sluckey wrote: ↑Thu Jun 13, 2024 3:11 pm
Did you verify the FWB diodes are all good? How much B+ voltage do you have at the choke?
FWIW, my most immediate reaction was that one or more diodes in the FWB are either bad or installed backwards.
Also, from the PT, you indicate one leg at 4VAC and the other at 228VAC. If all else is OK, you might want to verify the PT is not blown on the upper half. If you really only have 4VAC that might explain the very low bias voltage as you are tapping that leg for the bias supply. Disconnect the FWB and measure VAC across the high voltage secondary. Is the reading consistent with the PT spec? That PT (if you really do have a 261M6) is supposed to give 215VAC for the whole secondary. In other words it is 107.5-107.5, not 215-215. The 261M6 has no center tap, so it can't be 215-0-215 as you indicated on your schematic. Voltages might run a little high as the primary is 115V rated and it is common to see 120V or more at the wall socket. You say it is 311+4 = 315, which is 100V higher than expected and each leg of the secondary should be the same. Rectified VDC should be about 304, not 411, again about 100V too much. Something doesn't add up or you have a different PT than indicated.
You have a couple of problems. The bias reservoir cap has to charge through a load (as shown in the Valve Wizard article). Move your reservoir and 220k bleeder upstream of the standby so the bias will be there when you hit "play." You are also putting a heavy load on the bias circuit with the dual bias adjustments, so I would go with the circuit below using a 220n cap and 33k to ground. It has a 3x 10u cap arrangement for faster settling and low ripple. The single 100u is not a good design, as it will take far too long to charge.
After you do that, with 215 VAC on the FWB, you should be seeing about 300VDC on the reservoir in standby, and about -37V for the bias at mid sweep.
You do not have the required permissions to view the files attached to this post.
Last edited by martin manning on Fri Jun 14, 2024 11:57 am, edited 1 time in total.
It will be easier to just redo the power supply circuit then to check all the diodes and move the standby switch location etc. I've got some still in the package 1N5408 that I know are good, more heavy duty then is necessary but then I don't have to worry. I'll get this done and report back tomorrow.
RyanThomas13 wrote: ↑Fri Jun 14, 2024 2:07 pm
Looked everywhere and I don't have a 220n 400v, only 160v which is not enough. I will order this and build it as shown. Thanks again.
Use a .047µF and 56K. Those are the values used in hundreds of Marshalls and Ampegs. Those values will work well.
Yes, but this isn't one of those, and I don't think the bias voltage will be low enough. The plate voltage and dual bias pots make this more of a challenge.
Found an old 0.18uf 400v, should be good enough to use until the 220nf arrives, at least to see if its working. I'll get started on it and hopefully have it done today, or tomorrow at the latest. Once I get it working I'll be curious to see what affect 47nf vs. 220nf has on the bias voltage.
Should be a worthwhile experiment. I was thinking you might need to put the 10k (or larger) back in, or reduce the cap size a bit to get the bias voltage you need to be roughly in the center of the trimmers. 180n and a 10k might just do it.
Thought I would include some pics. It took my a couple more days than I had hoped but its mostly done. I probably won't leave the FWB as shown, but I just wanted to get it working.
So as tested I have -39vdc after the diode, thank god. I still have not taken it off of standby because I'm a bit concerned about the 365 vdc I have at the first power node, which seems too much for the 6K6. The choke isn't going to reduce voltage that much, and if I'm reading my schematic correctly the first dropping resistor isn't until after.
Here's the updated schematic I'm using.
6K6 power supply.png
Am I understanding this correctly?
You do not have the required permissions to view the files attached to this post.
