New build complete and powered up great!

[Philr228]

Hi again,

It's been a long time since my last post/questions and I always intended to build my 5E3 head very slowly commitments permiting!

So I finally completed everything and switched on at the weekend.

And wow!

My nerveousness was unfounded - right voltages from the word go with just rectifier and when adding the rest of the tubes.

Then to plugging in and connecting speaker, I was waiting for some hum or hiss (I even thought I'd not connected correctly) quiet as a mouse almost the quietest tube amp I have! And what a great sound, mostly very clean -what I was after though haven't turned it up in anger! This could be down to the Hammond 125E outpur tranny.

So I thought I would share my voltages with you for opinions, they all seem good but the cathode voltage/current is a little high - but sounds great tubes not red-plating!

With all Tubes Installed:
All Heaters: 6.6
B+1: 365
B+2: 322
B+3: 247

V1
pin 1: 129.3
pin 2: 0
pin 3: 1.83
pin 6: 116.6
pin 7: 0
pin 8: 1.83

V2
pin 1: 168.9
pin 2: 0
pin 3: 1.2
pin 6: 204
pin 7: 19
pin 8: 47.9

V3
pin 3: 365
pin 4: 325
pin 5: 0
pin 8: 22.2

V4
pin 3: 365
pin 4: 323
pin 5: 0
pin 8: 22

V5
pin 8: 368

Tell me what you all think, there's a couple of strange ones if you include the pin 7 on v2 with the Cathode voltage/current (43mA)

I have photo's I will share too if you can bear it!!

Let me know what you thing you've all been so helpful getting me this far!

Cheers

Phil

[loudthud]

Pin 7 of V2 won't measure properly to ground. The meter will pull the grid negative which will disturb the bias point of the tube. You'll hear a thump through the speaker. If you measure from pin 8 to pin 7 you'll see pin 7 is about 1.5V negative WRT pin 8 as it should be.

[Philr228]

Thanks Loudthud,

So I needn't get worried about that, I wasn't too concerned about that as the amp sounds so good and clear. What do you think regarding the cathode voltage and hence the bias current and the limit of the 6V6's, although again Fender ran them hot and the tone is good to my ears anyhow!

Anyone else??

Cheers

Phil

[Philr228]

Incidentally my cathode resistor is 253 Ohms and the 6V6's are EH6V6's therefore:

22.2/253 = 0.0877 Amps (43.9mA per 6V6 -a bit high) Is it safe to subtract 2-3 mA for screen current?

365-22.2 = 342.8

342.8V * 41.9mA = 14.4 Watts

Please give me your thoughts, I double checked for red-plating last night and in a darkened room it's so difficult to determine whether it's heater glow or a plate, i.e not glaringly obvious!

It still sounds great though can't wait for a band rehearsal to try ir for real!!!

[loudthud]

You are right on the edge of plate dissipation. You can get a good estimate of screen current from the B+1, B+2 and B+3 numbers. I get 2.8699mA per tube assuming 4.7K and 22K for the dropping resistors. I'm calling B+1 "V1" because the + sign would be confusing.

((V1-V2)/4.7K) - ((V2-V3)/22K) = screen current for both tubes

So, 342.8V * (41.9mA-2.87mA) = 13.38W

You might want to bump the 6V6 cathode resistor up to 270 ohms but there won't be a huge impact on tube life or tone either way. You might want to take power line voltage into account. Are you taking these measurements at the high end of expected line voltages you might encounter where the amp is played? The amp will be OK in that case, but if these measurements were taken with a low line voltage, a little more margin might be prudent.

[Philr228]

Thanks Loudthud for your reply.

This might be a lengthy garbbled response!!!

The first set of measurements I took were with a wall voltage of 239V (yes UK voltage) so a little on the low side as it's quite often up at 246V.

I remeasured my voltages as follows:

Wall volatge 243V

With all Tubes Installed:
All Heaters: 6.56
B+1: 371
B+2: 316
B+3: 232

V1
pin 1: 126.8
pin 2: 0
pin 3: 1.82
pin 6: 116.3
pin 7: 0
pin 8: 1.82

V2
pin 1: 150.2
pin 2: 0
pin 3: 1.22
pin 6: 188
pin 7: 17.65
pin 8: 46.2

V3
pin 3: 367
pin 4: 319
pin 5: 0
pin 8: 21.58

V4
pin 3: 367
pin 4: 319
pin 5: 0
pin 8: 21.58

V5
pin 8: 369.5

I applied your calculation for determining the screen current (you're right with regard to the 4.7k and 22k resistors) and this time got 3.94mA for each 6V6. Applying that the the screen voltage drop (367-21.58) and the cathode current (21.58/253) I do get a dissipation of 13.4 Watts.

Now I'm actually reasonable happy with this (stupidity, gullibility, inexperience all considered) although I would like to try the 270Ohm cathode resistor, certainly just to see how it sounds. As I said earlier the amp sounds great just as it is and I believe that they can and do run hot, just not sure about Electro harmonix 6V6EH's!

By the way for others interested I'm running a Hammond 372BX and a Sovtek 5Y3(??) to give me those B+ voltages.

Loudthud, thanks for your help again I'm a bit happier now that you explained the calculations, I think I know where I am.

Once again this forum has shown it's merit - we are all wiser with shared knowledge.

Phil

[tubeswell]

JJ6V6S will take that dissipation

[Philr228]

I've heard good things about the JJ's but overall the EH's seem to come out with better reviews from a sound point of view and are still pretty tough, are they tough enough for my bias current, only time will tell and possibily a 270Ohm resistor!

Here's a few pics of my version of a 5E3 basically stock but using modern parts and a few build changes.











I'm very happy with it, but have I created a monster and what mods (if any) am I likely to make!

Thanks for all your help guys from day one, back in May of this year!

Cheers

Phil