Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on November 16, 2025, 05:05:01 pm

Title: I'm looking for an old thread about tubes between PI and Power Section
Post by: kagliostro on November 16, 2025, 05:05:01 pm

Some time ago there was a thread about the use of tubes between PI and Power Section (as a buffer if I remember correctly)

I would like to re read it but I'm not able to find it, don't remember the author of the thread but to me seems that SILVERGUN contributed actively to that discussion

Does someone remember that thread and has a link for it ?

Many Thanks

Franco

Title: Re: I'm looking for an old thread about tubes between PI and Power Section
Post by: tubeswell on November 17, 2025, 04:26:04 am

Can't recall any specific thread myself but here are a couple of schematic examples:


1) Hiwatt DR 400 - the 12AT7 is the type of buffer tube you might be thinking about (between the (un-shown) PI and output tubes)


2) Ampeg SVT CL  - uses a 12 AX7 cathodyne and a pair of 12AU7s for the impedance buffer

Title: Re: I'm looking for an old thread about tubes between PI and Power Section
Post by: kagliostro on November 17, 2025, 10:24:08 am

Many thanks Tubeswell for the effort

I remember the schematics you are taking about

What I don't remember is the content of the discussion on that thread, I only remember It was interesting

Thanks again

Franco

Title: Re: I'm looking for an old thread about tubes between PI and Power Section
Post by: tubeswell on November 17, 2025, 10:53:47 am

The essence of the concept behind these sorts of buffers is to facilitate a low resistance path to ground for excess grid current in big output tube grids, whilst simultaneously addressing poor impedance bridging between the PI and these big output tubes that would otherwise occur if there was no such buffer.


The low resistance path for grid current is achievable because the grid leak and the cathode load resistor for the buffer stage is one and the same, I.e., it’s an ‘active’ grid leak resistor.

Title: Re: I'm looking for an old thread about tubes between PI and Power Section
Post by: kagliostro on November 17, 2025, 02:37:04 pm

OK, perfect Tubeswell

Good explanation

The reason for which I'm investigating about the use of buffers in that position is because of the use of PPIMV

I arrived to the conclusion that the less impact in loading the PI is using a Frondelli PPIMV (also RoobRobinette say it is a good one), but also if the effect is lower than other kind of PPIMV when the wiper is on the low part, near the ground, the load for the PI increases and tonal response may vary, so the idea to use a buffer betwen PI and the Power Section (a SS Buffer not a tube one) and the search for all the info I can collect

Franco

Title: Re: I'm looking for an old thread about tubes between PI and Power Section
Post by: kagliostro on November 17, 2025, 07:02:14 pm

Desperate times call for desperate measures.

I couldn't find anything, so I asked ChatGPT for help.

And... BINGO!

https://el34world.com/Forum/index.php?topic=15679.0 (https://el34world.com/Forum/index.php?topic=15679.0)

https://el34world.com/Forum/index.php?topic=15798.0 (https://el34world.com/Forum/index.php?topic=15798.0)

and also this were found

https://el34world.com/Forum/index.php?topic=29679.0 (https://el34world.com/Forum/index.php?topic=29679.0)

https://el34world.com/Forum/index.php?topic=29489.0 (https://el34world.com/Forum/index.php?topic=29489.0)

https://el34world.com/Forum/index.php?topic=12213.0 (https://el34world.com/Forum/index.php?topic=12213.0)

Franco

Title: Re: I'm looking for an old thread about tubes between PI and Power Section
Post by: tubeswell on November 17, 2025, 08:54:05 pm

OMG You da man.


Amazing what one can forget after 12 or so years...

Title: Re: I'm looking for an old thread about tubes between PI and Power Section
Post by: TIMBO on November 18, 2025, 12:06:38 am

Hey mate,
This was what I was thinking...
Maxim Amplifier (https://www.aggh.net/discussion/index.php?topic=48458.msg540351#msg540351)

Title: Re: I'm looking for an old thread about tubes between PI and Power Section
Post by: kagliostro on November 18, 2025, 01:24:01 am

Oh, not strictly mandatory

I can easily forget someting i did the past week

 :l2: :l2: :l2:

Franco