5f1 problems

[swany100]

hi chris from the uk here i have recently built a 5f1 champ clone and have been playing it for two weeks now and happy with the sound but a bit too clean for me so i decided
to fit a 22uf cap across the first cathode resister but on power up amp seems to have
lost volume and when turning it up it squeals around 11 to 12 can any body help
regards
chris

[MWJB]

Hi Chris,

Sounds like you have an oscillation issue, adding the 22uf cap to V1 pin 3 should increase gain & make the amp louder (5F1s normally had this cap as stock).

Check that you have fitted the cap the right way round (waisted band nearest the tube socket pin, arrows to ground).

Best thing is for you to post some pics of the build, showing circuit board & wiring to tube sockets and input jacks & pot.

[swany100]

thanks for looking mwjb i have the cap the correct way round
while messing with volume on amp and guitar i have found that if i set amp no more than 6 and guitar around five i get a nice sweet overdrive but anymore than that and it oscillates and speaker cuts out
the amp works fine without it but hard to overdrive the sound
i have used a 600 champ pt and diode rectifier and think voltages may be high

6v6
p3 370
p4330
p50.060
p820.50

12ax
p1 195
p3 1.4
p6 186
p8 1.5

b+394
cap1 384
cap2 330
cap3 288

[MWJB]

The voltages are fine, in fact o the low side for a solid state rectifier. The problem most likely lies in the length & routing of the hook up wires running to the tube sockets.

You say that you used a Chap 600 PT, I trust that you have not used the champ 600 chassis? What board layout (Mojo/Weber etc) and chassis have you used?

Pics would really, really help.

[swany100]

thanks for the reply mwjb i have just taken some pics
dont want to sound like a complete fool but i dont know how to post them could you please advise

[MWJB]

I wish I could, I have no idea myself!

When you click reply, click "go advanced" try the little paperclip (attachement) sign at the top amongst the edit icons, or just copy a link to where the pics amight be hosted like photobox?

[swany100]

pics of 5f1 build

hope there ok

[MWJB]

Scrap the board, it's not a sympathetic layout for the Champ 600 chassis, note on a 5F1 the pots & jacks are on one side of the chassis, tubes & output jacks at the other side, components sit in between. Either hardwire parts direct to tube sockets, or use a few terminal strips to better arrange components & keep them closer to where they terminate.

E.g. 0.022 coupling cap at 12AX7 pin 1 connects directly to vol pot RH tab, 0.022 cap from 12AX7 pin 6 goes to power tube pin 1, then a 1.5k resistor links that to pin 5. Add a 470ohm screen grid resistor from 6V6 pin 6 to pin 4.

There seems to be an alien material between input jacks & ground?

Those LCRs are way too big for this chassis, use some reasonable sized TAD, F&T or even Rifa, or HL caps from RSWWW. It'll take you less than an hour to change them all if they need it in 20yrs time ;-)

[swany100]

5f1 fixed

thanks mwjb for all the info and advice i moved the 2 68k resistors and wired direct to tube and imputs { problem solved } thanks.....