peavey Iron

[xtian]

Or use a MOSFET as CF.

[norburybrook]

yes it is, it says -53v on the schematic.

So basically that tap isn't used on the SSS? , is it cathode biased? I thought cathode bias was for smaller wattage amps.


M

[norburybrook]

But you can always add the small extra transformer for the CF power supply, or perhaps use the Peavey's bias winding (looks like it is separate from the HT) and a voltage tripler to get the negative voltage. You could then use the PI node for the driver's positive voltage.

yes it is, it says -53v on the schematic.

So basically that tap isn't used on the SSS? , is it cathode biased? I thought cathode bias was for smaller wattage amps.


M

[martin manning]

The CF driver stage is arranged so that the cathode voltage (at its output) is negative (the voltage at the bottom of the cathode resistor is way-negative) and the DC value is at the right level to bias the power stage in fixed bias.

[norburybrook]

very clever.....and.........the question de jour......

why is this better than a regular fixed bias?

Does it affect the sound, and if so in what way?


M

[larsvictor]

Peavy triumph 120W PT:
US version is: #70518719
European version is: #70518705

TAD has a replacement (for 160€!):

Primary:
0V Black
220V Grey
230V Brown
240V White

Secondary:
195V Red
0V Pink
195V Red

23V Orange
0V White
23V Orange

6,3V Yellow
0V Yellow

They use the first secondary without the CT (full wave bridge rectifier). Using the second secondary winding without the CT in a voltage tripler you are in a range of -150 VDC. That would be enough for the CF (you have to decrease the value of the cathode resistors).

This is a fixed bias circuit too.

Lars

[martin manning]

This is a fixed bias circuit too.

Quite right!

[norburybrook]

ah...this is all good stuff.

I didn't read the schematic properly so didn't realise it was a 195-0-195 transformer.

so with that in mind I'd have to use a full bridge rectifier to get some decent voltage out of it anyway.

you say they only use one side of the secondary....looking at the schematic though it looks like both 195 are connected to the bridge rectifier...where's the other end from then and what is the unused end doing?


good thing is I paid less for the whole amp than the cost of a single transformer


I'm sure I'll be able (with the help here) to make something great out of this

TBC


M

[martin manning]

The first and second secondaries Lars refers to are the HT and the bias winding.

[norburybrook]

oops double post

[norburybrook]

The first and second secondaries Lars refers to are the HT and the bias winding.

ah, I understand what you mean now.

The first secondary would be as is,a full wave bridge rectified signal and the bias tap would be used without a center tap with a voltage Tripler to get 3x the 53 volts, so somewhere in the ~150v range.


what would the voltage tripler look like? I've seen a doubler where it uses both cycles to double the voltage with diodes and capacitors, but how do you do triple? how do you get 3x the voltage......doesn't that go against the laws of thermo dynamics?

M

[larsvictor]

For a negative output the direction of the diodes and capacitors must be reversed.

[norburybrook]

@Lars, thanks for that, I had a google last night and did some reading up on voltage multiplying

I'm going to order a chassis and boards from Taylor and have a go at the Wonderland, it seems like a simpler build and will fit in his chassis obviously. The SSS is a huge amp and O haven't the facility to make a chassis.



so looking at the wonderland, it looks like a ODS without the OD but with reverb.


there's no choke either it seems!


so with the PT I have from my donor amp.

Primary:
0V Black
220V Grey
230V Brown
240V White

Secondary:
195V Red
0V Pink
195V Red

23V Orange
0V White
23V Orange

6,3V Yellow
0V Yellow


I'll need to make my own rectifier board as it's very different from the twin transformer.

so could anyone help me with a layout for this?

I know I'll need a full wave bridge rectifier without the CT to get my main DC voltage it's showing 508v DC on the peavey schematic , the wonderland uses the standard, and familiar to me two phase rectifier, so I'm in unchartered waters for a start.

Then there's the bias, which is very simple on the wonderland board as there's just a single 50v secondary feed from the transformer.

the peavey has 23-0-23 so I'm unsure how that gets to be -56vDC on the schematic.


I'm still finding it hard to visualise a layout of components from a schematic.



Marcus

[larsvictor]

Marcus,

the Wonderland has a choke too. Look at the standby switch in the layout. The returning wire from the choke should go to + terminal of the first (left) 20µF filter cap.

If you want ~500V on the plates and a little less for the screens use a full wave bridge rectifier for the HV like in the peavy schematic.

For the bias you can do the same with both 23V windings. CT not grounded. FWBR, but of course with the positive terminal of the rectifier grounded.
You will have ~ -77 VDC at the negative terminal of the rectifier. Some resistors and a small potentiometer and that´s it (10k - 10k pot - 27k to ground).

Lars

[norburybrook]

thanks Lars,

as I said I could do with seeing what that would actually look like on a layout so I could make a board

I'm learning as I go with all this stuff and each new build gives me a little more knowledge , this will raise the bar for me as I'm using a donor amp so having to go off piste


Thanks again for your help

marcus