5e3 build - fuse keeps blowing

[Bruce / Mission Amps]

Sounds bad to me.
I stopped using the Sprague 16uF-475v and 20uF-500v caps a couple years ago because I kept running into bad, brand new ones or having them go bad in less then 8 months to a year.
I use the less expensive TAD 22uF-500v caps from, probably, China... I hate doing it but I've not had a bad one yet.

You should not use the resistor to increase the bias voltage of the power tubes... instead use a couple 5 watt, 10v-15v zener diodes in the PT's high voltage center tap to reduce the overall voltage to the amp's B+ rail.

[AppleScrappler]

Thanks again Bruce! I'm going to try some different caps instead of Sprague. I've also read about the zener diode trick but was never entirely sure how to do it properly so i didn't risk it. Then just now I ran into this quote that you posted elsewhere on the forum:


"However, if you want to lower the B+ rail a little, pick up some $1.00 five watt 10v zener diodes and use a one, two or three of them in series with the red/yellow center tap wire of the PT before it is grounded.... the zener diode's cathodes pointing to ground.
Each 10v zener will NOT drop 10v but it will drop enough with a few to get you a little lower at very low cost.
Once the B+ is such that your JJ6V6s are running a little cooler, and you are satisfied, I'd just leave it alone and play. "

one question: won't the diodes get really hot?

So thanks for the advice! I know this amp is legendary for its overdriven tone but I really like its clean sound with a strat - especially when the neck and bridge pickups are on together in series. Big honking clean tone that I just love. So I like a little more headroom from this amp too. Ultimately I should like to have a switch to lower the B+ so I can have the best of both worlds. I've read of someone doing that mod somewhere. I have a JAN 5771 in the first tube slot (the 12AX7 was just over the top and the 12AU7 I got just sounded awful) and it tames the beast a little and sounds great as well. Also I've really grown to like Tung-Sol 6V6s. great bluesy breakup. The JJ's are a bit crunchier sounding and I like those as well.

Now I'm getting that itch to build another amp... can't wait to scratch it!

[ThermionicScott]

You really ought to get a 12AY7 - old or new - for the V1 slot. It'll bring the gain down even more.

[AppleScrappler]

Yeah I've tried a 12AY7 too... I've experimented with a lot of tubes in V1. It just so happened that the 5771 I had was the best sounding tube of the bunch I was playing around with. but from time to time I switch tubes around and experiment to see how my tastes change. As we all know sometimes it's a particular brand or even down to a particular tube that sounds the way we want... so far all 12AX7's have been over the top and I think that'll be the case with all of them. But other lower gain tubes I'm always game to experiment with. It's a matter of acquiring them - those NOS tubes can be expensive to collect! this thread has been great! keep the comments coming. I love to hear from you guys!

[ThermionicScott]

Yeah, NOS prices are up there. Pulls with life left in them are the real bargain.

[AppleScrappler]

that's true, I'm pretty happy with many brands of new production power tubes for 6V6, 6L6, EL84, EL34 etc. it takes some searching but I've been able to find new productions of those tubes that I'm happy with which is good because they need changing most often (except for the rectifier). preamp tubes usually last me a few years (depending on how much use the amp gets, of course) before sounding bad. those and the rectifier I like to get NOS (though the new tung-sol 12AX7's have served me well) and play around with. I'll even often just use the solid state rectifier in place of the tube when I'm doing playing that isn't recorded or live just to save the strain on an NOS 5Y3. It has a great sound too but can lack sag and be a bit harsh. It all depends on mood i suppose.

anyway... I ordered some parts which should be here next week and I'm going to try a string of diodes to lower the B+ as well as possibly changing the cathode resistor value back to stock. First and formost, though, I'm going to put in the new capacitor and make sure the amp functions normally. I really hope there's no other issues... I'll let the thread know what happens. I tested the screen resistors just to be safe and they ARE fine and thouroughly inspected the tube sockets for shorts and made sure nothing was arcing. seems good to go

and here's a chuckle courtesy of xkcd:

[AppleScrappler]

ok so i replaced the bad filter cap with a new one and the amp works like it used to. The B+ is about 430vdc which seems higher than I remember it being when I built it. changing rectifier and power tube configurations only drops it by 10v at most so I don't think tubes are the issue (besides I'm using new 6V6's and a NOS 5Y3). The new filter cap I put in is rated for 450v so I feel I'm getting uncomfortably close to that. Sure I could just get caps rated for 500v or more (which I probably will do anyway) but I really want the B+ down... way down. Isn't that safest for the amp overall anyway? Ideally under 380v but above 340v. Isn't that about where this amp was designed to run anyway? Stringing the zener diodes from the PT CT to ground is one way to lower it but is seems like I'd need a lot of them and there's the heat from those to deal with... I read some ideas (like the copper pennies) but was just wondering if there's anything else I can do to lower the B+ or if I'm overreacting and it's ok to leave it where it is. Plate dissipation on the 6V6's is currently at 11.61 watts each so, although they're above the usual 70% max dissipation rule, they're still below where most people seem to run them and that is fine by me. any thoughts are welcome as usual!

