Ampeg Reverberocket R-12-R DIY project

Peegoo

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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.

Yes, you're correct.

Based on what I've learned about 'em, this is a very early Reverberocket. Another clue is on on the tube chart (the handwritten "R" after the model), because they did make R-12 Rockets before they made Reverberockets. They used a Rocket label in this one and wrote an R there. This info I got from Ken Fischer (RIP), who worked at Ampeg when these were being built.

Being that the Reverberocket was the first amplifier built with onboard reverb, it makes this particular amp pretty kewl.

I'll take a peek under her skirts tomorrow when I get some time, and record some voltages.

Here are pics of the PT and OT. It does have a PT-106 installed.

'926' on the PT is the EIA code for the manufacturer--Todd Electric Company of Yonkers, NY. '129' is the production date: 1961, week 29.

Ampeg-R12-R-PT.jpg


'682' on the OT is the EIA code for Electrical Windings Incorporated of Chicago, IL. '112' means the 12th week of 1961.

Ampeg-R12-R-OT.jpg


More info as I get time... Cheers!
 
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Lowerleftcoast

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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.
Your intuition is correct. When the circuit uses more current the voltage will sag and vice versa. So with no tubes in the amp, the B+ will be high. With tubes installed the voltage comes down.
A catastrophic failure. The tube has a dead short. The transformer will attempt to provide the current. The voltage will drop but the current will flow until something gives. *Something* can be a fuse, resistor, wire, circuit breaker, etc. If a fuse or circuit breaker is not tripped quick enough, the enamel insulation in the PT coil melts letting out the magic smoke. And intuitively we know it is not good to let the smoke out.
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?"
Yes but we have to know variables we seldom know. The winding companies don't give up their info easily. Sometimes they do try to be helpful and give some numbers under load. If you notice sometimes those numbers can't be accurate/believable so, we are still back at square one. Who can we trust for good info?
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.
Availability may be a problem but I would probably choose the 290BX or 40-18016 due to the known ~370 B+ in circuits with a little less current draw (using vintage 5Y3). With the extra current demand of the Reverberocket and some voltage drop across the OT resistance it will be close enough.

BTW there are 3 of the Stancor PM-4809 on ebay. A couple of them look NOS. One has a BIN at a reasonable amt. You can internet search for stancor catalog to see the specs on many of their products.
 

chas.wahl

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@Lowerleftcoast: Thanks for the notice, very kind of you. It was PM-8409, but I found it, and got one of them. One was $5 cheaper, but an auction situation, and the one I bought BIN has all stranded wires cloth/rubber insulated (tinned and apparently not used), while the other two had some wires (heaters) with plastic insulation and some cloth/rubber -- weird. All of them may, in fact, be unused. Cosmetically, the label is falling off, but it looks like the right thing. So, for $52 all in, I took a chance. We'll see if it fits the bill. Except for being rated 3A for 6.3 V heater (which should be just enough, based on calculation) it's exactly like the spec for the Ampeg PT-106 (4.5 A heater). Of course, it's for 117 V primary, but I have the 6.3/12.6 V transformer and everything else except the box for a BuckMinister* project (à la R.G. Keen and @robrob), if that's necessary. Obviously, throwing the caution given by @FenderLover regarding taking mfgrs' "data" to the winds, I'm placing some faith in Ampeg's, and betting that a transformer rated at a lower HT mA (90) than what the load seems to calculate to (116 or so) will drop the voltage at the 6V6 plates to somewhere close to that spec.

*note to those unfamiliar with @robrob's invaluable web pages: many of these are very image heavy, and if you click a link to somewhere in the middle, you need to be patient (wait for it) if presented with a blank screen while it loads all the images preceding the section you want to look at! Well worth it!
 

Peegoo

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Any chance of imposing on you to fact-check the voltages shown on the chart below (from service manual) at some point?

I poked around in the chassis and made up the following chart today. I used my trusty old (recently calibrated) Fluke 77 to record these measurements.

R12-R-Voltage-Chart-19-Jun22-BAD-INFO.jpg


IMPORTANT NOTE: I edited this post the next day because my heater readings were wrong. Here are the correct numbers; see my update with explanation a bit farther down in this thread:

R12-R-Voltage-Chart-20-Jun22.jpg
 
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chas.wahl

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Thank you very much @Peegoo, for taking the time to do this, on a Sunday. This is an amazing result, and a real boon to this conversation. My only quibble with that report would be that unless voltage standards changed between 1961 and 1962 (and I don't see any contrariwise indication on your schematic or tube chart of the primary voltage) the design primary was probably 117 V, since that's what's listed consistently on the Ampeg service manual. All I know about the voltage standards timeline is that the advent of 117 seems to have been "after WWII."

