Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on September 08, 2015, 04:10:08 am
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Hi guys, Couldn't pass this oldie up.
I am only going by the date stamp on the choke. This amp does not look like a production amp, maybe purpose built.
I traced out the circuit and looks fairly simple.
Some of the circuit has been disconnected so I'll figure that out at a later date, so I was hoping someone might have a similar circuit to compare.
The valve lineup is old and looks super cool, but on delivery three of the valves were broken.
The 5Z3, one of the 2A3 and the 6D6 the top cap had come off.
I have found some replacements, but was concerned with how well matched the 2A3s should be as their construction is different the todays valves with cathodes.Thanks
(http://i761.photobucket.com/albums/xx254/Timbo-08/003%202_2.jpg) (http://s761.photobucket.com/user/Timbo-08/media/003%202_2.jpg.html)
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The top cap for the 6D6 is for the grid connection. You could just solder the grid wire to the post coming out of the tube. If you have the top cap, just fill it with epoxy and set it down on top of the tube, threading the post through the top cap. Solder the top cap to the post.
The 2A3s heater is also the cathode. That 1K/3W and 32µF on the heater winding center tap is the cathode resistor/cap for the 2A3s. Having a matched pair is just as important as having a matched pair in any push/pull amp.
Some of the old Hammond organ amps used 2A3s.
Your amp was meant to have a field coil speaker. The coil would probably be approx. 5K, 6K, 7.5K, (something in that range) and was connected parallel to the B+ line, IOW, a big load on the supply. You will likely need a 25W or 30W resistor to simulate that missing field coil. B+ will be high without it.
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The cathode network shown for "V2", if right, is wrong.
It seems to be a conglomeration of a gain/mix stage (which is needed), and a cathodyne (which is reasonable), but won't do much of either as-drawn.
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Thanks guys, Flying a bit blind and only going by what is there.
I think it will be a case of just finding a schem that has a similar circuit. :icon_biggrin:
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> ....a similar circuit.
Good luck.
Them is strange tubes. Only popular for a short while. The 6A6 common-cathode twin triode is awkward for anything except how V3 is done: push-pull. And that is an excellent driver for the 2A3s.
The 6D6 at V1 is a remote-cutoff RF/IF tube which may distort in audio uses (which may not be bad for guitar). 0.1u cathode cap seems tiny, 10uFd may do better for you.
I actually "can" make-out V2, *IF* the 8uFd does not go to ground. Then it is an unbiased cathodyne. The cathode will only come up (and plate come down) about 7V DC. However the drive into V3 only needs to be like 2V. So it will work, at unity gain. That gives input sensitivity at "PU" of about 2V, which is high, but perhaps in sight of OLD style (78 RPM, giant magnet) phono pickups. Then the sensitivity at V1 could be in the e-guitar zone. Oh wait, this type mixer has real loss, so more like PU=4V and MIC=0.1V.
_I_ (67 years later) would wire resistive mixer (or omit PU) and wire V2 as a paraphase (one side gain, the other side via a divider to give unity inverting).
http://www.valvewizard.co.uk/paraphase.html (http://www.valvewizard.co.uk/paraphase.html)
http://www.ampbooks.com/mobile/amplifier-calculators/paraphase/ (http://www.ampbooks.com/mobile/amplifier-calculators/paraphase/)
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Your a good man PRR, I found this and had a quick read.
http://home.earthlink.net/~jeremyepstein/pdf/sp15_2a3.pdf (http://home.earthlink.net/~jeremyepstein/pdf/sp15_2a3.pdf)
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BTW, I would not obsess about 2A3 matching in self-bias. Somewhere (RDH4?) there is a test with one 2A3 push-pull with one type 50. The 2A3 "is" an overgrown '50, very overgrown, so you could say a '50 is a half-lame 2A3. The difference of power and THD was not large. And since the *point* of this amp, today, is not monster power or super-clean "Fi", but old-skool charm, I say use any non-dead 2A3.
The "1K 3W" 2A3 cathode resistor looks small, unless the B+ is well below 300V (it may be). Book says 820r and that could be over 4W (use 10W).
To match-check 2A3: change cathode resistor to 2K (2W will do for test). Put in one tube, read cathode voltage. Put in other tube, repeat. If they are within 30% even 40% of each other, it will play fine.
