Advice needed...

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TriodeLuvr
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Re: Advice needed...

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Much better!
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Re: Advice needed...

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Ok, looks like the heater circuit is operating correctly. I'm measuring 6.2 VAC (RMS), side to side at all nodes in the heater circuit. The meter I'm using always gives the AC voltage in RMS values. Not sure if I can change that, but it's always something I have to keep in mind. With a supply voltage of 115VAC I've got 96VAC at the anode of D1 and +117V at the cathode. My multi-cap values currently are:
C3-A= +117V / C3-B= +112.6V / C3-C= +107.2V
Look good?!
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Re: Advice needed...

Post by TriodeLuvr »

You want the meter to measure RMS. If it can also do that accurately at frequencies other than 60 Hz, that's a bonus!

The DC voltages look right to me, based on the AC voltage driving D1, plus the fact it's a half-wave rectifier (60 Hz ripple). Would you be able to replace D1 with a small, full-wave bridge rectifier? Some of them are about the size of a dime with four leads. Full-wave rectification would produce 120 Hz ripple, which would be filtered much more effectively by the caps. It might also bring the DC voltages up to where they should be with the reversed transformer.

Jack
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Re: Advice needed...

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TriodeLuvr wrote:
Sun 04/04/21 2:35 am
You want the meter to measure RMS. If it can also do that accurately at frequencies other than 60 Hz, that's a bonus!

The DC voltages look right to me, based on the AC voltage driving D1, plus the fact it's a half-wave rectifier (60 Hz ripple). Would you be able to replace D1 with a small, full-wave bridge rectifier? Some of them are about the size of a dime with four leads. Full-wave rectification would produce 120 Hz ripple, which would be filtered much more effectively by the caps. It might also bring the DC voltages up to where they should be with the reversed transformer.

Jack
He could make his own full wave bridge rectifier with 4 diodes. Or if he has a center tapped secondary, it takes as little as 2.
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Re: Advice needed...

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TriodeLuvr wrote:
Sun 04/04/21 2:35 am
The DC voltages look right to me, based on the AC voltage driving D1, plus the fact it's a half-wave rectifier (60 Hz ripple). Would you be able to replace D1 with a small, full-wave bridge rectifier? Some of them are about the size of a dime with four leads. Full-wave rectification would produce 120 Hz ripple, which would be filtered much more effectively by the caps. It might also bring the DC voltages up to where they should be with the reversed transformer.Jack
Yes, I believe I would be able to. If I can use the smaller of the three following rec's I have on hand it'd be prefect.
The one's I have on hand are: S4VB-20 (200V / 4A) The other three: 3J 3-f, LF084E & 2B 8-B, I can't ID. Also, when you say "bring the DC voltages up to where they should be with the reversed transformer" I'm wondering what you consider "appropriate". I'm familiar with the Max & Typical operation specs on datasheets but quite obviously there has to be more to it than that. Especially with an odd amp like this. Voltages for the 12AX7 & 6CU5's are currently almost exactly where they were when I started. I am aware that the Typical plate voltage for a 12AX7 is spec'd between +100 and +250 and the 6CU5 +120 with screens at +110.
Currently my 6CU5's plates are +110 and screens +115, grid 23mV & cathode +9. The 12AX7 plates are (p1)+59 (p2) +61, cathode 550mV, grid 9mV. Supply voltages from the multi cap are: C3-A: +118, C3-B: +115, C3-C: +109.
The only things that seem obvious to me is that the plate & screen voltages of the 6UCU5's are kinda upside down and the 12AX7 plates are pretty low. My thoughts: the cap surges to about +139 currently at power up. So I'm wondering what the addition of a full wave rec will do there. I have other thoughts about the plate and screen voltages but I'll hold onto them for now.
Thanks Jack
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Re: Advice needed...

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JMPGuitars wrote:
Sun 04/04/21 6:41 am
He could make his own full wave bridge rectifier with 4 diodes. Or if he has a center tapped secondary, it takes as little as 2.
I'm hoping one of these recs will do the job. A little easier than putting in single diodes. I'd have to figure out how to do that. But I'm wondering what the addition of the additional rectification or a full wave bridge rec will do to the voltages on the cap.
Thanks, JMP
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Re: Advice needed...

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The4thWatcher13 wrote:
Sun 04/04/21 5:16 pm
I'm hoping one of these recs will do the job. A little easier than putting in single diodes. I'd have to figure out how to do that. But I'm wondering what the addition of the additional rectification or a full wave bridge rec will do to the voltages on the cap.
Thanks, JMP
What power transformer are you using?

Thanks,
Josh
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Re: Advice needed...