Can't argue with a Fluke 77; and recently recalibrated.

Differences noted from 1962 service manual nominal specs:
Not a difference per se, but heaters seem to be elevated on V5/V6 cathode junction (21 V). Does that appear to be how the heaters are circuited? Heater circuit is, like most practice back then, not documented on the Ampeg schematics, and there's no layout.
V1b [preamp stage 1] is somewhat hotter at plate (172 vs 150 V)
V3a runs a bit higher at cathode (7.4 vs 4.2 V)
V3b is a bit hotter at plate (109 vs 100 V)
V4 (PI) runs cooler at plates (166/157 vs 200/195 V) and cathodes (1.6/1.5 vs 2.2/2 V)
V5 & V6 screens and plates all somewhat lower than the 1962 chart: (screens both 320 vs 335 V, plates 317/322 vs 345 V, and V5 screen higher than its plate)

It's interesting that there does seem to be some real-life "sag" in the middle B+ values compared to the service manual's nominal ones, but the ends of the chain are pretty much the same. Not as much sag as in the Verberwaffe build using the 300-0-300 HT secondary, which makes sense. This makes me more satisfied about the choice to go with a piece of vintage iron at the original spec.

I haven't had time to go through @Peegoo's schematic yet to compare component values to the 1962 schematic; but I will do that. I'm currently going over the Verberwaffe Fender-ish board layout, and penciling in the component numbers and missing/differing values so that I can understand it better. It seems to be pretty pragmatic and ingenious, but there are some very long under-board connections that I'd like to see if I can improve on.
 
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Lowerleftcoast

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heaters seem to be elevated on V5/V6 cathode junction (21 V). Does that appear to be how the heaters are circuited? Heater circuit is, like most practice back then, not documented on the Ampeg schematics, and there's no layout.
The schematic on Peegoo's amp doesn't show it but one of the schematics you provided did.

Ampeg R-12-R.pdf.jpg
 

Peegoo

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Not a difference per se, but heaters seem to be elevated on V5/V6 cathode junction (21 V). Does that appear to be how the heaters are circuited? Heater circuit is, like most practice back then, not documented on the Ampeg schematics, and there's no layout.

The schematic on Peegoo's amp doesn't show it but one of the schematics you provided did.

It does show as 6.3v on the schematic on my amp's back panel. Look just to the left of "5Y3" on the lower right.

I assumed the heater was DC so that's how I measured it as 21v. Many makers use DC heater voltage to help reduce noise, so I went with that. But after looking at the schematic, I popped it back open and took readings using AC autoranging, and I'm getting between 3.06 vAC and 3.46 vAC on all the tubes' heaters. That averages as 3.25v, which is about half the spec'd voltage of 6.3v. [Scratching head] What is going on here?

Then I considered how I was taking readings--and that was with the negative probe connected to the chassis common ground. Ah...half-wave AC? Gotta be. So I tested directly across the heaters' pins (7 and 8), and got a reading of 6.6 vAC. That explains that weirdness.

So everywhere you see 21v(DC) on my chart should read 6.6 vAC.

I updated my chart to reflect this. I also edited my previous post to ditch the old chart and replace it with the new one. Don't want bad info on the innerrwebz!

R12-R-Voltage-Chart-20-Jun22.jpg
 

chas.wahl

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I'm no expert certainly, but I think that @Peegoo got it right the first time. @Peegoo would you check to see where the heater CT terminates? I forgot about the schematic for the 1962 R-12-R, but it clearly shows the heater CT being connected to the junction of the 6V6s' cathodes (good catch @Lowerleftcoast), which in my book is heater elevation on a DC reference, per Merlin Blencowe (Grounding article) and others. This is not providing a DC heater supply (different animal), but simply changing the reference for the heater circuit's CT.

OTOH, @Peegoo's schematic shows heater circuit without a center tap. It would be good to know if there is one, and where it terminates, I guess.

And if the original iron was built/spec'd for 110 V, I would expect the voltage differences between measured and Ampeg spec values to be greater in magnitude than they are; but maybe not.
 
