JCM900 Rebuild into JCM800

[Mr_bibbles]

The JCM900 power supply clearly has all the kinks worked out to work with the JCM900 power transformer. Pdf64 makes a good point about the bias supply working when the amp is in Standby mode. This is critical ! Follow the Standby switch details like the JCM900 uses.

IT SHOULD BE NOTED BY ALL that the capacitor coupled bias supply such as the JCM900 uses (and slight variations that use a large value resistor in place of the 47nF cap) doesn't charge up very fast. Those 10uF caps are critical to how fast the supply comes up when power is applied. Don't increase their value !

Was able to test today, copied the 900 bias supply exactly and I get bias voltage on the 220K feed resistors without flipping standby!

BUT, I also get about 190VDC on the HT supply measured at the plates with the standby OFF...

MY theory is the single diode between the secondary and the HT fuse is half-wave rectifying the current and delivering it through the choke / OT center tap.... To correct this I Interrupted both secondaries with the DPDT standby switch, (while feeding the bias directly from the PT secondary tap), but then I got almost no bias voltage in standby mode, only about -0.4VDC. After flipping standby, I have good Bias voltage in this configuration, but it takes about 3 seconds to charge up after the flip, which as noted above could be a potentially damaging issue.

What is going on here?

[Helmholtz]

Please post a drawing of your wiring.

The JCM bias supply won't work with a DPDT.

For C15 I recommend an X2 cap rated for at least 350VAC.

[Mr_bibbles]

Please post a drawing of your wiring.

The JCM bias supply won't work with a DPDT.

For C15 I recommend an X2 cap rated for at least 350VAC.

I did wire it up as using only half the switch (SPST), and like I said it worked great but I had less than a volt of bias at the tubes in standby mode.

Here's a drawing, please excuse my chicken scratch

.

EDIT: I was biking home after posting this and realized, I could probably just move the standby switch from the secondary to AFTER the rectifier! Effectively, the standby would go between the rectifier and the OT / Choke resistor, which should charge bias correctly and shut off all HT to the amp in standby mode! (Another note, I have a fuse before the OT / Choke resistor not shown here)

[The Dude]

Move the standby switch to switch B+ after the rectifier.

Edit: Sorry, I was typing during your edit.

[Mr_bibbles]

Move the standby switch to switch B+ after the rectifier.

Edit: Sorry, I was typing during your edit.

Never apologize for being helpful dude! Even just confirming I was on the right track is immensely helpful

[g1]

BUT, I also get about 190VDC on the HT supply measured at the plates with the standby OFF...

MY theory is the single diode between the secondary and the HT fuse is half-wave rectifying the current and delivering it through the choke / OT center tap....

I'm not sure about this, and I don't think the 190VDC in standby is normal for a 900. I'm hoping someone will comment on this.

Here is another variation on the same type circuit. (No HV center tap). The standby switch grounds the bridge. The 330K from the bias diode to the bridge I believe is to get the bias voltage up in standby mode, but I may be mistaken.

[Helmholtz]

The capacitor coupled bias supply is more tricky than I thought. It's actually a charge pump.
See here: http://valvewizard.co.uk/bias.html

Moving the standby switch after the bridge will not work, because the bias circuit needs a current path through the load connected to B+.
This also requires the tubes to be in and heated.

[pdf64]

Shouldn’t the balancing resistors across the reservoir caps provide a suitable current path for the bias circuit to function?

[Helmholtz]

Shouldn’t the balancing resistors across the reservoir caps provide a suitable current path for the bias circuit to function?

Are there any with the OP's amp? If yes, I would think so.
The JCM 900 schematic I looked at didn't have them (no stacked caps).

[Mr_bibbles]

Are there any with the OP's amp? If yes, I would think so.
The JCM 900 schematic I looked at didn't have them (no stacked caps).

To clarify, I have the 800's filter caps installed into this amplifier, but I am copying the 900's bias supply tapped directly from the transformer. MY reservoir caps looks exactly like this except I added 2x 220K resistors across the two left caps to compensate for the lack of center tap, and we are now moving the standby switch (TBD location). We decided to go with the 800's robust 12x 500V capacitors instead of the 900's 4x 500V for sag reasons.

Moving the standby switch after the bridge will not work, because the bias circuit needs a current path through the load connected to B+.
This also requires the tubes to be in and heated.

From this explanation (and using the valve wizard "capacitor coupled bias supply diagram as reference) , it seems like adding a single resistor (r1) between the c1 capacitor and the Bridge rectifier's ground point should provide a complete circuit for the bias in standby mode, passing first through the resistor, then the diode, and back into the winding. EDIT: I already have R1 in there.

Then the reservoir caps and their balancing 220K's (reminder, these are in the supply but NOT on the triode schematic provided) provide a ground reference for the HT, which we would then interrupt AFTER the reservoir caps with the standby? OR would we interrupt at the bridge rectifier, and place the caps / OT CT / Choke resistor after the standby?

[Helmholtz]

If there are no balancing resistors I would add a 220k/2W resistor as a base load like Merlin mentions.
So the bias could be tested without tubes

[Mr_bibbles]

If there are no balancing resistors I would add a 220k/2W resistor as a base load like Merlin mentions.
So the bias could be tested without tubes

sorry was typing while you were posting, I would also need to add a path through a resistor to the bridge rectifiers ground as well, yes?

[Helmholtz]

The single pole standby switch needs to interrupt the AC before the bridge.

[Helmholtz]

BTW, your drawing looks ok.

[Mr_bibbles]

The single pole standby switch needs to interrupt the AC before the bridge.

makes sense! Interrupt the winding from the tap the bias supply is connected to, so that one wind goes ONLY to bias supply in standby mode, while the other winding is connected to the bridge, and connected from the bias supply through R1 on the valve king diagram... this "turns off" the rectifier, while till allowing one diode to pass AC back into the winding, allowing bias to charge while the rectifier only see current through one diode?