Ampeg Reverberocket R-12-R DIY project

[chas.wahl]

Over the years that I have been interested in this field of guitar amp circuits, I've developed a particular interest in the Ampeg Reverberocket R-12-R, a push-pull 6V6-powered design from the early '60s with an all-octal preamp tube complement, reverb, tremolo and a paraphase PI. It's way more complicated/complex than my building/troubleshooting skill set now, but I can't help myself from researching it, even though most of the amps I'm interested in are of the Fender Tweed persuasion.

During COVID, I've continued this research, and I've lately found myself thinking that I should do a project schematic for it, and start working on a layout. And here's the state of my brain: I found out last week that I had already done the schematic in CAD, during a month of 2020's winter COVID surge and the political angst and turmoil of the election -- and I'd completely forgotten about it! Really -- I have no memories of the process at all.

Since I lately tagged a year-and-a-half old thread I had started, based on some recent research, which resulted in some "likes", and mentioned this schematic which resulted in a request to see it -- here it is, along with the Ampeg schematic I have worked from, and also an Ampeg service manual that I found.

There were several iterations of the R-12-R (that is R-12 with [trem and?] reverb), starting with the octal preamp / 6V6 one presented here, and progressing through ones using 7591 or other power tubes, and even later preamp using 12A*7 tubes and more exotic compactrons, and so forth. I believe that Everett Hull kept trying to make the amp into a jazz/clean thing, which was his personal bent. But the ones that a lot of people seem to have a fondness or regard for are the ones at the starting point, like this one.

In a way I'm quite lucky to have found (in different locations) a schematic (from the Hoffman EL34World archives) that somebody marked up with the component numbers, and also the service manual that seems to be consistent with that schematic in every respect, (except an un-numbered resistor) and contains baseline voltages and resistances for the circuit, plus a lot of specs for the hardware.

{see attachments below}

I have done some modifications in my version of the schematic which are listed on the right hand side. I've also included, on the same PDF, the Ampeg schematic in question. And I added to the service manual the same schematic (again) and have annotated things in the service manual that reflect my research. I've also appended a page that has some of the research info, and a couple more about reverb tank characteristics. Obviously this is a work in progress, still. I have yet to double-check my own work, and I'm not asking anyone else to do that -- though I would of course appreciate knowing about any stupid errors you may find.

As for layout, there are some resources for this by people who've
1) owns or owned one (1961) and documented it (Troy D. Woods) https://www.pdf-archive.com/2016/05/30/ampeg-r12r
2) built a clone ("Sonny ReVerb" in thread Ampeg R12-R Reverberocket variant on Hoffman's site EL34world.com https://el34world.com/Forum/index.php?topic=22267.0
3) Hoffman also has a "layout" in his Schematics hierarchy that's just a circuit board with off-board destinations for terminals https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_r_12_r_layout_rev_a.pdf

Links for these currently work, though if you ask I can provide what's at these locations. I also have a trove of photos of various R-12-R amps that I've scarfed from google searches, which I could upload to a DropBox or OneDrive location -- though the organization of it is not stellar. However, most of it has filenames that are fairly descriptive.

Regarding layouts: I have an aversion to the Vox-like arrays of parallel or nearly parallel components, with sheafs of conductors running parallel to the long edges of the board(s). To me, a Fender-style layout makes a lot more sense, as in #2 above (though I have not delved into it enough to know whether all the components are there -- looks a bit sparse on first glance). So that's what I would prefer to do. I should look at Fender layouts that have both reverb and tremolo, to see how those portions of the circuit are handled, though the trem shares a tube with the first preamp stage in this case, which has a certain inherent gravity to it.

I'm inclined to build a head, rather than a complete combo, partly because the octal preamp tubes are reputed to be more microphonic (I assume that some isolation from speaker vibrations would help with that), and partly because I just don't have room for a bunch of combos.

As always, comments welcome!

 

[Lowerleftcoast]

If you don't have the OT yet, I vote for the 18Watt... mainly because Hammond has a paper layered one at a decent price.

Everything looks OK from my cursory look.

 

[telemnemonics]

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!

 

[chas.wahl]

^^I'm fairly certain that the schematic I've included is a copy of one of those -- and lots of the photos I have collected are of such schematics. They're often tattered on the top edges.