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JMPGuitars wrote:
Sun 04/04/21 7:53 pm
What power transformer are you using? Thanks,
Josh
Triad N68X iso-trans, run: wall-->secondary-->primary-->circuit, which dropped the voltage down a little bit. When installed initially I was using 120VAC on the primary and was getting 125VAC on the secondary. Which gave me +148, +143 & +136 on the +150V rated cap sections. I thought it'd be good to know what would happen if this was plugged into a socket that actually had 120VAC on it. Currently I'm using 115VAC 'cuz that's what the filament trans (Triad F-16X) is rated for. I can swap it back, of course, if need be.
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Re: Advice needed...

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The4thWatcher13 wrote:
Sun 04/04/21 9:04 pm
Triad N68X iso-trans, run: wall-->secondary-->primary-->circuit, which dropped the voltage down a little bit. When installed initially I was using 120VAC on the primary and was getting 125VAC on the secondary. Which gave me +148, +143 & +136 on the +150V rated cap sections. I thought it'd be good to know what would happen if this was plugged into a socket that actually had 120VAC on it. Currently I'm using 115VAC 'cuz that's what the filament trans (Triad F-16X) is rated for. I can swap it back, of course, if need be.
That's not Center Tapped, so you would need 4 diodes to make a full wave bridge rectifier. You can read more here for examples: https://www.electronics-tutorials.ws/diode/diode_6.html

I don't think Triad makes a PT with secondary CT in that voltage range. Hammond very likely does, if you'd rather a simpler SS rectifier.
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Re: Advice needed...

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OK, I decided to do a few quick calculations on this to confirm everything. The voltages you're reporting look good until the last output to the small signal stages. I assume the 12AX7 and 6AV6 together draw about 3 or 4 mA. That should drop the last voltage node to about +85V. So, before going further, maybe you could check to see how much current those stages are drawing and make sure they're connected after the 10K, not before. Current consumption can be checked by measuring the voltage across each cathode resistor and calculating with Ohms Law. Just post the voltages if you're not sure how to do that.

On my simulator, changing the rectifier to a FW bridge, the voltage on C1 should increase from +117V to about +125V. Everything else follows suit. The voltage at C2 increases from +112V to about +121V. I would consider this a better voltage for the 6CU5s. Voltage at C3 increases from +85V to about +91V. These last voltages are highly dependent on the current drawn through the 10K resistor.

Ripple throughout the circuit improves considerably with the bridge rectifier. I suspect the rectifier change will be essential in order to control hum once the additional high-gain stage (6AV6) is operational. At worst, it shouldn't stress the caps more than when the circuit was being operated directly from the line. The reversed isolation transformer has dropped the voltage quite a bit, and that will compensate.

Jack
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Re: Advice needed...

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This is the type of bridge I was thinking about. They're so cheap now, less than $2 for a 400V 1A version. Just get one of these if there's an electronics outlet near you.

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Re: Advice needed...

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JMPGuitars wrote:
Sun 04/04/21 9:19 pm
That's not Center Tapped, so you would need 4 diodes to make a full wave bridge rectifier. You can read more here for examples: https://www.electronics-tutorials.ws/diode/diode_6.html
I don't think Triad makes a PT with secondary CT in that voltage range. Hammond very likely does, if you'd rather a simpler SS rectifier.
Indeed, the iso-trans I'm using is not center tapped. That's a great article, thanks for the link. I'm still trying to get it, tho, so I applied it to what I'm working on. I found part of the explanation confusing, so I connected the AC output of the iso-trans (98VAC) to the S4VB, FWBRec alone, and got +80VDC.
The article says that the average DC output is higher with the FWBR, so I'm assuming that when connected to the load of the circuit the DC-Vrms (B+) will rise. But by how much?! I'm a little leary just connecting the FWBR to the circuit to see how much the DC will rise.
Should I be? Or is the answer in the article, and I'm just missing it.
Sorry about this Josh, I turned into a question machine. :?
Thanks man!
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Re: Advice needed...

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The S4VB isn't suitable for this if it's only rated for 200V peak reverse voltage. When the circuit charges the first cap to 117V, and then the voltage from the transformer reverses, the bridge sees 252V in reverse. That's the sum of the +117VDC on one side of the bridge plus -135V on the other side, which is the peak value of the 96V you're measuring at the input to the diode.

Jack
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Re: Advice needed...

