Idle Power For Vibrochamp

[Amp Kat]

Just going over these numbers and realized yesterday how high the voltages are compared to the schemo. Of course this amp being much different than other Fenders because it's autobiased. The transformer voltage at current puts the plate voltage at 425 and the schematic says 340. I found about 110 to 115 VAC pretty good as far as the heaters being at 6.3vac and the rectifier at about 5vac. Of course with a 5AR4 you can tack on about 25 volts to that. The main problem is that at lets say wall voltage of 122vac and the plates at(5Y3) 400Vvdc the 470 cathode resistor yields about 25 to 28 volts across it so 53ma's X 400 and 21 watts of dissipation at idle ! I'm thinking this can't be right and most likely the cause for the blown OT on one amp and bad tone on the other and that's with a bypass cap. These should idle just a little over or 3/4 maybe of the total power. The number I like is arould 10 watts and that's still pretty hot. Increasing the Cathode resistor to 1K yields 28/1000=28ma's@400=11.2 watts so the question is do you try to lower the plate voltage of course one step is the 5Y3 or do you adjust the Cathode resistor to compensate for increased line voltage ? Just wondering what you do instead of varying the line voltage which many aren't capable of.

[bob p]

The classic recommendation would be the variac, or better yet, a bucking transformer setup. R.G. has an excellent article about building the Vintage Voltage Adapter at the GEO website.

If you just want to lower plate voltage, you could use the Zener diode trick, though this wouldn't help with the 6.3 and 5 VAC voltages.

As a matter of practice, I think that everyone who has a vintage amp should own a bucking transformer to bring the line voltages under control. A 120VAC-12VAC transformer rated at 1 to 2 A is an inexpensive part that can be picked up at Radio Shack, and the rest just involves buying a case, a power cord, and an AC receptacle. Personally, I think that plugging your vintage amp into a bucking transformer is an easier approach than modifying the circuits of all of your vintage amps, but that's just my preference. YMMV.

[Amp Kat]

Thanks Bob and I've done RG's bucker several times for my own personal amps but it's hard to sell a customer on it as it's something extra they have to lug around and many just don't want to do that. It really is the correct answer though because it takes care of all three problems in one shot and eliminates modding three circuits.

[bob p]

Yeah, convincing somebody to cart an extra piece of gear around can be hard to do. Getting them to always remember it may be harder still.

Sometimes I've installed the bucking transformer on the inside of a combo amp and rewired the power cord in the amp. That way the user can't forget to take it along.

If the 6.3 VAC and 5.0 VAC voltages are in-line and you only need to bring the B+ into spec, then you might want to consider the zener diode trick. Just in case anyone reading this needs a reference, Aiken has tech info about this type of approach on his back-biasing page.

[MWJB]

The figures you have for the VC are correct and normal, anyone who has heard and liked the sound of a stock BF/SF Champ has heard an amp running around these voltages and 20W plate dissipation, they go on and on and on like this for years. It is not the cause of "bad tone" and I seriously doubt that it is the cause of the blown OT.

If you want to get plate dissipation back to around the 14W mark try an 800-1000ohm cathode resistor...let your ears guide you not the calculator. Plate voltage will rise some (maybe 440v+ uncorrected so watch the filter caps voltage rating - it probably wants new ones anyway). Whilst you're in there replace the RCA speaker jack for 1/4" for a better positive contact on the plug tip, sometimes they get a little slack...this is potentially more dangerous for the OT.

I've never seen a BF/SF Champ that runs 340-350v.

[MWJB]

Further... I doubt that you'll see as much as 25v when switching to a 5AR4, more like 10-ish volts?

If you're getting 28mA with the 1K cathode resistor (sorry, I should have paid more attention first time I read your post) have a listen, then listen again at say 35mA to 40mA (660-800ohms?) and go with what sounds best. I'd be concerned that with guitar 28mA wil be a shade on the cold side (might be fine for a harp player though).

Use the rectifier that sounds best (listen to sag/envelope), voltage drop/variation is not going to be as significant a factor compared to a push pull amp with a bigger current draw.

If you significantly drop the B+ voltage (<400v uncorrected) it won't sound like a tolex Champ any more. Heaters are fine at 6.9VAC or under, lots of vintage amps run up to 7VAC, they most probably did this when people heard them & decided to buy them because of the sound they made.

[Amp Kat]

Mark you are right about the bad tone of the SF was due to a shorted Cathode Cap as I didn't test that amp,just played it. Not sure about what actually caused the OT to go bad on the BF one but we installed a Mercury and I raised that Cathode resistor to 1k and it sounds great so I may leave it. But the thing I did realize is it still doesn't red plate at the 21 watts but it just seems like it would with only a 12 watt 6V6 tube. The SF was red plating but of course basically a mu amp with the shorted cathode cap. I will try the 600 then 800 ohm setup and it seems like that break point is always at right about 820 ohms when you hear the change. Thanks for your input as always very good stuff

[bob p]

the first time that I was presented with a tolex champ that had a bad Ck on the power stage, i guessed that it sounded like a failing OT. as it turns out, i was barking up the wrong tree.

if i had to make a list of problems that come up in champs, i think that cathode bypass cap failures would have to top the list. the operating voltage on the typical SF champ was right at the voltage rating on the cap, and the caps were typically mounted up against the cathode resistor and they took a lot of heat, which is a recipe for failure. when replacing that cap, i'd use a 50V rather than a 25V cap, and I'd mount it as far from the resistor as is practical.

regarding the tone of the SFC and SFVC, to my ears they always seem to have a harsh component to them, which i think is partly attributable to replacement speaker selection and partially due to B+ voltages. of course, this is an entirely subjective statement.

[Amp Kat]

when replacing that cap, i'd use a 50V rather than a 25V cap, and I'd mount it as far from the resistor as is practical.

regarding the tone of the SFC and SFVC, to my ears they always seem to have a harsh component to them, which i think is partly attributable to replacement speaker selection and partially due to B+ voltages. of course, this is an entirely subjective statement.

I did exactly that Bob and pulled the Sprague Atom way to the side. The one that came out had a deep brown spot on the positive side but how many caps have you checked that are dead shorted. Man that's a good bit of heat. Mark stated that he has hardly never seen a Champ style at 340v on the plates, so what about you ? The extra heater voltage really doesn't bother me and it could use some new filters but that high plate voltage rubs me the wrong way as maybe I'm just anal so I guess we can deal with that too. I can certainly see also some dynamic and frequency reponse differences in speaker changes so yes I agree on that and the 1/4" plug change because that could very well have trashed that OT if the connection got either loose are non-conductive.

[bob p]

i don't get too excited about heater voltages; even though the amps that were designed to run at 110 or 115 VAC invariably have higher heater voltages when faced with today's modern wall voltages, they're always within the normal operating range defined in the tube data sheets. (high normal)

i've never seen an unmodified tolex era champ that had voltages that low either. that is not to say that voltages that low would be at all bad. i think that they would actually be better, as i don't like the sound of the tolex champs at high voltages. i prefer to dial them down a bit, and i think that the 360 VDC zone sounds quite good for that circuit.

if you don't mind modding the amp, you could perform the voltage lowering either with a zener diode, or with a suitable vitreous enamel power resistor between the PT center tap and ground. personally, i prefer the external mod/vintage voltage adapter.

[Amp Kat]

personally, i prefer the external mod/vintage voltage adapter.

That really is the way to go after all it's the way it was designed !