If you don't have the OT yet, I vote for the 18Watt... mainly because Hammond has a paper layered one at a decent price.

Everything looks OK from my cursory look.

Thank you. I did already get a ClassicTone 40-18037 OT, which is a Marshall 18 watt replacement, also paper-layered. Haven't got a PT though. Fliptops wants $140, and I just haven't gotten there yet.

 

[Lowerleftcoast]

Have you considered a toroidal PT? It would be less cost. It would be more modern looking or non traditional looking or ?

 

[chas.wahl]

No, I haven't considered toroidal. I have looked a bit for more traditional E-I types. The PT specs are a bit odd: 350-0-350 at 90 mA, with a 6.3 V secondary that delivers 4.5 A. I realize that the octal tubes, especially the 6SN7s, suck a lot of current. Everything connected (tubes, pilot, humdinger) adds up to about 2.9 A, and I'm wondering whether one of the 700 V transformers that's only rated 3 A for the 6.3 V heater would be OK.

 

[sdcb27]

My Reverber is one amp that ill never sell and had it something like 15-20 years

 

[Lowerleftcoast]

I have looked a bit for more traditional E-I types.

Most amplifiers run the heater current very close to the PT specs. I suppose that helps to keep the voltage near the 6.3 mark.

I wonder if this might be a good donor for the reverberocket.
Idk the current capability of the PT that was used in the Hammond organ AO-29 chassis. I have always assumed it was around 90mA to 100mA because most PTs from that era powering 2 x 6V6 seemed to be in that range. It used a 5U4 rectifier so the 5v winding can do 3A. It had many preamp tubess so the 6.3v winding can handle most anything. If memory serves it was 320-0-320. Modern wall voltage might push up the secondary voltage where it is close.

 

[chas.wahl]

I certainly appreciate your help and advice @Lowerleftcoast -- it's true that the PT is the most expensive component in a project like this, and possibly the most critical to get right or suffer the consequences. I'm somewhat leery of committing to something whose spec are not clearly published (even though there's always the problem of expected and actual performance differing, something we see in threads so frequently). I do understand that the specs don't mean much if the heater and HT loads engendered by an amp don't correspond well with the rated loads published by the manufacturer: if I get a transformer rated for 200 mA HT and only load it with 100 mA, it's going to put out more voltage than I want, or is good for the tubes and other circuit components -- and similarly in inverse for the heater circuit. Correcting with dropper resistors adds to the effective load, of course. So it's a complicated dance.

The person who built a clone (Verberwaffe on EL34world.com) used a ClassicTone (now defunct) PT rated at 330-0-330, and intended, according to Tube Depot's description of that now unavailable item, as being appropriate for a bunch of amps including, say, the Blackface Deluxe Reverb -- something with both reverb and trem, but a somewhat less current-hungry tube complement. I sent that person a PM on the EL34world forum, about his experience since posting (about 5 years ago). He did have it up and running, and first reports were good. But my concern is that giving away 20 V per side right off the top is maybe not ideal.

Then we have the (what seems to me to be a problem) with manufacturers who are doing reverse engineered vintage transformers, and the fact that nowadays voltages are higher than they used to be: are the mfgrs just winding the same number of turns at the same gauge they find in a real vintage model, or are they compensating, in their engineering, for the change in voltages to make a product that has the same output. Part of this is that, in the old days, a lot of engineering was more casual, the specs not available or not shared. Also, "shortfall" in performance might contribute to what people liked in an amp, or might be seen as a problem that should be fixed.

So, when looking for transformers, especially PTs, should one get one that overachieves a bit (a lot of people feel that "stiffer" is better in the modern world, but also a lot of people complain about too-high voltages), or attempt to get one that very closely matches the (when available) original specs or observed performance of vintage amps? I'm inclined to try to find a PT that matches closely the performance in Ampeg's service manual: 700 VAC CT at 90 mA, with 6.3 V heater winding at 4.5 A. If I have to choose, then, as you've observed, I might go with lesser heater current capacity. I can find, for prices nearer $100 to $115 PTs with the 700 to 710 VAC, 100 mA or so, and 3A heater. If I want more heater current, then the HT mA creeps upward. Right now, a good candidate looks like a Hammond 273DZ, its "shortcoming" being the 3 A heater. But in fact, an alternative strategy might be to look at a PT with somewhat lower advertised HT voltage than desired at a somewhat higher mA rating, and a 3.5 A or 4 A heater winding. There are some of those around too.