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That "1948 Recording Amp" could be a root of your amp.
6N7 is a variant 6A6. Here it is done as a floating paraphase.
6J7 is a great audio tube and maybe what your amp should have at V1 instead of 6D6. (Though at guitar level and cleanliness it may not be "better".)
We see fine preamp values (for 6J7), a resistive mixer, reasonable decoupling. The dual chokes are an iron-age thing, today we'd favor one or none and beef-up the caps.
5Z3 is a 5U4 on a 4-pin base. I see a price of $20. Short-answer may be to adapt an octal socket to a 4-pin base and run 5U4 for now.
The 5T4 was a very fine rectifier, much better performance than 5U4; I dunno why 5U4 ever got popular. Sadly 5T4 is not readily available today. 5Z4 is also good, and I can score 5Z4 for $8. Check pinouts; I had a grocery-store amp which had been bodged three times as rectifiers failed and were replaced with other types.
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I missed an ebay 2A3 and the seller PM me and he has a few that he would sell me, I ask him if he had a match pair and looks like its a goer. He is a local bloke so cheep shipping. He also has the 6D6 and 5Z3 these also were cheep. Thanks
The cathode resistor on the 2A3s is actually about 25w, at the time of tracing this didn't look right and when I copied and pasted, that was what I copied but the 1k was probably just under 1k
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Some interesting circuits, still waiting for the new valves to arrive...................
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OK guys, Time to kick start this project.
PRR, your schem looks great and a good place to start.
I have gutted the chassis, leaving the valve sockets for the moment but looking to replace them.
I have done a test on the OT and it looks like 2ohms will reflect 5K P-P.
I have a field coil speaker that is 2ohms with 1500ohm coil.
The tube sockets look to be in good order, keep 'em or chuck 'em
I want to slide on tradition with this one, by keeping the outer look as original as possible and the insides as modern as possible with an eyelet board and all new components.
So when a future someone flips it over and sees the modern twist, they'll say "I didn't expect to see this." OR will they :dontknow:
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It's almost suicidal to restore a pre 1950 Aussie amp without redoing all wiring - so I'd be far less concerned to know that someone did all that restoration work 'on the inside'.
Radio and Hobbies, May 1939 had a 2A3 PP with 6C5 PP driver stage and 6C5 cathodyne. Then in June 1952 had similar configuration in Playmaster No.5. Note that the general configuration was quite common, and was in the Williamson amp.
Do you have an insulation resistance meter? The older the transformers and chokes get, the more concern there is that they can cope.
Ciao, Tim
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Thanks Tim, One of the boys found this http://i1255.photobucket.com/albums/hh629/joebog1/2A3_zpseltgfdgi.jpg (http://i1255.photobucket.com/albums/hh629/joebog1/2A3_zpseltgfdgi.jpg)
and think it will be a great help.
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As per the above schematic, I have copied the power supply, power amp and phase inverter.
The preamp is much the same as the original, with the 6A6 as a channel mixer.
Some component values will need to be tweaked.
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Hi guys, A small amount of progress.........
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TIMBO
Nice sockets and connections, but ...... don't you think it will be nice to paint that rust chassis ?
Franco
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don't you think it will be nice to paint that rust chassis ?
Can't speak for Timbo, but in the states we call that *rust*; patina, very sought after in some circles, Rat-Rods come to mind :icon_biggrin:
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Thanks guys.
K, I hope the "RUST" contains many years of collected MOJO and even a few poltergeist to add a bit of spirit.
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This is where it lives................
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Oh, I see, here some architect uses rusted iron parts on buildings (doors or other particoulars) and fix the action of the rust with a special treatment (not the standard rust converter) that didn't change the aspect but stops corrosion
Franco
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> architect uses rusted iron parts
I just saw a TV car-show. The guy took fresh new steel, hit it with the grinder, and brushed on magic acid. Instant rust! When the rust is just-ripe, he washed and dried it, shot clear spray over it. Bah.
My garden gate sagged. I needed BIG flat angle braces. I realized that a 16"x24" carpenter's framing square was cheaper than a 9"x9" angle bracket, and a lot more support. The steel square was cheap, the aluminum square cost much more. I got 4 cheep steel squares. They had paint so they would not rust in your truck, but exposed in the yard they are rusting-up nice and natural.
