Advice needed...

[The4thWatcher13]

OK, I've got a Triad F-16X 6.3V - 3A filament transformer on it's way. https://www.mouser.com/ProductDetail/553-F16X
The hunt around here is still on for the recommended tubes, ...and I'll be reading, of course. : )

[The4thWatcher13]

Well, I found two sets of 6AV6's, two of which tested new and the others test rather good as well. I haven't found any 6Cu5's so I guess those are the next order of business. Hopefully the filament trans will get here this week.

[TriodeLuvr]

There's a similar discussion going on elsewhere about a phono amp that doesn't have enough gain for guitar. One concept that's been raised is the possibility of using a 12AX7, but only heating one side of the filament. IOW, 6V power would be connected either between pins 4-9 or 5-9. The advantage is that when the tube eventually wears out, you can move the power to the other pin, and you've got a new section to use. This also uses half the current of the 6AV6. I wanted to throw this out as another possibility.

Jack

[The4thWatcher13]

There's a similar discussion going on elsewhere about a phono amp that doesn't have enough gain for guitar. One concept that's been raised is the possibility of using a 12AX7, but only heating one side of the filament. IOW, 6V power would be connected either between pins 4-9 or 5-9. The advantage is that when the tube eventually wears out, you can move the power to the other pin, and you've got a new section to use. This also uses half the current of the 6AV6. I wanted to throw this out as another possibility. Jack

Thanks Jack, that sounds interesting. Considering that this will be bread-boarded before hand, trying out both ways may be in the cards.
I appreciate the info.
Mark

[The4thWatcher13]

OK, sports fans, all of the actors have reached the stage and it's time to get this show on the road. After all, </i>"the play is the thing...."!</i><br>So, the cast is as follows: 2- 6CU5's, 1- 6AV6's & 1- 12AX7.
I tested the new 6CU5's and paired them accordingly. I've modified the heater circuit of the schematic that I submitted earlier in the thread to what I think is required and have included it in this post. On the schematic there are also two red dots; which indicate the removal of wiring to ground through the original heater circuit. The only question I had in my mind is whether it was better to put the heater trans on the primary or secondary side of the isolation transformer. I've been trying to figure out the answer to that question on my own, and although I've learned some new stuff, I'm no closer to knowing the answer to that particular question.
So that's where it stands as of today. All input is welcome!
Thanks again,
M

[TriodeLuvr]

I would run the filament transformer directly off the line like it's drawn in your schematic. Looking forward to hearing your report when the amp is up and running.

Jack

[The4thWatcher13]

I would run the filament transformer directly off the line like it's drawn in your schematic. Looking forward to hearing your report when the amp is up and running. Jack

Thanks Jack, I'm on it and smooth sailing so far!
M

[The4thWatcher13]

Ut oh, a foul wind doth blow, matey. Me thinks I found something fishy! (..ok, enough of the sailing puns) I noticed what I think is a mistake in the way I drew my schematic of the heater string. Since all of my tubes are now 6 volt heaters they no longer need to be arranged in series. So I've changed my schematic accordingly. I'm in the process of changing the wiring as well. Should be up by tomorrow.

[TriodeLuvr]

Ut oh, a foul wind doth blow, matey. Me thinks I found something fishy! (..ok, enough of the sailing puns) I noticed what I think is a mistake in the way I drew my schematic of the heater string. Since all of my tubes are now 6 volt heaters they no longer need to be arranged in series. So I've changed my schematic accordingly. I'm in the process of changing the wiring as well. Should be up by tomorrow.

I just realized your schematic shows two shorts across the output of the filament transformer. I'm assuming that's just a paper error.

[The4thWatcher13]

I just realized your schematic shows two shorts across the output of the filament transformer. I'm assuming that's just a paper error.

I think I know what you mean, I've changed it. How's that?
(see above)

[TriodeLuvr]

Much better!

[The4thWatcher13]

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?!

[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

[JMPGuitars]

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.

[The4thWatcher13]

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

Bridge Recs.jpg

[The4thWatcher13]

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

[JMPGuitars]

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

[The4thWatcher13]

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.

[JMPGuitars]

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.

[TriodeLuvr]

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