Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on May 29, 2023, 07:29:18 am
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Hi all,
A friend of mine who collects 1930s and 40s amps gave me a 1934 Rickenbacher Type 2 Electro amplifier to service the other day. It's a rare one and I'm not at all understanding what's going on with the rectification stage.
After the rectifier (5Z3) there is a choke, with the FC after it, and filter caps to ground between each as you would expect to see. However:
1. The 5V filament winding is center-tapped;
2. The choke has three wires, one of which goes to that 5V center tap;
3. There is no jumper from the 5V filaments to the choke, as normally seen. I'm assuming DC is going to the choke through the center tap. But why this arrangement?
Can you help me understand this? It's unlike anything I've seen before.
Thank you for any insight you can provide! There are no schematics on this online that I could find, and none with this weird choke/rectifier arrangement. I'm just starting to trace out the circuit before I do anything with the amp. I appreciate the help.
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I'm assuming DC is going to the choke through the center tap. But why this arrangement?
That was very common during that era when all the rectifier tubes had a directly heated cathode. IOW, the filament is also the cathode. If you take the B+ from either end of the filament (as we normally do with a 5Y3 or 5U4) you will have 5VAC riding along with the pulsing B+. No big deal because the big filter caps we use today just filter out that 5VAC component while smoothing the 120Hz B+ ripple. Long time ago it was common practice to use a 5VAC center tapped filament winding and take the B+ from the center tap. This arrangement cancelled the 5VAC component from 'riding' along with the B+ pulses. Now those small 4µF filter caps don't have to worry about removing the 5VAC component.
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Ahha, that makes perfect sense. Thanks so much for that. I appreciate it.
One other thing I noticed was the unused 2.5v filament winding. I looked at a couple different but similar early 30s amp circuits that use push pull 47s. All of them used two 2.5v windings — one for the 47s which don't have cathodes (just 1.75a heaters) and one for the other tubes.
In my case one of those windings is unused, and so a single winding with a center tap elevated 400 ohms over ground is powering them all.
I'm not familiar with the weird 47 tube topology and that just seems strange to me ... but I'm sure there will be a lot of things I haven't seen before as I go through the amp and trace out the circuit.
Thanks again for the help Sluckey.
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... the 47s which don't have cathodes (just 1.75a heaters)
I'm not familiar with the weird 47 tube topology and that just seems strange to me
The 47 has a special coating on the filament which will emit electrons when the filament is heated. Thus the filament is also the cathode. The same filament coating is used in that 5Z3 rectifier tube.
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Those 47s must get crazy hot. 1.75A heaters, wow.
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Those 47s must get crazy hot. 1.75A heaters, wow.
At 2.5V. 2.5V*1.75A = 4.375 Watts
6F6 == 6.3V*0.7A = 4.41 Watts
EL84 == 6.3V*0.76A = 4.8 Watts
36L6 == 35V*0.15A = 5.25 Watts
Yes, the 6V6 is thriftier for more plate power.
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Hi all,
I spent the morning working on a schematic. Sorry for the crudity; it took a while.
I was wondering if anyone could give this schematic a once-over and let me know how it looks.
Also: For all I know the tubes named aren't correct; there were no ID markers on the sockets. But from the circuit they seem to add up.
The lines are very light; open a full size version if possible. If still too small, I can email one, etc. Attached are PDF and JPEG versions.
I really appreciate the help!
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Hi all,
I just tried to post a reply to my earlier thread with a jpg schematic I drew up this morning. When I tried to post the site locked up and now the old thread appears dead. Weird, sorry.
So, I'm going to start a new thread.
Attached is the schematic I (very crudely) drew up this morning. How is it looking to everyone? I could use a few sets of eyes to see if it all makes sense. I appreciate it!