I'm content to keep working on a practical layout that I like, and put off sinking money into a PT until I feel that it's about the most appropriate one I can get. I might just spring, in the end, for the Fliptops PT-106, but on the website and in my correspondence with the proprietor so far requesting more info, I haven't seen any clear specs that convince me that's about the best match to the Ampeg specs.

Has anyone devised a (safe and reasonably conclusive) method of bench testing PTs "in the raw" to measure performance with a given set of HT load and heater circuit loads? Something simpler than building your amp and then seeing where things end up? If they have, I'm not aware of it.

 

[ok_state_blues]

Are you set on using tube rectification or diode? I have had 2 good builds with Antek toroidal PT's. If your go with diodes you could try

AS-1T275 - 100VA 275V Transformer

Toroidal transformers and power supplies

www.antekinc.com

With diodes, it should get you to where you are wanting to go. Also here is a spreadsheet from the old sewatt website from other user inputs about Anteks offerings

 

[King Fan]

I’m the opposite of an expert, but in 95% of what I see here, people struggle with their modern PTs having too much HV on modern wall power — especially, as you say, the 'cloned' PTs. I’d be strongly inclined to try that idea of ~320-330V HV at 90-100 mA. What did you say your B+ target is? Something like the 350 on that schematic? That’s a low bar, easy to hit too high…. I had to build a bucking transformer to get several of my amps that low even with non-cloned PTs.

 

[chas.wahl]

@ok_state_blues : I'm inclined to use a tube rectifier, but I'm just old-fashioned. I realize that I could do with a lot less HT voltage if I didn't have that 5Y3, but that's a sacrifice I'm loath to make "currently".

@King Fan : Per the service manual B+ at OT primary (10k nominal) is 350, at 6V6 plates is 345, and at 6V6 screens is 335. The EL34world member who built the Verberwaffe clone and used the CT 40-18028 PT had somewhat lower voltages, like over 20 V less (V5 and V6 are the power tubes) -- that's one of the things I asked him about in my PM there:

That OT is 330-0-330 at 120 mA, and has a 3A 6.3 V heater winding. A somewhat "browner" amp.

His values for B+ nodes A–F all run from 13 to 20-something volts lower than Ampeg's schematic shows. And his line voltage is over 122 (about what I have at home). Note, though, that his heater voltages aren't saggy (nor too high)! That's a good sign, I think. Compared to a lot of the "my B+ is too high" day-to-day we see here and even on EL34world, this is not a terrible result. Maybe browner than Everett Hull would like, but possibly more "tweedy" in a good way. Just not "correct".

 

[Snfoilhat]

Looking forward to where this goes!

Looking at the tube set, I added it and got 2.8 A on the 6.3VAC heater circuit. I don't know and am wondering at a lot of the same questions posed above. Is there any benefit to having a power transformer rated for a good bit more than the expected heater current? In a world where so many people post about subbing 6L6GCs into their 6V6GT amps with no worries about breaking the PT, I wonder at how to take the published specs on a given PT.

I don't know if this will suit what you're going for, but I was happy to find this transformer when building some recent amps with higher heater current needs (higher than a Princeton Reverb, say) but aiming for a low B+

Edit: I forgot about the voltage drop from the 5Y3 -- this would probably end up way too low in the R-12. My mistake!

And it's < $70

 

[chas.wahl]

Looking at the tube set, I added it and got 2.8 A on the 6.3VAC heater circuit.

I get 2700 mA (2x300 + 2x600 + 2x450) and added 150 mA for a pilot light and 50 mA for a Blencowe Humdinger to balance the heaters. So 2.9 A, with no addition for dropping resistors if the heater over-volts. That's all on my schematic.

. . .
And it's < $70

I'm not that much of a cheapskate! In fact, it seems to me that a deal that seems too good to be true probably is.

 

[Lowerleftcoast]

Has anyone devised a (safe and reasonably conclusive) method of bench testing PTs "in the raw" to measure performance with a given set of HT load and heater circuit loads? Something simpler than building your amp and then seeing where things end up? If they have, I'm not aware of it.

I am not aware of it either.

