huge voltage drop after diode rectifier

Status
Not open for further replies.
Jump to Latest
B

bagudan

Member
Joined 2010
#1
Hi

I´ve just completed a tainwreck express clone per this ceriatone layout:

http://www.ceriatone.com/images/layoutPic/tWCloneLayout/ExpressionCeriatone.jpg

Everything was fine starting it up with a bulb limiter, but I found the voltages very low, but I just thought it was the bulb

So I plugged it in the wall

heater voltage is 6,9 with tubes installed

secondary pt voltage is around 350 v ac

but when checking the rectified voltage, just after the diodes, at the 33 uf cap, I only get 40 v dc?

This isn´t going to deliver smooth overdriven tones, for sure? But why would the diodes do that? I used 1n4007s for the power supply diodes

I probably should mention, that the power supply is taken from an old marshall jcm900

Best regards, Kasper
 
#2
First, this is wrong forum, here we discuss HiFi.
Surely you have the standby switch turned on? There must also be a centertap on the HT winding for this circuit to work, is it connected? That´s the two grey wires to the star ground
🙂
Cheers,
Seppo
 
Last edited:
#10
What is HT with no valves installed ?

If that is OK I would present the PSU with a resistive load to check that all is well.

It could be a fault in the PSU or one valve pulling too much current.

IME one of the HV caps may be faulty.
 
Last edited:
#12
Hey, I think I have your answer. If the power transformer is from a Marshall 900 series the high voltage has no center tap so you need a bridge rectifier. The clue was the high voltage reading. If it were a center tapped transformer the high voltage across the winding would be closer to 700V.
 
#13
Havent thought about this projec´t for months, but You solved the problem MelB

Thanks a million. I redid the power supply, so it is now a bridge rect. The B+ I s around 409 vdc, wich is ok, a tad high though. The bias is another story. I should be able to dial to -33vdc, but it is way too hot. There is serious redplating going on. Maybe this is because of the differently rectified voltage on the powersupply board?
 
#14
If you need more BIAS range... Adjust the 220K resistor that feed the BIAS circuit..
Try lowering it....this should get you more range...
For starters bring it down to roughly 180K by putting a 1MEG in parallel with the 220K..
From there you can monkey with the value till the bias range is acceptable..
 
#16
I managed to get the bias right, well at least when measuring from chassis to pin 5 on the powertubes. It´s around -35mv. But I still get serious redplating. I had to swap the 220 k bias resistor for a 50k before I t worked. The anodes on the powertubes were around 400v an the screens around 300v?
 
#17
I just powered it up with a bulb limiter (40w bulb)

Here are my voltages

V1: pin1-152v, pin3-160mv, pin6-127 v, pin8 -5mv
V2: pin1-147v, pin3-9mv
v3: pin1-148v, pin2-0,5v, pin3-0,6v,pin6-150v, pin7-o,5v, pin8-0,6v

v4&v5: pin3-160, pin4-157, pin5 - -81mv

The bulb flashes shortly, but then it goes dim after af few seconds. it glows, but not bright.

any ideas?
 
#20
I´m puzzled. Maybe modifying the rectifier from full wave to bridged is problematic? I´ve triple checked every component, but the only thing I can think of, that could be wrong, is the bridged rectifier. My PT has no High voltage center tap, so MelB helped me by suggesting a bridged rectifier. He was right. The voltages suddenly were fine, but maybe I implemented the change in a bad way. I´ve attached my layout for the bias cirquit
 

Attachments

Status
Not open for further replies.
Top Bottom