Ampeg Reverberocket R-12-R DIY project

[chas.wahl]

@Lowerleftcoast : I'm afraid you've lost me a bit in your last paragraph.

The "glaring problem" with the "way hot triode" is, I think not such a problem as you may believe. If you look at the schematic and the voltage chart I posted from the service manual, it becomes clear that pins 1/2/3 are the "V1b", side of that tube; not the first stage, but the tremolo. The voltage chart from the service manual says that pins 2 & 3 have voltages 310 and 6.2 V when the trem is off, and 100 and 0.8 V when it's enabled. So, while the voltages from the Verberwaffe R-12-R clone vary from that a bit, they're not out of the ballpark. The input stage is pins 4/5/6. Those voltages reported for the Verberwaffe are right on the money, agreeing well with those in the service manual for that triode.

All the other preamp tubes have the "a" side as 1/2/3, and the "b" side as 4/5/6. I don't really know why the first 6SN7 is different, but it is -- this reminds me of that conversation hereabouts recently having to do with the "b" side of a double triode (at least in the 12A*7 world) being "preferred" for hum (or the lessening thereof). Because I found this confusing while doing my version of the schematic, I actually marked up the service manual voltage chart to indicate the V*a or V*b correspondence. And both schematics (Ampeg's and mine) have the pin numbers on the tube diagrams.

Now, I'll have to go back to Blencowe's Preamp book to figure out what your comments about the MPD mean.

 

[King Fan]

I may query Mercury, though I seem to remember when I looked at their offerings, the prices seemed quite a bit higher than what one can get a Hammond for. Most of the transformers I have around are from Hammond via Hawk.

Right. I wasn’t so much suggesting you buy one as you use theirs as a datapoint. I don’t know if their reverse engineering is accurate, but it’s another bit of info.

As far as voltages on vintage amps, I find them pretty golden. Reports don’t always agree, and you have to allow for modern wall voltages, but if it’s a vintage PT and decent tube set, that’s reference grade info.

Finally, that B+ calculator at thesubjectmatter is always about right on B+ for me, but the joker may be the octals here and their current needs, and how to fudge factor the effect of extra load on the B+…

 

[chas.wahl]

I took a look at Mercury Magnetics (which I've seldom done, for the reasons exemplified here), and just for a point of reference, did a search for the (Triad) #6452 -- the PT found in many 5E3 Tweed Deluxes. The "hit" for this ends up being their model listed as described as "stock B+", a "clone" of a 1954 model, they say same PT used for 1950–1957 (though I've seen a 1960 Tweed Deluxe with the Triad 6452) with HT 380-0-380 unloaded. In terms of "apples-to-apples" that doesn't seem very useful to me, since I think that most mfgrs (unless this is a newbie mistake) list voltages loaded as indicated by the current ratings. Moreover, MM doesn't list the current rating for the HT winding, either, so how would one even calculate the capacity without that and/or the DCR? Does the VA rating of 60 mean anything; as in 60 VA*/760 V = ~79 mA?*
*now that I'm re-reading ValveWizard on Transformers, I see that the answer to this is "probably not" since VA does not necessarily mean watts, and probably also means a rating for the whole transformer, as opposed to just the HT secondary.

Furthermore, they have another PT listed for Tweed Deluxe "lowest B+" with unloaded HT 290-0-290. The spec sheet for that has less information (no VA rating for instance, and no HT current rating either). Then they have a cryptic note "do not ground green and grn/yel wire!" {6.3 V filament} -- what the heck does that mean? Isn't the center tap for grounding?

We're back to the question, did MM "clone" the 6452 by simply replicating it, with no modification to adjust the primary winding for the general change in line voltage between now and 70 years past? And if so, how is that "stock B+"? Making a clone sound like an all-original 5E3 sounds when played on 121 V today? Is that desirable? And what is the "lowest B+" supposed to calibrate to? "Lowest" could be "zero", no? Or does it mean "what a 5E3 played like in 1954"? That could be bad too -- maybe we love the vintage tweed because higher voltages came along and juiced them up somewhat.

