Build a Pro Reverb from an AO-43 chassis--am I crazy?

[Lectroid]

So for my next build, I have a Hammond AO-43 chassis with good working iron but  not sure if the PT is strong enough.  I know I can build a Deluxe Reverb AB763 with 6V6s, but my hope is to be able to put together a Pro Reverb AA165 circuit, which wants 420 VDC on the plates of two 6L6s.

I've read everything I could find on this site about AO-43 projects, especially Toxophilite's, so I have a fair sense of how to do the layout.  I think the PT should put out enough heater current.

But--I'm not sure how to estimate the total B+ voltage I can expect.  As in, if I decrease the current draw on the PT, could that increase the max B+ voltage available?  Are there any rules of thumb for evaluating a PT and what total B+ voltage it might put out?

I fired it up with a 5U4 I had on hand and got 335VAC-0-335VAC from the transformer at 120VAC from the wall VAC.  At pin 8 of the rectifier tube, I measured 425 VDC completely unloaded.

Some questions:

• Would I get a higher max voltage by using a 5AR4/GZ34 over my 5U4?  I've read the two data sheets and it seems like either tube is in the ballpark for a Pro Reverb as far as current-handling capacity.
• In the Hammond organ, this PT also drove another 4-5 dual triodes that wouldn't be present in my build.  Would their absence add to the available current. (?)
• If I left out the tremolo circuit and its tube, would that also add to the available current?
• I assume that if I used a SS rectifier plug-in, it would increase the max B+ voltage available for the 6L6s.  But how much?

Any consensus on this?  Is it possible to wring 420 B+ volts out of this PT?  Or should I give it up and go ahead with the AB763 DR?


Thanks,

Rich

[sluckey]

Build the DR with SS rectifier and don't worry about if the PT will support the 6L6s. When it's running swap in a couple 6L6s and rebias. Then you can tell us is the Hammond PT is strong enough to support 6L6s.   

[Latole]

Hammond AO-43 Conversion to Guitar Amp Rebuild - Dr. Z Inspired

[vampwizzard]

Field manual on the PT says

Based on the fuse values and assuming hammond followed 25% protection factor, that 315-0-315 winding should have about 200mA available to it.

FUSE VALUES
6.3V - 5A
5V - 3A
315-0-315V - 250mA (1/4A)

That's a whole lot of current. Im with Sluckey here.. build a 6V6 version of the amp and then try biasing it in with 6L6's. If it doesnt work youve still got a working amp.

with a SS recitifer you'll get 1.414 ideal rectified voltage. 315 would be up near 445.

FWIW, we've had people on here build 6L6 amps on this hardware (allegedly). they did an AA964 and stated that the PT was roughly the same dimensions as the vibrolux replacement.

[pdf64]

… with a SS recitifer you'll get 1.414 ideal rectified voltage. 315 would be up near 445.
…

Just to note for Lectroid, the fully loaded winding voltage (and hence the resulting rectified DC) will sag, probably at least 10%.
As usual, Merlin accurately explains this stuff, have a read of http://www.valvewizard.co.uk/bridge.html

[vampwizzard]

Even so, this should put them in 6L6 compatible territory, no?

Per Merlin: 1.3x 315 ~410, minus 2V for diode drop ~408?

Using thesubjectmatter's transformer calc on a pro reverb (6 preamp tubes, 2 power) im looking at 185ma requirement for a 6L6GC, and they estimate the B+ voltage to be 425 with the choke.

[sluckey]

Field manual on the PT says
6.3V - 5A
5V - 3A
315-0-315V - 250mA (1/4A)

That sounds like a useful manual to have. Is it available in pdf format? Maybe free?

[vampwizzard]

came off another forum. Silver Jubilee conversion. T302. Image attached.

[vampwizzard]

These appear to be fuse ratings and not transformer ratings.

160mA on the schematic, probably more because they give it after a dropping resistor.

[sluckey]

Ahh. That pic came from the Hammond L Series Service Manual. I have that SM but never bothered to look at page 5-21.

[Lectroid]

Great info, all.

Thanks to vampwizard-pdf-ValveWizard for the discussion of the B+ rule of thumb, I knew there had to be one.

@pdf,
Thanks for the ref to the the VW's rectifier discussion.  Thought I'd read everything there but I missed that one.

Even so, this should put them in 6L6 compatible territory, no?

Per Merlin: 1.3x 315 ~410, minus 2V for diode drop ~408?

Using thesubjectmatter's transformer calc on a pro reverb (6 preamp tubes, 2 power) im looking at 185ma requirement for a 6L6GC, and they estimate the B+ voltage to be 425 with the choke.

Being conservative, I think I can get around 420 DC based on the math.  And since this PT also drove 5 other 12A*7 tubes which it no longer has to supply, and I'm leaving off the Vibrato channel and the tremolo, overall, I'll have  so that sounds like 4 preamp tubes and 2 power tubes. 

I'm thinking I'm still in the ballpark for the Pro Reverb/6L6s.

Build the DR with SS rectifier and don't worry about if the PT will support the 6L6s. When it's running swap in a couple 6L6s and rebias. Then you can tell us is the Hammond PT is strong enough to support 6L6s.   

@sluckey,
I hear your advice about just building it and then trying different power tubes.  But given the different parts values in the PR power supply nodes, etc., would it be better to build the PR circuit and then try it both ways?  Would the DR parts values work okay with the higher PR voltages?

Thanks again to everyone, you've cleared up a lot for me.

[sluckey]

Would the DR parts values work okay with the higher PR voltages?

I don't think you will get 440V even with a SS rectifier. You "may" get 420V by using 6V6s and not biasing them hot. Plug in 6L6s and the B+ will drop even more, probably less than 400V.

