Modifying and "securing" a ancient sunn 2000s?

[Balticnoiseforge]

Hallo i got a sunn 2000s with dual rectifiers at home, and was thinking of adding a impendence selector so i can get access to all 3 taps, and at the same time add a PPIMV and also add some protection mods for the transformers.

But since it is such a old amplifier i wanted to ask here first what would be best approach.

Installing a impendence selector is quite straight forward so no worries there, polarity switch is also disabled - but i still have the dpst on / off switch living in Europe, but solved that by having a 3 prong cord with a dpdt switch.

Also thought about adding tube rectifier protection diodes, BY2000 at 2kV and 2a should be enough right? Rather than using diodes in series.

Also thought about adding flyback voltage diodes to the output tubes, i generally use R3000 which are 3kV but only 200mA, never been quite sure about mA rating for flyback voltage diodes. So what would a suitable diode be if not using 1n4007 in series?

Or are there better ways to protect output transformer etc that would be more suited?

And lastly, a PPIMV.
Thought that the "improved PPIMV" circuit with coupling caps after the dual gang pot would be most suited for the layout since it would let me keep grid leak resistors, and not having bias on the pots for such a big amp, but amp already has quite big 0.25uF coupling caps, i generally go for x10 but that would be 2.5uF capacitors - beyond 1uF are there really any audible difference? So would using 2.2uF capacitators make sense?

Quite basic question i suppose, but want to make sure i get it right.

Have a great day!

Link to schematic: https://el34world.com/charts/Schematics/files/sunn/Sunn_2000s.pdf

[danhei]

I can’t speak to your other mods but I don’t love PPIMVs in amps with negative feedback, as the MV changes the amount of feedback which significantly alters the sound and response of the amp. And overdriving the unmodified cathodyne phase inverter in he 2000s may not sound great anyway.

[Balticnoiseforge]

I can’t speak to your other mods but I don’t love PPIMVs in amps with negative feedback, as the MV changes the amount of feedback which significantly alters the sound and response of the amp. And overdriving the unmodified cathodyne phase inverter in he 2000s may not sound great anyway.

Never had that issue when using PPIMV in typical Fender circuits, albeit they didnt have a cathodyne phase inverter or such - but yeah it is possible that slamming the phase inverter with full signal doesnt sound great, but its a very loud amplifier and a master volume could be needed, or atleast worth to try!

I also dont see any good locations for a pre phase inverter master volume.

[tubeswell]

about adding flyback voltage diodes to the output tubes, i generally use R3000 which are 3kV but only 200mA, never been quite sure about mA rating for flyback voltage diodes. So what would a suitable diode be if not using 1n4007 in series?

Use 3 x 1N4007 in series on each half of the primary - cathodes (banded end) pointing away from the CT towards the OT winding ends

  "improved PPIMV" circuit with coupling caps after the dual gang pot would be most suited for the layout since it would let me keep grid leak resistors, and not having bias on the pots for such a big amp, but amp already has quite big 0.25uF coupling caps, i generally go for x10 but that would be 2.5uF capacitors - beyond 1uF are there really any audible difference? So would using 2.2uF capacitators make sense?

Strictly speaking, make the extra coupling caps 5 x bigger than the exisiting 0.25uF if you want to 'maintain' the pass band - same voltage rating tho.

[Balticnoiseforge]

Yeah iam aware of how to do it, but what would determine max required current for a diode in that position? I tend to like only using 1 diodes on each side, such as the R3000 that has 3kV.

But havent used it for bigger amplifiers, would the current rating even matter when used in that position? Or is it only voltage and vF?

[SEL49]

The R3000 is rated for 250mA but don't worry about the forward current rating. The diode is installed reverse polarity and only conducts if you have a flyback pulse that exceeds 3000V. Sometimes the diode may be destroyed if you get a big flyback pulse but that's OK. The whole idea of the diode is to protect the tube and OT from big negative spikes that may occur when to OT is not connected to a speaker or other proper load.

[Balticnoiseforge]

The R3000 is rated for 250mA but don't worry about the forward current rating. The diode is installed reverse polarity and only conducts if you have a flyback pulse that exceeds 3000V. Sometimes the diode may be destroyed if you get a big flyback pulse but that's OK. The whole idea of the diode is to protect the tube and OT from big negative spikes that may occur when to OT is not connected to a speaker or other proper load.

Ah okay! Aware if there would be any difference in how much current typical diodes can take in reverse before breaking down? Or i suppose it would depend on voltage and current.

But either way, their current rating doesnt matter in this application then and ok to use?

Have been into old Ampegs where the string soldering of x3 1n4007 have caused breakdowns, so always thought using one diode is better.

[tubeswell]

Yeah iam aware of how to do it, but what would determine max required current for a diode in that position? I tend to like only using 1 diodes on each side, such as the R3000 that has 3kV.

But havent used it for bigger amplifiers, would the current rating even matter when used in that position? Or is it only voltage and vF?

The diodes are a low resistance path for voltage transients from the collapsing magnetic field, which is a self-induction thing that happens with inductors when they are suddenly switched off. Bigger and more tubes = more current going in, which has got to come out. You’ve got 2 x 6550s on each side of the primary. Ampeg SVT CL uses 1A 3kV each side - for 3 x 6550s on each side. So if you copy that, you’re in safe margins.

[Balticnoiseforge]

Thanks for all replies, i just thought of one more thing - would it be possible to change the 1m grid leak resistor prior to the 6an8a and after the treble potentiometers output for a 1meg potentiometer to achieve a pre phase inverter master volume?

Or would that throws off the pentode?

[SEL49]

You can do that.

[Balticnoiseforge]

You can do that.

Ah okay! Wonder why i didnt think about that at first, since it would be easiest one to implement.