Paging Larry/SDM/Techs...Marshall 9005 Poweramp circuit ?

MacGaden · Post by **MacGaden** » Thu Aug 27, 2009 5:02 pm

I just got one of these very cheap.

Can anyone explain the schematic to me ? It looks weird, what are the extra tube stages after the PI for ?

Schematic here:

TIA

Structo · Post by **Structo** » Thu Aug 27, 2009 7:34 pm

Good question, not sure I have seen that before either.
Do you have any pictures of the amp you could share?

MacGaden · Post by **MacGaden** » Fri Aug 28, 2009 1:24 am

Structo wrote:Good question, not sure I have seen that before either.
Do you have any pictures of the amp you could share?

Not yet, but I´ll take it apart in the weekend..

Those extra stages after the PI looks very strange. What are they for ?

mensur · Post by **mensur** » Fri Aug 28, 2009 10:36 am

MacGaden wrote:I just got one of these very cheap.

Can anyone explain the schematic to me ? It looks weird, what are the extra tube stages after the PI for ?

Schematic here:

TIA

That extra 12AX7 is extra driver tube that deliver more current to amp when switched in triode mod.

MacGaden · Post by **MacGaden** » Fri Aug 28, 2009 8:39 pm

mensur wrote:
MacGaden wrote:I just got one of these very cheap.

Can anyone explain the schematic to me ? It looks weird, what are the extra tube stages after the PI for ?

Schematic here:

TIA
That extra 12AX7 is extra driver tube that deliver more current to amp when switched in triode mod.

Interesting.. Tell me some more. How does that work ?

SDM · Post by **SDM** » Sat Aug 29, 2009 5:33 am

Local FB around those stages gives them a low output impedance to easier drive what follows, also a wider/flatter frequency response along with consistent performance less dependent on the particular 12AX7 used.

MacGaden · Post by **MacGaden** » Sat Aug 29, 2009 9:23 am

SDM wrote:Local FB around those stages gives them a low output impedance to easier drive what follows, also a wider/flatter frequency response along with consistent performance less dependent on the particular 12AX7 used.

Thanks, i haven´t had a chance to hear it yet, but: It should sound cleaner that a direct power amp in a 50 wat Marshall ?
That might be OK, as I may use it for a W/D/W setup. If it doesn´t I could always turn it into something else, PTP and Sozo´s..

mensur · Post by **mensur** » Mon Aug 31, 2009 7:43 am

Its good advice to use ECC81/82 instead of ECC83 because they will deliver more current to the output.It's not going to cleaner sounding, it will have more damping, better dynamic's, smoother and richer harmonics hence it's low impedance driver.

sl0000 · Post by **sl0000** » Sat Apr 11, 2020 4:06 pm

I thought that they might be there to minimize the distortion. PR1 "sets balance" which I assumed was to help make the pushing and the pulling of the power stage equal on both sides. It might help with increasing the max power before distortion so both power tubes start to clip at the same time. I think trying to do this in a normal phase inverter will cause more problems than it would solve. It would be hard to do it without creating frequency response or biasing issues.

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Paging Larry/SDM/Techs...Marshall 9005 Poweramp circuit ?

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