5F1 with 5AR4 rectifier - excessive hum

[star looper]

Hi!

Short story, skip to Problems below, or continue.

Iīve been lurking here for a while collecting information for my SE build. Eventually Iīve ordered the filter caps and put together an amp with parts that have been sitting on the shelf for a couple of years or so. Converted a tube radio to a "5D2" some years ago wich worked but sounded a bit too shrill with the radio OT. This is my second build in a nice chassi with most parts except for the rectifier tube and first filter cap and 6V6 chatode resistor/cap are as specified in the schematics.

I happened to have 5 new Sovtek 5AR4 and thatīs why I chose that configuration, and also for itīs known slow start which makes it ok to skip the standby switch. Unfortunately the fuses burned with the first rectifier tube. Went through the circuit and found that B+ was from pin 2 on rectifier tube. Donīt know if that is a problem actually(both pins connect to same part in the tube from different ways) but I changed to pin 8 as it is named cathode. Tried again, fine for a 30 sec and then crackle in the speaker and flashes in the tube. Went through the circuit again and again, and checked the transformers thoroughly. Third rectifier tube worked(at least the second was suspicious), aside from pin 2 and 8 mix up the build was ok from start. Also used V1b for input and V1a for second stage since I thought aI, gI and kI was A side... Working anyhow.

The amp sounds really nice clean with my strat(Tonerider Pure Vintage neck pu, and Tonerider AC4 humbucker in bridge pos) on the low strings, full and thight bass, the top end is too glassy for my liking though and thereīs no tone control on the strat. At the moment the V1 bypass cathode cap is 2uF(forgot to order the standard value but since Iīve recently seen recommendation for smaller values to avoid mushiness I was eager to try the caps I already got) and that increases the treble but not the bass as I understand, maybe no cap at all is what I need since Iīm not looking for crunch in the first place. One canīt reduce treble only with this cap, I believe. Using an 8" Blue Bulldog Celestion. Played for an hour or so mostly between 9 and 12 a clock, too loud for serious testing at home above that. Works nicely with my HM-3 distortion pedal too.

Iīm far from fully understanding the circuit, and grounding/loops is hard for me to grasp(hence my nick which I hope you all see the irony behind). And probably thatīs why hum now is a problem, and which I want to ask help for. Everything stargrounding from PT bolt except for preamp tube and volume which are grounded at tube base. Cliff jacks sleeves to PT bolt.

Problems:

1. Above 9 oīclock thereīs excessive hum with or without guitar connected.

2. Whole amp is getting quite hot.

3. Filter caps seem to drain themselves without "bleeder", down below 20V in a few seconds when turning off the power switch.

4. As said above, top end with single coil is too glassy, ice-picking.

Maybe point 3 is most important, after all maybe some major hazardous connection is made, is it the tubes that consume the voltage from the caps after switching off? Attaching pics of the amp and circuit which I hope will reveal some mistakes regarding grounding(at the moment of taking the pics, the NFB, 4 Ohm tap yellow wire, was not connected, it is now. OT is positioned by "earphone" and oscilloscope for best best placement, and input tube was aimed for furthest position away from PT, yet close to input jack. Point 2, my readings are about 45-50C degrees outside, PT hottest aside from the tubes, is it normal? 4. Where to shunt some high Hz, volume pot to ground? Tried my stomp box equalizer and with th 10K fader -15dB, and the 4.5K maybe -3dB which was better for a rough start. Though my Gibson sounds pretty good as is, still I play more with the strat. And finally, what to do with the hum which starts to annoy about 9 oīclock.

As said above, everything stargrounded from PT bolt except for input tube and volume which are grounded at tube base. Cliff jacks sleeves to PT bolt.



Parts:

Mojo 760EX, 230V primary, 325-0-325 secondary, PT
Mojo 771, 7K, 8 and 4 Ohm, OT
Sovtek 5AR4
6V6 russia no name
Sovtek 12AX7 WA
Cliff jacks
2x100r artificial 6.3V CT to output tube cathode.
Filter caps, 47, 16, 16uF
2x 1M pots, the one out of circuit is in place for future power scaling.
Terminal strips point to point.
Brown resistor is for manually discharging caps.

Voltage readings seem ok:

B+ 397
Plate 388
Screen 356
Cathode(680r) 24.3
3rd filter cap 312
12AX7 plates 209, 212 highest on input
12AX7 chatodes 1.56, 1.52
100r both 3.2
Rectifier fil 5.2

Poked around a bit with a chop stick, no difference... Just downloaded the Valve Wizard ground guide. Remember reading it some time ago but didnīt grasp it. Iīve built three stomboxes which I successfully implemented the one point chassi grounding at input, but I canīt figure how to apply the idea in a tube amp.

