Brand new 5E3 gets its first power up...(long)

[PremiumPlus]

And it sounds delicious. I've never played an old Deluxe before and built it on reputation and internet clips I've seen. Now I understand why its so popular! what a touch sensitive, expressive amp. I don't have my cab or speaker (Mojo cab with Celestion Blue) yet so I'm playing thru a 1x12 open back EVM12L.
There is a straaange problem with the standby though; it hums when I put the amp on standby from operate. When I switch to standby I get a significant hum that diminishes slowly to silence. It sounds like an open guitar cord is plugged in and someone is holding on to it. But it's dead silent and sounds great during play. I put the switch directly between the rectifier and the first filter stage. Is it possible that the 5 volt and 6.3 volt heaters are out of phase with respect to each other? I need to get back to the bench and see if there is a leakage path somewhere. All the negative sides of the filter caps are tied together on a bus that goes to a star ground, and that bus is then tied to the brass control plate behind the pots and jacks. I don't think I have a ground issue because the amp is so quiet in operation. But that hum is entering somewhere when I open the standby switch.
Incidentally, the amp hummed a bit in first operation and so I tied the 6.3 volt center tap to the 6L6 cathodes and that quieted it down. I thought that might fix the 'after standby' hum but it didn't. The amp is nice and quiet in operation. I thought after 40+ years of experience with tube amps that culminated with an EE degree (went back to school late in life, another story) that I had seen it all. I stand humbled...help...:
I'm going to try moving the standby to the HV center tap and see what happens.

[mbratch]

Sounds like fun PP. Isn't it exciting to get that thing working the first time and hearing it? I remember how thrilled I was a few weeks ago when I first powered up my 5E3 kit.

I was going to suggest switching the HV CT for standby, but you already thought of it. I use that method and it works well.

I have the mojo kit with their cabinet. Their cabinet is quite nice. I got the lacquered version. It is very well done. I think he builds them in batches, so depending upon where your order falls between batches that likely determines the lead time.

[loudthud]

Does swapping tubes around or the volume controls affect the hum?

[PremiumPlus]

Okay, all is well. When I wired the standby I only cut the B+ going to the main filters, and left the stupid output transformer primary hooked to the rectifier tube socket! I moved the wire and it's great now.
Now I'm having fun dialing in my output tube bias. I've gone from a 250 ohm cathode resistor up to a 275 ohm to cool them down a little. Voltages:
Output tube plates: 404.
Power supply nodes: 407,372,277.
The cathode resistor is dropping 24 1/2 volts across it and gets real hot. According to my calculations it's dissipating just under two watts. The tubes aren't redplating but they are good and hot!
Do these numbers fall in line with what a 5E3 should be? I'd appreciate it if those who know could give me some feedback...But I'll say this...it sure sounds like a great amp!

[mbratch]

Hey PP... what are the specs on your PT (output voltage)?

404 V at the plate, and 24.5V on the 275 ohm cathode means that your power dissipation at idle at the 6V6 plates is (approximately): 404 * (24.5/275) / 2 = 18 watts (if I did my math right). That's a bit high for 6V6s which are spec'ed at a max of 14 watts (although some vintage Fender amps do drive them pretty hard!).

What brand/type of rectifier tube do you have in there?

[tubeswell]

You could always use a 10W cathode resistor if you're worried about the heat.

At the risk of opining on the tabooish subject of plate voltages, to me your voltages are a bit high for some 6V6 tubes (but JJ6V6s could handle that). Other people have different views. How rugged is your heater circuit? You could try a couple of 6L6s in there (if you had at least 3A, and could change the load resistance through the OT to 4k instead of 8k)

I built one last year and preferred to run the plates lower (around 350-360) with 6V6s. I kept a ~250R cathode resistor and really liked the sound of that. You could also try zenering down the B+ to see if you like the sound with lower voltages.

[PremiumPlus]

Here are some shots of the nearly complete chassis. I just cleaned up and completed the output jacks, and repaired the standby error. Plus I changed the cathode resistor from 250 up to 275 ohms. Sounding good!

[PremiumPlus]

Thanks guys,
mbratch, the p trans spec is 370-0-370-50, with 5 volt and 6.3 volt (w/ct) heater windings. I actually expected higher voltages after rectification/filtering than I got. It's a Heyboer. The heater current spec is unknown, I'll have to email them and see. But it was purchased thru the Weber site, and spec'd for a Fender Deluxe/Deluxe Reverb, so I think it would handle the heater requirements of 6l6s. It's an RCA 5Y3 rectifier tube.
As far as heat, I calculate that the cathode resistor is dissipating about two watts, and it's a five watt resistor. I'm fine there. What concerns me is the tubes idling at 18 watts...That's like 50% more than they max out at, according to my book. Wouldn't they be redplating at that number? I need to rethink my calculations, don't we need to include screen current too? My tubes weren't redplating with the 250 ohm resistor and they are cooler now with the 275 ohm in there. But I'm still concerned. Maybe I will try dropping a zener on the centertap of the HV. I think I've got a 5 watt 50 volt one around somewhere...but that might be too much...what do you think?
I'm gonna quit writing and go back to playing!