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Lowerleftcoast

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@Peegoo had it right both times. Good catch on measuring AC heater volts Peegoo. Yes this is elevating the 6.3VAC heaters to the DCV at the 6V6 cathodes. Not shown on Peegoo's schematic but is shown on the '62 schematic.
And if the original iron was built/spec'd for 110 V, I would expect the voltage differences between measured and Ampeg spec values to be greater in magnitude than they are; but maybe not.
Peegoo's B+ voltages are lower than I anticipated.

EDIT: @Peegoo notice on the '62 schematic the CT from the 6.3v secondary has an arrow to(x). Co-responding to that (x) is an (x) at the junction of the cathode resistor and pin 8 of the 6V6 tubes. Your schematic does not show a CT on the 6.3v secondary.
 
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Peegoo

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@chas.wahl & @Lowerleftcoast

It looks like this:

Heater-Circuit-Diagram.jpg


It also has a 3rd wire (not shown on the diagram here) coming from the PT secondary, connected to pin 8 on both 6V6 tubes. It reads 6.6 vAC when testing for AC across pins 7 & 8 on each 6V6.

..
 

chas.wahl

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It also has a 3rd wire (not shown on the diagram here) coming from the PT secondary, connected to pin 8 on both 6V6 tubes. It reads 6.6 vAC when testing for AC across pins 7 & 8 on each 6V6.
To me that sounds like a center tapped heater, elevated on the 6V6 cathodes. Thanks!

Per Merlin Blencowe's Heaters article:
1655810381777.png


What his article doesn't seem to say explicitly is that if you measure the heater circuit for DC, you get the voltage that the cathodes are operating at, and if you switch to AC you get the 6.3 V power for the heaters. Does that sound about right?
 
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King Fan

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Back at the overall voltages, I may be missing something. I see this on the schematic:

1655819448899.png
1655819775211.png

and this as measured on your amp, @Peegoo ? Or is 'this amp' from some other source? As I say, I may be missing something.

1655819517243.png

If I'm not missing the boat, the schematic thinks we'll drop 5V from B+ to the plate voltage, but your measured plate voltages are ~320, roughly a drop of 30V from 'circuit point A.' Are you measuring that pin-to-chassis, or plate to cathode? Or am I just lost in space?
 

Lowerleftcoast

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What his article doesn't seem to say explicitly is that if you measure the heater circuit for DC, you get the voltage that the cathodes are operating at, and if you switch to AC you get the 6.3 V power for the heaters. Does that sound about right?
Right. DC and AC can both be in the same wire at the same time.

Sometimes the terminology "AC riding on DC" is used when talking about rectified DC. The reservoir cap and the large filter caps are there to remove the AC content. The DC cannot make it through the cap and the AC easily passes right through the cap to ground. When the amp needs a cap job due to hum, it is because the filter caps are not doing their job. Some of the AC is not passing through the caps to ground.

The signal from the guitar is AC. As the signal is amplified by the tube both AC and DC are at the plate and cathode of the tube. The coupling caps block the DC but allow the signal to pass to the next stage.

Elevating the 6.3v heater might help the tubes not pick up hum. The heater element, inside the tube, is in close proximity to the cathode. The cathode needs to be heated for the tube to function. Because of the close proximity, the cathode can pick up the 60Hz sine wave from the AC heater voltage inducing hum in the amplifier. Tubes are designed to withstand a difference of DC. Elevating from *0* to a higher value reduces the voltage the tube *sees*. Ideally having the heater elevated to the same DC voltage the tube is using will keep current from flowing so the 60Hz sine wave will not get picked up by the cathode. Elevating the heater voltage is more important in amplifiers that have higher levels of DC on the cathodes of the tube. Amplifiers with cathode followers and amplifiers with cathodyne phase inverters may have cathode DC voltages high enough to necessitate elevating the heaters. Other amplifiers might benefit as well and on cathode biased amps, such as the Reverberocket, there is a convenient place to get that elevated voltage. It may or may not reduce hum, but it won't hurt anything, so why not?

@Peegoo you want to use the DCV measured from the 6V6 pin8 to ground on your voltage chart. It is an important number for calculating the bias numbers of the 6V6 tubes. As long as the 6.3 AC voltage is on the heater pins and pilot lamp you are golden there. You can just report the heater voltage once for the entire amp on the chart.
 

chas.wahl

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Back at the overall voltages, I may be missing something. I see this on the schematic:

View attachment 996118
View attachment 996123

and this as measured on your amp, @Peegoo ? Or is 'this amp' from some other source? As I say, I may be missing something.

