I thought I had this figured out...

[dorrisant]

Ok... so read a bit of this first: https://ampgarage.com/forum/viewtopic.p ... eavey+5150

So now, I have a Schecter Hellwin on the bench... Oh Boy! I hate it. Anyway, it came to me not working fresh from an auction site. It had a 220v mains PT so that wasn't gonna work. Customer wants it bad so I send the good but 220v PT to Patrick @ MM. After a reasonable time, he sends a replacement wired for 120v. All good but the bias. This thing is running EL34s with 450v on the plates and average of 60mA for each power tube. It has a switch for EL34/6L6 operation. Change that to 6L6... now it idles at 55mA per tube. Still too high. Check the bias voltage... -32v. No dice! I'm starting to think the winding for the bias may be wound wrong at this point. Bias circuit seems like it has been run from a 13-0-13 tap... This, by way of deduction must be the relay supply as well. All of the relays are working so no need to worry... Checking the EL34/6L6 switch... it seems to be changing the voltage for the OT CT. It is not changing the bias voltage on pins 5. So, if I just get the bias a bit more negative then it should work for both EL34 and 6L6. Now I notice that the HT winding is connected to a full wave bridge rectifier. Not a problem, just like in the thread at the link above, right? Wrong! I used basically the same supply that I posted as a pdf on the 2nd page. (posted below too). I thought I was gonna breeze right through this... Ha ha!

What gives?

I tried everything I could think of so far. I even tried the normal way of tapping the HT... HT tap -> 220k -> diode -> etc.. Nothing as far as DC.
With the cap fed circuit, I measure 185vac and 223vdc where the cap is connected to the HT secondary winding. On the other end of the class xy cap (@ the reversed diode) I measure 87vac and 53vdc. After the reversed diode there is less than 1 vac and -7 vdc. I am at a loss and so is the customer.

And, there is NO SCHEMATIC TO BE FOUND, change my mind.

I should have never accepted this amp. No more cheap PCB amps!

[T Wilcox]

I had a similar but different issue when I used a trainwreck PT with no 50v tap on an ab763 build. I tried the merlin bias with issues then posted here and member Sluckey posted a schem with the standard 50watt marshall bias supply that I used successfully.
This is the link he posted that I used http://sluckeyamps.com/phoenix/phoenix.pdf

[dorrisant]

Thanks, but that is where you can pull your bias supply from the HT where you are using a full wave rectifier. This application that I'm working on uses a full wave BRIDGE rectifier. This has to be treated differently. If you read far enough into the link I posted above, you will see that you have to feed the bias circuit with a Class XY type cap. That is what I'm dealing with here and this time it is not working.

[martin manning]

The resistance to ground after the diode is too low. Try increasing the 10k from the pot to ground to 100k.

[dorrisant]

The resistance to ground after the diode is too low. Try increasing the 10k from the pot to ground to 100k.

Screenshot_20200627-123114_EBookDroid.jpg

Merlin Bias Supply for Bridge Rect.pdf

Are you referring to the 10k to ground in that diagram?

[martin manning]

Yes, the one off the blue wire from the pot.

[dorrisant]

I ran the 10k all the way up to 10M with a decade box... no dice. Still same voltages Maybe up by 1/2 vdc at the output.

[dorrisant]

Ok... back on this. Does anyone see why this wouldn't work for a bias supply?

https://www.cedist.com/products/transfo ... pen-50-vct

I can get the bias voltage down to -34 stock. I'm hoping to get it down into the -50s with this.

[Littlewyan]

Before you spend more money I think we should go through the Merlin circuit and see what's happening. If you're measuring DC where you've connected your new bias circuit to the HT Secondary then something bizarre is going on. I would trace that part of the circuit and draw it out if you can.

Just a quick note, I recall when I used this circuit in one of my amps that I had to be careful with the standby switch. I can't remember the exact issue, but a paragraph on Merlin's site explains it well.

Notice that this charging path includes the HT load represented by R3. This means that before the valves have warmed up, there will be no bias voltage present unless you have a bleeder resistor across the HT. It is therefore a good idea to include a bleeder, and 100k to 220k (2W) is ideal. (Also, the HT reservoir capacitor must be larger than C1, but it always is anyway, so don’t worry about it).

[Phil_S]

I may be out in left field. As a amateur, I don't always understand everything. Is it possible to employ a doubler in the Merlin bias circuit? A 2x multiplier will actually give 2.88x. From there, it would be necessary to tame the voltage by changing the resistor values. It may be a long shot, but I am thinking it would relieve the clutter of adding a separate transformer.

[sluckey]

Look at page 9 of this pdf for an explanation of how a cap coupled bias circuit works. May give a clue as to why your circuit is not working. I'm betting it's not wired exactly like the Merlin circuit.

http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[thetragichero]

if i remember the capacitor-coupled bias circuit i had to use when i repurposed a fender excelsior pt for a fixed bias design, the magnitude of capacitance effects the voltage you're getting, correct?
larger cap = more voltage to work with
(i believe this is the tldr summary of sluckey's pdf that i have just saved)

[martin manning]

Just a quick note, I recall when I used this circuit in one of my amps that I had to be careful with the standby switch. I can't remember the exact issue, but a paragraph on Merlin's site explains it well:

Notice that this charging path includes the HT load represented by R3. This means that before the valves have warmed up, there will be no bias voltage present unless you have a bleeder resistor across the HT.

Littlewyan makes a good point. Are you measuring with standby set to play, and if not, is there a bleeder resistor across the reservoir cap?

Can you draw out the original low voltage and bias circuit?

[dorrisant]

I'll post some pics by tomorrow. Iv'e used this exact setup before... Merlin bias ckt fed from fwb. Doubler would be no good with this since it doesn't really produce anything to double yet.

[Littlewyan]

if i remember the capacitor-coupled bias circuit i had to use when i repurposed a fender excelsior pt for a fixed bias design, the magnitude of capacitance effects the voltage you're getting, correct?
larger cap = more voltage to work with
(i believe this is the tldr summary of sluckey's pdf that i have just saved)

Spot on. Although dorrisant is getting very weird readings. I don’t think they should be reading any dc between the cap and diode.