You clearly have had great success dealing with MM in person and in practice, but you might understand why review of what they publish doesn't leave me with a feeling of confidence, in terms of using their products as a reference. Hammond seems to have more consistency, as did ClassicTone's technical info.

I guess what I'd like to get a reading on, if I'm interested in designing/building a R-12-R clone that's really close to the original performance, is whether to
1. do what many people seem to do: find transformers that exceed the putative specs or the design conditions that may be inferred, with some margin of "fudge" upward, or
2. try to do the Goldilocks approach: not too wimpy, but also not too brutish either.

I'm currently inclined to the latter. As has been discussed many times, people do #1 and then have problems with the amp being too hot, too stiff, with heater voltages that are alarming and reduce tube life, etc. I think that some mfgrs are starting to get this, and offer lower output, rather than "stout enough for anything you'll throw at it (like putting 6L6s and a 5U4 in your 6V6/5Y3 amp) and keep you from coming back whining about how you burned up your PT". But it seems to have taken them awhile to catch on, no?

 

[FenderLover]

Probably not adding anything too valuable here, but the the V1b is not drawing any current because the switch is open. Calculating from the cathode voltages and Rk for each circuit, and just figure in a nominal 2mA for the LFO V1b, I get 94.47mA total. I'd go with B+ tranny 120-150mA.

 

[King Fan]

You clearly have had great success dealing with MM in person and in practice

Heh, I sent them exactly one email and got one back. I don’t vouch for their cloning skillz or their vintage amp research in any way. And frankly, I found their *unloaded* voltages not just what I was looking for. I’m just saying they were fast and professional in replying. Data are data; then we look for patterns and logic…

 

[FenderLover]

It would also be cool to hit the 6V6 plates with 325V. I think that's the magic on the old octal circuits, running the 6V6's lower. Another fine example is the Martin 112, a 5E3 running with lower than Fender B+ voltage. Tranny:[email protected] and a 5Y3 should get you there.

 

[chas.wahl]

Though I can understand why this is happening (I posted "less accessible" PDFs in the thread starter, rather than images) I wish people would refer to the Ampeg voltages rather than the ones for the Verberwaffe R-12-R clone I uploaded as an image. That was built with a substitute PT and OT, and as I pointed out, the PT particularly deviates from the service manual's specs and has lower voltages. Here's a screen clip of Ampeg's chart:

So, in order to calculate and total up the PT's current requirement reliably, all I need to do is find cathode voltage over the cathode resistor value (Vk/Rk)? For power tubes and preamp tubes? No screen current or plate current considerations? Of course wouldn't that requirement change at a different B+ voltage? Less voltage, more current? Does the 5Y3 consume current from the HT circuit that needs to be counted?

When I calculate Vk/Rk that way for values in the chart above, I come up with roughly similar values as @FenderLover posted previously, except for V4 (the PI), where there is a shared cathode resistor of 1k and an "unbalance" resistor on V4a as well. So does that count (as in the case of the power tubes) as doubling the 1k to 2k (effective) for each triode? If so, then the current there is about half what @FenderLover posted. I make a total of 116 mA this way:

I made a brief search for the unicorn Martin 112's (the 5E3 done right) schematic, but find nothing; must be a limited-membership club's official secret.

 

[FenderLover]

The biggest difference by far is the 6V6 current, the rest being small differences. Since the Ampeg B+ is higher I would tend toward [email protected] with the 5Y3.

Remember the plate, tube, and cathode circuits are all in series for each stage. The plate current is the cathode current. (That is a glaring error seen on many Fender schematics - if you calculate the plate current and cathode current using the given voltages, they do not match!) Regarding the output stage, the screen and plate currents both come from the cathode current. Therefore the G2 (and plate) currents are inclusive in the cathode current.

