5F6A keeps cutting out...

[yankeerob]

Hello chaps - long time no speak...

Having a real ballache of a time with a recent 5F6A build - the amp functions fine and then just cuts out with no obvious signs... still working but very low power...

I at first thought it might be the PI overdriving the output stage; pushing it into clamping so I threw a signal straight into PI with the presence cranked to full and sure enough the waveform was pretty awful so I swapped the GNF return from the 4 ohm tap to the 8 and saw a great improvement and then further improved this by dropping the feedback resistor from 56K to 47K (I like to have as much open-ness as poss) and now I'd got to where I could throw as much signal into the PI and the clipping was nice and even and I was no longer experiencing the positive cycle pushing the wave down to where the negative cycle was clipping before the positive - so that seemed OK to me... I then measured the AC voltage from the signal generator with the output well into clipping (also measuring the output AC voltage at around 21V into an 8 ohm load) and found this to be around 2.7V... given the OT is rated for 60W I thought 55W max would be a good limit.

I then thought - let's find out what the front end is producing - so I cranked up mid, bass, treble and both channels to full and found that the front end was producing about 17.5V and it still wasn't clipping and the wave still looked pretty damned good - ah ha I thought - so set about attenuating the front end - bear in mind I had a 12AX7 in V1 but the reasoning behind this is although everyone knows that a 5F6A should have a 12AY7 in V1 I know full well someone is eventually gonna come along and throw a 12AX7 in there... So I first tried attenuating the output from the treble wiper by bleeding some of it to ground but the waveform looked awful so I then went to the next obvious place and wound up putting a 56k or 68K between the input to V2 and ground and hey presto I had an attenuated output from the treble wiper and a good looking waveform - measured around 4 or so V and thought so far, so good...

Re-soldered the wire between the treble wiper and the PI input, biased it up with KT66's in - played it for a round an hour yesterday - giving it a good thrashing - and thought I'd cracked the case. Nice clean sound and a bit of distortion if you hit it hard....

Came in today and thought I'd better test it with 6L6's - the JJ KT's actually produce a fab clean sound but maybe a bit too hi-fi for my liking (although I have to say that if a good clean sound is your bag - they do work very well) so I threw in a brand new pr of 5881WXT's and biased it up... plugged in, cranked it up and sure enough it happens again... it starts with a slight little crackle on heavy transients and then just goes really low power - I'm thinking it might have something to do with the pots I'm using for the bias - I have had one of the same brand (1/4W Citec's from Farnell/Newark) fail in it's role as a vol pot after it'd been soldered and de-soldered a couple times and wonder it they might be failing due to having adjusted the range a good coupla times (- I've got it so I can bias 6L6, KT66 or KT77's - EL34's don't seem to want to play in this set-up even though I have enough range to bias them up - maybe the 5K OT?)

Pic below shows the bias supply setup - that's a 0.25W 22K... you can't see the link between the wiper and the bias feed resistors which goes under the board... I've checked this and all seems sound - I get a range of around -59-40V thereabouts... (I need the high end for the 66's)...



I'm wondering should I start using a 0.5W preset instead? Any other suggestions?

[phantomman]

Have you tried a 12AT7 in V1, Rob? Seems to me it might strike a happy medium, with its modest Mu factor. And it wouldn't clobber your cascaded second-stage V2 quite so hard. I think the extra resistor tying the treble wipe to ground puts you on the right track but it might not be necessary at all with a 12AT7. Just a thought......

[Offshore Angler]

After an hour? Check the voice coil on the speaker.

[yankeerob]

Have you tried a 12AT7 in V1, Rob? Seems to me it might strike a happy medium, with its modest Mu factor. And it wouldn't clobber your cascaded second-stage V2 quite so hard. I think the extra resistor tying the treble wipe to ground puts you on the right track but it might not be necessary at all with a 12AT7. Just a thought......

Well the treble wiper to ground distorts the waveform (wish I had a digi-scope with capture so you could see what I mean) but tying a 56 or 68K from where the coupla 270K's join and go onto grid 1 of V2 to where the two cathode followers of V2 join and go onto earth is perfect location and as I say the waveform is pristine coming outta the treble wiper - so that's a good mod for anyone wanting tame the gain on a 5F6A (I've certainly made a note in my book any road...)

You are absolutely correct in that you can also just put a 12AY or AT in there - the thing I have be aware of is that some kid might come along and throw an AX in there - have the thing going into clamping and then go around telling everyone what a crap amp I sold 'em - this way I know it won't happen under any circumstances... but I gratefully take yer point

After an hour? Check the voice coil on the speaker.

You haven't seen these badass drivers - I keep telling everyone about 'em - they're called Tayden F Series - made in Gt Yarmouth - and they're basically copies of the old cast chassis Fane drivers that used to come in the old 200W HiWatt cabs (a la P Townshend) and they kick some serious booty - they're clean, loud and punchy as hell - there's two of 'em in the cab I've been using and it hasn't seen any crazy square waves or enough power to blow 'em - I've had 'em make us a pair with ally dust caps just for the look - they'll arrive on Wed... but having said that I will nonetheless have a look...

I really do suspect there's something amiss with the pots so we're gonna try another brand - I've got some 1M Alpha's that we've used in 5E3's and we're also gonna try some others from RS - I have had a 500K Citec fail in another amp and it was weird 'cause it measured OK - both across the track and through the wiper all the way 'round but it was the same symptom if I remember correctly (which I have to confess is getting harder and harder to do!!!) - but thanks for all yer comments - I have an email in drafts that I will finish today Rog... Will let ya'll know...

[yankeerob]

I was wrong - just a duff PI

I have however come up with a way to run a 12AX7 in V1 on a straight 5F6A without ever clamping the output grids...



Just put a 56 or 68K where asterisked and you'll be able to crank it flat out and get a pretty damned exceptional overdrive sound and have more travel in the volume pots... of course it should be noted I now wire 'em with a single jack to a 68K grid stopper to pin 2 of V1 and then have a 10K between pin 2 and pin 7... (always linked) and sometimes will put a 330K in instead of a 270K from the wiper of the bright pot...

It's a simple effective way of wiring a 5F6A so the channels are already linked (without oscillations between the triodes on V1 which will likely occur if you just put a straight link between the grids) it's still pretty hot but never too hot... keeps it's clarity and takes a drive pedal better because you're never pushing the front end too much...

I now - before connecting the output of the treble pot to the PI input - inject a signal into the PI, adjust the signal gain so the power stage has gone well into clipping, measure the voltage of the output stage and adjust the signal so it's well within the rating of the OT but is still clipping healthily... then I measure the output voltage of the treble wiper with a 0.01V signal into the input jack and all tone and vol controls full up - with the mod resistor in this usually will wind up being about double the voltage that I'd decided was a safe continuous signal (which is about right as the guitar is not a continuous signal) - it's a little suck it and see but I've found 56 or 68K usually gets the job done and has produced some real sweet sounding amps and I never have to problem with grid clamping anymore