works for you 6sl7 stages. I know is hard to negociate for this type of tube. Tung Sol seems to be a pretty decent one still
Mfg
Catalin

[GregS]

Txstrat:
I connected the input of my pre-amp to grid 4.

[txstrat]

Txstrat:
I connected the input of my pre-amp to grid 4.

Yep, me too.
Which grid is your input for the PI?
And maybe you have some suggestions for the resistor values?

[GregS]

Yep, me too.
Which grid is your input for the PI?
And maybe you have some suggestions for the resistor values?

Sorry, I meant the input of the PI.

These are the resistor values:
Plates: 100K
Grid leaks: 1M
Bias resistor: 750R
Total tail resistance: 27K + 4.7K = 31.7K

Measured voltages at idle:

HT: 446V
Plates: 311V, 310V
Grids: 55.4V, 54.9V
Cathodes: 86.9V
Junction of bias resistor and tail resistance: 84.7V

Hope this helps.

Greg

[txstrat]

Hope this helps.

Absolutely. Thanks alot.
Next monday I'm gonna haul the amp (which is a head btw) from our rehearsal room to my place to fool around with it.

27K plus 4.7K is quite a large resistance, isn't it? I was thinking about values close to a standard fender PI. What do you say?

Matt

[txstrat]

I'll try your suggested values.
One more thing.
What about the neg. feedback resistor? Do you think 10K is a good value to start from?

[GregS]

27K plus 4.7K is quite a large resistance, isn't it? I was thinking about values close to a standard fender PI. What do you say?

The reason for the larger value is for better balance of the circuit. You wouldn't want it any larger than this as you'd lose headroom by dropping too much voltage across that resistance. There is some information on Aiken's site on designing long-tail pairs which is what I used when I designed mine. It's also convenient that he uses a 6SL7 in his example. The link is below.

Designing Long-Tail Pairs - The Load Line Approach

What about the neg. feedback resistor? Do you think 10K is a good value to start from?

It depends also on what the resistance to ground is (shunt resistance) of the voltage divider. In my case I have a shunt resistance of 4.7K and a series resistance of 150K. This gives an attenuation ratio of 0.03. However, the circuit I based my power amp on used 100K as the series resistor. I chose a larger value for less NFB as I thought it would make the amp more dynamically responsive.

Greg

[txstrat]

I tried a 680 ohms bias resistor which is recommended on Radall Aikens website as a good 'standard' value. Good call.
The tail was 22k and 4.7k to ground. NFB was 10k.
The amp was veeery clean but it felt weak and not at all dynamic.
I changed the tail to 10k and 100 ohms to ground and the NFB to 22k.
Well, as you described it, Juan, it now pushes like a motherf...er and the sound is still what I'd call B-15N.
Actually a little tighter than than. But that's what I was aiming for.
Thanks to all for your contributions.

Matt