Advice needed...

[The4thWatcher13]

If you change to 6.3V filaments, the preamp tube would need to be a 6AV6, not the 12AV6 (assuming you don't use another 12AX7 instead). The reason for this is that the 12AV6 does not have a center-tapped filament like the dual triodes. It won't operate on a 6V filament supply. Jack

Thanks Jack, I've come up short for a filament trans so far but I'm still looking. Same for tubes but there's a good chance I've got em. I Just need to keep looking. I found the right trans online so that may be the way to go, it's cheap, but only after I exhaust my search. I did find a 6.3-6A and a 6.3 - 0.600A ..of course.

[JMPGuitars]

Thanks Jack, I've come up short for a filament trans so far but I'm still looking. Same for tubes but there's a good chance I've got em. I Just need to keep looking. I found the right trans online so that may be the way to go, it's cheap, but only after I exhaust my search. I did find a 6.3-6A and a 6.3 - 0.600A ..of course.

What are you looking for exactly?

[TriodeLuvr]

The filament transformer doesn't need to be exact (amperage), but 6A might be too much. The transformers are rated for the voltage they deliver at full load. Running a 6A transformer at ~3A will probably over-voltage the filaments, especially if it's an older transformer intended for 115/117V operation. A 3A transformer would be perfect.

6CU5s are pretty cheap on eBay. I checked on that before suggesting them as replacements.

Jack

[The4thWatcher13]

What are you looking for exactly?

The 6.3V - 3A filament transformer. I thought I might have one in storage. But there's one on mouser for $13 bucks. Can't beat it, if you can't find it!

[The4thWatcher13]

The filament transformer doesn't need to be exact (amperage), but 6A might be too much. The transformers are rated for the voltage they deliver at full load. Running a 6A transformer at ~3A will probably over-voltage the filaments, especially if it's an older transformer intended for 115/117V operation. A 3A transformer would be perfect.
6CU5s are pretty cheap on eBay. I checked on that before suggesting them as replacements. Jack

That may be my "go to" if I can't find any around here. I've got lots of tubes. I was well on my way to becoming a tube hoarder! But I've got it under control, ...sort of. Problem is the vast majority are un-organized. Doh!

[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