RobRob Deluxe Micro Build - Measured Voltages - Advice Needed

Lancer X

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Hi folks:

I'm finishing up my build of @robrob 's 5E3MM Deluxe Micro Mod design. Made guitar sounds on the first try, so yay for that!

My measured voltages versus those that @robrob reported follow:

My BuildRobRob
AC mains in120V AC125V AC
Rectifier (EZ81) in297V AC328V AC
Rectifier out384V417V
B382V400V (w/ added 220 Ohm resistor)
B+1364V390V
B+2292V304V
V1A (12AY7) Plate170V150V
V1A Cathode3.3V2.3V
V1B Plate136V146V
V1B Cathode2.2V2.4V
V2 (12AU7) Plate348V390V
V2 Cathode14.7V7.4V


I'm still learning about amp design, so not really sure how these differences will affect tone or if they are even significant. Can anyone advise?

In particular, I'm concerned about the V2 cathode voltage being so much higher. I'll look for bad solder joints, etc.
 
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dan40

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Double check your resistor values in that area to be certain you didn't make a mistake. I like to measure each resistor before installing to eliminate issues like this. You can try measuring them in circuit but some of them will not read accurately because they are in parallel to others on the board. In this case you need to unsolder one end to get an accurate reading. I will also recommend that you try a different tube because different tubes will bias up differently when installed in the amp.

Does the amp sound good otherwise?
 

Lancer X

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Double check your resistor values in that area to be certain you didn't make a mistake. I like to measure each resistor before installing to eliminate issues like this. You can try measuring them in circuit but some of them will not read accurately because they are in parallel to others on the board. In this case you need to unsolder one end to get an accurate reading. I will also recommend that you try a different tube because different tubes will bias up differently when installed in the amp.

Does the amp sound good otherwise?
Thanks. Yeah, I was checking all my resistor values as I stuffed the board.

I took a look at the typical voltages for the original 5E3, and I'm actually pretty close. Perhaps a little on the high side, but I'm running at a non-vintage 120V AC. @robrob 's wall voltage was 125.3, and he was also using a different transformer, so no big surprise we're different.

So, I don't think anything is wrong per se. In particular, the power tube cathode was showing 20V on the 5E3, which is more in line with my build. (Granted these are different tubes, but @robrob was trying to model the 5E3.)

I haven't really cranked up the amp yet, since I'm waiting on a few Keps nuts to properly mount the speaker in the chassis, and these voltage differences gave me pause. It's certainly making pleasant guitar-y noises. When I get a change to properly listen, I'll post back. Thanks again.

5E3_Voltages.png
 
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Lancer X

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Came across some more info. Earlier in his writeup on the 5E3Micro, @robrob stated:

For a solid state rectifier I believe the sweet spot comes from 250-0-250V of high voltage which gave me a B+1 of 390V, B+2 of 304, 150 and 146V on the V1 plates, 2.3 and 2.4V on the V1 cathodes; 375V on the V2 plate and 11.8V on the V2 cathode. I built my Deluxe Micro using an EZ81 tube rectifier and a 300-0-300V transformer gave 416v B+1 which is a little high so a 275-0-275V power transformer is perfect.

This leads me to wonder if the later stated V2 cathode of 7.4 volts was a typo.

So if anything, my V1A plate voltage might be a little hot, and I could adjust my B+2 with a different value voltage dropping resistor. As you say, I'll give it a good listen before changing anything.
 

robrob

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Yes, it looks like the power tube 7.4 volts is incorrect. It should be near 11 volts for my plate voltage.

@Lancer X all your voltages are close enough to function correctly and sound good but the power tube is running at about 109% with 100% being max safe. I would run it like that and if you burn up a power tube too quickly then you could increase the cathode resistor to 1200 ohms and that should get you under 100%.
 

Lancer X

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Yes, it looks like the power tube 7.4 volts is incorrect. It should be near 11 volts for my plate voltage.

@Lancer X all your voltages are close enough to function correctly and sound good but the power tube is running at about 109% with 100% being max safe. I would run it like that and if you burn up a power tube too quickly then you could increase the cathode resistor to 1200 ohms and that should get you under 100%.

Thank you so much for taking the time to review these values, @robrob !!

The amp sounds absolutely fantastic!! The two Micro Mod preamp option switches (cathode bypass and negative feedback) also create a ton of flexibility - anything from Tweed grit to Blackface cleans. So cool!

However, at master volume levels above about 1 o’clock, the amp currently makes extreme distortion and breaks up into absolute oblivion. Clearly pushing the power tube too hard as you pointed out. I’m going to dive in and make sure I don’t have a bad resistor or something, as well as looking at the tube specs so I know what you’re referring to.

Thanks again, @robrob !
 

