5F1 with 5AR4 rectifier - excessive hum

[star looper]

I had mixed up lug 1 and 3 on the tone pot. Now it´s working fine, nice treble attenuating at 9 o clock with the neck pu single coil, and the treble sweep feels fine too all over.

Also removed the 22uF cathode cap on the second stage of the preamp, lost some overdrive but volume is more suited for homerecording. I will experiment with 2x68k in parallell at input.

I´m almost satisfied with the amp at this stage, though I miss some sparkle, mojo, tone is a bit dead. 6V6 is a no name brand, maybe a good brand will help. And I know that the 12ax7wa is a bit boring. I have a Tungsol in another amp that is great and I will try it.

And I´m thinking of lower the output tube cathode resistor from 680 to 620 or 560 Ohm, what can one expect soundwise?

[jmaf]

I will experiment with 2x68k in parallell at input.

...

6V6 is a no name brand, maybe a good brand will help. And I know that the 12ax7wa is a bit boring. I have a Tungsol in another amp that is great and I will try it.

And I´m thinking of lower the output tube cathode resistor from 680 to 620 or 560 Ohm, what can one expect soundwise?

The parallel resistors at input won't do anything noticeable to your tone or drive, it's just there for high frequency filtering, beyond hearing.

JJ 6V6's sounded cool on a tiny amp I retubed recently. They're cheaper lately too.

As for 12AX7 you've got a whole world to experiment with, but some current production gold pin JJ's(inherited from the mythical and often counterfeited ECC803S) sound great, I substituted Svetlana for three gold pins on a friend's amp and it is in another league entirely now.

Lowering to 560 ohm you'll change it very little, you can probably expect more high end and less compression, plus a slightly hotter tube(sounds better IMO, but tube will live less). Why not go stock at 470 OHMs?

[star looper]

The parallel resistors at input won't do anything noticeable to your tone or drive, it's just there for high frequency filtering, beyond hearing.

Ok, that´s why I didn´t notice anything particular.

JJ 6V6's sounded cool on a tiny amp I retubed recently. They're cheaper lately too.

As for 12AX7 you've got a whole world to experiment with, but some current production gold pin JJ's(inherited from the mythical and often counterfeited ECC803S) sound great, I substituted Svetlana for three gold pins on a friend's amp and it is in another league entirely now.

Thanks for the tip, I´ll check those goldpins, interesting.

Lowering to 560 ohm you'll change it very little, you can probably expect more high end and less compression, plus a slightly hotter tube(sounds better IMO, but tube will live less). Why not go stock at 470 OHMs?

Do you have any idea of how much shorter the lifespan will be? Changing the tube once a year is ok, I think, if playing an hour a day.

Plate voltage 388V with 680 Ohm(24V), won´t stock cook the tube?

[jmaf]

Do you have any idea of how much shorter the lifespan will be? Changing the tube once a year is ok, I think, if playing an hour a day.

Plate voltage 388V with 680 Ohm(24V), won´t stock cook the tube?

If you play at home for an hour a day at a reasonable volume they'll probably last several years.

I just multiplied 363.7 VDC by your 0.0357 A cathode current and you're already running at the limit there with that cathode resistor....I don't think you can go any lower, sorry about that earlier advice.

[star looper]

If you play at home for an hour a day at a reasonable volume they'll probably last several years.

I just multiplied 363.7 VDC by your 0.0357 A cathode current and you're already running at the limit there with that cathode resistor....I don't think you can go any lower, sorry about that earlier advice.

Thanks, no problem, the iron isn´t even heated yet!

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The 680 Ohm cathode resistor is specified 2W, and my calculatings makes .85W, is this enough safety margin?

[MWJB]

You can go to 470ohms for the 6V6 cathode resistor if you want to hear what more current will do, use a 5W resistor. You are nowhere near cooking your power tube, I would stick with 470ohms right up to 400vdc (to ground) on the plate, maybe a little over.

[jmaf]

You can go to 470ohms for the 6V6 cathode resistor if you want to hear what more current will do, use a 5W resistor. You are nowhere near cooking your power tube, I would stick with 470ohms right up to 400vdc (to ground) on the plate, maybe a little over.