Sometimes you do want to fix-up and polish-up so it looks like 1948 was only yesterday. My neighbor is working on a 1948 Hudson boss-sedan which looks like 1950 (not perfect, but doesn't show 65+ years age). OTOH his 1936 Chevy had a tree growing through it for many decades. He put new seats over the tree-hole, new V-8 with mail-box air-scoop, and drives it often.
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Some progress...
A new circuit board
(http://i761.photobucket.com/albums/xx254/Timbo-08/007_6.jpg) (http://s761.photobucket.com/user/Timbo-08/media/007_6.jpg.html)
Some prewiring
(http://i761.photobucket.com/albums/xx254/Timbo-08/004%203_4.jpg) (http://s761.photobucket.com/user/Timbo-08/media/004%203_4.jpg.html)
A neat fit
(http://i761.photobucket.com/albums/xx254/Timbo-08/008_3.jpg) (http://s761.photobucket.com/user/Timbo-08/media/008_3.jpg.html)
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:thumbsup:
Those mustard caps looks very cool !
Franco
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Hi K, I debated about the mustards, as I had intended to use orange drops (with the idea that this amp may need all the help it can get)
The orange drops are a bit big and bulky.
The mustards were in the amp and look to be in good order.
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Hi guys, Some great results. :guitar1
Working off "sheet 4" this circuit works fine.
With nothing plugged in quiet as.
With guitar plugged in "guitar input", low volume with some hum but sounds good (couldn't try full volume as it is late)
With guitar plugged into "mic input" this also works well with increased volume compared to the other input.
Voltages look great, but not experienced with these valves I'm not sure of their happy voltage.
Dropping resistors values between A and B and B and C were a suck and see with a 22k between A and b and 1k between B and C my starting point. The 22k reduced to 2.2k to increase V1, V2 and V3 plate voltages.
According to the calculator I could be getting about 7w/tube, this could be a bit high.
Voltages "sheet 5"
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Hi guys, It seems that the voltages are a bit low on the 6A6 and 6D6 is there any reason not to do the PSU like this.
(http://i761.photobucket.com/albums/xx254/Timbo-08/1948%202A3%20AMPLIFIER.bmp.jpeg) (http://s761.photobucket.com/user/Timbo-08/media/1948%202A3%20AMPLIFIER.bmp.jpeg.html)
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Is the 20k 20w correct? in your 1st post appears in series after the socket, now it's in parallel?
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Hey Shooter, I am using this schem for the circuit as the original circuit is unknown.
(http://i1255.photobucket.com/albums/hh629/joebog1/2A3_zpseltgfdgi.jpg) (http://s1255.photobucket.com/user/joebog1/media/2A3_zpseltgfdgi.jpg.html)
But the original PSU was similar to what I drew in the previous post. Thanks
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Hi guys, I think the end is nigh for this one.
After having the field coil in series as per the RCA schematic I felt the current draw through it may have been excessive as the bell that covers the coil was warm to touch.
I have settled for the schematic below and adding the 10k dropping resistor put node "B" into the 360v range as per the RCA schem.
I feel that this is a good supply voltage for V1 and V2.
A couple of things I'm not sure of........
The difference between the plate and grid voltage on V1.
The plate voltages on V3 were originally similar but for some reason they have changed even when swopping tubes.
V1 and V2 have become microphonic.
Also there was a switch that was used to turn the mic off and on, this has made a great BOOST sw.
Missed the voltage on V3 cathode,5.54v
(http://i761.photobucket.com/albums/xx254/Timbo-08/1948%202A3%20AMPLIFIER_1.png) (http://s761.photobucket.com/user/Timbo-08/media/1948%202A3%20AMPLIFIER_1.png.html)
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A few pics.
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Got the plate voltages on V3 sorted by adding a 500k grid leak resistor to Pin3.
Shorted the 10k/2w resistor and have a much higher voltage for the PI and preamp.
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. My neighbor is working on a 1948 Hudson boss-sedan which looks like 1950 (not perfect, but doesn't show 65+ years age). OTOH his 1936 Chevy had a tree growing through it for many decades. He put new seats over the tree-hole, new V-8 with mail-box air-scoop, and drives it often.
Pictures PRR would be cool to see these cars.. ;-)