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]

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]

Originally Posted by Bernardduur

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]

Originally Posted by Merlinb

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]

Originally Posted by Bernardduur

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]

Originally Posted by Merlinb

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]

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]

Originally Posted by Jazz P Bass

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]

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.

[Steve Conner]

Reducing the NFB might stop the symptoms, but it's not necessarily the cure. A proper cure would be to get rid of the extra RC worth of phase shift that you introduced, then the amp will be stable with the original NFB network.

As Merlin said, you can get rid of either C3/C4 or C5/C6 in your original schematic by appropriate rejigging, and maybe adding an extra negative rail.

It's very important to use as few AC coupled stages as possible inside the NFB loop. Look at the Williamson amp for an example, it has about as many as can be tolerated.

[Bernardduur]

Originally Posted by Steve Conner

Reducing the NFB might stop the symptoms, but it's not necessarily the cure. A proper cure would be to get rid of the extra RC worth of phase shift that you introduced, then the amp will be stable with the original NFB network.

As Merlin said, you can get rid of either C3/C4 or C5/C6 in your original schematic by appropriate rejigging, and maybe adding an extra negative rail.

It's very important to use as few AC coupled stages as possible inside the NFB loop. Look at the Williamson amp for an example, it has about as many as can be tolerated.

Even now it works perfectly?

[Steve Conner]

Yep, unless you can prove to yourself that it's unconditionally stable and doesn't have a big gain peak at some subsonic frequency.

[Jazz P Bass]

I agree Steve. Not that you took the words out of my mouth. Far from it.
AC coupling in a NFB is tricky.
What I stated holds true , to agree, on a working amp.
The fact the "popping " got worse at a lower resistance tells me the amp was not functionong properly.

[Bernardduur]

Originally Posted by Jazz P Bass

I agree Steve. Not that you took the words out of my mouth. Far from it.
AC coupling in a NFB is tricky.
What I stated holds true , to agree, on a working amp.
The fact the "popping " got worse at a lower resistance tells me the amp was not functionong properly.

OK; great to know. So removing the whole buffer would be key here. Would this be negative on the amp as I will be driving 8 powertubes? (as I never work on amps with more then 4 powertubes )

[Steve Conner]

No, if you really want to have buffers, you can change to DC coupled CFs. Then there will be no more AC coupled stages in the loop than there were before.

I'm not sure the buffers are needed anyway. If the PI was already able to drive the power tube grids all the way up to 0V, then it's hard to improve on that. Unless you install some hefty buffers DC-coupled to the power tube grids, and try to drive them into AB2. But in AB1 the grids need next to no power to drive them, so I don't know why some amps used these CFs that are limited, by AC coupling or large stopper resistors ala SVT, to work in AB1. Maybe it was to overcome stray capacitance in the wiring to so many tubes, that would destabilize the amp at HF. Or maybe Merlin knows some other reason.

The RDH4 has a whole math-heavy chapter on how to stabilize a power amp with almost any number of RC-coupled sections inside the feedback loop, but my personal approach has always been to minimize the number of sections to start with, and check amps that I work on for any signs of LF instability or peaking.

[Bernardduur]

Originally Posted by Steve Conner

No, if you really want to have buffers, you can change to DC coupled CFs. Then there will be no more AC coupled stages in the loop than there were before.

I'm not sure the buffers are needed anyway. If the PI was already able to drive the power tube grids all the way up to 0V, then it's hard to improve on that. Unless you install some hefty buffers DC-coupled to the power tube grids, and try to drive them into AB2. But in AB1 the grids need next to no power to drive them, so I don't know why some amps used these CFs that are limited, by AC coupling or large stopper resistors ala SVT, to work in AB1. Maybe it was to overcome stray capacitance in the wiring to so many tubes, that would destabilize the amp at HF. Or maybe Merlin knows some other reason.

The RDH4 has a whole math-heavy chapter on how to stabilize a power amp with almost any number of RC-coupled sections inside the feedback loop, but my personal approach has always been to minimize the number of sections to start with, and check amps that I work on for any signs of LF instability or peaking.

Great! Thanks all for the insight, I've learned a lot! I'll try the amp without the CF. I already tried the DC coupled CF's but they gave me too much compression

Think I'll return to my ECC85 in the PI stage!

[jbrew73]

before you rebuild the driver section you might try putting the feedback network back to your original specs and reversing the primary wires and see what happens. you may just have positive feedback.

