5f2: rectifiers, tubes, and OT primary impedence

[JoeM]

425, yikes...

- Scott

But isnt that cathode biased? The effective plate V would be less.

[ThermionicScott]

But isnt that cathode biased? The effective plate V would be less.

A little less, but I'm guessing the dissipation was still way up there, unless the screen voltage was lowered a bunch.

[JoeM]

Yeah, I had a VC for a while and tubes didnt last a real long time in it.

[Chuck H]

Yup. That amp had an RCA in it that I blew up during the the crank it up test. The Ei I replaced it with handled it with no problems. And "Yikes!" is right. This is a class A amp after all. To have it idle at 100% dis at such Vp is quite insane. I had thought to put a 6L6 in instead but the OT is pretty small. I fugured if I could get a 6V6 that would take it that would be best.

And yes, cathode bias helps for dis but that high Vp is still on the plate under max current. It was an internal arc that killed the original 6V6.

[ThermionicScott]

And yes, cathode bias helps for dis but that high Vp is still on the plate under max current. It was an internal arc that killed the original 6V6.

I wonder if the innards warped under the heat and shorted.

A couple years ago, I recapped a friend's all-original 1966 Champ which was running a B+ of 438V. The poor RCA 6V6GTA glowed red and was microphonic after dissipating 17 watts for who knows how long. A testament to overbuilt/underrated American tubes.

To resolve that issue, I used a 4x20uF cap can and built a Pi filter ahead of the OT using a 2.2K/5W power resistor. That brought the B+ down to a reasonable 360V and improved the hum immensely. That amp has a long happy life ahead of it.

- Scott

[Chuck H]

+++
The Pi filter works. I'd have done that 'cept it was ten years ago and I just wasn't quite where I am now with design. My consideration at the time was a big zener but since current 6V6's, though probably not as durable over all, handle high voltage as well or better than NOS I decided to give it a listen. I was a little surprised that it worked I must admit. But it sounded good and no red plating. I think 405Vp should work fine with current production tubes. If it didn't we'd have noticed a rash of "hey my Champ blew up" threads, since they all run even higher than that.

[ThermionicScott]

Yeah, I wonder if newer tubes have lower gm, and that's what keeps the dissipation in check with higher voltages. Maybe not.

At any rate, my friend's Champ was working fine, but I figured if wall voltages continue to creep up, things could get dangerous. When I buy incandescent bulbs, a lot of them have 130V ratings now.

Cm9, if you have any JJ 6V6 tubes on hand, I know those will tolerate higher voltages and dissipation without any trouble. You might even decide you like the sound at 400V.

- Scott

[cminor9]

yeah, got some JJ and EH 6V6s laying around. Either one will work fine. I actually prefer the EHs to the JJs in all my other amps. I'll keep it where it is for a while before I start experimenting. It's not gonna redplate any tubes, so that's good enough for now.

Just finished up the tolex on the cab last night so today or tomorrow will be the day put it all together, cab + baffle + chassis. It's hard to be patient, but I'd rather let everything set up right before assembling it.

[cminor9]

well, put it together and fired it up. It sounds pretty good! It is so durn quiet, I cannot even tell if its working until I plug something into it. All of my other builds are quiet but there is always just a little bit of hum. Is that normal for a 5F2-a or did all of the stars align just right for me on this build?

EH and JJ tubes are a tied up now, so I put in an old CBS 6V6. Played with it for a while, and then the 6V6 started redplating. So I put the zener in. Now, I am running about 370 on the plates, 345 on the screens and all seems to be well in 6V6-land.

Are these supposed to get kinda thin and ratty when dimed? I really aimed for a replica here, so that means there is a Weber sig 8" speaker in there, 4 ohms and 15w. Sounds pretty until you turn it all the way up and hit it with some hot pickups, but then the OD is nowhere near as good as the 5E3. Maybe I'll try the 8 ohm tap and run it through the 12" Weber Blue Dog in my 5E3 to see if it's just the smaller, cheaper speaker.

I do like how responsive and dynamic this amp is though. I have been running it just a dot or two shy of dimed and I can go from clean to grind just by varying my attack, not to mention using the guitar's volume knob. It's also nice because I can run it hard and it's still pretty quiet. Probably won't be enough to play with a drummer, which is OK for this amp.