[Lectroid]

@sluckey,

Bear with me, I think I see what you all were getting at.

The max B+ available would drop because--at an equal bias voltage--the 6L6s can handle more current than the 6V6s, and that will give more of a voltage drop across them, right?  Am I seeing that right?  (Of course the 6L6s would be biased hotter in actual operation.)

In the end, building the DR stock and then swapping in 6L6s might give me more billable watts over the DR with 6L6s? 

So at best, I'd get sort of pumped-up Deluxe Reverb / de-tuned Pro Reverb?  (Not a bad thing!).

And how much should I worry about the DR parts values if I swap in 6L6s?

Thanks again.  As soon as Iwork up a new schematic, I'll start a new thread to get a sanity check.

[sluckey]

The PS dropping resistors will not affect the power hungry output tubes. I would just build the DR but use the 1K and 4.7K dropping resistors as found in most of the 6L6 circuits.

Just to be clear, are you just gonna build the normal preamp, no reverb, no tremolo?

[pdf64]

…
The max B+ available would drop because--at an equal bias voltage--the 6L6s can handle more current than the 6V6s, and that will give more of a voltage drop across them, right?  Am I seeing that right?  (Of course the 6L6s would be biased hotter in actual operation.)
...

I suggest to avoid directly swapping 6L6 for 6V6 in a fixed bias amp. They may be rather under biased, and idle rather hot.

In AB operation, how much current the valves draw at high signal output will depend on the OT impedance.
With a Pro Reverb type 4k ish OT, it will be more than with a DR’s 6k6 ish OT. Maybe up to 80% more. Hence the HT would probably be pulled lower, possibly stressing the PT.

With repurposed PTs / OTs, it’s probably a good idea not to stray too far from the original equipment’s power supply / power amp.

[PRR]

......Merlin accurately explains this stuff, have a read of http://www.valvewizard.co.uk/bridge.html

His Designing Power Supplies for Valve Amplifiers has been out of print for years. I heard "somewhere" that he was updating it. Ha, it is already listed on Lulu:
https://www.lulu.com/en/us/shop/merlin-blencowe/designing-power-supplies-for-valve-amplifiers-second-edition/hardcover/product-y495e4.html


(For the next two days there is a 10%-off code on the top of the page.)

[kagliostro]

As I don't know if what they sell is a revised edition, if you buy it remember this Errata Corrige





I corrected what was necessary and about the bias supply I printed the image, cut it and glued it on the book superimposed on the original

Franco

[dude]

“For what it’s worth”
With that PT, I ran three 12ax’s, two EL34’s with a 5Ar4, with the 34’s biased at 70%, l got 395 on the plates. With SS plug, got 410; Playing hard for an hour, PT was kinda warm to the touch with 5ar4 but just slighty warm with SS.

[kagliostro]

... PT was kinda warm to the touch with 5ar4 but just slighty warm with SS.

I think that was due to the fact that there was no consumption on the rectifier heater wiring

Franco

[pdf64]

... PT was kinda warm to the touch with 5ar4 but just slighty warm with SS.

I think that was due to the fact that there was no consumption on the rectifier heater wiring

Franco

That only takes 10W loading off the total on the PT.
Whereas the increase in HT voltage, which for a given OT will result in a similar % increase in peak anode current, might easily lead to a loading being pretty much as it was with the valve rectifier.

[Lectroid]

@sluckey,

I plan to build a one-channel amp with reverb but no tremolo

@pdf,

I suggest to avoid directly swapping 6L6 for 6V6 in a fixed bias amp. They may be rather under biased, and idle rather hot.

I get that the different tube types must be biased at different voltages.  My question to sluckey was phrased as an all-things-being-equal hypothetical so I could understand why he predicted a voltage drop--because the 6L6 flows more current and hence drops more voltage than 6V6s in the same circuit.  Also, thanks for your comments on OTs and how the whole power stage functions together.

@dude,
Thanks for your adding your real-world AO-43 experience.  Along with sluckey, you've convinced me that the iron won't support a full-blown Pro Reverb.  But it will make a pretty hot Deluxe Reverb with power to spare, hopefully up in the 18 - 20 watt range.

Final question, a tossup: would a 5AR4 give any increase in B+ voltage over using a 5U4, in a given circuit

Thanks again everyone, you guys are the best!

[dude]

"Final question, a tossup: would a 5AR4 give any increase in B+ voltage over using a 5U4, in a given circuit"


Yes,

[Lectroid]

Thanks, dude. 

btw did you ever post a schematic of your AO-43 conversion?  I've looked but haven't found it.

[PharmRock]

"Final question, a tossup: would a 5AR4 give any increase in B+ voltage over using a 5U4, in a given circuit"


Yes,

Check out the attachments in this thread. Everything you ever wanted to know about rectifiers.
https://el34world.com/Forum/index.php?topic=20455.0

[dude]

btw did you ever post a schematic of your AO-43 conversion?  I've looked but haven't found it.

Used Sluckey’s schematic in Doug’s schematics here. Never used a program for layout just this by hand. I cut down an AE bd. I think 2, 3/8” x about 7”, installed turrets, l had to lower the bd.1/4” from bottom of chassis so l could replace pots, l also had to make a lot of adjustments as l went along due to the narrow chassis. I probably made some minor changes to this layout, as things got tight. But everything worked out great. I love this 6v6 Plexi with EL34’s and cathode bias. (added a switch), I use all AO-43 iron except an Allen Amps 30 watt OT. Used a SS plug, plates about 410v. I also raised the PT on standoffs but it l did it again, l’d just top mount with hand made mounts.

[Lectroid]

Thanks again, dude.

PharmRock, thanks for the rectifier cheat sheet.  Very informative.