I have a few attachments I could offer on the Martin 112, but don't want to derail your thread. I will offer a few words though and one attachment. DeArmond was commissioned to build the Martin 112 by rebadging the DeArmond R15 with cosmetic and model number changes.
Differences from 5E3:
650K Tone
350K volumes
330 B+ (lower than 5E3)
40uF first filter
Large OT
no 1K5 output grid stoppers
Modular build, plastic coated input and inverter boards
See Vintage Guitar article May 2014

Here's a poor image of the schematic:

 

[Lowerleftcoast]

The "glaring problem" with the "way hot triode" is, I think not such a problem as you may believe.

Ah yes. I was looking at the *clean* schematic you provided for the Reverberocket that reverses the pins. Looking at the Verberwaffe schematic I see clearly you are correct... in the words of Emily Litella *Never mind*.

Now, I'll have to go back to Blencowe's Preamp book to figure out what your comments about the MPD mean.

MPD = Maximum Plate Dissipation. It is how *hot* the 6V6 tubes are biased. At 100%MPD The 6v6 should put out 14 of it's 14 Watts. At 75%MPD The 14 Watt tube puts out 10.5 Watts.

My remarks come from my experience with the 5E3. In reviewing the DeArmond schematic @FenderLover posted, I am willing to back off on pontificating how the other manufacturers *should* approach gain and tone in their massaging of 6V6 tubes. That said, the *schematic voltages* you provided about the Reverberocket would run the 6V6 tubes at 100%MPD. The 5E3 runs at 100%MPD as well. Ime, the 5E3 begins to sound lackluster when the bias is less than 100%MPD.

The voltages posted of the Verberwaffe placed it <75%MPD. This Verberwaffe is not getting enough B+ to make it to 100%MPD when it has the 250R bias resistor. The ClassicTone 40-18028 and 40-18016 have datasheets that show same same voltage and current ratings. The difference, imo, must be from using the 300v taps on the 40-18028. I just don't buy the extra current draw of the Verberwaffe, or some current hungry 6V6, drops the B+ of the ClassicTone PT by over 30 volts.

Then they have a cryptic note "do not ground green and grn/yel wire!" {6.3 V filament} -- what the heck does that mean? Isn't the center tap for grounding?

The older amplifiers heater wiring does not use the heater CT. One of the green wires attaches to the chassis. The other green wire attaches to the tube pin(s). It would short half the winding to attach both a green and CT to ground.

try to do the Goldilocks approach: not too wimpy, but also not too brutish either.

This. The 5E3 seems to have a *range* where it just sounds best. Imo, the 5E3 does best with a B+ between ~350 and ~385.

Does the 5Y3 consume current from the HT circuit that needs to be counted?

IDK.

I think your current figures put you pretty close. Remember this is cathode biased so the current draw does not swing much. Which means a PT with a higher current rating is not going to change much other than your voltage estimate.

I bet the ClassicTone PTs mentioned would work well.

(I used the Robinette bias calculator to determine the %MPD of the above.)

FYI, the ClassicTone 40-18016 heater voltage on the three 5E3 I built were between 6.37 and 6.51 VAC. Wall voltage 119 to 120 at time of reading.

 

[chas.wahl]

Thanks for your time spent on this @Lowerleftcoast and @FenderLover for much eludication leading, of course, to more questions.

I don't wish to knock they guy who built the Verberwaffe R-12-R clone in any way (he's the only one I can find who's done one) -- his knowledge of amp building is obviously top-shelf, and he's got a 'scope! He made the intelligent (IMO) choices to make the board layout more Fendery, use a Blackface-style chassis instead of a gopher coffin to reduce the crowding, and his work looks very fastidious and clean. However: as @Lowerleftcoast surmises, he did, apparently, use the 300 V (white-red) taps on the HT winding (his schematic says 290V each side at rectifier), and the 120 V (brown-black) primary.

