Separate names with a comma.
Discussion in 'Shock Brother's DIY Amps' started by jvande7, Oct 14, 2020.
Do you happen to remember what type of resistors you used?
Cement block probably ordered from mouser.
EDIT: Mouser # CP0005R1000KE14
5W 100mOhm Vishay/Dale
I have no affiliation with the above mentioned companies.
I've never even measured heated voltage.
You could use a pair of diodes, one in each direction which will knock off about 0.7V. As long as the heater current is less than the current rating of the diodes you should be good to go (1N4007 will handle 1A and should be sufficient here).
But as most people have said, I wouldn't sweat over 6.8VAC on the heaters.
Squawker beat me to the suggestion, I'd also say that diodes was the best and easiest solution.
That chart seems to be saying it's much easier to ruin them with less voltage.
I've seen full wave rectifiers in heater circuits, although I think the scope was to reduce noise.
@Squawker @Bendyha I read that diodes could theoretically add hum in some situations. Not sure how much truth there is to that, but I decided to go with resistors instead.
I did use four 5w 9v zener diodes to lower the B+ though. It worked perfectly (dropped me from 383v to 352v), but the more I play with the amp the more I feel the diodes need to go because the 6v6 is only dissipating around 11.2w, and the overdrive isn't quite right. In original config (383v B+) it was dissipating around 13.8w, and I was worried but I've since been taught that the 6V6 is a14w tube, not 12w. And I understand you want to dissipate 95-100%+ of the tubes rating when using a single-ended class A amplifier.
Does it make more sense to adjust cathode resistor value to set bias? In this case I went with dropping the B+ because I wanted to lower it from 383v which seems high. And anyone I've ever read about changing the 470R resistor value always ended up putting it back to 470R in the end.
Another option - if you've been at all interested in getting a variac and are having trouble justifying it this could be what pushes you over the edge
^^^This. Especially with B+ at 383 V as documented in OP's first post (layout shows 340, and 295 rather than 330 at pin 4, both of which are 12% or so over "spec", though within Fender's "20%"). Arguably cheaper than a Variac would be the (Robinette) BuckMinister approach, with a 6.3/12.6 bucking transformer, a couple switches and a fuse. Knock down both B+ and both sets of heater voltages by ~7% or 14% at one fell swoop.
Plus, all this can happen outside your amp enclosure, so zero heat buildup (except ambient).
What I normally do in a cathode biased output stage is to select a resistor that is higher in resistance than the normal value. Then I parallel a smaller (wattage) resistor across it to drop the resistance down to where I think it should be. So you can have a relatively easy way of having adjustable bias on a cathode biased amp.
The watt or so with the resistors is not going to change the temperature all that much.
@jvande7, if you want to properly go down the tube life rabbit hole, read Robert Tomer's "Getting the Most Out of Vacuum Tubes": http://www.tubebooks.org/Books/Atwood/Tomer 1960 Getting the Most Out of Vacuum Tubes.pdf (Just Google it if this link doesn't work.)
It's a really good read on the likeliest causes of premature tube failure (defined as less than 5,000-10,000 hours), and what to do about them. Just be forewarned that you may end up with an amp that no longer resembles a 5F1 -- or any other Fender amp.
As mentioned before, it's worth putting this in perspective: if I wear out a $25 power tube in "only" 1,000 hours (that's an hour a day for the better part of three years), then my cost comes out to a couple of pennies for each of those hours. Cheap thrills, really!
This might help.
Let’s pull up a few thousand feet and survey the whole battlefield.
1. I don’t worry about heater voltage per se, but B+ *and* heater voltage vary together... what’s your B+?
2. I run good ($$) NOS output tubes, and experts (as above) since the '20s point out tubes last longer if not above 6.3.
3. Your amp will sound different, possibly better, if wall voltage is say 110-115 instead of 120-125.
Variacs are nice and have other uses, but a simple and possibly cheap option is a 'vintage voltage adapter.'
RG Keen tells all here. Rob has parts and options for building a switchable version. Here's one I built.
My original B+ was 383v and 6V6 tube dissipation was about 13.82w
I installed four zener diodes at the PT CT to ground, which brough B+ down to 352v and tube dissipation became 11.2w
I would love my B+ to be around 350v like it is, but I would like to bias the 6V6 hotter, which seems to indicate I should change the value of the 470R cathode resistor, but I fear I'll lose the famous character of the amp based on various forum posts I've read where folks change the 470R a bunch and then ultimately put it back to 470R citing poor tone as the reason.
At this point I'm ready to just return it to the original 383v / 13.82w configuration and call it good. Sure it'll kill tubes more frequently but even if I get 1,000 hours from a tube... I play less than one hour most days, and not at all on some days. Like someone said above that's 3 years if I play an hour each day. I can certainly afford to re-tube every few years, and realistically the tubes will last longer than that since I'm just a bedroom player and my actual average playing time is far less than 1 hour per day (when averaged over 1+ years).
I'm interested in getting a variac, but I don't see how that's going to help me here. I've already successfully dropped my B+ to the level I want. How would the variac help me with heater voltage and/or output tube bias?
Transformers are ratio devices. Lower the AC coming in, each secondary will drop proportionately to the AC feed's reduction. The heaters are a secondary.
In your example, 6.8V is about 8% higher than spec. If you were to reduce the AC by about 10 V, that would drop the heaters roughly to 6.3V.
At the same time, your high voltage (B+) secondaries would be about 8% lower and so would your filtered B+, to a first approximation. 92% of 383 would be 352, right where you went with the Zener method. Remove the Zeners and you are right where you were with B+ around 350 and your heaters are now taken care of too.
You'd want a 120V:10V transformer with 250 VA rating or more, again rough values. Put the 10V winding in series with the 120V winding. Feed the combined series winding with 120V, feed the amp from the neutral and the junction of 120 & 10.
Safety first. Follow all standard precautions for mains wiring - a fuse in the adapter's box, a metal shell enclosure bonded to both the wall outlet ground and the adapted outlet's ground, yadds yadda yadda.
Second, be sure the 10V winding and the 120V are phased the same to do it this way.
There are already links in this thread for a slightly different approach but it's basically the same idea.
The arrangement I've put forth is EXACTLY how a variac adjusted to 92% works: a tapped autotransformer.
As a bonus, your adapter box can be used with any amp, and your '50s tweeds will be very happy you did.
Excellent. I can get a 5amp variac for about $60, so maybe I'll give that option a try.
The only bummer is that my 6V6 bias will still be cold, and I can easily add a parallel resistor to rebias, but then if I ever run the amp without the variac the bias will be too hot.
Which makes me want to just get the Hammond 290AX transformer which has 550v secondary, but the 6.3v heater still runs at the same 2.25 amps my current PT runs at, so I'd still have the slightly high heater voltage issue.
Yeah, I feel ya. You should be able to source a surplus 10V mains transformer and a box big enough to hold everything somewhat less than that, but the Variac has the Choose Your Adventure feature which the adapter won't.