21H amp rebuild, tech recommendations?

[dweisskeys]

Hello,

I inherited a 21h last winter that sat for years. It was in pretty bad shape. After replacing the can cap the tone was markedly better. A short while after the amp was running very hot, crackling, all kinds of issues. I had a tech repair it and now it's rock solid reliable, but the tonal detail it had before is gone. It's real dull. Some (but not all ) caps were changed and some other things cleaned up but the Leslie's lost it's mojo. I'm guessing it needs an undoing of whatever went into it and complete restoration. I'm not up to the task. Who's the right tech?

Thanks,
Daniel

[Jyvoipabo]

Hello,
Have you checked the dividing network ?
Capacitors may need to be replaced.
JP

[kziss]

Hello,
I inherited a 21h last winter that sat for years....After replacing the can cap the tone was markedly better. A short while after the amp was running very hot, crackling, all kinds of issues. I had a tech repair it and now it's rock solid reliable, but the tonal detail it had before is gone. It's real dull.
Daniel

Hi Daniel.
Apparently some Leslie 21H cabinets have a 32H Series 3 amplifier in them.

On the 32H Series 3 schematic, in dotted lines it shows a 0.003 treble cut capacitor wired between the plate pins 2 and 5 of the 6SN7 tube, and immediately below this it says "31H cab only".

The 0.003 uf treble cut capacitor noticeably mellows or dulls down the treble response so therefore it is a good idea to examine your amplifier to see if the tech has installed the 0.003 uf treble cut capacitor between the pin 2 and the pin 5 of the 6SN7 tube.

If the 0.003 uf capacitor is there, then you should unsolder it and remove it so that the treble response is increased.

Also have a look at the uf value of the capacitor wired across the pins 3 of the two sets of output tubes (5881 in 32H Series 3 amplifier, and 6V6 in 21H amplifier ).

The capacitor here should be a 0.0047 uf very high voltage rated capacitor.

Check to see that the tech has not accidentally wired a higher uf value capacitor here because a higher uf value capacitor would more noticeably dull down the treble response.

Another possibility is that the 0.0047 uf capacitor was absent before your amplifier was worked on, and if so, then the tech might have installed the 0.0047 uf capacitor in accordance with the stock 21H / 32H Series 3 amplifier schematic.

The absence of the 0.0047 uf capacitor may allow a brighter treble response, but it's absence can also allow ultra high frequency oscillation to occur in the power amplifier which can cause crackling and other strange sounds, and the power sucking ultra high frequency oscillation can also cause the amplifier to run hot.

Try unsoldering the 0.0047 uf capacitor and then listen to the amplifier to see if the old mojo and treble detail is restored.

If this is the case, then you could try replacing the stock 0.0047 uf capacitor value with a smaller uf value capacitor such as a 500 pf or 0.001 uf capacitor in order to allow more treble response whilst still suppressing the ultra high frequencies which cause oscillations and instability and overheating.

The capacitor here needs to be very high voltage rated such as 2 or 3 kilo Volts in order to avoid becoming damaged by the high voltage transients present at the 5881 / 6V6 tube plate pins 3.

WARNING: make sure that the amplifier is completely removed from the AC mains power and make sure that the power supply filter electrolytic capacitors are fully discharged before doing any work with the amplifier.

Here is the Leslie 21H amplifier schematic:


Here is the Leslie 32H Series 3 schematic:


All the best.
Kon.

[dweisskeys]

It is a 32H amp. Fascinating. Thanks for all this great info! -D

Hi Daniel.
Apparently some Leslie 21H cabinets have a 32H Series 3 amplifier in them.

On the 32H Series 3 schematic, in dotted lines it shows a 0.003 treble cut capacitor wired between the plate pins 2 and 5 of the 6SN7 tube, and immediately below this it says "31H cab only".

The 0.003 uf treble cut capacitor noticeably mellows or dulls down the treble response so therefore it is a good idea to examine your amplifier to see if the tech has installed the 0.003 uf treble cut capacitor between the pin 2 and the pin 5 of the 6SN7 tube.

If the 0.003 uf capacitor is there, then you should unsolder it and remove it so that the treble response is increased.

Also have a look at the uf value of the capacitor wired across the pins 3 of the two sets of output tubes (5881 in 32H Series 3 amplifier, and 6V6 in 21H amplifier ).

The capacitor here should be a 0.0047 uf very high voltage rated capacitor.

Check to see that the tech has not accidentally wired a higher uf value capacitor here because a higher uf value capacitor would more noticeably dull down the treble response.

Another possibility is that the 0.0047 uf capacitor was absent before your amplifier was worked on, and if so, then the tech might have installed the 0.0047 uf capacitor in accordance with the stock 21H / 32H Series 3 amplifier schematic.

The absence of the 0.0047 uf capacitor may allow a brighter treble response, but it's absence can also allow ultra high frequency oscillation to occur in the power amplifier which can cause crackling and other strange sounds, and the power sucking ultra high frequency oscillation can also cause the amplifier to run hot.

Try unsoldering the 0.0047 uf capacitor and then listen to the amplifier to see if the old mojo and treble detail is restored.

If this is the case, then you could try replacing the stock 0.0047 uf capacitor value with a smaller uf value capacitor such as a 500 pf or 0.001 uf capacitor in order to allow more treble response whilst still suppressing the ultra high frequencies which cause oscillations and instability and overheating.

The capacitor here needs to be very high voltage rated such as 2 or 3 kilo Volts in order to avoid becoming damaged by the high voltage transients present at the 5881 / 6V6 tube plate pins 3.

WARNING: make sure that the amplifier is completely removed from the AC mains power and make sure that the power supply filter electrolytic capacitors are fully discharged before doing any work with the amplifier.

Here is the Leslie 21H amplifier schematic:
[ATTACH=CONFIG]30885[/ATTACH]

Here is the Leslie 32H Series 3 schematic:
[ATTACH=CONFIG]30884[/ATTACH]

All the best.
Kon.