A Champ and a shaggy dog wrong rectifier tale.

[holio cornolio]

Hello all, I'm something of a noob to this world but I have an issue which requires a long story to fully explain. If you're not feeling patient, better switch off now!
A few years back I discovered an old tweed champ at a guitar store. I plugged in a nice SG special and wow, what a sound!! It was a superb almost marshally crunch, very full and quite british sounding, and this was my first experience of a champ - just a volume control and nothing else - the perfect amp (if only they made them bigger!). The amp was all stock except for having been re-coned fairly recently. Sadly it was way beyond my budget, but the Champ bug had bitten and I had to have one. Fast forward a couple of years and research had brought me to Torres UK, and I was lucky enough that my wife bought me a tiny boxer kit as a birthday gift and gave me some free - non chore time - to build it. With no electronic knowledge, but an ability to solder ok and follow instructions reasonably well, it was quickly built and worked first time. And it sounds great, but it is nothing like that first champ I tried, it's all tweedy and gritty when dimed, but not a hint of that crunch I was looking for.... I've given the amp a couple of years and some serious conal abuse to break it in, but it's still resolutely tweed sounding - more characteristically bassman overdrive than anything else - it's also way darker sounding than that first champ I tried, not necessarily warmer - but definitely darker). I never put in the Torres tone 'stack' which even to my uneducated eyes seemed to be more of a passive filter so it's pretty much exactly as a 50s champ would be laid out. I have messed with power tubes, and currently have a nice NOS Mazda 6v6 power tube, but never changed the preamp tube (still the same EHX 12 AX7) and I never bothered with the rectifier until yesterday. Looking at all the old champ schematics, it used a 5y3, but I got a 5AR4 with my kit, and from what I've read, it's probably a bit over specified for the amp. I can't find my torres manual at the moment, which has the schematic in it, but I did have a little trouble getting the amp biased to begin with, as it seemed to run very hot, but I thought that had more to do with me misreading the multimeter than anything else. Anyway, I want to try dropping in a 5Y3 into the rectifier slot, any reason why I shouldn't? I guess I'll have to check the bias again (maybe I'll get it right this time), but in the opinion of those people who actually know what they're doing with amps, is this change going to help me get closer to that crunchy response and early break up I want??? Or am I barking up the wrong tree??
If you got this far, thanks for your patience!
P.S the other changes from 50s champ spec that I know of are: ceramic 8 ohm jensen speaker (could run a 4 ohm if necessary as have 3 taps on output tranny).
all non electro caps are orange drops (have thought about recapping with PIO but too much of a PITA!).

[Albert Kreuzer]

A 5Y3 is the correct rectifier for a champ, so I'd go for it. If you have a 470 Ohm cathode resistor, it's probably running too hot with the 5AR4 anyway.
The biggest influence on sound is the speaker IMO. Try a 4 Ohm P8R Jensen Alnico, I have installed some of these in champs and the owners like them a lot.
Coupling caps are way down on my list of useful mods. We've had several threads on this topic recently. Not to open a can of worms here, but I like Mallory 150s in Fender amps. The difference to orange drops is very subtle though (could also be just my imagination).

[holio cornolio]

Yes it does have a 470ohm resistor on the cathode. As I recall, I did mess around with resistor values at the cathode, and stuck something else in there (the kit came with 3 different resistors for the cathode), but by the time I was at the point of checking bias I was so excited to get playing through it that I didn't check it right. From what I recall, the manual intructed me to measure voltage at the plate, and then resistance across a 1 ohm resistor placed across the cathode (or something like that...) to get a value for current (which should have been a similar number in milliamps to the voltage number???) but i measured current instead and got a stupid reading, ditched my multimeter as faulty and went ahead and played anyway. By the time I realised my mistake I was too happy it was making any sound at all to worry that it was the right sound. As I understand it it's a more or less self biasing system anyhow so..... . I just ordered a nos sylvania . hytron 5y3gt so we'll see how that sounds....
Thanks for yr help Mr Kreuzer. Some day I'll get that P8R, but for the time being the C8R will have to do....

[Albert Kreuzer]

Yes, a 1 Ohm resistor on the cathode gives you 1mV for 1mA flowing through it. But you already have a cathode resistor in your champ, so measure voltage across your 470 Ohm, should be around 19V.
19/470=40.4 (mA). Shouldn't get hotter than that with a 6V6 and 320..350V on the plate.
If you up the resistor, you'll get more voltage across it, so best to upgrade the 25/25 cap to a 40V or 63V one.

[holio cornolio]

Hi - maybe you can help with this? I have 290V at the plate, which sounds a little weak. there's 300V getting to the rectifier, so it's the transformer that's down on power, but I guess within 10% tolerance is ok? Anyhow, I'm getting 16V across that 470 Ohm resistor (34mA) which according to my recollection of the Torres manual, means the bias is about spot on, but is that voltage at the plate an issue? I would have thought a lower plate voltage would mean a 'browner' sound, or is a little knowledge, in this instance becoming a confusing factor??
All this is with the 5AR4 still in place - I didn't get the 5y3 just yet, but I'll recheck with that installed too.
Thanks again for your help.
Matt

[Albert Kreuzer]

Yes, the plate voltage is a little low-ish... but no reason to worry.
You'll get slightly less output power and longer tube life. Try the 5Y3 and decide if you like it better than the 5AR4. In any case you are on the safe side and won't blow up anything.