[loudthud]

There is a filter cap that feeds B+ to both sides of the 12AX7. Move it's ground to the preamp ground point. The input jack ground should also go to the preamp ground point. I see two knobs on your amp but the 5F1 doesn't have a tone control. Did you add a tone control? Check this thread: http://music-electronics-forum.com/t16311/ in post 10 I attached a 5F1 schematic that diagrams an ideal ground. Your ground may be ok if you fix the two things above.

Edit: Your plate dissipation calculates to 18.8 Watts. A little higher that you would like. You need to increase the cathode resistor of the 6V6. If it's 470 ohms, perhaps 560 will cool the tube down a little. Target is 12W.

[star looper]

Thank you!

Great help, thank you loudthud!

No hum in the signal chain now at all. I put the capīs negative and the input ground at the second stage side of the preamp tube. Though thereīs a bit of background hum still, is this because of the tubes positions, or is there any more to do?

Thereīs a 680 Ohm cathode resistor at the output tube, 24V, which makes 35mA. Donīt know the screen current, it shouldnīt be that much I think. Plate 388V minus 24 times .035 = 12.7W.

No tone control, just prepaired the chassi with the pot for future mods.

Yes, seen your schematics, thought it was for switchcraft jacks, hence I skipped that one. And read some simple solutions from MWJB, grounding at input tube which suit my tiny chassi and from there I started...

Today the "glassiness" was not that of a big problem, still want to know how to cut some dB:s of the highs.

[hack]

Increase the value of the catyhode resistor

Great help, thank you loudthud!

No hum in the signal chain now at all. I put the capīs negative and the input ground at the second stage side of the preamp tube. Though thereīs a bit of background hum still, is this because of the tubes positions, or is there any more to do?

Thereīs a 680 Ohm cathode resistor at the output tube, 24V, which makes 35mA. Donīt know the screen current, it shouldnīt be that much I think. Plate 388V minus 24 times .035 = 12.7W.

No tone control, just prepaired the chassi with the pot for future mods.

Yes, seen your schematics, thought it was for switchcraft jacks, hence I skipped that one. And read some simple solutions from MWJB, grounding at input tube which suit my tiny chassi and from there I started...

Today the "glassiness" was not that of a big problem, still want to know how to cut some dB:s of the highs.

Increasing the value of the cathode (not catyhode) resistor on the preamp tube from 25uF to to 50uF may help.

[Tom Phillips]

...Filter caps seem to drain themselves without "bleeder", down below 20V in a few seconds when turning off the power switch....is it the tubes that consume the voltage from the caps after switching off? ...

Yes it is the tubes. The heaters are still warm when the power is turned off and the tubes continue to draw current. Mostly the power tube. If you continue playing guitar through the amp when you turn the switch off you will hear the sound die away as the voltage drops. This is expected behavior.
Cheers,
Tom

[Chuck H]

Today the "glassiness" was not that of a big problem, still want to know how to cut some dB:s of the highs.

Increasing the value of the cathode (not catyhode) resistor on the preamp tube from 25uF to to 50uF may help.

I know your only trying to help so I am compelled to tell you that this is misinformation.

[loudthud]

Check the tone control on the 5F10 Harvard. (There are other examples in tweed amps.) You can just solder the two caps between the tone pot and the volume pot. That will help knock the treble down on that Strat.

I see you have a virtual center tap on the heater supply connected to the 6V6 cathode. That's good. It will get rid of some of the 60Hz hum. There is a brown wire from the rectifier socket that goes to the terminal strip where the + side of the first filter cap is connected. Route that wire away from the pots and any other signal carrying wires or components. That wire carries 120Hz hum that sounds more like a buzz. Also the red wire to the output transformer. Try to get that as far as possible form the capacitor that goes to pin 5 of the 6V6.

[star looper]

Thank you guys, very welcome information, again!

I wonder if a larger value(maybe std 25uF) of the preamp cathode cap would saturate the tone and drench a bit of the highs when playing the thinner strings? Will the gain, overdrive, increase much(now using 2uF). Probably Iīm going to try without any cap at all today, but I donīt like solder/desolder things that much, and I donīt have any larger cap at the moment, hence Iīm asking.

I was thinking of using Power Scaling or VVR(Variable Voltage Regulator) at the "tone" pot later on. Anyone endorsing that in this circuit, or is it a waste of money and tone(maybe this is for another thread)?

Do the 100 ohm resistors do equal good on 50Hz like 60Hz, or are there any values that is better for 50Hz?

The residue hum are probably more like a buzz, 100Hz then I think, is it beyond the virtual center tap thing, I donīt understand the math of, or the term elevate, regarding the vct? Iīll check the actual Hz soon.

Speaking of the OT wires, does it matter which way(blue or red) the primary is connected in relation to the secondary, couldnīt find any info about my 771 OT, except for the secondaries?