[loudthud]

Do you just have one 220K grid leak resistor for all the output tubes? I'm not sure what the effect is but most amps you see with 6 or 8 output tubes (like the Fender you copied) use much lower resistance like 33K. Steve, what does RDH4 say about that? That's why I like the direct coupled cathode followers with a 33K cathode resistor and you can use a much higher grid leak on the input to the cathode follower and a reasonable coupling cap.

[imaradiostar]

If you haven't the room for cathode followers try a mosfet source follower- I've been playing with them recently and I really like the results. the LND150 is cheap and you can use it for all kinds of helpful things in a tube amp without effecting the "tube" tone of the amp.

jamie

[Bernardduur]

Originally Posted by loudthud

Do you just have one 220K grid leak resistor for all the output tubes? I'm not sure what the effect is but most amps you see with 6 or 8 output tubes (like the Fender you copied) use much lower resistance like 33K. Steve, what does RDH4 say about that? That's why I like the direct coupled cathode followers with a 33K cathode resistor and you can use a much higher grid leak on the input to the cathode follower and a reasonable coupling cap.

OK, good point. Dunno...... I used 2 220k's (for both pairs).......

I checked the positive feedback thingie and no, I was not getting any positive feedback.

[Steve Conner]

Originally Posted by loudthud

Do you just have one 220K grid leak resistor for all the output tubes? I'm not sure what the effect is but most amps you see with 6 or 8 output tubes (like the Fender you copied) use much lower resistance like 33K. Steve, what does RDH4 say about that? That's why I like the direct coupled cathode followers with a 33K cathode resistor and you can use a much higher grid leak on the input to the cathode follower and a reasonable coupling cap.

Good point Loudthud. That's probably the reason. The spec for maximum grid leak resistance in the tube datasheet is per tube. So if you have a push-pull amp with 4 tubes per side, then you should use one-quarter of the resistance. The buffers are then needed to drive this low resistance without signal loss.

With just one 220k resistor per side, you're flirting with thermal runaway. I think EL34s were specced for a maximum resistance of 100k in the first place.

[Enzo]

Jazz, the last thing we need is an Enzo emoticon, especially for Steve and Merlin who know an awful lot more about this stuff than I do.

[Jazz P Bass]

My sincerest apologies to Enzo, Steve & Merlin.
As I posted it tongue in cheek, I did not mean to offend.

[Bernardduur]

Originally Posted by Steve Conner

Good point Loudthud. That's probably the reason. The spec for maximum grid leak resistance in the tube datasheet is per tube. So if you have a push-pull amp with 4 tubes per side, then you should use one-quarter of the resistance. The buffers are then needed to drive this low resistance without signal loss.

With just one 220k resistor per side, you're flirting with thermal runaway. I think EL34s were specced for a maximum resistance of 100k in the first place.

Wow, that's why I love it here! I'll try!

[Bernardduur]

It works! I lowered the resistors to 22k (approx 1/4 of 100k) and voila, it works like a charm with very sweet sound..... I'm in love!

I use 6L6GC's btw

[Smitty]

The 820/100R resistive divider IMHO works well with a Super Reverb because the initial feedback voltage is so low. Super is 40 watts into 2 ohms [8.9v=SQRT(40*2)]. The Super divides down about a volt at the tail at rated output.


One of the reasons why the Twin Reverb feels too stiff to so many players is due to the massive increase in negative feedback compared to the Super. Leo used the same 820/100 divider so the voltage feedback goes way up to about two volts at the tail at rated output.

The amp in question has eight output tubes. With the 820/100 there's probably some instability because of too much begative feedback. At 4 ohms 2K7/100 would result in about a volt at the tail and 3K6/100 for 8 ohms.

[Bernardduur]

Originally Posted by Smitty

The 820/100R resistive divider IMHO works well with a Super Reverb because the initial feedback voltage is so low. Super is 40 watts into 2 ohms [8.9v=SQRT(40*2)]. The Super divides down about a volt at the tail at rated output.


One of the reasons why the Twin Reverb feels too stiff to so many players is due to the massive increase in negative feedback compared to the Super. Leo used the same 820/100 divider so the voltage feedback goes way up to about two volts at the tail at rated output.

The amp in question has eight output tubes. With the 820/100 there's probably some instability because of too much begative feedback. At 4 ohms 2K7/100 would result in about a volt at the tail and 3K6/100 for 8 ohms.

Thanks!