MWJB, he's at almost 13 watts quiescent now...how hot do you go on 6V6's?

[star looper]

You can go to 470ohms for the 6V6 cathode resistor if you want to hear what more current will do, use a 5W resistor. You are nowhere near cooking your power tube, I would stick with 470ohms right up to 400vdc (to ground) on the plate, maybe a little over.

Had no 470 ohm, put a 560 ohm/4W in. Have not played it yet, just measured. I´m confused with the readings, plate is now 380V(388V before) and 21.8V over the 560 resistor. I thought the lower value of the resistor would increase the voltage of the plate, please correct me here if I´m wrong?

As I understand the mA raised a couple of notches and there´s now about 14W instead of 13W with the 680 ohm resistor(not taken the screen current into account).

Still using a bypass cap of 47uF,50V, as I did with the 680 ohm, is this ok?

[MWJB]

In P-P cathode biased 14W to 16W, in S-E cathode bias ~20W

[jmaf]

In P-P cathode biased 14W to 16W, in S-E cathode bias ~20W

Goodness. I never had the guts to go there....13 W is the most I've ever adjusted a 6V6 to in S-E . I have this 6V6 amp to be fixed here(for myself). I'll experiment

[jmaf]

Had no 470 ohm, put a 560 ohm/4W in. Have not played it yet, just measured. I´m confused with the readings, plate is now 380V(388V before) and 21.8V over the 560 resistor. I thought the lower value of the resistor would increase the voltage of the plate, please correct me here if I´m wrong?

As I understand the mA raised a couple of notches and there´s now about 14W instead of 13W with the 680 ohm resistor(not taken the screen current into account).

Still using a bypass cap of 47uF,50V, as I did with the 680 ohm, is this ok?

The transformer also has DC resistance in its windings, the more current you pull, +B goes lower as well. You're at 14 Watts idle now. MWJB goes up to 20W - I've never gone there.

[star looper]

The transformer also has DC resistance in its windings, the more current you pull, +B goes lower as well. You're at 14 Watts idle now. MWJB goes up to 20W - I've never gone there.

I see, thank you!

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Played the amp, and I liked the difference in sound(treble less harsh, and overall a bit more saturated, I think), with 560 ohm resistor, that much that I will go for stock when I can. I assume a 2W resistor(which I have at home) is too low?

[MWJB]

2W will just about do. I tend to use 5W for 6V6 cathode resistors.

[star looper]

Ok, thanks for info!

[star looper]

About finished fine tuning the amp!

Think I will contribute with my findings and thoughts.

The 6V6 cathode resistor is now 470 ohm and bypass cap is 25uF, which gives the most saturated tone of the tried resistor values, more Ampere. Thought it enhanced the sound of the thinnest strings, fatter, crunchier. The sound of the thick strings became a tad mushy with chords(this was with input tube cathode cap 4.7uF, which now is 3uF), just a tad but enough to consider it as a drawback. Put some new strings on and stepped up a notch from .009 to .010. Low end .046 as before. What a difference, much improved tone! Not sure if it´s the increased mass or the freshly steel that contributes most to the better tone. My string set before was quite old but had good sustain left, but now the amp and guitar is way changed. More balanced, fat, full sounding(both tone and volume knob at 11 o´clock on the amp, neck pickup on the strat).

When I started the project I played with my 8" blue bulldog speaker, but changed later on to WGS Reaper 30, which has both more low end and more high end. The pronounced high end was a problem, not any more, tonepot now at 11 o´clock, before about 9. Also using a JJ 6V6S and a Tungsol preamp tube at the moment.

I have a EI 12AX7, which I tried, it has a distinct character, which I want to share thoughts on, it compresses very early compared to other 12AX7 I´ve tried, it´s like using a pedal compressor. Very nice for clean rythm and spanky soloing(tried before I changed strings). Downsides are my tube flashes when amp is turned on and that´s why I switched it, and it´s notorious for often being microphonic, though not a problem for my home studio purposes. Go find a good item!

Now voltage readings are:

B+ 385
Plate 372
Screen 330
Cathode(470r) 19.8
3rd filter cap 289
12AX7 plates 187, 190 highest on input

This means 42mA through 6V6 output tube which gives about 14.8W

Now I need a cab for the head!