[jmaf]

I wonder if a larger value(maybe std 25uF) of the preamp cathode cap would saturate the tone and drench a bit of the highs when playing the thinner strings? Will the gain, overdrive, increase much(now using 2uF). Probably Iīm going to try without any cap at all today, but I donīt like solder/desolder things that much, and I donīt have any larger cap at the moment, hence Iīm asking.

The bigger the cap, the more you boost the lower frequencies, up to a point where it doesn't make a difference anymore. IME from 25 uF and up you won't hear much of a difference, I've tried 250uF and electrically I know it is acting differently but my ears couldn't tell much difference. From 2 uF to 25uF you will notice more output.

Do the 100 ohm resistors do equal good on 50Hz like 60Hz, or are there any values that is better for 50Hz?

Equally good.

The residue hum are probably more like a buzz, 100Hz then I think, is it beyond the virtual center tap thing, I donīt understand the math of, or the term elevate, regarding the vct? Iīll check the actual Hz soon.

Some hum is expected in any tube amp. About elevating heaters, some tubes run the cathodes at a potential tens of volts above ground(e.g. the long tail phase inverter), as to bias the grid negative with respect to it(it's tied to ground). The heaters are very close to the cathode and if the potential between them gets too high it starts to modulate the electron flow, the result is 60hz hum in the signal. Plus, it's good for tube life when the cathode and heaters are not at a very different potential, one doesn't try to steal ions from the other.

Speaking of the OT wires, does it matter which way(blue or red) the primary is connected in relation to the secondary, couldnīt find any info about my 771 OT, except for the secondaries?

If you invert the primary leads the negative feedback will become positive and it'll squeal soon as the tubes heat up.

[star looper]

Thank you, for the info, jmaf!

Some hum is expected in any tube amp. About elevating heaters, some tubes run the cathodes at a potential tens of volts above ground(e.g. the long tail phase inverter), as to bias the grid negative with respect to it(it's tied to ground). The heaters are very close to the cathode and if the potential between them gets too high it starts to modulate the electron flow, the result is 60hz hum in the signal. Plus, it's good for tube life when the cathode and heaters are not at a very different potential, one doesn't try to steal ions from the other.

Iīll try to see if I can get a clear picture of that some day, at the moment I canīt.

If you invert the primary leads the negative feedback will become positive and it'll squeal soon as the tubes heat up.

I found some notes, red(primary) is on the same side as green 8 Ohm(secondary), though I happened to put blue to plate and red to B+, no squeal. Yellow, 4 Ohm tap, is used for NFB.

---

Also found some scrap electronics and got a 22uF which I tried on second stage preamp cathode, loud! Though, nice saturation which Iīll have to try out some more tomorrow.

Pushing the B+ or the red wire, from OT, a little bit sideways made no audible difference regarding the hum/buzz.

[star looper]

Check the tone control on the 5F10 Harvard. (There are other examples in tweed amps.) You can just solder the two caps between the tone pot and the volume pot. That will help knock the treble down on that Strat.

Tried the 5F10 tone circuit. It kills the excessive highs. Though, most of the sweep increases the highs, I guess this must be an active tone circuit, only a very tiny part of the pot reduce the highs. Iīd like to have more reduction of the highs, which cap value to change(now using .0005uF mica to grid, and .005uF "mustard" to ground)?

[loudthud]

Is the pot a 1 meg? Audio or linear taper? I'm thinking it should be audio taper. If it kills too much high end, reduce the .005 to .002 or .001uF.

[Bruce / Mission Amps]

Tried the 5F10 tone circuit. It kills the highs. Though, most of the sweep increases the highs, I guess this must be an active tone circuit, only a very tiny part of the pot reduce the highs. Iīd like to have more reduction of the highs, which cap value to change(now using .0005uF mica to grid, and .005uF "mustard" to ground)?

Wow.. I've never really heard that complaint before..... I use the 5F10-5F11 etc., tone control on lots of simple amps... but with the 1 meg audio taper tone pot, frequently I use a .0068uF to .01uF grounded cap and sometimes a 680pF bright cap in the tone control.
Also.. if you use a cathode bypass cap on the second stage with negative feedback voltage applied at the same spot... you just dumped all the NFB voltage to ground and there is no NFB applied.. that is in conjunction with the boost from the cap on the cathode resistor.
That makes the preamp stage go to full output with a lot more gain.
Look at the way the negative feedback is applied to the preamp stage of a Princeton Reverb... that circuit enjoys both broadband gain boost plus the negative feedback still works

[star looper]

Is the pot a 1 meg? Audio or linear taper? I'm thinking it should be audio taper. If it kills too much high end, reduce the .005 to .002 or .001uF.

Itīs 1M, and audio taper.

I meant that it kills some of the excessive highs(thatīs good), and I want a bit more reduction of the highs.

[star looper]

Wow.. I've never really heard that complaint before.....

Are you suggesting something?

Thanks, for info!