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TriodeLuvr wrote:
Mon 04/05/21 12:00 am
The S4VB isn't suitable for this if it's only rated for 200V peak reverse voltage. When the circuit charges the first cap to 117V, and then the voltage from the transformer reverses, the bridge sees 252V in reverse. That's the sum of the +117VDC on one side of the bridge plus -135V on the other side, which is the peak value of the 96V you're measuring at the input to the diode. Jack
OK, I think I've got it. Thanks Jack!
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Re: Advice needed...

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So, I had a new Rectron RS60 FWBR on hand and though it wasn't the form factor I was considering it's peak reverse voltage is 400VDC, 6A.
I mounted it on a bracket and hooked it up. Now I've got:
C3-A: +125V / C3-B: +121V / C3-C: + 116V
V4(6CU5) +116 plate, +120 scr, +9.5 K, 22mV g.
V3(6CU5) +116 plate, +120 scr, +9.4 K, 23mV g.
V2(12AX7) +63 p1, +65 p2, 557mV K's, 10mV g's.
The B+ went up for sure and the DC on the scope looks better as well.
Output tube plates went up by +6V, screens up by +5V. 12AX7 plates up by +4V. Surge at startup is +135Vrms, ..but that's a peak voltage of +190, ..yes? I'm wondering if multicap voltages as written on the can are rated at peak or rms values :roll: Well, I'll check tomorow...
RS604.jpg
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Re: Advice needed...

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The4thWatcher13 wrote:
Mon 04/05/21 11:19 pm
So, I had a new Rectron RS60 FWBR on hand and though it wasn't the form factor I was considering it's peak reverse voltage is 400VDC, 6A.
I mounted it on a bracket and hooked it up. Now I've got:
C3-A: +125V / C3-B: +121V / C3-C: + 116V
V4(6CU5) +116 plate, +120 scr, +9.5 K, 22mV g.
V3(6CU5) +116 plate, +120 scr, +9.4 K, 23mV g.
V2(12AX7) +63 p1, +65 p2, 557mV K's, 10mV g's.
The B+ went up for sure and the DC on the scope looks better as well.
Output tube plates went up by +6V, screens up by +5V. 12AX7 plates up by +4V. Surge at startup is +135Vrms, ..but that's a peak voltage of +190, ..yes? I'm wondering if multicap voltages as written on the can are rated at peak or rms values :roll: Well, I'll check tomorow...RS604.jpg
It's alive! Those numbers all sound correct. I was wrong earlier about the 12AX7 - the 220K anode resistors really limit the current. I'm used to seeing these tubes draw a little more current than that. Anyway, the voltage drop across the 10K power supply resistor indicates total current of about 0.5 mA being drawn by the 12AX7, and the voltage at the two anodes equates to 0.25 mA each. So, that adds up correctly.

The filter caps are fine, in terms of the voltage being applied. Terms like "RMS" and "peak" only apply to AC voltages. The caps are rated for DC, and that's what you're measuring. +135V on the first cap during a cold start is within its rating.

When you add the 6AV6, voltage at the last cap will probably drop about 5 or 10 volts, depending on how the 6AV6 is biased. That should still be enough for everything to work OK, but you can tweak it later if necessary.

Jack
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Re: Advice needed...

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The4thWatcher13 wrote:
Mon 04/05/21 11:19 pm
V4(6CU5) +116 plate, +120 scr, +9.5 K, 22mV g.
V3(6CU5) +116 plate, +120 scr, +9.4 K, 23mV g.
Your screen voltage should be lower than your plate voltage. Do you have screen resistors in there now? If so, what value?

Thanks,
Josh
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Re: Advice needed...

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JMPGuitars wrote:
Tue 04/06/21 6:06 am
The4thWatcher13 wrote:
Mon 04/05/21 11:19 pm
V4(6CU5) +116 plate, +120 scr, +9.5 K, 22mV g.
V3(6CU5) +116 plate, +120 scr, +9.4 K, 23mV g.
Your screen voltage should be lower than your plate voltage. Do you have screen resistors in there now? If so, what value?

Thanks,
Josh
According to the schematic, there are no screen resistors. The screens are connected to the same B+ node as the CT of the output transformer, so anodes and screens will always be "upside down." The charts for the 50C5/6CU5 indicate that this shouldn't be a problem. At these voltages and grid bias, the anodes are operating much closer to the dissipation rating than the screens. Don't know if this is optimal in terms of output power or distortion, but it's a safe operating point for the tubes.

Jack
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Re: Advice needed...

Post by The4thWatcher13 »

OK, it was late and my brain was stuck in AC land. I believe that, pursuant to my previous question, that since the multi-cap is now getting fully rectified DC from the bridge rec that an rms value is no longer a consideration regarding the +135 surge value on the cap. :oops:
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Re: Advice needed...

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:?
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