That would appear to be a conscious choice, since his schematic indicates 290 V at the rectifier legs; yet he has 340 V at the B+ "A" node (OT primary CT) -- only 10 V shy of the Ampeg's "A" node. I'm wondering what could explain that. It's quite possible that his 5Y3 is a modern-production one (like his "made in Russia" 6V6s), with lower voltage drop. Above I've added a portion of a photo from his parallel "cab build" thread (5Y3 on the right) showing chassis topside, but it's not clear to me what that tube is. FWIW, I intend to use a vintage 5Y3 -- one thing I do have is all the tubes for this build, and they're all NOS or previously used (I hope in good working order).

Still, if we go down the B+ chain, all the Verberwaffe voltages are lower than Ampeg's:

All the dropping resistors in the B+ chain are the same for Ampeg R-12-R and Verberwaffe. So, I don't really understand two things: 1) why Ampeg specs a 350-0-350 PT to achieve 350 V at the OT and 2) why I'm being counseled to use a PT with a lower voltage rating than that, if my intent is to build something as much like the 1962 version Ampeg produced as possible. If I were to do that, shouldn't I reduce the dropping resistor value between screens and PI (22k) or between OT and screens (1k) to maintain the "stock" values for the rest of the circuit? Obviously that sort of thing might work out OK, since the guy that built the Verberwaffe seemed to be happy with it (at least based on his report in the cab build thread, which is admittedly sort of "preliminary" or "early days"). From his post #12:

The other thing that's bugging me is why Ampeg specified a PT supplying current-hungry octals and an amp with reverb and tremolo (except for the octals, sort of like a proto-AB763 circuit) at only 90 mA HT current.

@Lowerleftcoast says that a cathode-biased pair of 6V6's should end up with 350–380 V on plates, while @FenderLover suggests starting with a 300-0-300 V primary at 150 mA. Are these suggestions consistent, or alternative points of view? In the back of my untutored brain, I have a sneaking suspicion that there's something like a voltage-current tradeoff: Ampeg chooses a higher-voltage PT with lesser current rating (maybe when overtaxed, the apparent voltage spec drops), while @FenderLover counsels a lower-voltage PT with higher current rating (the PT won't drop voltage under typical load/use conditions). Is that what's going on, or something else?

I don't mind the Martin 112 interlude at all -- it's an interesting take on the 5E3, and one I'd never heard of; as if Jim Marshall had also copied the Tweed Deluxe (well, maybe different than that). I do notice that additional changes Martin/DeArmond/Rowe made to the 5E3 are lower-value coupling caps, which could be significant.

But in reaction to the above discussion and the mention of Martin 112: my own suspicions are, based on a lot of reading over the years, but not very much real-life experience admittedly, that I have witnessed a lot of DIY angst related to PTs that are too "stiff", and am somewhat predisposed to prefer ones that are more "compliant" or even "yielding" -- but I could be entirely wrong about that. To me, a higher-voltage under-currented PT sounds plausible enough as a strategy. Maybe that was not really Everett Hull's intention (or his designer's, was that Jess Oliver?) -- might have simply been a "here's what's available" decision, not dissimilar to the one I'm facing. But I'm not in the "this is the only thing available" situation so much as "gotta decide between high V low current or lower V higher current, or just a "browner" amp (which may be what @FenderLover is recommending, and the Verberwaffe builder has done).

I do have a few questions left over from my previous post with my (fumbling perhaps) calculation of current draw from the various tube sections:
1. Is it correct that the V3a (reverb driver) triode really draws about 13 mA?
2. From a Ia = Vk/Rk standpoint, have I calculated current draw for the PI triodes and 6V6 (both of which have shared cathode resistors) correctly; that is, by doubling the shared resistor value to arrive at "Rk"?
3. Admittedly I'm obsessing over preamp current, which is not the lion's share of it; but if I'm correct about the calculation, then does the total look about right for a PT's current rating? In this respect I am confused, going back a bit (to post #16 here), as to why the "thesubjectmatter" calculator turns up 435 V at the filter cap when a 350-0-350 PT is used, and 406 V when a 325-0-325 PT is used. Are those "no load" voltages? If so then why does the calculator express that as being "at first filter capacitor"? The voltages on both Ampeg's schematic and in the Verberwaffe documents seem to have much lower values, even accounting for the secondary winding voltage rating.