[holio cornolio]

Could I have done something stupid wiring in the power supply, or are some transformers just weaker than others?

[Albert Kreuzer]

As you probably have 240V mains, everything stupid I can think of (regarding transformer wiring) would lead to MORE plate voltage. So I suppose it's the transformer. You can verify this by disconnecting all secondaries and measuring all AC voltages.

[holio cornolio]

Yep 240 volt mains here. Sorry, this could well be my last total noob question, but maybe you can elucidate on the following: I've spent this evening, whilst kids are in bed, going over the circuit, and reckon that this torres kit is pretty much the 5f1 circuit. The main difference I can find, is that the component values on the power supply are all (nearly) trebled. So the electrolytics, instead of being 16uf, 8uf and 8uf, are 47uf, 22uf and 22uf, and instead of the 10K and 22k resistors on the power rail, it has 2.2k and 4.7k. Why would the power supply need amending in this way? Is it just because of the 240 volt thing?? In any case I guess it's academic as more or less the right voltage is getting to the right places. I've also read that putting a 25uf bypass cap in parallel with the 1.5K resistor on the input stage (to terminal 3 on the 12ax7) will give a bit more gain. does that sound feasible??
Thanks once again for all your help. Having made this amp up entirely devoid of knowledge, I actually feel like I'm starting to understand what might be going on in there!

Oh bollocks - no it's not. I've just noticed that the bypass cap on the 470ohm cathode resistor is 47uf. Why would that need to be doubled in value over the original????

[Albert Kreuzer]

I've spent this evening, whilst kids are in bed, going over the circuit, and reckon that this torres kit is pretty much the 5f1 circuit. The main difference I can find, is that the component values on the power supply are all (nearly) trebled. So the electrolytics, instead of being 16uf, 8uf and 8uf, are 47uf, 22uf and 22uf, and instead of the 10K and 22k resistors on the power rail, it has 2.2k and 4.7k. Why would the power supply need amending in this way? Is it just because of the 240 volt thing?? In any case I guess it's academic as more or less the right voltage is getting to the right places.

More filter capacity makes a "stiffer" power supply, which in some cases means better. Also reduces hum.
You have to take care though not to exceed the rectifier's max (which is 50µF for the 5AR4, 32µF for the 5Y3). Considering that old electrolytics had +80/-20% tolerance, 47µF will be no real danger for the 5Y3.
The smaller dropping resistors compared to the original look to me like an attempt to compensate for the low B+ voltage. Has nothing to do with the 240V primary.

I've also read that putting a 25uf bypass cap in parallel with the 1.5K resistor on the input stage (to terminal 3 on the 12ax7) will give a bit more gain. does that sound feasible??

Yes.

Oh bollocks - no it's not. I've just noticed that the bypass cap on the 470ohm cathode resistor is 47uf. Why would that need to be doubled in value over the original????

Better bass response. Remember, electrolytics were expensive back then, and the champ was a budget amp. So they took the smallest caps they could get away with.

You could try and change all the caps to the original values and listen if you notice a difference. Then decide which you like better.

[holio cornolio]

So I've just got the 5Y3 which I've put in, as well as a gain selected 12ax7, and I think it's getting there. It certainly sounds a bit crunchier than it did, with a slightly earlier breakup on the volume control. My next step will be to replace that 47uf bypass cap on the cathode resistor with a 22uf (to decrease bass response - hey it's got to be a factor right?!) and put in that bypass cap on the input stage to get a bit more input gain, then, maybe one month, when the kids don't need new shoes / clothes / school trips or whatever, I'll treat myself to that P8R!
Interestingly, I've variously been doing the measurements either during my lunch hour at work, or in the late evenings at home. I actually get a higher plate voltage from the mains at home, but the amp is a LOT noisier. At work, plate voltage is about 10V (at 290 - 300) down but the amp is way quieter, even with all the fluorescent lighting, computers, fax machines and a bunch of analytical lab equipment around.
Thanks for all your help and advice on the above, it's much appreciated!

Matt

[holio cornolio]

AHHH!! What have I done??? Well - I put that 25uf cap across the input resistor to tag 3 on the premp tube, but the cap was a bit big to put on the board (seriously - it's 25/25, but it's huge) so I mounted it off board on some tag strip, but inadvertantly (???) soldered the +ve end to the stand off tag, so both ends of the cap were to ground. Amp worked fine for a minute or so, then started to distort badly and die. Quickly turned off and saw my stupid mistake, but reassembly without the cap gives the same horrible sound. A quick output tube swap and all is fine, but could this miswire on the preamp have fried the output??

[Albert Kreuzer]

I can imagine three possibilities (not sure if I read your post right):

1. If you are talking about pin 3 of the 12AX7, you shorted the cathode of the tube to ground. This could have caused the tube to draw too much current and burn out.
The output stage is not affected by this, you just killed the 12AX7 (but you said output tube swap, right?).
2. The cap shorted something else and caused the output tube to burn out.
3. The output tube died all by itself, not caused by your mistake.

Probably #3.

If the amp works fine with a new tube (and the cap in the right place), all is good.

[holio cornolio]

yeah it's the output tube that's been swapped. I've put a cheapy sovtek or ehx 6v6 (can't remember which) in there for now just in case. Bit upset as that was a nice NOS Mazda that just died (got killed?) Anyhow, all seems ok, so if it stays that way I'll stick something nicer in... thanks. I've left the cap off for now as I feel it's somehow jinxed me!!