Jan 16, 2018 The tube layout doesn't match the Gibson schematic. This amp only has 4 tubes. (2) 6AQ5's, (1) 6C4, & (1) I don't know but a 12AX7 fits. Any help would be appreciated. I plan to get some 6AQ5's and a new 6C4, but I'm not sure about the 4th tube. Any other insight is appreciated as far as recommended cap replacements. Click the link above for Tube amp info, Schematics, Board building information, Projects, Mods, Transformer diagrams, Photo's, Sound clips. There are hundreds of pages of Tube amp information on my library page.
This is the schematic of the amp to which I referI'm astonisched about the really high value of the plate resistors of the 12ax7 tubea high value gives a high gain amplification stage, but I'm confused about, didn't remember to have seen so high values on a 12ax7 tubeSomeone has an idea about this unusual choice? Which can be the reason for that on this circuit??ThanksFrancop.s.: If someone has a link to a demo of THIS model of GA-5T with the 6bm8 tube I'll be glad to have it, thanks again(I was thinking I listened to some demo, but now I'm not so sure, Gibson build more than one version of the GA-5T).
AM radios mostly used 470K to maximize gain, allowing the treble to fall short of bass-mids. You don't want extended treble in an AM radio (anything over 3KHz is mostly static hiss/crash), you do want (in the US.) to bring-up weak stations so the buyer thinks it is a good radio.Gibson apparently took this line of design.(.) I might want to listen to Boston or NYC, 400-500 miles away. In Europe, long-range is less important, because so many different countries and languages are so close together.
![Skylark Skylark](http://protoboards.theshoppe.com/images/amps/GA_5T.jpg)
I remember the tube radio set of my grandparents. The loss of highs and distortion added warmth, especially from speech. Luther Perkins guitar sound was jumping out of the set.If you are worried about high frequency loss, you can follow Gibson idea of not following their schematic and it might just sound like a Gibson. Maybe bring down the value of both 470k plate resistors to 220k and bypass the volume pot with 150pf. If you have an ECL82 with transformers, you could always euro-gibsonize with a single EF86 up front and keep the tremolo circuit as is without negative feedback. How do you guys insert a quote in a middle of a post?At 220k/2k2 you can estimate the plate current to be around a milliamp for both sections so, 10k instead of 22k will only bring up your available preamp supply around 10 volts. You might be happy with both 220k/2k2 sections.
If you end-up with too much white noise, you can bring it down to 100k/1k5 like a Fender Champ or reduce the feedback resistor a little. There's a possibility that the 0.05uF bypass capacitor in the LFO circuit might not be enough to prevent speaker thumping if you are using a bigger speaker than a 8 inch. I was wondering why aiming for a specific preamp plate voltage regardless of the plate load.
But.In case you accept your first posted schematic with a 220k/2k2 change in both 12ax7 sections, I would expect a voltage drop around 100-110V across the 220k plate load. A voltage drop of around 20 volts across the 22k and 12 volts across 1k.So, if you start with 256V B+ you will end-up with 224v for your 12ax7 preamp supply and about 110V on the plates. This is the average «first stage» preamp supply for 50's Gibson amps and many others. Maybe darker than Fender tone-wise, but with a 150 picofarad bypass volume cap and some 6 inches speakers I would not worry so much about darkness.However, if you insist, you can gain about 20 volts for your preamp supply and better overall filtering if you insert a choke BEFORE the plate supply. I'm talking about a small 3-4H, 75ma, 100 ohm choke. Same type used to filter the screens in a Twin, bandmaster etc.
But it will be used to filter the whole amp. First node: no connection.
After the choke, you feed the OPT and the LFO. Then, you will still have that 1k resistor that will feed the screen and both 12AX7 sections.
3 filter capacitors, all 40µF caps. This will also get rid of the majority of single-ended hum. However, single-ended hum may not be a big problem with 6' speakers. For this reason and an economical one, I would return to the first option. I think a B+ around 266v.(at least 256v).
On the GA-5T B+ is around 278v?? 278V, 256V- what is the difference?Yes, 22V or 8.5% on voltage and 18% on power. So?If a GA-5T would do the job, a 256V GA-5T should do the same job, or close-enough that the audience won't laugh you off the stage.
I don't think the tone will be very different (and could maybe improve). Tube life is slightly better. You can't get an exact copy of the GA-5T's speaker, and even they varied 8.5% unit-to-unit and over the decades (even Alnico fades, slowly). Put a 'hot' speaker in it (or rig a Full Stack) and you will never miss the 22 Volts.