@FenderLover : if you have a less fuzzy image of the Martin 112 schematic (it's basically readable, but there is some dropout, such as "is that a decimal point?) could you please make that available?

Thanks,

 

[FenderLover]

...I have witnessed a lot of DIY angst related to PTs that are too "stiff", and am somewhat predisposed to prefer ones that are more "compliant" or even "yielding" -- but I could be entirely wrong about that....

This does seem to be in keeping with the era - many examples out there: 12W amps often had 15W speakers (many tweeds); PT current specs are often on the edge of barely sufficient (Princeton). And like the Martin schematic posted, no output G1 or G2 resistors. It was the wild west, and these things flew by the seat of their pants.

To reiterate another possibility, the Ampeg documentation may not be the gospel truth. Like the many inaccuracies in Fender schematics - If you variac your wall voltage down to vintage spec, you often don't get what the schematic says (Fender 6G3 is way off - I have one), and the schematic voltages often conflict with each other (as I noted earlier). Yet, people want to 'clone' to an inaccurate schematic as if it was gospel truth, and then wonder why their voltages are 'off'.

The easy answer is sometimes the hardest: find someone willing to measure their voltages. Didn't someone post earlier that they have two of these? IMO, being 10-20V different on the supply is pretty negligible compared to the difference in feel produced when the current and filtering are too over-rated. (Although in many cases, a little bump in current capacity often produces a 'better' amplifier.) In light of all the thorough research you've done, original transformer specs should be just as questionable as all the other data. You'll have to temper your research with common sense, experience, gut feel, and what is available.

 

[FenderLover]

1. Is it correct that the V3a (reverb driver) triode really draws about 13 mA?
2. From a Ia = Vk/Rk standpoint, have I calculated current draw for the PI triodes and 6V6 (both of which have shared cathode resistors) correctly; that is, by doubling the shared resistor value to arrive at "Rk"?
3. Admittedly I'm obsessing over preamp current, which is not the lion's share of it; but if I'm correct about the calculation, then does the total look about right for a PT's current rating? In this respect I am confused,....

1) yes - the tube maxes out at 20mA. This is curious why they would use a 6SN7 in the V2 position while barely providing enough current to operate. Gibson did the same thing, I suspect others too.
2) yes
3) I think you are still in a reasonable window, but the real confusion comes from 'what is real' in terns of conflicting information. Documentation practices back in the day serve to confuse. Today, we have CAD systems that lave logical connections to the physical; the data match each other or there is an error. Back then it was all done by pencil with manual correlation.

 

[King Fan]

^^^^ this. I agree with you, @chas.wahl , that we can’t take the voltages from that el34 build as vintage correct, but I agree with FL we can’t take Ampeg's too literally either. This is why I mentioned actual measurements on existing vintage amps.

The easy answer is sometimes the hardest: find someone willing to measure their voltages. Didn't someone post earlier that they have two of these?

Was it our smart friend @telemnemonics ? Do I recall @BobbyZ has Reverberockets? I dunno if the 6V6 version was built differently, but others will.

Two stories about PT current. I built my PR — on purpose — with a vintage correct ~70mA PT. Voltages came in perfectly — right about where thesubjectmatter said they would, high and vintage — and the amp sounds great.

But for my 5G9 (with fewer tubes, lower schematic B+) I wanted to find a PT to fit my giant 5F7 cutout, and this was after the great PT shortage began and Classictone folded. I ended up with the huge Hammond 290D2x (300 mA ). Wired with the 305V HT, B+ was only 5-10V above what the TSM calculator estimated and about where you’d get if you ran a 1960 PT on modern wall voltage. I built a bucking transformer to get B+ down to schematic 370V. Only thing I can now blame on the 300mA is the amazing lack of sag, which works for this big tweed just like the 70mA works for the streetwise little PR.

