Holden Slave mods

[Colonel_Sanders]

Hello again!

I'm modifying a Holden Slave (old KT88 tube P.A. amp with mixer) for guitar use at the moment and have found something interesting! Its the first time I've run into a different phase inverter than the Schmidt, but I'd like to change it to a Schmidt cause I'm building it into a high gain Plexi sort of amp.

Just wondering if there is anything I should watch out for when converting one phase splitter to another. I can't foresee any problems but better safe than buying a new amp for my client .

Cheers,
C_S.

[black_labb]

the holdens have a standard schmidt (just found out thats another name for a LTP, you learn something everyday) but they are followed by a driver/cathode follower for each side of the signal. i'd probably leave it in, unless you dont have room for extra tubes. is this a 4xkt88 version?

post some pics, im always interested in the old holden's/wasps as i have 2

[Colonel_Sanders]

Oh, the driver/cathode follower is what confused me. Never seen it before... Is it neccesary? I wanted to chop in a cathode follower and passive EQ.

2 x KT88 version. They're great amps eh!

Cheers.

[tubeswell]

The driver-cathode follower stages are there to boost the signal going into each KT88 signal grid. If you take them away you should still be able to get a good level of gain into the KT88 signal grid from a LTP (depending on how the LTP is set up). My guess is that, seeing as how the slave amp was designed with the driver and CF stages that way, they weren't aiming for anything that would produce undesirable distortion, rather they were probably aiming for a strong clean signal going into the output stage grid. If you remove those stages and use the heater uptake that was intended for those bottles for something in front of the PI, you will be able to get a decent guitar amp out of it.

[black_labb]

the drivers dont boost the signal as such (usually the gain is just under 1), they give a lower impedance output for the big bottles.

while they are not neccesary, a high gain amp could benefit from it by stopping the PI from distorting at high volumes. i'd personally leave the holden/wasp preamp and driver in there, and build a preamp where the small preamp (gain and volume control) are now.

if the chassis is like the one at the bottom of the page here http://www.ozvalveamps.elands.com/holden.htm then you should be able to put 3 tube sockets at the top and put your circuit board/tagstrips... there is probably a number of holes for pots down the bottom behind the faceplate. should be a good amp for modding. its what i planned to do with mine, but i like them and thier history too much to gut them.

only think is you may want to shield bits of the preamp, as high gain can get noisy, and the wasp chassis's dont seem to offer alot.

[tubeswell]

Ah I see - its not a driver-cathode follower, it just a cathode follower (I should've looked at the damn scheme before opening my big mouth). When you said driver/cathode follower initially, I was envisaging something like a direct-coupled pair.

[black_labb]

Ah I see - its not a driver-cathode follower, it just a cathode follower (I should've looked at the damn scheme before opening my big mouth). When you said driver/cathode follower initially, I was envisaging something like a direct-coupled pair.

probalby my fault, i didnt explain it too well.

[Colonel_Sanders]

Hey guys, cheers for the replies. I've already quoted and I don't think the client can afford another tube and socket! I'll try it without the cathode follower first and see how bad it is. Let you guys know how I get on!

Tubeswell, do you have schematic for the Slave?

[black_labb]

theres a circuit for a 200w slave on the link i posted, should be roughly the same circuit as that one.

why not buy the extra socket, as you will have less work to do if you leave the PI stage as it is (so you can leave the board there, just do the preamp). either way though.