My 5 F1 kit build

modulator · 2017-11-01T14:05:29+00:00

5F1 BUILD PART 1 So my kit did arrive after a very long time. I think it would have been faster to order from overseas, and split the opinions about qualit...

My 5 F1 kit build

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OK, I have my head wrapped around this now. Immediate plan is to use a 241 to 223 VAC supply bucking transformer to raise filament voltages, and then decide on zeners to tune B+ as required. My stash of junk refuses to disgorge a 240-20 V transformer. The latest one is from an old UPS. Maybe time to buy a Variac.

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I installed JJ 6V6S and JJ 12AY7 tubes. It does make a difference. (The original 12AX7B tube caused a short “buzz” at startup, when the system came on song after about 7 seconds.)

I find the overdrive to be smoother and more pleasant, at the same volume settings. The amp is still too loud to play with overdrive, though, but the sound is now less “distortion” and more coherent at around ¾ volume. Still a mess at full blast. Not much difference in available volume.

I am also changing to a 240 VAC to 222 VAC bucking transformer for the AC supply. With this, I measure the following all over the amp:

5U4GB RECTIFIER + JJ 12AY7 AND JJ 6V6S TUBES
2 x 1KOHM 5 WATT DROPPING RESISTORS IN PARALLEL TO B+ (495 OHM DROPPER)
INPUT: 222 V AC
703 V AC ON DIODE PINS FROM POWER TRANSFORMER
22 V OVER DROPPER RESISTORS (44 mA)
B+: 377 V DC
CAP 2, AFTER 10 K RES (B+2): 322 V DC
CAP 3, AFTER 22 K RES (B+3) 264 V DC

6V6S PIN 3 – PLATE: 366 V DC
6V6S PIN4 - GRID 2: 322 V DC
6V6S PIN 5 – GRID 1: 10 mV
6V6S PIN 8 - CATHODE: 19.9 V DC ( 40 mA)
19 V OVER 470 OHM RESISTOR (40 mA)
6V6S IDLE DISSIPATION: 13.8 WATT

12AY7 PIN 6 – ANODE/PLATE 1: 132.5 V DC
12AY7 PIN 1 – ANODE/PLATE 2: 135 V DC
12AY7 PIN 8 – CATHODE 1: 1.85 V DC
12AY7 PIN 3 – CATHODE 2: 1.8 V DC

DIODE FILAMENT: 4.93 V AC
TUBES FILAMENT: 6.3 V AC

Could be the amplifier sounds brighter like this. Might also have less breakup at the same volume setting.

I have a “FENDER “CHAMP-AMP” LAYOUT MODEL 5F1” diagram indicating B+ as 340 V. It also shows 150 V at the 12AX7 plates (never had close to that). Not sure how this compares to the 135 V I now have for the 12AY7?

Now to “fix” B+.

Voltages sorted, I hope. To reduce B+, I snipped off one of the two 1000 Ohm droppers, leaving me with one 5 Watt 991 Ohm dropper resistor between the diode and the B+ terminal. I measure the following:

240 TO 222 VAC BUCKING TRANSFORMER
5U4GB RECTIFIER TUBE + JJ 12AY7 AND JJ 6V6S TUBES
1 KOHM 5 WATT DROPPING RESISTOR IN PARALLEL TO B+ ( 991 OHM)
DROP 40 V OVER 991 OHM DROPPER RESISTOR (40 mA, 1.6 WATT)
B+: 341 V DC
CAP 2, AFTER 10 K RES (B+2) 293 V DC
CAP 3, AFTER 22 K RES (B+3): 241 V DC
6V6S PIN 3 – PLATE: 330 V DC
6V6S PIN4 - GRID 2: 292 V DC
6V6S PIN 5 – GRID 1: 5 mV
6V6S PIN 8 - CATHODE: 17.9 V DC (38 mA)
17.7 V OVER 470 OHM RESISTOR (37.6 mA)
6V6S IDLE DISSIPATION: 11.73 WATT
12AY7 PIN 6 – ANODE/PLATE 1: 120 V DC
12AY7 PIN 1 – ANODE/PLATE 2: 125 V DC
12AY7 PIN 8 – CATHODE 1: 1.6 V DC
12AY7 PIN 3 – CATHODE 2: 1.7 V DC
DIODE FILAMENT: 4.9 V AC
TUBES FILAMENT: 6.27 V AC

Comparing this to the mentioned Fender Champ schematic specifications, I am at the correct design voltages, but the 12AY7 cathodes have higher voltages than specified for the 12AX7 in the diagram (1.6 V/1.7 V vs. 1.5 V specified) and the plate voltages are lower (125 V / 120 V vs. 150 V specified). I am not going to worry about this now. Can I take it that the 12AY7 runs pretty cool biased, with low gain?
The 1 K dropper is supposed to dissipate only 1.6 Watt at idle, but it is hot to the touch. Will clamp on a bit of aluminium heatsink, will see if I can bolt that to the chassis. Ideally require a 10 Watt resistor now. Or two 500 Ohm 5 Watt resistors is series. Less heat per unit.
Question: At startup, if I probe the B+ terminal, the voltage starts to show after a second or two, goes in a varying way up to 481 V, jiggles around some, and finally settles on the B+ value?
I the interests of full disclosure, I measured B+ with a guitar plugged in and strummed hard. No sag at B+1. B+2 (293 V) does drop a few Volts when strumming hard, and B+3 (241 V) drops too, but less.
If I understand correctly, I can replace the 1 kOhm B+ dropper with a chassis mounted 40 V or so Zener, cathode to chassis, anode to center tap. I have read that this size Zener might induce noise, which can be reduced with a 0.1 microFarad capacitor over it. Your experience?