[ThermionicScott]

That B+ is pretty high -- are you using the Mercury Magentics PT that puts out 380-0-380?

The 70% rule applies to fixed-bias amps, but cathode-bias amps are run at higher idle plate dissipations (85% or more) because the bias shifts cooler (higher voltage at cathode) when it's amplifying large signals.

- Scott

[AppleScrappler]

it's a Heyboer that's 370-0-370 sold through ted weber made specifically for a 5E3. anything else wrong that might be demanding that much power? i could swear it was lower earlier on in the amps life (380-390 range). I haven't changed anything in it until this incident and am pretty sure switching the one cap didn't change anything. who knows. is it normal that a PT with that HV rating would reach 430 like that? is it dangerous to the PT? i don't want to burn it out. I suppose i'll use the diodes to drop B+ but i need to rig up a good way to dissipate the heat

[ThermionicScott]

Sounds like it's behaving as would be expected. The problem is that tweed deluxes are supposed to have a 350-0-350 or lower secondary voltage.

[AppleScrappler]

Thanks again for all the feedback! speaking of... I just completed the mod to add negative feedback to the tone control and also removed the bypass cap of the second preamp stage. I put a 56k resistor in for the feedback loop though I may put a 100k pot in there so fool around with it. Of course, being somewhat of a purist at heart, I have these mods on a switch that puts the amp back to stock settings. I'm quite interested in the extra headroom the mods give though and the potential that the amp can be a cleaner amp to gig with for jazzier shows or restaurant background gigs etc. More versatility in one amp is a big bonus for me right now being that I'm traveling a lot and may not always be able to have all my equipment with me at my disposal. but "that's the life of an army wife" ...except I'm an army husband. but still.... yeah. any comments on experience with these mods? I finished the work but won't test it until at least tomorrow

[guitarmike2107]

You seem to have the wrong PT, 380 - 0 - 380 is hot for a tweed deluxe

I would get the right PT since they aren’t that expensive, then use that over voltage PT to build something more appropriate.
I thought the standard deluxe PT was 325-0-325
I would also complain to the supplier, if they don’t know then they can never fix the issue for future customers.

I actually got duped on the PT on the first amp I ever build, also a tweed deluxe, mine was way under voltage and current.
I still have the e-mail were they told me the current rating would magically double and the voltage was correct. Back in those days I didn’t have internet access to know any better.

[AppleScrappler]

ok so I should make some corrections. The PT I have is rated for up to 25 watts and was meant to work for anything up to the range of a blackface deluxe and similar amps. I was led to believe (NOT BY TED WEBER!) that it would simply be overrated and therefore be better protected from burning out. Weber has served me fine in the past and has great customer service. This is more the fault of myself and poor advice I got a couple of years ago.

I have this exact PT at home:


I assume since it's 660v from red to red that it's 330 - 0 - 330. is that correct deduction on my part? and if I put this transformer in I should still expect it to create a B+ of around 370 - 380 correct? I'm just not too sure why the B+ voltage ends up being so much higher than what the sheet tells me. I thought maybe that has to do with what the components in the amp demand. I'm just confused I suppose. If I use the zener diodes and drop the B+ on the PT that's in the amp now to acceptable limits (which would be what? 380v? 360v?) is there any other reason to change the PT?

Now the OT I have in the amp is one that is paired to go with the PT that's in there now. In other words it's for a blackface deluxe and rated at up to 25 watts for an 8 ohm speaker. Is there any reason to change that too? I have a 15 watt Weber speaker in there now. aside from the speaker and transformers which I bought from weber the rest of the parts I bought from all over the internet. this wasn't a kit. so I put no blame on that company for getting the wrong type of transformer. that's all on me. thanks so much for fielding my stupid questions guys. I wish I could just intern with an amp guru and learn all of this well. or find classes that specialize in vacuum tube circuitry

[ThermionicScott]

Use that Weber PT, switch to a 250-300Ω cathode resistor, and you'll be golden. The one you have now is the type that the vintage amp nerds buy when they want the replacement PT to fry the tubes like their original 1957 model did with today's wall voltages.

As for the OT, it has a 6.6K primary whereas the original used an 8K. Shouldn't be a huge problem though, as many "5E3" kits ship with this iron.

- Scott

[ThermionicScott]

You should experiment with negative feedback on the output section as well. This would drop the overall gain a little, and reduce the intermodulation distortion that can make jazz chords sound unpleasant.

- Scott