O.T.B. Plush 3000B

[RicharD]

What a beast.  Beautiful sparkle blue puffy padded exterior like the old Kustom amps.  6 - 6L6GC, 2 - 12AX7, & 1 - 12AT7.  the chassis has a ton of rust.  All the 6L6's look gassed.  2 of the sockets have been replaced.  Transformers are massive and reek of Dynaco although there are no Dynaco markings so I'd guess not.  The chassis is quite reminiscent of an old Sunn I worked on.  I lit it up with a lamp limiter w/o tubes and the PSU did come up.  The OPT measures 23.8:1 which at 4 ohms is roughly 2K.  Rock on.  All the important thangs appear to be good.  No docs anywhere on the web.  From what I've read, Plush was around about 5 years.  Most of their stuff is Peavey knock-offs.  Apparently this particular amp was constantly changing whereas some have a voltage regulator tube.  This one does not.

[DummyLoad]

cool looking amp! 

[andrew_k]

Chris Martin aka "Sir HC" on various forums runs http://www.plush-amps.com/ and has a few in his basement if I remember correctly. If you're looking for particular data he would be worth contacting.

Sick amp. Want.

[simonallaway]

Man, the last time I saw upholstery like that was at my own bachelor party 

[OldHouseScott]

I recapped and retubed a Plush 3000B a few years ago (in red sparkle!) and here's the reversed schematic I made at that time. Note it has the gas-discharge voltage-regulator tube, and two chokes. It also had a PCB for the output tubes IIRC. Maybe this will help.

[RicharD]

Thanks for the schemo.  This one does not have the VR tube.  It actually looks pretty simplistic.  I'm thinking recap and retube will probably make it happy.  Cross that bridge when the parts come in.

[RicharD]

The PSU is entirely different.  I don't recall ever seeing a rectifier arrangement such as this. 

[PRR]

> I don't recall ever seeing a rectifier arrangement such as this.

It's diabolical.

The CT winding, 2 caps, and FWB create a "center"-tapped cap-input filter. The center is used only to enforce a split-voltage on the two caps, presumably because the HV is past the 400V-440V which is safe on a single electro cap.

The 2 extra diodes don't change that. So what do they do?

With the diodes, the choke gets, not smoothed DC, but rectified DC, big half-sines. So the choke works as a choke-input filter.

DC Voltage for cap-input: VAC * 1.414
DC Voltage for choke-input: VAC * 0.9

So if the cap-input goes to 500V, the choke-input goes to 318V.

It makes sense if you need two voltages. They probably up-ed the plate voltage because the OT was hi-fi rated for 75W at 50Hz and they wanted to go 150W at 80Hz. At 6K/pair load the 6L6 does not need G2 or driver voltage as high as plate. And 500V increases cap costs. So they used whatever gimmick to drop 500V plate voltage for the other stages. A series gas-tube works, but costly, and anti-regulates.

A drawback of choke-input is that there is a minimum current. Any less and it turns into a cap-input. When current is hot-cathode tubes, no current for 10 seconds after switch-on, those "choke input" caps must stand the full 500V. And we see you note "525V" parts, premium price over standard 450V rating.

[RicharD]

>It's diabolical.

The CT winding, 2 caps, and FWB create a "center"-tapped cap-input filter. The center is used only to enforce a split-voltage on the two caps, presumably because the HV is past the 400V-440V which is safe on a single electro cap.

The 2 extra diodes don't change that. So what do they do?

With the diodes, the choke gets, not smoothed DC, but rectified DC, big half-sines. So the choke works as a choke-input filter.

I think I understand.... but not really.  Is it sorta like a voltage doubler sitting atop a bridge?  I can't wait to get the new caps and tubes in so I can light this thang up.  I do have a set of 10kV scope probes so i can (and will) grab full telemetry with diagrams.

[PRR]

> like a voltage doubler sitting atop a bridge?

No. It is a FWB, but loaded two ways: cap-input and choke-input.

[birt]

that is a very cool, big and nearly empty amp :p

i see holes for 2 more power tubes and 3 more preamp tubes.. makes me wonder if they had an even bigger beast like that?

[RicharD]

I know they made a 4000B which eye ASSuME was perhaps a bigger beast.  It is a fairly sparse circuit and most of it's hidden on the PCB with the pots. 

I pulled the PI/Bias board and found the trace side COMPLETELY corroded.  I mean BAD.  Solid green.  All I wanted to do was replace the bias cap and diode for good measure.  Instead, I got to completely disassemble the PCB which was a wchore.  Because of the corrosion, the solder didn't wanna melt.  I went over every trace with a dental tool then flowed solder on the traces, and repopulated the board with all new parts.  JOY!  Fortunately the main audio board is not in the same condition.  Obviously this amp sat in a damp garage somewhere.  One of those spare holes is right above the bias/PI board.  That board sits horizontal and musta took on some water.  The main audio board sits approximately 15 degrees off of vertical and wasn't exposed.  I was really dreading pulling it.  It's fine and dandy. 

[JonO]

Have a Plush P1000, dead copy of a Twin Reverb except with an out for slave amp/recording