Hello forum members, as you may or may not know, I am a fairly new member, and I am beginning to be more active on the site. Anyways, about two years ago I purchased a 1950's Danelectro re-branded Montgomery Wards Airline 8511 guitar amp in working condition with brand new Sprague Atom electrolytic capacitors and good NOS tubes. So far, I have been very happy with the tone from this bargin-bin tube amp, but at the moment, I am wondering if there are any ways I could possibly increase the gain of the preamp, which features a single 12au6 feeding into a 50c5 output tube. (Before I raise any red-flags about transformerless, non-isolated power-supplies, I have installed an isolation transformer and a three prong cord) I have tried NOS 12aw6's as a substitute for supposedly more gain or sensitivity, but I am happy with the tone I am getting from my Telefunken 12au6 I have in the amp right now. Attached to this post is a picture of the amp's original schematic, and I was wondering if any of you more experienced members may have any suggestions as to how I may increase the gain of the 12au6 stage. Thanks. Oh, and sorry for the huge image.
First I would check all of the film or paper capacitors, making sure they're good and not leaky. Probably easier to just replace the 4 of them. Check the voltages listed on the schematic. After that, you could insert a 6C4 before the 12AU6 to get more gain, but you might not get any more sound. With a SE 50C5 you're only gonna get about 2 watts out. A 6C4 is half of a 12AU7.
The amp checks out 100% fine except for the filament voltages are a small percentage high due to the fact that the wall voltage has increased since the 50's, but the caps and resistors all check out just fine. Also, yes it is indeed a very simple minimalist circuit which utilizes a 12au6, a tube not truly suited for audio, but I am simply wondering if any component additions or value changes could increase gain. I'm also open to tube substitutions. Due to the fact that the amp is 100% percent original besides filter caps and tubes, I would prefer not to modify it excessively by changing the general circuit or adding tubes. I have been using clean boosts in front of it, but I simply would like to increase the max gain. And I don't expect more output, as I am already getting into the clipping range, I just want even more gain and distortion. Thanks for the suggestions.
It's a 12AU6 pentode, not a 12AU7 twin triode, and it's grid leak, AKA contact, biased. Notice the huge grid to ground resistance value.
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bparnell57,
1st, this thread belongs on the musical instruments "board".
In order to get more gain out of the pentode, it's necessary to increase the value of the tube's plate loading resistor. That, in turn, requires a "taller" B+ rail. The 50C5 O/P tube can't deal with such a B+ rail.
My suggestion is you leave well enough alone.
Moving to Instruments & Amps forum per forum policy.Okay, first off, sorry for posting in the wrong forum, and thanks for correcting that kevinkr, and also, I see that I am unable to raise the b+, but do you know of any tube substitutions that I could make other than other 12au6's or 12aw6's. I was debating putting in a 12 volt to 6 volt filament transformer just for this tube so that I could use a 6hs6 or a 6ah6, which I believe both of those are possible subs with higher possible gain and sensitivity in the same circuit, but are unavailable in 12 volt filament versions. Thanks again.
If you install a filament transformer to provide for a change in voltage amplifier pentode, you will have add additional resistance to the series heater string. Ohm's Law tells us to use an 84 Ω part. Given today's higher average AC mains voltages, I'd use a tad larger value. A 5 W. rated part will be satisfactory.
Oh, that is true, I must account for the resistance of the 12 volt winding, now, would the new effective resistance/voltage dropped be 12 volts plus whatever the resistance of the winding would drop, or are those technically in parallel? Also, the harmony H400, a very similar amp, uses a 12v to 12v filament transformer on it's 12au6 with now dropping resistor whatsoever, so if the resistance of said transformer's winding was sufficient, I wouldn't need a dropping resistor, as the winding would dissipate that energy?
You have a 150 mA. series heater string. If you put a transformer in place of the 12AU6, 1.89 W. (Volt-Amperes) are available to energize anything and that computation doesn't account for the inevitable insertion losses.
Stop thinking guitar and start thinking Physics and engineering. In a series circuit, the same current flows everywhere.
Stop thinking guitar and start thinking Physics and engineering. In a series circuit, the same current flows everywhere.
First off, the truth is that the current is rarely even as low as 150 mA on these tubes, and it varies greatly by brand and production run, with probably about + or - 15%. Second, I understand how the current is flowing in the series-filament wiring in this amp, as I have used this topology in designs before that functioned flawlessly, and third, I actually am thinking of gain, which leads to distortion, which is related back to GUITAR. Just because I am a newcomer who happens to be one of the incredibly rare 16 year olds that has self taught themself enough about vacuum tube technology and circuit design to design an amp from scratch with a few datasheets and a calculator, doesn't mean I know everything about this. I don't need to be told how to think, I just came here for advice. Thanks for your input, and if you meant no-offense, consider it taken. Now, back to the discussion, in order to account for losses, do you believe that I should lower the value of the series resistor? Also, how does the winding's resistance not become a factor in this circuit? I know ohms law, but still... Thanks
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You are searching something that this amp is unable to provide in any way. A front distortion or booster is the best solution, indeed : it simply gives this amp the additional required preamp stages that it has not, along with pre-clipping if needed.
Any tweaking to the circuit to increase gain will be unsatisfactory and nearly useless in that field. Moreover, it will destroy the original condition of that vintage Valco/Supro/Harmony/Silvertone kind of amp, which works flawlessly as you wrote it.