Short version: I trust that B+ calculator, and for B+, I find HT counts more than mA. Low current isn’t fatal (tho it works the PT harder and may give more vintage sag). High current only adds moderately to B+, but may modernize and beef up your note attack. The mA effects are more subtle overall than the HT effects.

 

[telemnemonics]

^^^^ this. I agree with you, @chas.wahl , that we can’t take the voltages from that el34 build as vintage correct, but I agree with FL we can’t take Ampeg's too literally either. This is why I mentioned actual measurements on existing vintage amps.



Was it our smart friend @telemnemonics ? Do I recall @BobbyZ has Reverberockets? I dunno if the 6V6 version was built differently, but others will.

Two stories about PT current. I built my PR — on purpose — with a vintage correct ~70mA PT. Voltages came in perfectly — right about where thesubjectmatter said they would, high and vintage — and the amp sounds great.

But for my 5G9 (with fewer tubes, lower schematic B+) I wanted to find a PT to fit my giant 5F7 cutout, and this was after the great PT shortage began and Classictone folded. I ended up with the huge Hammond 290D2x (300 mA ). Wired with the 305V HT, B+ was only 5-10V above what the TSM calculator estimated and about where you’d get if you ran a 1960 PT on modern wall voltage. I built a bucking transformer to get B+ down to schematic 370V. Only thing I can now blame on the 300mA is the amazing lack of sag, which works for this big tweed just like the 70mA works for the streetwise little PR.

Short version: I trust that B+ calculator, and for B+, I find HT counts more than mA. Low current isn’t fatal (tho it works the PT harder and may give more vintage sag). High current only adds moderately to B+, but may modernize and beef up your note attack. The mA effects are more subtle overall than the HT effects.

Sorry, I sold one and the other us currently not working, so I cannot measure voltages.
@BobbyZ maybe...

 

[Lowerleftcoast]

I don't wish to knock they guy who built the Verberwaffe R-12-R clone in any way (he's the only one I can find who's done one) -- his knowledge of amp building is obviously top-shelf, and he's got a 'scope! He made the intelligent (IMO) choices to make the board layout more Fendery, use a Blackface-style chassis instead of a gopher coffin to reduce the crowding, and his work looks very fastidious and clean.

Agreed, it looks like some fine work there.

It's quite possible that his 5Y3 is a modern-production one (like his "made in Russia" 6V6s), with lower voltage drop. Above I've added a portion of a photo from his parallel "cab build" thread (5Y3 on the right) showing chassis topside, but it's not clear to me what that tube is.

I see he has TungSol 6V6 reissue tubes and probably has a new production TungSol rectifier tube. There have been several reports on TDPRI of the TS 6V6 red plating in 5E3 and Champ circuits. The Reverberocket is designed to run just as hot as those. Assuming he had this trouble, a logical choice would be to use the red/white PT taps to stop the red plating. Whether the new production 5Y3 is dropping as much voltage as the old stock is unknown to me. It appears it does not, which seems to be expected from the new versions. I would expect his amp will sound good. Good enough to get a pat on the back but it will not sound like an original.

1) why Ampeg specs a 350-0-350 PT to achieve 350 V at the OT and 2) why I'm being counseled to use a PT with a lower voltage rating than that, if my intent is to build something as much like the 1962 version Ampeg produced as possible. If I were to do that, shouldn't I reduce the dropping resistor value between screens and PI (22k) or between OT and screens (1k) to maintain the "stock" values for the rest of the circuit?

1 it was probably easily sourced.
2 reducing B+ makes a lot of heat. We don't want to deal with that. Basically it is easier to start at the right place to end at the right place. I have used an old Stancor PM8409 350-0-350 90mA PT in a 5E3. The B+ came in around 390something. Imo, the Reverberocket requiring another 20mA will still end up a little higher B+ than what you want. But just a bit. If the Reverbrocket indeed came with a 700CT 90mA PT, this may have been the one they used as a replacement or maybe the original. These come up on ebay. If your goal is to make a clone, I would seriously consider this PT. I would probably install a 270R bias resistor for the old stock 6V6 or run JJ 6V6 with the original 250R value and leave all the other values alone.

why Ampeg specified a PT supplying current-hungry octals and an amp with reverb and tremolo (except for the octals, sort of like a proto-AB763 circuit) at only 90 mA HT current.