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Oops, fumble fingers posted before I finished editing the above. The dropper to B+ is one 991 Ohm resistor, no parallel anymore.

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Been a long time. I am making up a cabinet for the amp, posting a picture of the initial woodwork.

guidothepimmp

Sweet. Keen to see how this comes out

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Patience. My current place of residence is jinxed beyond even my pessimistic cynic belief. Who cares, I am working through it.

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The picture above did not go far. The wood cracked on both sides (being cut from the same plank), and I installed straps to avoid future problems. I re-thinked the mesh cage idea a bit, and came up with this:

You have no idea how long it took me to paint that pattern on the top of these valves:

Not done yet, some final finishing to do, add a leather carry strap, but at least the valves are now protected.

V8

modulator You have no idea how long it took me to paint that pattern on the top of these valves:

😆

I'm digging the sahara desert level dry humour in this thread! Keep it coming 👍️

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My post of 2August 2020 referred: After all this time, I read you get directly and indirectly heated rectifier tubes. Using the 5U4GB rectifier, a directly heater type, explains my measuring a voltage spike at startup. The voltage spike is probably not good for the other two valves. Or the first filter capacitor, although the dropper resistor should help there. In the interests of valve life, I should use a 5AR4 rectifier, but then I start all over with the B+ that is too high.

I at long last obtained 560 Ohm resistors for the 6V6 bias resistor. I fitted one measuring 540 Ohm.
I have previously lifted the cathode bypass capacitor on the first stage, not an improvement. I was under the impression I ordered smaller capacitors to use here, but, dang, I did not?
I rigged in an old 10 uF capacitor, while I look for a selection of capacitors. I was thinking 4.7 uF, to tighten up the flab, but may want smaller.
I fitted the 5AR4 rectifier again, and probed the following, with a JJ 12AY7 and JJ 6V6s:

AC at switch 217 VAC
Drop over Rectifier – B+
dropper Resistor 38 V
B+1 350 V
B+2 304 V
B+3 250 V

6V6 valve:
Heaters 6.06 VAC
Plate Voltage 341 V
Grid Voltage 304 V
Cathode Voltage 19.4 V

So, a drop of 19.4 V over the 6V6 Cathode resistor, giving me an idle dissipation of 11.6 Watt. I am happy. The amplifier is still a bit “congested”, but, that is the nature of the beast. Ideally I need an AC supply of 220 - 222 VAC, to lift the heater Voltage a bit, but as is now, it is within the 5% variation.

V8

modulator That chassis build is neat - looks factory 👏

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V8
Thanks!
As mentioned, the chassis is not really deep enough, but it was free. The advantages of machine tools, chassis punches, I guess in the end the workshop becomes it's own hobby, costing much more than the hobbies one supports with it. The better option would, of course, be to CAD out the chassis (confident in your layout!) and have a CNC sheet metal place just cut punch and bend it up.
My future work (if ever) will use existing donor amps. I was planning to build this year, but, it is April already, and the plans are still just ideas. I really do not "need" another guitar amplifier, a small Stereo would make more sense, but, yah.

V8

modulator I guess in the end the workshop becomes it's own hobby, costing much more than the hobbies one supports with it.

So much truth in this - but the joys of fixing shit outwieghs it...(as I'm told by every woodworker/metalworker I know 😁 )

modulator My future work (if ever) will use existing donor amps.

I was trawling through the Gear Junkies stash of broken stuff - there's sooo much that can be re-purposed/re-used - but collecting it, is easier than actually doing something with it. I'll be content just to get in a few more minutes practice (that's my excuse!)

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V8
I would love to dig through Gear Junkie's junk, but, will probably not get time to use any of it.
Transformers, always welcome...

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You know how it is, at some stage it “is time”. I felt it is time to do the Master Volume.
I used an Alpha 250 kOhm Audio potentiometer (ex Ibanez guitar), looked at schematics of the Tiny Terror and Peavey Royal 8, snipped and soldered, and, now have a working Master Volume.

The amplifier will not get loud with a clean tone, but now I can fully open the original Volume pot (now working as “Gain”) and turn down the Master, and not blow out the windows or just have a mess of incoherent sound. Almost Fun.

So far, so good.

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The eagle eyed will notice I twisted the two wires going to the Output Transformer. The good RobRobinette mentions this might not be a good idea. I untwisted them, and re-routed to keep them separate.

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Another thing with this kit, the feedback resistor supplied was the 22 kOhm value on all the schematics, for a 4 Ohm impedance speaker. I have an 8 Ohm fitted, so, I should fit a bigger feedback resistor?
I tried a 47 kOhm, it is a bit hairy, so I went with a 33 kOhm instead.

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I also at long last fitted a 2.2 microFarad cathode resistor bypass capacitor to the first gain stage. Typical Fender value is 25 microFarad, resulting in a bit of wooly tone. Playing through a 10” Celestion G10 Creamback, at late night noodling volume, is quite acceptable. I might try a 1 microFarad bypass capacitor in future.