Still going with this – The amplifier has almost two years’ use on it now. I have located a 1 MegOhm pot for the Tone, and shall fit it as soon as it arrives.
My “final setup” in August 2018, with a 5AR4 rectifier, was as follows:
26 V Bucking transformer on mains input.
5AR4 Rectifier tube.
2 X 103 Ohm ¼ Watt resistors at 12AX7 filaments to ground.
500 K log pot + 500 nanoFarad and 47 picoFarad caps for the tone control and “treble bleed”.
2 x 1 kOhm 5 Watt dropping resistors in parallel between rectifier and B+ pin.
With an input voltage of 205 V AC at the fuse I have:
646 V AC on diode pins
Diode output before dropper resistors: 380 V DC
B+: 360V DC (40 mA) (12 WATT)
1 (6P6P PIN4 - GRID 2): 321 V DC
2 (CAP 3, AFTER 22 K RES): 279 V DC
3 (6P6P PIN 8 - CATHODE): 18.5 V DC ( 40 mA)
4 (12AX7B PIN 6 – ANODE/PLATE 1): 185 V DC
5 (12AX7B PIN 1 – ANODE/PLATE 2): 188 V DC
6 (12AX7B PIN 3 – CATHODE 2): 1.35 V DC
7 (12AX7B PIN 8 – CATHODE 1): 1.31 V DC
DIODE FILAMENT: 4.65 V AC
TUBES FILAMENT: 5.82 V AC
B+ Droppers are warm.
I have been using the amplifier with a 5U4GB rectifier tube, with B+ 348 V, and 17.8 V on pin 8 of the 6P6P tube. I need to find a 560 Ohm resistor to lower the 6P6P valve idle current some more.
I like the amplifier, nice clean, open low volume amplifier, but it cannot handle much overdrive, just gets to be a noisy mess. I am happy with the overall tone, but the lack of control when driven limits it’s use. I also find it to be unable to cleanly render chords without smearing the sound a bit. It seems to require a better balance between gain and volume. I do not use the “LOW” input at all – might as well remove it.
While I was playing around with speakers for the other amplifiers, I decided to force a 10 inch in this 8 inch cabinet to hear if it gives a slightly “bigger” sound.
A Celestion G10 Greenback is looking for a job, but, for me, with this amplifier and cabinet, it did not qualify. Compared to the Mod 8-20, it lacks “sparkle”. Sweet, full sound, but no Sparkle. Sad. Looking at the frequency response graphs of the speakers I have on hand, I decided to try the Celestion Tube 10 I removed from the Laney Cub 10. Supposed to be a “no-good cheap” item, but I needed to check it in here before I tried a “no-good cheap” Celestion Ten Thirty. Surprise. It has more sparkle than the Greenback. Well, not exactly “sparkle”, but more high-frequency output. It lacks bass.
New Baffle. Large hole for a small box.
The speaker is a tight fit… The Greenback looked monstrous in there, with it’s large magnet.
I cannot compare tone to the original Celestion 8-15 I started with before I got the Jensen Mod 8-20, that was a long time ago. The 8-15 was OK, but the Mod 8-20 is much better here. The Tube 10 does have a bigger sound than the Jensen Mod 8-20, slightly less sparkle. I would like to have a 10 inch speaker combining the Mod 8-20 sparkle and Greenback warmth. Looking at the Jensen Mod 8-20 frequency response curve, it has a dip in the midrange. As I play “clean” at low volume, this charateristic might contribute to why I like it: Boosted low and high frequencies, compensating for the lack of ear-response to those at low volumes?
I also tried the Celestion Ten-Thirty.
This is a surprise: Big sound. Huge sound. The little 5F1 now sounds like a real amplifier, full, dark, strong, smooth. No sparkle. The speaker does not lack in treble, twisting the tone knobs can supply more treble brightness than I should require, but pushing volume as well results in a harsh, hard, unpleasant treble. For playing the middle strings around the middle of the neck, treble and volume turned down a bit, very nice. I prefer a more sparkly, less harsh treble, but this speaker has potential here.
Another plus was being able to get some useful overdrive, but here is not much leeway here. Too much too soon, too little too late. Too loud then anyway.
As for the Jensen Mod 8-20, while I was making this baffle, I fittted it in my Marshall G15 RCD solid state amp, very nice, sounds better than the Celestion 8-15 “upgrade”. I initially bought the Celestion 8-15 for the 5F1 build, with a Celestion Ten-Thrity for the G15 RCD. Not much space for the Ten-Thirty in there, so I never did try it, but I might someday make a baffle and try it anyway. I was getting to be convinced that 8 inch speakers are just the ticket for low volume bedroom use, but his 10 inch experiment has proved that there is indeed more than one way to make noise.
The “to do” list for this project currently stands at:
- Fit the 1 MegOhm tone pot.
- Find a 560 Ohm 5 Watt resistor to replace the industry standard 470 Ohm bias resistor.
- Fit a master volume. This should help to get some overdrive at lower volume.
- Try lower gain pre-amp tubes. This might help to get better overdrive without making it all a mess.
- Find a way to fix the mains supply - B+ - filament voltage issue.
- It actually needs a total re-build with new filter caps and so on. And an extra “pre” filter cap to lower hum.
- Build an enclosure. Eventually. Works well as is.
8.
This would be an ideal “bedroom” amplifier if it had useable overdrive and a master volume. So, it looks as if this already modified F51 build might eventually turn out to be a 5F2A with master volume.