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#1 Help before I lose my sanity

Posted: Sat Jan 14, 2023 5:26 pm
by RhythMick
... though some might say it's too late.

A friend and I have discussed building a guitar amp and though I've only built hifi amps before we've started to develop a prototype. Really simple, starting with SE similar to Fender Champ 5F1 but with some of the crap parts of the design amended (no death cap, secondary fuses, better filtering, ...). The objective is to help him understand valve amps by designing and building a prototype which we can tweak to hear the effect on the tone.

I'm in the early part of the build and running early tests as I go. Doing this part on my own to get the basics right, and glad of it. Although it's my first guitar amp I've built many hifi amps - certainly not an expert but this should be easily within my range. However I'm failing at the second test. OK I've had a bad cough for what seems like 6 weeks, might not be at my best but I can't for the life of me see what's going wrong here.

I'm using the Hammond 290CEX power transformer. All wires have been cut and soldered to turret board so nothing can accidentally short out. I've measured DCR (per spec within the limits of my multimeter) and tested to ensure there are no shorts across any of the windings or to the shield/case.

With the 5V secondary across the rectifier pins 2 & 8, nothing else connected, rectifier tube in (5R4WGB), apply power gradually through the Variac. No problems, at full UK mains I get 5V across the heaters and visible glow in the valve. Test 1 complete.

Test 2 adds connections from Pin 8 to CLC filter, bleed resistor across the second cap, ground bus to return current to the 325-0-325 centre tap. Power up through Variac again, but the fuse in the Variac blows. Check everything, seems ok. Replace fuse in variac, remove secondary fuses from 325v secondaries - this should be as per test 1 with heaters the only circuit flowing current. Power up again, fuse blows again ??!!

Diagram attached shows exactly what I have connected for this test. Note that nothing is connected to pins 4 and 6 of the rectifier - the fuses are physically removed and I've even removed the wires to make triple sure. I've checked again that there is no short between the windings of the transformer. Heck I even removed the valve base to check for possible shorts - there were none, but I changed it and checked the new one. Nothing connected to pins 4 and 6, just as per the diagram,
Test 2 - Rectifier Heaters CLC no HV Fuses.png
Test 2 - Rectifier Heaters CLC no HV Fuses.png (58.74 KiB) Viewed 4581 times
With any luck writing this down and then resting (trying to stop coughing) will pop it into my head, but if anyone wants to steer me towards the stupid mistake I'm obviously making I'd be very grateful.
290CEX Wire Colours.png
290CEX Wire Colours.png (36.95 KiB) Viewed 4581 times
290CEX DCR and Specs.png
290CEX DCR and Specs.png (71.29 KiB) Viewed 4581 times

#2 Re: Help before I lose my sanity

Posted: Sat Jan 14, 2023 5:38 pm
by RhythMick
With the rectifier tube removed, measuring resistance from pin 2 to the ground bus shows 75K (after the current from the multimeter charges up the caps it settles at 75K). Obviously it's connected through the transformer winding to pin 8 and on through the CLC.

#3 Re: Help before I lose my sanity

Posted: Sat Jan 14, 2023 6:25 pm
by Nick
Are you winding the Variac up from 0 or just turning on when set to 240v?

#4 Re: Help before I lose my sanity

Posted: Sat Jan 14, 2023 6:28 pm
by ed
not sure this is related to your problem..but...cap input for 5R4WGB is max at 4uf

#5 Re: Help before I lose my sanity

Posted: Sat Jan 14, 2023 6:37 pm
by RhythMick
ed wrote: Sat Jan 14, 2023 6:28 pm not sure this is related to your problem..but...cap input for 5R4WGB is max at 4uf
Thanks Ed yes I saw that. Also the transformer DC resistance is lower than spec. Inrush current? The Variac fuse is 2A slo blo.

However, as I say only the 5V winding should have current through it. The 325-0-325 winding is ONLY connected from CT to ground bus, but the rectifier plates are not connected.

It will be something stupid and obvious.

#6 Re: Help before I lose my sanity

Posted: Sat Jan 14, 2023 6:51 pm
by RhythMick
Nick wrote: Sat Jan 14, 2023 6:25 pm Are you winding the Variac up from 0 or just turning on when set to 240v?
Hmmm... First time I wound up from zero. Repeat tests I just switched on.

#7 Re: Help before I lose my sanity

Posted: Sat Jan 14, 2023 7:31 pm
by Nick
Did the fuse go on the wind up from 0?

#8 Re: Help before I lose my sanity

Posted: Sat Jan 14, 2023 7:38 pm
by RhythMick
Nick wrote: Sat Jan 14, 2023 7:31 pm Did the fuse go on the wind up from 0?
I'll repeat the test. I may have only done the wind up with heaters only, which was successful.