View attachment 996119

If I'm not missing the boat, the schematic thinks we'll drop 5V from B+ to the plate voltage, but your measured plate voltages are ~320, roughly a drop of 30V from 'circuit point A.' Are you measuring that pin-to-chassis, or plate to cathode? Or am I just lost in space?
Unless I'm wrong, the 321 V is measured at the 6V6 screens (at "B"), not plates. I'm not smart enough to know why, on the schematic(s) there's a 5V drop from B+ on OT center tap to the plates (resistance-inductance of half the primary winding?) but the "A" voltage is taken where shown on the lower image of @Peegoo's schematic, no?

@Peegoo has also posted voltages from tube pins:
1655828380822.png

And I don't know why there should be a 30–33 V drop from his "A" voltage to his plates. Or why that's about the same as that for the screens, if there's a 1k dropping resistor between "A" and "B" on the B+.

We're really giving your poor Reverberocket the third degree, aren't we @Peegoo ?
 
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Peegoo

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If I'm not missing the boat, the schematic thinks we'll drop 5V from B+ to the plate voltage, but your measured plate voltages are ~320, roughly a drop of 30V from 'circuit point A.' Are you measuring that pin-to-chassis, or plate to cathode?

"This amp" means readings I took with the amp idling, pin to chassis.

@Peegoo you want to use the DCV measured from the 6V6 pin8 to ground on your voltage chart. It is an important number for calculating the bias numbers of the 6V6 tubes. As long as the 6.3 AC voltage is on the heater pins and pilot lamp you are golden there. You can just report the heater voltage once for the entire amp on the chart.

I understand that; all tube heaters are connected in parallel with the lamp, and read 21 vDC. However, this amp has had work done on it throughout its lifetime and I wanted to measure everything to see if there was anything out of whack. I know there probably isn't (other than the fairly big voltage drop from spec at circuit point A) because the amp runs quietly and works fine at all control settings. It does run hot though.
 

King Fan

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Unless I'm wrong, the 321 V is measured at the 6V6 screens (at "B"), not plates. I'm not smart enough to know why there's a 5V drop from B+ on OT center tap to the plates (resistance of half the primary winding?) but the "A" voltage is taken where shown on the lower image of @Peegoo's schematic, no?
Sorry, not trying to be a pain. But still confused. I'm taking 6V6 pin 3 as the plate readings, and pin 4 for the screens. Yes, it would be typical to drop ~5V going through the OT. But if point A (what we often call "B+", whether taken at rectifier pin 2 or at the positive of the first filter cap) is ~350, why is plate voltage ~320?

What am I missing?

1655828058629.jpeg

1655828344504.jpeg


Also, let me echo @Lowerleftcoast and suggest you list the 6V6 pin 8s in DC volts. I assume this is 21V as you measured initially. This isn't about the DC elevation voltage; as LLC says, this *cathode voltage* is key to knowing about output tube bias and operation.
 
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Peegoo

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Right. DC and AC can both be in the same wire at the same time.

Sometimes the terminology "AC riding on DC" is used when talking about rectified DC. The reservoir cap and the large filter caps are there to remove the AC content. The DC cannot make it through the cap and the AC easily passes right through the cap to ground. When the amp needs a cap job due to hum, it is because the filter caps are not doing their job. Some of the AC is not passing through the caps to ground.

The signal from the guitar is AC. As the signal is amplified by the tube both AC and DC are at the plate and cathode of the tube. The coupling caps block the DC but allow the signal to pass to the next stage.

Elevating the 6.3v heater might help the tubes not pick up hum. The heater element, inside the tube, is in close proximity to the cathode. The cathode needs to be heated for the tube to function. Because of the close proximity, the cathode can pick up the 60Hz sine wave from the AC heater voltage inducing hum in the amplifier. Tubes are designed to withstand a difference of DC. Elevating from *0* to a higher value reduces the voltage the tube *sees*. Ideally having the heater elevated to the same DC voltage the tube is using will keep current from flowing so the 60Hz sine wave will not get picked up by the cathode. Elevating the heater voltage is more important in amplifiers that have higher levels of DC on the cathodes of the tube. Amplifiers with cathode followers and amplifiers with cathodyne phase inverters may have cathode DC voltages high enough to necessitate elevating the heaters. Other amplifiers might benefit as well and on cathode biased amps, such as the Reverberocket, there is a convenient place to get that elevated voltage. It may or may not reduce hum, but it won't hurt anything, so why not?

That is an outstanding explanation of heater voltage schemes.
 




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