EDIT... EL34 fixed this thread and I merged this message with the original thread... sluckey
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That drawing is very hard on my eyes. It's a HUGE pic for the little amount of circuitry. By the time I expand the view to be able to see the lines clearly I can only see a tiny portion of the drawing and I no longer feel I'm looking at a schematic. I need a monitor the size of my house! But when I go to the bedroom to look at one portion, I can no longer see the part that's in the kitchen. Might be a good pic for the IMAX! :icon_biggrin:
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You need to reduce the size of the images before posting them
I took your image and reduce it to 1024 x 683 and attached it to this post
Read this forum post on image sizing
https://el34world.com/Forum/index.php?topic=16961.0
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In the original drawing I missed the 57 cathode resistor ... found it when I started recapping. Attached is a better, clearer schematic.
Any thoughts? Is this a paraphrase phase inverter? I would love to learn about this. The amp was gone through A LOT, pretty much all caps/resistors replaced probably in the 60s, and I mainly want to find out, before powering up, if there are any circuit issues. Thank you!
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Yes, paraphase inverter. Use a light bulb limiter when you first turn it on.
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I don't have one, but I run stuff through a Sencore isolation transformer/variac/analyzer, set to read current on the meter. I usually start at 0VAC, monitor the current and slowly turn up if it doesn't shoot high. Good enough?
Thanks for reaching out. See anything obviously wrong?
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Are you sure about the connections of the 50K on the 57 tube?
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I don't know if it's correct, but it's the way I received it; see pix. It's the two 100K resistors paralleled. The red dot is pin one; the 50K goes from pin 3 to pins 4 and 5, which are jumpered. Thanks!
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Are you sure about the connections of the 50K on the 57 tube?
The drawing is confusing.
It "could" be reasonable to drop G2 down to 1/6th of B+ in a voltage gain stage (no real current needed).
If I was working on amps of this vintage, I'd want a collection of period schematics to compare to.
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Apologies for my sucky drawing skills. It’s probably not presented in the clearest way possible, but it is accurate to how the amp came to me.
The amp was recapped/resistored a long time ago and simply because most parts are not original, I don’t want to trust that everything I see is correct… it’s possible it isn’t.
Yeah, I agree on comparing circuits.
I’ve been poring over the internet for days trying to find anything similar, but haven’t really gotten very close. There’s very little info online about these early amps. I’m going to keep looking … I’m in no hurry to power it up.
Thanks.
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Assume it "is" Mystery Amp #2 (http://prewaramps.org/media/m2schematic.JPG), with different tubes and values doing the same thing. Pentode mic amp, twin triode, push-pull power tubes.
http://prewaramps.org/schematics.htm
This drawing reads clearly (except the smudged "." in the 2nd cathode network). Because different small pentode, it seems not to have a divider for G2, only a dropper (and an anonymous cap).
It does have the same 500k:15: divider for the phase inverter.
It omits the added triode input, and of course power cathode resistor and hookup changed to suit the power tubes. '53 became 6A6 which became 6N7, so all the same.
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Thanks for this.
I'm going to rework my schematic to try and match the conventional drawing scheme. This is a big help ....
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I re-worked the schematic, but the phase inverter looks different. I double and tripled checked the wiring vs. the amp and the original schematic I used, and they match.
Can anyone give me their input on this circuit as drawn? {see next message-- PRR}
Thank you!
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PLEASE DISREGARD above schematic.
Here is an updated schematic. I realized I had just inverted the two halves of the 53 tube in the most recent schematic. I fixed them. Here is a clearer version of the schematic. Hopefully I got it right. Can anyone give it a set of eyes?
Thanks.
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I replaced the output transformer earlier today (10K primary impedance into 4 ohms) and just finished powering up the amp for the first time. It plays, with issues.
On initial startup it's about as loud as I would expect it to be. After about a minute of playing, the volume suddenly drops very low and it gets flubby and distorted. Ive seen this before in bad caps, but all caps and resistors are new. Possible gassy tube somewhere? They tested good without excessive gas (I have a good tester) but I don't know what else would cause that.
It's drawing less than an amp at 110VAC wall voltage, so it's running cool.
Here are the voltages I came up with. Any thoughts on dividing and conquering this?