Lowerleftcoast

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at master volume levels above about 1 o’clock, the amp currently makes extreme distortion and breaks up into absolute oblivion.
Is the oblivion blocking distortion???
Did you follow Rob's design exactly with the 470k control grid stopper on V2?
The coupling cap after the plate of V1b can be reduced to help quell distortion. (On a 6V6 Deluxe I have reduced that .022uF cap to .0033uF. YMMV on a micro but, I would reduce that value to .01uF (or less) right off the bat. I am not sure why Rob shows a .1uF on the micro. The Firefly schematic on Rob's micro site shows a .022uF there like an original 5E3.)

Happy hunting.:)
 

Lancer X

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Oh for cripes sake. I just realized my problem.

@robrob ’s circuit and component values for his 5E3 Micro Mod design are slightly different from the regular 5E3 Micro circuit and values. I built the regular 5E3M circuit, and just slapped in the Cathode Bypass and Negative Feedback mods from the 5E3MM design assuming it was identical to the regular 5E3M design. It isn’t…

What a dope. It’s no wonder things are a bit off…. 🤪🥸
 

Lancer X

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My post above is incorrect. @robrob ’s 5E3MM circuit isn’t different. It’s based on a diode rectifier, while I’m using an EZ81. The only value difference was the power tube cathode resistor.

I found another mistake that’s likely affecting my power tube bias. I mistakenly wired my power transformer center tap to the ground side of the power tube cathode resistor and bypass cap, instead of the intended ground/anode of the B+ filter cap. (Dammit, Jim - I’m a materials scientist, not an electrical engineer…) 🤓

I’ll reroute the center tap tomorrow and re-measure my power tube bias. Finger crossed! Worst case, I’ll drop in a different cathode resistor.

Thanks again to @robrob! After some head scratching, I now understand that he was referring to Maximum Plate Dissipation, and that I can/should aim for 100% of that value (5.5W for a 12AU7) when adjusting my output tube bias. Score one for insomnia-borne bonus reading time… 😴
 

Lowerleftcoast

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I can/should aim for 100% of that value (5.5W for a 12AU7) when adjusting my output tube bias.
I can't speak for the 12AU7. The 6V6, in cathode bias, seem to sound best when they are pushed. The 12AU7 is one of the easiest tubes to get a hold of. Not much of loss should it fry. I liked Rob's advice. If it is not red plating, run it past the 100% figure and see how long it can take it. Sure, that will put you in the *Make it take it* club but those guys have a smile on there face while the tube simmers. You might be surprised how long a tube can outdo the datasheet especially in cathode bias.
I mistakenly wired my power transformer center tap to the ground side of the power tube cathode resistor and bypass cap, instead of the intended ground/anode of the B+ filter cap.
Electrically that is the same place. The suggestion to route the PT CT to the first cap is to keep the rectifier/PT/reservoir cap current loop as short as possible. As long as you are not experiencing ripple current hum/buzz you are good to go. There are a lot of amplifiers that have long distances for the rectifier/PT/reservoir cap current loop that have no noise issues. (Fender, Marshall, etc.)
 

Lancer X

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Electrically that is the same place. The suggestion to route the PT CT to the first cap is to keep the rectifier/PT/reservoir cap current loop as short as possible. As long as you are not experiencing ripple current hum/buzz you are good to go. There are a lot of amplifiers that have long distances for the rectifier/PT/reservoir cap current loop that have no noise issues. (Fender, Marshall, etc.)

Oh man. I woke up this morning and as soon as the thought entered my mind, I thought oh man that’s just a ground bus. I don’t think that would actually affect the cathode resistance…. 😒 Don’t theorize at 3am, y’all…
 

Lancer X

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You guys are not going to believe the actual cause of the problem. I spent all day learning about cathode bias testing, etc. and trying to figure out if my bias was out of whack. My eventual results:

Plate to Cathode 338V
Voltage across Cathode Resistor 13.8 ohms
Cathode Resistor actual 810 ohms

The result? I’m at 5.7W plate dissipation. 103.6% of ideal, but didn’t really explain the amp’s weird master volume behavior. (If I turned master up over 1 o’clock, the distortion spiked exponentially and the sound cut out.

Everything was measuring within tolerance. The only thing I could think of was that there was some gunk stuck in the master pot? Worked it around for a while and then plugged in the Strat. Sure enough, the loud crackles were in a different place, and the volume cut in and out randomly. Crud in the brand new volume pot. Maybe solder flux or something? I couldn’t believe it - hours and hours down the drain. 🤯 (Well, I guess I learned a good bit about cathode biasing at least…)

Worked the knob around a bit more, and the problem was almost gone. This amp sounds amazing, and at any volume.

What’s your fave cleaner to use on dirty pots?

Thanks, everyone - and especially to @robrob . I’ll make some videos when I get a few free moments.
 




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