[ThermionicScott]

well, put it together and fired it up. It sounds pretty good! It is so durn quiet, I cannot even tell if its working until I plug something into it. All of my other builds are quiet but there is always just a little bit of hum. Is that normal for a 5F2-a or did all of the stars align just right for me on this build?

Sometimes the gods smile on our feeble efforts. Mine is very quiet, with just a faint hum that lets me know it's on.

Are these supposed to get kinda thin and ratty when dimed? I really aimed for a replica here, so that means there is a Weber sig 8" speaker in there, 4 ohms and 15w. Sounds pretty until you turn it all the way up and hit it with some hot pickups, but then the OD is nowhere near as good as the 5E3. Maybe I'll try the 8 ohm tap and run it through the 12" Weber Blue Dog in my 5E3 to see if it's just the smaller, cheaper speaker.

I do like how responsive and dynamic this amp is though. I have been running it just a dot or two shy of dimed and I can go from clean to grind just by varying my attack, not to mention using the guitar's volume knob. It's also nice because I can run it hard and it's still pretty quiet. Probably won't be enough to play with a drummer, which is OK for this amp.

So you've got it hooked up using the 6K primary, with a 4-ohm speaker plugged into the 4-ohm tap? Once thing worth checking is that your negative feedback is applied correctly. If the OT is hooked up backward, then the amp may be prone to oscillation and bad-sounding clipping.

Some folks really like the stock circuit, but I really prefer mine with the grid-stoppers I mentioned earlier. THe distortion is tighter, and the blattiness (which I had despaired was inevitable with single-ended amps) is mostly gone. It's something you should definitely try!

(I need to start posting soundclips of my amps.)

- Scott

[cminor9]

So you've got it hooked up using the 6K primary, with a 4-ohm speaker plugged into the 4-ohm tap? Once thing worth checking is that your negative feedback is applied correctly. If the OT is hooked up backward, then the amp may be prone to oscillation and bad-sounding clipping.

Oops, that is an 8K primary. I must have misread the spec. It's hooked up right and is using the 4 ohm tap, I double checked. It sounds good at about halfway up. Really nice chimey drive when I turn up the guitar volume. Up more than about 3/4, it just sounds ratty with the guitar full up. My suspicion is that it's how it should be, since other places on the dial it sounds pretty good and these things weren't meant to be run this way. I mean, for the right kinda feel, it could be a useful tone. Like if I were Jack White maybe.

I'll try to record all of that since "ratty" is not a meaningful term.

Some folks really like the stock circuit, but I really prefer mine with the grid-stoppers I mentioned earlier. THe distortion is tighter, and the blattiness (which I had despaired was inevitable with single-ended amps) is mostly gone. It's something you should definitely try!

I think I understand the term blatty, and yes, I agree. The grid stoppers, I might try those. No 6.8k resistors, but I have some 270k. Maybe I'll start there.

[cminor9]

ok, here are some sound clips:

1: vol 9:00, tone 5:00 (full treble). Guitar starts at about half vol, then full up
2: vol 12:00, tone 5:00 (full treble). Guitar starts at about half vol, then full up
3: vol 5:00, tone 5:00 (full treble). Guitar starts at about half vol, then full up
4: vol 12:00, tone 5:00 (full treble), tone full up, guitar 1/3 up, tone pot maybe 3/4 up

All samples are on an Ibanez guitar with Bill Lawrence L-500 pickups. I used the neck pickup only. All samples were recorded on my iPhone, which was about 20 feet away from the amp.

You can definitely hear it get ratty on #3. The sound clips aren't to show off any playing, just to put across the amp's tones. I just played some chords mostly. I wish people would do this instead of just wailing away. I want to hear what the guitar/amp sounds like!


1 12_31_10 8_01 PM.mp3
2 12_31_10 8_04 PM.mp3
3 12_31_10 8_07 PM.mp3
4 12_31_10 8_50 PM.mp3

[Chuck H]

Sounds about right, if a little spanky. This amp would probably benefit from reduced Vp for the heavy OD tones. JMHO. The cleans and very slightly dirties are great though. +++

[ThermionicScott]

Ah yeah, 7 or 8K is the more traditional primary Z for these amps. Lower, and you might get more power, but the waveform would be lopsided (more even harmonics).

- Scott