[tubeswell]

Thanks guys,
mbratch, the p trans spec is 370-0-370-50, with 5 volt and 6.3 volt (w/ct) heater windings...

Maybe I will try dropping a zener on the centertap of the HV. I think I've got a 5 watt 50 volt one around somewhere...but that might be too much...what do you think?

The 5E3 tranny was 325-0-325. The Deluxe Reverb was a BF amp and had much higher B+. If you want, you could zener it down with a bunch of 5W zeners in series (say 5 x 9V or 4 x 12V). Soldering them on a terminal strip would help with the heat dissipation too (not that it would be likely to be a problem). Depends what you like the sound of.

[mbratch]

I'm a bit surprised to see a B+ greater than 400V with a cathode resistor of 275ohms and a vintage 5Y3. (The new Sovteks, for example, generate a much higher voltage.) I would have expected to see you hit 400V if you had a larger cathode resistor. But that's only based upon my single-amp experience (see below).

Tubeswell, don't Deluxe Reverbs run the plates typically at around 420V or so on 6V6's? So in this case I wasn't so sure his 404V would hurt. Also, if 6L6's are put in, is he in danger of drawing too much current from the PT? I'm not sure what the current spec of the PT would need to be. You know more about this stuff than I do...

I have a 384-0-384 PT and had gone through the gamut of all sorts of mods to control it. I finally did 3 11volt, 5watt zeners in series off the HV CT and that gave me all the "right" measurements: 350V plate voltage (to spec) on the 6V6's and a plate dissipation of around 11 watts, still a little high but not bad. And I was using a 270 ohm cathode resistor. After all that, the amp lost quite a bit of that chime I was used to hearing on examples of 5E3 recordings. So I took the zeners out and went with 470 ohms on the cathode resistor. This took the plate voltage to 395V, about the same power dissipation, but the chime was back. However, this left me feeling perplexed about what the proper configuration for a 5E3 would be. Maybe what we all love about a 5E3 isn't what a 5E3 really sounded like back "in the day"?

[PremiumPlus]

Thanks for the replies, guys!
I have a 50 volt, 5 watt zener floating around the shop somewhere...it might even be in my diodes drawer (imagine that...). But maybe first I'll try a big cathode resistor like you, mbratch. I sure don't want to lose any chime, the tone of the amp is really very good considering how hot the tubes are running. I'm always amazed when electronics parts, especially tube era design, survive the harshest operating conditions. Try overdriving a solid state output device to 50% over max sometime!
Anyway, I'll try another resistor first. A good friend from our church just turned me onto about 15 pounds of vintage electronics parts; really cool stuff like mil-spec switches, old wirewound high power resistors, pots, caps, and misc. hardware. I'm sure I saw a 10 watt resistor around 500 ohms in there somewhere...
I'll post back when I find out some more. Thanks for the help.

[mbratch]

The possible issue with the 500 ohm resistor in your case is it will raise the plate voltage even further (I'm still a little surprised it's at 404V with a 270 ohm resistor and a vintage 5Y3). If you have a spare 5Y3, you might try that as well (as long as it isn't a new product Sovtek). To bring the plate dissipation down, you may need to do both the zener and the increased cathode resistor.

[PremiumPlus]

ALLRIGHT! I installed a 50 volt zener in series with a 100 ohm resistor and the B+ is right at 349 volts on the plates. My tubes are dissipating 12.05 watts...not that I'm nitpicking!
I ran the amp up and down on a variac as I played it and there definitely is a sweet spot at 350 volts. I took the chassis to church yesterday and plugged into a 2x12 with G12H30 speakers. People were stopping what they were doing and saying how "pure" and "clear" and "sweet" the tone was. I can't wait to hear it when the cab and Celestion Blue get here in a couple of weeks.

[mbratch]

OK, now you have me wanting to go back and try my zeners again. When I had originally gotten my plate volts down to 350 I thought the sound I was getting was duller and not as nice. Maybe I was mistaken... The perceptions really mess with you when you're sitting there trying different things!

[straycat]

Buy a PT transformer from Bruce at Mission Amps and all of your voltages will be correct



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