Price most likely. The PT will get a little hotter but it can handle the extra current draw and not burn up, imo.

I have witnessed a lot of DIY angst related to PTs that are too "stiff", and am somewhat predisposed to prefer ones that are more "compliant" or even "yielding" -- but I could be entirely wrong about that.

These cathode biased amps do not have the voltage swing of other amps so *stiff* *compliant* *yielding* are not much in the equation. The fixed bias amps are where this makes a difference.

@Lowerleftcoast says that a cathode-biased pair of 6V6's should end up with 350–380 V on plates, while @FenderLover suggests starting with a 300-0-300 V primary at 150 mA. Are these suggestions consistent, or alternative points of view?

Hah. Maybe both.
The push pull Harvard 5F10 and Princeton 6G2 show (low 300's) 6V6 plate voltage but they are fixed bias so that is apples/oranges.

As I mentioned, I have been looking at this from my knowledge of a 5E3. To get the 5E3 experience the voltages have to be in the right range. The DeArmond info posted will not be able to drive the 6V6 into distortion like the 5E3 circuit will. My hunch is the Revererocket can be coerced into power tube distortion like the 5E3. It might even be better equipped to do so because it has less signal loss in the circuit compared to the 5E3.

The question is what B+ will be attained. Will a 300-0-300 150mA rated PT end up with a similar B+ as a 350-0-350 90mA rated PT in a circuit that draws ~116mA? I don't think they will.

TDPRI member jsnwhite619 measured an original 5E3 at 396 B+. He also says he has used several Hammond 290BX (330-0-330 @138mA [email protected] [email protected]) and they come in around 370B+. He has been quite a resource and posts audio clips. Check out his *College Dropout* mostly modded Harvard content.

 

[chas.wahl]

You guys are the greatest, even if it makes my head spin. I'm really saddened to hear that @telemnemonics doesn't have a working R-12-R to record voltages -- both for @telemnemonics and for me.

A question I've asked that hasn't been answered is: what happens when I underload a PT HT winding; does the voltage increase over spec? And the converse; over-loading causes voltage drop (until failure)? Sorry to be obtuse, but while that seems logical to me, confirmation of that is a basic void in my understanding.

Is there a way to determine "if I take a PT with voltage rating X and current rating Y, what happens to the voltage when I load it with Z mA instead?"

I think that Hammond consistently provides X volts at Y mA figures; I say this because many of their transformer datasheets also give unloaded voltages for the secondary windings, and DCR values too. I hope that they mean "when powered by the primary voltage indicated". I'm not averse to erring a bit on over-voltage and correcting with resistors (@Lowerleftcoast's warning about heat is taken under advisement; but the normal dropping resistors contribute heat too), but those add to the load, so there has to be enough cushion in the current to do this and stay within the performance envelope. I am contemplating a head, and would try to think of ways to design it with some more venting than enclosures typically have. I'm thinking a head configuration like a TrainWreck or a Marshall (or is it Vox) with the chassis along the bottom edge.

@King Fan: "that EL34 build" was posted on Hoffman's site, but apart from the transformer substitutions and a confessed insertion of a Brown Fender tonestack, it's a straight R-12-R 6V6 -- I assume you know that, but I want to make that clear, so some other reader doesn't get off on the wrong track.

Re @King Fan's first story about PT current: one project I have in mind (to precede this Reverberocket, as an intro to push-pull) is a 5E3, in a 16-1/2" long gopher coffin chassis (already in hand) using a transformer I also have -- an older version of Hammond's 290AX that's rated 330-0-330 at 81 mA; this is exactly the spec that @tubeswell has reported the original to be (and I'm hopeful that this is borne out). It's a smaller (5F2-A-size) format than the 290BX that Hammond sells for the 5E3; but I have that covered with a modification to the laydown hole to take a plate that would allow either PT to be installed.