You are doing what I did when searching for the *right* 5E3 voltages. At that time there were several members that listed their voltages and the Classic Tone 40-18016 nailed the voltages fairly well. It even fit the chassis cutout. (Plates of the 6V6 run 351 to 364 on the three 5E3 I built. Wall voltage 119 to 120 at time of the readings.)

It seems odd that the CT 40-18028 with the same HT specs as the 40-18016 would supply 30 to 40 volts less than the 40-18016 in a similar circuit. (Even with the V1 displaying that 11.2 volts on pin3. Maybe there is more wrong with his build than just V1.)

I hear you though, trying to predict the right voltages when there are so many variables and so much misinformation. Maybe we need a voltage chart listed by PT for known builds.

 

[chas.wahl]

It seems odd that the CT 40-18028 with the same HT specs as the 40-18016 would supply 30 to 40 volts less than the 40-18016 in a similar circuit. (Even with the V1 displaying that 11.2 volts on pin3. Maybe there is more wrong with his build than just V1.)

Were you using vintage or newer-production 5Y3s? I hear that the newer ones don't have as high a voltage drop.

Also, doesn't it make a difference what the connected load is? A 5E3 has only 2 preamp tubes, while the R-12-R has 4, and more current-hungry ones to boot. The preamp tubes for a 5E3 have a plate current for all sections of 8.4 mA, while the R-12-R has 45.2. Yeah, that's not a huge increase when you add in the load from 2 x 6V6s, but it's more than 5x the load for preamp tubes, and I would expect that to up the load and drop the voltage, no? Am I missing something?

For instance: I'm no PT-calculating whiz (though I may have to school myself on this), but if I go to the https://www.thesubjectmatter.com/calcptcurrent.html calculator and use the 325-0-325 button for PT (no 330) and then choose 6V6 for power tubes, select 2 power and 3 preamp tubes (the calculator counts everything as 12AX7 I think, and a 12AY7 has ~double the current of a 12AX7), I get a current requirement of 81 mA (which I remember @tubeswell as having said was the original 5E3 PT HT current spec, exactly). If I go back and change the number of preamp tubes to 8 (the max available, and the difference is actually greater than that), it returns a current requirement of 120+, or about a 50% increase. Oddly, the voltage at first filter cap is reported as 406 in both cases.
If I change the PT voltage to 350-0-350, then the mA requirements are 77 and 117 respectively, with a filter cap B+ voltage of 435 for each. Maybe I just don't know how to use the tool -- correct me if I'm doing something wrong.

I'm glad you brought this 5E3/40-18016 thing up though, because I notice the only difference between CT 40-18016 and the 40-18028 was that the latter has a quintuple-value primary voltage. I should be careful to avoid buying more boat anchor than I need.

 

[SerpentRuss]

Your schematic is very well done. I'm following as well. These old amp clones are always interesting with questions like where does the mojo live and will I lose it if I add this or substitute that. Good luck!

 

[King Fan]

Not a perfect analogy (those thirsty octals), but I used 305 HT windings on my 5G9 and still have to buck down the wall volts to get B+ down to 350.

Did you say you were able to find datapoints on actual B+ values in vintage versions of this amp?

Also does Mercury make a PT for this amp? They don’t publish specs, of course, but they were willing to share HT voltage data when I asked about their 5G9 versions…

 

[chas.wahl]

Did you say you were able to find datapoints on actual B+ values in vintage versions of this amp?

Also does Mercury make a PT for this amp? They don’t publish specs, of course, but they were willing to share HT voltage data when I asked about their 5G9 versions…

I have not found any voltage reports for actual amps (though I haven't really focused on that) I thought that I was extremely lucky to have found the service manual that matched the schematic I'm interested in, with fairly complete "spec" voltage and resistance values throughout. That's all in one of the PDFs I uploaded in post #1.

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.

 

[Lowerleftcoast]

Were you using vintage or newer-production 5Y3s?

I use vintage 5Y3 tubes.

As I mentioned, the glaring problem with the voltage chart you posted is the way way hot triode in V1.

Wait a minute.
There is something more than meets the eye with the numbers on that voltage chart. I just figured the plate dissipation from the posted numbers. It shows less than 75%MPD. The plate voltage would be even lower if it were biased at 100%MPD. Are we sure the 300v tapped set of secondaries of that 40-18028 weren't used?