Diode rectification

[Mrcads]

Hello

I was wondering if it is possible to squeeze 480 VDC out of a this transformer:
http://www.classictone.net/40-18053.pdf
This would be 175-0-175 on the center tap.
I was trying to go by this schematic:
https://schematicheaven.net/marshallamp ... w_2203.pdf
The most I got out with this diode arrangement, was 122 VDC. Seems like only D2 and D4 have current going through them, D3 and D5 go to ground.
If i feed the AC through D3 and D5, I get 245 VDC, which is only half the glory.

[danman]

That PT is only used with the 100 watt models that employ a voltage doubler rectification setup. With the correct configuration it should land somewhere in that area depending on tubes and biasing. Do a google search for Marshall voltage doubler and you will find some articles that explain the details behind it.

[Mrcads]

That PT is only used with the 100 watt models that employ a voltage doubler rectification setup. With the correct configuration it should land somewhere in that area depending on tubes and biasing. Do a google search for Marshall voltage doubler and you will find some articles that explain the details behind it.

Thanx Danman!

I thought that would be the case. I haven't been able to find a 100 W Marshall PT with center tap > 360V

[neikeel]

As Danman said:
-the 100w Marshalls after the 45/100 all used a voltage doubler with the centre tap feeding the centre point of the filter cans.

My Hiwatt has a non-centre tap PT and has 360vac for (360x1.4) around 500v

You can get the correct PT from George here (Heyboer), Brian Wallace at Marstran (also Heyboer) and ******, all will give around the correct 490v on the output anodes for a 100w Marshall. Part number is Dagnall T2562.

[Mrcads]

That PT is only used with the 100 watt models that employ a voltage doubler rectification setup. With the correct configuration it should land somewhere in that area depending on tubes and biasing. Do a google search for Marshall voltage doubler and you will find some articles that explain the details behind it.

Well... I made it happen. Around 490 VDC, with 4 6550C's sucking the juice out of the caps. Thanx for the tips!

Now another issue... I think it's called the negative feedback loop. Where in the world are R15, R16, R17 supposed to get the power from? I get 300+ on the plates on V3, but no voltage on the grid or anode. It's supposed to be 40-50 Volts an all three, but only R16 has around 5 Volts.

It looks like that circuit starts at the 4 ohm tap of the OT, goes through the negative feedback resistor, where it splits to the Presence pot, and the 10K resistor.
All help is appreciated.

[danman]

I'm nor sure which resistor you are referring to as I don't know which layout you are using. You said you had 300v on the plates but nothing on the grid or anode. The anode is the plate, so I'm gonna assume you meant the cathode. If you can post a link to the layout I may be able to give a better answer. One last thing...be sure that your presence pot is grounded properly or you won't see the expected voltages through that part of the circuit. One lug of the pot is grounded and the capacitor also attaches to ground.

[Mrcads]

I'm nor sure which resistor you are referring to as I don't know which layout you are using. You said you had 300v on the plates but nothing on the grid or anode. The anode is the plate, so I'm gonna assume you meant the cathode. If you can post a link to the layout I may be able to give a better answer. One last thing...be sure that your presence pot is grounded properly or you won't see the expected voltages through that part of the circuit. One lug of the pot is grounded and the capacitor also attaches to ground.

Thanx danman. I meant no current on grid or cathode.
https://schematicheaven.net/marshallamp ... w_2203.pdf
I swear,it looks just like my 2204 build which is actually working. I'm sure I'm looking at the damned thing, i just can't see the obvious.

[Mrcads]

This is what I’m using for selectors in the back. I used the same concept to hook up the OT output as it is described for the PT.
viewtopic.php?f=3&t=2528

[danman]

R15 is your 33k slope resistor and is part of the tone control. It is not involved in the negative feedback loop. A common mistake in new builds is using a 470k in place of 470 ohm at r16. Be sure this resistor measures 470 ohm and you may need to unsolder one end of it to get an accurate reading. Don't trust the color bands as I have seen those to be wrong. R16 and 17 don't really get their power from the power supply like the dropping resistors in the B+ line do. They do develop a DC voltage across them though and must be properly grounded at the tail end which is the presence pot. Double check all connections from PI to presence pot to assure you have proper grounding. I have seen times where a solder joint looks good but the electrical connection is actually bad.

[Mrcads]

Spot on. A friend of mine pointed out the 470R resistor. Sure enough, it was x1000 times higher. Everything works as it should, except a very high pitch sound, when I turn the presence under 6. Other than that,the amp sounds like a monster. 4x 6550 tubes with 45% emission and 4 preamp tubes, at 2.5 on the volume knob, the whole house is shaking!

[harleytech]

With my Metro 100 watt Superbass build, I'm getting 481v with the 40-18053...

[danman]

Be sure that the wire leading from the board to the presence pot runs out from the end of the board and then straight to the pot. Running it out of the side (long edge) and then to the pot may introduce some oscillation from being in close proximity to the tone pot wires. The presence wire should be separated from the tonestack wiring as much as possible and running it to the end of the board before heading to the pot helps accomplish this. I believe that George mentioned this detail in the 100 watt build instructions and may have some pics of the wire routing.