If it is not already done, I suggest you to use a separate mains insulation transformer for safety...
... So you can consider my advice above about your small amp... If you are willing to :
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A+!
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Okay, thanks for your input. I guess I am faced with the fact that there is basically nothing I can do, but that explains why my efforts to find any suggestions to increase gain have been futile. Also, I accept your advice. And I will say that those are some very nice amps that you have built. Thanks to everyone for their input, if it wasn't for all of you, I would still be looking for a nonexistent way to increase gain.
Unfortunately, in gain increasement : yes, nothing to do.
The 12AU6 is at its maximum possibilites here :
- No Rk, No Ck, grid leak bias : maximum gain has been already expected via this solution.
- R3 = 390K, this is quite high. Try 470K or 680K, even more, but you won't notice significative gain improvement because of the plate voltage drop, and it may be the opposite.
Note that you may then hear more "dirtyness" in the sound, and if you go further in R3's value, leading finally to distortion, yes, but which will be awful... Is it what you expect ?
- Trying to increase screen voltage will increase plate current so the plate voltage will drop. Again, no significative gain improvement should be expected there.
Moreover, look at C7 5-80pF variable cap. You may know (or have discovered) that or not, but this capacitor is used to compensate residual 60Hz hum in the amp, so you may have to re-adjust it for better hum results. So modifying the screen supply may have consequences on the residual hum.
IMHO, your small amp ought to stay as original, flawlessly working as possible. And if its cosmetic condition is good to mint, it has certainly a collector value...
A+!
A+!
Yes, I realize that the variable capacitor is used to compensate for the residual 60 Hz hum, but with my adjustments I have noticed little change? Without an oscilloscope handy, how can I adjust this to be in a relatively good capacitance to correct the hum? Would putting a volt meter set to AC at the speaker terminals with no guitar plugged in be suitable, or would I do it by ear, or is an oscilloscope a must? Thanks again for all of your advice, I will continue to use my clean boost and dirt pedals as a source of gain. I will not change R3 as I am happy with the harmonic content and distortion present in the amp. Thanks for all the advice.
If you have a FLUKE 87-III or V measuring in TRMS AC millivolts, have a try, but I think it won't give you a good result.
combining both is probably the best... I would make a test with a headphone set connected instead of the speaker, in order to magnify the hum, and so be able to adjust it more easily. That's the method that I use for adjusting the filament hum balance on vintage Fenders, with complete success. See if you need to short to GND the input of the amp or not.
It is possible by construction, though... Nonetehless, I would have a look to these points :
1 - the tubes used in the amp have filament-to-cathode hum. The 50C5 is known to be subject to that issue, and this often occurs in serial filament heating wiring scheme...
2 - HV filter caps are not-so-fresh, there is no HV choke coil...
3 - shielding is approximative, there is a magnetic induction between power and output transformer...
4 - GND scheme is bad or absent...
And maybe nothing really better can be expected in term of hum, the amp being built as is...
A+!
I understand that the tubes have internal hum, but this is not relevant to the variable cap as this is intended to introduce 60hz AC at a phase shift of exactly one half of a period, thus canceling out the hum if at exactly the same amplitude. I just did it by ear with the speaker and the hum is quite low even at full volume with my hands on the strings or chassis, but with them off, it is quite bad. I believe the outlet I am testing it through right now is wired backwards, so this may be the issue, as I cannot flip the 3 prong cord. Thanks again for all of your help guys.
Have you replaced the supply electrolytics? If not any assessment of the amplifier's hum and noise is pointless as those caps are now at some small fraction of their original value, miracle that they are working at all.
As I said, they were replaced about 2 years ago with Sprague Atoms, and they are still going strong. Speaking of which, talk about miracles, I just purchased a 1960's Eico multi-meter and the original C cell battery which is soldered in still tests a 1.49 volts and still works accurately, that battery is old enough to say "radio A battery" on it... talk about a miracle... and speaking of electrolytics, I have a 1952 bell 2122B mono hifi amp, with a pair of 6V6's at 12 watts, with original caps, which are "dry" electrolytics from sangamo, and it is 99% humless at full volume on the phono inputs with the added filter choke I added when I purchased it... I guess those dont dry out, huh... 😛
I picked up a Silvertone 1451 on ebay a few months ago and it appears to be nearly identical. My adjustable cap does nothing as confirmed via my ears and scope. Could be broke.
Changed the original dual filter cap and will do the 50c5 cathode cap. Picked up a bunch of (alleged) NOS USA-made tubes which are dirt cheap. Still need to install the iso tranny and put it back together. I got a Weber Signature which is just about the only 4ohm 6" I could find.
I also wanted more gain but I measured the actual amplification factor of the preamp tube and it was about 95! I might increase the cathode resistor on the power tube to see if I can get it to cold-clip. A clean boost may be the only way for any real distortion. A very cool amp either way. Vintage 1959 gold sparkle. 😎
Changed the original dual filter cap and will do the 50c5 cathode cap. Picked up a bunch of (alleged) NOS USA-made tubes which are dirt cheap. Still need to install the iso tranny and put it back together. I got a Weber Signature which is just about the only 4ohm 6" I could find.
I also wanted more gain but I measured the actual amplification factor of the preamp tube and it was about 95! I might increase the cathode resistor on the power tube to see if I can get it to cold-clip. A clean boost may be the only way for any real distortion. A very cool amp either way. Vintage 1959 gold sparkle. 😎
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