My First Tube Amp: Check my work?

[Wigwam Jones]

I have to ask a favor again. I want to be sure I've got this right before I start to solder it up. Please be patient, this will be my first tube amp, and it's very basic, but I need it for a confidence-booster before I embark on the 'real thing'.

I don't care if it sounds good or bad, as long as it makes sound. I'm just going to build it on a slab of wood and test it, take some photos and then take it apart again. I should be able to re-use most of the components at some point; so this is in the nature of an experiment.

Here's the schematic:


amp_diagram by Wigwam Jones, on Flickr

I got this from "Practical Amplifier Diagrams," by Jack Robin and Chester Lipman, 1947. It's a one-watt, one-channel design using a single tube (117NGT), which is a diode (rectifier) and power pentode in one envelope.

First of all, I know this is a transformerless design, which is inherently dangerous. Rather than risk any problems, I bought a Triad N68X Isolation transformer, which I intend to build into the circuit, along with a fuse.

This is the parts list. The naming conventions are not what is used currently, so I had asked some questions of the AK intelligencia in the past about what the current values should be. I then placed my order and I have the parts in-hand with the exception of the 1-meg potentiometer and the SPST switch, which should be along shortly.


parts_list by Wigwam Jones, on Flickr

So here are my actual parts, compared to the parts list above. Have I done this correctly?

C-1: Capacitor, Polyester Metalized, Radial, .047uf, 400V
C-2: Capacitor, Aluminum Electrolytic 10uf 50V 20% Temp .85 Radial
C-3: Capacitor, Polyester Film .022uf Tol 10% 400VDC Radial
C-4, C-5: Capacitor, Aluminum Electrolytic 47uf ED Type A 200V
C-6: Capacitor, Ceramic Disc, General Purpose 20% .0050uf 1000V

R-1: 1 Meg Ohm Volume Control (not yet obtained, will get from Apex, Jr).
R-2: Resistor, Carbon Film 100 Ohms, 1 watt 5% Radial
R-3: Resistor, Carbon Film 1 Kilohms 5% Radial Car-Core
J: RCA jack (salvaged)
SW: SPST 115v switch (not yet obtained, will get from Apex, Jr).
T: 3000 ohms output transformer (salvaged from old Reel-to-Reel tape player)

I am using a 117P7GT tube in place of the 117N7GT that the diagram suggests; I did not see any difference in the tube specs and I got 6 NOS 117PGT tubes for 99 cents, so yeah, I went for it. I am also using an octal socket that has screw-down terminals for each of the pins to make it easy for me to take this apart again later.

Bearing in mind that this is not intended to be 'high fidelity' in any way. Just to pass sound in one end and have it come out the other amplified and relatively unmolested.

I would appreciate any advice or corrections you might offer. I am not looking for perfection, but I'd prefer not to set the house on fire by doing something totally stupid like using the wrong kind of capacitor or something.

Thank you!

 

[GordonW]

Tube is fine. The 'P7 is listed as a direct sub for the 'L7, on NJ7P tube database.

The only problem with your component values is in C3. It asks for a .08uf. You have a .022uf. I'd use either a .068uf or a .1uf there. Either should work fine.

Or, if you have one, you can just parallel a .047uf 400v cap with the .022 you got. That will be close enough.

Other than that, it looks pretty much fine from here...

Regards,
Gordon.

 

[Wigwam Jones]

Tube is fine. The 'P7 is listed as a direct sub for the 'L7, on NJ7P tube database.

The only problem with your component values is in C3. It asks for a .08uf. You have a .022uf. I'd use either a .068uf or a .1uf there. Either should work fine.

Or, if you have one, you can just parallel a .047uf 400v cap with the .022 you got. That will be close enough.

Other than that, it looks pretty much fine from here...

Regards,
Gordon.

Thanks, Gordon! I'm sorry that screen-shot is so hard to read - I believe the actual value for C-3 is ".02 mfd 400v paper cond." It *does* look like a .08 on here, doesn't it? Sorry about that! So a .022 uf would be OK, yes?

And thank you once more. You've been a big help to me and I appreciate it!

 

[Dadbar]

Maybe this is so obvious.....you ought to put in a fuse. I would also be uncomfortable having the input ground being one simple cap away from the input AC power line. Use one of those caps for C1 rated to fail open.

 

[Wigwam Jones]

Maybe this is so obvious.....you ought to put in a fuse. I would also be uncomfortable having the input ground being one simple cap away from the input AC power line. Use one of those caps for C1 rated to fail open.