What I mistrust about the "thesubjectmatter" calculator is a) that I can't put in enough preamp tubes for this R-12-R amp because it assumes every preamp tube is a 12AX7 (octals use twice as much current); and b) the B+ voltage it reports seems to be way high to me. For example, if I put in a standard 5E3: 2 6V6s in PP and 3 preamp tubes (because a 12AY7 uses twice the current of a 12AX7), with a 325-0-325 PT, I get 406 V B+. I'm pretty sure that's incorrect, based on paying attention to what people put in their 5E3 builds, and what sort of B+ they're reporting. I'd expect it to be down in the 350–360 V zone, as @Lowerleftcoast has mentioned. Moreover, the reported HT B+ doesn't change at all if I change the number of preamp tubes -- as if their number and load is ignored in calculating B+. Finally, c) the HT voltage only increases 39 V if I choose a diode rectifier instead of the 5Y3. The HT mA of 73 seems about right. Maybe that 5Y3 model in the calculator is a modern one; dunno, doesn't seem to be explicit.

@FenderLover's remonstrations about the unreliability of mfgrs' schematic data, in terms of being supported by real-life examples or "adding up" on paper is dispiriting, more bad news; but forewarned is forearmed, I guess.

Long and short: I'd really like to find a way to get a PT that will deliver 350 V to the OT with the loads imposed as designed. Maybe the 273DZ with 700 V at 104 mA and 6.3 V winding at 3A isn't it, but it seemed to quack the right way to me. Alternatives that occur to me after the above discussion: a) Hammond 290BX (5E3) with 660 V @138 mA, again 3A 6.3 V heater, or Hammond 290CX (AB763 Deluxe Reverb reissue) with 650 V @ 180 mA, 4.5 A 6.3 V heater. You can see I'm going down the B+ voltage ladder and up the B+ mA one, with heater winding going along for the ride. If anyone has a better candidate, and can provide some reasoning to support why that might be so, then I'm all ears.

 

[Peegoo]

Here are a few pics of my 1961 Reverberocket.

These pics are pretty big, so save 'em and you can blow 'em up.

It was re-capped by Jeff Bober (Budda/East) about 12 years ago. I installed a Weber 5Y3 Copper Cap rectifier to firm up the lows and make the amp less 'squishy'.

Schematic on the inside rear panel. Any resemblence to the Shroud of Turin is purely coincidental:

 

[Peegoo]

Seems like both my R12R came with a printed schematic glued to the inside of the back panel?

Cool amps though, well worth working up understanding and building!

I'm really saddened to hear that @telemnemonics doesn't have a working R-12-R to record voltages -- both for @telemnemonics and for me.

What voltages do you need?

 

[chas.wahl]

^^^ Thank you so much for checking in @Peegoo. You deserve more than cookie for sure. That's only the 2nd R-12-R I've got any documentation of. Any chance of imposing on you to fact-check the voltages shown on the chart below (from service manual) at some point? The whole service manual is one of the PDF attachments to post #1 in this thread. If that's too big an ask, then just the B+ voltages indicated on the schematic's chain (at OT CT and on the lower-voltage side of each dropping resistor) would be a great start, in terms of most of this discussion. I do realize that those may not be that easy to find in that chassis/layout! If it's any help, the other PDF, my rendition of the schematic, has all the information for components, number correlated with those in the manual, values, and voltages. It's not easy to print, since it's so wide, but I could print it at reasonable size for you, and even do different kinds of information in different colors.

A quicker question, does your PT have a model marking? (PT-106 is what the documents call it, which I think are from 1962, like the schematic I've been working with).

Also, looking a bit more closely at your schematic, it seems that most of the component IDs are written in ink, not printed. You may have a prototype. The 272 is probably a sequential number for all the amps Ampeg did that year.

I'm going to be checking your schematic against the one I've been working with.

 

[FenderLover]

@Peegoo It would be super cool if you could make a voltage chart like post #27.
edit: chas beat me to it