Also wondering about the 18ish DC volts on all the heaters. Does this stem from the 47s using the heaters as cathodes?
Attached is an updated schematic. Thank you for your help!
Ted
EDIT: All solder joints reflowed, tubes securely seated, chopsticking didn't reveal any intermittents, etc.
Wall voltage: 110VAC
HV secondary AC 375-0-375
B+ 412
B+1 395
B+2 320
Heater AC: 2.5VAC
47-1
Plate 317
G1
G2 316
Heaters/cathode: 17.8DC
47-2
Plate 315
G1 -
G2 316
Heaters/cathode 17.8DC
53
Plate 1 (pin 2) 155
Grid 2 (pin 3) -
Plate 2 (pin 6) 74
Grid 2 (pin 5) 2.6
Shared cathode: 4
Heaters: 17.6 DC
57
Plate 100
G2 40
G3 1.4
Cathode 1.4
Heaters 17.8 DC
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Possible gassy tube somewhere?
Also wondering about the 18ish DC volts on all the heaters. Does this stem from the 47s using the heaters as cathodes?
90 year old tubes have to be suspect number 1.
Notice the 270Ω resistor on the filament center tap? That's the cathode resistor for the 47s. So the entire filament string is elevated. If you want an accurate filament voltage reading for each tube then connect your two meter probes to the two tube filament pins.
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I was thinking tubes, Ive had old radios cut out on me after a minute of power and it turned out to be gassy or otherwise bad tubes. These ones test strong though.
I don't exactly have a huge pile of 53s, 57s and 47s laying around, but I have a friend two towns over who has about 100 early 30s radios. I asked him to raid his collection and pull the stuff I need. I just want to eliminate them as variables.
Thanks!
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Just got a clue.
The first grid of the top 47 jumps from nothing up to 200V when the power cuts out.
That pin could have voltage on it if the .1uf from the second 53 plate (pin 6) was bad. I replaced it though and still the same issue.
So looking at a phase inverter issue but, where should I start?
Thanks.
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Found the issues!
First, the first 47 must have had an internal short that only appeared when it got hot. I found ONE crusty old smoke stained 47 in my stash, gave it a quick test and now the amp is stable with lots of volume.
ONly problem left was/is the incredible distortion i was getting even at low volumes. Put the speaker back in the cabinet and now it's mostly gone, clear tone! The speaker had been reconed at some point. Remains to be seen if it will need another recone job but it seems to have all but removed the distortion!
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The pot at the input won't be 5k or wired that way.
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Yeah, I switched it out fir a 500k. At 5k it was doing next to nothing.
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For the sake of completeness, here's an update.
Things have changed with the amp over the past day or so.
I initially thought it was a Type II, the second amp Rickenbacher made. At least that's what it was sold as. Anyway, I couldn't find any definitive info that pointed 100 percent to it being a Type II because it also bore some hallmarks of a Type I, the first commercially sold guitar amp ever. The info just wasn't there to be able to make a call either way.
Yesterday I found a collector in Detroit who actually has a Type I .. the first actual one I've seen after a ton of looking. He's somewhat of an expert on early Rics. Looking at his photos and asking questions filled in a lot of blanks, and for a few reasons I think I can now pretty definitively say that it's a Type I, though it differs from his with a different preamp tube (his is a 56) and his also has an interstage transformer instead of a tube and PI circuit, as mine does. Due to some stuff on the chassis, the chassis itself and the cabinet construction, it appears according to him to be a later stage Type I, just before the Type II was released, and dates to late 1932 probably. I agree with him, stuff that can be quantified and verified now point to a Type I, right on the transition into Type IIs. Very exciting.
My friend the owner's goal was to get the circuit back to as much originality as possible, and I think now I'm about as close as I can get. Yesterday I:
1. Removed the power switch as the original had none. No problem as it will always run through a switched bucking transformer anyway.
2. Removed one of the two jacks, as the original only had one.
3. Removed the volume pot. It's not original either.
Here's an updated schematic, with voltages.