I intend to insert a fuse and an isolation transformer. I may not have said that clearly enough; if so, my apologies. I agree that safety is paramount!

 

[gadget73]

I'd also stick a cap, maybe 0.01uf between the input jack and R1 to block any possible DC coming in from your input source. it would also serve to protect the source if the tube went funky and developed grid leakage.

 

[Wigwam Jones]

I'd also stick a cap, maybe 0.01uf between the input jack and R1 to block any possible DC coming in from your input source. it would also serve to protect the source if the tube went funky and developed grid leakage.

Thanks! I want to be clear though; do you mean to stick the cap in-line between the RCA input and R1 (the volume pot)? Or would this be placed between them and bleed to chassis ground? Any particular type or voltage carrying capacity you'd recommend for the cap?

And just FYI, I don't intend for this amp to be a permanent fixture, nor to hook it up to anything vital. I just want to prove to myself that I can indeed put together a tube amp and have it amplify sound and not sound too much like trash can lids being slammed together. Then I'll take it apart again and start on my 'real' tube amp building.

I tend to start REALLY slow with things I am not comfortable with...

 

[larryderouin]

Put it INLINE. You want it to block any potential DC from the amp backing up into your source and possibly damaging it. No sense in damaging even an inexpensive piece of gear that's running ok.

Larry

 

[tri-comp]

Just a thought:
When you have 6 of them why not use a few in parallel?
The output transformer should handle 2 or 3 without problems at this power-level.
Of course the transformer has to have a gap in the core and come from a single-ended amp in the R-to-R from where it was salvaged.
May give you around 3 watts of output with 3 tubes and put less demand on the speaker-efficiency.

rgds,

/tri-comp

Tube-data attached

 

[Wigwam Jones]

Put it INLINE. You want it to block any potential DC from the amp backing up into your source and possibly damaging it. No sense in damaging even an inexpensive piece of gear that's running ok.

Thanks, I will take your advice. Should I worry about the type of capacitor or will any .01uf cap do the trick?

 

[Wigwam Jones]

Just a thought:
When you have 6 of them why not use a few in parallel?
The output transformer should handle 2 or 3 without problems at this power-level.
Of course the transformer has to have a gap in the core and come from a single-ended amp in the R-to-R from where it was salvaged.
May give you around 3 watts of output with 3 tubes and put less demand on the speaker-efficiency.

rgds,

/tri-comp

Tube-data attached

It's a very clever idea, but perhaps a bit too complicated for my first-ever build. However, I will still have the tubes after I'd done the first iteration. Nothing saying I can't do something like that a bit farther down the road. Thanks!

 

[larryderouin]

.01cap for this build anything of that value over 200V in your stash will be fine.

 

[Wigwam Jones]

.01cap for this build anything of that value over 200V in your stash will be fine.

Thanks!

 

[Squidward]

C-1: Capacitor, Polyester Metalized, Radial, .047uf, 400V
C-2: Capacitor, Aluminum Electrolytic 10uf 50V 20% Temp .85 Radial
C-3: Capacitor, Polyester Film .022uf Tol 10% 400VDC Radial
C-4, C-5: Capacitor, Aluminum Electrolytic 47uf ED Type A 200V
C-6: Capacitor, Ceramic Disc, General Purpose 20% .0050uf 1000V

Sounds good, if I were shooting for "perfection" I'd probably upgrade C6 to some high-voltage film cap.

R-1: 1 Meg Ohm Volume Control (not yet obtained, will get from Apex, Jr).
R-2: Resistor, Carbon Film 100 Ohms, 1 watt 5% Radial
R-3: Resistor, Carbon Film 1 Kilohms 5% Radial Car-Core
J: RCA jack (salvaged)
SW: SPST 115v switch (not yet obtained, will get from Apex, Jr).
T: 3000 ohms output transformer (salvaged from old Reel-to-Reel tape player)

I find it hard to believe that you can salvage an output transformer and an RCA jack from an old R2R, but you can't find a switch on it anywhere to sub in for the SPST. I've often gotten fuse holders from old R2Rs, as well. Salvaging is a fun part of the experience. Just don't ask my wife why the basement is full of half disassembled chassis!

Looks good so far!

 

[Wigwam Jones]

Sounds good, if I were shooting for "perfection" I'd probably upgrade C6 to some high-voltage film cap.

Agreed. As soon as I've actually proven to myself that I can do this, I intend to move on to amp #2, which I believe will be the stereo 50C5 I've been bugging people with questions about. I will re-use the isolation transformer, but I'll buy some higher-quality parts for it.

