Post PI buffers........

[Bernardduur]

Ok, so I'm having a hard time getting a post PI buffer duo to work. I've read in some sources that this buffer placed post the PI will make the sound even sweeter. I've seen em used in some multi-powertube amps as the Fender Studio bass, Super Twin Reverb, the HiWatt 400W and the Ampeg SVT (if I'm correctly). I've also seen in some books the use of the tube buffer.

When I want to add it to my newest build all I get is some compressed sounds from it with a sort of tremolo feeling in attack / decay. I've used both 12AT7's as 12AX7's for this tube.

When I remove the buffer the amp works perfectly.

Output section

The PSU that goes with that

Any thoughts? Should I just remove the buffer and keep the amp the other way? Or try another design? I tried several different designs, all give the same outcome. I triple checked my build and it's fine!

[loudthud]

Do you have just one pair of output tubes? What are the DC voltages you get at the output of the cathode followers with 12AX7s and 12AT7s? It seems like they wouldn't be biased very hot with 2.2K bias resistors. Have you looked at the output with an oscilloscope? Have you tried adjusting and/or removing the feedback?

[Bernardduur]

I use 8 output tubes, 6L6GC's.

I use the double bias adjustment to bias both sides differently although now they are biased normal and equal.

I'll check the voltages today!

I've tried adjusting the feedback. No solution. My scope is with a friend so I have to check that later!

I've borrowed the idea from the Fender Studio Bass and the Super Twin Reverb, as can be found here

[Jazz P Bass]

Post PI

That schematic has a multitude of voltage points labelled.
Can you try to identify which one (ones) are off on your build.
That tremelo effect sounds like your bias voltage is changing.
Monitor pin 5 & verify that it is solid.

[Bernardduur]

The voltages I get:

V4: 474V
V3: 473V
V2: 424V
V1: 414V

Voltage at the end of the cathode follower = 85V

Bias is stable. As I called it a tremolo don't see it as a 'real' tremolo. See it more as a compressor that kicks in when I play and acts 'tremolo-y' as the attack and delay increase and decrease.

Could it be that the voltage over the anode of the 12AT tube is too high?

Edit; got the V+ down to 385V and still the problem remains

[Merlinb]

When I want to add it to my newest build all I get is some compressed sounds from it with a sort of tremolo feeling in attack / decay. I've used both 12AT7's as 12AX7's for this tube.

It could be squegging, as you have quite a bit of low frequency phase shift in there. Have you tried removing the feedback altogether? It would be pretty simply to DC couple the LTP to the buffers, which would eliminate some phase shift, and other nasties that coupling caps introduce.

[Bernardduur]

It could be squegging, as you have quite a bit of low frequency phase shift in there. Have you tried removing the feedback altogether? It would be pretty simply to DC couple the LTP to the buffers, which would eliminate some phase shift, and other nasties that coupling caps introduce.

Please explain...... my knowledge of abbrevations is still really small! With feedback you mean the 470k?

[Merlinb]

Please explain...... my knowledge of abbrevations is still really small! With feedback you mean the 470k?

Ah, I mean R29, the 820R global negative feedback resistor. Have you tried disconnecting this, and seeing if the problem stops?

[Steve Conner]

I agree with Merlin, if you've added another AC coupled stage inside the power amp's NFB loop, you're asking for trouble with instability. Try disconnecting the NFB, see if that makes the problem stop.

[Bernardduur]

Ah, I mean R29, the 820R global negative feedback resistor. Have you tried disconnecting this, and seeing if the problem stops?

Yep; if I disconnect this resistor the problem stops but the unit will distort a bit.

[Jazz P Bass]

Feedback component

Merlinb, Steve Conner.
My hat is off to both of you!
(we need an ENZO emoticon!)
Any tube amp can be tweaked with this resistor.
I would suggest a temporary log / B pot in place of the present resistor.
Wire the pot with one leg connected to the wiper.
That leaves two nodes to connect to.
Adjust the resistance until you, hopefully, get what you want.
At the extreme, the Mesa Boogie Nomad 45 has a switch to disconnect the feedback resistor.

[Bernardduur]

Merlinb, Steve Conner.
My hat is off to both of you!
(we need an ENZO emoticon!)
Any tube amp can be tweaked with this resistor.
I would suggest a temporary log / B pot in place of the present resistor.
Wire the pot with one leg connected to the wiper.
That leaves two nodes to connect to.
Adjust the resistance until you, hopefully, get what you want.
At the extreme, the Mesa Boogie Nomad 45 has a switch to disconnect the feedback resistor.

OK, I'll try......

Any recommendations on the value?

[Jazz P Bass]

Tweak potentiometer

R29 is labeled as 820 ohms.
A 1000 ohm pot will do.
A pot that has too high of a resistance value will make it harder to find the exact value that you require.
It really is academic.

[Bernardduur]

I tried it with a 5k potmeter. The popping goes but I get a distorted misbiased sound from it at higher resistances..... at lower the poppings gets worse.

[Bernardduur]

OK, I've changed some values as can be found here and now it works perfectly fine!

Can these values be that easily transferable to my position? Using a 12AT7 with these values? I tried to follow the whole piece but got a bit lost at the load lines.