#9 Re: Help before I lose my sanity

Posted: Sat Jan 14, 2023 7:40 pm
by andrew Ivimey
This may not be helpful but having built several 'fender champs' the 'simple' psu is a feature of its erm, magic. One ecc83 and one el84 ( or 6sl7/ 6v6) really doesn't need much to get rocking but otoh it is only a tiny on/off sort of amp. 8 inch speaker sounds crap too!

The fender deluxe is hugely more versatile. I tried variations but concluded that the tried and tested circuit and layout really is easier to copy and quite critical if you want quality sound and a truly useful rugged little amp.

#10 Re: Help before I lose my sanity

Posted: Sat Jan 14, 2023 7:45 pm
by RhythMick
andrew Ivimey wrote: Sat Jan 14, 2023 7:40 pm This may not be helpful but having built several 'fender champs' the 'simple' psu is a feature of its erm, magic. One ecc83 and one el84 ( or 6sl7/ 6v6) really doesn't need much to get rocking but otoh it is only a tiny on/off sort of amp. 8 inch speaker sounds crap too!

The fender deluxe is hugely more versatile. I tried variations but concluded that the tried and tested circuit and layout really is easier to copy and quite critical if you want quality sound and a truly useful rugged little amp.
Thanks Andrew. We're planning to use this as a head amp plugging into various speakers initially I think 12" 16ohm. You may well be right about the Champ not being the best sound, after all Fender released many further models so one might assume there were improvements. Or cost cutting maybe. However the most important part of this exercise is the tweaking and learning what makes the difference, so I don't want to just copy one. Also we are aiming to get power tube distortion at home listening levels.

#11 Re: Help before I lose my sanity

Posted: Sat Jan 14, 2023 7:51 pm
by RhythMick
RhythMick wrote: Sat Jan 14, 2023 7:38 pm
Nick wrote: Sat Jan 14, 2023 7:31 pm Did the fuse go on the wind up from 0?
I'll repeat the test. I may have only done the wind up with heaters only, which was successful.
Interesting.

Firstly, after a rest I reviewed the physical connections against the circuit I posted. The earth to power ground was missing.

I added that (at the far end of the ground bus, where the guitar jack will connect), though it won't make any difference yet.

Second I repeated the test, with the 5R4WGB in circuit no secondary fuses. Variac ramp up. No problems, 5V across nicely glowing heaters.

Finally I set the Variac to 230V and did a straight power on test. Again no problems.

Could the missing earth/ground connection have caused that problem?

#12 Re: Help before I lose my sanity

Posted: Sat Jan 14, 2023 7:58 pm
by RhythMick
And the final test for tonight, with plate connections restored and secondary fuses in place, DC voltage across the bleed resistor measures 452V with about 160mV ripple.

#13 Re: Help before I lose my sanity

Posted: Sat Jan 14, 2023 8:40 pm
by andrew Ivimey
Good to have a variac.

One problem might be that getting a Champ right in the first place might be part of the process. Guitar amps are not like hifi amps. If you build something hifi-ish and put a guitar through it, it can sound pleasant. But guitar amps have enormous gain and Champs use one valve to get the sort of gain that goes into wildness and feedback through the output valve and speaker. 12 inches always sounds better!

And, a Champ's power supply or indeed Deluxe are ridiculously simple- in hifi you just wouldn't do that.

Looking at Vox and WEM it's the same. It looks ludicrously simple yet it works. Copy them and disregard how they made them/ where they put components and you can have a handful of snakes.

#14 Re: Help before I lose my sanity

Posted: Sat Jan 14, 2023 8:50 pm
by RhythMick
andrew Ivimey wrote: Sat Jan 14, 2023 8:40 pm Good to have a variac.

One problem might be that getting a Champ right in the first place might be part of the process. Guitar amps are not like hifi amps. If you build something hifi-ish and put a guitar through it, it can sound pleasant. But guitar amps have enormous gain and Champs use one valve to get the sort of gain that goes into wildness and feedback through the output valve and speaker. 12 inches always sounds better!

And, a Champ's power supply or indeed Deluxe are ridiculously simple- in hifi you just wouldn't do that.

Looking at Vox and WEM it's the same. It looks ludicrously simple yet it works. Copy them and disregard how they made them/ where they put components and you can have a handful of snakes.
Thanks. At least with a prototype it's pretty easy to revert to the standard (remove the LC) and listen to the difference. And that's the point of the exercise.

#15 Re: Help before I lose my sanity

Posted: Sat Jan 14, 2023 8:52 pm
by andrew Ivimey
Good luck!