I find it hard to believe that you can salvage an output transformer and an RCA jack from an old R2R, but you can't find a switch on it anywhere to sub in for the SPST. I've often gotten fuse holders from old R2Rs, as well. Salvaging is a fun part of the experience. Just don't ask my wife why the basement is full of half disassembled chassis!

I will look again! I was thinking about the rotary on-off volume switch that is on it, but a) it looks pretty well buried in the chassis and b) I did not know what the value of the pot was. How can I tell, do you know?

Looks good so far!

Thanks! I was wondering if the half-wave rectifier is going to introduce too much DC on the isolation transformer and possibly damage it. I thought about placing a SS bridge rectifier in back of the transformer to protect it, then a dropping resistor to get the voltage back down. What do you think?

 

[mfhale]

I will look again! I was thinking about the rotary on-off volume switch that is on it, but a) it looks pretty well buried in the chassis and b) I did not know what the value of the pot was. How can I tell, do you know?

I'd put an ohmmeter on it and turn the volume all the way up.

 

[Wigwam Jones]

I'd put an ohmmeter on it and turn the volume all the way up.

Thanks! If I can get it out I will do that (I am not sure I can; it appears to be an early version of surface-mounting and it is spot-welded to the chassis).

 

[Squidward]

I think your fears of blowing up the iso transformer with a half-wave rectification scheme are unfounded. You have a new transformer, and if you had problems you'd probably hear mechanical buzzing before anything fatal happened. I doubt you'll even have that!

You can make an iso transformer from a pair of identical transformers, such as a pair of 6 or 12 volt transformers from radio shack. Hook the low voltage secondaries together. When you plug in one primary, the other will be hot as well. Yes, you're using the secondary as a primary and vice-versa on one, but it works.

Trick question: Why does the pentode in the schematic only have four elements?

 

[Wigwam Jones]

I think your fears of blowing up the iso transformer with a half-wave rectification scheme are unfounded. You have a new transformer, and if you had problems you'd probably hear mechanical buzzing before anything fatal happened. I doubt you'll even have that!

Thanks! As you know, I'm a babe in the woods on this. I happened to read online about a fellow who was installing the same Isolation transformer on his guitar amps (the Triad I have) and he mentioned that half-wave rectification puts DC on the transformer and can cause overheating. So that's why I asked.

You can make an iso transformer from a pair of identical transformers, such as a pair of 6 or 12 volt transformers from radio shack. Hook the low voltage secondaries together. When you plug in one primary, the other will be hot as well. Yes, you're using the secondary as a primary and vice-versa on one, but it works.

Very cool! I went ahead and bought a Triad isolation transformer because I believe I'll be using it again and thought it would be handy to have, plus it cost 11 bucks as I recall. However, I will keep that advice handy!

Trick question: Why does the pentode in the schematic only have four elements?

Um, because it's a Beam Power Tetrode and not a Pentode? I may be insane, you know. Pentodes were created to solve the problem of secondary emissions (I have no idea what that means, I just Googled it).

So would this be a SET amp? (Single-ended Tetrode)? Yeah, I know, I know, SET means 'Triode' but hey, Tetrode starts with a T also. :yes:

 

[dcgillespie]

Squid is just messin' with you! All Beam Power tubes or Power Pentodes have five elements (besides the heater), but the third "grid" is not always shown on all schematics. The third grid is called a Suppressor Grid in a Power Pentode, or a Beam Forming Element in a Beam Power tube, and is usually internally connected to the cathode, as is the case with this tube. Both elements act to repell electrons back to the plate, that if knocked off the plate in the process of bombardment, might otherwise be attracted to the screen grid rather than back to the plate if these elements did not exist. Without these elements, you have a Power Tetrode, which were known for much less power output capability (because of electron flow lost to the screen grid), and generally quirky screen grid behavior when the plate voltage was dropped low with the application of signal.

Any references you have seen to the 117P7GT being a Beam Power Tetrode is a factual error. There can be no such thing as a Beam Power Tetrode because any Beam Power tube has five elements in it as described above. The 117P7GT contains a Beam Power pentode section.

As for your amp, since you are using an isolation transformer, you will want to eliminate C1, by just shorting it out -- particularly since you are adding a new .01 cap in series with the hot signal from the input jack to the volume control. C1 is there precisely because as shown, it is a "hot chassis" design, and is included to prevent a direct connection to the ground side of the pickup, and the metal tone arm it was likely installed in back in the day. With an isolation transformer, the capacitor is simply no longer necessary, and will likely add some level of hum if included.

Good luck with your project!

Dave